KR20180127173A - Isolated switching power supply for three-phase AC - Google Patents

Abstract

The present invention relates to an isolated switching power source, into which three-phase AC is inputted, capable of performing efficient power factor improvement and power conversion by means of a simple configuration. The power source includes: input terminals (R, S, T); positive electrode and negative electrode output terminals (P, N); three transformers (Tr, Ts, Tt) each of which the primary coil is connected to each of the input terminals; three switching elements (Qr, Qs, Qt) which allow or block conduction through the current paths between the other end of the primary coils of the three transformers and an input-side reference potential terminal; a sub-capacitor connected between one end of the secondary coils of the three transformers and the negative electrode output terminal; rectifier elements (D1, D2, D3) connected between the other end of the secondary coil of the three transformers and the positive electrode output terminal; rectifier elements (D4, D5, D6) connected between the other end of the secondary coils of the three transformers and the negative electrode output terminal; a smoothing capacitor; and rectifier elements (D7, D8, D9) connected between the input-side reference voltage terminal E and the input terminals (R, S, T).

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an isolated switching power supply for three-

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an insulated switching power supply for converting a three-phase alternating current into a direct current.

Conventionally, an insulated converter is known as a switching power supply for converting an alternating current into a direct current. Various schemes are proposed, but a configuration in which a DC / DC converter is disposed after AC / DC conversion is generally performed by smoothing not only single-phase and three-phase but also an alternating-current voltage by a rectifying smoothing capacitor through a rectifying circuit Patent Documents 1 to 7). In order to improve the power factor, a two-stage configuration in which a power factor correction device (PFC) and a DC / DC converter are combined is also known. Patent Documents 6 and 7 disclose an apparatus configured to perform boosting and power factor correction for three-phase alternating current output of an alternator of a wind power generator.

[Patent Literature]

- Patent Document 1: Japanese Patent Application Laid-Open No. 7-31150

- Patent Document 2: JP-A-8-331860

- Patent Document 3: Japanese Patent Laid-Open No. 2002-10632

- Patent Document 4: Japanese Patent Application Laid-Open No. 2005-218224

- Patent Document 5: Japanese Patent Application Laid-Open No. 2007-37297

- Patent Document 6: JP-A-2013-128379

- Patent Document 7: JP-A-2014-23286

There has been a problem that the circuit becomes complicated when the switching power supply having the existing power factor improving function is made to have a two-stage configuration. In addition, the forward converter typically requires an external choke coil in addition to the transformer.

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and it is an object of the present invention to provide an insulated switching power supply to which three-phase alternating current is inputted.

In order to achieve the above object, the present invention provides the following configuration. The reference numerals in parentheses are used for reference in the following drawings.

According to an aspect of the switching power supply of the present invention,

(a) first, second and third input terminals (R, S, T) to which three-phase alternating current is inputted,

(b) an anode output terminal P and a cathode output terminal N,

(c) a primary coil Lr1, Ls1, Lt1 and secondary coils Lr2, Ls2, Lt2, one end of each primary coil being connected to the first, second and third input terminals R, S Second, and third transformers Tr, Ts, Tt connected to the first, second, and third transformers T,

(d) Each current path between the other end of each of the primary coils Lr1, Ls1, Lt1 of the first, second, and third transformers Tr, Ts, Tt and the input- Second, and third switching elements Qr, Qs, and Qt that are on and off controlled by one control signal Vg to turn on or off the switching elements Q1, Q2,

(C1) connected between one end of each of the secondary coils Lr2, Ls2 and Lt2 of the first, second and third transformers Tr, Ts and Tt and the negative output terminal N, ),

(f) is connected between the other end of each of the secondary coils (Lr2, Ls2, Lt2) of the first, second and third transformers (Tr, Ts, Tt) and the anode output terminal Second, and third rectifying elements D1, D2, and D3 for conducting currents flowing from the other end of the secondary coil to the anode output terminal P,

