JP2008131721A - Switching power supply - Google Patents

Abstract

A low-cost switching power supply device is provided that has a high output voltage stability and a high switching frequency even when a low-cost comparator with a relatively large delay time is used in a control circuit.
A primary winding N1 of a transformer 18 and a switching element Q1 are connected in series to a DC input power supply 10, and a switching element drive circuit 16 is connected to the switching element Q1. The voltage signal of the DC input power supply 10 is input to the integrating circuit 32, and the output voltage of the integrating circuit 32 and the output voltage control signal from the feedback control circuit 24 are input to the switching element driving circuit 16 for comparison, and the switching element driving circuit 16 is compared. The output of the rectifying / smoothing circuit 23 is controlled to a predetermined value by controlling the switching element drive pulse width. The output voltage of the integration circuit 32 is a value obtained by superimposing the voltage of the input proportional voltage generation circuit 30 that generates a voltage proportional to the voltage signal of the DC input power supply 10 on the voltage obtained by integrating the voltage signal of the input power supply 10.
[Selection] Figure 1

Description

  The present invention relates to an input voltage feedforward control type switching power supply device for converting a DC voltage into a desired voltage and supplying it to an electronic device.

  Conventionally, as a method of improving the output voltage stability of a switching power supply device when the input power supply voltage of the switching power supply device changes rapidly, the pulse width of the switching element is adjusted with respect to the input power supply voltage of the switching power supply device. An input voltage feedforward control which is a control method is known. For example, the step-down chopper circuit disclosed in Patent Document 1 discloses an example in which input voltage feedforward control is applied.

  Here, FIG. 5 shows an example of a switching power supply device in which the conventional input voltage feedforward control is applied to an isolated forward converter. FIG. 6 shows operation waveforms of the switching power supply device of FIG. 5, and shows two waveforms when the input voltage is low (A) and when the input voltage is high (B).

  In this switching power supply device, an integrating circuit 12 comprising a series circuit of a resistor 11 and a capacitor 13 is provided in parallel with the output terminal of the DC input power supply 10, and one terminal of the resistor 11 is connected to the + side terminal of the DC power supply 10. One terminal of the capacitor 13 is connected to the negative terminal of the DC input power supply 10. The middle point of the resistor 11 and the capacitor 13 is connected to the reset circuit 14 and to the inverting input terminal of the comparator 17 of the switching element drive circuit 16. The other terminal of the reset circuit 14 is connected to one terminal of the capacitor 13 and the negative terminal of the DC input power supply 10.

  Further, the + side terminal of the DC input power source 10 is connected to the terminal on the side of the primary winding N1 of the transformer 18 where the dot is attached, and the one without the dot is connected to the drain of the switching element Q1 which is a MOS-FET. ing. The source of the switching element Q1 is connected to the negative terminal of the DC input power supply 10, and the gate is connected to the output of the comparator 17 of the switching element drive circuit 16.

  The secondary winding N2 of the transformer 18 has a dot-attached terminal connected to the output terminal 20 of the power supply device, and a terminal without a dot connected to the drain of the forward-side synchronous rectifier element Tr1 that is a MOS-FET. It is connected. Further, the source of the forward side synchronous rectifier element Tr1 is connected to the source of the flywheel side synchronous rectifier element Tr2, and is connected to the output terminal 21 of the power supply device via the choke coil L1. The drain of the flywheel side synchronous rectification element Tr2 which is a MOS-FET is connected to the terminal of the secondary winding N2 of the transformer 18 which is marked with a dot. An output capacitor C1 is provided between the drain of the flywheel side synchronous rectifier element Tr2 and the output 21 side terminal of the choke coil L1. The outputs of the synchronous rectifier driving circuit 22 are connected to the gates of the forward-side synchronous rectifier element Tr1 and the flywheel-side synchronous rectifier element Tr2, respectively, and are configured to be turned on complementarily. A circuit connected to the secondary side of the transformer 18 constitutes a rectifying / smoothing circuit 23.

  Furthermore, the output terminal 20 is connected to the inverting input terminal of the error amplifier 25 of the feedback control circuit 24 in order to control the output voltage Vo of the switching power supply device. The reference voltage 26 is connected to the non-inverting input terminal of the error amplifier 25, and the output of the error amplifier 25 is connected to the non-inverting input terminal of the comparator 17.

