JP5790010B2 - Switching power supply - Google Patents

Description

  The present invention relates to a current resonance type switching power supply device.

  A switching power supply device used for a flat panel display or the like is often a half-bridge type using two switching elements and a current resonance type that can further reduce switching loss. Since the current resonance type switching power supply device is usually used in combination with PFC (Power Factor Correction), it is often used at a constant input voltage.

  FIG. 7 is a circuit diagram showing a configuration of a DC-DC converter in a conventional switching power supply apparatus. The operation of the DC-DC converter shown in FIG. 7 will be described. First, when the voltage of the DC power supply Vin is applied, the control circuit 10 starts operating by a starter circuit (not shown). The control circuit 10 includes an oscillation circuit 11, a D-type flip-flop circuit 13, dead time generation circuits 14 and 15, a level shift circuit 16, and buffer circuits 17 and 18, and the switch elements Q1 and Q2 have a dead time. And alternately turn on and off.

  When the switch element Q2 is turned on, a current flows through a route of Vin → Q2 → Lr → P → Cri → Vin. This current is supplied from the output terminals + Vo and −Vo to the load via the exciting current flowing through the exciting inductance Lp on the primary side of the transformer T, the primary winding P, the secondary winding S2, the diode D2, and the capacitor Co. The resultant current is combined with the generated load current. The former current is a sinusoidal resonance current between (reactor Lr + exciting inductance Lp) and the current resonance capacitor Cri, and has a resonance frequency lower than the on-period of the switch element Q2, so that a part of the sinusoid is a triangular wave. Is observed as a current. The latter current is a sinusoidal resonance current in which resonance elements of the reactor Lr and the current resonance capacitor Cri appear.

  When the switch element Q2 is turned off, voltage pseudo resonance is generated by (reactor Lr + excitation inductance Lp), the current resonance capacitor Cri, and the voltage resonance capacitor Crv due to the energy of the excitation current stored in the transformer T. At this time, the resonance frequency by the voltage resonant capacitor Crv having a small capacity is observed as the voltage across the switch element Q1 and the switch element Q2. That is, the current of the switch element Q2 moves to the voltage resonance capacitor Crv when the switch element Q2 is turned off. When the voltage resonant capacitor Crv is discharged to zero volts, the current is transferred to the diode D8. This is because the energy by the exciting current stored in the transformer T charges the current resonance capacitor Cri via the diode D8. By turning on the switch element Q1 during this period, a zero volt switch of the switch element Q1 can be realized.

  When the switch element Q1 is turned on, a current flows through a path of Cri → P → Lr → Q1 → Cri using the current resonance capacitor Cri as a power source. This current is an excitation current flowing through the excitation inductance Lp of the transformer T and a load supplied from the output terminals + Vo and −Vo to the load via the primary winding P, the secondary winding S1, the diode D1, and the smoothing capacitor Co. It becomes the combined current with the current. The former current is a sinusoidal resonance current of (reactor Lr + exciting inductance Lp) and the current resonance capacitor Cri, and has a resonance frequency lower than the ON period of the switch element Q1, so that a part of the sine wave has a triangular wave shape. Observed as current. The latter current is a sinusoidal resonance current in which resonance elements of the reactor Lr and the current resonance capacitor Cri appear.

  When the switch element Q1 is turned off, the voltage pseudo resonance is generated by the (reactor Lr + excitation inductance Lp), the current resonance capacitor Cri, and the voltage resonance capacitor Crv due to the energy of the excitation current stored in the transformer T. At this time, the resonance frequency by the voltage resonant capacitor Crv having a small capacity is observed as the voltage across the switch element Q1 and the switch element Q2. That is, the current of the switch element Q1 moves to the voltage resonance capacitor Crv when the switch element Q1 is turned off. When the voltage resonance capacitor Crv is charged to the voltage of the DC power supply Vin, a current is transferred to the diode D9. This is because the energy by the excitation current stored in the transformer T is regenerated to the DC power source Vin via the diode D9. By turning on the switch element Q2 during this period, the zero volt switch of the switch element Q2 can be realized.

