JP2011145875A - Controller and control method for switching power supply - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a controller for a switching power supply, controlling a ripple amount of a load current in addition to a peak value of the load current of the switching power supply constant. <P>SOLUTION: The controller 420 includes: a current detection circuit I_Det detecting the load current of a switching transistor SW; a first comparator Comp1 receiving a signal outputted from the current detection circuit I_Det to detect that the load current reaches a target peak value; a second comparator Comp2 for detecting whether or not the ripple amount of the load current is large; an OFF time control circuit controlling an OFF time of the switching transistor SW by whether or not the ripple amount of the load current is large; and a flip-flop circuit changing over ON/OFF of the switching transistor SW by output signals from the first comparator Comp1 and the OFF time control circuit. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a control device and a control method for a switching power supply, and more particularly to a control device and a control method for a switching power supply including a switching transistor for controlling a current supplied to a load such as an LED.

  FIG. 1 shows an example of a conventional switching power supply device. The switching power supply device 100 uses a comparator to turn off the switching transistor when the current flowing through the switching transistor exceeds a set value, and turn on the switching transistor again after a predetermined time has elapsed. By repeating this operation, the current flowing through the LED as a load is controlled.

  Specifically, the switching power supply device 100 includes a switching power supply 110 that is a power conversion unit and a control device 120 that is a control unit. The switching power supply 110 includes an input capacitor C, a coil L, a switching transistor SW, and a diode D. In FIG. 1, an LED is connected between a coil L and a diode D as a load. The control unit 120 includes an LED load current detection circuit I_Det (for example, a resistor), a comparator Comp, a flip-flop circuit F, and an oscillator OSC. Here, a direct current or an alternating current power source as a power supply source provided outside is connected to the node Vin, and the LED receives a stable current supply from the switching power supply device 100.

  In the switching power supply 110 in which the LED is connected as a load between the coil L and the diode D as shown in FIG. 1, when the switching transistor SW is turned on, current flows from the node Vin to the LED, the coil L, and the switching transistor SW. Flows (see solid line). Since there is the coil L, the current value increases with time. Thereafter, when the switching transistor SW is turned off, the current flowing in the coil L changes the path from the switching transistor SW to the diode D (see dotted line). At this time, since the direction of the voltage applied to the coil L is reversed, the current decreases with time. In the circuit shown in FIG. 1, when the switching transistor SW is turned on, the amount of increase in the current flowing through the coil L is expressed by the following formula (1), where the current flowing from the output terminal Vout2 to the switching transistor SW is plus, and ΔIL is Since it becomes positive, the current value increases.

On the other hand, when the switching transistor SW is turned off, the increase amount of the current flowing through the coil L is expressed by the following formula (2), and ΔIL becomes negative, so that the current value decreases.

  In this circuit system, a method of controlling the peak of the load current flowing through the LED load to a constant value is common. At this time, the load current flowing through the switching transistor SW is detected to determine the end of the ON operation. On the other hand, in order to detect the load current when the switching transistor SW is off in order to determine the end time of the off operation, an element for current detection is added to the path of the LED, the coil L, and the diode D. There is a need. If the voltage is Vin = AC100V, in order to detect the current at these positions, the circuit becomes complicated and high breakdown voltage components are required, which is disadvantageous in terms of cost. Therefore, the load current is detected when the switching transistor SW is turned on, and current control is performed such that the switching transistor SW is turned off when the current value exceeds a certain value, and the switching transistor SW is turned off at a constant time or operating frequency is constant ( In general, the operation is performed in the off time that does not depend on the current value, such as the sum of the on time and the off time is constant.

