JP2011019372A - Switching power supply apparatus and semiconductor device for switching power supply - Google Patents

Abstract

PROBLEM TO BE SOLVED: To trigger and reset a load which requires a large output power while suppressing degradation in effect of overload protection function.SOLUTION: A switching power supply apparatus 50 converts a DC input voltage VIN that is input to an input unit 6 into a DC output voltage VO for output. The switching power supply apparatus 50 includes an overload detection circuit 15 which determines whether or not the output voltage VO is higher than a first threshold voltage, an overload control circuit 13 which causes a switching device 1 to operate continuously if the output voltage VO is higher than the first threshold, and causes the switching device 1 to perform an overload protection operation if the output voltage VO is lower than the first threshold, and a resetting detection circuit 11 which determines whether or not the output voltage VO is higher than a second threshold which is lower than the first threshold. During the overload protection operation, the overload control circuit 13 increases a power to be supplied to a load 8 if the output voltage VO is higher than the second threshold, as compared with the power when the output voltage VO is lower than the second threshold.

Description

  The present invention relates to a switching power supply device and a semiconductor device used therefor, and more particularly to a switching power supply device having an overload protection function.

  Conventionally, in a switching power supply device, in order to prevent a large current from being output when the output voltage drops due to a load abnormality such as a load short circuit, or a current exceeding a certain value is accidentally applied to an electronic device as a load. An overload protection function is often required to prevent flow. In addition, many methods are known as methods for realizing this overload protection function.

  There are two types of overload protection methods, a latch stop type and a self-reset type. The latch stop type is a protection method in which the switching device does not oscillate again unless the input voltage is dropped after detecting the overload state and stopping the switching operation of the switching device. On the other hand, the self-recovery type is a protection method in which after the overload state is detected and protection is performed, the switching power supply device operates normally again when the overload state disappears.

  If there is a switching power supply that requires latch-stop type protection for a safe stop in the event of an overload, even if the protection is activated during an overload, the self-reset type that operates again when the overload condition is resolved Some switching power supply devices require protection, and the required protection method differs depending on the equipment connected to the output of the switching power supply device and the usage environment.

  Hereinafter, a conventional switching power supply device having a self-recovery type overload protection function will be described.

  As an example of the self-recovery type, when an overload condition is detected, an overload protection function is generally known that reduces the energy supply to the output unit by reducing the period during which the switching device oscillates to a certain rate. .

  FIG. 8 is a circuit diagram showing a configuration example of a conventional switching power supply apparatus 100. 8 includes a transformer 102, an output unit 105, an input unit 106, an output voltage generation circuit 107, an overload protection operation capacitor 109, an output voltage detection circuit 110, and switching power control. Semiconductor device 126 for use.

  Transformer 102 includes a primary winding 102A and a secondary winding 102B.

  The output voltage generation circuit 107 is a rectifying / smoothing circuit including a diode 103 and a capacitor 104, and is connected to the secondary winding 102B. The output voltage generation circuit 107 is connected to the output voltage detection circuit 110 and the load 108.

  The output voltage detection circuit 110 outputs a control signal for controlling the output voltage VO on the secondary side to be constant from the secondary side to the feedback signal control circuit 114 of the control circuit 112 on the primary side. The load state is transmitted to the control circuit 112.

  The semiconductor device 126 includes a switching device 101 and a control circuit 112 for the switching device 101.

  The semiconductor device 126 includes, as external input terminals, a drain terminal that is an input terminal of the switching device 101, a VDD terminal that functions as an internal power supply terminal and an overload protection operation terminal, and an FB terminal that is a feedback signal input terminal. And a GND terminal of the control circuit 112 and a source terminal functioning as an output terminal of the switching device 101.

  The overload protection operation capacitor 109 is a capacitor for overload protection operation connected to the VDD terminal, and at the same time serves to stabilize the power supply of the control circuit 112.

  The control circuit 112 includes a feedback signal control circuit 114, a clamp circuit 119, an overload protection circuit 120, a primary current detection circuit 122, a comparator 123, a constant current source 124, and a drive circuit 142.

  The constant current source 124 is a starting constant current source for supplying a circuit current when the switching power supply device 100 is started, and supplies a starting current to the VDD terminal via the switch 125 when the switching power supply device 100 is started.

  The primary current detection circuit 122 detects the on-voltage of the switching device 101 generated by the product of the drain current ID flowing through the switching device 101 and the on-resistance of the switching device 101, thereby obtaining the drain current ID flowing through the switching device 101. To detect. The primary current detection circuit 122 generates a voltage signal corresponding to the current value of the switching device 101 by converting the detected current value of the switching device 101 into a voltage signal, and the generated voltage signal is compared with the comparator 123. Output to.

  The feedback signal control circuit 114 converts the feedback current IFB that is a current signal input to the FB terminal into a voltage signal, and outputs the converted voltage signal to the comparator 123.

  The clamp circuit 119 is a clamp circuit for determining the maximum value of the voltage signal output from the feedback signal control circuit 114. The clamp circuit 119 determines the maximum value of the drain current ID flowing through the switching device 101 and realizes the overcurrent protection function of the switching device 101. As a result, the maximum value of the primary side drain current ID is limited, so that the maximum power supplied to the secondary side load 108 is limited. That is, the clamp circuit 119 determines the overload protection level.

  The comparator 123 outputs a reset signal to the reset terminal of the RS flip-flop circuit 128 when the voltage signal output from the feedback signal control circuit 114 becomes equal to the voltage signal output from the primary current detection circuit 122. To do.

  The drive circuit 142 drives the switching device 101. The drive circuit 142 includes an oscillation circuit 116, a gate driver 117, a NAND circuit 118, and an RS flip-flop circuit 128.

  The oscillation circuit 116 outputs a maximum duty cycle signal 116A that determines the maximum duty cycle of the switching device 101 and a clock signal 116B that determines the oscillation frequency of the switching device 101. Maximum duty cycle signal 116 </ b> A is input to NAND circuit 118, and clock signal 116 </ b> B is input to a set terminal of RS flip-flop circuit 128.

  The NAND circuit 118 receives the output signal from the counter circuit 121 that receives the output signal from the start / stop circuit 127, the maximum duty cycle signal 116A, and the Q signal output from the RS flip-flop circuit 128. The output signal of the NAND circuit 118 is input to the gate driver 117. Thereby, the switching operation of the switching device 101 is controlled.

  The overload protection circuit 120 detects that the load 108 is in an overload state. The overload protection circuit 120 performs an overload protection operation by controlling the voltage at the VDD terminal when an overload condition is detected.

  Specifically, the overload protection circuit 120 switches the switching device 101 in a normal operation mode in which the switching device 101 is continuously operated when the load 108 is not in an overload state (in a normal load state or a light load state). Make it work. Further, the overload protection circuit 120 sets the switching device 101 in an overload protection operation mode that repeats an operation period in which the switching device 101 operates and a stop period in which the switching device 101 stops when the load 108 is in an overload state. Make it work.

  The overload protection circuit 120 includes an overload control circuit 113 and an overload detection circuit 115.

  The overload detection circuit 115 uses the signal output from the feedback signal control circuit 114 to detect whether or not the load 108 is in an overload state. Specifically, when the power required by the load 108 in the normal load state and the light load state is smaller than the maximum power that can be supplied from the primary side to the secondary side, the signal from the feedback signal control circuit 114 is Since it becomes lower than the threshold value IFB (OLP), the overload detection circuit 115 sets the overload detection signal OLP to a low level (hereinafter referred to as “L”). On the other hand, when the load 108 is overloaded and the power required by the load 108 is larger than the maximum power that can be supplied from the primary side to the secondary side, the signal output from the feedback signal control circuit 114 is greater than the threshold IFB (OLP). Therefore, the overload detection circuit 115 sets the overload detection signal OLP to a high level (hereinafter referred to as “H”).

  Further, when the load 108 returns from the overload state to the normal load state, the feedback current IFB output from the output voltage detection circuit 110 increases. As a result, the signal output from the feedback signal control circuit 14 becomes lower than the threshold value IFB (OLP) of the overload detection circuit 115, so the overload detection circuit 115 switches the overload detection signal OLP from H to L. As a result, the overload protection operation mode is released, and the switching device 101 operates in the normal operation mode.

  The overload control circuit 113 selects the overload protection operation mode when the overload detection signal OLP output from the overload detection circuit 115 is H. Further, in the overload protection operation mode, the overload control circuit 113 charges and discharges the overload protection operation capacitor 109 via the VDD terminal, so that the operation period and the stop period in the overload protection operation mode are set. The length is determined and the operation period and the stop period are switched.

  The overload control circuit 113 includes a counter circuit 121, a switch 125, and a start / stop circuit 127.

  The start / stop circuit 127 controls the start and stop of the semiconductor device 126. The start / stop circuit 127 controls the voltage of the VDD terminal by turning on or off the switch 125. Further, the start / stop circuit 127 outputs a signal for switching the stop period and the operation period in the overload protection operation mode to the NAND circuit 118 via the counter circuit 121 according to the voltage of the VDD terminal.

  Specifically, the start / stop circuit 127 outputs a signal for continuously operating the switching device 101 to the NAND circuit 118 via the counter circuit 121 in the normal operation mode (the overload detection signal OLP is L). At this time, the switching device 101 operates at the oscillation frequency of the oscillation circuit 116.

  In the normal operation mode, the start / stop circuit 127 turns on the switch 125. As a result, current is supplied from the constant current source 124 to the VDD terminal via the switch 125. The constant current source 124 supplies current from the drain terminal to the VDD terminal via the switch 125 in order to maintain the constant voltage even after the voltage at the VDD terminal reaches the constant voltage.

  On the other hand, when the overload detection signal OLP from the overload detection circuit 115 is H, the start / stop circuit 127 performs the overload protection operation after the voltage at the VDD terminal reaches the stop voltage VDD (OFF). By turning on the switch 125, the overload protection operation capacitor 109 is charged. The start / stop circuit 127 turns off the switch 125 after the voltage at the VDD terminal reaches the start voltage VDD (ON). As a result, the voltage of VDD decreases due to the circuit current of the semiconductor device 126.

  Thus, when the overload detection signal OLP from the overload detection circuit 115 is H, the voltage at the VDD terminal repeatedly rises and falls between the start voltage VDD (ON) and the stop voltage VDD (OFF).

  The start / stop circuit 127 outputs a signal indicating whether the overload protection operation capacitor 109 is being charged or discharged to the counter circuit 121.