(g) connected between the other end of each of the secondary coils (Lr2, Ls2, Lt2) of the first, second and third transformers Tr, Ts, Tt and the negative output terminal N, Fourth, fifth and sixth rectifying elements D4, D5 and D6 for conducting currents flowing from the negative output terminal N to the other end of the secondary coil,

(h) a smoothing capacitor (C2) connected between the positive output terminal (P) and the negative output terminal (N), and

(i) is connected between the input-side reference potential stage (E) and each of the first, second and third input terminals (R, S, T) Seventh, eighth and ninth rectifying elements D7, D8 and D9 for conducting the currents respectively flowing through the first, second and third input terminals R, S and T, respectively.

According to the present invention, an isolation type switching power supply is provided which is capable of efficient power factor correction and power conversion by means of a simple constitution in which three-phase alternating current is input, while improving the efficiency of the transformer.

1 is a schematic diagram showing an example of a circuit configuration for an embodiment of a switching power supply according to the present invention.
FIGS. 2A and 2B are diagrams for explaining the power factor improving action by the input three-phase alternating current and the switching operation.
FIG. 3 is a diagram schematically showing a current flow in the On-period in the T-mode of the circuit configuration shown in FIG.
Fig. 4 is a diagram schematically showing the potential relationship in the On-period in the secondary side of the circuit configuration shown in Fig.
5 is a diagram schematically showing a current flow in the Off-period in the T-mode of the circuit configuration shown in FIG.
6 is a diagram schematically showing the potential relationship in the Off-period in the secondary side of the circuit configuration shown in Fig.

Hereinafter, an embodiment of the switching power supply according to the present invention will be described with reference to the drawings showing embodiments.

(1) Circuit configuration

BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a diagram schematically showing an example of a circuit configuration for an embodiment of an insulated switching power supply according to the present invention; FIG.

The switching power supply of the present invention is a power conversion device that receives a three-phase AC power output from an AC generator and outputs DC power to a load. For example, in wind power generation, when the windmill rotates and the axis of the alternator rotates, three-phase alternating-current power is output from the three-phase stator coil Y-connected in the alternator.

The switching power supply of the present invention is a power conversion device and also has a function as a power factor improving device. The power factor correction device aims at making the waveform of the input current sinusoidal waveform equal to the input voltage and matching the phases to make the power factor 1.

The switching power supply of the present invention is of an insulation type that electrically insulates the input side and the output side. To this end, three transformers (Tr, Ts, Tt) corresponding to each phase are provided. The three transformers (Tr, Ts, Tt) each have one primary coil and one secondary coil. It is preferable that the three transformers (Tr, Ts, Tt) have the same electromagnetic characteristics, and it is preferable to use a three-phase reactor. Lr1, Ls1 and Lt1 denote the primary coils of the respective transformers, and Lr2, Ls2 and Lt2 denote the secondary coils of the respective transformers.

The start and end of winding of each coil are indicated by black circles. The term "one end" and "tartan" as used herein mean both a case where the combination of the "winding start end" and the "winding end" and a case where the combination of "winding end" and "winding start end" .

One end (start winding end in this example) of the primary coils Lr1, Ls1 and Lt1 of the transformer on the input side is connected to a first input terminal R, a second input terminal S, And the third input terminal T, respectively. In the present application, each phase of three-phase alternating current is referred to as R-phase, S-phase, and T-phase. And reference symbol E denotes an input-side reference potential stage.

The secondary side of the transformer is provided with a positive output terminal (P) and a negative output terminal (N) which are two terminals for outputting a DC voltage. The negative output terminal N is a secondary-side reference potential terminal. An output voltage is applied to a load (not shown) connected between the positive output terminal P and the negative output terminal N, and an output current flows.

