JPH0556638A - Switching power supply - Google Patents

Abstract

PURPOSE:To reduce the cost of a partial resonance type switching regulator and diminish loss. CONSTITUTION:The series circuit of a reactor 2, the primary winding 4 of a transformer 3 and a first insulated gate type FET 5 is connected to a DC power 1. A capacitor 9 is connected in parallel through a second FET 10 in parallel with the first FET 5. A reactor 22 is bonded in parallel with a secondary winding 13 through a diode 23. The reactor 22 stores energy for an OFF period, and discharges it to the load side for an ON period. The first FET 5 is ON-OFF controlled for a fixed period. A second FET 5 is On-controlled for one part of an OFF period.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a switching power supply device capable of zero volt switching.

[0002]

2. Description of the Related Art A switching regulator in which a transformer and a switching element are connected in series, an alternating current is generated on the secondary side of the transformer by controlling the switching element to be turned on and off, and a direct current output is obtained by rectifying this Widely used. However, in this type of switching regulator, the current waveform and the voltage waveform overlap each other during the OFF conversion period and the ON conversion period of the switching element, resulting in power loss.

Resonant switching regulators have been studied to solve this type of problem. In the resonance type switching regulator, the voltage waveform between both terminals of the switching element is determined by the resonance between the inductance of the transformer and / or the inductance of the resonance reactor and the parasitic capacitance of the switching element and / or the capacitance of the resonance capacitor, and The voltage waveform between both terminals becomes a sine wave, and zero volt switching is possible. However, when the output voltage is controlled, there are problems that the switching frequency changes, that a high voltage is applied to the switching element, and that control becomes complicated.

On the other hand, a capacitor is connected in parallel with the switching element, and at the time of turn-off, the capacitor is charged by resonance operation to absorb the surge voltage, and at the same time delay the rise of the voltage between both terminals of the switching element to reduce the switching loss. After that, a partial resonance type switching regulator in which the energy of the capacitor is fed back to the power supply has been proposed.

[0005]

However, in the conventional partial resonance type switching regulator, the power loss caused by returning the energy of the resonance capacitor to the power supply causes a decrease in efficiency. Further, the conventional partial resonance type switching regulator inevitably has a high cost due to the special circuit configuration.

[0006] Therefore, an object of the present invention is to provide a switching power supply device which can achieve high efficiency and low cost.

[0007]

The present invention for achieving the above object provides a DC power source, a primary winding of a transformer connected between one end and the other end of the DC power source, and the primary winding. A first switching element connected in series to the winding, and a diode integrally formed with the first switching element or an independent diode, the first switching element connected in parallel to the first switching element Of the first switching element and a parasitic capacitance of the first switching element or an independent capacitor, which is connected in parallel to the first switching element, and the inductance of the primary winding and / or Alternatively, the resonance inductance formed of an independent inductance element and the first switching element are connected in parallel via the second switching element and A second capacitor having a larger capacitance than the first capacitor and a diode integrally formed with the second switching element or an independent diode, and connected in parallel with the second switching element. A second diode connected to the transformer, a secondary winding of the transformer, a rectifying / smoothing circuit connected to the secondary winding, and a first control for controlling on / off of the first switching element. Second control for turning on the second switching element between a pulse and a time point after the start of the off period of the first switching element and a time point before the end of the off period. A switch control circuit for forming a pulse, a reactor connected in parallel with the secondary winding, and a reactor connected in series with the reactor and based on an induced voltage in the secondary winding. The present invention relates to a switching power supply device, comprising: a third diode having a direction that is forward biased by an induced voltage in the secondary winding during a period in which no current flows in the flow smoothing circuit. ..

As described in claim 2, the second capacitor can be connected in parallel with the primary winding via the second switching element.