  The counter circuit 121 counts the number of times that the signal output from the start / stop circuit 127 becomes H. Specifically, after activation, when the H signal is input from the activation stop circuit 27 to the counter circuit 121, the counter circuit 121 operates the switching device 101 by outputting an H output signal to the NAND circuit 118. . Further, the counter circuit 121 counts every time the signal from the start / stop circuit 127 is alternately switched from H to L during the overload protection operation mode, and the number of H signals output from the start / stop circuit 127 is counted. After counting a predetermined number of times, an H output signal is output to the NAND circuit 118. That is, the counter circuit 121 switches between H and L of the signal output to the NAND circuit 118 every time the overload protection operation capacitor 109 is charged and discharged a predetermined number of times. Thereby, switching of the operation period and the stop period during the overload protection operation mode is controlled.

  Normally, the stop period of this overload protection operation mode is longer than the operation period. During the operation period of the overload protection operation mode, the switching device 101 operates at the oscillation frequency of the oscillation circuit 116 as in the normal operation mode. Further, the switching device 101 is completely stopped during the stop period.

  In the conventional example, the voltage at the VDD terminal is lowered due to the circuit current. However, when the internal power supply terminal and the overload protection operation terminal of the control circuit 112 are provided separately, this overload protection operation In order to reduce the voltage at the terminal, the current is drawn from the capacitor connected to the overload protection operation terminal instead of the circuit current.

  The operation of the switching power supply apparatus 100 configured as described above will be described with reference to FIGS. 9, 10, and 11. 9, 10 and 11 are time charts showing operation waveforms of respective parts of the switching power supply apparatus 100 shown in FIG.

  FIG. 9 is a diagram showing operation waveforms of the conventional switching power supply apparatus 100 of FIG.

  The input unit 106 shown in FIG. 8 receives a DC input voltage VIN generated by, for example, rectifying and smoothing a commercial AC power supply. This input voltage VIN is applied to the drain terminal of the semiconductor device 126 via the primary winding 102A of the transformer 102.

  Then, the start-up current generated by the start-up constant current source 124 flows into the overload protection operation capacitor 109 connected to the VDD terminal via the switch 125, and charges the overload protection operation capacitor 109. As a result, the voltage at the VDD terminal rises. When the voltage at the VDD terminal reaches the start voltage VDD (ON) set by the start / stop circuit 127 at time t1 shown in FIG. 9, the switching operation of the switching device 101 is started.

  Hereinafter, the operation from time t1 to time t2 shown in FIG. 9 will be described.

  When the switching operation is started, energy is supplied to the primary winding 102A of the transformer 102, so that a current flows through the secondary winding 102B. Immediately after the start-up, the output voltage VO of the output unit 105 on the secondary side does not rise, and the feedback current IFB does not flow, so the overload detection signal OLP output by the overload detection circuit 115 is H, The switching device 101 operates in an overload protection operation mode.

  Since the overload detection signal OLP is H, the start / stop circuit 127 turns off the switch 125 for supplying a constant current to the VDD terminal. At this time, since no current is supplied to the VDD terminal, the voltage at the VDD terminal starts to decrease due to the circuit current as shown in FIG.

  Further, the current flowing through the secondary winding 102B is rectified and smoothed by the diode 103 and the capacitor 104 to become DC power. This DC power is supplied to the load 108. Further, by repeating the switching operation, the output voltage VO gradually increases and the output current IO also increases.

  When the output voltage VO reaches the set voltage at time t2, the feedback current IFB flowing through the feedback signal control circuit 114 increases. When the feedback current IFB increases and the signal output from the feedback signal control circuit 14 falls below the threshold IFB (OLP) of the overload detection circuit 115, the output signal from the overload detection circuit 115 switches from H to L. . As a result, the overload protection operation mode is canceled and the normal operation mode is entered.

  At this time, the start / stop circuit 127 turns on the switch 125 when the overload detection signal OLP is switched from H to L. Therefore, current supply from the constant current source 124 to the VDD terminal is performed. As a result, the voltage at the VDD terminal starts to rise again after time t2. Thereafter, when the voltage at the VDD terminal reaches the starting voltage VDD (ON), the VDD terminal holds a constant voltage (starting voltage VDD (ON)).

  Next, the operation from time t2 to time t3 when the load 108 is in the normal load state will be described.

  From time t2 to time t3, the signal output from the feedback signal control circuit 14 is equal to or less than the threshold IFB (OLP) of the overload detection circuit 115, and the switching device 101 operates in the normal operation mode.

  Further, when the feedback current IFB flowing through the FB terminal increases, the feedback voltage VFB input to the comparator 123 decreases, so that the drain current ID flowing through the switching device 101 decreases. By applying such negative feedback, the output voltage VO is stabilized.

  For example, after the output voltage VO is stabilized, when the output current IO flowing to the load 108 decreases, the feedback current IFB increases, the feedback voltage VFB input to the comparator 123 decreases, and the drain flowing to the switching device 101 The current ID is reduced.

  Further, when the output current IO flowing through the load 108 increases, the feedback current IFB decreases, the feedback voltage VFB input to the comparator 123 increases, and the drain current ID flowing through the switching device 101 increases as the output current IO increases. growing. Further, when the feedback voltage VFB rises and reaches the voltage defined by the clamp circuit 119, overcurrent protection functions and the drain current ID is clamped at the maximum current value ILIMIT.

  Next, the operation from time t3 to time t5 when the load 108 switches from the normal load state to the overload state will be described.

  From time t3 to time t4, the output current IO becomes equal to or higher than a specified value, and the load 108 becomes overloaded. Here, since the drain current ID is clamped at the maximum current value ILIMIT and cannot be increased any more, the output voltage VO begins to decrease and the feedback current IFB also decreases. At this time, the signal output from the feedback signal control circuit 14 becomes larger than the threshold IFB (OLP) of the overload detection circuit 115, so that the overload detection signal OLP is switched from L to H.

  Next, since the overload detection signal OLP is H from time t4 to time t5, the start / stop circuit 127 turns off the switch 125 that supplies current from the drain terminal to the VDD terminal. As a result, the voltage at the VDD terminal decreases due to the circuit current. At this time, the switching power supply device 100 continues to supply power to the secondary load 108 at the maximum current value ILIMIT of the drain current ID.

  When the voltage at the VDD terminal decreases to the stop voltage VDD (OFF) of the start / stop circuit 127 at time t5, the signal output from the start / stop circuit 127 to the counter circuit 121 switches from H to L. Therefore, the output signal of the counter circuit 121 is also switched from H to L. Thereby, since the L signal is input to the NAND circuit 118, the switching operation of the switching device 101 is stopped. That is, the switching device 101 shifts from the operation period in the overload protection operation mode to the stop period in the overload protection operation mode.

  Next, the operation from time t5 to time t7 will be described.

  From the time t5 to the time t6 is a stop period during the overload protection operation mode, and the switching device 101 stops operating.

  Further, when the voltage at the VDD terminal decreases to the stop voltage VDD (OFF) at time t5, the switch 125 is turned on by a signal from the start / stop circuit 127, whereby current supply to the VDD terminal is resumed. As a result, the voltage at the VDD terminal starts to rise.

  After that, when the voltage at the VDD terminal reaches the starting voltage VDD (ON) at time t6, the switch 125 is turned off again by the starting / stopping circuit 27. Thereby, the voltage at the VDD terminal starts to decrease due to the circuit current of the semiconductor device 126. At this time, an H signal is input to the counter circuit 121.

  The counter circuit 121 receives the L and H signals, and continues to stop the switching device 101 until the number of times H is counted by the number set internally.

  Next, the operation in the operation period in the overload protection operation mode from time t7 to time t8 will be described. From time t7 to time t8, the load 108 is in an overload state.

  At time t7, the counter circuit 121 finishes counting the number of times set internally. Accordingly, since the H signal is input to the NAND circuit 118, the switching device 101 resumes the switching operation. That is, the operation period is shifted from the stop period in the overload protection operation mode.

  In addition, the overload detection signal OLP from the overload detection circuit 115 remains H and does not switch to L during the period until the voltage at the VDD terminal decreases from the start voltage VDD (ON) to the stop voltage VDD (OFF). When this happens, the switching device 101 shifts to a stop period during the overload protection operation mode. That is, the overload protection operation mode is continued.

  Here, since the load 108 is still in an overload state after time t7, the output voltage VO and the output current IO hardly increase, and the feedback current IFB output by the output voltage detection circuit 110 also increases. Absent. As a result, the overload detection signal OLP output from the overload detection circuit 115 at time t8 also remains H. As a result, the output signal from the start / stop circuit 127 switches from H to L again, so that the L signal is input to the NAND circuit 118 via the counter circuit 121. Therefore, the switching operation of the switching device 101 is stopped. That is, the stop period during the overload protection operation mode is entered.

  After that, the start / stop circuit 127 turns the switch 125 on and off again, thereby repeatedly increasing and decreasing the voltage at the VDD terminal, thereby realizing an overload protection operation.

  Usually, the operation period of the switching device 101 in this overload protection operation mode is made shorter than the stop period, thereby reducing supply of output power to the load 108 in an overload state and reducing stress on the load 108. Yes.

  Next, the operation of the conventional switching power supply device 100 when the overload state of the load 108 continues and when it is canceled during the overload protection operation mode will be described with reference to FIGS. 10 and 11.

  FIG. 10 is a diagram illustrating operation waveforms of the conventional switching power supply device 100 when the overload state of the load 108 continues.

  Here, in the operation period in the overload protection operation mode, the return from the overload state is determined. Further, this operation period is determined by the time during which the voltage at the VDD terminal decreases from the start voltage VDD (ON) to the stop voltage VDD (OFF).

  When the feedback current IFB does not increase during the operation period (time t10 to t11) during the overload protection operation mode and the signal output from the feedback signal control circuit 14 does not exceed the threshold value IFB (OLP), the time t11 Then, the operation enters the stop period during the overload protection operation mode again.

  Such a condition of the operation period in the overload protection operation mode is also applied when the switching power supply device 100 is activated, and immediately after the activation, the voltage at the VDD terminal decreases from the activation voltage VDD (ON) to the stop voltage VDD (OFF). By the time, when the signal output from the feedback signal control circuit 14 is not lower than the threshold value IFB (OLP) and the output signal of the overload detection circuit 115 remains H, the overload protection operation mode is stopped. It enters and becomes a start-up failure.

  Next, the operation when the overload state of the load 108 is released and the normal operation mode is restored will be described with reference to FIG.

  When the operation period in the overload protection operation mode is entered at time t12 after the overload state of the load 108 is released, the switching device 101 performs a switching operation. Thereby, power supply from the primary side to the secondary side is started, and the output voltage VO and the output current IO begin to rise. Thereafter, when the output voltage VO and the output current IO rise to desired values, the feedback current IFB also starts to rise. At time t13, the signal output from the feedback signal control circuit 14 exceeds the threshold IFB (OLP) of the overload detection circuit, and the overload detection signal OLP is switched from H to L.

  Here, in the operation period in the overload protection operation mode from time t12 to time t13, the switch 125 is off, so the voltage at the VDD terminal continues to decrease.

