JP2001045751A - Insulated switching power supply control circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an insulated switching power supply control circuit that is small and the output change of which is minimal with respect to load fluctua tion. SOLUTION: This circuit is provided with an output control circuit 32, that comprises a reference triangular-wave voltage generating circuit, to which a timing capacitor CT for generating reference triangular waves is connected and a comparator that compares the reference triangular-wave voltage with a first output voltage information signal VFB fed back from the output of a power supply via a photocoupler PC1 for controlling of an output voltage and generates control pulse signals. Also, a discharge current control circuit is incorporated that has the functions of locking the control pulses to the 'off' state during the fall time of the reference triangular waves and of extending the minimum 'off' time by reducing the discharge current of the timing capacitor CT, when a second output voltage information signal VF, an average voltage of the control pulses, becomes equal to or lower than a given value. Furthermore, a bias circuit 25 is provided for increasing the second output voltage information signal VF to a given value or larger during the period, when at least the first output voltage information signal VFB is outputted.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a forward-type switching power supply control circuit for controlling the output voltage by changing the duty of a pulse output.

[0002]

2. Description of the Related Art As a conventional switching power supply control circuit, for example, there has been a circuit as shown in FIG. In this circuit, a transformer 1 has a primary winding 2 and a secondary winding 3 and a third winding 4. The input voltage VIN of a power supply 30 is connected to the primary winding 2 of the transformer 1. This is transformed and output from the secondary winding 3 and the third winding 4. A rectifying diode 5 and a smoothing coil 6 are connected in series to the secondary winding 3 of the transformer 1, and a rectifying diode 7 and a smoothing capacitor 8 are connected in parallel. The rectified output voltage Vo (current Io) is connected to the load Lo via the output terminal To.

The rectified output voltage Vo is connected to an error amplifier 9 which is a differential amplifier.
Is connected to the reference voltage Vre of the reference voltage source 10.
f is applied. The output of the error amplifier 9 is connected to the cathode side of the light emitting diode LD1 provided in the photocoupler PC1, and to the anode side of the light emitting diode LD1.
The output voltage Vo is applied via the resistor 11.

A phototransistor PT1 is provided in series with a resistor 12 using a stable voltage source Vr as a power supply.
The phototransistor PT1 is provided as a light receiving element of the photocoupler PC1, and operates by receiving light emitted from the light emitting diode LD1. Thereby, the phototransistor P
The collector of T1 includes a first output voltage information signal V based on a feedback voltage related to the rectified output voltage Vo of the transformer 1.
FB occurs. The first output voltage information signal VFB is an output of the error amplifier 9, that is, the output voltage Vo and the reference voltage VFB.
It changes according to the difference of ref.

Next, the output control circuit 32 shown in FIG. 6 will be described with reference to FIG. The output control circuit 32 is connected to the voltage source Vr and emits a pulsed output signal OUT. In FIG. 7, the voltage source Vr and the timing resistor RT are connected in series, and a predetermined voltage source Vr and the resistor RT are connected. An arbitrary charge / discharge setting reference current IR is set by utilizing this. The voltage source Vr is connected to a charging current source IS1 for supplying a current proportional to the charge / discharge setting reference current IR. Further, the voltage source Vr is a current proportional to the charge / discharge setting reference current IR, or a voltage source Vr similar to the charge / discharge setting reference current IR.
And a discharge current source IS2 for flowing a current proportional to a current set by an arbitrary constant voltage source and an arbitrary resistor.

A current source for charging IS1 and a current source for discharging IS2
The capacitor CT is a timing capacitor for generating a triangular wave that is charged by the charging current source IS1 and discharged by the discharging current source IS2. At both ends of the capacitor CT, a reference triangular wave voltage Vosc is generated. Switching switches SW1 and SW2 for switching between charging and discharging are provided between the charging current source IS1 and the capacitor CT and between the discharging current source IS2 and the capacitor CT, respectively.
The charge / discharge of T is switched.

