JPH11318075A - Transformer insulating type dc-to-dc converter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce switching losses and noise, and improve efficiency in a transformer insulating type DC-DC converter. SOLUTION: This transformer insulating type DC-DC converter is provided with a diode 11, whose one end is connected with a connecting point of a primary winding 2a of a transformer 2 and a transistor 3, a capacitor 12 for resonance which is connected between the other end of the diode 11 and a cathode terminal of a DC power source 1, a primary winding 13a of an auxiliary transformer 13 and an auxiliary transistor 14, which are connected in series between the connecting point of the capacitor 12 for resonance and the diode 11, and a secondary winding 13b of the auxiliary transformer 13 and a diode 15, which are connected in series between an anode terminal and a cathode terminal of the DC power source 1. The auxiliary transistor 14 is turned on or off simultaneously with the transistor 3. Discharging energy of the capacitor 12 for resonance is regenerated in the DC power source 1 via the auxiliary transformer 13 and the diode 15, when the auxiliary transistor 14 is turned on.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a transformer-isolated DC-DC converter capable of reducing switching loss and noise and improving efficiency.

[0002]

2. Description of the Related Art A DC power supply, a primary winding of a transformer, and a switching element are connected in series, and the switching element is turned on and off, so that a DC power supply is supplied from a secondary winding of the transformer via a rectifying and smoothing circuit. Transformer-isolated type D that supplies a DC output of a constant voltage different from the voltage of
2. Description of the Related Art C-DC converters have been widely used in power supply circuits of electronic devices and the like. For example, the conventional transformer-insulated DC-DC converter shown in FIG. 6 includes a DC power supply 1 such as a battery or a capacitor input type rectifier circuit and a primary winding 2a.
A transformer 2 having a secondary winding 2b, a primary winding 2a of the transformer 2 connected in series to both ends of the DC power supply 1, and a transistor 3 as a switching element;
A rectifier smoothing circuit consisting of rectifier diodes 4 and a smoothing capacitor 5 is connected to the secondary winding 2b, a load 6 connected in parallel to the smoothing capacitor 5, a control pulse signal V _B to the base terminal of the transistor 3 Granting Transistor 3
And a control circuit 7 for turning on and off the. Although not specifically shown, the control circuit 7 includes an oscillation circuit section for generating a triangular wave voltage having a constant period, an error amplification circuit section for arithmetically amplifying an error voltage of the terminal voltage of the load 6 with respect to the reference voltage, and an error amplification circuit section. a comparing circuit for comparing the error output voltage and the triangular wave voltage of the oscillator circuit portion of the circuit, and generates a control pulse signal V _B proportional to the time width of the output voltage of the comparator circuit is applied to the base terminal of the transistor 3 And a control pulse generation circuit section. This transformer-insulated DC-
In the DC converter, the control circuit 7 controls the transistor 3
The pulse width of the control pulse signal V _B for the grant to the base terminal is changed according to the terminal voltage of the load 6, the transistor 3
By controlling the ON / OFF period of the DC power supply 1, a DC output V _O having a constant voltage different from the voltage E of the DC power supply 1 is supplied to the load 6.

[0003]

[SUMMARY OF THE INVENTION Incidentally, in the trans-insulated DC-DC converter shown in FIG. 6, the collector of the transistor 3 as shown in FIG. 7 at the time of turn-on and turn-off of the transistor 3 - emitter voltage waveform V _CE large switching losses based on overlapping portion W of the collector current waveform I _C of the transistor 3 is a drawback to occur. Further, since the rise of the collector-emitter voltage waveform V _CE and the collector current waveform I _C of the transistor 3 is steep, spike-shaped surge voltages V _sr and surge current I _sr are generated, and noise based on these is generated. There were drawbacks. Therefore, power loss inside the converter increases due to the occurrence of switching loss, noise, and the like, and the efficiency of the converter decreases.

Accordingly, the present invention provides a transformer-isolated DC that can reduce switching loss and noise and improve efficiency.
-To provide a DC converter.