  Patent Document 1 describes an overload protection circuit for a switching power supply that is intended to prevent the overload detection signal from being dependent on the input voltage even when the input voltage is widened. This overload protection circuit rectifies and smoothes AC power, applies it to the primary winding of the transformer, turns it on and off by the switching element, and rectifies and smoothes the switching voltage induced in the secondary winding of this transformer and outputs it. A control signal is sent to the switching element in a direction to reduce the error voltage by comparing the output voltage of the converter unit with the reference voltage, and when the input voltage increases, the pulse width is narrowed. A pulse width control circuit to be controlled, a means for detecting a peak component of the current flowing in the primary winding, and a rectifying / smoothing on / off polarity switching voltage induced in a bias winding provided on the primary side of the transformer A rectifying / smoothing unit, and adding the input voltage correction signal detected by the rectifying / smoothing unit to the load current signal detected by the current detecting means. It is intended to send to the overload protection terminal of the pulse width control circuit to.

  According to this overload protection circuit, the input voltage dependency of the overload detection signal is canceled by the input voltage correction signal of the rectifying and smoothing unit using the bias winding, so that even if the voltage of the commercial power supply is widened, it is accurate. Overload protection can be performed.

  Patent Document 2 describes a resonant switching power supply that has a problem of performing overcurrent control with a simple circuit. The switching power supply includes two switching elements, a resonance circuit, a transformer, an output rectifying / smoothing circuit, and an overcurrent protection circuit. The two switching elements are connected in series, and both ends of the series circuit are led to a DC power source and are driven alternately. The transformer includes at least a primary winding and a secondary winding. The resonant circuit has a resonant capacitor and a resonant inductor. The primary winding of the resonant capacitor, resonant inductor and transformer are connected in series, and both ends of the series circuit are connected between the connection point of the two switching elements and one end of the series circuit constituted by the two switching elements. Yes. The output rectifying / smoothing circuit is connected to the secondary winding of the transformer. The overcurrent protection circuit detects a current flowing on the secondary winding side of the transformer and gives an overcurrent protection operation to the switching element based on the detection signal.

  In this switching power supply, by operating two switching elements connected in series alternately, the input DC power supply is switched, and the switching output is supplied to the primary winding of the resonance circuit and the transformer. Between the connection point of the two switching elements and one end of the series circuit constituted by the two switching elements, a series in which a resonance capacitor and a resonance inductor constituting the resonance circuit and a primary winding of the transformer are connected in series. Since both ends of the circuit are connected, a pseudo sine wave current corresponding to the resonance frequency of the resonance circuit flows through the primary winding of the resonance circuit and the transformer by the alternating operation of the two switching elements. At this time, an induced voltage is generated in the secondary winding coupled to the primary winding. This induced voltage is converted into a direct current by an output rectifying / smoothing circuit connected to the secondary winding of the transformer and output.

  The overcurrent protection circuit detects a current flowing on the secondary winding side of the transformer and gives an overcurrent protection operation to the switching element based on the detection signal. The current that flows on the secondary winding side of the transformer varies depending on the circuit system of the output rectifying and smoothing circuit. When the output rectifying and smoothing circuit is a choke input type including an output choke coil, a trapezoidal current flows through the secondary winding of the transformer. The trapezoidal current becomes a triangular wave when the output voltage drops to zero volts. Further, the peak value is substantially equal to the output current. Therefore, by detecting this current and converting it into a voltage signal or the like, it is possible to perform overcurrent control of a constant current voltage drooping characteristic.

  When the output rectifying and smoothing circuit is a capacitor input type that does not include an output choke coil, a sinusoidal current flows in the secondary winding of the transformer. Therefore, the overcurrent control of the constant current voltage drooping characteristic can be performed by detecting this current and converting the average value into a voltage signal or the like.

JP-A-5-344712 JP-A-10-229673

CQ Publisher Power Circuit Design 2009

  As described above, the current resonance type switching power supply is usually used in combination with the PFC, but the PFC part has a large part cost and a large board area, and the load capacity not subject to the harmonic regulation is a ratio. In a small region (for example, PIN 75 W or less), current resonance may be used without PFC. However, when used without PFC, the input voltage is not constant. In particular, in the world wide input (AC85V to AC276V), since the input voltage changes greatly, there is a problem that the maximum output voltage changes greatly according to the input voltage.

