JP6800098B2 - Insulated switching power supply for three-phase AC - Google Patents

Description

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

Conventionally, an isolated converter is known as a switching power supply that converts alternating current into direct current. Although various methods have been presented, the DC / DC converter is arranged after AC / DC conversion by rectifying the AC voltage with a rectifier circuit and smoothing it with a smoothing capacitor, not limited to single-phase and three-phase. (Patent Documents 1 to 7). A two-stage configuration in which a power factor improving device (PFC) and a DC / DC converter are combined to improve the power factor is also known. Patent Documents 6 and 7 describe a device for boosting and improving the power factor with respect to the three-phase AC output of a wind power generator.

Japanese Unexamined Patent Publication No. 7-31150 Japanese Unexamined Patent Publication No. 8-331860 JP-A-2002-10632 Japanese Unexamined Patent Publication No. 2005-218224 Japanese Unexamined Patent Publication No. 2007-37297 Japanese Unexamined Patent Publication No. 2013-128379 Japanese Unexamined Patent Publication No. 2014-23286

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

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

In order to achieve the above object, the present invention provides the following configurations. The reference numerals in parentheses are the reference numerals in the drawings described later and are provided for reference.

-One aspect of the switching power supply of the present invention is
(A) The first, second and third input terminals (R, S, T) where three-phase alternating current is input, and
(B) Positive electrode output end (P) and negative electrode output end (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 the first, second and third input ends (R, S). , T), the first, second and third transformers (Tr, Ts, Tt), respectively,
(D) Each current path between the other end of each primary coil (Lr1, Ls1, Lt1) of the first, second and third transformers (Tr, Ts, Tt) and the input side reference potential end (E). The first, second and third switching elements (Qr, Qs, Qt), which are controlled on and off by one control signal (Vg) so as to conduct or cut off the
(E) Connected between one end of each secondary coil (Lr2, Ls2, Lt2) of the first, second and third transformers (Tr, Ts, Tt) and the negative electrode output end (N), respectively. The first, second and third sub-capacitors (C1, C2, C3) and
(F) Connected between the other end of each secondary coil (Lr2, Ls2, Lt2) of the first, second and third transformers (Tr, Ts, Tt) and the positive electrode output end (P), respectively. , The first, second and third rectifying elements (D1, D2, D3) that conduct the current flowing from the other end of the secondary coil to the positive electrode output end (P), respectively.
(G) Connected between the other end of each secondary coil (Lr2, Ls2, Lt2) of the first, second and third transformers (Tr, Ts, Tt) and the negative electrode output end (N), respectively. The fourth, fifth, and sixth rectifying elements (D4, D5, D6) that conduct the current flowing from the negative electrode output end (N) to the other end of the secondary coil, respectively.
(H) It is characterized by having a smoothing capacitor (C4) connected between the positive electrode output end (P) and the negative electrode output end (N).

According to the present invention, in an isolated switching power supply that receives three-phase alternating current to improve the power factor and perform power conversion, a simple configuration can be obtained, 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 of the present invention. 2 (a) and 2 (b) 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 the current flow during the 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 the current flow during the 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.

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

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

The switching power supply of the present invention is a power conversion device that receives three-phase AC power output by an AC generator as an input 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, the three-phase AC power is output from the Y-connected three-phase stator coil in the AC generator.

The switching power supply of the present invention is not only a power conversion device but also a function as a power factor improving device. The purpose of the power factor improving device is to make the waveform of the input current the same sine wave waveform as the input voltage and to match the phases so that the power factor is 1.

The switching power supply of the present invention is an insulated type that electrically insulates the input side and the output side. For this purpose, three transformers Tr, Ts, and 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. The reference numerals Lr1, Ls1, and Lt1 indicate the primary coil of each transformer, and the reference numerals Lr2, Ls2, and Lt2 indicate the secondary coil of each transformer.

The winding start end of each coil is indicated by a black circle. In the present specification, the terms "one end" and "the other end" of a coil mean a combination of "winding start end" and "winding end" and a combination of "winding end" and "winding start end". It shall include any of.