  In the switching power supply device 100 using the conventional circuit system shown in FIG. 1, only the peak value of the load current is controlled. Therefore, when the load resistance value and the input voltage fluctuate, the amount of ripple in the current changes, and the load current There was a disadvantage that the average value of fluctuated. Here, the current ripple amount means the difference (Peak to Bottom) between the current value at the decrease start point and the current value at the increase start point, which are adjacent to each other in the current that repeatedly increases and decreases at the same or different periods. FIG. 2 shows the output voltage waveform of the current detection circuit I_Det of FIG. 1 and the current waveform of the coil L together with the on / off operation of the switching transistor. VRef represents a reference voltage corresponding to the peak value of the load current. Although the coil current increases or decreases in response to the ON / OFF of the switching transistor SW, the current flows through the current detection circuit I_Det only during the time when the switching transistor is ON, so that the output voltage waveform is as shown in FIG. Here, when the number of connected LEDs is changed, the voltage applied to the coil L varies, and the slope of the coil current waveform changes according to the above equations (1) and (2). For this reason, in conventional current control with a fixed off time or operating frequency, if the number of LED lamps changes, the slope of the coil current waveform changes, and the load resistance value also changes. As a result, the average current value fluctuated. This means that the brightness per LED varies depending on the number of lamps, and the number of lamps and the brightness are not proportional. A similar phenomenon occurs when the input voltage supplied to the node Vin varies.

  The present invention has been made in view of such problems, and an object of the present invention is to add a complex current detection element for detecting a current when the switching transistor is off or a high withstand voltage current detection element. The object is to provide a control device and a control method for a switching power supply that can control the ripple amount of the load current in addition to the peak value of the load current of the switching power supply.

  In order to achieve such an object, a first aspect of the present invention includes an input terminal connected to an external power supply source, first and second output terminals connected to a load, A switching power supply control device comprising a switching transistor for controlling a load current supplied between the first output terminal and the second output terminal, wherein the control device is configured to turn on the switching transistor. A control device that detects an on time and an off time of the switching transistor by detecting the load current, and turns off the switching transistor when the load current exceeds a first reference current, and (1) the switching The load current when the transistor is on is greater than a second reference current smaller than the first reference current, and the first reference current If the following time is the same as the ON time of the switching transistor, the OFF time is extended from a predetermined OFF reference time, and (2) the load current exceeds the second reference current, In addition, if the time that is equal to or less than the first reference current is less than the ON time of the switching transistor, the OFF time is shortened from the OFF reference time.

  According to a second aspect of the present invention, in the first aspect, the current detection circuit that detects the load current when the switching transistor is on and outputs a voltage corresponding to the detected load current; The voltage output from the current detection circuit is compared with the first reference voltage corresponding to the first reference current. When the voltage exceeds the first reference voltage, the output signal is at the H level, and the voltage is A first comparator whose output signal is at L level when it is equal to or lower than the first reference voltage, and a voltage output from the current detection circuit are compared with a second reference voltage corresponding to the second reference current. When the voltage exceeds the second reference voltage, the output signal is at an H level, and when the voltage is equal to or lower than the second reference voltage, the second comparator has the output signal at an L level; First compare A flip-flop circuit that is reset when the output signal of the transistor becomes H level and turns off the switching transistor, and an off time that sets the flip-flop circuit after the off time has elapsed since the flip-flop circuit turned off the switching transistor A control circuit, and the off-time control circuit determines the off-time from the off-reference time when the time when the output signal of the second comparator is at the H level is less than the on-time of the switching transistor. It is characterized by a shortened value.

  According to a third aspect of the present invention, in the second aspect, the off-time control circuit includes: a third comparator having an output terminal connected to a set terminal of the flip-flop circuit; and the third comparator. A first capacitor connected between the inverting input terminal and the ground; and a second capacitor connected between the non-inverting input terminal of the third comparator and the ground, wherein the first capacitor is the switching capacitor. When the transistor is turned on, it is charged with the output signal of the flip-flop circuit, and the off time is when the time when the output signal of the second comparator is at the H level is less than the on time of the switching transistor. During the period when the switching transistor is on and the output signal of the second comparator is at L level. Ri, characterized in that it is shortened by charging the second capacitor.

  According to a fourth aspect of the present invention, there is provided a switching power supply comprising the control device according to any one of the first to third aspects and the switching power supply.

  According to a fifth aspect of the present invention, there is provided an input terminal connected to an external power supply source, first and second output terminals connected to a load, the first output terminal, and the A switching power supply comprising a switching transistor for controlling a load current supplied between the second output terminal and (a) detecting the load current when the switching transistor is turned on; b) turning off the switching transistor when the load current exceeds a first reference current; (c) the load current is greater than a second reference current less than the first reference current; and If the time that is equal to or less than the first reference current is the same as the on-time of the switching transistor, the off-time of the switching transistor is set to a predetermined off-reference. If the time during which the load current is greater than the second reference current and less than or equal to the first reference current is less than the on time, the off time is set to the off reference time. And (d) turning on the switching transistor after elapse of the off time from when the switching transistor is turned off.

