Power conversion circuit

Abstract

A second power conversion circuit (40) is provided with an input terminal (41) and a transformer (43) having a primary-side winding (44) and a secondary-side winding (45). The second power conversion circuit (40) is also provided with: a first bidirectional tap switch (TS1) having a first end connected to the input terminal (41) and a second end of connected to a first terminal (44A) of the primary-side winding (44); a second bidirectional tap switch (TS2) having a first end connected to the input terminal (41); and a second control unit (46) for controlling the first bidirectional tap switch (TS1) and the second bidirectional tap switch (TS2). The primary-side winding (44) has a tap (44C) between the first terminal (44A) and a second terminal (44B) of the primary-side winding (44). A second end of the second bidirectional tap switch (TS2) is connected to the tap.

Description

Power Conversion Circuit

[0001] The present disclosure relates to power conversion circuits.

[0002] The charging device described in Patent Document 1 includes a non-insulated converter and an insulated converter. The non-insulated converter converts input AC power into DC power. The DC power output from the non-insulated converter is input to the insulated converter. The insulated converter includes a transformer. The non-insulated converter includes a switch element.

[0003] JP 2014-53992 A

[0004] In a charging device such as that disclosed in Patent Document 1, the AC voltage input to the charging device may change. In this case, the voltage input to the non-insulated converter also changes. Therefore, in order to maintain a constant output voltage of the charging device, it is necessary to adjust the ON and OFF periods of the switching elements of the non-insulated converter in accordance with the change in the voltage input to the non-insulated converter. Alternatively, it is necessary to add a non-insulated converter downstream of the isolated converter to adjust the voltage. As a result, depending on the voltage input to the charging device, it may be necessary to control the switching elements in the non-insulated converter and isolated converter that make up the charging device in a state where switching losses are large.

[0005] In order to solve the above-described problems, one aspect of the present disclosure provides a power conversion circuit comprising: a transformer having a primary winding and a secondary winding, the primary winding having a first terminal and a second terminal; an input terminal; a first bidirectional switch having a first end connected to the input terminal and a second end connected to the first terminal of the primary winding; a second bidirectional switch having a first end and a second end, the first end connected to the input terminal; and a control unit that controls the first bidirectional switch and the second bidirectional switch, wherein the primary winding has a tap between the first terminal and the second terminal of the primary winding, and the second end of the second bidirectional switch is connected to the tap.

[0006] This makes it possible to suppress an increase in switching loss when the amplitude of the input voltage to the power conversion circuit fluctuates.

[0007] FIG. 1 is a circuit diagram of a power conversion device. FIG. 2 is a circuit diagram of a first power conversion circuit. FIG. 3 is a circuit diagram of a second power conversion circuit. FIG. 4 is a diagram showing the waveform and sectors of three-phase AC power. FIG. 5 is a space vector diagram in SVPWM. FIG. 6 is a space vector diagram in SVPWM. FIG. 7 is a sequence diagram of a first switching pattern. FIG. 8 is a sequence diagram of a second switching pattern. FIG. 9 is a sequence diagram of a third switching pattern. FIG. 10 is a sequence diagram of a fourth switching pattern.

[0008] <One Embodiment of a Power Conversion Circuit> Hereinafter, one embodiment of a power conversion circuit will be described. Note that in the drawings, components may be shown enlarged to facilitate understanding. The dimensional ratios of the components may differ from those in the actual drawings or from those in other drawings.

[0009] (1. Overall Configuration) As shown in FIG. 1 , the power conversion device 10 includes an input-side low-pass filter 20, a first power conversion circuit 30, a second power conversion circuit 40, a rectifier circuit 50, and an output-side low-pass filter 60. The power conversion device 10 also includes three external input terminals 11 and a pair of external output terminals 12. The power conversion device 10 is a so-called three-phase isolated AC-DC converter. That is, the power conversion device 10 is capable of converting three-phase AC power input to the external input terminals 11 into DC power and outputting it from the external output terminals 12. The second power conversion circuit 40, which will be described later, is interposed on the power path from each external input terminal 11 to each external output terminal 12, thereby electrically isolating the external input terminal 11 side from the external output terminal 12 side.

[0010] The three external input terminals 11 of the power conversion device 10 are a first external input terminal 11A, a second external input terminal 11B, and a third external input terminal 11C. Three phases of three-phase AC power input from a three-phase AC power supply 80 are input in a one-to-one correspondence to each external input terminal 11. The three-phase AC power supply 80 is a three-phase, three-wire commercial power system in which three AC power supplies are Y-connected.