  On the other hand, when the overload detection signal OLP is switched to L at time t13, the start / stop circuit 127 turns on the switch 125, so that the supply to the VDD terminal is resumed and the voltage at the VDD terminal reaches the start voltage VDD (ON). To rise. Further, the feedback current IFB increases and the feedback voltage VFB input to the comparator 123 decreases, so the drain current ID flowing through the switching device 101 decreases. By applying such negative feedback, the output voltage VO is stabilized. At this time, the voltage at the VDD terminal is controlled to be held at the starting voltage VDD (ON). In this way, it is possible to return to the normal operation mode.

  In the above conventional example, the internal control circuit power supply terminal and the overload protection operation terminal are shared by the VDD terminal, but in reality, the internal control circuit power supply terminal and the overload protection operation terminal are Even if they are provided separately, there is no significant difference in overload protection operation.

  A technique close to the above conventional example is disclosed in Patent Document 1.

JP-A-7-28532

  However, as described above, the output voltage during the operation period in the overload protection operation mode, which is the period until the voltage at the VDD terminal decreases from the start voltage VDD (ON) to the stop voltage VDD (OFF). If VO does not rise, the operation again shifts to the stop period during the overload protection operation mode. That is, even when the overload state of the load 108 is released, a malfunction occurs in which the overload protection operation mode is continued.

  Further, when the switching power supply device 100 is started up, the voltage at the VDD terminal rises and reaches the starting voltage VDD (ON) and then decreases from the starting voltage VDD (ON) to the stop voltage VDD (OFF). If the output voltage VO does not rise during the operation period in the overload protection operation mode, which is a period of time, the normal operation mode cannot be entered and an overload protection operation mode is entered.

  This malfunction occurs, for example, when the load 108 requires a large output voltage. In this case, since the rise of the output voltage VO is delayed, the output voltage VO does not rise during the operation period in the overload protection operation mode. Also, when the output power required by the load 108 is large, the output voltage VO is difficult to rise, and this malfunction occurs.

  Although these malfunctions occur at the time of start-up, when the load 108 requires a larger output power than at the start-up when the load 108 recovers from the overload once the overload protection operation is started, There is a case in which the malfunction cannot be recovered and the overload protection operation mode is continued.

  Conventionally, in order to deal with this problem, the operation period in the overload protection operation mode is extended by increasing the capacity of the overload protection operation capacitor 109. As a result, the power that can be supplied to the secondary side during the operation period increases, and the load 108 that requires a larger output power can be handled.

  However, when the capacity of the overload protection operation capacitor 109 is increased as described above, the following problems arise.

  FIG. 12 is a diagram showing operation waveforms in the overload protection operation mode when a certain overload protection operation capacitor 109 is used. FIG. 13 is a diagram showing operation waveforms during an overload protection operation when the overload protection operation capacitor 109 having a larger capacity than the overload protection operation capacitor 109 used in FIG. 12 is used.

  Comparing the waveforms shown in FIG. 12 and FIG. 13, the waveform of the voltage at the VDD terminal shown in FIG. 13 has a longer triangular period than the waveform of the voltage at the VDD terminal shown in FIG. That is, both the operation period and the stop period in the overload protection operation mode per cycle are long. The cycle of this overload protection operation is proportional to the size of the capacity of the overload protection operation capacitor 109. For example, when the capacitance value of the overload protection operation capacitor 109 is increased 10 times, the overload protection operation is performed. The period is also 10 times.

  Further, when the output current IO in the operation period in the overload protection operation mode shown in FIG. 12 and FIG. 13 is compared, the operation period in the overload protection operation mode becomes longer, so that it is supplied to the output unit 105. Since the power increases, the output current IO when the capacity of the overload protection operation capacitor 109 is increased is larger than the output current IO shown in FIG.

  At this time, since the stop period during the overload protection operation mode also becomes longer, on average, the amount of current supplied to the secondary load 108 is the same. However, since the power supplied to the load 108 increases during the operation period in the overload protection operation mode per cycle, the output current IO temporarily increases. When this large current flows through the component, it causes a great stress, which may cause the component to be destroyed and deteriorated. That is, the effect of the overload protection function is reduced.

  Thus, in the conventional switching power supply device 100, there is a problem that the effect of the overload protection function is reduced by increasing the capacity of the overload protection operation capacitor 109.

  Further, when the overload protection operation capacitor 109 is integrated on the semiconductor device, the capacitance value of the overload protection operation capacitor 109 cannot be changed. That is, the length of the operation period during the overload protection operation mode is fixed. As a result, in such a case, there is a problem that a malfunction occurs at the time of start-up and recovery when a load 108 that requires a large amount of power is used.

  In the above description, the switching power supply device having a self-recovery type overload protection function has been described as an example. There is a problem that occurs.

  Accordingly, a first object of the present invention is to provide a switching power supply device and a switching power supply semiconductor device capable of starting and returning a load that requires a large output power while suppressing a decrease in the effectiveness of the overload protection function. To do.

  It is a second object of the present invention to provide a switching power supply device and a switching power supply semiconductor device having a latch-type overload protection function that can suppress the occurrence of a malfunction at startup.

  In order to achieve the above object, a switching power supply according to the present invention is a switching power supply that converts a DC input voltage input to an input unit into a DC output voltage and outputs the DC voltage, and converts the input voltage. A voltage conversion circuit for converting to a voltage; a switching device connected to the voltage conversion circuit; and a voltage conversion circuit connected to the load, and rectifying and smoothing the conversion voltage to thereby convert the output voltage. An output voltage generation circuit for generating, a drive circuit for driving the switching device, an overload detection circuit for determining whether or not the output voltage is greater than a first threshold, and the output voltage by the overload detection circuit If it is determined that the threshold value is greater than the first threshold value, the drive circuit is controlled to continuously operate the switching device, and the overload detection circuit is controlled. When the output voltage is determined to be smaller than the first threshold, an overload control circuit that controls the drive circuit so that the switching device performs an overload protection operation, and the output voltage is the first voltage A return detection circuit that determines whether or not the second threshold value is less than a second threshold value, and the switching device operates during the overload protection operation when the switching device operates during the overload protection operation. And a stop period during which the switching device stops during the overload protection operation, and the overload control circuit further includes a case where the output voltage is greater than the second threshold value during the overload protection operation. Compared with the case where the output voltage is smaller than the second threshold value, the power supplied to the load is increased.

  According to this configuration, the switching power supply according to the present invention is lower than the first threshold even when the output voltage does not rise to the first threshold for canceling the overload protection operation during the overload protection operation. It is determined whether or not the overload state has been released by determining whether or not it has risen to the second threshold value. Thereby, even if the output voltage has not risen to the first threshold value, the switching power supply device according to the present invention increases the power supplied to the load when the overload state is released. Thereby, the switching power supply according to the present invention can start and restore a load that requires a large output power.

  Furthermore, the switching power supply according to the present invention has a large output power without requiring adjustment of the amount of power supplied by the overload protection capacitor even when the load is, for example, a driver and the required output power changes. Can be activated and restored. Therefore, the switching power supply device according to the present invention starts and restores a load that requires a large output power while suppressing a reduction in the effectiveness of the overload protection function by increasing the capacitance value of the overload protection operation capacitor. it can.

  Further, when the overload protection operation capacitor is integrated in the semiconductor device and the overload protection period is determined internally, the operation period during the overload protection operation is fixed. Even in such a case, the switching power supply according to the present invention can start and recover a load that requires a large output power.

  In the overload protection operation, the overload control circuit makes the operation period longer when the output voltage is larger than the second threshold than when the output voltage is smaller than the second threshold. By doing so, you may increase the electric power supplied to the said load.

  The switching power supply device further includes a capacitor, and the overload control circuit determines a length of the operation period using a time during which the capacitor is charged or discharged from the first voltage to the second voltage. The overload control circuit supplies the first voltage to the capacitor during the operation period when the output voltage is greater than the second threshold value during the overload protection operation. May be longer than when the output voltage is smaller than the second threshold.

  The return detection circuit may include a return signal detection circuit that directly detects the output voltage.

  According to this configuration, since the power supply designer can easily set the second threshold, the switching power supply device according to the present invention can expand the degree of freedom in design. Furthermore, the accuracy of the output voltage that can be recovered from the overload can be increased by arbitrarily setting the second threshold value.

  The drive circuit generates a first signal for driving the switching device, and the return detection circuit has an on-pulse width or on-duty for turning on the switching device in the first signal larger than a third threshold value. The output voltage is determined to be greater than the second threshold, and if the on-pulse width or on-duty is less than the third threshold, the output voltage may be determined to be less than the second threshold. .

  According to this configuration, it is not necessary to add an external terminal to the semiconductor device included in the switching power supply device according to the present invention, and space saving and the number of parts around the external terminal can be reduced.

  The driving circuit includes an oscillation circuit that generates a clock signal, the driving circuit generates a first signal that turns on the switching device in a cycle of the clock signal, and the return detection circuit includes the first detection circuit. When an on-pulse width or on-duty for turning on the switching device in a signal is greater than a third threshold, it is determined that the output voltage is greater than the second threshold, and the on-pulse width or on-duty is the third threshold If smaller, the output voltage is determined to be smaller than the second threshold, and the return signal detection circuit generates a set signal that is a pulse signal when the on-pulse width or on-duty is larger than the third threshold. Flip in which the set signal is input to the set terminal and the clock signal is input to the reset terminal The overload control circuit is configured to supply the first voltage to the capacitor during the operation period of the overload protection operation while the flip-flop circuit is in a set state. The period may be longer than when the on-pulse width or on-duty is smaller than the third threshold.

  The voltage conversion circuit may be a transformer having a primary winding, a secondary winding, and an auxiliary winding, and the return detection circuit may detect the output voltage from the auxiliary winding.

  The switching power supply device further includes a capacitor, and the overload control circuit determines a length of the operation period using a time during which the capacitor is charged or discharged from the first voltage to the second voltage. In the overload protection operation, the overload control circuit is configured such that when the output voltage is greater than the second threshold, the first voltage and the first voltage are greater than when the output voltage is less than the second threshold. By increasing the difference from the second voltage, the operation period when the output voltage is larger than the second threshold may be made longer than when the output voltage is smaller than the second threshold.

  The overload control circuit causes the overload detection circuit to determine that the output voltage is greater than the first threshold when the output voltage is greater than the second threshold during the overload protection operation. Thereby, the electric power supplied to the load may be increased by continuously operating the switching device.

  The switching power supply device further includes a capacitor, and the overload control circuit determines a length of the operation period using a time during which the capacitor is charged or discharged from the first voltage to the second voltage. In the overload protection operation, when the output voltage is larger than the second threshold, the overload control circuit holds the voltage of the capacitor so that the output voltage is lower than the second threshold. The operation period when the output voltage is large may be longer than when the output voltage is smaller than the second threshold.