Next, the operation of the conventional switching power supply device will be described. First, the reset circuit 14 resets the capacitor 13 in the integration circuit 12 at a predetermined period T (timing in FIG. 6A). A predetermined period T of the reset circuit 14 is a switching frequency of the switching power supply device. When the capacitor 13 in the integrating circuit 12 is reset, the switching element driving circuit 16 turns on the switching element Q1 (timing in FIG. 6A).
Thereafter, the capacitor 13 in the integrating circuit 12 is charged from the DC input power supply 10 through the resistor 11 in the integrating circuit 12. The charging voltage of the capacitor 13 in the integration circuit 12 is output to the switching element control circuit 16 as the integration circuit output voltage Vi.

  The output voltage Vi of the integrating circuit 12 and the output voltage control signal voltage Vs of the error amplifier 25 of the feedback control circuit 24 are compared by the comparator 17 in the switching element driving circuit 16, and the charging voltage of the capacitor 13 is the output voltage control signal voltage. When it becomes equal to Vs, the switching element Q1 is turned off.

  Here, when the voltage Vin of the DC input power supply 10 is low, the rising speed of the charging voltage Vc of the capacitor 13 in the integrating circuit 12 is slow, so the timing at which the switching element Q1 is turned off is a relatively late timing within the period T. (Timing (b) in FIG. 6A). On the other hand, when the voltage of the DC input power supply 10 is high, the rising speed of the voltage Vc of the capacitor 13 in the integrating circuit 12 is fast, so the timing at which the switching element Q1 is turned off is relatively fast (as shown in FIG. 6B). (Timing (c)).

  By repeating the above operation, a pulsed voltage is applied to the primary side of the transformer 18 and transmitted to the secondary side. The pulsed voltage generated on the secondary side of the transformer 18 is converted into direct current by the rectifying and smoothing circuit 23 and becomes the output voltage Vo of the switching power supply device.

  The switching element driving circuit 16 controls the output voltage Vo of the switching power supply device by controlling the voltage pulse width Ton by the output voltage Vi of the integrating circuit 12 and the output voltage control signal voltage Vs. As for the output voltage control signal voltage Vs, the reference voltage Vref of the reference voltage 26 in the feedback control circuit 24 and the output voltage Vo of the switching power supply device are input to the error amplifier 25 in the feedback control circuit 24, and the switching power supply device It is determined by the error amplifier 25 in the feedback control circuit 24 so that the output voltage Vo becomes equal to the voltage Vref of the reference voltage 26 in the feedback control circuit 24.

  Next, how the output voltage of the switching power supply device is controlled when the switching power supply device of FIG. 5 operates ideally will be described.

  The output voltage of the switching power supply device of FIG. 5 is determined as shown in Equation (1) when an ideal operation is performed. From equation (1), when the voltage Vin of the DC input power supply 10 is low, the ON time Ton of the switching element Q1 is lengthened, and when the voltage of the DC input power supply 10 is high, the ON time Ton of the switching element Q1 is shortened. By performing the control, the output voltage Vo of the switching power supply device is controlled to be constant.

  Here, Vo: output voltage of the switching power supply device Vin: input power supply voltage, N1: number of turns on the primary side of the transformer 18, N2: number of turns on the secondary side of the transformer, T: switching period, Ton: on time of the switching element .

  In the switching power supply device of FIG. 5, when the capacitor 13 of the integrating circuit 12 is reset, the switching element Q1 is turned on. After the reset, the charging current Ic flows from the DC input power supply 10 through the resistor 11 in the integrating circuit 12, and the voltage of the capacitor 13 increases. The charging current Ic can be expressed by Expression (2) when the charging voltage Vc of the capacitor 13 is sufficiently smaller than the input power supply voltage Vin. In addition, the charging voltage Vc of the capacitor 13 is expressed by Expression (3). From equation (3), it can be seen that the charging voltage Vc of the capacitor 13 is proportional to the voltage Vin of the DC input power supply 10 and the charging time t.

  Here, Ic: charging current of the capacitor 13 of the integrating circuit 12, Vc: voltage of the capacitor 13 of the integrating circuit 12, Ci: capacitance of the capacitor 13 of the integrating circuit, Ri: resistance value of the resistor 11 of the integrating circuit 12, t: This is the charging time of the capacitor 13 of the integrating circuit 12.