One end of each of the three switching elements Qr and Qs Qt is connected to the other end (winding end in this example) of the primary coils Lr1, Ls1 and Lt1 of each transformer. The other end of each switching element Qr, Qs, Qt is connected to the input-side reference potential terminal E. Each of the switching elements Qr, Qs and Qt has a control terminal and each of the control terminals conducts a current path between the other terminal of the primary coils Lr1, Ls1 and Lt1 and the input- Respectively.

The respective control terminals of the three switching elements Qr and Qs Qt are controlled by a common control signal Vg. The control signal Vg is, for example, a PWM signal having a pulse waveform of a predetermined frequency and an impact coefficient. That is, the three switching elements Qr, Qs, and Qt are controlled to be always turned on and off simultaneously. In the illustrated example, the switching elements Qr, Qs and Qt are n-channel type MOSFETs (hereinafter referred to as "FET Qr", "FET Qs" and "FET Qt"), one end being the drain, the other end being the source, to be. In this case, the control signal Vg is a voltage signal.

Further, switching elements Qr, Qs, and Qt other than FETs, such as IGBTs and bipolar transistors, may be used.

In the secondary side, one capacitor (hereinafter referred to as a " secondary winding ") is provided between one end (winding start end in this example) of the secondary coils Lr2, Ls2, and Lt2 of each transformer and a negative output end N Quot; capacitor ") C1 are connected. A smoothing capacitor C2 is connected between the positive output terminal P and the negative output terminal N. [

The first, second and third diodes D1 and D2, which are examples of rectifying elements, are provided between the other end (winding end in this example) of the secondary coils Lr2, Ls2 and Lt2 of each transformer and the anode output terminal P, , And D3 are connected to one another. Each anode of the diodes D1, D2 and D3 is connected to the other end of each of the secondary coils Lr2, Ls2 and Lt2 and each cathode is connected to the anode output terminal P. [ The diodes D1, D2 and D3 conduct currents respectively flowing from the other ends of the secondary coils Lr2, Ls2 and Lt2 of the respective transformers to the anode output terminals P in the case of forward bias, .

It is preferable that the diodes D1, D2, and D3 operate at a high speed with a small forward voltage drop.

The fourth, fifth and sixth diodes D4, D5 and D6, which are examples of rectifying elements, are connected between the other end of the secondary coils Lr2, Ls2 and Lt2 of each transformer and the negative output terminal N, It is connected. The cathodes of the diodes D4, D5 and D6 are connected to the other ends of the secondary coils Lr2, Ls2 and Lt2, and the respective anodes are connected to the negative output terminal N. [ The diodes D4, D5 and D6 conduct currents respectively flowing from the negative output terminal N to the other ends of the secondary coils Lr2, Ls2 and Lt2 of the respective transformers in case of forward bias, do.

In the circuit of Fig. 1, the input-side reference potential stage E and the seventh and eighth embodiments, which are examples of the rectifying elements, are provided between the first input terminal R, the second input terminal S and the third input terminal T, 8th diode (D7, D8, D9) are connected, respectively. Each cathode of the diodes D7, D8 and D9 is connected to the input-side reference potential terminal E and each anode is connected to each of the first, second and third input terminals R, S and T, respectively. Each of the diodes D7, D8 and D9 conducts currents flowing respectively to the first, second and third input terminals R, S and T from the input-side reference potential terminal E in the case of forward bias, In the case of bias, each is shut off.

Also, although not shown, it includes a control unit for generating a control signal Vg. For example, the control unit detects the magnitude of the input voltage and the DC output voltage, determines the impact coefficient of the control signal Vg based on the detected input voltage and the output voltage, and then generates the control signal Vg based thereon. As a main part of the control unit, it is preferable to use PWMIC.

The PWM IC outputs a pulse-shaped control signal Vg having a constant impact coefficient, for example, by inputting a DC signal having a constant voltage corresponding to the determined one impact coefficient and a carrier triangular wave signal having a constant frequency to the comparator. In the present invention, such a control signal Vg is referred to as a " control signal having a constant impact coefficient ". This control signal is an example.

