JP3522405B2 - Flyback type and forward type switching power supply - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a flyback type and forward type switching power supply device.

[0002]

2. Description of the Related Art A flyback system has been known as a main circuit system of a switching power supply device. FIG. 14 shows an example of a flyback type switching power supply device.
Shown in. The voltage of the AC power source 21 is the diode bridge 20.
2 and the capacitor 206 rectify and smooth it, and restore it to an unstable DC voltage. This unstable DC voltage is connected to the series circuit of the primary winding 207f of the flyback transformer 207 and the switch element 204. Switch element 2
04 is switched according to a predetermined signal, and when the switch element 204 is turned on, the flyback transformer 2
The primary winding 207f of No. 07 is excited to store energy, the switch element 204 generates a flyback voltage when it is off, and the capacitor 2 is passed through the diode 210.
11 is charged and a DC voltage is output. Then, the above operation is repeated every time the switch element 204 is turned on and off.

When the switch element 204 is turned on, the energy Q1 stored in the primary winding 207f of the flyback transformer 207 is such that L1 is the inductance of the primary winding 207f of the transformer 207 and I1 is the primary winding. 20
If the current flowing through 7f,

[0004]

## EQU1 ## Q1 = (1/2) L1 × I1 Is represented.

[0005]

However, since the capacitor 206 is directly connected to the diode bridge 202 (referred to as a capacitor input type rectifier circuit), the input current seen from the power supply line is a harmonic wave as shown in FIG. It has a large power consumption and a poor power factor, which has become a big problem in recent years.

Further, when the power is turned on, the capacitor 2 is suddenly changed.
Since 06 is charged, there is a problem that a rush current of several tens of times the rated voltage flows, which has many adverse effects on the power supply line.

Immediately after the power is turned on, as shown in FIG. 16, there is no voltage for resetting the current flowing through the primary winding 207f of the flyback transformer 207 when the switching element 204 is on, and the next on In some cases, the current that had been flowing in the primary winding 207f last time was taken over to increase the current, which gave considerable stress to the switch element 204, and in the worst case, the switch element 204 may be destroyed.

An object of the present invention is to solve the above problems, to suppress harmonics, and to suppress inrush current to the smoothing capacitor when the power is turned on. Flyback type and forward type. It is to provide a switching power supply device.

[0009]

According to a first aspect of the present invention, there is provided a rectifier circuit for rectifying an AC power supply voltage, and a series circuit of an inductor and a switch element, wherein the rectifier circuit is provided when the switch element is turned on by switching. A circuit for connecting the unstable DC voltage obtained by to the inductor to store energy in the inductor, and a series circuit of a capacitor and a first diode, which is connected in parallel to the inductor, A circuit for storing energy of the inductor in the capacitor when the switch element is turned off, and a circuit for connecting the voltage of the capacitor to the primary winding through the second diode when the switch element is turned on by switching. , The energy of the capacitor is stored in the primary winding, and the switch element switches When it is turned off by the flyback transformer, a flyback transformer for generating a flyback voltage in the secondary winding, a rectifying / smoothing circuit for rectifying / smoothing the voltage appearing in the secondary winding of the flyback transformer, and the rectifying / smoothing circuit. A control circuit for switching the switch element based on the DC voltage obtained by the smoothing circuit, a resistor connected in parallel to the second diode, the inductor, the capacitor, and the primary winding together with a soft resistor. A resistor that forms a start circuit, and when the AC power is turned on, the switch element is maintained in an off state, and after the AC power is turned on, a predetermined time that is larger than a time constant determined by the resistance value of the resistor and the capacitor of the capacitor. When has passed,
And a switch element drive control circuit for starting switching of the switch element by the control circuit.

With this configuration, when the AC power source is turned on with the switch element in the off state, the current becomes
The inductor flows through the path of the inductor, the capacitor, the primary winding, and the resistor, and the capacitor is charged with the time constant determined by the capacitor and the resistor. Start switching. Then, when the switch element is turned on by the control circuit, the unstable DC voltage obtained by the rectifier circuit is connected to the inductor, energy is stored in the inductor, and this energy is stored in the capacitor when the switch element is turned off. The voltage of this capacitor is stored and connected to the primary winding through the second diode when the switch element is turned on, and when the switch element is turned off, a flyback voltage is generated in the secondary winding.
The voltage appearing in the secondary winding is rectified and smoothed by the rectification and smoothing circuit.