  In the switching power supply device of FIG. 5, the switching element Q1 is turned off when the voltage Vc of the capacitor 13 becomes equal to the voltage of the output voltage control signal voltage Vs. That is, the following relationship is established in the switching power supply device of FIG.

Charging time t of capacitor 13 = on time Ton of switching element Q1 (4)
Voltage Vc of capacitor 13 of integrating circuit 12 = output voltage control signal voltage Vs (5)
From the above, the following relationship is obtained between the output voltage Vo of the switching power supply apparatus and the output voltage control signal voltage Vs.

  From the equation (6), the switching power supply device of FIG. 5 that operates ideally performs the control so that the output voltage control signal voltage Vs becomes a constant value, so that the output voltage Vo is changed even if the input voltage varies. It can be seen that it can be constant.

  Here, a problem when the switching power supply device of FIG. 5 is configured with actual parts will be described with reference to FIG. The comparator 17 in the switching element drive circuit 16 ideally has a delay time of zero, but in the case of an actual component, it has a delay time Td for comparison judgment. If the comparator 17 in the switching element driving circuit 16 has a delay time Td, the timing until the switching element Q1 is actually turned off after the voltage Vc of the capacitor 13 in the integrating circuit 12 becomes equal to the output voltage control signal voltage Vs. Will be delayed.

  Since the delay time Td of the comparator 17 in the switching element driving circuit 16 delays the timing until the switching element Q1 is turned off, the on-time Ton of the switching element Q1 becomes longer. As a result, the output voltage of the switching power supply device is increased. Vo will be raised. Therefore, the feedback control circuit 24 performs control to reduce the output voltage control signal voltage Vs so that the output voltage Vo of the switching power supply device becomes equal to the reference voltage Vref.

  That is, the switching power supply device of FIG. 5 shortens the on-time Ton of the switching element Q1 when the input power supply voltage Vin is high, and lengthens the on-time Ton of the switching element Q1 when the input power supply voltage Vin is low. Control is performed so that the output voltage Vo of the switching power supply device is constant.

Here, the ON time Ton of the switching element Q1 is the time until the output voltage Vi of the integration circuit 12 (the voltage Vc of the capacitor 13 in the integration circuit 12) reaches the output voltage control signal voltage Vs and the delay time Td of the comparator 17. Is the sum of Since the delay time Td of the comparator 17 is constant regardless of the input power supply voltage Vin, the on-time of the switching element Q1 increases as the input voltage increases (when the on-time Ton of the switching element is controlled to be shorter). The ratio of the delay time Td of the comparator 17 to Ton increases. Therefore, the feedback control circuit 24 controls the output voltage control signal voltage Vs to decrease by δV when the input voltage Vin is high, compared to when the input voltage Vin is low. Value.
JP-A-3-183357

  However, in the switching power supply device of FIG. 5 using the comparator 17 having a delay time in the feedforward control, when the input power supply voltage Vin of the switching power supply device fluctuates rapidly, that is, more rapidly than the response speed of the feedback control circuit 24. In such a case, the time until the feedback control circuit 24 changes the output voltage control signal voltage Vs, the output voltage Vo of the switching power supply device is affected by the change of the input power supply voltage Vin, and the predetermined value is Don't be.

  That is, in a switching power supply apparatus that feedforward-controls an output voltage with an input voltage, the switching power supply apparatus when the input power supply voltage changes rapidly unless the delay time Td of the comparator 17 is sufficiently small with respect to the switching period T. There was a problem that the output voltage stability deteriorated.

  On the other hand, in order to reduce the size and performance of the switching power supply device, it is necessary to increase the switching frequency. When the switching power supply device of the input voltage feedforward control is increased in frequency, a control circuit with a small comparator delay time Td is used. This has led to an increase in cost of the control circuit.

  The present invention has been made in view of the above-described conventional technology. Even when a low-cost comparator with a relatively large delay time is used in the control circuit, the output voltage stability of the switching power supply device can be changed when the input power supply voltage changes suddenly. An object of the present invention is to provide a switching power supply device that can increase the switching frequency and increase the switching frequency and can realize high performance of the power supply device at low cost.