  According to a sixth aspect of the present invention, in the fifth aspect, the steps (a) to (d) are repeated until the off-time is not continuously extended or shortened.

  According to the control device and the control method of the switching power supply according to the present invention, the load current is detected when the switching transistor is turned on, and the comparison between the ripple amount and the target value is performed to convert the ripple amount from the target value. If it is larger, the off time is shortened, and if it is less than the target value, the off time is extended, so that the ripple amount of the load current can be controlled to be constant in addition to the peak value of the load current of the switching power supply.

It is a figure which shows an example of the conventional switching power supply device. FIG. 2 is a diagram illustrating an output voltage waveform of a current detection circuit I_Det of FIG. 1 and a current waveform of a coil L together with an on / off operation of a switching transistor. FIG. 2 is a diagram illustrating an output voltage waveform of a current detection circuit I_Det and a current waveform of a coil L in FIG. 1 together with an on / off operation of a switching transistor when the number of LEDs is increased or decreased. It is a figure which shows the control apparatus of the switching power supply based on embodiment of this invention with a switching power supply. It is a figure for demonstrating operation | movement of the control apparatus 420 which concerns on this embodiment. It is a figure for demonstrating operation | movement of the control apparatus 420 which concerns on this embodiment. It is a figure which shows the Example of an off time control circuit. It is a figure which shows the signal of the point shown by AD in FIG.

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

  FIG. 4 shows a switching power supply controller according to an embodiment of the present invention, together with the switching power supply. The switching power supply 110 is the same as the conventional one, and an input terminal Vin connected to a power supply source provided outside, a first output terminal Vout1 and a second output terminal Vout2 connected to a load such as an LED. And a switching transistor SW for controlling a load current supplied between the first output terminal Vout1 and the second output terminal Vout1. The control device 420 receives a current detection circuit I_Det that detects the load current of the switching transistor SW and a signal output from the current detection circuit I_Det, and detects that the load current has reached a target peak value. Comparator Comp1, a second comparator Comp2 for detecting the magnitude of the load current ripple, an off-time control circuit for controlling the off-time of the switching transistor SW based on the magnitude of the load current ripple, and a first comparator Comp1 and a flip-flop circuit that switches on / off the switching transistor SW according to an output signal from the off-time control circuit.

  The operation of the control device 420 according to this embodiment will be described with reference to FIGS.

  FIG. 5 is an operation waveform when the ripple amount of the load current is smaller than the target value. In this case, the output voltage of the current detection circuit I_Det at the moment when the switching transistor SW is turned on becomes higher than the bottom voltage VRef2 (corresponding to “second reference voltage”).

  When the output voltage of the current detection circuit I_Det increases and exceeds the peak voltage VRef1 (corresponding to “first reference voltage”), the output signal of the first comparator Comp1 becomes H level, and the flip-flop circuit F is reset. As a result, the switching transistor SW is turned off.

  Since the output voltage of the current detection circuit I_Det at the moment when the switching transistor SW is turned on becomes higher than the bottom voltage VRef2, Comp2 becomes H level. When the switching transistor SW is turned off, the output voltage of the current detection circuit I_Det becomes the bottom voltage VRef2. Therefore, the output signal of the second comparator Comp2 is L level. That is, the time during which Comp2 is at the H level and the time during which the switching transistor SW is on are the same. In this case, the off-time control circuit extends the off-time of the switching transistor SW by a predetermined length from the off-reference time. To do. The load current decreases in proportion to the extended length, and the ripple amount increases toward the target value (peak voltage VRef1-bottom voltage VRef2).