Charge / discharge changeover switches SW1, SW2
Is connected to the capacitor CT to detect the upper limit detection comparator C
OMH and a lower limit detection comparator COML are connected. Further, the upper limit setting voltage VH and the lower limit setting voltage VL are input to the upper limit detection comparator COMH and the lower limit detection comparator COML, respectively. By the upper limit detection comparator COMH and the lower limit detection comparator COML,
The upper and lower limits of the waveform of the reference triangular wave voltage Vosc generated in the capacitance CT are set. Upper limit detection comparator COM
H, each output of the lower limit detection comparator COML is a capacitance C
An output is inverted by a voltage generated at T, and an RS flip-flop R for generating charge / discharge switching timing is provided.
It is input to each of the R and S terminals of the SFF.

The output Q of the flip-flop RSFF is connected to a changeover switch SW1 and to a switch SW2 via a logical NOT circuit NOT, and the changeover switches SW1 and SW2 are turned on and off alternately.

Further, the reference triangular wave voltage Vosc is connected to the inverting input terminal of the comparator COMP, and is compared with the first output voltage information signal VFB input to the non-inverting input terminal of the comparator COMP. And the comparator C
The output of the OMP and the output Q of the flip-flop RSFF are ANDed through an AND circuit AND, whereby a control pulse signal having a duty ratio based on the comparison result is output.
Output from terminal.

The output control circuit 32 has a VF (to be described later).
A discharge current control circuit 36 is provided which gradually reduces the discharge current source IS2 when the terminal voltage falls below a predetermined threshold voltage Vth. Further, a latch circuit 38 that is reset every period of the reference triangular wave voltage Vosc is provided.

In the circuit shown in FIG.
The third winding 4 has an input voltage VI generated in the primary winding 2.
A pulse voltage is generated by transforming N by the turns ratio, and is rectified and smoothed by the rectifying diode 13 and the smoothing capacitor 14 connected to the winding 4. Vcc. The switching element 16 is connected to the primary winding 2 and the switching element 1
The OUT terminal of the output control circuit 32 is connected to the gate of No. 6, and the switch element 16 is turned on and off by the control pulse voltage of the OUT terminal, and the transformer 1 is driven.

Further, an average value circuit 15 composed of a resistor 17 and a capacitor 18 for outputting an average control pulse voltage is connected to the OUT terminal of the output control circuit 32. The output of the average value circuit 15 is V
An output of the average voltage of the control pulse voltage by the average value circuit 15 is connected to the F terminal and is input to the VF terminal as a second output voltage information signal VF.

Further, a resistor 19 is connected in series with the switch element 16 to convert a current flowing through the switch element 16 into a voltage. This resistor 19 is a current detection resistor,
9 is applied to a latch circuit 38 reset every cycle of Vosc, and when the detected voltage of the resistor 19 exceeds a specified value, the latch circuit 38 forcibly turns off the switch element 16 so that a pulse-by-pulse It constitutes a current protection circuit.

In the conventional switching power supply control circuit having the above configuration, the output current is reduced by the operation delay time of the output latch circuit 38 even when the output voltage Vo is reduced by the overcurrent protection operation. It may increase. In the conventional example of FIG.
When the second output voltage information signal VF becomes equal to or less than the specified value Vth, it has a function of reducing IS2 to extend the minimum off-time of the control pulse, thereby suppressing an increase in output current.

[0015]

However, in such a conventional switching power supply control circuit, when the output load current Io changes suddenly, the transient fluctuation of the output voltage Vo increases. was there.

Here, first, as shown by the static second output voltage information signal VF and the load current Io characteristic in the conventional switching power supply control circuit shown by the broken line in FIG. In a load current region (a region where the current of the coil 6 becomes continuous) in which the overcurrent protection operation is performed beyond the rating, the second output voltage information signal VF indicates the output voltage Vo.
The voltage becomes almost proportional to. Therefore, the overcurrent protection function of the discharge current control circuit 36 operates to suppress an increase in the current Io.