(2) Description of operation

The operation of the circuit configuration shown in Fig. 1 will be described with reference to Figs. 2 to 6. Fig. Except for the transient operation at the time of starting and stopping the circuit, the operation in the case where the circuit is in the normal state will be described.

(2-1) Three-phase AC input and power factor improvement

First, referring to FIGS. 2A and 2B, the power factor improving operation by the switching operation for the three-phase AC input will be described.

FIG. 2A shows the voltage waveforms for the R-phase, S-phase, and T-phase of the input three-phase alternating current. The phase that becomes the highest potential and the phase that becomes the lowest potential are sequentially changed every 120 degrees of phase. The frequency of the three-phase alternating current is, for example, about several Hz to 100 Hz for an alternator of a wind power generator. On the other hand, the switching frequency, that is, the frequency of the control signal Vg in Fig. 1 is several kHz to several hundreds kHz, which is sufficiently higher than the frequency of the three-phase alternating current.

For example, a period in which the T-phase becomes the lowest potential (referred to as "T-mode") will be described. The period during which the R-phase becomes the lowest potential (referred to as "R-mode") and the period during which the S-phase becomes lowest potential (referred to as "S-mode") are the same and will not be described herein.

In the T-mode, the R-phase becomes the highest potential in the first half of the period, and the S-phase becomes the highest potential in the latter half. In FIG. 1, the input current flows due to the phase-to-phase voltage of the positive and negative electric potentials of the positive electric potential and the electric current does not flow during the off-period during the ON-period of the FETs Qr and Qs Qt.

In the following description of the operation, as an example, a case where the input current flows by the RT phase voltage or the ST phase voltage from the R-phase or S-phase of the highest electric potential to the T-phase of the lowest electric potential in the On-period The operation is the same for other cases, so the explanation thereof is omitted).

Here, the phase-to-phase voltage between the R-phase and the T-phase is referred to as "RT phase-to-phase voltage" Further, for example, the input current flowing by the RT phase voltage vrt is denoted by "angle ".

The power factor improving operation will be described with reference to FIG. 2B shows a waveform of the PWM control signal Vg at the point A on the time axis t in FIG. 2A, the RT phase-to-phase voltage vrt and the phase difference between the first input terminal R and the third input terminal T On-current (angular) is schematically shown. Since the switching frequency is sufficiently high compared to the frequency of the three-phase alternating current, the RT-phase voltage (vrt) in one On-period can be regarded as a constant voltage in pulse form. Therefore, the value of the on-current (angular) time point is determined by the RT phase-to-phase voltage vrt and the inductance L on the current path between the first input terminal R and the third input terminal T, = vrt / L? (where? is the switching frequency). The current (irt) increases linearly in the On-period. In the Off-period, the current (irt) becomes zero.

Since the inductance L on the current path and the switching frequency? Are integers, the value at the time point of the current in the On-period is the instantaneous value of the RT-phase voltage vrt at the time of the On- ). Since the instantaneous value of the RT phase voltage vrt is located on the locus of the sinusoidal wave, the current flowing through the primary coil in the On-period also plots the locus of the sinusoidal wave. This means that the input current is sinusoidal with the same phase as the input voltage. Therefore, improvement of the power factor of the primary side is realized.

In this circuit, the current path including the inductance applied with the phase-to-phase voltage of the three-phase AC is conducted and blocked by using the PWM control signal having the constant frequency and the duty factor to obtain the sinusoidal input current whose phase matches the input voltage .

(2-2) Detailed description of the operation of the primary and secondary sides of the On-period

3 schematically shows a current flow (dotted line with an arrow) for the On-period of the time point in the T-mode in the circuit configuration shown in Fig. 1 (for example, in the vicinity of point A in Fig. 2).