  Here, the off reference time may be a fixed value or an off time of the previous cycle. When the off reference time is a fixed value, the predetermined length to be extended or shortened is a variable value determined by calculation from the amount of ripple or the like. If the off reference time is the off time of the previous cycle (not limited to the previous cycle), the predetermined length to be extended may be a fixed value or a variable value determined by calculating from the amount of ripple, etc. It may be. In order to prevent the circuit from becoming complicated, it is preferable that the off reference time is the off time of the previous cycle, and the predetermined length to be extended is a fixed value.

  When the flip-flop circuit F is set by the off-time control circuit after the off-time has elapsed and the output voltage of the current detection circuit I_Det at the moment when the switching transistor SW is turned on again is higher than the bottom voltage VRef2, the off-time is further extended. As described above, when the time when the output signal of the second comparator Comp2 is at the H level is the same as the ON time of the switching transistor, the OFF time is gradually increased, so that the ripple amount of the load current is gradually set to the target. It can be close to the value.

  FIG. 6 shows operation waveforms when the ripple amount of the load current is larger than the target value. In this case, the output voltage of the current detection circuit I_Det at the moment when the switching transistor SW is turned on is lower than the bottom voltage VRef2.

  When the output voltage of the current detection circuit I_Det increases and exceeds the peak voltage VRef1, the output signal of the first comparator Comp1 becomes H level, the flip-flop circuit F is reset, and the switching transistor SW is turned off.

  The output signal of the second comparator Comp2 is initially at L level. When the output voltage of the current detection circuit I_Det exceeds the bottom voltage VRef2, it changes to H level and continues until the switching transistor SW is turned off. In this case, the time during which the output signal of the second comparator Comp2 is at the H level is shorter than the ON time of the switching transistor SW. In this case, the OFF time control circuit shortens the OFF time of the switching transistor SW by a predetermined length from the OFF reference time. The load current increases in proportion to the shortened length, and the ripple amount decreases toward the target value (peak voltage VRef1-bottom voltage VRef2).

  When the output voltage of the current detection circuit I_Det at the moment when the switching transistor SW is turned on again after the off time has elapsed is also lower than the bottom voltage VRef2, the off time is further shortened. In this way, when the time when the output signal of the second comparator Comp2 is at the H level is less than the ON time of the switching transistor, the OFF time is gradually shortened, so that the ripple amount of the load current is gradually reduced to the target value. Can be approached.

  In the above description, the load current is converted into a voltage by the current detection circuit, and the voltage is compared with the peak voltage VRef1 and the bottom voltage VRef2. However, if the description is given focusing on the load current itself, it can be expressed as follows. Can do. The control device 420 according to the present embodiment detects the load current when the switching transistor SW is on, and controls the on time and off time of the switching transistor SW. The load current is the peak current ("first reference" Switching transistor SW is turned off. The off time is the same as the on time of the switching transistor when the load current when the switching transistor is on is not less than the bottom current (corresponding to the “second reference current”) smaller than the peak current and not more than the peak current. If so, it is extended from the off reference time. On the other hand, if the load current when the switching transistor is on exceeds the bottom current and is less than the peak current for less than the on time, the time is shortened from the off reference time. By repeating such control, the ripple amount can be brought close to the target value. If the off-time is not continuously extended or shortened, it can be determined that the difference between the actual ripple amount and the target value is minimized.

  Finally, an embodiment of the off-time control circuit will be described with reference to FIGS. Signals at points indicated by A to D in FIG. 7 are shown in FIG. The off-time control circuit 700 includes a third comparator Comp3 whose output terminal is connected to the set terminal of the flip-flop circuit F, and a first capacitor C1 connected between the inverting input terminal of the third comparator Comp3 and the ground. And a second capacitor C2 connected between the non-inverting input terminal of the third comparator Comp3 and the ground. The first capacitor C1 forms a ramp circuit, and is charged by the output signal of the flip-flop circuit F when the switching transistor SW is turned on. When it is off, it is discharged by the constant current source I3. As a result of the discharge, when the inverting input becomes smaller than the non-inverting input given by the second capacitor C2, the output signal of the third comparator Comp3 becomes H level, and the flip-flop circuit F is set.