In claim 1, only when the current of the pulse generator and the power supply line is smaller than a predetermined current set value, the output of the pulse generator is output to the switch element to turn on the switch element. A pulse control circuit, and when the DC voltage on the secondary side of the flyback transformer reaches a predetermined voltage, control of the pulse control circuit is stopped and switching of the switch element by the control circuit is started. And a drive control circuit.

The pulse control circuit outputs the output of the pulse generator to the switch element to turn on the switch element only when the current of the power supply line is smaller than a predetermined current setting value, and the secondary side of the flyback transformer is turned on. When the DC voltage of 1 reaches a predetermined voltage, the first drive control circuit stops the control of the pulse control circuit and starts the switching of the switch element.

According to claim 1, a voltage dividing circuit for dividing the unstable DC voltage obtained by the rectifying circuit, a pulse generator, and a current detection value of a power supply line are set current values obtained by the voltage dividing circuit. Only when it is smaller, the pulse control circuit that outputs the output of the pulse generator to the switch element to turn on the switch element, and the DC voltage on the secondary side of the flyback transformer have reached a predetermined voltage. At this time, stop the control by the pulse control circuit,
A second drive control circuit for starting switching of the switch element by the control circuit can be provided.

Only when the current detection value of the power supply line is smaller than the current setting value obtained by the voltage dividing circuit, the pulse control circuit outputs the output of the pulse generator to the switch element to turn on the switch element and turn on the switch element. When the DC voltage on the secondary side reaches a predetermined voltage, the second drive control circuit stops the control by the pulse control circuit and starts switching of the switch element by the control circuit.

According to a fourth aspect of the present invention, there is provided a rectifier circuit for rectifying an AC power supply voltage, and a series circuit of an inductor and a switch element, wherein when the switch element is turned on by switching, the unstable circuit obtained by the rectifier circuit. A direct current voltage is connected to the inductor, a circuit for storing energy in the inductor and a series circuit of a capacitor and a first diode are connected in parallel to the inductor, and the switching element is turned off by switching. At this time, when the switching element is turned on by the circuit for storing the energy of the inductor in the capacitor and the switching, the voltage of the capacitor is connected to the primary winding via the second diode and the energy is supplied to the secondary winding. For rectifying and flattening the voltage that appears in the transformer and the secondary winding of the transformer. A rectifying and smoothing circuit, a control circuit for switching the switch element based on a DC voltage obtained by the rectifying and smoothing circuit, a resistor connected in parallel to the second diode, the inductor, and the capacitor. A resistor that forms a soft start circuit together with the primary winding, and keeps the switch element off when the AC power is turned on, and is determined by the resistance value of the resistor and the capacitor of the capacitor after the AC power is turned on. When a predetermined time greater than the time constant
And a switch element drive control circuit for starting switching of the switch element by the control circuit.

With this configuration, when the AC power source is turned on with the switch element kept in the off state, the current becomes
The inductor flows through the path of the inductor, the capacitor, the primary winding, and the resistor, and the capacitor is charged with the time constant determined by the capacitor and the resistor. Start switching. Then, when the switch element is turned on by the control circuit, the unstable DC voltage obtained by the rectifier circuit is connected to the inductor, energy is stored in the inductor, and this energy is stored in the capacitor when the switch element is turned off. The voltage of this capacitor is stored and connected to the primary winding through the second diode when the switching element is turned on, and energy is supplied to the secondary winding. The voltage appearing in the secondary winding is rectified and smoothed by the rectification and smoothing circuit.

In claim 4, the pulse generator and the output of the pulse generator are output to the switch element only when the current of the power supply line is smaller than a predetermined current set value to turn on the switch element. Pulse control circuit,
When the DC voltage on the secondary side of the transformer reaches a predetermined voltage, the control of the pulse control circuit is stopped,
A first drive control circuit for starting switching of the switch element by the control circuit can be provided.

The pulse control circuit outputs the output of the pulse generator to the switch element to turn on the switch element only when the current of the power supply line is smaller than a predetermined current set value, and the DC of the secondary side of the transformer is turned on. When the voltage reaches a predetermined voltage, the control of the pulse control circuit is stopped and the switching of the switch element is started by the first drive control circuit.