  According to the present invention, a transformer primary winding and a switching element are connected in series to a DC input power source, a switching element driving circuit is connected to the switching element, and a rectifying / smoothing circuit connected to the secondary winding of the transformer. A feedback control circuit that feeds back an output voltage to the switching element drive circuit; a voltage signal of the input power supply is input to an integration circuit; and an output voltage of the integration circuit and an output voltage control signal from the feedback control circuit are A switching power supply apparatus that performs feed-forward control for controlling the output of the rectifying and smoothing circuit to a predetermined value by controlling the switching element drive pulse width of the switching element drive circuit by inputting to the switching element drive circuit for comparison. The output voltage of the integration circuit is obtained by integrating the voltage signal of the input power supply. The pressure, a switching power supply a value obtained by superimposing the voltage of the input proportional voltage generating circuit for generating a voltage proportional to the voltage signal of the DC input power source.

  The input proportional voltage generation circuit is a resistance element connected in series to a capacitor in the integration circuit.

  In the switching power supply apparatus, a tertiary winding is provided in the transformer, and an input to the integration circuit is replaced with a voltage signal of the DC input power supply, and an output of the tertiary winding is input to the integration circuit.

  Further, the voltage generated by the input proportional voltage generation circuit is approximately equal to the voltage increase of the integration circuit due to the response delay time of the switching element drive circuit.

  The present invention relates to a switching power supply that performs feedforward control of an output voltage using an input voltage, canceling a change in an output voltage control signal accompanying a change in the input power supply voltage, and even when the input power supply voltage fluctuates rapidly. It is possible to improve the output voltage stability. Accordingly, it is possible to increase the switching frequency by using an inexpensive comparator having a relatively long delay time, and it is possible to provide a low-cost and small high-performance switching power supply device.

  Furthermore, according to the second aspect of the present invention, the above effect can be obtained with a simple circuit configuration.

  According to the third aspect of the present invention, high-efficiency control and high-precision control are possible regardless of the value of the input voltage.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 shows a switching power supply device according to a first embodiment of the present invention. The same members as those in the circuit shown in FIG. In this switching power supply device, an integrating circuit 32 including a series circuit of a resistor 11 and a capacitor 13 is provided in parallel with the output terminal of the DC input power supply 10, and one terminal of the resistor 11 is connected to the + side terminal of the DC power supply 10. One terminal of the capacitor 13 is connected to the negative terminal of the DC input power supply 10. Connected between the resistor 11 and the capacitor 13 is an input proportional voltage generation circuit 30 that generates a voltage proportional to the DC input power supply 10 to the voltage generated by the capacitor 13. A voltage obtained by superimposing the voltage from the input proportional voltage generation circuit 30 on the voltage of the capacitor 13 is input to the switching element drive circuit 16 as the output voltage of the integration circuit 32. The capacitor 13 is connected to the reset circuit 14, and the other terminal of the reset circuit 14 is connected to one terminal of the capacitor 13 and the negative terminal of the DC input power supply 10.

  The switching element driving circuit 16 includes a comparator 17, and the output voltage of the integrating circuit 32 on which the voltage of the input proportional voltage generating circuit 30 is superimposed is connected to the inverting input terminal of the comparator 17. The output of an error amplifier 25 described later is connected to the non-inverting input terminal of the comparator 17.

  Further, the + side terminal of the DC input power source 10 is connected to the terminal on the side of the primary winding N1 of the transformer 18 where the dot is attached, and the one without the dot is connected to the drain of the switching element Q1 which is a MOS-FET. ing. The source of the switching element Q1 is connected to the negative terminal of the DC input power supply 10, and the gate is connected to the output of the comparator 17 of the switching element drive circuit 16.

  The secondary winding N2 of the transformer 18 has a dot-attached terminal connected to the output terminal 20 and a dot-free terminal connected to the drain of the forward-side synchronous rectifier element Tr1 that is a MOS-FET. Yes. Further, the source of the forward side synchronous rectifier element Tr1 is connected to the source of the flywheel side synchronous rectifier element Tr2 which is a MOS-FET, and is connected to the output terminal 21 via the choke coil L1. The drain of the flywheel side synchronous rectifier element Tr2 is connected to the terminal of the secondary winding N2 of the transformer 18 with a dot. An output capacitor C1 is provided between the drain of the flywheel side synchronous rectifier element Tr2 and the output 21 side terminal of the choke coil L1. The outputs of the synchronous rectifier driving circuit 22 are connected to the gates of the forward-side synchronous rectifier element Tr1 and the flywheel-side synchronous rectifier element Tr2, respectively, and are configured to be turned on complementarily. A circuit connected to the secondary side of the transformer 18 constitutes a rectifying / smoothing circuit 23.