  The feature of the present embodiment is that the off time is extended or shortened by controlling the charge amount of the second capacitor C2. If the charge amount of the capacitor C2 increases and the non-inverting input increases, the output signal of the third comparator Comp3 becomes H level earlier. That is, the off time is shortened. Therefore, when the time during which the output signal of the second comparator Comp2 is at the H level is less than the ON time and the OFF time needs to be shortened, the switching transistor SW is ON and the output signal of the second comparator Comp2 is Over a period of time at L level, the constant current source I1 is turned on using the AND circuit to temporarily charge the second capacitor C2.

  Since the second capacitor C2 is always discharged by the constant current source I2 arranged in parallel, the off time is gradually extended unless the constant current source I1 is turned on. In order to supplement the discharge by the constant current source I2 and charge the second capacitor C2, it is necessary to make the constant current source I1 larger than the constant current source I2.

400 switching power supply 420 control device 700 off-time control circuit I_Det current detection circuit F flip-flop circuit Comp1 first comparator Comp2 second comparator Comp3 third comparator C1 first capacitor C2 second capacitor I1, I2, I3 Constant current source

Claims (6)

Supplied between an input terminal connected to an external power supply source, first and second output terminals connected to a load, and between the first output terminal and the second output terminal. A switching power supply control device comprising a switching transistor for controlling the load current
The control device is a control device that detects the load current when the switching transistor is on and controls an on time and an off time of the switching transistor,
When the load current exceeds a first reference current, the switching transistor is turned off;
(1) A time when the load current when the switching transistor is turned on is greater than a second reference current smaller than the first reference current and less than or equal to the first reference current. If it is the same as the time, the off time is extended from a predetermined off reference time, and (2) the load current is greater than the second reference current and less than or equal to the first reference current. Is less than the ON time of the switching transistor, the OFF time is shortened from the OFF reference time. A current detection circuit that detects the load current when the switching transistor is on and outputs a voltage corresponding to the detected load current;
The voltage output from the current detection circuit is compared with a first reference voltage corresponding to the first reference current. When the voltage exceeds the first reference voltage, an output signal is at an H level, and the voltage A first comparator in which the output signal is at L level when is less than or equal to the first reference voltage;
The voltage output from the current detection circuit is compared with a second reference voltage corresponding to the second reference current. When the voltage exceeds the second reference voltage, an output signal is at an H level, and the voltage A second comparator in which the output signal is at L level when is less than or equal to the second reference voltage;
A flip-flop circuit that is reset when the output signal of the first comparator becomes H level and turns off the switching transistor;
An off time control circuit that sets the flip flop circuit after the off time has elapsed since the flip flop circuit turned off the switching transistor;
The off-time control circuit has a value obtained by shortening the off-time from the off-reference time when the time when the output signal of the second comparator is at the H level is less than the on-time of the switching transistor. The control device according to claim 1. The off-time control circuit includes:
A third comparator having an output terminal connected to the set terminal of the flip-flop circuit;
A first capacitor connected between the inverting input terminal of the third comparator and ground;
A second capacitor connected between the non-inverting input terminal of the third comparator and the ground;
The first capacitor is charged with an output signal of the flip-flop circuit when the switching transistor is on,
The off-time is when the time when the output signal of the second comparator is at the H level is less than the on-time of the switching transistor, and the output signal of the second comparator is on. 3. The control device according to claim 2, wherein the control device is shortened by charging the second capacitor over a period of time when L is at an L level. A control device according to any one of claims 1 to 3,
A switching power supply comprising the switching power supply. Supplied between an input terminal connected to an external power supply source, first and second output terminals connected to a load, and between the first output terminal and the second output terminal. A switching power supply comprising a switching transistor for controlling a load current
(A) detecting the load current when the switching transistor is on;
(B) turning off the switching transistor when the load current exceeds a first reference current;
(C) If the load current is greater than a second reference current smaller than the first reference current and less than or equal to the first reference current is equal to the on-time of the switching transistor, The switching transistor has an off time extended from a predetermined off reference time, and the load current exceeds the second reference current and is less than or equal to the first reference current less than the on time. If there is, a step of setting the off time to a value shortened from the off reference time;
(D) including a step of turning on the switching transistor after the off time has elapsed since the switching transistor was turned off.   6. The control method according to claim 5, wherein steps (a) to (d) are repeated until the off-time is not continuously extended or shortened.

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