JPH11262253A - Switching power unit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the conversion efficiency of a switching power unit and to surely operate the zero-voltage switch of the unit. SOLUTION: A switching power unit 1 alternately executes first switching operations, by which an output current is discharged from a secondary winding 2b via a capacitor 11 in a closed circuit 18 incorporating the winding 2b, by switching its input DC via a primary winding 2a by setting a first switching element 3 to a turned-on state, and second switching operations by which the capacitor 11 is charged by making the current discharged from a transformer 2 to the closed circuit 18, by turning on the switching element 15 after turning off the first switching element 3. After the remaining energy has been discharged, regenerated power is regenerated to the supplying source side of the input DC, by making a discharged current in the direction opposite to the current discharged from the capacitor 11 to flow into the closed circuit 18 via the second switching element 15. The regenerated electric energy is controlled by controlling the turning-on time of the second switching element 15.

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 for generating a DC voltage by switching.
More specifically, the present invention relates to a switching power supply device that generates a DC voltage by a so-called zero volt switch system while increasing a utilization rate of a secondary winding of a transformer.

[0002]

2. Description of the Related Art A power supply device 41 shown in FIG. 6 is conventionally known as a switching power supply device for generating a DC voltage by a zero volt switch system while increasing the utilization rate of a secondary winding of a transformer of this type. The power supply device 41 includes a transformer 2 and an FE that is turned on / off by a switching control signal SS output from a switching control circuit (not shown) on the primary winding 2a side of the transformer 2.
T3 and a capacitor 4 equivalently connected in parallel with the FET3. It should be noted that the capacitor 4 is provided with a parasitic capacitance of the FET 3 or an FET separately from the parasitic capacitance.
3 comprises a capacitor connected in parallel.
Further, the power supply device 41 includes, for example,
Capacitor 11, capacitor 11 and secondary winding 2b
12 connected in series in a closed circuit 42 including
And a choke coil 13 and a capacitor 14 for smoothing.
And

In the power supply device 41, when the FET 3 is turned on, a current I11 flows in the primary winding 2a in the direction shown in FIG. At this time, the current I13 returning from the winding start terminal of the secondary winding 2b to the winding end terminal via the load and the current I14 returning to the winding end terminal via the capacitor 14, as shown in FIG. And the current I12 of
As a result, the output voltage VO of the device is generated, and the capacitors 11 and 14 are charged. In this case, as shown in the figure, the capacitor 11 has a positive potential on the secondary winding 2b side and a negative potential on the diode 12 side. Further, the transformer 2 has a gap formed in its internal core, and also functions as an inductance. Therefore, energy is stored in the transformer 2 by the flow of the current I11. Next, when the FET 3 is turned off, a current I15 based on the stored energy of the transformer 2 and the stored energy of the capacitor 11 is released through the secondary winding 2b. At this time, the current I15 in the direction shown in FIG.
By flowing through a closed circuit 42 including a diode 12, a capacitor 11, and a winding start terminal of the secondary winding 2b,
The capacitor 11 is discharged. In this state, the stored energy still remains in the capacitor 11, and the polarity of the capacitor 11 maintains the polarity shown in FIG.

Next, when all the energy stored in the transformer 2 is released, a discharge current I16 based on the charge energy of the capacitor 11 flows in the same direction as the current I15. At this time, the regenerative current I17 flows in the primary winding 2a in the direction shown in FIG. As a result, the energy stored in the capacitor 4 is regenerated to the input DC supply source side, so that the voltage across the capacitor 4 reaches 0V or near 0V. Therefore, by controlling the FET 3 to be in the ON state in this state, a zero volt switch is achieved, so that the power loss and the noise generated due to the short circuit of the capacitor 4 during the switching by the FET 3 are reduced. Then, F
When the ET3 is controlled to the ON state, the current I12 is output from the secondary winding 2b in the same manner as the above-described operation. As described above, in the power supply device 41, the current value flowing through the secondary winding 2b is averaged by flowing the current through the secondary winding 2b both when the FET 3 is turned on and off, whereby the secondary winding 2 The use rate of the transformer 2 is improved to reduce the size of the transformer 2 and to realize a zero volt switch of the FET 3.