Further, as described in claim 3, first and second switching elements for conversion are arranged on both sides of the primary winding, and third and fourth switching elements are provided for the first and second switching elements. Capacitors can be connected in parallel via the elements. When the correspondence between claim 3 and the embodiment of FIG. 6 is shown, the first, second, third and fourth switching elements are 31, 6, 35 and 11, and the first, second and third switching elements are the same. The third, fourth, and fifth diodes are 32, 7, 36, 12,
23, and the first, second, third, and fourth capacitors are 3
3,8,37,9.

[0010]

According to the first and second aspects of the present invention, the first switching element is a conversion switching element, which is turned on / off like a general separately excited switching regulator. The resonance inductance and the first capacitor resonate at a relatively high frequency. Therefore, the voltage of the first switching element changes along the sine wave during the short turn-off period and the turn-on period of the first switching element. Second
Capacitor and the second switching element control the energy release of the first capacitor. The reactor connected in parallel to the secondary winding has a function of storing energy released from the first capacitor during the off period. The energy stored here is released to the load side during the ON period.

The first and second switching elements of claim 3 share the voltage during the off period. Therefore, the first and second
It is possible to use a low breakdown voltage switching element as the switching element.

[0012]

[First Embodiment] A switching power supply device according to a first embodiment of the present invention will now be described with reference to FIGS. Figure 1
In, for example, N-channel insulation in which a resonance reactor 2, a primary winding 4 of a transformer 3 and a substrate are connected to a source between one end and the other end of a DC power supply 1 composed of a rectifying and smoothing circuit connected to an AC power supply. A gate type field effect transistor, that is, a series circuit with FET 5 is connected. F
ET5 comprises a first switching element 6, a first diode 7 and a first capacitor 8 shown equivalently. The first switching element 6 is composed of the body of the FET 5. The first diode 7 is a diode built in the FET 5, and is the source of the first switching element 6.
Anti-parallel connection is made between the drains. The first capacitor 8 is a parasitic capacitance between the drain and source of the first switching element 6.

A second capacitor 9 is connected in parallel with the first FET 5 via a second FET 10. The second capacitor 9 has a larger capacitance than the first capacitor 8. The second FET 10 is also an N-channel insulated gate field effect transistor in which the substrate is connected to the source, and contains the second diode 12 in addition to the second switching element 11. The FET 10 also has a parasitic capacitance, but the illustration is omitted.

The secondary winding 13 of the transformer 3 has a pair of output terminals 1 through a rectifying / smoothing circuit 18 including two diodes 14 and 15, a reactor 16 and a capacitor 17.
It is connected to 9 and 20. A pair of output terminals 19, 20
A load 21 is connected between them.

A reactor 22 for storing energy in the off period is connected in parallel to the secondary winding 13 via a diode 23. The direction of the diode 23 is such that the secondary winding 1
It is in the direction to be reverse biased with the voltage of 3.

First and second switching elements 6 and 11
A control circuit 24 for turning on / off the switch is connected to the gates of the first and second FETs 5 and 10, that is, the control terminals of the first and second switching elements 6 and 11 by lines 25 and 26, respectively. .. Further, the control circuit 24 is connected to the output terminals 19 and 20 in order to control the output voltage constant.

FIG. 2 shows the control circuit 24 of FIG. 1 in detail.
The control circuit 24 includes a voltage detection circuit 40 connected to the output terminals 19 and 20, an error amplifier 41, and a reference voltage source 4
2, a PWM control IC 44 including a PWM pulse forming circuit 43, a NOT circuit 45, first and second delay circuits 46 and 47, and first and second AND gates 48 and 4
9 and first and second drive circuits 50 and 51.
The voltage detection circuit 40 is composed of a voltage dividing circuit, and this voltage dividing point, that is, the detection line is connected to one input terminal of the error amplifier 41. The error amplifier 41 outputs a signal corresponding to the difference between the reference voltage of the reference voltage source 42 connected to the other input terminal and the detection voltage of the voltage detection circuit 40. The PWM control circuit 43 connected to the error amplifier 41 includes a triangular wave generator and a voltage comparator. The comparator compares the triangular wave having a constant cycle with the output signal of the error amplifier 41 to generate a square wave having a constant cycle. A commercially available MB3759, μPC494, or the like can be used as the PWM control IC 44. One input terminal of the first AND gate 48 is directly connected to the PWM control circuit 43, and the other input terminal of the first AND gate 48 is connected to the PWM control circuit 4 via the first delay circuit 46.
Connected to 3. One input terminal of the second AND gate 49 is connected to the PWM control circuit 43 via the NOT circuit 45.
And the other input terminal is connected to the second delay circuit 47.
It is connected to the NOT circuit 45 via. The output terminals of the first and second AND gates 48 and 49 are the first and second
Are connected to the first and second FET control lines 25 and 26 via the driving circuits 50 and 51 of the.