[0011] The pair of external output terminals 12 is a first external output terminal 12A and a second external output terminal 12B. An arbitrary load 70 can be connected between the first external output terminal 12A and the second external output terminal 12B. The load 70 is, for example, an electronic device driven by DC power.

[0012] The input low-pass filter 20 includes a first inductor L1, a second inductor L2, and a third inductor L3, as well as a first capacitor C1, a second capacitor C2, and a third capacitor C3.

[0013] A first end of the first inductor L1 is connected to the first external input terminal 11A. A first end of the first capacitor C1 is connected to the second end of the first inductor L1. A first end of the second inductor L2 is connected to the second external input terminal 11B. A first end of the second capacitor C2 is connected to the second end of the second inductor L2. A second end of the second capacitor C2 is connected to the second end of the first capacitor C1.

[0014] A first end of the third inductor L3 is connected to the third external input terminal 11C. A first end of the third capacitor C3 is connected to the second end of the third inductor L3. A second end of the third capacitor C3 is connected to the second end of the first capacitor C1.

[0015] 2, the first power conversion circuit 30 includes three input connection terminals 31 and a pair of output connection terminals 32. Furthermore, as shown in FIG. 3, the first power conversion circuit 30 includes a plurality of bidirectional switches TSW and a first control unit 33.

[0016] As shown in FIG. 1 , the multiple input-side connection terminals 31 include a first connection terminal 31A, a second connection terminal 31B, and a third connection terminal 31C. The first connection terminal 31A is connected to the second end of the first inductor L1. The second connection terminal 31B is connected to the second end of the second inductor L2. The third connection terminal 31C is connected to the second end of the third inductor L3. Therefore, three-phase AC power is input to the input-side connection terminals 31 of the first power conversion circuit 30 via the external input terminal 11 and the input-side low-pass filter 20. The pair of output-side connection terminals 32 include a fourth connection terminal 32A and a fifth connection terminal 32B. High-frequency AC power converted by on / off control of the multiple bidirectional switches TSW can be output from the pair of output-side connection terminals 32.

[0017] As shown in Fig. 2, each bidirectional switch TSW has two switch elements. Each switch element is an N-channel MOSFET (Metal Oxide Semiconductor Field Effect Transistor). That is, each switch element has a body diode. The bidirectional switch TSW is composed of two switch elements connected in series such that the anode terminals of the body diodes are connected to each other. In other words, each bidirectional switch TSW has two switch elements whose source terminals are connected to each other.

[0018] The multiple bidirectional switches TSW include a first high-side bidirectional switch HS1, a first low-side bidirectional switch LS1, a second high-side bidirectional switch HS2, a second low-side bidirectional switch LS2, a third high-side bidirectional switch HS3, and a third low-side bidirectional switch LS3.

[0019] The first high-side bidirectional switch HS1 connects the first connection terminal 31A and the fourth connection terminal 32A. Specifically, the first high-side bidirectional switch HS1 has an eleventh switch element S11 and a twenty-first switch element S21. The drain terminal of the eleventh switch element S11 is connected to the first connection terminal 31A. The source terminal of the twenty-first switch element S21 is connected to the source terminal of the eleventh switch element S11. The drain terminal of the twenty-first switch element S21 is connected to the fourth connection terminal 32A.

[0020] The first low-side bidirectional switch LS1 connects the first connection terminal 31A and the fifth connection terminal 32B. Specifically, the first low-side bidirectional switch LS1 has a 24th switch element S24 and a 14th switch element S14. The drain terminal of the 24th switch element S24 is connected to the first connection terminal 31A. The source terminal of the 14th switch element S14 is connected to the source terminal of the 24th switch element S24. The drain terminal of the 14th switch element S14 is connected to the fifth connection terminal 32B.

[0021] The second high-side bidirectional switch HS2 connects the second connection terminal 31B and the fourth connection terminal 32A. Specifically, the second high-side bidirectional switch HS2 has a thirteenth switch element S13 and a twenty-third switch element S23. The drain terminal of the thirteenth switch element S13 is connected to the second connection terminal 31B. The source terminal of the twenty-third switch element S23 is connected to the source terminal of the thirteenth switch element S13. The drain terminal of the twenty-third switch element S23 is connected to the fourth connection terminal 32A.