  When a transformer designed for world-wide input with current resonance is used, there is a problem that input correction (overload protection level is constant even if the input voltage changes) becomes difficult for overload protection. As described in Non-Patent Document 1, page 214, the current flowing in the current resonance capacitor Ci (or primary winding P) for detecting overcurrent is an excitation current (circulating current) and a load current to the secondary side. It becomes the sum of. In order to reduce a switching loss due to a resonance phenomenon, a current resonance type switching power supply needs to increase an exciting current (circulating current) as the input voltage is wider. In particular, at the upper limit of the input voltage, the primary-side load current decreases as the input voltage increases, so that the proportion of the excitation current occupies a relatively large value, making it difficult to detect load current.

  The overload protection circuit described in Patent Document 1 detects an input voltage by some method and changes the overcurrent detection level accordingly. In the current resonance type switching power supply, the above-described excitation current occupies a large proportion. There is a problem that desired power supply characteristics cannot be obtained.

  FIG. 8 shows an example of operation waveforms at the time of no load and maximum load when the input voltage is high with a wide input in the conventional switching power supply device. As shown in FIG. 8A, a triangular wave excitation current (circulating current) flows through the current resonance capacitor when there is no load. As shown in FIG. 8B, the combined current of the excitation current and the load current flows through the current resonance capacitor in a sine wave shape at the maximum load, but the peak value of the current greatly changes even when the maximum load is taken. I understand that there is not. For this reason, it is difficult to perform overload protection in the method of detecting the current peak value. This is not a problem when the PFC is present and the input voltage is constant, since the excitation current (circulating current) is small, but it becomes a significant problem when the input is wide.

  Since the switching power supply described in Patent Document 2 can make the excitation current zero, it can be predicted that only the load current can be confirmed, and constant overload protection can be performed regardless of the input voltage, It is considered that desired power supply characteristics can be obtained. However, as a problem of this method, it is necessary to use an expensive current transformer. In addition, it is possible to perform signal transmission between the primary and secondary by performing IV conversion with a resistor without using a current transformer, but in this case, it is necessary to obtain various safety distances, and board layout constraints And the board area increases. If possible, it is desirable not to use an expensive current transformer and not to transmit signals between the primary and secondary, considering the total power supply cost.

  The present invention solves the above-mentioned problems of the prior art, and suppresses the influence of excitation current and supports a wide range of input voltages when performing overload protection on the primary side for miniaturization and cost reduction. It is an object of the present invention to provide a switching power supply device that performs stable and stable overload protection.

In order to solve the above problem, a switching power supply according to the present invention is connected to both ends of a DC power supply, and includes a first series circuit in which a first switch element and a second switch element are connected in series, and the first A second series circuit that is connected in parallel to one of the switch element and the second switch element, and in which a resonant capacitor, a resonant reactor, and a primary winding of the transformer are connected in series; and a secondary winding of the transformer A rectifying / smoothing circuit for rectifying and smoothing a line voltage; a control circuit for alternately turning on / off the first switch element and the second switch element based on an output voltage of the rectifying / smoothing circuit; and the second series circuit. a current detection unit for detecting a current flowing through said first in synchronism with at least one of the oN period of the switching element and the second switching element, based on the detected current by the current detecting section Characterized in that it comprises a load current extractor which averages the Ku voltage value to extract the load current value, an overcurrent protection unit which performs an overcurrent protective operation based on the load current value extracted by the load current extractor And

  According to the present invention, when performing overload protection on the primary side for miniaturization and cost reduction, it is possible to suppress the influence of excitation current and to perform stable overload protection corresponding to a wide range of input voltages. it can.