Waveforms of (A), (B) and (C) of FIG. 3 show voltage waveforms at points A, B and C of FIG. From the PWM control circuit 43, a square wave pulse (PWM pulse) shown in FIG.
Occurs repeatedly with a period T. When the output voltage becomes higher than the reference value, the pulse width becomes narrow as shown by the broken line. Conversely, when the output voltage becomes lower than the reference value, the pulse width becomes wider. This is the same as the operation of a general PWM-controlled switching regulator. First AND gate 4
Since the pulse of FIG. 3A and this delayed pulse are input to 8, the first control pulse of FIG. 3B is output from here. On the other hand, since the inversion pulse of FIG. 3A and this delay pulse are input to the second AND gate 49,
The second control pulse shown in FIG. 3C is output. First
The first and second control pulses of the second and second AND gates 48 and 49 are applied to the gates of the first and second switching elements 6 and 11 via the drive circuits 50 and 51. Figure 2
The first and second delay times Td of the first and second delay times Td correspond to the periods t1 to t2 and t3 to t4 of FIG. 3, and the total inductance Lr of the primary winding 4 and the reactor 2 and the first capacitor 8 of FIG.
This corresponds to a period of approximately π / 2 (90 degrees) of a sine wave according to the resonance frequency determined by the electrostatic capacitance Cr of.

Next, the operation of the circuit shown in FIG. 1 will be described with reference to FIG. The waveforms in FIG. 4 show the states of the respective parts in FIG. Figure 1
The operation of the circuit is as follows: t0 to t1 section of FIG.
Section, t2 to t3 section, t3 to t4 section, t4 to
The t5 section and the t5 to t6 sections can be divided into six sections.

In the section from t0 to t1, the first switching element 6 is in the ON state in response to the control signal (first control pulse) Vg1, and the second switching element 11 is in the OFF state. Further, the first diode 7, the second diode 12, and the smoothing diode 15 are in the non-conducting state, and the rectifying diode 14 and the diode 23 are in the conducting state. Therefore, the circuit including the power supply 1, the reactor 2, the primary winding 4, and the first switching element 6 is used to
The current Iq1 of the switching element 6 of 6 flows as shown in FIG. This current Iq1 increases with time due to the inductance Lr of the reactor 2 and the primary winding 4. The voltage Vs of the secondary winding 13 is almost constant as shown in FIG. 4, and based on this voltage Vs, a current flows in a closed circuit composed of the secondary winding 13, the diode 14, the reactor 16, the capacitor 17 and the load 21. Is flows as shown in FIG. Also,
During the off period of the first switching element 6, the reactor 22
On the basis of the energy stored in, the current Ib is applied to the closed circuit composed of the reactor 22, the diode 14, the reactor 16, the capacitor 17, the load 21 and the diode 23.
Flows as shown in FIG.

