JP2018196273A - Insulated switching power supply for three phase alternating current - Google Patents

Abstract

To efficiently improve a power factor and convert electric power by a simple structure in an insulated switching power supply in which a three phase AC is inputted.SOLUTION: An insulated switching power supply for three phase AC comprises: input ends R, S and T; positive and negative output ends P and N; three transformers Tr,Ts and Tt having each primary coil connected to each input end; three switching elements Qr, Qs and Qt for passing and intercepting each current path between the other end of each primary coil of the three transformers and the input side reference potential end; a sub capacitor connected between one end of each second coil of the three transformers and the negative electrode output end; first, second and third rectifier elements D1, D2 and D3 connected respectively between the other end of each secondary coil of the three transformers and the positive electrode output end; fourth, fifth and sixth rectifier elements D4, D5 and D6 connected respectively between the other end of each secondary coil of the three transformers and the negative electrode output end; and a smoothing capacitor connected between the positive output end and the negative output end.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an isolated switching power supply that converts three-phase alternating current into direct current.

  Conventionally, an insulating converter is known as a switching power source for converting alternating current into direct current. Various schemes have been presented, but not limited to single-phase and three-phase, DC / DC converters are arranged after AC / DC conversion by roughly rectifying AC voltage with a rectifier circuit and smoothing with a smoothing capacitor. (Patent Documents 1 to 7). In order to improve the power factor, a two-stage configuration combining a power factor corrector (PFC) and a DC / DC converter is also known. Patent Documents 6 and 7 describe devices for boosting and improving the power factor for the three-phase AC output of an AC generator for wind power generation.

JP-A-7-31150 JP-A-8-331860 JP 2002-10632 A JP 2005-218224 A JP 2007-37297 A JP 2013-128379 A JP 2014-23286 A

  In a conventional switching power supply having a power factor correction function, there is a problem that the circuit becomes complicated when the two-stage configuration is used. In addition, the forward converter requires an external choke coil in addition to the transformer.

  In view of the above problems, an object of the present invention is to perform efficient power factor improvement and power conversion with a simple configuration in an isolated switching power supply to which three-phase alternating current is input.

  In order to achieve the above object, the present invention provides the following configurations. In addition, the code | symbol in parenthesis is a code | symbol in drawing mentioned later, and attaches | subjects it for reference.

-One aspect of the switching power supply of the present invention is
(A) first, second and third input terminals (R, S, T) to which three-phase alternating current is input;
(B) a positive electrode output terminal (P) and a negative electrode output terminal (N);
(C) Each includes a primary coil (Lr1, Ls1, Lt1) and a secondary coil (Lr2, Ls2, Lt2), and one end of each primary coil is connected to the first, second and third input terminals (R, S). , T) respectively connected to the first, second and third transformers (Tr, Ts, Tt);
(D) Each current path between the other end of the primary coil (Lr1, Ls1, Lt1) of each of the first, second and third transformers (Tr, Ts, Tt) and the input-side reference potential terminal (E) First, second, and third switching elements (Qr, Qs, Qt) that are turned on and off by one control signal (Vg) so as to conduct or cut off,
(E) Sub connected between one end of each secondary coil (Lr2, Ls2, Lt2) of the first, second and third transformers (Tr, Ts, Tt) and the negative output end (N) A capacitor (C1),
(F) Connected between the other end of each secondary coil (Lr2, Ls2, Lt2) of each of the first, second and third transformers (Tr, Ts, Tt) and the positive output terminal (P). , First, second and third rectifying elements (D1, D2, D3) for conducting currents flowing from the other end of the secondary coil to the positive output terminal (P), respectively;
(G) Connected between the other end of each secondary coil (Lr2, Ls2, Lt2) of each of the first, second and third transformers (Tr, Ts, Tt) and the negative output terminal (N). , Fourth, fifth and sixth rectifying elements (D4, D5, D6) for conducting currents flowing from the negative output terminal (N) to the other end of the secondary coil, respectively;
(H) A smoothing capacitor (C2) connected between the positive electrode output terminal (P) and the negative electrode output terminal (N).

  According to the present invention, a simple configuration can be achieved in an isolated switching power supply that receives a three-phase alternating current and performs power factor improvement and power conversion, and the efficiency of the transformer can be improved.