[On-period: primary side]

On the trans primary side, when the control signal Vg becomes On voltage in the On-period, both the FET Qr, the FET Qs, and the FET Qt are turned On and the current path becomes conductive.

In the primary coil of the transformer (Tr), the input current (irt) flows to the following path by the RT phase voltage (vrt).

- input current (irt): first input terminal (R) -> transformer (Tr) primary coil -> FET Qr -> diode (D9)

The input current ist flows to the next path by the ST-phase voltage vst at the primary coil of the transformer Ts.

- Input current (ist): Second input S → Trans (Ts) Primary coil → FET Qs → Diode (D9) → Third input (T)

Here, the current that flows through the diode D9 and returns to the input side, such as the current returning from the third input terminal T to the input side, is referred to as the next "reflux." If there is no diode D9, the reflux is returned from the FET Qt to the third input T through the primary coil of the transformer Tt. When the reflux flows through the primary coil of the transformer Tt, a power loss occurs. In this circuit, since the diode D9 is provided, the primary winding of the transformer Tt does not flow and the power loss due to the reflux does not occur.

[On-period: secondary side]

In FIG. 3, for convenience of explanation, the other end of the secondary coil of the transformer Tr, Ts, Tt is assumed to be a point, b point, and point c respectively and a secondary coil common to the three transformers (The winding start end in this example) is taken as d point. Further, the anode output terminal P is defined as point f, and the cathode output terminal N is defined as e point. The point f is also one end of the smoothing capacitor C2. The point e is a common end of the sub capacitor C1 and the smoothing capacitor C2, and is also a secondary side reference potential stage.

Fig. 4 is a diagram schematically showing the potential relationship between the points a to f of the trans-secondary side in the On-period. The operation of the trans-secondary side in the On-period will be described with reference to Fig.

In the steady state, the sub capacitor C1 and the smoothing capacitor C2 are charged at predetermined both-end voltages VC1 and VC2, respectively.

An electromotive force Vr is generated in the secondary coil as the input current flows to the primary coil of the transformer Tr (in this specification, "electromotive force" and "reverse electromotive force" are used in the meaning of voltage). The electromotive force (Vr) is the direction of the high potential at the point d and the direction of the low potential at the point a. Since the diode D1 becomes a reverse bias with respect to this electromotive force Vr, no current flows. On the other hand, the diode D4 becomes a forward bias and conducts.

Here, the potential relation diagram of the On-period shown in Fig. 4 will be referred to. The secondary coil a point of the transformer Tr becomes the same potential as the secondary side reference potential point e because the diode D4 conducts. When the electromotive force Vr of the transformer Tr exceeds the both-end voltage VC1 of the sub-condenser C1, the on-period current iron flows in the direction to charge the sub-capacitor C1 to the next path.

- iron: Trans (Tr) Secondary coil d point → Sub-capacitor (C1) → Diode (D4) → Trans (Tr) Secondary coil a point

Further, when the electromotive force Vr does not exceed the d-point potential which is one end of the sub-capacitor C1, the current iron does not flow (see Vr 'in Fig. 4).

The input current (ist) flows to the primary coil of the transformer (Ts) to generate the electromotive force (Vs) in the secondary coil. The electromotive force (Vs) is the direction of the high potential at point d and the direction of low potential at point b. The diode D2 is reverse biased with respect to the electromotive force Vs, so that no current flows. On the other hand, the diode D5 conducts.

Here, the potential relation diagram of the On-period shown in Fig. 4 will be referred to. The secondary coil b point of the transformer Ts becomes the same potential as the secondary side reference potential point e when the diode D5 conducts. When the electromotive force Vs of the transformer Ts exceeds the both-end voltage VC1 of the sub-condenser C1, the On-period current ison flows in the direction to charge the sub-condenser C1 to the next path.

- Current (ison): Trans (Ts) Secondary coil d point → Sub-capacitor (C1) → Diode (D5) → Trans (Ts) Secondary coil b point

Further, when the electromotive force Vs does not exceed the d-point potential at one end of the sub-capacitor C1, the current ison does not flow (see Vs' in Fig. 4).