It is a circuit diagram which shows the structure of the switching power supply device of the form of Example 1 of this invention. It is a block diagram which shows the structure of a part of content of the control circuit in the switching power supply device of the form of Example 1 of this invention. It is a figure which shows the output of the reference voltage generation circuit in the switching power supply device of the form of Example 1 of this invention. It is a wave form diagram of each part which shows operation | movement of the switching power supply device of the form of Example 1 of this invention. It is a block diagram which shows another example of a structure of a part of content of the control circuit in the switching power supply device of the form of Example 1 of this invention. It is a circuit diagram which shows the structure of the switching power supply device of the form of Example 2 of this invention. It is a circuit diagram which shows the structure of the DC-DC converter in the conventional switching power supply device. In the conventional switching power supply device, it is a figure which shows the example of an operation waveform at the time of the no load at the time of a maximum load when a wide input and input voltage are high.

  Embodiments of a switching power supply apparatus according to the present invention will be described below in detail with reference to the drawings.

  Embodiments of the present invention will be described below with reference to the drawings. First, the configuration of the present embodiment will be described. FIG. 1 is a circuit diagram showing a configuration of a switching power supply apparatus according to Embodiment 1 of the present invention.

  The low-side switching element Q1 corresponds to the first switch element of the present invention. The high-side switching element Q2 corresponds to the second switch element of the present invention. These switching elements Q1, Q2 are, for example, MOSFETs. The series circuit in which the switching element Q1 and the switching element Q2 are connected in series corresponds to the first series circuit of the present invention, and is connected to both ends of the DC power supply. However, the DC power supply in this embodiment is constituted by a power supply circuit that outputs a DC voltage obtained by full-wave rectifying a commercial AC power supply and smoothing it with a smoothing capacitor. DC voltage is output across the series circuit.

  The series circuit in which the resonant capacitor Ci, the resonant reactor Lr, and the primary winding Lp of the transformer T1 are connected in series corresponds to the second series circuit of the present invention. Here, the resonant reactor Lr may be configured by a leakage inductance (leakage inductance) of the primary winding Lp of the transformer T1. A voltage resonant capacitor Cv is connected in parallel to the switching element Q1. The second series circuit of the present invention is connected in parallel to either the first switch element or the second switch element. In this embodiment, as shown in FIG. 1, the first switch element (switching element) Q1) is connected in parallel. The case where the second series circuit is connected in parallel to the second switch element will be described later.

  The series circuit including the diodes D1 and D2 and the smoothing capacitor Co corresponds to the rectifying and smoothing circuit of the present invention, and is connected in parallel to the secondary windings S1 and S2 of the transformer T1, and is connected to the secondary windings S1 and S2 of the transformer T1. The voltage is rectified and smoothed. The DC voltage of the smoothing capacitor Co obtained by this rectifying and smoothing circuit becomes the output voltage of the switching power supply device shown in FIG. 1, and DC power is supplied from Output.

  In addition, the switching power supply device illustrated in FIG. 1 includes a control circuit 10a. The control circuit 10a alternately turns on / off the switching element Q1 and the switching element Q2 so that the output voltage is held at a predetermined value based on the output voltage of the rectifying / smoothing circuit described above.

  Specifically, the output voltage is detected by the error amplifier 20, and the detected output voltage is sent to a feedback terminal (FB terminal) of the control circuit 10a on the primary side via a photocoupler. The control circuit 10a adjusts the oscillation frequency of the internal oscillation circuit in accordance with the output voltage, and alternately turns on / off the switching element Q1 and the switching element Q2. Thereby, the control circuit 10a changes the charging / discharging period to the resonant capacitor Ci, and controls the amount of power induced to the secondary side of the transformer T1.

  Here, the switching element Q1 is turned off when the LO terminal voltage is Low, and is turned on when the LO terminal voltage is High. The switching element Q2 is turned off when the HO terminal voltage is Low, and turned on when the HO terminal voltage is High.

  The capacitor C1 and the resistor ROCP are connected in parallel to the capacitor Ci in order to shunt the current flowing through the current resonance capacitor Ci, which contributes to reduction of power consumption. The capacitor C1 and the resistor ROCP correspond to the current detection unit of the present invention and detect the current flowing through the second series circuit. That is, the current flowing through the second series circuit is shunted by the resonance capacitor Ci and the capacitor C1. At this time, the current flowing through the capacitor C1 is proportional to the current flowing through the resonant capacitor Ci and flows through the resistor ROCP.