[0005]

However, the conventional power supply device 41 has the following problems. First, in the power supply device 41, in order to realize the zero volt switch of the FET 3, the discharge current I16 based on the energy stored in the capacitor 11 is caused to flow through the secondary winding 2b and the diode 12, so that the input DC Regenerated on the supply side of In this case, the amount of power regenerated on the supply source side is determined by the energy stored in the capacitor 11 and the amount of F
The timing is determined by the timing when the ET3 is controlled to the ON state. On the other hand, the FET 3 is controlled to be turned on at a timing such that the output voltage VO becomes a constant voltage. Therefore, the regenerative power is not always large enough to maintain the voltage between both ends of the capacitor 4 connected in parallel to the FET 3 at 0V. For this reason, the regenerative power more than necessary for the zero volt switch of the FET 3 is regenerated to the input DC supply source side, thereby lowering the conversion efficiency of the device, or conversely, the regenerative power is too small to make the zero volt switch reliable. There is a problem that it is not performed. Secondly, in the power supply device 41, when the current I15 flows from the capacitor 11 via the secondary winding 2b, or when the discharge current I16 flows to flow the regenerative current I17 to the primary winding 2a, I15 and 16 flow through the diode 12. Therefore, power is lost by the diode 12 at that time. For this reason, the power supply device 41 has a problem that the conversion efficiency of the device is reduced and the cost of the device is increased due to the heat radiation of the diode 12.

The present invention has been made to solve such a problem, and has as its main object to provide a switching power supply capable of improving the conversion efficiency of the device and ensuring the zero volt switch.

[0007]

According to a first aspect of the present invention, there is provided a switching power supply device for controlling an input direct current through a primary winding of a transformer by controlling a first switching element to an on state. Thereby, a first switching operation of discharging an output current from the secondary winding via a capacitor connected in series in a closed circuit formed by the second switching element including the secondary winding of the transformer, and After controlling the first switching element to the off state, the second switching element is controlled to the on state to form a closed circuit, and the capacitor is charged by flowing an emission current based on the residual energy of the transformer into the closed circuit, In addition, the discharge current discharged from the capacitor in the opposite direction to the discharge current after the residual energy is released
A switching power supply device that alternately executes a second switching operation of regenerating regenerative power based on a discharge current to a supply side of an input DC through a primary winding by flowing the regenerative power based on a discharge current through a switching circuit through a switching element. Accordingly, the on-time of the second switching element is controlled to control the amount of regenerative power regenerated to the supply source side.

In this switching power supply, first, an output current is output from the secondary winding by switching the first switching element. In this case,
The DC voltage is supplied to the load, and the capacitor in the closed circuit is charged. Next, after the first switching element is controlled to be in the off state, the second switching element is controlled to be in the on state. At this time, an emission current based on the remaining energy of the transformer flows out of the secondary winding into the closed circuit. In this case, the emission current passes through the second switching element and charges the capacitor. This capacitor has a positive potential on the side to which the current flows, and maintains this polarity thereafter. In this case, while the emission current flows through the diode in the conventional power supply device 41, the emission current flows in the second switching element in this switching power supply device. By using such a method, it is possible to reduce the power loss at that time. Next, when the residual energy of the transformer has been released, the capacitor outputs a discharge current in a direction opposite to the discharge current to the closed circuit through the second switching element and the secondary winding.

Next, the closed circuit is cut off by controlling the second switching element to an off state. In this state, the transformer excited by the current flowing through the secondary winding tries to release its excitation energy,
Release at least to the primary winding side. At this time, the regenerative current flows through the primary winding of the transformer toward the input DC supply source, so that the voltage between both ends of the capacitor equivalently connected in parallel to the first switching element is reduced. In this state, the first switching element is controlled to be on. This achieves zero volt switching. In this case, in this power supply device, by controlling the time during which the second switching element is turned off, the time during which the discharge current of the capacitor is output can be controlled. Therefore, the amount of excitation energy of the transformer is controlled, whereby the amount of regenerative power regenerated to the input DC supply source side can be controlled. For this reason, energy necessary and sufficient for zero volt switching of the first switching element can be regenerated to the primary winding side. As a result, it is possible to ensure the zero volt switch while preventing unnecessary regeneration of regenerative power to the primary winding side. In addition, the conversion efficiency of the device can be improved by optimizing the regenerative power and reducing the power loss by the second switching element.