FIG. 1 is a diagram schematically showing a circuit configuration example of 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 during an ON period in the T mode of the circuit configuration shown in FIG. FIG. 4 is a diagram schematically showing the potential relationship during the ON period on the secondary side of the circuit configuration shown in FIG. FIG. 5 is a diagram schematically showing a current flow during an OFF period in the T mode of the circuit configuration shown in FIG. FIG. 6 is a diagram schematically showing the potential relationship during the off period on the secondary side of the circuit configuration shown in FIG.

  Embodiments of a switching power supply according to the present invention will be described below with reference to the drawings showing examples.

(1) Circuit Configuration FIG. 1 is a diagram schematically showing an example of a circuit configuration of an embodiment of an insulated switching power supply according to the present invention.

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

  The switching power supply of the present invention is a power conversion device and also has a function as a power factor correction device. The power factor correction apparatus aims to make the waveform of the input current the same sine wave waveform as the input voltage and to make the power factor 1 by matching the phases.

  The switching power supply of the present invention is an insulating type that electrically insulates the input side and the output side. For this purpose, three transformers Tr, Ts, Tt corresponding to each phase are provided. Each of the three transformers Tr, Ts, and Tt includes one primary coil and one secondary coil. It is preferable to use three transformers Tr, Ts, and Tt having the same electromagnetic characteristics, and it is preferable to use a three-phase reactor. Symbols Lr1, Ls1, and Lt1 indicate primary coils of each transformer, and symbols Lr2, Ls2, and Lt2 indicate secondary coils of each transformer.

  The winding start end of each coil is indicated by a black circle. In this specification, when “one end” and “the other end” are referred to as a coil, it means a combination of “winding end” and “winding end”, and a combination of “winding end” and “winding end”. Any of these shall be included.

  One end (the winding start end in this example) of the primary coils Lr1, Ls1, Lt1 on the input side is the first input end R, the second input end S, and the first input end, which are three terminals to which a three-phase AC voltage is input. Each of the three input terminals T is connected. In the present specification, each phase of the three-phase alternating current is referred to as an R phase, an S phase, and a T phase. Reference E indicates an input-side reference potential end.

  On the secondary side of the transformer, a positive output terminal P and a negative output terminal N, which are two terminals from which a DC voltage is output, are provided. 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 electrode output terminal P and the negative electrode output terminal N, and an output current flows.

  One end of each of the three switching elements Qr, Qs, Qt is connected to the other end (the winding end in this example) of the primary coils Lr1, Ls1, Lt1 of each transformer. The other end of each switching element Qr, Qs, Qt is connected to the input side reference potential end E. Each switching element Qr, Qs, Qt is provided with a control end, and each control end is configured to conduct or cut off the current path between the other end of the primary coils Lr1, Ls1, Lt1 and the input side reference potential end E. Each is on / off controlled.

  The control terminals of the three switching elements Qr, Qs, and Qt are controlled by a common control signal Vg. The control signal Vg is a PWM signal having a pulse waveform with a predetermined frequency and duty ratio. That is, the three switching elements Qr, Qs, and Qt are always turned on and off simultaneously. In the illustrated example, the switching elements Qr, Qs, and Qt are n-channel MOSFETs (hereinafter referred to as “FETQr”, “FETQs”, and “FETQt”), one end being a drain, the other end being a source, and the control end being a gate. is there. In this case, the control signal Vg is a voltage signal.

  In the case of switching elements Qr, Qs, Qt other than FETs, such as IGBTs or bipolar transistors, rectifying elements such as diodes that serve as current return paths must be connected in antiparallel.

  Further, a capacitor (hereinafter referred to as a “sub-capacitor”) is provided between one end (the winding start end in this example) of the secondary coils Lr2, Ls2, and Lt2 of each transformer and the negative output terminal N that is the secondary reference potential end. C1) is connected. Further, a smoothing capacitor C2 is connected between the positive electrode output terminal P and the negative electrode output terminal N.

  Between the other end (the winding end in this example) of the secondary coils Lr2, Ls2, and Lt2 of each transformer and the positive output terminal P, first, second, and third diodes D1, D2 that are examples of rectifying elements are provided. , D3 are connected to each other. The anodes of the diodes D1, D2, and D3 are connected to the other ends of the secondary coils Lr2, Ls2, and Lt2, respectively, and the cathodes are connected to the positive output terminal P. Each diode D1, D2, D3 conducts current flowing from the other end of the secondary coil Lr2, Ls2, Lt2 of each transformer to the positive output terminal P when forward biased, and cuts off current when reverse biased. To do.