  Furthermore, the output terminal 20 is connected to the inverting input terminal of the error amplifier 25 of the feedback control circuit 24 in order to control the output voltage Vo of the switching power supply device. The reference voltage 26 is connected to the non-inverting input terminal of the error amplifier 25, and the output of the error amplifier 25 is connected to the non-inverting input terminal of the comparator 17.

  FIG. 2 shows an example of the input proportional voltage generation circuit 30 of the switching power supply device of FIG. Here, a resistance element 34 is used as a circuit that generates a voltage proportional to the DC input power supply 10. The resistance element 34 is set to have a sufficiently small value with respect to the resistance 11 in the integration circuit 32. At this time, the current flowing through the resistance element 34 is Ic in the above equation (2). Therefore, the voltage of the circuit that generates a voltage proportional to the DC input power supply 10, that is, the correction voltage Va generated in the resistance element 34 is obtained by the following equation.

Va: Correction voltage of a circuit that generates a voltage proportional to the input power supply
Ra: Value of a resistance element constituting a circuit that generates a voltage proportional to the input power supply Vin: DC input power supply voltage Ic: Charging current of the capacitor of the integration circuit Ri: Value of the resistance of the integration circuit Ri (7) Since Ra is a constant, it can be seen that the correction voltage Va of the circuit that generates a voltage proportional to the DC input power supply 10 is proportional to the input power supply voltage Vin.

  Next, the operation of the switching power supply device of FIG. 2 will be described based on FIG. The output voltage Vi of the integrating circuit 32 in the switching power supply device of FIG. 1 is superimposed on the voltage Vc of the capacitor 13 in the integrating circuit by a correction voltage Va of a circuit that generates a voltage proportional to the input voltage Vin of the DC input power supply 10. It is the value. The correction voltage Va of the circuit that generates a voltage proportional to the DC input power supply 10 is small when the DC input power supply voltage Vin is low, and is large when the DC input power supply voltage Vin is high.

  When the comparator 17 of the switching element drive circuit 16 has a delay time, the switching power supply device of FIG. 5 sets the output voltage control signal voltage Vs high when the input power supply voltage Vin is low, as shown in FIG. Thus, when the input power supply voltage Vin is high, the output voltage control signal voltage Vs is set to be low. However, in the switching power supply device shown in FIG. 1, even if the power supply device includes such a comparator 17, the correction voltage Va is added to the integration circuit output voltage Vi to be corrected, thereby changing the input power supply voltage Vin. On the other hand, by adding the correction voltage Va proportional to the fluctuation of the input power supply voltage Vin, it is possible to control the output voltage control signal voltage Vs so as to be almost equal at all times.

  According to the switching power supply device of this embodiment, the output voltage control signal voltage Vs can be controlled to be constant only by adding a simple circuit such as the resistance element 34 to the integrating circuit 32. This can be performed accurately with an inexpensive circuit configuration, and the stability of the output voltage of the switching power supply device can be improved when the input power supply voltage in the switching power supply device by feedforward control changes suddenly.

  Next, a second embodiment of the present invention will be described with reference to FIG. Here, the same members as those in the above embodiment are denoted by the same reference numerals, and the description thereof is omitted. The switching power supply device shown in FIG. 4 is different from the above embodiment in that the transformer 18 is provided with the tertiary winding N3 and the output voltage of the tertiary winding N3 is input to the integrating circuit 32. As a result, this switching power supply device can improve the conversion efficiency of the power supply device in a switching power supply device with a high voltage input. Moreover, in the switching power supply device with a low input voltage, the control accuracy of the output voltage of the switching power supply device can be improved.

  That is, when the DC input power supply 10 has a high voltage, when the input voltage Vin is directly input to the integration circuit 32 as in the above embodiment, the high input voltage is applied to the integration circuit 32, and the resistance in the integration circuit 32 is reduced. 11 increases the loss. The loss due to the resistor 11 increases in proportion to the square of the input voltage Vin. Therefore, in the power supply device of this embodiment, the transformer 18 is provided with the tertiary winding N3, and the output voltage of the tertiary winding N3, which is lower than the input voltage Vin, is input to the integrating circuit 32. For this reason, even if the input voltage Vin is high, the loss in the resistor 11 of the integrating circuit 32 can be reduced by setting the output voltage of the tertiary winding N3 low.