[0005]

According to a first aspect of the present invention, there is provided a transformer-insulated DC-DC converter in which a DC power supply, a primary winding of a transformer, and a switching element are connected in series. By performing the on / off operation, a DC output of a constant voltage different from the voltage of the DC power supply is supplied from the secondary winding of the transformer via the rectifying and smoothing circuit to the load. In the transformer-insulated DC-DC converter, a first rectifier element having one end connected to a connection point between the primary winding of the transformer and the switching element;
A resonance capacitor connected between the other end of the first rectifier element and the other end of the DC power supply; a connection point between the resonance capacitor and the first rectifier element; A primary winding and an auxiliary switching element of an auxiliary transformer connected in series between the secondary winding and a second winding of the auxiliary transformer connected in series between one end and the other end of the DC power supply. A rectifier element, wherein the auxiliary switching element is turned off from the on state at the same time as or earlier than when the switching element is turned off from the on state,
The switching element is turned on from the off state simultaneously or later when the switching element is turned on from the off state,
When the auxiliary switching element changes from the off state to the on state, the resonance capacitor charged during the off period of the switching element is discharged toward the primary winding of the auxiliary transformer. The voltage generated in the secondary winding is regenerated to the DC power supply via the second rectifier.

When the auxiliary switching element is switched from the on state to the off state at the same time as or before the switching element is switched to the off state while the switching element is on, the current flowing from the primary winding of the transformer to the switching element. Is switched to the current flowing through the resonance capacitor via the first rectifying element, and the resonance capacitor is charged in a sine wave shape. As a result, the voltage at both ends of the switching element rises in a sine wave form from 0 V, so that when the switching element is turned off, zero voltage switching (ZVS) is performed, and the switching loss is reduced. Further, when the switching element is turned on from the off state, the current flowing through the switching element increases at a gentle slope from 0 due to the leakage inductance of the transformer, so that when the switching element is turned on, zero current switching (ZCS) occurs and switching loss occurs. Is reduced. As described above, the switching loss at the time of the ON / OFF operation of the switching element can be reduced.
At the same time, the spike-like surge voltage and current generated when the switching element is turned off and turned on are
The resonance action of the resonance capacitor and the leakage inductance of the transformer is absorbed and the falling and rising of the voltage and current waveforms of the switching element become gentle, so that noise based on the surge voltage and current can be reduced. When the auxiliary switching element is turned on from the off state at the same time as or after the turning on of the switching element, the resonance capacitor charged during the off period of the switching element is discharged toward the primary winding of the auxiliary transformer. As a result, the voltage generated in the secondary winding of the auxiliary transformer is regenerated to the DC power supply via the second rectifier. Therefore, when the auxiliary switching element is turned on, the discharge energy of the resonance capacitor is regenerated to the DC power supply via the auxiliary transformer and the second rectifying element, so that the power loss generated inside the converter is reduced and the efficiency of the converter is reduced. Can be improved.

Furthermore, in the transformer-insulated DC-DC converter according to the second aspect of the present invention, a current limiting reactor is connected in series with the primary winding of the transformer. In this transformer-isolated DC-DC converter, when the switching element is turned on, the surge current flowing from the primary winding of the transformer to the transistor is absorbed by the self-inducing action of the current limiting reactor, so that the transformer has no leakage inductance. Even in the case of a simple transformer, zero current switching is ensured, and switching loss and noise at the time of turning on the switching element can be more reliably reduced as compared with the invention according to claim 1.

According to a third aspect of the present invention, in the transformer-insulated DC-DC converter, the first rectifying element having one end connected to a connection point between the primary winding of the transformer and the switching element; A capacitor for resonance connected between the other end of the first rectifying element and the other end of the DC power supply; and between a connection point between the resonance capacitor and the first rectifier and the other end of the DC power supply. A primary winding and an auxiliary switching element of an auxiliary transformer connected in series to a secondary winding and a second rectifying element of the auxiliary transformer connected in series between a pair of output lines of the rectifying and smoothing circuit. Wherein the auxiliary switching element is changed from the on state to the off state at the same time as or earlier than when the switching element is changed from the on state to the off state, and the switching element is changed from the off state to the on state. Then, at the same time or later, the state is changed from the off state to the on state, and when the auxiliary switching element is changed from the off state to the on state, the resonance capacitor charged during the off period of the switching element is connected to one of the auxiliary transformers. The secondary winding of the auxiliary transformer is discharged toward the secondary winding.
Is supplied to the load through the rectifying element.