[0022] The second low-side bidirectional switch LS2 connects the second connection terminal 31B and the fifth connection terminal 32B. Specifically, the second low-side bidirectional switch LS2 has a 26th switch element S26 and a 16th switch element S16. The drain terminal of the 26th switch element S26 is connected to the second connection terminal 31B. The source terminal of the 26th switch element S26 is connected to the source terminal of the 16th switch element S16. The drain terminal of the 16th switch element S16 is connected to the fifth connection terminal 32B.

[0023] The third high-side bidirectional switch HS3 connects the third connection terminal 31C and the fourth connection terminal 32A. Specifically, the third high-side bidirectional switch HS3 has a fifteenth switch element S15 and a twenty-fifth switch element S25. The drain terminal of the fifteenth switch element S15 is connected to the third connection terminal 31C. The source terminal of the twenty-fifth switch element S25 is connected to the source terminal of the fifteenth switch element S15. The drain terminal of the twenty-fifth switch element S25 is connected to the fourth connection terminal 32A.

[0024] The third low-side bidirectional switch LS3 connects the third connection terminal 31C and the fifth connection terminal 32B. Specifically, the third low-side bidirectional switch LS3 has a 22nd switch element S22 and a 12th switch element S12. The drain terminal of the 22nd switch element S22 is connected to the third connection terminal 31C. The source terminal of the 12th switch element S12 is connected to the source terminal of the 22nd switch element S22. The drain terminal of the 12th switch element S12 is connected to the fifth connection terminal 32B.

[0025] The first control unit 33 controls each bidirectional switch TSW. Specifically, the first control unit 33 controls the on / off of each switch element by inputting switching signals to the gate terminals of two switch elements included in each bidirectional switch TSW. The switching signals include an eleventh switching signal SG11 to a sixteenth switching signal SG16 and a twenty-first switching signal SG21 to a twenty-sixth switching signal SG26. The eleventh switching signal SG11 to the sixteenth switching signal SG16 correspond one-to-one to the eleventh switching element S11 to the sixteenth switching element S16, respectively. The twenty-first switching signal SG21 to the twenty-sixth switching signal SG26 correspond one-to-one to the twenty-first switching element S21 to the twenty-sixth switching element S26, respectively.

[0026] Each bidirectional switch TSW can be in four on / off states depending on the on / off combination of each switch element. The following description will be given taking the first high-side bidirectional switch HS1 and the first low-side bidirectional switch LS1 as examples.

[0027] The first state is a bidirectionally on state. In the bidirectionally on state, the eleventh switch element S11 and the twenty-first switch element S21 of the first high-side bidirectional switch HS1 are on and on, respectively. In the bidirectionally on state, the first high-side bidirectional switch HS1 allows a current to flow from the first connection terminal 31A to the fourth connection terminal 32A, and also allows a current to flow from the fourth connection terminal 32A to the first connection terminal 31A.

[0028] In the first low-side bidirectional switch LS1, when the first low-side bidirectional switch LS1 is in the bidirectional on state, the fourteenth switch element S14 is on and the twenty-fourth switch element S24 is on. In the bidirectional on state, the first low-side bidirectional switch LS1 allows a current to flow from the first connection terminal 31A to the fifth connection terminal 32B, and also allows a current to flow from the fifth connection terminal 32B to the first connection terminal 31A.

[0029] The second state is a positive on state. In the positive on state, the eleventh switch element S11 is on and the twenty-first switch element S21 is off in the first high-side bidirectional switch HS1. In the positive on state, the first high-side bidirectional switch HS1 allows current to flow from the first connection terminal 31A to the fourth connection terminal 32A through the body diode of the twenty-first switch element S21. On the other hand, the first high-side bidirectional switch HS1 does not allow current to flow from the fourth connection terminal 32A to the first connection terminal 31A.

[0030] Furthermore, when the first low-side bidirectional switch LS1 is in the on state in the positive direction, the fourteenth switch element S14 is on and the twenty-fourth switch element S24 is off. In the on state in the positive direction, the first low-side bidirectional switch LS1 allows current to flow from the fifth connection terminal 32B to the first connection terminal 31A through the body diode of the twenty-fourth switch element S24. On the other hand, the first low-side bidirectional switch LS1 does not allow current to flow from the first connection terminal 31A to the fifth connection terminal 32B.