During the period from t1 to t2, the control signal Vg1 of the first switching element 6 is at low level. Therefore, t1 ~
In the t2 period, the first and second switching elements 6 and 11 are
Is off and the diodes 7, 12, 14, 15, 23 are
Maintains the same state as during the period t0 to t1. t1 to t2
When the first switching element 6 is turned off during the period, the first
Is connected in series to the reactor 2 and the primary winding 4, resonance occurs due to these inductance Lr and capacitance Cr, and the voltage of the capacitor 8, that is, the voltage Vq1 of the first switching element 6 rises to a sine wave. climb. In addition, in FIG. 4, the rising of the voltage Vq1 is shown by a straight line for simplification of illustration. The current at the time of this resonance is the power source 1 and the reactor 2 and 1
The current Iq1 shown in FIG. 4 is shaded in the waveform of the current Iq1 in the circuit composed of the secondary winding 4 and the first capacitor 8. The current flowing through the first switching element 6 is the current Iq1 in FIG. 4 with the shaded portion in the period t1 to t2 removed. Therefore, the voltage V of the first switching element 6
During the rising period of q1, the current of the first switching element 6 is zero, and the power loss of the first switching element 6 is reduced.

During the period from t2 to t3, the first and second
Switching elements 6 and 11 are off, diodes 7 and 1
2, 14 are non-conducting, and the diodes 15, 23 are conducting. After the capacitor 8 is charged to the input voltage Vin at time t2, the charging current of the capacitor 8 further flows due to the inertia based on the inductance of the reactor 2 and the primary winding 4, and this voltage Vq1 is the input voltage Vin and the flyback voltage Vf. And (Vin + Vf). Since the voltage Vq1 of the capacitor 8 becomes higher than the input voltage Vin, the reverse voltage is applied to the primary winding 4, and the induced voltage in the secondary winding 13 is also reversed. , The diode 14 becomes non-conductive. During this off period, the energy stored in the reactor 16 is supplied to the load 21.

At time t3, the voltage Vq1 of the first capacitor 8 becomes higher than the charging voltage of the second capacitor 9 and the second diode 12 becomes conductive. As a result, a current due to inertia based on the inductance Lr of the reactor 2 and the primary winding 4 also flows in the second capacitor 9. The current Iq2 in FIG. 4 shows this. Second capacitor 9
The voltage Vc varies depending on charging or discharging. However, since the capacitance C is sufficiently large, the fluctuation of the voltage Vc is extremely small. Therefore, in FIG. 4, the voltage Vc is shown as a constant value. The control signal Vg2 is applied to the second switching element 11 at time t3, but since it has the opposite polarity to the current due to the inertia of the inductance Lr, the current due to the inertia flows through the diode 12. The current due to inertia gradually decreases as shown by Iq2 in FIG. 4, and becomes zero at time t4. During the ON period of the diode 12, the voltage Vq1 of the FET 5 is clamped and maintained at a substantially constant value. A voltage obtained by subtracting the input voltage Vin from the voltage Vc of the capacitor 9 is applied to the primary winding 4, and this acts as a reset voltage of the transformer 3. The voltage Vs of the secondary winding 13 is kept substantially constant corresponding to the voltage of the primary winding 4. Since the substantially constant voltage Vs is applied to the reactor 22, the current Ib of the reactor 22 gradually increases.

When the inertial current due to the energy release of the inductance Lr becomes zero at time t4, the discharge current of the capacitor 9 becomes the capacitor 9, the second switching element 11,
It flows in the path of the primary winding 4, the reactor 2, and the power supply 1. Although the second switching element 11 is ON-controlled from the time point t3 in FIG. 4, it actually functions from the time point t4, and therefore may be ON-controlled up to now.

When the second switching element 11 is turned off at time t5, the primary winding 4, the reactor 2, the power source 1, and the capacitor 8 are formed by the energy accumulated in the inductor Lr of the reactor 2 and the primary winding 4. A resonance circuit is formed, the charge of the capacitor 8 is discharged, and the FE
The voltage Vq1 at T5 gradually decreases. Since a downward voltage is generated in the secondary winding 13 during the charge discharge period of the capacitor 8, the diode 23 is in a conductive state, and a part of the energy discharged by the capacitor 8 is transferred to the reactor 22. When the discharge of the electric charge of the capacitor 8 is completed, a current tends to flow in the direction for reversely charging the capacitor 8 based on the stored energy of the inductance Lr. However, since the diode 7 becomes conductive, the voltage of the capacitor 8, that is, the FET 5 The voltage Vq1 is kept substantially at zero volts. t
At time point 6, the control signal Vg1 for the first switching element 6 is again generated.
Is given and this is turned on. At this time, since the voltage Vq1 of the FET 5 is zero volt, turn-on with less loss is achieved. After the time point t6, the same operation as the period from t0 to t6 is repeated.