According to a fourth aspect, the voltage dividing circuit for dividing the unstable DC voltage obtained by the rectifying circuit, the pulse generator, and the output of the pulse generator are used as the current detection value of the power supply line. A pulse control circuit for outputting to the switch element and turning on the switch element only when it is smaller than the current setting value obtained by the voltage circuit; and when the DC voltage on the secondary side of the transformer reaches a predetermined voltage, A second control for stopping control by the pulse control circuit and starting switching of the switch element by the control circuit;
And a drive control circuit.

Only when the current detection value of the power supply line is smaller than the current setting value obtained by the voltage dividing circuit, the pulse control circuit outputs the output of the pulse generator to the switch element to turn on the switch element and turn on the switch element. When the DC voltage on the secondary side reaches a predetermined voltage, the second drive control circuit stops the control by the pulse control circuit and starts switching of the switch element by the control circuit.

[0021]

[0022]

[0023]

[0024]

[0025]

DETAILED DESCRIPTION OF THE INVENTION

<First Embodiment> FIG. 1 shows a first embodiment of the present invention. This is an example of a flyback type switching power supply. In FIG. 1, 101 is an AC power supply. 1
Reference numeral 02 denotes a diode bridge for full-wave rectification, which full-wave rectifies the voltage of the AC power supply. 103 is an inductor and 104 is a switch element.
An unstable DC voltage obtained by full-wave rectification is connected to the series circuit of the inductor 103 and the inductor 103. 105 is a diode, 106 is a capacitor, and the diode 105
A series circuit in which the anode and the capacitor 106 are connected in series is connected in parallel to the inductor 103. 108
Is a diode, and the voltage at the node between the diode 105 and the capacitor 106 is connected to the series circuit of the diode 108 and the primary winding of the flyback transformer 107. Reference numeral 109 is a resistor, which is connected in parallel with the diode 108.

Reference numeral 110 is a diode and 111 is a capacitor for rectifying and smoothing the voltage on the secondary side of the flyback transformer 107. Reference numeral 112 is a load resistance. Reference numeral 113 denotes a control circuit, which changes the obtained variation of the DC voltage into a pulse width to switch the switch element 104. Reference numeral 114 denotes a timer, and the timer time is from the time when the AC power is turned on to the time when the flyback transformer 107 is set
The time required for the secondary side to reach a predetermined voltage is set, and the AC power supply 101 is activated when it is turned on.

The operation when the AC power source is turned on will be described with reference to FIG. When the AC power supply 101 is turned on with the switch element 104 turned off, the timer 114 is activated, and the current flows through the path of the inductor 103, the capacitor 106, the primary winding of the flyback transformer 107, and the resistor 109. The capacitor 106 is charged. In this charging, the inductance of the inductor 103 and the primary winding of the flyback transformer 107 is small, so that the capacitor 10 is almost charged.
The time constant of the capacitance of 6 and the resistance value of the resistor 109 is used. Then, when the timer 114 times out,
The control circuit 113 starts switching of the switch element 104. In this way, the inrush current when the power is turned on is suppressed and the power system is not disturbed. Switch element 1
No switching element 10 is applied to 04.
When the switching operation of No. 4 is started, the operation is almost a steady operation. By selecting a resistor 109 having a sufficiently large value, it is possible to prevent the inrush current without lowering the efficiency.

Next, the operation after the start of the switching operation will be described with reference to FIGS.

FIG. 2 shows the same electric circuit as in FIG.
3 is a time chart showing the timing of each part of FIG. When the output voltage of the diode bridge 102 is EO and the switch element 104 is turned on for the period of T1, a current flows through the inductor 103. The current I1 flowing through the inductor 103 is as follows, where L1 is the inductance of the inductor 103:

[0030]

## EQU00002 ## I1 = (E0 / L1) T1 (1) and energy Q1 in the inductor 103, that is,

[0031]

## EQU3 ## Q1 = (1/2) × L1 × I1 ² (2) is stored.

Since the energy Q1 of the inductor 103 is stored in the capacitor 106 during the period T2 immediately before the period T1, the voltage of the capacitor 106 flybacks when the switch element is turned on during the period T1. The current I2 is applied to the primary winding 107f of the transformer 107 and flows through the diode 108 into the primary winding 107f. If the inductance of the primary winding 107f is L2, the current I2 becomes

[0033]

Equation 4] I2 = (1/2) L2 × I2 2 ... (3) next, the primary winding 107f, energy Q2, i.e.,

[0034]

[Equation 5] Q2 = (1/2) L2 × I2 ² (4) is stored.