  In the overload protection operation, the overload protection circuit includes the first threshold value when the output voltage is greater than the second threshold value than when the output voltage is less than the second threshold value. The electric power supplied to the load may be increased by reducing.

  In the overload protection operation, the overload protection circuit oscillates the switching device when the output voltage is larger than the second threshold than when the output voltage is smaller than the second threshold. The power supplied to the load may be increased by increasing the frequency or increasing the peak value of the current flowing through the switching device.

  The switching power supply according to the present invention is a switching power supply that converts a DC input voltage input to an input unit into a DC output voltage and outputs the voltage, and converts the input voltage into a converted voltage. An output voltage generation circuit which is connected between the circuit, a switching device connected to the voltage conversion circuit, and the voltage conversion circuit and a load, and generates the output voltage by rectifying and smoothing the conversion voltage A drive circuit that drives the switching device, an overload detection circuit that determines whether the output voltage is greater than a first threshold, and the output voltage that is greater than the first threshold by the overload detection circuit If it is determined that the output voltage is large, the drive circuit is controlled to operate the switching device continuously, and the output voltage is controlled by the overload detection circuit. An overload control circuit that stops the switching device when it is determined that the output voltage is smaller than the first threshold, and a return detection that determines whether the output voltage is larger than a second threshold smaller than the first threshold. The overload control circuit further includes, when the switching power supply device is activated, when the output voltage is smaller than the first threshold and larger than the second threshold, the output voltage is the second The power supplied to the load may be increased as compared with a case where it is smaller than the threshold value.

  According to this configuration, it is possible to suppress the occurrence of a malfunction when the switching power supply device having the latch stop type overload protection function is started.

  The switching power supply semiconductor device according to the present invention includes the drive circuit, the overload detection circuit, the overload control circuit, and the recovery detection circuit formed on one semiconductor chip.

  A switching power supply semiconductor device according to the present invention includes the switching device, the drive circuit, the overload detection circuit, the overload control circuit, and the recovery detection circuit formed on one semiconductor chip. May be provided.

  The present invention can be realized not only as such a switching power supply device but also as a control method for a switching power supply device including a characteristic means included in the switching power supply device as a step. It can also be realized as a program for causing a computer to execute. Needless to say, such a program can be distributed via a recording medium such as a CD-ROM and a transmission medium such as the Internet.

  Furthermore, the present invention can be realized as a semiconductor integrated circuit (LSI) that realizes part or all of the functions of such a switching power supply device.

  As described above, the present invention can provide a switching power supply device and a switching power supply semiconductor device capable of starting and returning a load that requires a large output power while suppressing a decrease in the effectiveness of the overload protection function. In addition, the present invention can provide a switching power supply device having a latch-type overload protection function and a semiconductor device for switching power supply that can suppress the occurrence of malfunction of the overload protection operation at the time of startup.

It is a circuit diagram which shows the structure of the switching power supply device which concerns on Embodiment 1 of this invention. It is a figure which shows the relationship between the peak current IDp of the drain which flows into a switching device, and the feedback current IFB in the switching power supply which concerns on Embodiment 1 of this invention. It is a circuit diagram which shows an example of the return detection circuit of the switching power supply device concerning Embodiment 1 of this invention. It is a figure which shows the relationship between the feedback electric current IFB and output voltage VO, and the state of load in the switching power supply which concerns on Embodiment 1 of this invention. It is a figure which shows the operation | movement waveform of each part in the switching power supply which concerns on Embodiment 1 of this invention. It is a circuit diagram which shows the structure of the switching power supply device which concerns on Embodiment 2 of this invention. It is a figure which shows the operation | movement waveform of each part in the switching power supply device concerning Embodiment 2 of this invention. It is a circuit diagram which shows the structure of the conventional switching power supply apparatus. It is a figure which shows the operation | movement waveform of the conventional switching power supply device. It is a figure which shows the operation | movement waveform when continuing the overload protection operation mode in the conventional switching power supply device. It is a figure which shows the operation | movement waveform when returning from overload protection operation | movement in the conventional switching power supply device. It is a figure which shows the operation waveform at the time of an overload protection operation mode when the capacity | capacitance of the capacitor for an overload protection operation is small in the conventional switching power supply device. It is a figure which shows the operation waveform at the time of an overload protection operation mode when the capacity | capacitance of the capacitor for an overload protection operation is large in the conventional switching power supply device.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

(Embodiment 1)
Even if the output voltage VO does not rise to the first threshold value for canceling the overload protection operation mode during the overload protection operation mode, the switching power supply device 50 according to Embodiment 1 of the present invention It is determined whether or not the overload state is released by determining whether or not the threshold value has risen to a second threshold value that is lower than the threshold value. As a result, the switching power supply device 50 according to the first embodiment of the present invention has the overload protection operation mode when the overload state is canceled even if the output voltage VO has not risen to the first threshold value. By extending the inside operation period, the power supplied to the load 8 can be increased.

  Thereby, the switching power supply according to the present invention can start and restore the load 8 that requires a large output power. Furthermore, since the switching power supply device 50 according to the present invention does not need to increase the capacitance value of the overload protection operation capacitor 9, it is possible to suppress a reduction in the effectiveness of the overload protection function.

  FIG. 1 is a block diagram showing a circuit configuration example of a switching power supply device 50 using a semiconductor device 26 which is a switching power supply control IC according to the first embodiment of the present invention.

  1 includes a transformer 2, an output unit 5, an input unit 6, an output voltage generation circuit 7, an overload protection operation capacitor 9, an output voltage detection circuit 10, and a switching power source control. For example, and a return signal detection circuit 30. The switching power supply device 50 converts the DC input voltage VIN input to the input unit 6 into a DC output voltage VO and outputs it to the output unit 5.

  The transformer 2 includes a primary winding 2A and a secondary winding 2B. The transformer 2 corresponds to the voltage conversion circuit of the present invention, and converts the input voltage VIN into a converted voltage. Note that the function of the voltage conversion circuit can also be realized by using a coil instead of the transformer 2.

  One end of primary winding 2 </ b> A is connected to input unit 106. The secondary winding 2 </ b> B is connected to the output voltage generation circuit 7.

  The output voltage generation circuit 7 is connected between the secondary winding 2 </ b> B and the load 8. The output voltage generation circuit 7 generates an output voltage VO by rectifying and smoothing the converted voltage converted by the transformer 2. Specifically, the output voltage generation circuit 7 generates a DC output voltage VO by rectifying and smoothing an AC voltage induced in the secondary winding 1 b, and a load 8 connected to the output unit 5. The output voltage VO is supplied. The output voltage generation circuit 7 includes a diode 3 and a capacitor 4.

  The output voltage detection circuit 10 transmits a feedback current IFB, which is a control signal for controlling the secondary output voltage VO to be constant, from the secondary side to the feedback signal control circuit 14 of the control circuit 12 on the primary side. To do. This output voltage detection circuit 10 detects the level of the output voltage VO, and based on the level of the output voltage VO, a feedback current IFB proportional to the level of the output voltage VO is fed via the FP terminal to the feedback signal control circuit 14. Output to.

  The semiconductor device 26 includes the switching device 1 and its control circuit 12.

  For example, the semiconductor device 26 is a semiconductor integrated circuit (LSI) including the switching device 1 and the control circuit 12 formed on one semiconductor chip. Only the control circuit 12 may be formed on one semiconductor chip. Further, each processing unit included in the control circuit 12 may be formed on a plurality of semiconductor chips.

  This semiconductor device 26 has, as external input terminals, a drain terminal that is an input terminal of the switching device 1, a VDD terminal that functions as an overload protection operation terminal and an internal power supply terminal, and an FB terminal that is a feedback signal input terminal. And a return signal input terminal, and a source terminal that functions as a GND terminal of the control circuit 12 and an output terminal of the switching device 1.

  The overload protection operation capacitor 9 is connected to the VDD terminal. The overload protection operation capacitor 9 serves as an overload protection operation capacitor and also serves as a power supply stabilization capacitor for the semiconductor device 26.

  The switching device 1 is connected in series with the primary winding 2A.

  The control circuit 12 controls the switching operation of the switching device 1. The control circuit 12 includes a feedback signal control circuit 14, a clamp circuit 19, a part of the overload protection circuit 20, a primary current detection circuit 22, a comparator 23, a constant current source 24, and a drive circuit 42. Including.

  The constant current source 24 is a starting constant current source for supplying a starting circuit current, and supplies the starting current to the VDD terminal via the switch 125 at the time of starting or the like.

  The primary current detection circuit 22 detects the drain current ID flowing through the switching device 1. The primary current detection circuit 22 converts the detected drain current ID into a detection current signal 61 that is a voltage signal, and outputs the converted detection current signal 61 to the comparator 23.

  The feedback signal control circuit 14 converts the feedback current IFB flowing into the FB terminal into a feedback signal 62 that is a voltage signal, and outputs the converted feedback signal 62 to the comparator 23. Further, the feedback signal control circuit 14 converts the feedback current IFB flowing into the FB terminal into the feedback signal 63 and outputs the converted feedback signal 63 to the overload protection circuit 20.

  The comparator 23 compares the feedback signal 62 output from the feedback signal control circuit 14 with the detected current signal 61 output from the primary current detection circuit 22. The comparator 23 outputs a reset signal 64 for resetting the RS flip-flop circuit 28 to the reset terminal of the RS flip-flop circuit 28 when the feedback signal 62 and the detected current signal 61 become equal.

  The clamp circuit 19 is a clamp circuit for determining the maximum value of the feedback signal 62. The clamp circuit 19 determines a maximum current value ILIMIT that is the maximum value of the drain current ID flowing through the switching device 1 and functions as overcurrent protection for the switching device 1. Further, the feedback current IFB changes, and the feedback current when the drain current ID flowing through the switching device 1 reaches the maximum current value ILIMIT becomes the first feedback current value IFB1.

  FIG. 2 is a diagram showing the relationship between the drain current ID flowing through the switching device 1 and the feedback current IFB in the switching power supply device 50 of the present invention.

  As shown in FIG. 2, the drain current ID increases as the feedback current IFB decreases. Further, when the feedback current IFB becomes the first feedback current value IFB1, the drain current ID is clamped to the maximum current value ILIMIT. A value lower than the first feedback current value IFB1 is set as the threshold value IFB (OLP) of the overload detection circuit 15.

  The drive circuit 42 generates a pulse signal 68 that drives the switching device 1. The drive circuit 42 includes an oscillation circuit 16, a gate driver 17, a NAND circuit 18, and an RS flip-flop circuit 28.