[0031] The third state is a negative on state. In the negative on state, in the first high-side bidirectional switch HS1, the eleventh switch element S11 is off and the twenty-first switch element S21 is on. In the negative on state, the first high-side bidirectional switch HS1 allows current to flow from the fourth connection terminal 32A to the first connection terminal 31A through the body diode of the eleventh switch element S11. On the other hand, the first high-side bidirectional switch HS1 does not allow current to flow from the first connection terminal 31A to the fourth connection terminal 32A.

[0032] Furthermore, when the first low-side bidirectional switch LS1 is in the on state in the negative direction, the fourteenth switch element S14 is off and the twenty-fourth switch element S24 is on. In the on state in the negative direction, the first low-side bidirectional switch LS1 allows a current to flow from the first connection terminal 31A to the fifth connection terminal 32B through the body diode of the fourteenth switch element S14. On the other hand, it does not allow a current to flow from the fifth connection terminal 32B to the first connection terminal 31A.

[0033] The fourth state is the off state. In the off state, the eleventh switch element S11 and the twenty-first switch element S21 of the first high-side bidirectional switch HS1 are off. In the off state, the first high-side bidirectional switch HS1 does not allow a current to flow from the first connection terminal 31A to the fourth connection terminal 32A, nor does it allow a current to flow from the fourth connection terminal 32A to the first connection terminal 31A.

[0034] In the first low-side bidirectional switch LS1, when the first low-side bidirectional switch LS1 is in the off state, the fourteenth switch element S14 is off and the twenty-fourth switch element S24 is off. In the off state, the first low-side bidirectional switch LS1 does not allow a current to flow from the first connection terminal 31A to the fifth connection terminal 32B, nor does it allow a current to flow from the fifth connection terminal 32B to the first connection terminal 31A.

[0035] 3 , the second power conversion circuit 40 includes a pair of input terminals 41 and a pair of output terminals 42. The second power conversion circuit 40 also includes a fourth inductor L4, a first bidirectional tap switch TS1 as a first bidirectional switch, a second bidirectional tap switch TS2 as a second bidirectional switch, a transformer 43, and a second control unit 46.

[0036] As shown in FIG. 1 , the pair of input terminals 41 includes a first input terminal 41A and a second input terminal 41B. The first input terminal 41A is connected to the fourth connection terminal 32A. The second input terminal 41B is connected to the fifth connection terminal 32B. The pair of output terminals 42 includes a first output terminal 42A and a second output terminal 42B. AC power can be output from the pair of output terminals 42. Furthermore, as shown in FIG. 3 , the first input terminal 41A is connected to a first end of a fourth inductor L4.

[0037] The transformer 43 includes a primary winding 44 and a secondary winding 45. The primary winding 44 has a first terminal 44A, a second terminal 44B, and a tap 44C. The tap 44C is a terminal located between the first terminal 44A and the second terminal 44B, for example, in the middle.

[0038] A first terminal 44A of the primary winding 44 is connected to the second end of the fourth inductor L4 via the first bidirectional tap switch TS1. A tap 44C of the primary winding 44 is connected to the second end of the fourth inductor L4 via the second bidirectional tap switch TS2. A second end of the primary winding 44 is connected to the second input terminal 41B.

[0039] A first end of the secondary winding 45 is connected to the first output terminal 42A. A second end of the secondary winding 45 is connected to the second output terminal 42B. As shown in Fig. 1, each output terminal 42 is connected to the external output terminal 12 via a rectifier circuit 50 and an output low-pass filter 60. The primary winding 44 and the secondary winding 45 are electrically insulated from each other.

[0040] As shown in FIG. 3 , each bidirectional tap switch has two switch elements. Each switch element is an N-channel MOSFET. That is, each switch element has a body diode. Each bidirectional tap switch is composed of two switch elements connected in series so that the anode terminals of the body diodes are connected to each other. In other words, each bidirectional tap switch has two switch elements whose source terminals are connected to each other.

[0041] The first bidirectional tap switch TS1 has a first switch element SW1 and a second switch element SW2. The drain terminal of the first switch element SW1 is connected to the second end of the fourth inductor L4. The source terminal of the first switch element SW1 is connected to the source terminal of the second switch element SW2. The drain terminal of the second switch element SW2 is connected to the first end of the primary winding 44.