When the auxiliary switching element is switched from the on state to the off state at the same time as or before the switching element is switched to the off state with the switching element turned on, the current flowing from the primary winding of the transformer to the switching element. Is switched to the current flowing through the resonance capacitor via the first rectifying element, and the resonance capacitor is charged in a sine wave shape. As a result, the voltage at both ends of the switching element rises in a sine wave form from 0 V, so that zero-voltage switching is performed when the switching element is turned off, and the switching loss is reduced. Also,
When the switching element is changed from the off state to the on state, the current flowing through the switching element increases at a gentle slope from 0 due to the leakage inductance of the transformer, so that zero current switching is performed when the switching element is turned on, and the switching loss is reduced. As described above, the switching loss at the time of the ON / OFF operation of the switching element can be reduced. At the same time, the spike-like surge voltage and current generated when the switching element is turned off and turned on are absorbed by the resonance action of the leakage inductance of the resonance capacitor and the transformer, and the falling and rising of the voltage and current waveforms of the switching element are gradual. Therefore, noise based on the surge voltage and the current can be reduced. Further, when the auxiliary switching element is turned on from the off state at the same time as or after the turning on of the switching element, the capacitor charged during the off period of the switching element is discharged toward the primary winding of the auxiliary transformer, Thereby, the voltage generated in the secondary winding of the auxiliary transformer is supplied to the load via the second rectifier. Therefore, when the auxiliary switching element is turned on, the discharge energy of the resonance capacitor is supplied to the load via the auxiliary transformer and the second rectifying element, so that the power loss generated inside the converter is reduced and the efficiency of the converter is improved. can do.

Further, in the transformer-insulated DC-DC converter according to claim 4 of the present invention, a current limiting reactor is connected in series with the primary winding of the transformer. In this transformer-isolated DC-DC converter, when the switching element is turned on, the surge current flowing from the primary winding of the transformer to the transistor is absorbed by the self-inducing action of the current limiting reactor, so that the transformer has no leakage inductance. Even in the case of a simple transformer, zero current switching is ensured, so that switching loss and noise at the time of turning on the switching element can be reduced more reliably than in the invention according to claim 3.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a transformer-insulated DC-DC converter according to the present invention will be described below with reference to FIGS. However, in FIG. 1, substantially the same portions as those shown in FIG. 6 are denoted by the same reference numerals, and description thereof will be omitted. As shown in FIG. 1, the transformer-isolated DC-DC converter according to the present embodiment includes a primary winding 2a of a transformer 2.
And a diode 11 as a first rectifying element having an anode terminal (one end) connected to a connection point of the transistor 3;
A resonance capacitor 12 connected between the cathode terminal (the other end) of the diode 11 and the cathode terminal (the other end) of the DC power supply 1; a connection point between the resonance capacitor 12 and the diode 11; Are connected in series between the primary winding 13a of the auxiliary transformer 13 and the auxiliary transistor 14 as an auxiliary switching element, and between the anode terminal (one end) and the cathode terminal of the DC power supply 1. A secondary winding 13b of the auxiliary transformer 13 and a diode 15 as a second rectifier are provided. As the transformer 2 and the auxiliary transformer 13, a leakage transformer having a leakage inductance is used. Further, in the control circuit 7, when the control pulse signal V _{B1 applied} to the base terminal of the transistor 3 changes from a high level to a low level, the control pulse signal voltage V _B2 applied to the base terminal of the auxiliary transistor 14 becomes high. The control pulse signal voltage V _B2 applied to the base terminal of the auxiliary transistor 14 when the control pulse signal voltage V _B1 applied to the base terminal of the transistor 3 changes from the low level to the high level. An auxiliary transistor control circuit unit (not shown) for switching from a high level to a high level is added. As a result, the auxiliary transistor 14 changes from the on state to the off state at the same time as the transistor 3 changes from the on state to the off state, and changes from the off state to the on state at the same time as the transistor 3 changes from the off state to the on state. Other configurations are substantially the same as those of the transformer-insulated DC-DC converter of FIG.

Next, a transformer-insulated DC-D shown in FIG.
The operation of the C converter will be described. FIG. 2 (A) and
As shown in (B), when both the transistor 3 and the auxiliary transistor 14 are on before t ₁ , a current I ₀ flows through the primary winding 2 a of the transformer 2 and the transistor 3. At this time, a voltage having the same polarity as the voltage of the primary winding 2a is induced in the secondary winding 2b of the transformer 2 and the rectifier diode 4 is in a conductive state.
From b, the smoothing capacitor 5 is charged via the rectifier diode 4 and DC power is supplied to the load 6. On the other hand, before t ₁ , the resonance capacitor 12 is not charged, and the voltage at both ends is 0V.