Further, since no current flows through the secondary coil of the transformer Tt, no electromotive force is generated in the secondary coil. No current flows in the secondary coil of the transformer (Tt).

The supply current to the load is only the discharge current from the smoothing capacitor C2.

The operation of the On-period of this circuit is summarized as follows. In the transformer in which the input current flows in the primary coil, an electromotive force is generated in the secondary coil. When the generated electromotive force exceeds the voltage of the sub-capacitor, a current flows in the direction of charging the sub-capacitor. On the other hand, in a transformer in which no current flows in the primary coil, no current flows in the secondary coil.

In the normal forward mode, magnetic energy is accumulated in an external choke coil in the On-period, and magnetic energy is accumulated in the transformer in the On-period in the case of the ordinary flyback method. On the other hand, in this circuit, energy is accumulated in the sub-capacitor C1 by the current (iron and ison) flowing in the secondary side in the On-period. As a result, an external choke coil is unnecessary in this circuit.

(2-3) Detailed description of the operation of the primary and secondary sides of the Off-period

Fig. 5 schematically shows the current flow (dotted line with arrows) for the Off-period of the T-mode in the circuit configuration of Fig.

[Off-period: primary side]

On the transformer primary side, when the control signal Vg is turned off, both the FET Qr, the FET Qs and the FET Qt are turned off and the switch is opened. Each current path of the primary coil of each transformer is cut off, and the current becomes zero. As a result, a reverse electromotive force is generated in each of the primary coil and the secondary coil of the transformer Tr, Ts, Tt.

[Off-period: secondary side]

Fig. 6 is a diagram schematically showing the potential relationship between the points a to f in the trans-secondary side in the Off-period. The operation of the secondary side in the Off- period will be described with reference to FIG.

The reverse direction electromotive force (Vr) generated in the secondary coil of the transformer Tr is a low potential at the point d and a high potential at the point a. Since the diode D4 is reverse biased with respect to this reverse electromotive force Vr, no current flows.

Here, the potential relationship diagram of the Off- period shown in Fig. 6 will be referred to. When the secondary coil a point potential of the transformer Tr rises with respect to the d point potential by the backward electromotive force Vr and exceeds the f point potential which is one end (first output end [P]) of the smoothing capacitor C2 The diode D1 is forward biased by the reverse electromotive force Vr, and the current iroff flows to the next path.

- Current (iroff): Trans (Tr) Secondary coil a point → Diode (D1) → Load (or smoothing capacitor [C2]) → Sub-capacitor (C1) → Trans (Tr) Secondary coil d point

The reverse direction electromotive force (Vs) generated in the secondary coil of the transformer (Ts) is a low potential at the point d and a high potential at the point b. Since the diode D5 is reverse biased with respect to such a reverse electromotive force Vs, no current flows.

Here, the potential relation diagram of the Off-period shown in Fig. 6 will be referred to. When the secondary coil b point potential of the transformer Ts rises with respect to the d point potential by the backward electromotive force Vs and exceeds the f point potential which is one end (first output end [P]) of the smoothing capacitor C2, The diode D2 becomes forward biased by the reverse electromotive force Vs and the current isoff flows to the next path.

- Transformer (Ts) Secondary coil b point → Diode (D2) → Load (or smoothing capacitor [C2]) → Sub-capacitor (C1) → Trans (Ts) Secondary coil d point

The current (iroff, isoff) in the Off-period flows in the direction of discharging the sub-capacitor C1. The off-period current (roff, isoff) corresponds to the flyback current of the flyback method. In the conventional flyback method, the magnetic energy stored in the transformer is released. In the case of this circuit, the energy accumulated in the sub-condenser C1 is released.

Since no current flows through the primary coil in the On-period of the transformer (Tt), no backward electromotive force occurs in the Off-period and no current flows in the Off-period.