  The diodes D1, D2, and D3 are preferably those that have a small forward voltage drop and perform high-speed operation.

  Further, fourth, fifth, and sixth diodes D4, D5, and D6, which are examples of rectifying elements, are connected between the other ends of the secondary coils Lr2, Ls2, and Lt2 of each transformer and the negative output terminal N, respectively. Has been. The cathodes of the diodes D4, D5, and D6 are connected to the other ends of the secondary coils Lr2, Ls2, and Lt2, respectively, and the anodes are connected to the negative output terminal N. Each of the diodes D4, D5, and D6 conducts current flowing from the negative output terminal N to the other end of the secondary coils Lr2, Ls2, and Lt2 of each transformer when forward biased, and cuts off when reverse biased.

  In addition, although not shown, a control unit that generates a control signal Vg is provided. For example, the control unit detects the magnitudes of the input voltage and the DC output voltage, determines the duty ratio of the control signal Vg based on the detected input voltage and output voltage, and generates the control signal Vg based on the duty ratio. It is preferable to use PWMIC as the main part of the control unit.

  The PWMIC, for example, inputs a constant voltage DC signal corresponding to one determined duty ratio and a carrier triangular wave signal having a constant frequency to a comparator to thereby control a pulse having a constant duty ratio. The signal Vg is output. In the present invention, such a control signal Vg is referred to as a “control signal having a constant duty ratio”. Such a 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. The operation when the circuit is in a steady state will be described with the exception of the transient operation when the circuit is started and stopped.

(2-1) Input three-phase alternating current and power factor improving action First, the power factor improving action by the switching operation for the input three-phase alternating current will be described with reference to FIGS.

  FIG. 2A shows voltage waveforms of the R phase, S phase, and T phase of the input three-phase alternating current. The phase having the highest potential and the phase having the lowest potential are sequentially switched every 120 °. For example, in the case of an AC generator for wind power generation, the frequency of the three-phase AC is about several Hz to 100 Hz. On the other hand, the switching frequency, that is, the frequency of the control signal Vg in FIG. 1, is several kHz to several hundred kHz, which is sufficiently higher than the three-phase AC frequency.

  As an example, a period (referred to as “T mode”) in which the T phase is the lowest potential will be described. Note that the period during which the R phase is at the lowest potential (referred to as “R mode”) and the period during which the S phase is at the lowest potential (referred to as “S mode”) are the same and will not be described.

  In the T mode, the R phase has the highest potential in the first half of the period, and the S phase has the highest potential in the second half. In FIG. 1, an input current flows due to an interphase voltage between a positive potential phase and a negative potential phase during the on period of the FETs Qr, Qs, and Qt, and no current flows during the off period.

  In the following description of the operation, as an example, a case will be described in which an input current flows from the R phase or S phase having the highest potential to the T phase having the lowest potential during the ON period due to the voltage between the RT phase or the voltage between the ST phases ( Since the operation is the same, the description is omitted).

  Here, for example, the interphase voltage between the R phase and the T phase is referred to as “RT phase voltage” or the like, and is expressed as “vrt”. Also, for example, an input current that flows due to the RT phase voltage vrt is represented as “irt”.

  The power factor improving action will be described with reference to FIG. 2B shows, as an example, the waveform of the PWM control signal Vg at point A on the time axis t in FIG. 2A, the RT phase voltage vrt, and the first input terminal R and the third input terminal T. An on-current irt flowing between them is schematically shown. Since the switching frequency is sufficiently higher than the frequency of the three-phase alternating current, the RT phase voltage vrt in one ON period can be regarded as a pulse-like constant voltage. Accordingly, the value of the start point of the on-current irt is irt = vrt / Lω (ω is a switching frequency) by the RT phase voltage vrt and the inductance L on the current path between the first input terminal R and the third input terminal T. Determined by. The current irt rises linearly during the on period. During the off period, the current irt is zero.

  Since the inductance L and the switching frequency ω on the current path are constants, the value of the start point of the current irt during the on period is determined by the instantaneous value of the RT phase voltage vrt at the start point of the on period. Since the instantaneous value of the RT phase voltage vrt is scattered on the locus of the sine wave, the current irt flowing through the primary coil during the ON period also draws the locus of the sine wave. This means that the input current is a sine wave with the same phase as the input voltage. Thereby, the power factor improvement on the primary side is realized.