[0042] The second bidirectional tap switch TS2 has a third switch element SW3 and a fourth switch element SW4. The drain terminal of the third switch element SW3 is connected to the second end of the fourth inductor L4. The source terminal of the third switch element SW3 is connected to the source terminal of the fourth switch element SW4. The drain terminal of the fourth switch element SW4 is connected to the tap 44C of the primary winding 44.

[0043] Each bidirectional tap switch can be in four on / off states depending on the on / off combinations of the switch elements. The following describes the first bidirectional tap switch TS1 as an example. The first state is a bidirectional on state. In the bidirectional on state, the first switch element SW1 is on and the second switch element SW2 is on. In the bidirectional on state, the first bidirectional tap switch TS1 allows current to flow from the second end of the fourth inductor L4 to the first terminal 44A of the primary winding 44, and also allows current to flow from the first terminal 44A of the primary winding 44 to the second end of the fourth inductor L4.

[0044] The second state is a positive on state. In the positive on state, the first switch element SW1 is on and the second switch element SW2 is off. In the positive on state, the first bidirectional tap switch TS1 allows current to flow from the second end of the fourth inductor L4 to the first terminal 44A of the primary winding 44 via the body diode of the second switch element SW2. On the other hand, the first bidirectional tap switch TS1 does not allow current to flow from the first terminal 44A of the primary winding 44 to the second end of the fourth inductor L4.

[0045] The third state is a negative on state. In the negative on state, the first switch element SW1 of the first bidirectional tap switch TS1 is off and the second switch element SW2 is on. In the negative on state, the first bidirectional tap switch TS1 allows current to flow from the first terminal 44A of the primary winding 44 to the second end of the fourth inductor L4 via the body diode of the first switch element SW1. On the other hand, the first bidirectional tap switch TS1 does not allow current to flow from the second end of the fourth inductor L4 to the first terminal 44A of the primary winding 44 of the transformer 43.

[0046] The fourth state is the off state. In the off state, the first switch element SW1 and the second switch element SW2 of the first bidirectional tap switch TS1 are off. In the off state, the first bidirectional tap switch TS1 does not allow a current to flow from the second end of the fourth inductor L4 to the first terminal 44A of the primary winding 44 of the transformer 43, nor does it allow a current to flow from the first terminal 44A of the primary winding 44 to the second end of the fourth inductor L4.

[0047] The second control unit 46 controls the on / off of each switch element by inputting switching signals to the gate terminals of the two switch elements of each bidirectional tap switch. Specifically, the second control unit 46 inputs a first switching signal SG1 to the gate terminal of the first switch element SW1. The second control unit 46 inputs a second switching signal SG2 to the gate terminal of the second switch element SW2. The second control unit 46 inputs a third switching signal SG3 to the gate terminal of the third switch element SW3. The second control unit 46 inputs a fourth switching signal SG4 to the gate terminal of the fourth switch element SW4.

[0048] As shown in FIG. 1 , the rectifier circuit 50 is a full-wave rectifier circuit composed of four diodes. Specifically, the rectifier circuit 50 includes a first diode 51, a second diode 52, a third diode 53, and a fourth diode 54. The anode terminal of the first diode 51 is connected to the first output terminal 42A of the second power conversion circuit 40. The cathode terminal of the first diode 51 is connected to the cathode terminal of the third diode 53. The anode terminal of the third diode 53 is connected to the second output terminal 42B of the second power conversion circuit 40 and the cathode terminal of the fourth diode 54. The anode terminal of the fourth diode 54 is connected to the anode terminal of the second diode 52. The cathode terminal of the second diode 52 is connected to the first output terminal 42A of the second power conversion circuit 40 and the anode terminal of the first diode 51.

[0049] The cathode terminal of the first diode 51 and the cathode terminal of the third diode 53 are connected to the first external output terminal 12A via the output-side low-pass filter 60. The anode terminal of the second diode 52 and the anode terminal of the fourth diode 54 are connected to the second external output terminal 12B. Therefore, the first diode 51 allows current to flow from the first output terminal 42A to the first external output terminal 12A. The fourth diode 54 allows current to flow from the second external output terminal 12B to the second output terminal 42B. The third diode 53 allows current to flow from the second output terminal 42B to the first external output terminal 12A. The second diode 52 allows current to flow from the second external output terminal 12B to the first output terminal 42A.