The switching power supply according to claim 2 is
2. The switching power supply device according to claim 1, further comprising a unidirectional element connected in parallel with the second switching element and capable of passing an emission current immediately after the first switching element is controlled to be turned off. And

There is no need to connect a diode in parallel with the second switching element as long as the second switching element can be controlled immediately after the first switching element is turned off. However, in such a configuration, if the timing of controlling the second switching element to the on state is too early, the current discharged from the secondary winding when the first switching element is controlled to the on state is closed. The short circuit phenomenon that flows in the circuit occurs. On the other hand, if the timing of controlling the second switching element to be in the off state is too late, a situation occurs in which a closed circuit for discharging the remaining energy of the transformer is not formed.
In this power supply device, the current based on the remaining energy of the transformer passes through the diode connected in parallel to the second switching element until the first switching is controlled to the off state and a closed circuit is formed. Through the closed circuit. Next, when the second switching element is controlled to be turned on, the current flows through the closed circuit via the second switching element. Therefore, the timing for controlling the second switching element to be in the on state only needs to be after the first switching element has been controlled to be in the off state, which simplifies the control and short-circuits in the closed circuit. The phenomenon can be reliably prevented.

The switching power supply according to claim 3 is
3. The switching power supply device according to claim 1, wherein the second switching element is constituted by a field effect transistor, and is controlled to be in an on state in a state where energy charged in an internal capacitance of the second switching element is emitted by an emission current of the transformer. It is characterized by that.

In the case where the FET as the second switching element is controlled to be turned on, if energy is accumulated in a noise removing capacitor or a parasitic capacitor connected to the FET in parallel, the FET is controlled to be turned on. When power is lost, power is lost due to shorting of these capacitors. In this power supply device, when the first switching element is controlled to be turned off and a current based on the remaining energy of the transformer flows in the closed circuit, the internal diode of the FET or the unidirectional element connected in parallel to the internal diode is turned off. By discharging the residual energy through the capacitor, the voltage across these capacitors is reduced, and then the second switching element is controlled to the ON state. For this reason, the second switching element can be controlled to be in the ON state by the zero volt switch method, so that the power loss due to the second switching element can be reduced.

According to a fourth aspect of the present invention, there is provided a switching power supply.
The switching power supply according to any one of claims 1 to 3, wherein an off-timing for the second switch is controlled based on an input DC voltage value.

The timing for controlling the second switch means to be in the off state can employ various methods. On the other hand, when a capacitor is equivalently connected in parallel to the first switching element, the voltage across the capacitor increases as the input DC voltage increases. In this switching power supply device, for example, when the input DC voltage is high, the ON time of the second switching element is controlled to be long, and when the input DC voltage value is low, the ON time is controlled to be short. That is, when the input DC voltage is high, the regenerative current for discharging the capacitor through the primary winding of the transformer is controlled to be relatively large. As a result, the voltage across the capacitor can be reliably reduced to 0 V or near 0 V, so that it is possible to regenerate an appropriate amount of regenerative power to the primary winding while ensuring the zero volt switch. .

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A preferred embodiment of a switching power supply according to the present invention will be described below with reference to the accompanying drawings. Note that the same components as those of the power supply device 41 are denoted by the same reference numerals, and redundant description will be omitted.

As shown in FIG. 1, a power supply device 1 includes a FET 3 corresponding to a first switching element in the present invention and connected in series to a primary winding 2a of a transformer 2,
3 includes a capacitor 4 and a diode 5 connected in parallel to each other. The diode 5 is an FET
3 and a diode called a so-called body diode or a parasitic diode, or an independent diode connected in parallel to the FET 3 separately from the body diode and the like. In this case, F
ET3 is controlled by a switching signal SS output from a switching control circuit (not shown), and its switching duty ratio and frequency are feedback-controlled to stabilize the output voltage VO to a voltage value E2.

A capacitor 1 is connected to the secondary winding 2b.
1, a diode 12, a choke coil 13 and a capacitor 14 are provided in the same manner as in the power supply device 41, and the FET 15 is connected in parallel to the diode 12 and is connected to the diode 12 in parallel with the second switching element in the present invention. It comprises a capacitor 16 and a control circuit 17 for controlling on / off of the FET 15 according to the voltage value of the input DC input voltage VIN. Note that the diode 12
Corresponds to the unidirectional element of the present invention,
, A diode called a so-called body diode or a parasitic diode, or an independent diode connected in parallel to the FET 15 separately from the body diode and the like. The capacitor 16 is constituted by a parasitic capacitance existing inside the FET 15 or a capacitor connected in parallel to the FET 15 separately from the parasitic capacitance.