[0026]

[Second Embodiment] Next, a switching power supply device according to a second embodiment shown in FIG. 5 will be described. However. In FIG. 5 and FIG. 6 to be described later, parts that are substantially the same as those in FIG.

In FIG. 5, the second capacitor 9 is the second FE.
It is connected in parallel to the reactor 2 and the primary winding 4 via T10. The rest of FIG. 5 is the same as FIG.
Vg1, Vg2, Vq1, Iq1, Vs, Is showing the state of each part
, Ib, Iq2, Vc and Vq2 are substantially the same as in FIG. However, the value of the voltage Vc of the capacitor 9 is Vf lower than that in FIG.

The operation of the first switching element 6 during the ON period is the same as that of the circuit shown in FIG. When the first switching element 6 is off, the resonance of the inductor Lr of the reactor 2, the primary winding 4 and the first capacitor 8 causes the voltage Vq1 of the capacitor 8 to gradually rise, and zero volt switching is achieved. After that, the diode 12 becomes conductive with the flyback voltage Vf, and the capacitor 9 is charged with the flyback voltage Vf. Since the capacity of the capacitor 9 is sufficiently larger than that of the capacitor 8, the flyback voltage Vf
The voltage Vc hardly changes even by the charging by.
When the charging of the capacitor 9 by the flyback voltage is completed, the capacitor 9, the switching element 11 and the primary winding 4
A closed circuit of the reactor 2 and the reactor 2 forms a discharge circuit of the capacitor 9. After that, when the switching element 11 is turned off, the resonance operation similar to that of the circuit of FIG. 1 occurs, and the turn-on of zero volt switching is achieved. Since the operation of the circuit of FIG. 5 is substantially the same as that of FIG. 1, the same effect can be obtained.

[0029]

[Third Embodiment] The switching power supply device shown in FIG.
The circuit of FIG. 1 has two FETs 30, 34 and a capacitor 37.
And are added. FET is switching element 3
1 and a built-in diode 32, and a capacitor 33 composed of a parasitic capacitance, and is connected between the power supply 1 and the reactor 2. Capacitor 37 is FET3 via FET34
0 connected in parallel. The FET 34 includes a switching element 35, a built-in diode 36, and a parasitic capacitance (not shown). The gate of the switching element 31 is connected to the first control signal line 25 similarly to the gate of the switching element 6. The gate of the switching element 35 is connected to the second control signal line 26 similarly to the switching element 11.

The switching elements 6 and 31 are simultaneously turned on by the first control signal Vg1 shown in FIG. 7 during the period from t0 to t1. As a result, the current Iq1 flows through the circuit including the power supply 1, the switching element 31, the reactor 2, the primary winding 4, and the switching element 6. The operation of the output stage of the secondary winding 13 during the period from t0 to t1 is the same as that in FIG.

When the switches 6 and 31 are turned off at t1 in FIG. 7, a series resonance circuit of the inductor Lr of the reactor 2 and the primary winding 4 and the two capacitors 8 and 33 is formed, and the two capacitors 8 and 33 are formed. Charging begins. After that, at time t2, the voltage of the two capacitors 8 and 33 becomes 1/2 of the input voltage Vin. Although only the voltage Vq1 and current Iq1 of the capacitor 8 are shown in FIG. 7, the voltage and current of the capacitor 33 also change.
The voltage of the capacitors 8 and 33 rises as a sine wave due to resonance, and the current flowing through the switching elements 6 and 31 is the waveform of the current Iq1 shown in FIG. 7 with the shaded portion in the period t1 to t2 removed. Therefore, the current flowing through the switching elements 6 and 31 is zero, and the power loss of the switching elements 6 and 31 is reduced.