As shown in FIGS. 2A and 2B, the control pulse signal voltages V _B1 and V _B2 applied from the control circuit 7 to the base terminals of the transistor 3 and the auxiliary transistor 14 at t ₁ are both high. When the transistor 3 and the auxiliary transistor 14 simultaneously change from the on state to the off state, the current I _{0 of the} primary winding 2 a of the transformer 2 flowing through the transistor 3 is supplied via the diode 11 to the resonance capacitor. The current is switched to the current flowing through No. 12. Therefore, the current I _TR1 flowing through the transistor 3 is
It becomes 0 as shown in (D). At this time, the current flowing through the resonance capacitor 12 increases sinusoidally, and the resonance capacitor 12 is charged with the polarity shown in FIG. 1, and the voltage of the resonance capacitor 12 rises sinusoidally from 0V. Thus, as shown in FIG. 2C, the voltage V _TR1 across the transistor 3 increases in a sine wave form from 0V. For this reason, when the transistor 3 is turned off, zero voltage switching is performed with little overlap between the voltage waveform and the current waveform. At the same time, as shown in FIG. 2 (E), the voltage V _TR2 across the auxiliary transistor 14 rises in a sine wave form from 0V.
The auxiliary transistor 14 also performs zero voltage switching when turned off. At t ₂ , when the resonance capacitor 12 is charged to the voltage E of the DC power supply 1 and the voltage V _TR1 across the transistor 3 becomes equal to the voltage E of the DC power supply 1 as shown in FIG. Secondary winding 2b
Becomes 0 V, the rectifier diode 4 is reverse-biased and becomes non-conductive, and the charge charged in the smoothing capacitor 5 during the ON period of the transistor 3 is discharged to the load 6.

As shown in FIGS. 2A and 2B, at t ₃ , the control pulse signal voltages V _B1 and V _B2 applied from the control circuit 7 to the base terminals of the transistor 3 and the auxiliary transistor 14 are both low. When the transistor 3 and the auxiliary transistor 14 simultaneously change from the off state to the on state, the voltage V _TR1 across the transistor 3 and the auxiliary transistor 14 as shown in FIGS. , V _TR2 immediately drop to 0V. At this time, the primary winding 2a of the transformer 2 is connected to the transistor 3
As shown in FIG. 2D, the current I _TR1 flowing through the transistor 2 increases at a gentle slope from 0 due to the leakage inductance of the transformer 2, so that when the transistor 3 is turned on, the overlap between the voltage waveform and the current waveform is small, and zero current switching is performed. Become. At the same time, the resonance capacitor 12 starts discharging toward the primary winding 13a of the auxiliary transformer 13, and the resonance capacitor 12 and the leakage inductance of the auxiliary transformer 13 resonate, so that the resonance capacitor 12 and the auxiliary transformer 13 A resonance current flows in a closed circuit formed by the primary winding 13a and the auxiliary transistor 14. As a result, the current I _TR2 flowing through the auxiliary transistor 14 increases sinusoidally from 0 as shown in FIG. 2 (F), so that the auxiliary transistor 14 also performs zero current switching at the time of turn-on. Further, a voltage is generated in the secondary winding 13b by the resonance current flowing through the primary winding 13a of the auxiliary transformer 13, and
The voltage generated in the next winding 13b is regenerated to the DC power supply 1 via the diode 15. Thus, when the auxiliary transistor 14 is turned on, the discharge energy of the resonance capacitor 12 is regenerated to the DC power supply 1 via the auxiliary transformer 13 and the diode 15, so that no power loss occurs when the resonance capacitor 12 is discharged. In t _4, the current I _TR1 flowing through the transistor 3 is equal to the current I ₀ of the primary winding 2a of the transformer 2 as shown in FIG. 2 (D), 1 of the transformer 2 is later from the DC power source 1 Tsugimaki A current I ₀ flows through the line 2a and the transistor 3. As a result, a voltage having the same polarity as the voltage of the primary winding 2a is induced in the secondary winding 2b of the transformer 2, and the rectifier diode 4 is turned on.
The smoothing capacitor 5 is charged via the
Is supplied with DC power.