  In this circuit, the current path including the inductance to which the three-phase AC interphase voltage is applied is turned on and off using a PWM control signal having a constant frequency and duty ratio, so that the sine is matched in phase with the input voltage. Wave input current can be obtained.

(2-2) Details of operation on primary side and secondary side in on-period FIG. 3 schematically shows a current flow (dotted line with an arrow) in the on-period in the T mode in the circuit configuration shown in FIG. ing.

[On period: Primary side]
On the transformer primary side, when the control signal Vg is turned on during the on-period, all of the FETQr, FETQs, and FETQt are turned on and the current path is conducted.

The input current irt flows through the following path in the primary coil of the transformer Tr due to the RT phase voltage vrt.
Input current irt: first input terminal R → transformer Tr primary coil → FETQr → FETQt → transformer Tt primary coil → third input terminal T

The input current ist flows in the primary coil of the transformer Ts through the following path by the ST phase voltage vst.
Input current ist: second input terminal S → transformer Ts primary coil → FETQs → FETQt → transformer Tt primary coil → third input terminal T

  Here, the current returning to the input side, such as the current flowing from the primary coil of the transformer Tt to the third input terminal T, is hereinafter referred to as “reflux”.

[On period: Secondary side]
In FIG. 3, for convenience of explanation, the other end of the secondary coil of the transformers Tr, Ts, and Tt (the winding end in this example) is a point, b point, and c point, respectively, and one end of the secondary coil common to the three transformers. Let (the winding start end in this example) be the d point. Further, the positive electrode output terminal P is set as point f, and the negative electrode output terminal N is set as point e. The point f is also one end of the smoothing capacitor C2. Point e is a common end of the sub-capacitor C1 and the smoothing capacitor C2 and a secondary reference potential end.

  FIG. 4 is a diagram schematically showing the potential relationship between points a to f on the transformer secondary side during the ON period. The operation of the transformer secondary side during 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 with predetermined both-end voltages VC1 and VC2, respectively.

  When the input current irt flows through the primary coil of the transformer Tr, an electromotive force Vr is generated in the secondary coil. The electromotive force Vr has a direction in which the d point side has a high potential and the a point side has a low potential. Since the diode D1 is reverse-biased with respect to the electromotive force Vr, no current flows. On the other hand, the diode D4 becomes forward biased and becomes conductive.

Here, reference is made to the potential relationship diagram of the on period shown in FIG. The secondary coil a point of the transformer Tr has the same potential as the secondary side reference potential end e point because the diode D4 conducts. When the electromotive force Vr of the transformer Tr exceeds the voltage VC1 across the sub-capacitor C1, a current iron during the ON period flows in the following path in the direction of charging the sub-capacitor C1.
Current iron: transformer Tr secondary coil d point → sub capacitor C1 → diode D4 → transformer Tr secondary coil a point

  Further, when the input current ist flows through the primary coil of the transformer Ts, an electromotive force Vs is generated in the secondary coil. The electromotive force Vs has a direction in which the d point side has a high potential and the b point side has a low potential. Since the diode D2 is reverse-biased with respect to the electromotive force Vs, no current flows. On the other hand, the diode D5 becomes forward biased and becomes conductive.

Here, reference is made to the potential relationship diagram of the on period shown in FIG. The secondary coil b point of the transformer Ts has the same potential as the secondary reference potential terminal e point because the diode D5 is conductive. When the electromotive force Vs of the transformer Ts exceeds the voltage VC1 across the sub-capacitor C1, an on-state current ison flows in the direction of charging the sub-capacitor C1 through the following path.
Current ison: transformer Ts secondary coil d point → sub capacitor C1 → diode D5 → transformer Ts secondary coil b point

  Further, the transformer Tt generates an electromotive force Vt in the secondary coil due to the flow of reflux through the primary coil. The electromotive force Vt has a low potential on the d point side and a high potential on the c point side, and is opposite to the transformers Tr and Ts. Since the diode D6 is reverse-biased with respect to the electromotive force Vt, no current flows.