  FIG. 2 is a block diagram showing a configuration of part of the contents of the control circuit 10a in the switching power supply device of this embodiment. The switch 22 is provided between the PL terminal and the CL terminal in the control circuit 10a, and is closed when the LO terminal voltage is Low and opened when the LO terminal voltage is High. The switch 22 and the capacitor C2 connected to the CL terminal outside the control circuit 10a correspond to the load current extraction unit of the present invention.

  The load current extraction unit of the present invention converts the current detected by the current detection unit into a voltage in synchronization with an on period of at least one of the first switch element (switching element Q1) and the second switch element (switching element Q2). While converting to a signal, the voltage value of a voltage signal is averaged and a load current value is extracted.

  In the present embodiment, the load current extraction unit converts the current detected by the current detection unit into a voltage signal in synchronization with the ON period of the first switch element (switching element Q1). That is, the load current extraction unit takes in the current detected by the current detection unit into a voltage signal by the resistor ROCP from the PL terminal via the resistor R3, and switches the switch 22 in synchronization with the ON period of the switching element Q1. By opening and closing, a voltage signal synchronized with the ON period of the switching element Q1 is output to the CL terminal.

  Therefore, the voltage signal output to the CL terminal charges and discharges the capacitor C2 in synchronization with the ON period of the switching element Q1. As a result, the CL terminal voltage is obtained by averaging the voltage signal. That is, when converting the current detected by the current detection unit into a voltage signal, the load current extraction unit converts the voltage signal into a voltage signal including a positive / negative sign, and averages the voltage value of the voltage signal without rectification. To extract the load current value.

  The reference voltage generation circuit 24, the comparator 26, and the overpower protection circuit 28 illustrated in FIG. 2 correspond to the overcurrent protection unit of the present invention, and perform an overcurrent protection operation based on the load current value extracted by the load current extraction unit. I do. Here, each configuration of the overcurrent protection unit will be described.

  The reference voltage generation circuit 24 generates and outputs a second reference voltage value based on the input Vsen voltage. Here, the Vsen voltage is a voltage having a value corresponding to the resistance division of the resistors R1 and R2 generated from the input voltage (DC power supply voltage). In other words, the reference voltage generation circuit 24 generates the second reference voltage value based on the voltage value of the DC power supply.

  FIG. 3 is a diagram showing the output of the reference voltage generation circuit 24 in the switching power supply device of this embodiment, and shows the voltage value of the second reference voltage that is output with respect to the input Vsen voltage. Basically, as shown in FIG. 3, the reference voltage generation circuit 24 decreases the second reference voltage value with respect to the increase in the voltage value of the DC power supply (increase in the Vsen voltage), and decreases the voltage value of the DC power supply. A voltage having a complementary relationship such that the second reference voltage value is increased with respect to (decrease in Vsen voltage) is output.

  This is because the current flowing to the primary side becomes smaller when the input voltage is higher, so that the reference voltage generation circuit 24 adjusts so that the second reference voltage becomes smaller when the Vsen voltage becomes higher, and appropriate overcurrent detection is performed. This is to make it possible.

  However, the reference voltage generation circuit 24 may have nonlinear characteristics in accordance with actual switching power supply characteristics. For example, as shown in FIG. 3A, the second reference for the Vsen voltage with a predetermined Vsen voltage value as a boundary. The slope of the voltage value may be changed, or the second reference voltage value may simply have a negative slope with respect to the Vsen voltage as shown in FIG.

  The comparator 26 compares the second reference voltage value generated by the reference voltage generation circuit 24 with the CL terminal voltage, and outputs a low level voltage when the voltage value of the CL terminal voltage is less than the second reference voltage value. When the voltage value of the CL terminal voltage is equal to or higher than the second reference voltage value, a high level voltage is output.

  The overpower protection circuit 28 performs an overcurrent protection (overpower protection) operation when a high level voltage is output from the comparator 26. The protection operation by the overpower protection circuit 28 may be a conventional operation, for example, by stopping the oscillation and stopping the power supply to the secondary side, or by forcibly increasing the oscillation frequency. The power supply to the secondary side may be limited. The specific operation by the overpower protection circuit 28 is not an essential part of the present invention, and is not limited to the method described above.