From t2 to t3, the capacitors 8 and 33 are charged by the inertia of the current flowing through the inductance Lr of the reactor 2 and the primary winding 4, and the voltage Vq1 is Vin /
It becomes 2 + Vf / 2.

When the voltage of the capacitors 8 and 33 becomes higher than the voltage of the capacitors 9 and 37 at time t3, the diodes 12 and 36 become conductive, and the capacitors 37 and 3 are turned on.
6, the current Ic flows through the closed circuit of the reactor 2, the primary winding 4, the diode 12, the capacitor 9 and the power supply 1. This current Ic becomes smaller as the discharge of the stored energy of the inductance Lr progresses, becomes zero at the time t4, and the diodes 12 and 36 become non-conductive. However, since the switching elements 11 and 35 are on-controlled, the capacitors 9, 37 are a closed circuit including the capacitor 9, the switching element 11, the primary winding 4, the reactor 2, the switching element 35, the capacitor 37, and the power supply 1. A discharge is generated and a reset voltage is applied to the primary winding 4.

When the switching elements 11 and 35 are turned off at time t5, the two capacitors 9 and 37 having large capacitances are connected.
Is separated, and a resonance circuit of two capacitors 8 and 33 having a smaller capacity than this and an inductance Lr is formed, and a closed circuit consisting of the capacitor 8, the primary winding 4, the reactor 2, the capacitor 33, and the power supply 1 is formed. 8, 33
Are discharged, and the voltage Vq1 across both ends gradually decreases. When the voltage of the capacitors 8 and 33 becomes zero volt and then the current for charging in the opposite direction flows, the diodes 7 and 32
Pass through. Therefore, when the switching elements 6 and 31 are turned on again at t6, this voltage Vq1 is zero volt, and no switching loss occurs. The operation of the output stage of the secondary winding 13 is substantially the same as that of the first embodiment, and similar operational effects can be obtained.

[0035]

MODIFICATION The present invention is not limited to the above-described embodiments, and the following modifications are possible, for example. (1) The diodes 7, 12, 32, and 36 can be independent diodes instead of the built-in diodes of the FET. (2) As the capacitors 8 and 33, independent capacitors can be connected without using the parasitic capacitance of the FETs 5 and 30. (3) Instead of the FETs 5 and 30, a combination circuit of a switching element such as a bipolar transistor, a thyristor, or a junction FET, a diode, and a capacitor can be used. (4) When the inductance of the primary winding 4 is large, the reactor 2 can be omitted. (5) It is possible to delay the rising of the control pulse shown in FIG. 3C at time t4.

[0036]

As is apparent from the above, according to the invention of each claim, zero volt switching can be easily achieved and the efficiency can be improved.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a switching power supply device according to a first embodiment.

FIG. 2 is a block diagram showing the control circuit of FIG. 1 in detail.

FIG. 3 is a waveform diagram showing a state of each part of FIG.

FIG. 4 is a waveform diagram showing a state of each part of FIG.

FIG. 5 is a circuit diagram showing a switching power supply device according to a second embodiment.

FIG. 6 is a circuit diagram showing a switching power supply device according to a third embodiment.

7 is a waveform chart showing a state of each part of FIG.

[Explanation of symbols]

 1 Power Supply 2 Reactor 3 Transformer 4 Primary Winding 5, 10 FET 6, 11 Switching Element 7, 12 Diode 8, 9 Capacitor 22 Reactor

Claims (3)