Here, reference is made to the potential relationship diagram of the on period shown in FIG. When the c-point potential of the secondary coil of the transformer Tt rises with respect to the d-point potential by the electromotive force Vt and exceeds the f-point potential that is one end (first output terminal P) of the smoothing capacitor C2, the diode D3 is forward biased. The current iton flows through the following path.
Current iton: transformer Tt secondary coil point c → diode D3 → point f → load → e point → sub capacitor C1 → transformer Tt secondary coil point d

  The current iton is supplied to the load. Therefore, the current iton corresponds to the forward current in the forward method. The supply current to the load is also combined with the discharge current from the smoothing capacitor C2.

  The current iton during the ON period flows in the direction of discharging the sub capacitor C1 contrary to the currents iron and ison. Therefore, whether the sub-capacitor C1 is actually charged or discharged depends on the magnitude of the current iron or ison and iton.

  The operation during the ON period of this circuit is summarized as follows. In a transformer in which an input current flows in the primary coil, when the electromotive force generated in the secondary coil 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 reflux flows through the primary coil, the electromotive force generated in the secondary coil is added to the voltage of the sub capacitor. When the added voltage exceeds the voltage of the smoothing capacitor, a current flows to the smoothing capacitor and the load.

  As can be seen from the potential relationship during the ON period in FIG. 4, the potential at point c (point f) when iton flows is the voltage VC1 across subcapacitor C1 and the electromotive force Vt of transformer Tt with respect to the potential at point e. The sum is raised by the voltage VC1 across the sub-capacitor C1.

  In the normal forward method, magnetic energy is stored in the external choke coil during the ON period, and in the normal flyback method, magnetic energy is stored in the transformer during the ON period. In contrast, in this circuit, energy is accumulated in the sub-capacitor C1 by currents iron and ison flowing on the secondary side during the ON period. Further, energy is supplied to the load by the current iton flowing on the secondary side during the ON period. As a result, this circuit does not require an external choke coil.

(2-3) Details of Primary Side and Secondary Side Operations in Off Period FIG. 5 is a diagram schematically showing a current flow (dotted line with an arrow) in the off period in the T mode in the circuit configuration of FIG. is there.

[Off period: Primary]
On the transformer primary side, when the control signal Vg is turned off, the FETQr, FETQs, and FETQt are all 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, back electromotive voltages are generated in the primary and secondary coils of the transformers Tr, Ts, and Tt, respectively.

[Off period: Secondary side]
FIG. 6 is a diagram schematically showing a potential relationship between points a to f on the transformer secondary side in the off period. The operation on the secondary side in the off period will be described with reference to FIG.

  The counter electromotive voltage Vr generated in the secondary coil of the transformer Tr has a direction in which the d point side has a low potential and the a point side has a high potential. Since the diode D4 is reverse-biased with respect to the back electromotive voltage Vr, no current flows.

Here, reference is made to the potential relationship diagram in the off-period shown in FIG. The secondary coil a point potential of the transformer Tr rises with respect to the d point potential by the back electromotive voltage Vr. When the point a potential exceeds the point f potential which is one end (first output end P) of the smoothing capacitor C2, the diode D1 becomes forward biased and the current iroff flows through the following path.
Current iroff: transformer Tr secondary coil point a → diode D1 → point f → load (or smoothing capacitor C2) → point e

  Further, the counter electromotive voltage Vs generated in the secondary coil of the transformer Ts is such that the d point side has a low potential and the b point side has a high potential. Since the diode D5 is reverse-biased with respect to the back electromotive voltage Vs, no current flows.

Here, reference is made to the potential relationship diagram in the off-period shown in FIG. Due to the counter electromotive voltage Vs, the potential at the secondary coil b of the transformer Ts rises with respect to the potential at the point d. When the b-point potential exceeds the f-point potential that is one end (first output terminal P) of the smoothing capacitor C2, the diode D2 becomes forward biased and the current isoff flows through the following path.
Current isoff: transformer Ts secondary coil b point → diode D2 → f point → load (or smoothing capacitor C2) → e point

  The off-period currents iroff and isoff flow in the direction of discharging the sub-capacitor C1. The off-period currents iroff and isoff correspond to the flyback current in the flyback method. In the normal flyback method, the magnetic energy stored in the transformer is released, but in the case of this circuit, the energy stored in the sub capacitor C1 is released.

  As can be seen from the potential relationship in the off period of FIG. 6, the point a potential or point b potential when iroff or isoff flows is the voltage VC1 across the sub capacitor C1 and the back electromotive voltage Vr or Vs with respect to the point e potential. It is what added and.