Next, the operation of the power supply device 1 will be described with reference to FIG.

In the power supply device 1, in the normal state after the power is turned on, as shown in FIGS. 2A and 2B, when the control signal SC is maintained at the low level and the FET 15 is in the off state at the time t1, the high state occurs. Level switching control signal SS
Is output, the FET 3 is controlled to be turned on. At this time, the current I1 flowing through the primary winding 2a is initially 0 A or a slightly negative current value, and gradually increases with time, as shown in FIG.
When the current I1 flows through the primary winding 2a, a current I2 substantially proportional to the current I1 is output from the secondary winding 2b of the transformer 2 as shown in FIG. The transformer 2 is excited by energy corresponding to. In this case, the current value of the current I2 is the sum of the current value of the current I3 supplied to the load as the output voltage VO and the current value of the current I4 for charging the capacitor 14, as shown in FIG. Further, the capacitor 11 has a large capacity, and in a normal state, the drain side of the FET 15 is charged to a positive potential, and the secondary winding 2b side is charged to a negative potential. For this reason, when the current I2 flows, a part of the charging energy of the capacitor 11 is output to the load or the capacitor 14 together with the energy released from the transformer 2, but the voltage across the capacitor 11 is small due to its large capacity. Only to drop. On the other hand, since the current I3 flows through the choke coil 13, the current value becomes a triangular-wave current whose current value gradually increases, and is supplied to the load.
, Part of the energy is stored as the excitation energy of the choke coil 13.

Next, at a time t2, as shown in FIG. 2A, a low-level switching control signal SS is output to control the FET 3 to be turned off, and immediately thereafter, the FET 3 is turned off. As shown in (1), the high-level control signal SC is output from the control circuit 17 so that the FE
T15 is controlled to the ON state. At this time, an emission current I5 is output from the secondary winding 2b based on the excitation energy remaining in the transformer 2. In this case, when the FET 15 has not reached the ON state, the emission current I5 is, as shown in FIG. 1, the winding end terminal of the secondary winding 2b, the diode 12, the capacitor 11, and the winding start of the secondary winding 2b. The capacitor 11 is charged through a closed circuit 18 including terminals. On the other hand, after the FET 15 is controlled to be turned on, the emission current I5 is output from the closed end terminal of the secondary winding 2b, the source and drain of the FET 15, the capacitor 11, and the winding start terminal of the secondary winding 2b. The capacitor 11 is charged through the circuit 18. In this case, by flowing the emission current I5 through the diode 12 first, the FET 15 can be controlled to be in the ON state by the zero volt switch. As a result, power loss at that time and noise generation are reduced. ing.
In this case, both ends of the secondary winding 2b are connected to the capacitor 1
1 is clamped to a voltage almost equal to the voltage across
The drain of FET 3 which is also the voltage across capacitor 4-
The source-to-source voltage VDS has a voltage value represented by the following equation, as shown in FIG. In the equation, E
1, E3, V2b, N1 and N2 are the voltage value of the input voltage VIN, the voltage across the secondary winding 2b when the secondary winding 2b is clamped, the number of turns of the primary winding 2a, and It means the number of turns of the next winding 2b. VDS = E1 + E3 = E1 + V2b × (N1 / N2) formula

During the period from time t2 to time t3, the energy stored in the choke coil 13 is also gradually released. At this time, the current IL released from the choke coil 13 is applied to the choke coil 13 and the capacitor 14. The current flows through a current path including the positive electrode and the negative electrode, the source and the drain of the FET 15, and the choke coil 13, and the current waveform has a triangular waveform whose current value gradually decreases. As a result, as shown in FIG.
The current I8 flowing through the ET 15 is a combined current of the emission current I5 emitted from the secondary winding 2b and the current IL emitted from the choke coil 13.