[Claims] 1. A DC power supply, a primary winding of a transformer connected between one end and the other end of the DC power supply, and a first switching element connected in series with the primary winding. A first diode formed of a diode integrally formed with the first switching element or an independent diode and connected in parallel to the first switching element; and a parasitic capacitance of the first switching element or an independent diode. A first capacitor connected in parallel to the first switching element, a resonance inductance including an inductance of the primary winding and / or an independent inductance element, Connected in parallel to the switching element via the second switching element, and has a larger capacitance than the first capacitor. A second capacitor that is formed integrally with the second switching element or an independent diode, and that is connected in parallel to the second switching element; A secondary winding, a rectifying / smoothing circuit connected to the secondary winding, and a first switching element for controlling ON / OFF of the first switching element.
Second control element for controlling to turn on the second switching element between a time point after the start pulse of the off period of the first switching element and a time point before the end time of the off period of the first switching element. Switch control circuit for forming a control pulse of the secondary winding, a reactor connected in parallel to the secondary winding, a reactor connected in series to the reactor, and a rectifying and smoothing circuit based on an induced voltage in the secondary winding. And a third diode having a direction in which it is forward-biased by an induced voltage in the secondary winding during a period in which no current flows. 2. A DC power supply, a primary winding of a transformer connected between one end and the other end of the DC power supply, and a first switching element connected in series to the primary winding. A first diode formed of a diode integrally formed with the first switching element or an independent diode and connected in parallel to the first switching element; and a parasitic capacitance of the first switching element or an independent diode. A first capacitor connected in parallel with the first switching element, a resonance inductance including an inductance of the primary winding and / or an independent inductance element, and the primary An electrostatic capacitance that is connected in parallel to the winding and the resonance inductance via a second switching element and has a larger electrostatic capacitance than the first capacitor. A second capacitor having a capacitance and a diode integrally formed with the second switching element or an independent diode, and a second capacitor connected in parallel to the second switching element. A diode, a secondary winding of the transformer, a rectifying / smoothing circuit connected to the secondary winding, and a first switching element that controls ON / OFF of the first switching element.
Second control element for controlling to turn on the second switching element between a time point after the start time of the off period of the first switching element and a time point before the end time of the off period of the first switching element. Switch control circuit for forming a control pulse of the secondary winding, a reactor connected in parallel to the secondary winding, a reactor connected in series to the reactor, and a rectifying and smoothing circuit based on an induced voltage in the secondary winding. And a third diode having a direction in which it is forward-biased by an induced voltage in the secondary winding during a period in which no current flows. 3. A DC power supply, a primary winding of a transformer connected between one end and the other end of the DC power supply, and a connection between one end of the DC power supply and one end of the primary winding. A first switching element, a second switching element connected between the other end of the primary winding and the other end of the DC power supply, and integrally formed with the first switching element. A first diode which is composed of a diode or an independent diode and is connected in parallel to the first switching element, and a parasitic capacitance of the first switching element or an independent capacitor, which is parallel to the switching element A first capacitor connected to the second switching element, and a diode integrally formed with the second switching element or an independent diode, and connected in parallel to the second switching element. A second diode connected to the second switching element in parallel with the second diode, a parasitic capacitance of the second switching element or an independent capacitor, and an inductance of the primary winding. And / or an independent inductance element, which is connected in series with the first and second switching elements in series, and a resonance inductance with respect to the first switching element via a third switching element. A third capacitor connected in parallel and having a larger capacitance than that of the first capacitor; and a diode integrally formed with the third switching element or an independent diode. A third diode connected in parallel with the second switching element, and with respect to the second switching element A fourth capacitor connected in parallel via four switching elements and having a larger electrostatic capacitance than the second capacitor; and a diode or an independent diode formed integrally with the fourth switching element. A fourth diode formed of a diode and connected in parallel to a fourth switching element; a secondary winding of the transformer; a rectifying / smoothing circuit connected to the secondary winding; And a first control pulse for on / off controlling the second switching element, a time point after the start time point of the off period of the first and second switching elements and a time point before the end time point of the off period. Between the third and fourth
A switch control circuit for forming a second control pulse for ON-controlling a switching element of the switch, a reactor connected in parallel with the secondary winding, and a reactor connected in series with the reactor and the secondary winding. A fifth diode having a direction that is forward biased by the induced voltage in the secondary winding during a period in which no current flows in the rectifying / smoothing circuit based on the induced voltage in the switching circuit. Power supply.