[0050] The output-side low-pass filter 60 includes a fifth inductor L5 and a fourth capacitor C4. A first end of the fifth inductor L5 is connected to the cathode terminal of the first diode 51 and the cathode terminal of the third diode 53. A second end of the fifth inductor L5 is connected to the first external output terminal 12A. A first end of the fourth capacitor C4 is connected to the second end of the fifth inductor L5. A second end of the fourth capacitor C4 is connected to the second external output terminal 12B.

[0051] (2. Definition of Sectors) As described above, three-phase AC power is input to the input-side connection terminal 31 of the first power conversion circuit 30 from the three-phase AC power supply 80 via the external input terminal 11 and the input-side low-pass filter 20. As shown in FIG. 4 , the three phase voltages of the three-phase AC power are a first voltage VA, a second voltage VB, and a third voltage VC, which are AC voltages of different phases. The first connection terminal 31A, the second connection terminal 31B, and the third connection terminal 31C are input with one-to-one correspondence to each other. Specifically, the first voltage VA is input to the first connection terminal 31A. The second voltage VB is input to the second connection terminal 31B. The third voltage VC is input to the third connection terminal 31C. The second voltage VB has a phase difference of 120° with respect to the first voltage VA. The third voltage VC has a phase difference of 120° with respect to the second voltage VB. The "phase difference of 120°" allows for an error of about ±1°.

[0052] In the following, the timing of the phase when the first voltage VA is maximum is defined as 0°. Furthermore, the timing of the phase when the first voltage VA is minimum is defined as -180°. Therefore, one cycle of the first voltage VA, the second voltage VB, and the third voltage VC is expressed as a phase range of -180° or more and less than 180°. For convenience, the timing of the voltage phase may also be expressed as a phase of 180° or more. When the voltage phase is expressed as a phase of 180° or more, X° is synonymous with (-180° + (X - 180°)). Here, sectors 1 to 6 are defined as six periods obtained by dividing one cycle into equal parts. Specifically, when the phase of the first voltage VA is defined as "θ°," sectors 1 to 6 are defined as the following periods spaced at 60° intervals:

[0053] Sector 1: -30°≦θ°<30° Sector 2: 30°≦θ°<90° Sector 3: 90°≦θ°<150° Sector 4: 150°≦θ°<180° and -180°≦θ°<-150° Sector 5: -150°≦θ°<-90° Sector 6: -90°≦θ°<-30° Furthermore, each of sectors 1 to 6 is subdivided into two periods. In other words, one cycle of three-phase AC power is subdivided into 12 periods. In the following, n is an integer between 1 and 6, and corresponds to the sector number. In this case, sector n is subdivided into two periods, sector na and sector nb. In this embodiment, each period is defined as follows:

[0054] Sector na: (X°-30°)≦θ°<X° Sector nb: X°≦θ°<(X°+30°) Note that X° is the midpoint value of the period of sector n. The "midpoint of the period" is the intermediate value between the endpoints of each sector expressed as a half-open interval. However, in the definition of sector 4a, X=180°.

[0055] (3. Regarding Vector Sequence) In the following, the potential difference between the fourth connection terminal 32A and the fifth connection terminal 32B is referred to as the primary voltage Vp. In other words, the primary voltage Vp is the potential difference between the first input terminal 41A and the second input terminal 41B of the second power conversion circuit 40. Furthermore, the current flowing between the fourth connection terminal 32A and the fifth connection terminal 32B is referred to as the primary current Ip. In other words, the primary current Ip is the current flowing between the second input terminal 41B and the first input terminal 41A of the second power conversion circuit 40. Specifically, the primary current Ip is the current flowing through the fourth inductor L4, each bidirectional tap switch, and the primary winding 44 of the second power conversion circuit 40. The direction in which the primary current Ip flows from the first input terminal 41A to the second input terminal 41B is referred to as the positive direction. The direction in which the primary current Ip flows from the second input terminal 41B to the first input terminal 41A is referred to as the negative direction.

[0056] Furthermore, the phase having the first voltage VA is referred to as the A phase, the phase having the second voltage VB is referred to as the B phase, and the phase having the third voltage VC is referred to as the C phase, with any one of these phases being referred to as the i phase. Of these three phases, the phase other than the i phase is referred to as the j phase. Hereinafter, the voltage difference obtained by subtracting the j phase voltage from the i phase voltage will be referred to as the "line voltage Vij."

[0057] The first control unit 33 controls the pulse width of each switching signal by space vector pulse width modulation (SVPWM).