  That is, the overcurrent protection unit as a whole generates the second reference voltage value based on the voltage value of the DC power supply, and the load current value (corresponding to the CL terminal voltage value) extracted by the load current extraction unit is the second reference voltage value. Overcurrent protection is performed when the voltage is higher than the specified value.

  Next, the operation of the present embodiment configured as described above will be described. A normal operation in which no overcurrent flows is the same as that of the conventional apparatus described in FIG. The capacitor C1 and the resistor ROCP are current detection units that detect the current flowing through the second series circuit as described above, and the current that originally flows through the resonant capacitor Ci is shunted to the capacitor C1 and converted into a voltage at the resistor ROCP.

  The load current extraction unit in the present embodiment converts the current detected by the current detection unit into a voltage signal in synchronization with the ON period of the first switch element (switching element Q1). That is, the load current extraction unit takes in the voltage signal converted by the resistor ROCP from the PL terminal via the resistor R3, and charges and discharges the capacitor C2 from the CL terminal in synchronization with the ON period of the switching element Q1. .

  FIG. 4 is a waveform diagram of each part showing the operation of the switching power supply device of the present embodiment. As described above, the switch 22 is closed when the LO terminal voltage is Low, and is opened when the LO terminal voltage is High. Therefore, as shown in FIG. 4, the current flowing through the resistor R3 becomes zero when the switching element Q1 is on, and has a value corresponding to the current flowing through the second series circuit when the switching element Q1 is off.

  The reason for dividing the waveform into the low side and the high side in this way is that the current flowing through the current resonance capacitor Ci becomes zero when the low side and the high side are added together. The switching power supply in the present embodiment uses the gate signal of the low-side MOSFET (switching element Q1).

  As shown in FIG. 4, the switching power supply according to the present embodiment charges and discharges the capacitor C2 connected to the CL terminal through the resistor R3 when the gate signal of the low-side MOSFET (switching element Q1) is Low. As a result, the CL terminal can be averaged from the minus direction.

  That is, the exciting current (circulating current) flows from the negative direction to the positive direction as shown in FIG. 4 and is therefore canceled out. The influence of the exciting current (circulating current) can be reduced, and only the load current is the capacitor. C2 is charged. Therefore, the CL terminal voltage connected to the capacitor C2 becomes a value based on the load current value sent to the secondary side.

  In this way, the load current extraction unit of this embodiment can extract only the load current and output a voltage corresponding to the load current value to the CL terminal.

  The comparator 26 compares the second reference voltage value generated by the reference voltage generation circuit 24 with the CL terminal voltage, and outputs a low level voltage when the voltage value of the CL terminal voltage is less than the second reference voltage value. When the voltage value of the CL terminal voltage is equal to or higher than the second reference voltage value, a high level voltage is output. The overpower protection circuit 28 performs an overcurrent protection (overpower protection) operation when a high level voltage is output from the comparator 26.

  That is, the overcurrent protection unit generates the second reference voltage value based on the voltage value of the DC power supply, and the load current value (corresponding to the CL terminal voltage value) extracted by the load current extraction unit is the second reference voltage value. In the above case, the overcurrent protection operation is performed.

  As described above, the switching power supply according to the first embodiment of the present invention suppresses the influence of excitation current (circulating current) when performing overload protection on the primary side for downsizing and cost reduction. In addition, stable overload protection can be performed in response to a wide range of input voltages.

  In other words, the switching power supply apparatus of this embodiment can cope with the case where the input voltage changes without the PFC and the world wide input, and the reference voltage generation circuit 24 is based on the voltage value of the DC power supply. Since two reference voltage values are generated, overcurrent detection can be performed with an appropriate detection threshold.

  In addition, the current flowing through the primary winding P is the sum of the exciting current (circulating current) and the load current to the secondary side. When the input voltage is high, the primary side load current decreases, so the proportion of the exciting current is relative. Since the load current extraction unit extracts only the load current value by reducing the influence of the excitation current (circulating current) even if the current becomes larger, the switching power supply device of this embodiment must appropriately detect the overcurrent. Can do.