  The oscillation circuit 16 generates a maximum duty cycle signal 16A that determines the maximum duty cycle of the switching device 1 and a clock signal 16B that determines the oscillation frequency of the switching device 1. The maximum duty cycle signal 16A is input to the NAND circuit 18, and the clock signal 16B is input to the set terminal of the RS flip-flop circuit 28.

  The NAND circuit 18 receives the control signal 66 output from the counter circuit 21, the maximum duty cycle signal 16 </ b> A, and the Q signal 65 output from the RS flip-flop circuit 28. The NAND circuit 18 generates a pulse signal 67 that is a negative logical product of the control signal 66, the maximum duty cycle signal 16A, and the Q signal 65.

  The gate driver 17 generates a pulse signal 68 obtained by inverting the logic of the pulse signal 67 generated by the NAND circuit 18, and outputs the generated pulse signal 68 to the gate of the switching device 1. Here, the pulse signal 68 has a pulse waveform that turns on the switching device 1 in the cycle of the clock signal 16B.

  The overload protection circuit 20 detects that the load 8 is in an overload state. The overload protection circuit 20 performs an overload protection operation by controlling the voltage at the VDD terminal when an overload state is detected. The overload protection circuit 20 performs a return operation when the overload is released.

  Specifically, the overload protection circuit 20 operates the switching device 1 in a normal operation mode in which the switching device 1 is continuously operated when the load 8 is not in an overload state (in a normal load state or a light load state). Make it work. Further, the overload protection circuit 20 operates the switching device 1 in an overload protection operation mode that repeats an operation period in which the switching device 1 operates and a stop period in which the switching device 1 stops when the load 8 is in an overload state. Make it work.

  The overload protection circuit 20 includes an overload detection circuit 15, a recovery detection circuit 11, an overload control circuit 13, and a recovery detection circuit 11.

  The overload detection circuit 15 detects whether or not the load 8 is in an overload state. Specifically, the overload detection circuit 15 determines whether or not the output voltage VO is larger than the first threshold, and when the output voltage VO is smaller than the first threshold, the load 8 is in an overload state. When the output voltage VO is smaller than the first threshold, it is determined that the load 8 is in a normal load state or a light load state.

  More specifically, the overload detection circuit 15 compares the feedback signal 63 from the feedback signal control circuit 14 with an overload detection threshold value IFB (OLP) corresponding to the first threshold value, thereby outputting an output. It is determined whether the voltage VO is greater than a first threshold value. When the load 8 is in a normal load state or a light load state and the power required by the load 8 is smaller than the maximum power that can be supplied from the primary side to the secondary side, feedback output from the feedback signal control circuit 14 Since the signal 63 becomes lower than the threshold value IFB (OLP), the overload detection circuit 15 determines that the load 8 is not in an overload state, and sets the overload detection signal OLP to L.

  On the other hand, when the load 8 is in an overload state and the power required by the load 8 is larger than the maximum power that can be supplied from the primary side to the secondary side, the feedback signal 63 output from the feedback signal control circuit 14. Becomes higher than the overload detection threshold IFB (OLP), the overload detection circuit 15 determines that the load 8 is in an overload state, and switches the overload detection signal OLP from L to H.

  When the load 8 returns from the overload state to the normal state, the feedback current IFB output from the output voltage detection circuit 10 increases. Thereby, since the signal output from the feedback signal control circuit 14 becomes lower than the threshold value IFB (OLP), the overload detection circuit 15 switches the overload detection signal OLP from H to L.

  The overload detection signal OLP generated by the overload detection circuit 15 is input to the start / stop circuit 27.

  The return detection circuit 11 detects that the output voltage VO has increased at the time of start-up and return from the overload protection operation mode. Specifically, the recovery detection circuit 11 determines whether or not the output voltage VO is greater than the second threshold value. When the output voltage VO is equal to or greater than the second threshold value, the return detection circuit 11 becomes H and is less than the second threshold value. In this case, a return signal 69 that is L is generated. The return signal 69 is output to the overload control circuit 13. Here, the second threshold value is a voltage value smaller than the first threshold value.

  The return detection circuit 11 includes a return signal detection circuit 30 and a return signal output circuit 29.

  The return signal detection circuit 30 directly detects the output voltage VO and generates a signal corresponding to the output voltage VO.

  The return signal output circuit 29 compares the signal generated by the return signal detection circuit 30 with the reference voltage Voth corresponding to the second threshold value to determine whether the output voltage VO is equal to or higher than the second threshold value. A return signal 69 indicating this is generated.

  FIG. 3 is a circuit diagram showing a configuration example of the return detection circuit 11. As shown in FIG. 3, the return signal detection circuit 30 includes resistors 31, 32, and 33, a bipolar transistor 34, and a photocoupler 35. The return signal output circuit 29 includes constant current sources 36 and 36A, a resistor 37, an N-type MOSFET 38, a return detection comparator 39, and a return detection reference voltage source 40. The photocoupler 35 includes a phototransistor 35A and a photodiode 35B.

  The resistors 31 and 32 generate a voltage 75 by dividing the output voltage VO on the secondary side. This divided voltage 75 is input to the base of the bipolar transistor 34. The bipolar transistor 34 is turned on when the output voltage VO on the secondary side becomes equal to or higher than a predetermined voltage value. When the bipolar transistor 34 is turned on and a current flows through the bipolar transistor 34 via the resistor 33, the current flowing through the photodiode 35B is controlled.

  When the current flowing through the photodiode 35B increases, the current flowing through the phototransistor 35A increases. Therefore, the current flowing out from the return signal detection terminal increases, and the current is pulled from the constant current source 36. As a result, the current flowing through the resistor 37 is reduced, so that the voltage 76 applied to the gate of the N-type MOSFET 38 is reduced.

  As a result, the voltage 77 input to the inverting input terminal of the return detection comparator 39 also decreases. The constant current source 36A limits the current flowing through the N-type MOSFET 38.

  The reference voltage Voth generated by the return detection reference voltage source 40 is input to the non-inverting input terminal of the return detection comparator 39. This reference voltage Voth is a voltage corresponding to the second threshold value.

  The return detection comparator 39 compares the voltage 77 with the reference voltage Voth, and generates a return signal 69 of H when the voltage 77 is lower than the reference voltage Voth and L when the voltage 77 is higher than the reference voltage Voth. To do.

  The overload control circuit 13 controls the drive circuit 42 so that the switching device 1 operates in the overload protection operation mode when the overload detection signal OLP is H, and operates normally when the overload detection signal OLP is L. Control is performed so that the switching device 1 operates in the mode.

  Further, in the overload protection operation mode, the overload control circuit 13 charges and discharges the overload protection operation capacitor 9 via the VDD terminal, so that the operation period and the stop period in the overload protection operation mode are set. The length is determined and the operation period and the stop period are switched.

  Further, in the overload protection operation mode, the overload control circuit 13 is more effective when the output voltage VO is larger than the second threshold than when the output voltage VO is smaller than the second threshold. The power supplied to the load 8 is increased by extending the operation period.

  The overload control circuit 13 includes a start / stop circuit 27, a counter circuit 21, and a switch 25.

  The start / stop circuit 27 controls the start and stop of the semiconductor device 26. This start / stop circuit 27 turns on or off the switch 25 to increase the voltage of the VDD terminal to the start voltage VDD (ON) before start-up, and after start-up, the start voltage VDD (ON) to the stop voltage VDD ( OFF). Further, the start / stop circuit 27 outputs a control signal 70 indicating whether the overload protection operation capacitor 109 is being charged or discharged to the counter circuit 121 in accordance with the voltage at the VDD terminal.

  Specifically, when the overload detection signal OLP output from the overload detection circuit 15 is L (in the normal operation mode), the start / stop circuit 27 outputs a signal for operating the switching device 1 continuously. The data is output to the NAND circuit 18 via the counter circuit 21. More specifically, an H control signal 70 is output to the counter circuit 21.

  When the overload detection signal OLP is L (in the normal operation mode), the start / stop circuit 27 turns on the switch 25 to supply current from the constant current source 24 to the VDD terminal. Further, the constant current source 24 supplies a current from the drain terminal to the VDD terminal via the switch 25 so that the startup voltage VDD (ON) is maintained even after the voltage of the VDD terminal reaches the startup voltage VDD (ON). Supply.

  On the other hand, when the overload detection signal OLP is H (in the overload protection operation mode), the start / stop circuit 27 repeats turning on and off of the switch 25 according to the voltage value of the VDD terminal, so that the VDD terminal The voltage at the VDD terminal is controlled by repeating the current supply to and interruption of the current. In addition, the start / stop circuit 27 outputs a control signal 70 corresponding to ON / OFF of the switch 25 to the counter circuit 21.

  Further, the start / stop circuit 27 controls the period during which the switch 25 is turned on based on the return signal 69 output from the return detection circuit 11 in the overload protection operation mode.

  Specifically, the start / stop circuit 27 detects that the overload detection signal OLP is H, and when the return signal 69 is L, the voltage at the VDD terminal decreases from the start voltage VDD (ON). After reaching the stop voltage VDD (OFF), the switch 25 is turned on to charge the VDD terminal and output the L control signal 70. Further, the start / stop circuit 27 lowers the voltage at the VDD terminal by the circuit current of the semiconductor device 26 by turning off the switch 25 after the voltage at the VDD terminal reaches the start voltage VDD (ON), and H The control signal 70 is output.

  With this operation, when the overload detection signal OLP is H and the return signal 69 is L, the voltage at the VDD terminal rises and falls between the start voltage VDD (ON) and the stop voltage VDD (OFF). And the control signal 70 repeats H and L.

  The counter circuit 21 counts the number of times that the control signal 70 output from the start / stop circuit 27 becomes H.

  Specifically, the counter circuit 21 performs switching by outputting an H control signal 66 to the NAND circuit 18 when an H control signal 70 is input from the start / stop circuit 27 to the counter circuit 21 after activation. The device 1 is operated.

  Further, the counter circuit 21 switches the control signal 66 from H to L when the control signal 70 is switched from H to L. Thereafter, in the overload protection operation mode, a control signal 70 in which H and L are repeated is input to the counter circuit 21. The counter circuit 21 outputs the H control signal 66 to the NAND circuit 18 again after counting the H signal a predetermined number of times set inside.

  That is, the counter circuit 21 switches between H and L of the signal output to the NAND circuit 18 every time the overload protection operation capacitor 9 is charged and discharged a predetermined number of times. Thereby, switching of the operation period and the stop period during the overload protection operation mode is controlled. The overload protection is realized by limiting the oscillation period of the switching device 1 by the stop period during the overload protection operation mode. Normally, the stop period during this overload protection operation mode is set longer than the operation period.