The operation of the off-period of this circuit is summarized as follows. In the transformer in which the input current flows into the primary coil in the On-period, a reverse-direction electromotive force occurs in the secondary coil in the Off-period. The generated electromotive force is added to the voltage of the sub-capacitor. When the added voltage exceeds the voltage of the smoothing capacitor, current flows to the load. On the other hand, in a transformer in which no current flows in the primary coil in the On-period, no current flows in the Off-period.

As can be seen from the potential relationship in the Off-period of FIG. 6, the a-point potential or the b-point potential when iroff or isoff flows is determined by the both-end voltage VC1 of the sub- Vr or Vs).

In addition, since the backward electromotive force generated in the secondary coil of the transformer Tr, Ts in the Off-period is suppressed by the voltage VC1 charged in the sub-capacitor C1 in the On-period, . As a result, since the reverse direction electromotive force generated in the primary side of the transformers Tr and Ts also becomes smaller, the withstand voltage required for the FET Qr and FET Qs on the primary side is relaxed.

As described above, the conventional forward type external choke coil is unnecessary in this circuit. Although the sub-condenser C1 and the diodes D4, D5, D6, D7, D8, and D9 are added to the conventional flyback method, they are advantageous over the choke coil in terms of cost and space.

Further, in this circuit, since the reflux diode is provided on the primary side, the reflux flows through the diode without flowing through the primary coil of the transformer and returns to the input terminal, thereby reducing the power loss of the transformer.

R, S, T inputs
E input side reference potential terminal
P positive output
N Negative output terminal (output side reference potential)
Tr, Ts, Tt Trance
Lr1, Ls1, Lt1 primary coils
Lr2, Ls2, and Lt2 secondary coils
Qr, Qs, Qt switching element (FET)
D1, D2, D3 rectifying element (output diode)
D4, D5, D6 rectifying element
D7, D8, D9 Rectifying Elements
C1 sub condenser
C2 smoothing capacitor

Claims (1)

(a) first, second and third input terminals (R, S, T) to which three-phase alternating current is inputted,
(b) an anode output terminal P and a cathode output terminal N,
(c) a primary coil Lr1, Ls1, Lt1 and secondary coils Lr2, Ls2, Lt2, one end of each primary coil being connected to the first, second and third input terminals R, S Second, and third transformers Tr, Ts, Tt connected to the first, second, and third transformers T,
(d) Each current path between the other end of each of the primary coils Lr1, Ls1, Lt1 of the first, second, and third transformers Tr, Ts, Tt and the input- Second, and third switching elements Qr, Qs, and Qt that are on and off controlled by one control signal Vg to turn on or off the switching elements Q1, Q2,
(C1) connected between one end of each of the secondary coils Lr2, Ls2 and Lt2 of the first, second and third transformers Tr, Ts and Tt and the negative output terminal N, ),
(f) is connected between the other end of each of the secondary coils (Lr2, Ls2, Lt2) of the first, second and third transformers (Tr, Ts, Tt) and the anode output terminal Second, and third rectifying elements D1, D2, and D3 for conducting currents flowing from the other end of the secondary coil to the anode output terminal P,
(g) connected between the other end of each of the secondary coils (Lr2, Ls2, Lt2) of the first, second and third transformers Tr, Ts, Tt and the negative output terminal N, Fourth, fifth and sixth rectifying elements D4, D5 and D6 for conducting currents flowing from the negative output terminal N to the other end of the secondary coil,
(h) a smoothing capacitor (C2) connected between the positive output terminal (P) and the negative output terminal (N), and
(i) is connected between the input-side reference potential stage (E) and each of the first, second and third input terminals (R, S, T) Seventh, eighth and ninth rectifying elements D7, D8, and D9 that conduct currents respectively flowing through the first, second, and third input terminals R, S, and T, respectively.
Switching power.

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