On the other hand, in a region where the load current Io is small and the current of the coil 6 is discontinuous, compared to a region where the current of the coil 6 is continuous, the duty control for stabilizing the output voltage Vo is performed by the coil 6. Is in a control state different from the region where the current is continuous, the output voltage Vo and the voltage of the second output voltage information signal VF are not proportional, and the second output voltage information signal VF changes in accordance with the change in the load current Io. It fluctuates lower. This is because the off-time of the control pulse becomes longer by the period during which no current flows through the coil 6, and the Du of the control pulse becomes longer.
This is because ty becomes relatively small. Accordingly, the VF voltage becomes equal to the threshold Vt at which the discharge current control circuit 36 starts operating.
When it is less than or equal to h, the minimum off-time of the control pulse becomes longer and the maximum on-duty becomes smaller. In particular, FIG.
As shown in (2), at the time of no load with the minimum load current Io, the VF voltage becomes the lowest even though the output voltage Vo is maintained at the predetermined voltage, and the second output voltage information signal VF is discharged. Operation start threshold value Vt of current control circuit 36
Below h, the minimum off-time of the control pulse voltage is the longest and the maximum on-duty is the smallest.

In the conventional switching power supply control circuit shown in FIG. 6, when the output load current Io suddenly changes from no load to the rated value as shown in FIG. 8, the output voltage Vo can be held only by the smoothing capacitor 8. Therefore, the output voltage Vo is reduced to some extent. Then, the output voltage Vo becomes Vr
When the error amplifier 9 recognizes that the voltage has decreased to ef or less,
The output voltage of the error amplifier 9 becomes high, and the photocoupler P
The current of the light emitting diode LD1 of C1 stops flowing. Then, the phototransistor PT1 of the photocoupler PC1
Does not flow, as shown in FIG. 8, the first output voltage information signal VFB rises to a voltage that does not cross the reference triangular wave voltage Vosc. As a result, the duty of the control pulse at that time increases to the maximum on duty determined by the minimum off time defined by Vosc, and the output voltage V
Feedback is applied to raise o and return to the original voltage.

However, when there is no load, the second output voltage information signal VF drops below the threshold value Vth, and the maximum on-duty at this time is minimum. Even when the output voltage Vo is to be increased, as shown in FIG.
ty is limited to the maximum on-duty at that time and is small, the force for increasing the output voltage Vo is weak, and the output voltage Vo is low.
However, there is a problem that it cannot rise immediately, resulting in an excessive fluctuation. On the other hand, in order to reduce the excessive fluctuation, the output smoothing capacitor 8 should have a large capacity. However, if the smoothing capacitor 8 is made large, there is a problem that the device becomes large.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems of the related art, and has as its object to provide an insulated switching power supply control circuit that is small in size and has a small variation in output with respect to a load variation.

[0021]

SUMMARY OF THE INVENTION The present invention provides a reference triangular wave voltage generation circuit having a timing capacitor for generating a reference triangular wave, and a first output voltage control feedback from an output of a power supply via a photocoupler. And a comparator for comparing the output voltage information signal with the reference triangular wave voltage to generate a control pulse signal. The fall time of the reference triangular wave is a time during which the control pulse cannot be turned on, and the average voltage of the control pulse. An insulation type switching power supply control circuit including a control circuit having a function of reducing a discharge current of the timing capacitor and increasing a minimum off time when a second output voltage information signal becomes equal to or less than a predetermined value. At least during a period in which the first output voltage information signal is being output, the isolated switching power supply control circuit is provided with a bias means for raising the second output voltage information signal to the predetermined value or more.

The insulated switching power supply control circuit of the present invention is designed so that the voltage of the second output voltage information signal VF does not decrease and the maximum duty does not decrease even in an output current region where the coil current is discontinuous. While the current flows through the phototransistor, the VF voltage is biased to a certain value or more to maintain the maximum Duty so as not to decrease, and when the current of the phototransistor stops flowing during the overcurrent protection operation, The function of releasing the bias of the VF voltage and suppressing the increase of the output current is achieved.

[0023] Further, the insulation type switching power supply control circuit of the present invention is provided as another means so that the VF voltage does not decrease and the maximum duty does not decrease even in an output current region where the coil current is discontinuous. While the load current in the region where the current of
The F voltage is biased to a certain value or more to keep the maximum duty so as not to be reduced, and when the coil current exceeds a predetermined load current in a region where the current of the coil is continuous, the bias of the VF voltage is released and an overcurrent is released. This is to perform the function of suppressing an increase in output current during the protection operation.