  Further, the start / stop circuit 27 overloads when the overload detection signal OLP is H and the return signal 69 is H (when the output voltage VO is larger than the second threshold value in the overload protection operation mode). In order to extend the oscillation period of the protection operation mode, the period during which the switch 25 is turned on is extended. Thus, current is supplied from the constant current source 24 to the overload protection operation capacitor 9 during the operation period in the overload protection operation mode, and the voltage at the VDD terminal is raised.

  Thus, the overload control circuit 13 compares the operation period when the output voltage VO is larger than the second threshold in the overload protection operation mode with the operation period when the output voltage is smaller than the VO second threshold. By increasing the length, the power supplied to the load 8 can be increased.

  In the example of the first embodiment of the present invention, the VDD terminal also functions as an overload protection operation terminal and an internal power supply terminal, and the voltage at the VDD terminal is reduced by the circuit current. The internal power supply terminal and the overload protection operation terminal may be provided separately. That is, in the example of the first embodiment of the present invention, the voltage of the VDD terminal (overload protection operation terminal) is reduced by using the circuit current, but the internal power supply terminal, the overload protection operation terminal, Is provided separately, the current is drawn from the capacitor connected to the overload protection operation terminal.

  Even when the overload protection operation capacitor 9 is built in the semiconductor device 26, the same effect can be obtained by performing the same control as in the first embodiment of the present invention.

  The operation of the switching power supply device 50 configured as described above will be described.

  For example, a DC input voltage VIN generated by rectifying and smoothing a commercial AC power supply is input to the input unit 6 shown in FIG. The input voltage VIN is applied to the drain terminal of the semiconductor device 26 via the primary winding 2 </ b> A of the transformer 2.

  When the switching power supply device 50 is started up, the constant current source 24 causes a starting current to flow from the drain terminal to the VDD terminal via the switch 25. As a result, the overload protection operation capacitor 9 connected to the VDD terminal is charged, and the voltage at the VDD terminal rises. When the voltage at the VDD terminal reaches the start voltage VDD (ON) set by the start / stop circuit 27, the start / stop circuit 27 outputs an H control signal 70. As a result, the counter circuit 21 outputs an H control signal 66 to the NAND circuit 18. Therefore, the switching device 1 starts a switching operation.

  When the switching operation is started, energy is supplied to the primary winding 2A of the transformer 2, and a current flows through the secondary winding 2B.

  The current flowing through the secondary winding 2 </ b> B becomes DC power by being rectified and smoothed by the diode 3 and the capacitor 4. This DC power is supplied to the load 8.

  Further, the output voltage VO and the output current IO are gradually increased by repeating the switching operation. The return signal detection circuit 30 detects that the output voltage VO has risen before the output voltage VO reaches the voltage set by the output voltage detection circuit 10. Thereafter, as the output voltage VO approaches the voltage set by the output voltage detection circuit 10, the current injected to the FB terminal also increases.

  FIG. 4 is a diagram illustrating a relationship between the feedback current IFB and the output voltage VO and the state of the load 8.

  As shown in FIG. 4, when the load 8 is in a normal load state from a light load, the output voltage VO is controlled at a constant voltage. Further, the feedback current IFB becomes smaller as the load 8 becomes heavier. At this time, the drain current ID flowing through the switching device 1 is controlled to increase as the feedback current IFB decreases. Thereby, the output power supplied to the load 8 is adjusted.

  Here, after the drain current ID flowing through the switching device 1 reaches the maximum current value ILIMIT and the power that can be supplied to the secondary side reaches the maximum value, the output current further increases and the power required by the load 8 is sufficient. When it disappears, the output voltage VO begins to decrease.

  Further, when the feedback current IFB increases and the signal output from the feedback signal control circuit 14 exceeds the threshold IFB (OLP) and an overload state is detected, the overload protection operation starts.

  On the other hand, when the load 8 is released from the overload state and returns to the normal operation mode, the feedback current IFB becomes smaller than the threshold value IFB (OLP) of the overload detection circuit 15 so that the constant voltage control of the normal output voltage VO is performed. Return to. Further, when the output voltage VO rises to near the output voltage set by the output voltage detection circuit 10 at the time of recovery from the overload state, the feedback current IFB starts to flow to the FB terminal.

  On the other hand, when the recovery signal 69 output from the recovery detection circuit 11 reaches a second threshold value lower than the first threshold value corresponding to the threshold value IFB (OLP), the recovery signal 69 output from the recovery detection circuit 11 is set to H when starting and when recovering from overload. Become. As a result, at the time of start-up and when returning from the overload state, the return signal 69 becomes H when the output voltage VO becomes higher than the second threshold before reaching the first threshold.

  FIG. 5 is an operation waveform diagram of each part in the switching power supply device 50 according to the first embodiment of the present invention.

  Here, the length of the operation period in the overload protection operation mode is determined by the time until the voltage at the VDD terminal decreases from the start voltage VDD (ON) to the stop voltage VDD (OFF). If the feedback signal 63 output from the feedback signal control circuit 14 does not exceed the threshold IFB (OLP) within the operation period in the overload protection operation mode, the stop period in the overload protection operation mode is entered again. Become.

  FIG. 5 shows an operation when the switching power supply device 50 is started up or returned from an overload state. Specifically, FIG. 5 shows that after the voltage at the VDD terminal reaches the starting voltage VDD (ON), the overload protection operation mode is switched from the stop period to the operation period in the overload protection operation mode. The operation until the load protection operation mode state is canceled is shown.

  At time t20, the voltage at the VDD terminal reaches the starting voltage VDD (ON). Thereby, the switching device 1 starts a switching operation. Therefore, output power is supplied to the secondary side, and the output voltage VO and the output current IO increase.

  As the output voltage VO increases as shown in FIG. 5, the current flowing through the photodiode 35B included in the return signal detection circuit 30 shown in FIG. 2 increases. Therefore, current increases in the phototransistor 35A. When the current flowing through the phototransistor 35A increases, the current flowing out from the return signal detection terminal increases, and the current is pulled from the constant current source 36. Thereby, the current flowing through the resistor 37 is reduced, and the voltage 76 applied to the resistor 37 is also reduced. As a result, the voltage 77 input to the return detection comparator 39 also decreases.

  At time t <b> 21 shown in FIG. 5, the voltage 77 input to the return detection comparator 39 becomes lower than the reference voltage Voth of the return detection comparator 39. As a result, the return signal 69 of the return detection comparator 39 is switched to H. When the start / stop circuit 27 receives the H return signal 69 output from the return detection comparator 39, the start / stop circuit 27 turns on the switch 25. As a result, a current is supplied from the constant current source 24 to the VDD terminal, so that the voltage at the VDD terminal starts to rise as shown in FIG.

  In this way, at time t21, the voltage at the VDD terminal stops decreasing to the stop voltage VDD (OFF). Therefore, since the operation period during the overload protection operation mode becomes long, the power that can be supplied to the load 8 on the secondary side increases. Thereby, since the load 8 can obtain sufficient power, the output voltage VO rises to a voltage set by the output voltage detection circuit 10. Therefore, the overload detection signal OLP of the overload detection circuit 15 switches from H to L, and returns from the overload protection operation mode to the normal operation mode.

  Next, a case where the overload state of the load 8 is not released and the overload protection operation mode continues will be described.

  When the overload state of the load 8 is not released, the output voltage VO hardly rises, so the return detection circuit 11 outputs an L return signal 69. At this time, the operation is the same as that of the conventional overload protection when the overload state continues. That is, the operation period during the overload protection operation mode is the same as the conventional time. Thus, when the load 8 continues the overload state, the same overload protection operation as before is performed, and the operation period during the overload protection operation mode is also the same as the conventional one, so that the effectiveness of the overload protection is weakened. Don't be.

  As described above, the switching power supply device 50 according to the first embodiment of the present invention detects that the load 8 is released from the overload state without increasing the capacity of the overload protection operation capacitor 9 and detects the load. Since the electric power supplied to 8 can be increased, the effectiveness of overload protection is not reduced.

  Further, the switching power supply device 50 detects that the output voltage VO has risen to the second threshold before the feedback current IFB of the feedback signal control circuit 14 increases, and detects the overload protection operation mode. Extend the operating period. Thereby, since the switching power supply 50 can increase the power that can be supplied to the secondary side, it can return from the overload protection operation mode without increasing the capacity even for the load 8 that requires a large output power. It becomes like this.

  In addition, when the switching power supply 50 is started, the switching power supply 50 detects that the output voltage VO has risen by the return detection circuit 11 in the same manner as when returning from the overload state, and the overload protection operation mode is detected. Extend the switching device operating period. Thereby, the switching power supply 50 can supply power until the output voltage VO on the secondary side rises, so that it can be started up without causing an overload protection malfunction.

  For example, even when the load 8 is a driver and the required output power is switched, the switching power supply 50 can detect from the output voltage that the load 8 is not in an overload state. Therefore, the switching power supply device 50 is effective against malfunction of overload protection.

  Further, by changing the dividing ratio of the resistors 31 and 32 of the return signal detection circuit 30 shown in FIG. 3, the power supply designer can freely select the level of the output voltage VO to be detected at the time of return. There is an advantage that the degree of freedom is large.

  Further, by using a Zener diode in the return signal detection circuit 30 shown in FIG. 3, the return signal can be detected with a simpler configuration. In other words, by connecting the cathode of the Zener diode to the load 8 and connecting the anode to the return signal output circuit 29, the Zener voltage of the Zener diode can be used as the second threshold voltage of the output voltage VO to be detected at the time of return. . Therefore, this second threshold voltage can be easily realized. In particular, when the power supply circuit does not insulate the primary side and the secondary side of the power supply like a chopper circuit, the return detection circuit 11 does not need to be insulated, so that the configuration using this Zener diode is effective.

(Embodiment 2)
Embodiment 2 of the present invention will be specifically described below with reference to the drawings.

  Switching power supply device 50A according to Embodiment 2 of the present invention is a modification of switching power supply device 50 according to Embodiment 1 described above. The switching power supply device 50A differs from the switching power supply device 50 according to the first embodiment in the configuration for determining whether or not the output voltage VO has increased to the second threshold value.

  FIG. 6 is a circuit diagram showing an example of a switching power supply device 50A according to Embodiment 2 of the present invention. Elements similar to those in FIG. 1 are denoted by the same reference numerals, and description of the same components is omitted.

  6 is different from the switching power supply device 50 shown in FIG. 1 in the configuration of the return detection circuit 11A.

  The recovery detection circuit 11A determines that the output voltage VO is greater than the second threshold when the on-pulse width of the pulse signal 68 output from the drive circuit 42 is greater than the third threshold, and the on-pulse width of the pulse signal 68 is equal to the first threshold. When the threshold value is smaller than 3, the output voltage VO is determined to be smaller than the second threshold value.

  The return detection circuit 11A includes a return signal detection circuit 30A and an RS flip-flop circuit 41.