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

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

One ends of each of the three switching elements Qr, Qs, and Qt are connected to the other ends (winding ends in this example) of the primary coils Lr1, Ls1, and Lt1 of each transformer. The other ends of the switching elements Qr, Qs, and Qt are 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 conducts or cuts off the current path between the other end of the primary coil Lr1, Ls1, Lt1 and the input side reference potential end E. Each is controlled on and off.

Each control end of the three switching elements Qr, Qs, and Qt is controlled by one common control signal Vg. The control signal Vg is, for example, a PWM signal having a pulse waveform having a predetermined frequency and duty ratio. That is, the three switching elements Qr, Qs, and Qt are controlled so as to always be turned on and off at the same time. 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 is the drain, the other end is the source, and the control end is the 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 and bipolar transistors, it is necessary to connect rectifying elements such as diodes that serve as current return paths in antiparallel.

Further, between one end (winding start end in this example) of the secondary coils Lr2, Ls2, and Lt2 of each transformer and the negative electrode output end N which is the secondary side reference potential end, the first, second, and third capacitors are used. C1, C2, and C3 (hereinafter referred to as "sub-capacitors") are connected to each other. Further, a smoothing capacitor C4 is connected between the positive electrode output end P and the negative electrode output end N.

Between the other ends (winding end in this example) of the secondary coils Lr2, Ls2, and Lt2 of each transformer and the positive electrode output end P, the first, second, and third diodes D1, D2, which are examples of rectifying elements, are located. , D3 are connected respectively. 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 each cathode is connected to the positive electrode output end P. The diodes D1, D2, and D3 conduct the current flowing from the other ends of the secondary coils Lr2, Ls2, and Lt2 of each transformer to the positive electrode output end P when the forward bias is applied, and cut off the current when the reverse bias is applied. To do.

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

Further, the 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 electrode output end N, respectively. Has been done. The cathodes of the diodes D4, D5, and D6 are connected to the other ends of the secondary coils Lr2, Ls2, and Lt2, and each anode is connected to the negative electrode output end N. Each diode D4, D5, D6 conducts a current flowing from the negative electrode output end N to the other end of the secondary coils Lr2, Ls2, and Lt2 of each transformer when the forward bias is applied, and cuts off the current when the reverse bias is applied.

Further, although not shown, it has a control unit that generates a control signal Vg. For example, the magnitudes of the input voltage and the DC output voltage are detected, the duty ratio of the control signal Vg is determined based on the detected input voltage and the output voltage, and the control signal Vg is generated based on the duty ratio. It is preferable to use a PWM IC as the main part of the control unit.

The PWM IC is, for example, a pulse-shaped control having a constant duty ratio by inputting a DC signal having a constant voltage corresponding to one determined duty ratio and a conveyed triangular wave signal having a constant frequency to a comparator. 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".

(2) Operation Description The operation of the circuit configuration shown in FIG. 1 will be described with reference to FIGS. 2 to 6. 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 AC and Power Factor Improving Action First, the power factor improving action by the switching operation on the input three-phase alternating current will be described with reference to FIGS. 2 (a) and 2 (b).

FIG. 2A shows voltage waveforms of R-phase, S-phase, and T-phase of input three-phase alternating current. The phase having the highest potential and the phase having the lowest potential are sequentially replaced every 120 ° of the phase. The frequency of the three-phase alternating current is, for example, several Hz to 100 Hz in the case 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 hundred kHz, which is sufficiently higher than the frequency of the three-phase alternating current.

As an example, the period during which the T phase reaches the lowest potential (referred to as “T mode”) will be described. The period in which the R phase has the lowest potential (referred to as "R mode") and the period in which the S phase has the lowest potential (referred to as "S mode") are the same, and thus description thereof will be omitted.

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 the interphase voltage between the positive potential phase and the negative potential phase during the on period of FETQr, Qs, and Qt, and no current flows during the off period.

In the following operation description, as an example, a case where an input current flows from the highest potential R phase or S phase to the lowest potential T phase by the RT-phase voltage or the ST-phase voltage will be described (other cases). The operation is the same, so the explanation is omitted).