When the switch element 104 is turned off, the energy Q1 of the inductor 103 becomes

[0036]

[Equation 6] T2 = (I1 / E1) L1 (5) is reset during the period and stored in the capacitor 106. In addition, the primary winding 1 of the flyback transformer 107
The current I2 flowing in 07f is cut off, and the energy Q
2 is transmitted to the secondary side, and a flyback voltage is generated.
A current I3 flows on the secondary side. If the inductance of the secondary winding 107s is L3, the current I3 becomes

[0037]

[Equation 7]

Then, the capacitor 111 is charged.

The energies Q1, Q2 and Q3 are:
Since there is a relation of Q1 = Q2 = Q3,

[0040]

[Equation 8]

It becomes

At AC input shown in FIG. 2 (effective value voltage Eef
f, RMS current Ieff), the input voltage e

[0043]

[Equation 9]

At the time of

[0045]

[Equation 10]

And

[0047]

[Equation 11]

It becomes

As shown in FIG. 4, the current flowing through the power supply is essentially the fundamental wave component and the harmonic components higher than the switching frequency component. For example, when the switching frequency is N times the fundamental wave component, The harmonic components included in the current are Nth or higher. This harmonic can be removed by a simple filter that only removes high frequency components, and the current waveform becomes a sine wave similar to the input voltage waveform.
In this way, harmonic current distortion can be suppressed and power factor can be improved.

<Second Embodiment> FIG. 5 shows a second embodiment of the present invention.
An embodiment of is shown. This embodiment is different from the first embodiment in the method of preventing inrush current. In the present embodiment, the current detection / pulse control circuit 514 detects the current flowing through the switch element 104, and the detected current value is a preset current setting value (for example, 1.5 of the peak current at the time of rating). When the doubled value or the value (Imax) which is larger than the maximum value of the values shown by the formula (10) is exceeded, the switch element is turned off. The control circuit 513 changes the obtained variation of the DC voltage into a pulse width to switch the switch element 104, and once the DC voltage reaches a predetermined voltage, the control circuit 513 is activated and the operation of the current detection / pulse control circuit 514 is stopped. It is what makes me.

The inrush current prevention method will be described in more detail with reference to FIG. 6 showing the configuration of the current detection / pulse control circuit 514. When the AC power supply 101 is turned on, one pulse from the pulse generator 601 is supplied to one input terminal of the AND gate 605 and the R terminal of the flip-flop 604 (see FIG. 7A). Since the detected current value is smaller than the current setting value immediately after the switch element 104 is powered on (see FIGS. 7B and 7C), the flip-flop 6
04 remains reset (see FIG. 7 (d)),
The level of one input terminal of the AND gate 605 becomes H level. Therefore, the pulse from the pulse generator 601 is output via the AND gate 605, and the switching element 1
04 is turned on (see FIG. 7E). Switch element 1
When 04 is turned on (see FIG. 8B), inductor 1
03 and the current flowing through the switch element 104 ΔI
1 is

[0052]

ΔI1 = (E0 / L1) t (13), which increases in proportion to time t (see FIG. 8A). Then, when the detected current becomes larger than the current setting value (see FIGS. 7B and 7C), the flip-flop 604 is set (FIG. 7D), and the AND gate 60.
The level of one input terminal of 5 becomes L level, and the switch element 104 is turned off (see FIG. 7E). Then, the current ΔI1 becomes

[0053]

ΔI1 =-(E1 / L1) t (14), which decreases in proportion to the time t (FIG. 8 (a),
(See (b)). After that, the pulse from the pulse generator 601 changes from H level to L level (see FIG. 7D).
After that, this cycle is repeated.

As shown in FIGS. 8 (a), 8 (b) and 8 (c), when E1 is not sufficiently charged, the current is Imax.
When the current reaches, the inrush current can be suppressed to Imax or less by turning off the switch element 104. During rated operation, pulse control does not work and steady operation is performed.

<Third Embodiment> FIG. 9 shows a third embodiment of the present invention.
An embodiment of is shown. Compared with the second embodiment, this embodiment is different in the current setting value set in the current detection / pulse control circuit. In the present embodiment, the current setting value of the current detection / pulse control circuit 914 is set to a value obtained by dividing the unstable DC voltage obtained by full-wave rectification by the bridge diode 102 with resistors 915 and 916. . Therefore, the current setting value is proportional to the input voltage value, and the input current can be a sine wave immediately after the power is turned on, as shown in FIG.