Next, as the exciting energy remaining in the transformer 2 decreases, the current I5 also decreases. When energy is completely released from transformer 2, transformer 2
, The voltage induced in the secondary winding 2b drops to 0V.
In this state, the capacitor 11 is used as a constant voltage source.
As shown in FIG.
A discharge current I6 in the direction opposite to the emission current I5 starts to flow in a closed circuit 18 including the FET 15, the secondary winding 2b, and the negative electrode of the capacitor 11. In this state, since the current value of the current IL emitted from the choke coil 13 is also reduced, the current I8 flowing through the FET 15 changes from a minus direction (a direction opposite to the current I6) to a plus direction (a direction of the current I6). Gradually increase. In this case, the choke coil 13
Of the current IL by the transformer 2
5 ends before the capacitor 14
The current I8 flowing through the FET 15 becomes positive earlier because the current flows in the opposite direction from the current IL through the path through the positive electrode, the choke coil 13, the FET 15 and the negative electrode of the capacitor 14. That is, in the FET 15,
In some cases, the emission current I5 emitted from the transformer 2 or the discharge current I6 output from the capacitor 11 and the current IL emitted from the choke coil 13 or the current output from the capacitor 14 cancel each other. Therefore, the emission of the emission current I5 by the transformer 2 and the emission of the current IL by the choke coil 13 are simultaneously terminated, and the timing when the discharge current I6 flows from the capacitor 11 and the current in the opposite direction to the current IL from the capacitor 14 are By making the flow start at the same time,
As a result, the current I8 flows in the FET 15 in the same direction. As a result, the current I8 can be set in the plus direction without the need to supply an extra current to the FET 15, and the conversion efficiency of the device can be prevented from lowering.

Next, at time t3 when the current I8 turns slightly in the positive direction (the direction of the current I6), the control circuit 17 outputs the low-level control signal SC to control the FET 15 to the off state. Is done. In this case, the control circuit 17 outputs a low-level control signal SC at a timing corresponding to the level of the voltage value E1 of the input voltage VIN. That is, if the voltage value E1 is high, the period from time t2 to time t3 is lengthened, and if it is low, the period is shortened. At this time, since the path of the discharge current I6 is cut off, the transformer 2 excited by the discharge current I6
In order to release the stored energy, as shown in FIG. 1 and FIG. 2D, the winding start terminal of the primary winding 2a, the positive and negative potential sides of the input DC, the source and drain of the FET 3, and The current path formed by the winding end terminal of the primary winding 2a passes through the regenerative current I9
Flows. For this reason, as shown in FIG. 2C, the drain-source voltage VDS of the FET 3, which is the voltage across the capacitor 4 equivalently connected in parallel to the first switching element, is reduced. At this time, the diode 5 prevents the voltage VDS between the drain and the source of the FET 3 from becoming a negative voltage, so that the breakdown voltage of the FET 3 is prevented. At the same time as the regenerative current I9 flows, the current I10 based on the energy stored in the transformer 2 is:
It flows through a path including the winding start terminal of the secondary winding 2b, the capacitor 11, the choke coil 13, the capacitor 14, and the winding end terminal of the secondary winding 2b. As a result, energy to be output to the load is stored in the capacitor 14.

Next, while the voltage across the capacitor 4 is reduced, the high-level switching control signal S
The output of S causes the FET 3 to be turned on. This ensures zero volt switching. In this case, in this power supply device 1, the FET 15
By controlling the timing of time t3, which is the timing at which the capacitor 11 is turned off, the discharge current I6 from the capacitor 11 is controlled.
Release time can be controlled. In other words, it is possible to control the amount of regenerative power regenerated to the input DC supply source side. For this reason, energy necessary and sufficient for zero-volt switching of the FET 3 can be regenerated to the primary winding 2a side. Therefore, the zero volt switch can be reliably performed while preventing unnecessary regeneration of the regenerative power to the primary winding 2a side. These processes are repeatedly executed to generate an output voltage VO.

As described above, according to the power supply device 1,
The control circuit 17 determines whether the input DC voltage E1
By controlling the off timing of the ET15, the FE
The energy necessary and sufficient to perform the zero volt switch of T3 is regenerated to the primary winding 2a side. For this reason,
As a result, it is possible to prevent unnecessary power from being regenerated to the primary winding 2a side. As a result, it is possible to improve the conversion efficiency of the device while ensuring the zero volt switch.
In order to regenerate electric power to the primary winding 2a side, the FET
Since the currents I5 and I6 flow through the power supply 15, the conversion efficiency of the device can be further improved as compared with the case where the current flows through the diode in the conventional power supply device 41, and heat dissipation measures are required. Cost can be reduced.

It should be noted that the present invention is not limited to the above-described embodiments of the present invention, and the configuration thereof can be changed as appropriate. Specifically, the power supply device 1 employs the equivalent basic circuit shown in FIG. 3, but the present invention is not limited to this. For example, as shown in FIG. Basically, a capacitor 11 can be connected between the source of the FET 15 and the secondary winding 2b. In addition, as shown in FIG.
It can also be connected between the source of T15 and the capacitor 14.