Here, for example, the interphase voltage between the R phase and the T phase is referred to as "RT interphase voltage" or the like and is expressed as "vrt". Further, for example, the input current flowing due to the RT interphase voltage vrt is expressed as "irt".

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

Since the inductance L and the switching frequency ω on the current path are constants, the value of the starting point of the current irt during the on-period is determined by the instantaneous value of the RT-phase voltage vrt at the starting point of the on-period. Since the instantaneous values of the RT interphase voltage vrt are scattered on the trajectory of the sine wave, the current irt flowing in the primary coil during the on period also draws the trajectory of the sine wave. This means that the input current is a sine wave with the same phase as the input voltage. As a result, the power factor improvement on the primary side is realized.

In this circuit, a current path including an inductance to which a three-phase AC interphase voltage is applied is conducted and cut off by using a PWM control signal having a constant frequency and duty ratio, so that a sine wave having a phase matching with the input voltage is cut off. The input current of the wave can be obtained.

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

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

The input current irt flows through the primary coil of the transformer Tr by the RT interphase voltage vrt in the following path.
・ Input current irt: 1st input end R → transformer Tr primary coil → FETQr → FETQt → transformer Tt primary coil → 3rd input end T

The input current is flows through the primary coil of the transformer Ts in the following path due to the ST phase voltage vst.
・ Input current ist: 2nd input end S → transformer Ts primary coil → FETQs → FETQt → transformer Tt primary coil → 3rd input end 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 end T, is hereinafter referred to as "reflux".

[On period: Secondary side]
In FIG. 3, for convenience of explanation, the other ends of the secondary coils of the transformers Tr, Ts, and Tt (winding ends in this example) are set as points a, b, and c, respectively, and the secondary coils of the transformers Tr, Ts, and Tt. Let one end (the winding start end in this example) be point d, point e, and point f. Further, the positive electrode output end P is defined as the h point, and the negative electrode output end N is defined as the g point. The h point is also one end of the smoothing capacitor C4. Point g is also the common end of the sub-capacitors C1, C2, C3 and the smoothing capacitor C4, and is the secondary side reference potential end.

FIG. 4 is a diagram schematically showing the potential relationship between points a to h on the secondary side of the transformer during the on period. The operation of the secondary side of the transformer during the on-period will be described with reference to FIG.

In the steady state, the sub-capacitors C1, C2, C3 and the smoothing capacitor C4 are charged with predetermined voltage across the ends VC1, VC2, VC3, and VC4, 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 high potential on the d point side and a low potential on the a point side. Since the diode D1 has a reverse bias with respect to this electromotive force Vr, no current flows. On the other hand, the diode D4 has a forward bias and conducts.

Here, the potential relationship diagram during the on-period shown in FIG. 4 is referred to. The secondary coil a point of the transformer Tr has the same potential as the secondary side reference potential end g point because the diode D4 conducts. When the electromotive force Vr of the transformer Tr exceeds the voltage VC1 across the sub-capacitor C1, the current iron during the on-period flows in the direction of charging the sub-capacitor C1 in the following path.
-Current iron: Transformer Tr secondary coil d point → Subcapacitor C1 → Diode D4 → Transformer Tr secondary coil a point

Further, when the input current is flows in the primary coil of the transformer Ts, an electromotive force Vs is generated in the secondary coil. The electromotive force Vs has a high potential on the e-point side and a low potential on the b-point side. Since the diode D2 has a reverse bias with respect to the electromotive force Vs, no current flows. On the other hand, the diode D5 has a forward bias and conducts.

Here, the potential relationship diagram during the on-period shown in FIG. 4 is referred to. The secondary coil b point of the transformer Ts has the same potential as the secondary side reference potential end g point because the diode D5 conducts. When the electromotive force Vs of the transformer Ts exceeds the voltage VC2 across the sub-capacitor C2, the current ison during the on-period flows in the direction of charging the sub-capacitor C2 in the following path.
-Current ison: Transformer Ts secondary coil e point → Sub capacitor C2 → Diode D5 → Transformer Ts secondary coil b point

Further, in the transformer Tt, an electromotive force Vt is generated in the secondary coil due to the reflux flowing through the primary coil. The electromotive force Vt has a low potential on the f-point side and a high-potential direction on the c-point side, which is opposite to that of the transformers Tr and Ts. Since the diode D6 has a reverse bias with respect to this electromotive force Vt, no current flows.