<Fourth Embodiment> FIG. 11 shows a fourth embodiment of the present invention. This is an example of a forward switching power supply. In FIG. 11, 101 to 10
6, 108, 109, 113 and 114 show the same parts as in FIG. 1107 is a transformer. The secondary voltage of the transformer is rectified by the diodes 1111 and 1113 and smoothed by the inductor 1112 and the capacitor 1114. Reference numeral 1115 is a load resistance.

The operation when the AC power source is turned on in this embodiment is essentially the same as the operation when the AC power source is turned on in the first embodiment, and therefore the description thereof is omitted.

Next, the operation after the start of the switching operation will be described. When the switch element 104 is turned on, a current flows through the inductor 103 and energy is stored. Since the energy Q1 of the inductor 103 is stored in the capacitor 106 immediately before the switching element 104 is turned off, the voltage of the capacitor 106 is applied to the primary winding of the transformer 1107 when the switching element 104 is on. Then, a voltage appears on the secondary side of the transformer 1107,
The current I3 passes through the diode 1111 and the inductor 11
12 and energy is stored in the inductor 1112. Then, when the switch element 104 is turned off, the energy of the inductor 103 is stored in the capacitor 106. No current flows in the primary winding of the transformer 1107. Therefore, the energy stored in the inductor 1112 is stored in the capacitor 1114. Since the present embodiment is configured in this way, it is possible to suppress harmonic current distortion as in the first embodiment.

<Fifth Embodiment> FIG. 12 shows a fifth embodiment of the present invention. This embodiment is different from the fourth embodiment in the method of preventing inrush current.
That is, in this embodiment, the current detection / pulse control circuit 514 is adopted as in the second embodiment. The operation at the time of turning on the AC power supply in this embodiment is essentially the same as that in the second embodiment, and therefore its explanation is omitted.

<Sixth Embodiment> FIG. 13 shows a sixth embodiment of the present invention. Compared to the fifth embodiment, the present embodiment is different in the current setting value set in the current detection / pulse control circuit. In the present embodiment, as in the third embodiment, the current detection / pulse control circuit 9
The current setting value of 14 is set to the unstable DC voltage obtained by full-wave rectification by the bridge diode 102 by the resistors 915,
The value obtained by dividing the pressure by 16 was used. Since the operation when the AC power supply is turned on in this embodiment is essentially the same as the operation when the AC power supply is turned on in the third embodiment, the description thereof will be omitted.

[0061]

As described above, according to the present invention,
Since it is configured as described above, it is possible to suppress harmonics and suppress inrush current to the smoothing capacitor when the power is turned on.

[Brief description of drawings]

FIG. 1 is an electric circuit diagram showing a first embodiment of the present invention.

FIG. 2 is an electric circuit diagram for explaining the operation of the switching power supply device according to the first embodiment.

FIG. 3 is a time chart showing an example of the timing of each part shown in FIG.

FIG. 4 is a waveform diagram showing an example of a waveform of a power supply line.

FIG. 5 is an electric circuit diagram showing a second embodiment of the present invention.

6 is a block diagram showing a configuration of a current detection / pulse control circuit 514 shown in FIG.

FIG. 7 is a time chart showing an example of the timing of each unit shown in FIG.

FIG. 8 is a time chart for explaining the relationship between on / off of a switch element and current ΔI1 and voltage E1.

FIG. 9 is an electric circuit diagram showing a third embodiment of the present invention.

FIG. 10 is a current detection / pulse control circuit 914 shown in FIG.
FIG. 3 is a waveform diagram showing an example of a current set value and a current waveform set in step 1.

FIG. 11 is an electric circuit diagram showing a fourth embodiment of the present invention.

FIG. 12 is an electric circuit diagram showing a fifth embodiment of the present invention.

FIG. 13 is an electric circuit diagram showing a sixth embodiment of the present invention.

FIG. 14 is an electric circuit diagram showing an example of a conventional flyback type switching power supply device.

15 is a waveform diagram showing an example of an input current waveform and an input voltage waveform of the device shown in FIG.

16 is a waveform diagram showing a current waveform of the device shown in FIG. 14 immediately after applying a current.