[0024]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows a first embodiment of an insulated switching power supply control circuit according to the present invention.
The switching power supply control circuit of this embodiment also has a configuration similar to that of the above-described conventional technology, and the transformer 1 has a primary winding 2 and a secondary winding 3 and a third winding 4. Input voltage VIN of the power supply 30
Are connected to each other, and are transformed to output a desired voltage from the secondary winding 3 and the third winding 4. A rectifying diode 5 and a smoothing coil 6 are connected in series to the secondary winding 3 of the transformer 1, and a rectifying diode 7 and a smoothing capacitor 8 are connected in parallel. The rectified output voltage Vo (current Io) is connected to the load Lo via the output terminal To.

The rectified output voltage Vo is connected to an error amplifier 9 which is a differential amplifier.
Is connected to the reference voltage Vre of the reference voltage source 10.
f is applied. The output of the error amplifier 9 is connected to the cathode side of the light emitting diode LD1 provided in the photocoupler PC1, and to the anode side of the light emitting diode LD1.
The output voltage Vo is applied via the resistor 11.

A phototransistor PT1 is provided in series with the resistor 12 using a stable voltage source Vr as a power supply.
The phototransistor PT1 is provided as a light receiving element of the photocoupler PC1, and operates by receiving light emitted from the light emitting diode LD1. Thereby, the phototransistor P
The collector of T1 includes a first output voltage information signal V based on a feedback voltage related to the rectified output voltage Vo of the transformer 1.
FB occurs. The first output voltage information signal VFB is an output of the error amplifier 9, that is, the output voltage Vo and the reference voltage VFB.
It changes according to the difference of ref.

The primary winding 2 is connected to the third winding 4 of the transformer 1.
Is generated by transforming the input voltage VIN generated at the winding ratio by the turns ratio, and is rectified and smoothed by the rectifying diode 13 and the smoothing capacitor 14 connected to the winding 4.
A voltage substantially proportional to N is supplied to the power supply Vcc of the drive circuit of the output control circuit 32. The output control circuit 32
Is the same as the conventional circuit shown in FIG. The switch element 16 is connected to the primary winding 2, the OUT terminal of the output control circuit 32 is connected to the gate of the switch element 16, the switch element 16 is turned on / off by a control pulse voltage of the OUT terminal, and the transformer 1 is turned on. Driven.

Further, an average value circuit 15 composed of a resistor 17 and a capacitor 18 for outputting an average control pulse voltage is connected to the OUT terminal of the output control circuit 32. The output of the average value circuit 15 is V
An output of the average voltage of the control pulse voltage by the average value circuit 15 is connected to the F terminal and is input to the VF terminal as a second output voltage information signal VF.

Also in this embodiment, a resistor 19 is connected in series to the switch element 16 to convert a current flowing through the switch element 16 into a voltage. This resistor 19 is a current detection resistor, and applies a detection voltage of the resistor 19 to a latch circuit reset every cycle of Vosc. When the detection voltage of the resistor 19 exceeds a specified value, the latch circuit forcibly switches. A pulse-by-pulse overcurrent protection circuit for turning off the element 16 is configured.

Further, the output control circuit 32 of this embodiment
The VF terminal of the second output voltage information signal VF is connected to a bias circuit 25 serving as a bias means for raising the second output voltage information signal VF to a threshold Vth or more. This bias circuit 25 has a phototransistor current detecting transistor 24 in which the emitter of the phototransistor PT1 is connected to the base as means for detecting the phototransistor current Ic1 of the photocoupler PC1, and the emitter of the transistor 24 has a ground potential. It is connected to the. Furthermore, resistors 22 and 23 are connected between the collector of the transistor 24 and the voltage source Vr.
The base of the VF voltage biasing transistor 21 is connected between 23. VF voltage bias transistor 2
The emitter 1 is connected to the voltage source Vr, and the collector is connected to the VF terminal via the bias voltage adjusting resistor 20.

As shown in FIG. 3, the operation of the switching power supply control circuit of this embodiment is such that when the output load current Io suddenly changes from no load to the rated value, the output voltage Vo decreases. When the error amplifier 9 recognizes that the output voltage Vo has dropped to Vref or less, the output voltage of the error amplifier 9 becomes high, and the current of the light emitting diode LD1 of the photocoupler PC1 stops flowing. Then, the current of the phototransistor PT1 of the photocoupler PC1 also stops flowing, and as shown in FIG. 3, the first output voltage information signal V
FB rises to a voltage that does not cross the reference triangular wave voltage Vosc.