  The counter electromotive voltage generated in the secondary coils of the transformers Tr and Ts in the off period is suppressed by the voltage VC1 charged in the sub capacitor C1 in the on period, and therefore, compared with the counter electromotive voltage without the sub capacitor C1. Become smaller. As a result, the back electromotive voltage generated on the primary side of the transformers Tr and Ts is also reduced, so that the withstand voltage required for the primary side FET Qr and FET Qs is reduced.

The counter electromotive voltage Vt generated in the secondary coil of the transformer Tt is in a direction in which the d point side is at a high potential and the c point side is in a low potential direction, and is opposite to the transformers Tr and Ts. Since the diode D3 is reverse-biased with respect to the counter electromotive voltage Vt, no current flows. When the d-point potential rises with respect to the c-point potential due to the counter electromotive voltage Vt and the d-point potential exceeds the voltage VC1 across the sub-capacitor C1, a current itoff flows through the following path.
Current itoff: transformer Tt secondary coil d point → sub capacitor C1 → diode D6 → transformer Tt secondary coil c point

  The current itoff in the off period flows in the direction of charging the sub capacitor C1 contrary to the currents iroff and isoff. Therefore, whether the sub capacitor C1 is actually charged or discharged depends on the magnitudes of the currents iroff, isoff, and itoff at that time.

  The operation during the off-period of this circuit is summarized as follows. In a transformer in which an input current flows through the primary coil during the on period, the counter electromotive voltage generated in the secondary coil during the off period is added to the voltage of the sub capacitor. When the added voltage exceeds the voltage of the smoothing capacitor, a current flows through the load. On the other hand, in a transformer in which reflux flows through the primary coil during the on-period, when the back electromotive voltage generated in the secondary coil during the off-period exceeds the voltage of the sub-capacitor, current flows in the direction of charging the sub-capacitor.

  As described above, in this circuit, current can be supplied to the load during both the on period and the off period. This circuit does not require an external choke coil in the normal forward method. In this circuit, a sub capacitor C1 and diodes D4, D5, and D6 are added, but these are more advantageous than choke coils in terms of cost and space.

  In this circuit, since current always flows through the three transformers in both the on period and the off period, the utilization efficiency of the transformer is improved.

R, S, T Input end E Input side reference potential end P Positive output end N Negative output end (output side reference potential)
Tr, Ts, Tt Transformer Lr1, Ls1, Lt1 Primary coil Lr2, Ls2, Lt2 Secondary coil Qr, Qs, Qt Switching element (FET)
D1, D2, D3 Rectifier element (output diode)
D4, D5, D6 Rectifying element C1 Sub capacitor C2 Smoothing capacitor

Claims (1)

(A) first, second and third input terminals (R, S, T) to which three-phase alternating current is input;
(B) a positive electrode output terminal (P) and a negative electrode output terminal (N);
(C) Each includes a primary coil (Lr1, Ls1, Lt1) and a secondary coil (Lr2, Ls2, Lt2), and one end of each primary coil is connected to the first, second and third input terminals (R, S). , T) respectively connected to the first, second and third transformers (Tr, Ts, Tt);
(D) Each current path between the other end of the primary coil (Lr1, Ls1, Lt1) of each of the first, second and third transformers (Tr, Ts, Tt) and the input-side reference potential terminal (E) First, second, and third switching elements (Qr, Qs, Qt) that are turned on and off by one control signal (Vg) so as to conduct or cut off,
(E) Sub connected between one end of each secondary coil (Lr2, Ls2, Lt2) of the first, second and third transformers (Tr, Ts, Tt) and the negative output end (N) A capacitor (C1),
(F) Connected between the other end of each secondary coil (Lr2, Ls2, Lt2) of each of the first, second and third transformers (Tr, Ts, Tt) and the positive output terminal (P). , First, second and third rectifying elements (D1, D2, D3) for conducting currents flowing from the other end of the secondary coil to the positive output terminal (P), respectively;
(G) Connected between the other end of each secondary coil (Lr2, Ls2, Lt2) of each of the first, second and third transformers (Tr, Ts, Tt) and the negative output terminal (N). , Fourth, fifth and sixth rectifying elements (D4, D5, D6) for conducting currents flowing from the negative output terminal (N) to the other end of the secondary coil, respectively;
(H) A switching power supply comprising a smoothing capacitor (C2) connected between the positive electrode output terminal (P) and the negative electrode output terminal (N).

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