[Explanation of symbols]

101 AC power supply 102 diode bridge 103 inductor 104 switch element 105, 108, 110 diodes 106,111 capacitors 107 transformer 107f primary winding 107s secondary winding 109 resistance 112 load resistance 113 control circuit 114 timer

Claims (6)

(57) [Claims] 1. A rectifier circuit for rectifying an AC power supply voltage, and a series circuit of an inductor and a switch element, wherein when the switch element is turned on by switching, an unstable DC voltage obtained by the rectifier circuit is output. A circuit for connecting energy to the inductor and storing energy in the inductor; and a series circuit of a capacitor and a first diode connected in parallel to the inductor, and when the switching element is turned off by switching, A circuit for storing energy in the capacitor, and when the switch element is turned on by switching,
The voltage of the capacitor is connected to the primary winding through the second diode, the energy of the capacitor is stored in the primary winding, and when the switch element is turned off by switching, the flyback voltage is applied to the secondary winding. A flyback transformer for generating, a rectifying / smoothing circuit for rectifying / smoothing the voltage appearing in the secondary winding of the flyback transformer, and a switching element of the switching element based on the DC voltage obtained by the rectifying / smoothing circuit. A control circuit for switching, a resistor connected in parallel to the second diode, the resistor forming a soft start circuit together with the inductor, the capacitor, and the primary winding; The switch element is maintained in the off state, and after the AC power is turned on, the resistance value of the resistor and the capacitor of the capacitor A flyback-type switching power supply device, comprising: a switch element drive control circuit that starts switching of the switch element by the control circuit when a predetermined time that is larger than the determined time constant has elapsed. 2. The pulse generator according to claim 1, wherein the output of the pulse generator is output to the switch element only when the current of the pulse generator and the power supply line is smaller than a predetermined current set value, and the switch element is turned on. When the DC voltage on the pulse control circuit to turn on and the secondary side of the flyback transformer reaches a predetermined voltage, control of the pulse control circuit is stopped and switching of the switch element by the control circuit is started. A flyback type switching power supply device comprising a first drive control circuit. 3. The voltage dividing circuit according to claim 1, which divides an unstable DC voltage obtained by the rectifying circuit, a pulse generator, and a current detected value of a power supply line is a current obtained by the voltage dividing circuit. A pulse control circuit that outputs the output of the pulse generator to the switch element to turn on the switch element only when the value is smaller than a set value, and the DC voltage on the secondary side of the flyback transformer becomes a predetermined voltage. And a second drive control circuit for stopping the control by the pulse control circuit and starting switching of the switch element by the control circuit when the flyback type switching power supply device is turned off. 4. A rectifier circuit for rectifying an AC power supply voltage, and a series circuit of an inductor and a switch element, wherein when the switch element is turned on by switching, an unstable DC voltage obtained by the rectifier circuit is supplied to the rectifier circuit. A circuit for connecting energy to the inductor and storing energy in the inductor; and a series circuit of a capacitor and a first diode connected in parallel to the inductor, and when the switching element is turned off by switching, A circuit for storing energy in the capacitor, and when the switching element is turned on by switching,
Transformer for connecting the voltage of the capacitor to the primary winding through the second diode to supply energy to the secondary winding, and rectifying and smoothing for rectifying and smoothing the voltage appearing in the secondary winding of the transformer A circuit, a control circuit for switching the switch element based on the DC voltage obtained by the rectifying and smoothing circuit, a resistor connected in parallel to the second diode, the inductor, the capacitor, and A resistor that forms a soft start circuit together with the next winding, and the time constant that is maintained by the resistance value of the resistor and the capacitor of the capacitor after the AC power is turned on and that keeps the switch element off. A switch element drive control circuit that starts switching of the switch element by the control circuit when a predetermined time longer than the predetermined time has elapsed A forward type switching power supply device comprising: 5. The pulse generator according to claim 4, wherein the pulse generator and the output of the pulse generator are output to the switch element only when the current of the power supply line is smaller than a predetermined current set value, and the switch element is output. A pulse control circuit for turning on, and stopping the control of the pulse control circuit when the DC voltage on the secondary side of the transformer reaches a predetermined voltage,
A forward-type switching power supply device comprising: a first drive control circuit that starts switching of the switch element by the control circuit. 6. The voltage dividing circuit for dividing the unstable DC voltage obtained by the rectifier circuit, the pulse generator, and the output of the pulse generator, the current detection value of the power supply line according to claim 4. A pulse control circuit that outputs to the switch element and turns on the switch element only when it is smaller than the current setting value obtained by the voltage dividing circuit, and the DC voltage on the secondary side of the transformer becomes a predetermined voltage. At this time, stop the control by the pulse control circuit,
And a second drive control circuit for starting switching of the switch element by the control circuit.