Here, the static VF-Io characteristic and the static Vo
As shown in FIGS. 2A and 2B, in the −Io characteristic, the phototransistor current Ic1 of the photocoupler PC1 flows in the range from when the output current Io is zero to when the overcurrent protection operation starts. In this state, since the bias transistor 21 is always on via the phototransistor current detection transistor 24, the voltage of the second output voltage information signal VF is biased to a potential equal to or higher than the threshold value Vth. This bias section corresponds to the load current Io
Is small, and the voltage of the second output voltage information signal VF maintains a voltage exceeding the threshold value Vth even in a region where the current of the coil 6 is discontinuous. Therefore, the output load current Io
When the load suddenly changes from no load to the rated load, the discharge current control circuit does not operate as in the above-described conventional technique, and the maximum ON D
Since the duty does not decrease, the power to increase the output voltage Vo is strong (the duty is larger than the conventional one), and the output voltage Vo increases in a relatively short period of time, so that the excessive fluctuation can be reduced.

On the other hand, in the range where the output current Io increases and the overcurrent protection function operates, as shown in FIG. 2A, since Ic1 does not flow, the phototransistor current detection transistor 24 is turned off. Since the bias transistor 21 is also turned off, the bias of the second output voltage information signal VF is eliminated, and the output control circuit 3
The voltage supply to the VF terminal 2 is only the average value circuit 15. As a result, the voltage of the second output voltage information signal VF becomes substantially proportional to the output voltage Vo, and the second output voltage information signal VF is supplied as it is in the same manner as in the related art. Then, the second output voltage information signal V
When F decreases, the discharge current control circuit operates to fulfill the function of suppressing an increase in the output current Io when the output voltage Vo decreases.

The switching power supply control circuit of this embodiment is designed so that the maximum on-duty of the output control circuit is not reduced even when the load is suddenly changed from the no-load state.
Since the voltage of the output voltage information signal VF is maintained at a voltage exceeding the threshold value Vth, the output voltage Vo can be quickly increased even if the output voltage suddenly changes. Therefore, the excessive fluctuation of the output voltage Vo can be reduced, and the output smoothing capacitor 8 can have a small capacitance, which contributes to downsizing of the device.

Next, a second embodiment of the present invention will be described with reference to FIGS. Here, the same components as those in the above embodiment are denoted by the same reference numerals, and description thereof is omitted.
In this embodiment, as a means for detecting the output current Io, a detection resistor 26 is inserted between the negative side of the output and the smoothing capacitor 8 so that the voltage generated at the detection resistor 26
7 has been entered. The current amplifier 27 has a load current Io
Is output, and the voltage V27 is compared with a predetermined reference voltage V28 by the comparator 29. Comparator 2
9 is the light emitting diode LD of the photocoupler PC2.
2 cathodes. Also, the photocoupler P
The collector of the phototransistor PT2 of C2 is connected to the voltage source Vr, the emitter is grounded via the resistor 42, and is connected to the base of the phototransistor current detecting transistor 40. Transistor 40
Has a grounded emitter, and the collector of the transistor 40 is connected to the voltage source Vr via the resistors 22 and 23. The resistor 22 and the resistor 23 are connected to the base of the VF bias transistor 21. The emitter of the VF voltage bias transistor 21 is connected to the voltage source Vr, and the collector is connected to the VF terminal of the output control circuit 32 via the bias voltage adjusting resistor 20.

The operation of the switching power supply control circuit of this embodiment is such that when the output voltage V27 of the current amplifier 27 is lower than the reference voltage V28, the output of the comparator 29 becomes low and the light emitting diode LD2 of the photocoupler PC2 emits light. ,
The current Ic2 flows through the phototransistor PT2. Conversely, when the voltage V27 of the current amplifier 27 is equal to or higher than the reference voltage V28, the output of the comparator 29 becomes high, and the current Ic2 does not flow through the phototransistor PC2.

Therefore, as shown in FIG. 5A, when the output current Io is zero, the output current signal voltage V27 changes to the reference voltage V27.
In a range lower than 28, the phototransistor current Ic2 of the photocoupler PC2 flows, and in this state, the bias transistor 40 is always on, so that the voltage of the second output voltage information signal VF becomes the threshold Vt.
h or more, the VF terminal of the output control circuit 32 maintains a voltage exceeding the threshold value Vth even in a region where the current of the coil 6 is discontinuous. Therefore,
Since the discharge current control circuit does not operate even when the output load current Io suddenly changes from no load to the rated load, the maximum ON Du
ty does not decrease.