  When the on-pulse width of the pulse signal 68 is larger than the third threshold, the return signal detection circuit 30A generates a set signal 81 that is a pulse signal, and outputs the generated set signal 81 to the set terminal of the RS flip-flop circuit 41. .

  The clock signal 16B output from the oscillation circuit 16 is input to the reset terminal of the RS flip-flop circuit 41. Further, a return signal 69 </ b> A that is a Q signal generated by the RS flip-flop circuit 41 is input to the start / stop circuit 27.

  In the operation period in the overload protection operation mode, when the on-pulse width of the pulse signal 68 is larger than the third threshold value, the return signal 69A of the RS flip-flop circuit 41 becomes H and the switch 25 is turned on by the start / stop circuit 27. . Therefore, current is supplied from the constant current source 24 to the overload protection operation capacitor 9, and the voltage at the VDD terminal rises. As a result, the overload control circuit 13 operates in the overload protection operation mode when the on-pulse width of the pulse signal 68 is smaller than the third threshold, compared to when the on-pulse width of the pulse signal 68 is smaller than the third threshold. Increase the period.

  Thereby, the overload control circuit 13 makes the operation period when the output voltage VO is larger than the second threshold in the overload protection operation mode longer than the operation period when the output voltage is smaller than the VO second threshold. By doing so, the electric power supplied to the load 8 can be increased.

  On the other hand, when the on-pulse width of the pulse signal 68 is smaller than the third threshold during the operation period in the overload protection operation mode, the return signal 69A is L, and the overload control circuit 13 performs normal overload protection. Perform the action. That is, after the voltage at the VDD terminal decreases and reaches the stop voltage VDD (OFF), the switch 25 is turned on to charge the VDD terminal and output the L control signal 70. Then, the start / stop circuit 27 lowers the voltage at the VDD terminal by the circuit current of the semiconductor device 26 by turning off the switch 25 after the voltage at the VDD terminal reaches the start voltage VDD (ON). The control signal 70 is output. By repeating these operations, an overload protection operation similar to the conventional one is performed.

  The operation of the overload control circuit 13 in the normal operation mode is the same as that in the first embodiment.

  FIG. 7 is a diagram illustrating operation waveforms of respective units in the switching power supply device 50A according to the second embodiment.

  At time t30, the voltage at the VDD terminal reaches the starting voltage VDD (ON) and then decreases. Further, after time t30, it is an operation period in the overload protection operation mode, and the switching device 1 performs a switching operation. As a result, electric power is supplied to the load 8 on the secondary side, and the output voltage VO starts to increase.

  Further, the ON pulse width (ON time Ton) of the pulse signal 68 becomes wider as the output voltage VO increases. At this time, the ON time Ton of the pulse signal 68 and the output voltage VO have a relationship represented by the following formula (1).

  Ton = (np / ns) VO / [(np / ns) VO + VIN] × T (1)

  In the above formula (1), VO is the output voltage, VIN is the input voltage, Ton is the ON time of the pulse signal 68, T is the oscillation period of the oscillation circuit 16, np is the number of turns of the primary side winding 2A of the transformer 2, and ns is This is the number of turns of the secondary winding 2B of the transformer 2. As shown in Expression (1), when the output voltage VO increases, the on-time Ton of the switching device 1 also increases.

  The return signal detection circuit 30A outputs a set signal 81, which is a short pulse signal, to the set terminal of the RS flip-flop circuit 41 when the ON time Ton becomes longer than the first time set in the return signal detection circuit 30A. To do. A clock signal 16B output from the oscillation circuit 16 is input to the reset terminal of the RS flip-flop circuit 41.

  Before the pulse is input to the set terminal, the RS flip-flop circuit 41 fixes the return signal 69A at L. However, after the pulse is input to the set terminal, the clock signal 16B is input to the reset terminal next. In the meantime, the RS flip-flop circuit 41 sets the return signal 69A to H.

  That is, the RS flip-flop circuit 41 determines that the ON time Ton of the pulse signal 68 has become larger than the first time, so that the output voltage VO becomes equal to or higher than the second threshold corresponding to the first time. Judging.

  In the example shown in FIG. 7, the ON time Ton of the pulse waveform 90 included in the pulse signal 68 is longer than the first time. As a result, the RS flip-flop circuit 41 outputs an H return signal 69A at time t31.

  The return signal 69A is input to the start / stop circuit 27. When the return signal 69A is L during the operation period in the overload protection operation mode, the start / stop circuit 27 turns off the switch 25. As a result, the current supply to the VDD terminal is stopped, and the voltage at the VDD terminal drops.

  On the other hand, when the return signal 69A is H during the operation period in the overload protection operation mode, the start / stop circuit 27 turns on the switch 25. As a result, the constant current source 24 supplies current to the VDD terminal, and the voltage at the VDD terminal rises.

  As shown in FIG. 7, in the conventional example, at the time of start-up or recovery from an overload state, the operation period in the overload protection operation mode is constant until the start-up voltage VDD (ON) decreases to the stop voltage VDD (OFF). The period was fixed. That is, if the output voltage VO does not rise to the set voltage during this operation period, the overload protection operation mode is automatically continued.

  On the other hand, in the switching power supply device 50A according to Embodiment 2 of the present invention, it is detected that the output voltage VO has risen to the second threshold using the on-pulse width of the pulse signal 68, and the output voltage VO is the second threshold. When the voltage rises to the current level, current is supplied to the VDD terminal. As a result, the voltage at the VDD terminal increases, so that the operation period during the overload protection operation mode is extended. Therefore, the switching power supply device 50A can increase the output power to the load 8 on the secondary side by the extension of this operation period.

  As a result, the feedback current IFB output by the output voltage detection circuit 10 increases. When the feedback signal 63 output from the feedback signal control circuit 14 becomes smaller than the threshold value IFB (OLP), the overload detection signal OLP is switched from H to L. Thus, the switching power supply device 50A can reliably return to the normal operation mode when the overload state of the load 8 is released.

  Further, at the time of activation of the switching power supply device 50A, the switching power supply device 50A detects that the output voltage VO has increased by using the on-pulse width of the pulse signal 68, as in the case of recovery from the overload state. Extend the operation period during the protection operation mode. As a result, the switching power supply device 50A can supply power to the secondary side until the output voltage VO rises, so that it can be started up without causing an overload protection malfunction.

  Next, a case where the overload protection operation mode continues without releasing the overload state will be described.

  In this case, as shown in the above formula (1), since the output voltage VO hardly increases, the ON time Ton of the pulse signal 68 becomes narrower than the first time. Therefore, since the return signal detection circuit 30A continues to output the L set signal 81, the return signal 69A generated by the RS flip-flop circuit 41 also maintains L. As a result, the start / stop circuit 27 continues to turn off the switch 25, so that the voltage at the VDD terminal continues to decrease due to the circuit current of the semiconductor device 26.

  Further, since the overload state is not released, the feedback current IFB does not increase. As a result, the feedback signal 63 output from the feedback signal control circuit 14 remains higher than the threshold value IFB (OLP) of the overload detection circuit 15.

  Therefore, when the voltage at the VDD terminal decreases to the stop voltage VDD (OFF), the stop period in the overload protection operation mode starts again. As described above, when the load 8 is not released from the overload state, the switching power supply device 50A performs the overload protection operation by repeating the operation period and the stop period in the overload protection operation mode having the same length as the conventional one. Therefore, the effectiveness of the overload protection function will not be weakened.

  As described above, since the switching power supply device 50A according to the second embodiment of the present invention can detect the output voltage VO from the on-pulse width of the pulse signal 68, an extra terminal is not added to the semiconductor device 26, and Without weakening the effectiveness of the overload protection function, it is possible to reliably start and recover from overload.

  Here, the switching power supply device 50A detects the increase in the output voltage VO using the on-time Ton of the pulse signal 68, but even if it detects the increase in the output voltage VO using the on-duty Don of the pulse signal 68. Good. Specifically, the return signal detection circuit 30A can detect an increase in the output voltage VO using the following equation (2).

    Don = (np / ns) VO / [(np / ns) VO + VIN] (2)

  Specifically, in the example of the second embodiment, the return signal detection circuit 30A outputs the H set signal 81 when the ON time Ton of the pulse signal 68 becomes wider than the first time. On the other hand, when the return signal detection circuit 30 </ b> A detects the output voltage VO from the on-duty Don of the pulse signal 68, the return signal detection circuit 30 </ b> A has a first on-state in which the on-duty Don is set by the return signal detection circuit 30. When it becomes larger than the duty value, the H set signal 81 is output. Note that the operation of the switching power supply 50 after outputting the set signal 81 is the same as when the output voltage VO is detected from the on-pulse width.

  In the switching power supply 50 of the first embodiment, the return signal detection circuit 30 directly detects the output voltage VO. However, the transformer 2 further includes a primary side auxiliary winding 2C. The increase in the output voltage VO may be detected from a voltage proportional to the output voltage VO appearing in the primary side auxiliary winding 2C. In this case, the same effect as the above configuration can be obtained.

  In addition to the methods described in the first and second embodiments, the operation period in the overload protection operation mode when the output voltage VO becomes equal to or higher than the second threshold value in the overload protection operation mode. As a method for increasing the length, the following first method or second method may be used. In these cases, the same effect as the above configuration can be obtained.

  Further, as a first method, the overload control circuit 13 is configured such that when the output voltage VO is equal to or higher than the second threshold during the overload protection operation mode, compared to the case where the output voltage VO is equal to or lower than the second threshold. The operation period of the overload protection operation mode may be extended by widening the voltage difference between the stop voltage VDD (OFF) and the start voltage VDD (ON).

  For example, when the output voltage VO becomes equal to or higher than the second threshold value during the overload protection operation mode, the overload control circuit 13 stops the stop voltage VDD (OFF) compared to when the output voltage VO is equal to or lower than the second threshold value. ) Threshold may be reduced. Thereby, the overload control circuit 13 can extend the operation period in the overload protection operation mode determined by the time until the voltage at the VDD terminal decreases to the stop voltage VDD (OFF).

  As a second method, the overload control circuit 13 performs both charging and discharging of the overload protection operation capacitor 9 when the output voltage VO becomes equal to or higher than the second threshold value during the overload protection operation mode. By not performing this, the voltage of the overload protection operation capacitor 9 may be held. At this time, after the overload protection operation mode is released, the voltage at the VDD terminal rises to the starting voltage VDD (ON).

  In the first embodiment and the second embodiment, when the output voltage VO becomes equal to or higher than the second threshold during the overload protection operation mode, the operation period in the overload protection operation mode is lengthened. Although the electric power supplied to the load 8 is increased, the electric power supplied to the load 8 may be increased by other methods.