As described above, in this embodiment, when the transistor 3 and the auxiliary transistor 14 are turned off and turned on, zero voltage switching and zero current switching are performed, so that switching loss can be reduced. Further, a spike-shaped surge voltage and a surge current generated when the transistor 3 and the auxiliary transistor 14 are turned off and on are absorbed by the resonance action of the resonance capacitor 11 and the leakage inductance of the transformer 2 and the auxiliary transformer 13, and Since the rise and fall of the voltage and current waveforms of the auxiliary transistor 14 become gradual, it is possible to reduce the noise based on the surge voltage and surge current at the time of the on / off operation of each of the transistor 3 and the auxiliary transistor 14. Further, when the auxiliary transistor 14 is turned on, almost all of the discharge energy of the resonance capacitor 12 is regenerated to the DC power supply 1, so that the resonance capacitor 12
No power loss occurs at the time of discharging the power, and the power loss occurring inside the converter can be reduced to improve the efficiency of the converter.

The transformer-insulated DC of the embodiment shown in FIG.
-The DC converter can be changed. For example, the transformer-insulated DC-DC converter of the embodiment shown in FIG. 3 is the transformer-insulated DC-DC converter shown in FIG. 1 in which a current limiting reactor 16 is connected in series with the primary winding 2a of the transformer 2. It is. In this case, an ideally manufactured transformer having no leakage inductance may be used as the transformer 2. Other configurations are substantially the same as those of the transformer-insulated DC-DC converter shown in FIG. Therefore, in the transformer-insulated DC-DC converter of the embodiment shown in FIG. 3, substantially the same operation and effect as in FIG. 1 can be obtained. Further, in the transformer-insulated DC-DC converter shown in FIG. 3, when the transistor 3 is turned on, a surge current flowing from the primary winding 2a of the transformer 2 to the transistor 3 is absorbed by the self-inducing action of the current limiting reactor 16, and The current I _TR1 flowing through 3 gradually increases from 0. Therefore, even when the transformer 2 is an ideal transformer having no leakage inductance, no surge current is generated. Therefore,
Zero current switching at the time of turning on the transistor 3 can be more reliably performed, and switching loss and noise at the time of turning on the transistor 3 can be further reduced.

In the transformer-insulated DC-DC converter of the embodiment shown in FIG. 4, the connection position of the series circuit of the secondary winding 13b and the diode 15 of the auxiliary transformer 13 in the embodiment of FIG. This is changed between a pair of output lines with the smoothing capacitor 5. Other configurations are the same as those of the transformer-isolated DC-
It is almost the same as a DC converter. In the transformer-isolated DC-DC converter shown in FIG.
At turn-on, the resonance capacitor 12 -the primary winding 13a of the auxiliary transformer 13 -the auxiliary transistor 14
A voltage is generated in the secondary winding 13b of the auxiliary transformer 13 by the resonance current flowing in the closed circuit formed by the above, and the voltage generated in the secondary winding 13b is supplied to the load 6 via the diode 15. Thereby, when the auxiliary transistor 14 is turned on, the discharge energy of the resonance capacitor 12 is supplied to the load 6 via the auxiliary transformer 13 and the diode 15, so that the discharge energy of the resonance capacitor 12 is discharged as in the case shown in FIG. No power loss occurs. Other operations are substantially the same as those in the embodiment shown in FIG. Therefore, in the embodiment shown in FIG. 4, substantially the same effects as in the embodiment shown in FIG. 1 can be obtained. Also, the transformer-isolated DC-DC converter shown in FIG. 4 can be modified in the same manner as shown in FIG. 3 as shown in FIG. Therefore, in the transformer-insulated DC-DC converter of the embodiment shown in FIG. 5, substantially the same operation and effect as in the case of FIG. 3 can be obtained.

The embodiments of the present invention are not limited to the above embodiments, and various modifications are possible. For example, in the above embodiments, the transistor 3 and the auxiliary transistor 14 are turned off and turned on at the same time. However, as shown by the broken line in FIG. 2B, the auxiliary transistor 14 is turned off before the transistor 3 is turned off. If the auxiliary transistor 14 is turned on after the transistor 3 is turned on, the same operation and effect can be obtained. Further, in each of the above-described embodiments, the form applied to the forward type DC-DC converter in which the rectifier diode 4 is in the conductive state when the transistor 3 is in the ON period is shown. However, the embodiments of FIGS. The present invention can also be applied to a flyback type DC-DC converter in which the rectifier diode 4 is in a non-conductive state when the transistor 3 is on. Further, in each of the above embodiments, a mode in which a bipolar transistor is used as a switching element has been described. It does not matter.

[0019]

According to the present invention, the switching loss and the noise can be reduced and the power loss generated inside the converter can be reduced, so that the conversion efficiency of the transformer-isolated DC-DC converter can be greatly improved. Become.