  Further, since it is not necessary to use an expensive current transformer as in the switching power supply described in Patent Document 2, it can be realized at a low cost, and overload protection is performed on the primary side. Therefore, it is not necessary to transmit signals and it is not necessary to acquire various safety distances, so that the size of the apparatus can be reduced. In particular, if an IC is formed together with the primary control circuit, the design is simple and the cost can be significantly reduced.

  In realizing the present invention, the overcurrent protection unit is not necessarily limited to the configuration shown in FIG. FIG. 5 is a block diagram showing another example of the configuration of part of the contents of the control circuit 10a in the switching power supply device of this embodiment.

  The amplitude adjustment circuit 23, the multiplication circuit 25, the comparator 26, and the overpower protection circuit 28 shown in FIG. 5 correspond to the overcurrent protection unit of the present invention, and are based on the load current value extracted by the load current extraction unit. Performs current protection. Here, each configuration of the overcurrent protection unit will be described.

  The amplitude adjustment circuit 23 is a circuit that adjusts the input Vsen voltage to an appropriate scale, and has a role of keeping the voltage value within the IC operating range, particularly when an IC is formed. The amplitude adjusting circuit 23 is not particularly related to the essential part of the present invention.

  The multiplication circuit 25 multiplies the CL terminal voltage by the Vsen voltage adjusted to an appropriate amplitude by the amplitude adjustment circuit 23 and outputs the result. Here, since the CL terminal voltage is based on the load current value sent to the secondary side, it can be said that (CL terminal voltage) × (Vsen voltage) = (similar to the secondary side load power).

  The comparator 26 compares the first reference voltage 27 with the output voltage from the multiplication circuit 25, and outputs a low level voltage when the output voltage value from the multiplication circuit 25 is less than the first reference voltage value. When the output voltage value by is equal to or higher than the first reference voltage value, a high level voltage is output. The overpower protection circuit 28 is the same as in FIG.

  That is, the overcurrent protection unit as a whole was obtained by multiplying the load current value (corresponding to the CL terminal voltage value) extracted by the load current extracting unit and the DC power supply voltage value (corresponding to the Vsen voltage value). When the multiplication value is equal to or higher than the first reference voltage value by the first reference voltage 27, the overcurrent protection operation is performed.

  Even if the overcurrent protection unit as shown in FIG. 5 is used, the switching power supply according to the present embodiment can obtain the same effect as the overcurrent protection unit shown in FIG. In particular, since the output value from the multiplication circuit 25 corresponds to the secondary side load power, there is no need to appropriately adjust the second reference voltage by the reference voltage generation circuit 24 as in the case of FIG. The overload protection operation can be appropriately performed by comparing the obtained multiplication value with the first reference voltage value which is a fixed value.

  FIG. 6 is a circuit diagram showing a configuration of the switching power supply device according to the second embodiment of the present invention. The difference from the configuration of the switching power supply device according to the first embodiment shown in FIG. 1 is that the components of the resonance circuit are changed to high-side MOSFETs. That is, in the present embodiment, the series circuit (corresponding to the second series circuit of the present invention) in which the resonant capacitor Ci, the resonant reactor Lr, and the primary winding Lp of the transformer T1 are connected in series is the case of the first embodiment. Unlike the above, the second switch element (switching element Q2) is connected in parallel.

  In this embodiment, instead of using a shunt capacitor, a current detection resistor ROCP is provided between the source and GND of the low-side MOSFET. In this embodiment, the resistor ROCP corresponds to the current detector of the present invention and detects the current flowing through the second series circuit. That is, the current flowing through the second series circuit flows to the resistor ROCP via the switching element Q1.

  The configuration of part of the contents of the control circuit 10b in the switching power supply device of this embodiment may be the same as that in FIG. 2 or FIG. 5 described in the first embodiment. However, the switch 22 opens when the LO terminal voltage is Low, and closes when the LO terminal voltage is High. The switch 22 and the capacitor C2 connected to the CL terminal outside the control circuit 10b correspond to the load current extraction unit of the present invention.