  For example, the overload control circuit 13 may increase the power supplied to the load 8 by facilitating the transition from the overload protection operation mode to the normal operation mode. Specifically, the overload control circuit 13 detects the overload when the output voltage VO is equal to or higher than the second threshold during the overload protection operation mode, compared to when the output voltage VO is lower than the second threshold. The threshold value IFB (OLP) of the circuit 15 may be lowered. Thereby, it becomes easy to return from the overload protection operation mode to the normal operation mode.

  Further, the overload control circuit 13 switches the switching device 1 from the overload protection operation mode to the normal operation mode when the output voltage VO becomes the second threshold value or more during overload protection operation. The power supplied to 8 may be increased. Specifically, the overload control circuit 13 causes the overload detection circuit 15 to output the output voltage VO larger than the first threshold when the output voltage VO becomes equal to or higher than the second threshold during the overload protection operation mode. May be determined. That is, the overload control circuit 13 may change the overload detection signal OLP output from the overload detection circuit 15 from H to L by resetting the overload detection circuit 15.

  Further, the overload control circuit 13 has a drain that flows to the switching device 1 when the output voltage VO is equal to or higher than the second threshold during the overload protection operation, compared to when the output voltage VO is lower than the second threshold. The peak value of the current ID may be increased, or the oscillation frequency of the oscillation circuit 16 may be increased. Thereby, since the power supply to the load 8 can be increased, the output voltage VO can be raised quickly.

  In the first and second embodiments, the switching power supply devices 50 and 50A having the self-recovery type overload protection function are described as an example. The present invention can also be applied to a switching power supply device having an overload protection function.

  In the latch type, the overload control circuit 13 performs an overload protection operation to stop the switching device 1 when the overload detection circuit 15 determines that the output voltage VO is smaller than the first threshold. In this latch type, since it is necessary to turn on the power again in order to start again after entering the latch for overload protection, the present invention is effective only at the time of starting. When the switching power supply device 50 or 50A is activated, the overload control circuit 13 is configured to output the second output voltage VO when the output voltage VO is smaller than the first threshold and larger than the second threshold. The power supplied to the load 8 is increased as compared with the case where it is smaller than the threshold value.

  As described above, the present invention can suppress the occurrence of malfunction at the time of startup of the switching power supply device having the latch stop type overload protection function. Also in this case, since the capacity of the overload protection operation capacitor 9 does not have to be increased, the effectiveness of the overload protection function is not reduced.

  The present invention can be applied to a switching power supply device and a semiconductor device, and is particularly useful for a switching power supply device having an overload protection function.

DESCRIPTION OF SYMBOLS 1,101 Switching device 2,102 Transformer 3,103 Diode 4,104 Capacitor 5,105 Output part 6,106 Input part 7,107 Output voltage generation circuit 8,108 Load 9,109 Overload protection operation capacitor 10,110 Output voltage detection circuit 11, 11A Return detection circuit 12, 112 Control circuit 13, 113 Overload control circuit 14, 114 Feedback signal control circuit 15, 115 Overload detection circuit 16, 116 Oscillation circuit 16A, 116A Maximum duty cycle signal 16B, 116B Clock signal 17, 117 Gate driver 18, 118 NAND circuit 19, 119 Clamp circuit 20, 120 Overload protection circuit 21, 121 Counter circuit 22, 122 Primary current detection circuit 23, 123 Comparator 24, 12 Constant current source 25, 125 Switch 26, 126 Semiconductor device 27, 127 Start / stop circuit 28, 41, 128 RS flip-flop circuit 29 Return signal output circuit 30, 30A Return signal detection circuit 31, 32, 33, 37 Resistance 34 Bipolar transistor 35 Photocoupler 35A Phototransistor 35B Photodiode 36, 36A Constant current source 38 N-type MOSFET
39 Recovery detection comparator 40 Recovery detection reference voltage source 42, 142 Drive circuit 50, 50A, 100 Switching power supply 61 Detection current signal 62, 63 Feedback signal 64 Reset signal 65 Q signal 66, 70 Control signal 67, 68 Pulse signal 69, 69A Return signal 75, 76, 77 Voltage 81 Set signal 90 Pulse waveform ID Drain current IFB Feedback current IFB1 First feedback current value IFB (OLP) Threshold ILIMIT Maximum current value IO Output current OLP Overload detection signal VDD (ON ) Start-up voltage VDD (OFF) Stop voltage VFB Feedback voltage VIN Input voltage VO Output voltage Voth Reference voltage

Claims (15)

A switching power supply that converts a DC input voltage input to an input unit into a DC output voltage and outputs the converted voltage.
A voltage conversion circuit for converting the input voltage into a conversion voltage;
A switching device connected to the voltage conversion circuit;
An output voltage generation circuit that is connected between the voltage conversion circuit and a load and generates the output voltage by rectifying and smoothing the conversion voltage;
A driving circuit for driving the switching device;
An overload detection circuit for determining whether the output voltage is greater than a first threshold;
When the overload detection circuit determines that the output voltage is greater than the first threshold, the drive circuit is controlled to operate the switching device continuously, and the output voltage is controlled by the overload detection circuit. Is determined to be smaller than the first threshold, an overload control circuit that controls the drive circuit so that the switching device performs an overload protection operation;
A return detection circuit for determining whether or not the output voltage is larger than a second threshold value smaller than the first threshold value,
In the overload protection operation, the switching device repeats an operation period in which the switching device operates during the overload protection operation and a stop period in which the switching device stops during the overload protection operation,
The overload control circuit further supplies the load when the output voltage is larger than the second threshold value than when the output voltage is smaller than the second threshold value during the overload protection operation. A switching power supply that increases power. In the overload protection operation, the overload control circuit makes the operation period longer when the output voltage is larger than the second threshold than when the output voltage is smaller than the second threshold. The switching power supply according to claim 1, wherein the power supplied to the load is increased by the switching power supply. The switching power supply device further includes:
With a capacitor,
The overload control circuit determines a length of the operation period using a time during which the capacitor is charged or discharged from the first voltage to the second voltage,
In the overload protection operation, the overload control circuit supplies the first voltage to the capacitor during the operation period when the output voltage is greater than the second threshold value. The switching power supply according to claim 2, wherein the output voltage is longer than when the output voltage is smaller than the second threshold. The switching power supply according to claim 1, wherein the return detection circuit includes a return signal detection circuit that directly detects the output voltage. The driving circuit generates a first signal for driving the switching device;
When the on-pulse width or on-duty for turning on the switching device in the first signal is greater than a third threshold, the return detection circuit determines that the output voltage is greater than the second threshold, and the on-pulse width The switching power supply according to claim 1, wherein when the on-duty is smaller than the third threshold, the output voltage is determined to be smaller than the second threshold. The drive circuit is
An oscillation circuit that generates a clock signal is provided.
The drive circuit generates a first signal that turns on the switching device in a cycle of the clock signal;
When the on-pulse width or on-duty for turning on the switching device in the first signal is greater than a third threshold, the return detection circuit determines that the output voltage is greater than the second threshold, and the on-pulse width Or when the on-duty is smaller than the third threshold, the output voltage is determined to be smaller than the second threshold;
The return signal detection circuit generates a set signal that is a pulse signal when the on-pulse width or on-duty is greater than the third threshold value,
A flip-flop circuit in which the set signal is input to a set terminal and the clock signal is input to a reset terminal;
In the operation period during the overload protection operation, the overload control circuit supplies the first voltage to the capacitor during the period in which the flip-flop circuit is in a set state, thereby reducing the operation period to the on-pulse width or The switching power supply device according to claim 3, wherein the on-duty is longer than when the on-duty is smaller than the third threshold. The voltage conversion circuit is a transformer having a primary winding, a secondary winding, and an auxiliary winding,
The switching power supply according to claim 1, wherein the return detection circuit detects the output voltage from the auxiliary winding. The switching power supply device further includes:
With a capacitor,
The overload control circuit determines a length of the operation period using a time during which the capacitor is charged or discharged from the first voltage to the second voltage,
In the overload protection operation, the overload control circuit is configured such that when the output voltage is greater than the second threshold, the first voltage and the second voltage are greater than when the output voltage is less than the second threshold. 3. The switching power supply according to claim 2, wherein the operation period when the output voltage is larger than the second threshold is made longer than that when the output voltage is smaller than the second threshold by increasing a difference from the voltage. apparatus. The overload control circuit causes the overload detection circuit to determine that the output voltage is greater than the first threshold when the output voltage is greater than the second threshold during the overload protection operation. The switching power supply device according to claim 1, wherein the power supplied to the load is increased by continuously operating the switching device. The switching power supply device further includes:
With a capacitor,
The overload control circuit determines a length of the operation period using a time during which the capacitor is charged or discharged from the first voltage to the second voltage,
In the overload protection operation, when the output voltage is larger than the second threshold value, the overload control circuit holds the capacitor voltage, so that the output voltage is larger than the second threshold value. The switching power supply unit according to claim 2, wherein the operation period is longer than when the output voltage is smaller than the second threshold. In the overload protection operation, the overload protection circuit reduces the first threshold when the output voltage is greater than the second threshold than when the output voltage is less than the second threshold. The switching power supply device according to claim 1, wherein the power supplied to the load is increased. In the overload protection operation, the overload protection circuit sets the oscillation frequency of the switching device when the output voltage is larger than the second threshold than when the output voltage is smaller than the second threshold. The switching power supply according to claim 1, wherein the power supplied to the load is increased by increasing or increasing a peak value of a current flowing through the switching device. A switching power supply that converts a DC input voltage input to an input unit into a DC output voltage and outputs the converted voltage.
A voltage conversion circuit for converting the input voltage into a conversion voltage;
A switching device connected to the voltage conversion circuit;
An output voltage generation circuit that is connected between the voltage conversion circuit and a load and generates the output voltage by rectifying and smoothing the conversion voltage;
A driving circuit for driving the switching device;
An overload detection circuit for determining whether the output voltage is greater than a first threshold;
When the overload detection circuit determines that the output voltage is greater than the first threshold, the drive circuit is controlled to operate the switching device continuously, and the output voltage is controlled by the overload detection circuit. Is determined to be smaller than the first threshold, an overload control circuit that stops the switching device;
A return detection circuit for determining whether or not the output voltage is larger than a second threshold value smaller than the first threshold value,
The overload control circuit further includes, when the switching power supply device is activated, when the output voltage is smaller than the first threshold and larger than the second threshold, and when the output voltage is smaller than the second threshold. A switching power supply device that increases the power supplied to the load as compared with the above. A switching power supply comprising: the drive circuit according to claim 1, the overload detection circuit, the overload control circuit, and the recovery detection circuit formed on one semiconductor chip. Semiconductor device. The switching device according to claim 1, the drive circuit, the overload detection circuit, the overload control circuit, and the recovery detection circuit formed in one semiconductor chip. A semiconductor device for a switching power supply.

Images