On the other hand, when the output current signal V27 is equal to the reference voltage V2
Since Ic2 does not flow in a section exceeding 8, the phototransistor current detection transistor 30 is turned off, and the bias transistor 21 is also turned off in conjunction therewith, so that no bias is applied to the VF terminal of the output control circuit 32. Voltage is supplied to the terminal only through the average value circuit 15. As a result, the voltage of the second output voltage information signal VF becomes substantially proportional to the output voltage Vo, and the second output voltage information signal VF is supplied as it is in the same manner as in the related art. Then, the discharge current control circuit operates when the second output voltage information signal VF decreases, and functions to suppress an increase in the output current Io when the output voltage Vo decreases.

The average value circuit 15 of the present invention is not limited to the structure of the above embodiment, as long as it can detect the average voltage of the control pulse signal. Further, the means for detecting the output current Io may detect the output current, convert the voltage with an appropriate method, convert the voltage, and compare the output current with a predetermined reference voltage.

[0040]

According to the present invention, it is possible to reduce the excessive fluctuation of the output voltage at the time of a sudden load change while maintaining the function of suppressing the increase of the output current due to the decrease of the output voltage at the time of the overcurrent protection operation. it can. In addition, the output smoothing capacitor can have a small capacitance, which contributes to downsizing of the device.

[Brief description of the drawings]

FIG. 1 is a schematic circuit diagram of a control circuit for an insulated switching power supply according to a first embodiment of the present invention.

FIG. 2 is a graph showing load current-output voltage information signal characteristics (A) and load current-output voltage characteristics (B) of the isolated switching power supply control circuit according to the first embodiment of the present invention.

FIG. 3 is a time chart showing operation waveforms of the isolated switching power supply control circuit according to the first embodiment of the present invention;

FIG. 4 is a schematic circuit diagram of a control circuit for an insulated switching power supply according to a second embodiment of the present invention.

FIG. 5 is a graph showing a load current-output voltage information signal characteristic (A) and a load current-output voltage characteristic (B) of the isolated switching power supply control circuit according to the second embodiment of the present invention.

FIG. 6 is a schematic circuit diagram of a conventional control circuit for an insulated switching power supply.

FIG. 7 is a schematic circuit diagram showing an output control circuit of a conventional control circuit for an insulated switching power supply.

FIG. 8 is a time chart showing operation waveforms of a conventional control circuit for an insulated switching power supply.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Transformer 2 Primary winding 3 Secondary winding 9 Error amplifier 15 Average circuit 25 Bias circuit 32 Output control circuit

Claims (3)

[Claims] 1. A reference triangular wave voltage generating circuit having a timing capacity for generating a reference triangular wave, a first output voltage information signal for output voltage control fed back from an output of a power supply via a photocoupler, and the reference A comparator for generating a control pulse signal by comparing the voltage with the triangular wave voltage, setting a fall time of the reference triangular wave to a time during which the control pulse cannot be turned on, and a second output voltage information signal that is an average voltage of the control pulse. In the isolated switching power supply control circuit including a control circuit having a function of reducing the discharge current of the timing capacity and increasing the minimum off-time when the discharge voltage falls below a predetermined value, at least the first output voltage information signal is output. And a bias means for raising the second output voltage information signal to the predetermined value or more during the period. Quenching power control circuit. 2. The method according to claim 1, wherein the bias unit monitors the photocoupler current, and controls the second output voltage information so that the discharge current of the timing capacitor does not change while the first output voltage information signal is being output. 2. The isolated switching power supply control circuit according to claim 1, wherein the signal is biased to a voltage equal to or higher than a predetermined value to keep the fall time and the minimum off time of the triangular wave constant. 3. The bias means monitors an output current, and when the output current information signal is equal to or less than a predetermined value, applies the second output voltage information signal input so that the discharge current of the timing capacitor does not change. Bias over a predetermined voltage,
2. The isolated switching power supply control circuit according to claim 1, wherein a fall time and a minimum off time of the triangular wave are kept constant.