  Other configurations are the same as those of the first embodiment, and redundant description is omitted.

  Next, the operation of the present embodiment configured as described above will be described. The basic operation is the same as that of the switching power supply device described in the first embodiment. The resistor ROCP is a current detection unit that detects the current flowing through the second series circuit as described above, and converts the current flowing through the resonance capacitor Ci into a voltage at the resistor ROCP when the switching element Q1 is turned on.

  The load current extraction unit in the present embodiment converts the current detected by the current detection unit into a voltage signal in synchronization with the ON period of the first switch element (switching element Q1). That is, the load current extraction unit takes in the voltage signal converted by the resistor ROCP from the PL terminal via the resistor R3, and charges and discharges the capacitor C2 from the CL terminal in synchronization with the ON period of the switching element Q1. .

  Other operations are the same as those in the first embodiment, and redundant description is omitted.

  As described above, according to the switching power supply according to the second embodiment of the present invention, when the component of the resonance circuit is on the high side, or the current detection resistor ROCP is not used without using the shunt capacitor. Even in the case of being connected in series to the side MOSFET, the same effect as in the first embodiment can be obtained.

  When a current transformer is used instead of the resistor ROCP, the power loss of the resistor ROCP is suppressed, and a low voltage is required between the windings. There is no need for intermediate pressure resistance, and an inexpensive one can be used.

  The switching power supply according to the present invention can be used for a current resonance type switching power supply.

10, 10a, 10b Control circuit 11 Oscillation circuit 13 D-type flip-flop circuit 14, 15 Dead time generation circuit 16 Level shift circuit 17, 18 Buffer circuit 20 Voltage detection circuit (error amplifier)
22 switch 23 amplitude adjustment circuit 24 reference voltage generation circuit 25 multiplication circuit 26 comparator 27 first reference voltage 28 overpower protection circuit C1, C2 capacitor Ci, Cri resonance capacitor Cv, Crv voltage resonance capacitor Co smoothing capacitors D1, D2, D8, D9 Diode Lp Primary winding Lr Resonant reactor Q1, Q2 Switching element R1, R2, R3, ROCP Resistance S1, S2 Secondary winding T, T1, T2 Transformer

Claims (5)

A first series circuit connected to both ends of the DC power source, wherein the first switch element and the second switch element are connected in series;
A second series circuit connected in parallel to one of the first switch element and the second switch element, wherein a resonance capacitor, a resonance reactor, and a primary winding of a transformer are connected in series;
A rectifying / smoothing circuit for rectifying and smoothing the voltage of the secondary winding of the transformer;
A control circuit that alternately turns on and off the first switch element and the second switch element based on an output voltage of the rectifying and smoothing circuit;
A current detection unit for detecting a current flowing in the second series circuit;
A load current extraction unit that extracts a load current value by averaging voltage values based on the current detected by the current detection unit in synchronization with an ON period of at least one of the first switch element and the second switch element. When,
An overcurrent protection unit that performs an overcurrent protection operation based on the load current value extracted by the load current extraction unit;
A switching power supply device comprising:   2. The switching power supply device according to claim 1, wherein the resonant reactor is configured by a leakage inductance of a primary winding of the transformer.   When the load current extraction unit converts the current detected by the current detection unit into a voltage signal, the load current extraction unit converts the voltage signal into a voltage signal including a positive / negative sign, and averages the voltage value of the voltage signal without rectification. 3. The switching power supply according to claim 1, wherein a load current value is extracted.   The overcurrent protection unit multiplies the load current value extracted by the load current extraction unit by the voltage value of the DC power source, and when the obtained multiplication value is equal to or greater than a first reference voltage value, the overcurrent protection is performed. The switching power supply device according to any one of claims 1 to 3, wherein the switching power supply device operates.   The overcurrent protection unit generates a second reference voltage value based on the voltage value of the DC power supply, and the overcurrent protection unit detects an overcurrent when the load current value extracted by the load current extraction unit is greater than or equal to the second reference voltage value. The switching power supply device according to any one of claims 1 to 3, wherein a current protection operation is performed.

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