[Brief description of the drawings]

FIG. 1 shows a transformer insulation type D showing an embodiment of the present invention.
Electric circuit diagram of C-DC converter

FIG. 2 is a waveform chart showing voltages and currents of respective parts of the circuit of FIG.

FIG. 3 is an electric circuit diagram showing a modified embodiment of the circuit of FIG. 1;

FIG. 4 is an electric circuit diagram of a transformer-insulated DC-DC converter showing another embodiment of the present invention.

FIG. 5 is an electrical diagram showing a modified embodiment of the circuit of FIG.

FIG. 6 is an electric circuit diagram showing a conventional transformer-insulated DC-DC converter.

FIG. 7 is a waveform chart showing an overlapping portion of a switching voltage waveform and a switching current waveform of the circuit of FIG. 6;

[Explanation of symbols]

1. . . DC power supply, 2. . . Transformer, 2a. . . Primary winding, 2b. . . 2. secondary winding; . . 3. transistors (switching elements); . . Rectifier diode, 5. . .
5. smoothing capacitor; . . Load, 7. . . Control circuit, 1
1. . . 11. diode (first rectifying element), . . 12. resonance capacitor; . . Auxiliary transformer, 13
a. . . Primary winding, 13b. . . Secondary winding, 14. . . 14. auxiliary transistor (auxiliary switching element); . .
15. diode (second rectifying element); . . Current limiting reactor

 ──────────────────────────────────────────────────の Continuation of the front page (72) Inventor Seiya Fukumoto 3-6-3 Kitano, Niiza-shi, Saitama Sanken Electric Co., Ltd.

Claims (4)

[Claims] 1. A DC power supply, a primary winding of a transformer, and a switching element are connected in series, and the switching element is turned on and off so that a secondary winding of the transformer is connected via a rectifying and smoothing circuit. In a transformer-insulated DC-DC converter for supplying a DC output of a constant voltage different from the voltage of a DC power supply to a load, a first rectifier having one end connected to a connection point between a primary winding of the transformer and the switching element. An element, a resonance capacitor connected between the other end of the first rectifying element and the other end of the DC power supply, the resonance capacitor and the first capacitor.
A primary winding and an auxiliary switching element of an auxiliary transformer connected in series between a connection point of the rectifying element and the other end of the DC power supply, and a serial connection between one end and the other end of the DC power supply A secondary winding of the auxiliary transformer and a second rectifier element, wherein the auxiliary switching element is turned off from the on state at the same time as or earlier than when the switching element is turned off from the on state, The switching element is turned on from the off state simultaneously or later when the switching element is turned on from the off state, and when the auxiliary switching element is turned on from the off state, the auxiliary element is charged during the off period of the switching element. The resonance capacitor is discharged toward the primary winding of the auxiliary transformer, so that the voltage generated in the secondary winding of the auxiliary transformer is reduced. Trans-insulated DC-DC converter, characterized in that regenerated to the DC power source through the second rectifying element. 2. The transformer-insulated DC-DC converter according to claim 1, wherein a current limiting reactor is connected in series with a primary winding of the transformer. 3. A DC power supply, a primary winding of a transformer, and a switching element are connected in series, and the switching element is turned on and off so that the secondary winding of the transformer is connected via a rectifying and smoothing circuit. In a transformer-insulated DC-DC converter for supplying a DC output of a constant voltage different from the voltage of a DC power supply to a load, a first rectifier having one end connected to a connection point between a primary winding of the transformer and the switching element. An element, a resonance capacitor connected between the other end of the first rectifying element and the other end of the DC power supply, the resonance capacitor and the first capacitor.
And a primary winding and an auxiliary switching element of an auxiliary transformer connected in series between a connection point of the rectifying element and the other end of the DC power supply, and a series connection between a pair of output lines of the rectifying and smoothing circuit. The secondary winding of the auxiliary transformer and the second
A rectifying element, wherein the auxiliary switching element is changed from the on state to the off state at the same time or earlier than when the switching element is changed from the on state to the off state, and simultaneously or simultaneously when the switching element is changed from the off state to the on state. Thereafter, when the auxiliary switching element is turned on from the off state, the resonance capacitor charged during the off period of the switching element is connected to the primary winding of the auxiliary transformer when the auxiliary switching element is turned on from the off state. A voltage generated in a secondary winding of the auxiliary transformer is supplied to the load via the second rectifying element.
-DC converter. 4. The transformer-isolated DC-DC converter according to claim 3, wherein a current limiting reactor is connected in series with the primary winding of the transformer.