Here, the potential relationship diagram during the on-period shown in FIG. 4 is referred to. When the secondary coil c point potential of the transformer Tt rises with respect to the f point potential due to the electromotive force Vt and exceeds the h point potential at one end (first output end P) of the smoothing capacitor C4, the diode D3 becomes a forward bias. , Current iton flows in the following path.
-Current iton: Transformer Tt secondary coil c point → Diode D3 → h point → Load → g point → Subcapacitor C3 → Transformer Tt secondary coil f point

The current iton is supplied to the load. Therefore, the current iton corresponds to the forward current in the forward system. The discharge current from the smoothing capacitor C4 is also merged with the supply current to the load. Since the sub-capacitor C3 is discharged by the current iton, the f-point potential is lowered by that amount, and the voltage VC3 across the sub-capacitor C3 is reduced accordingly.

As can be seen from the potential relationship during the on-period in FIG. 4, the point c (point h) potential when the iton flows is the voltage across the subcapacitor C3 VC3 and the electromotive force Vt of the transformer Tt with respect to the point g potential. It is an addition.

The electromotive force Vt of the transformer Tt is determined by the magnitude of the reflux of the primary coil. When the electromotive force Vt of the transformer Tt is small, the c point potential does not exceed the h point potential (see c'point potential). In this case, the current iton does not flow.

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

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. On the other hand, in this circuit, energy is stored in the subcapacitors C1 and C2 by the currents iron and ion flowing on the secondary side during the on period. Further, energy is supplied to the load by the current iton flowing to the secondary side during the on period. As a result, this circuit does not require an external choke coil.

(2-3) Details of operation on the primary side and secondary side during the off period FIG. 5 is a diagram schematically showing the current flow (dotted line with an arrow) during the off period in the T mode in the circuit configuration of FIG. is there.

[Off period: Primary side]
On the primary side of the transformer, when the control signal Vg is turned off, 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, counter electromotive forces are generated in the primary coil and the secondary coil of the transformers Tr, Ts, and Tt, respectively.

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

The counter electromotive force Vr generated in the secondary coil of the transformer Tr has a low potential on the d point side and a high potential on the a point side. Since the diode D4 has a reverse bias with respect to this counter electromotive force Vr, no current flows.

Here, the potential relationship diagram during the off period shown in FIG. 6 is referred to. The counter electromotive force Vr causes the secondary coil a point potential of the transformer Tr to rise with respect to the d point potential. When the a point potential exceeds the h point potential at one end (first output end P) of the smoothing capacitor C4, the diode D1 becomes a forward bias and the current iroff flows in the following path.
-Current iroff: Transformer Tr secondary coil a point → Diode D1 → h point → Load (or smoothing capacitor C4) → g point → Sub capacitor C1 → Transformer Tr secondary coil d point

Further, the counter electromotive force Vs generated in the secondary coil of the transformer Ts has a low potential on the e-point side and a high potential on the b-point side. Since the diode D5 has a reverse bias with respect to the counter electromotive force Vs, no current flows.

Here, the potential relationship diagram during the off period shown in FIG. 6 is referred to. When the secondary coil b point potential of the transformer Ts rises with respect to the e point potential due to the counter electromotive force Vs and exceeds the h point potential at one end (first output end P) of the smoothing capacitor C4, the diode D2 is forward biased. The current isoff flows in the following path.
-Current isoff: Transformer Ts secondary coil b point → Diode D2 → h point → Load (or smoothing capacitor C4) → g point → Sub capacitor C2 → Transformer Ts secondary coil e point

The currents iroff and isoff during the off period flow in the directions of discharging the subcapacitors C1 and C2, respectively. The currents iroff and isoff during the off period 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 subcapacitors C1 and C2 is released.