  Further, in the low-voltage input switching power supply device, in the case of the circuit configuration of the above embodiment, the input voltage Vin is directly input to the integration circuit 32, so that a low voltage is applied to the integration circuit 32. The amplitude of the voltage Vc of the capacitor 13 in the integrating circuit 32 cannot be set large. Accordingly, the output voltage Vi of the integrating circuit 32 cannot be set large, and the integrating circuit also compares the integrating circuit output voltage Vi with the output voltage control signal voltage Vs output from the feedback control circuit 24 by the comparator 17. Since the output voltage Vi and the output voltage control signal voltage Vs are small, the control accuracy by the comparator 17 is relatively low. In order to increase the control accuracy, a high-precision and expensive comparator 17 must be used.

  On the other hand, in the switching power supply device of this embodiment, since the tertiary winding N3 is provided in the transformer 18 and the output voltage of the tertiary winding N3 is input to the integrating circuit 32, the input voltage Vin is low. By setting the output voltage of the tertiary winding N3 high, the voltage applied to the integration circuit 32 can be increased, and the integration circuit output voltage Vi and the output voltage control signal voltage Vs can be set high. The control accuracy of the output voltage of the switching power supply device can be improved.

  The switching power supply device of the present invention is not limited to the above embodiment, and the configuration of the input proportional voltage generation circuit can set an appropriate circuit in addition to the resistor, and is proportional to the input voltage. Any circuit capable of outputting may be used.

1 is a schematic circuit diagram of a switching power supply device according to a first embodiment of the present invention. It is a circuit diagram which shows the example of the input proportional voltage generation circuit of the switching power supply device of 1st embodiment of this invention. It is a timing chart which shows operation | movement of the switching power supply device of 1st embodiment of this invention. It is a schematic circuit diagram of the switching power supply device of 2nd embodiment of this invention. It is a schematic circuit diagram which shows the switching power supply device of the conventional feedforward control. It is a timing chart which shows the case where the conventional switching power supply device is ideally controlled. It is a timing chart which shows the actual control of the conventional switching power supply device.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 DC input power supply 11 Resistance 12,32 Integration circuit 13 Capacitor 14 Reset circuit 16 Switching element drive circuit 17 Comparator 18 Transformer 20, 21 Output terminal 22 Synchronous rectification element drive circuit 23 Rectification smoothing circuit 24 Feedback control circuit 25 Error amplifier 26 Reference voltage 30 Input proportional voltage generation circuit Q1 Switching element TR1 Forward side synchronous rectifier TR2 Flywheel side synchronous rectifier

Claims (4)

A transformer primary winding and a switching element are connected in series to a DC input power source, a switching element driving circuit is connected to the switching element, and an output voltage of a rectifying and smoothing circuit connected to the secondary winding of the transformer is A feedback control circuit that feeds back to the switching element driving circuit, a voltage signal of the input power supply is input to the integrating circuit, and an output voltage of the integrating circuit and an output voltage control signal from the feedback control circuit are input to the switching element driving circuit; In the switching power supply device that performs feedforward control for controlling the switching element driving pulse width of the switching element driving circuit to control the output of the rectifying and smoothing circuit to a predetermined value,
The output voltage of the integration circuit is a value obtained by superimposing the voltage of the input proportional voltage generation circuit that generates a voltage proportional to the voltage signal of the DC input power supply on the voltage obtained by integrating the voltage signal of the input power supply. A switching power supply device.   2. The switching power supply device according to claim 1, wherein the input proportional voltage generation circuit is a resistance element connected in series to a capacitor in the integration circuit.   2. A tertiary winding is provided in the transformer, and the output of the tertiary winding is input to the integrating circuit instead of the voltage signal of the DC input power source being input to the integrating circuit. Or the switching power supply device of 2. 4. The switching power supply device according to claim 1, wherein the voltage generated by the input proportional voltage generation circuit is substantially equal to a voltage increase of the integration circuit due to a response delay time of the switching element driving circuit.

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