Furthermore, in the embodiment of the present invention, an example in which FETs are used as the first and second switch means has been described. However, the present invention is not limited to this, and various switching elements such as transistors are used. be able to.

[0029]

As described above, according to the switching power supply device of the first aspect, the on-time of the second switching element is controlled to control the amount of regenerative power regenerated to the input DC supply source side. Thereby, the first switching element can be reliably switched by the zero volt switch method. In addition, by supplying a current to the second switching element, power loss at that time can be reduced, and energy necessary and sufficient for zero volt switching of the first switching element is regenerated to the primary winding side. As a result, the conversion efficiency of the device can be improved.

Further, according to the switching power supply device of the second aspect, the unidirectional element which is connected in parallel with the second switching element and can pass the emission current immediately after the first switching element is controlled to be turned off. By having,
The control for controlling the second switching element to be turned off can be simplified, and the short circuit phenomenon in the closed circuit can be reliably prevented.

Further, according to the switching power supply device of the third aspect, the energy stored in the internal capacitance or the like of the field effect transistor as the second switching element is released by the discharge current of the transformer. By controlling to the ON state, power loss due to the second switching element can be reduced, and
Generation of noise can be reduced.

According to the switching power supply device of the fourth aspect, the second switch means is controlled to be in the off state based on the input DC voltage value, thereby ensuring the zero volt switch for the first switching element. And a proper amount of regenerative power can be regenerated to the primary winding side, so that the conversion efficiency of the device can be improved.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of a power supply device according to an embodiment of the present invention.

2A and 2B are diagrams for explaining the operation of the power supply device according to the embodiment of the present invention, wherein FIG. 2A is a voltage waveform diagram of a switching control signal SS, and FIG. 2B is a voltage waveform diagram of a control signal SC; (C) is a voltage waveform diagram of the drain-source voltage VDS of the FET 3, and (d) is a current I1, flowing through the primary winding 2a.
Ie is a current waveform diagram, (e) is a current waveform diagram of currents I2, I5, I6, and I10 flowing through the secondary winding 2b, and (f) is an FET.
FIG. 14 is a current waveform diagram of a current I8 flowing through the reference numeral 15;

FIG. 3 is an equivalent basic circuit diagram of the power supply device according to the embodiment of the present invention.

FIG. 4 is an equivalent circuit diagram of a power supply device in which the equivalent basic circuit diagram shown in FIG. 3 is modified.

FIG. 5 is an equivalent circuit diagram of another power supply device in which the equivalent basic circuit diagram shown in FIG. 3 is modified.

FIG. 6 is a circuit diagram of a conventional power supply device.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Power supply device 2 Transformer 2a Primary winding 2b Secondary winding 3 FET 4 Capacitor 11 Capacitor 12 Diode 15 FET 17 Control circuit

Claims (4)

[Claims] A first switching element that is turned on to switch an input direct current through a primary winding of a transformer, thereby forming a second switching element including a secondary winding of the transformer. A first switching operation for discharging an output current from the secondary winding via a capacitor connected in series in a closed circuit, and the second switching device after controlling the first switching device to an off state. Is controlled to an on state to form the closed circuit, and the capacitor is charged by flowing an emission current based on the residual energy of the transformer into the closed circuit, and the capacitor is charged after the residual energy is released. Discharge current discharged in the opposite direction to the discharge current from the closed circuit through the second switching element. A switching power supply device that alternately performs a second switching operation of regenerating regenerative power based on the discharge current to the input DC supply source side through the primary winding, the second power supply device comprising: A switching power supply device, wherein an on-time of a switching element is controlled to control an amount of regenerative electric power regenerated to the supply source side. 2. A device according to claim 1, further comprising a unidirectional element connected in parallel to said second switching element and capable of passing said emission current immediately after said first switching element is turned off. The switching power supply device according to claim 1. 3. The second switching element is constituted by a field effect transistor, and is controlled to be in an on state in a state where energy charged in an internal capacitance is released by an emission current of the transformer. The switching power supply device according to claim 1 or 2, wherein 4. The switching power supply according to claim 1, wherein an off-timing of said second switch is controlled based on a voltage value of said input direct current.