As can be seen from the potential relationship during the off period in FIG. 6, the a point potential when iroff flows is the sum of the voltage across the subcapacitor C1 VC1 and the counter electromotive force Vr to the g point potential. There is. Similarly, the b-point potential when isoff flows is the sum of the voltage across the sub-capacitor C2 VC2 and the counter electromotive force Vs to the g-point potential.

Although not shown in FIG. 6, since the subcapacitors C1 and C2 are discharged by the currents iroff and isoff, the d-point potential and the e-point potential are lowered by that amount, and the voltage across the ends VC1 and VC2 are reduced, respectively.

The counter electromotive force generated in the secondary coils of the transformers Tr and Ts during the off period is suppressed by the voltages VC1 and VC2 charged in the subcapacitors C1 and C2 during the on period, so that there is no subcapacitors C1 and C2. It is smaller than the back electromotive force of. As a result, the counter electromotive force generated on the primary side of the transformers Tr and Ts is also reduced, so that the withstand voltage required for the FETQr and FETQs on the primary side is reduced.

The counter electromotive force Vt generated in the secondary coil of the transformer Tt has a high potential on the f point side and a low potential on the c point side, which is opposite to the transformers Tr and Ts. Since the diode D3 has a reverse bias with respect to this counter electromotive force Vt, no current flows. When the f-point potential rises with respect to the c-point potential due to the counter electromotive force Vt and the f-point potential exceeds the voltage VC3 across the sub-capacitor C3, the current itoff flows in the following path to charge the sub-capacitor C3.
-Current itoff: Transformer Tt secondary coil f point → Sub capacitor C3 → Diode D6 → Transformer Tt secondary coil c point

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 force generated in the secondary coil during the off period is added to the voltage of the subcapacitor. 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 counter electromotive force generated in the secondary coil during the off period exceeds the voltage of the sub capacitor, a current flows in the direction of charging the sub capacitor.

As described above, in this circuit, it is possible to supply the current 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 system. Further, the sub-capacitors C1, C2, C3 and the diodes D4, D5, D6 are added as compared with the normal flyback method, but these are more advantageous than the choke coil in terms of cost and space.

Further, since the current flows through the three transformers during both the on period and the off period, the efficiency of using the transformers is improved.

R, S, T Input terminal E Input side reference potential end P Positive electrode output end N Negative electrode 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 Rectifying element (output diode)
D4, D5, D6 Rectifier elements C1, C2, C3 Subcapacitor C4 Smoothing capacitor

Claims (1)

(A) The first, second and third input terminals (R, S, T) where three-phase alternating current is input, and
(B) Positive electrode output end (P) and negative electrode output end (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 the first, second and third input ends (R, S). , T), the first, second and third transformers (Tr, Ts, Tt), respectively,
(D) Each current path between the other end of each primary coil (Lr1, Ls1, Lt1) of the first, second and third transformers (Tr, Ts, Tt) and the input side reference potential end (E). The first, second and third switching elements (Qr, Qs, Qt), which are controlled on and off by one control signal (Vg) so as to conduct or cut off the
(E) Connected between one end of each secondary coil (Lr2, Ls2, Lt2) of the first, second and third transformers (Tr, Ts, Tt) and the negative electrode output end (N), respectively. The first, second and third sub-capacitors (C1, C2, C3) and
(F) Connected between the other end of each secondary coil (Lr2, Ls2, Lt2) of the first, second and third transformers (Tr, Ts, Tt) and the positive electrode output end (P), respectively. , The first, second and third rectifying elements (D1, D2, D3) that conduct the current flowing from the other end of the secondary coil to the positive electrode output end (P), respectively.
(G) Connected between the other end of each secondary coil (Lr2, Ls2, Lt2) of the first, second and third transformers (Tr, Ts, Tt) and the negative electrode output end (N), respectively. The fourth, fifth, and sixth rectifying elements (D4, D5, D6) that conduct the current flowing from the negative electrode output end (N) to the other end of the secondary coil, respectively.
(H) A switching power supply characterized by having a smoothing capacitor (C4) connected between the positive electrode output end (P) and the negative electrode output end (N).

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