[0058] As shown in Figures 5 and 6, an active vector and a zero vector Iz are defined for control using SVPWM. In this embodiment, the active vector and the zero vector Iz are current vectors of the first power conversion circuit 30 in a predetermined switching state. When m is an integer between 1 and 6, the active vector is expressed as a space vector by the following mathematical expression 1. In this mathematical expression 1, "I" is the absolute value of the primary current Ip.

[0059]

[0060] The active vectors are roughly classified into positive direction active vectors and negative direction active vectors. Specifically, as shown in Fig. 5, the positive direction active vectors include a first positive direction active vector I1+ to a sixth positive direction active vector I6+. Each positive direction active vector is a current vector when the primary voltage Vp has a positive value and each bidirectional switch TSW is in the following switching state:

[0061] First positive active vector I1+: The second low-side bidirectional switch LS2 is on in both directions or in the positive on state, and the first high-side bidirectional switch HS1 is on in both directions or in the positive on state. At this time, the primary voltage Vp is the line voltage VAB.

[0062] Second positive active vector I2+: The first high-side bidirectional switch HS1 is on in both directions or in the positive on state, and the third low-side bidirectional switch LS3 is on in both directions or in the positive on state. At this time, the primary voltage Vp is the line voltage VAC.

[0063] Third positive active vector I3+: The third low-side bidirectional switch LS3 is on in both directions or in the positive on state, and the second high-side bidirectional switch HS2 is on in both directions or in the positive on state. At this time, the primary voltage Vp is the line voltage VBC.

[0064] Fourth positive active vector I4+: The second high-side bidirectional switch HS2 is on in both directions or in the positive on state, and the first low-side bidirectional switch LS1 is on in both directions or in the positive on state. At this time, the primary voltage Vp is the line voltage VBA.

[0065] Fifth positive active vector I5+: The first low-side bidirectional switch LS1 is on in both directions or in the positive on state, and the third high-side bidirectional switch HS3 is on in both directions or in the positive on state. At this time, the primary voltage Vp is the line voltage VCA.

[0066] Sixth positive active vector I6+: The third high-side bidirectional switch HS3 is on in both directions or in the positive on state, and the second low-side bidirectional switch LS2 is on in both directions or in the positive on state. At this time, the primary voltage Vp is the line voltage VCB.

[0067] 6, the negative direction active vectors include a first negative direction active vector I1 to a sixth negative direction active vector I6. Each negative direction active vector is a current vector when the primary voltage Vp has a negative value and each bidirectional switch TSW is in the following switching state.

[0068] First negative active vector I1−: The second high-side bidirectional switch HS2 is on in both directions or in the negative direction. The first low-side bidirectional switch LS1 is on in both directions or in the negative direction. At this time, the primary voltage Vp is the line voltage VAB.

[0069] Second negative active vector I2−: The first low-side bidirectional switch LS1 is on in both directions or in the negative direction. The third high-side bidirectional switch HS3 is on in both directions or in the negative direction. At this time, the primary voltage Vp is the line voltage VAC.

[0070] Third negative active vector I3−: The third high-side bidirectional switch HS3 is on in both directions or in the negative direction. The second low-side bidirectional switch LS2 is on in both directions or in the negative direction. At this time, the primary voltage Vp is the line voltage VBC.

[0071] Fourth negative active vector I4−: The second low-side bidirectional switch LS2 is on in both directions or in the negative direction. The first high-side bidirectional switch HS1 is on in both directions or in the negative direction. At this time, the primary voltage Vp is the line voltage VBA.

[0072] Fifth negative active vector I5−: The first high-side bidirectional switch HS1 is on in both directions or in the negative direction. The third low-side bidirectional switch LS3 is on in both directions or in the negative direction. At this time, the primary voltage Vp is the line voltage VCA.

[0073] Sixth negative active vector I6−: The third low-side bidirectional switch LS3 is on in both directions or in the negative direction. The second high-side bidirectional switch HS2 is on in both directions or in the negative direction. At this time, the primary voltage Vp is the line voltage VCB.

[0074] 5 and 6, the zero vector Iz has a seventh zero vector I7, an eighth zero vector I8, and a ninth zero vector I9. The zero vector Iz is a current vector when the primary voltage Vp is zero and the following switching state is in effect. Note that "the primary voltage is zero" allows for an error of about ±10 V, for example.

[0075] Seventh zero vector I7: The first high-side bidirectional switch HS1 is in a bidirectional on state or a positive on state, and the first low-side bidirectional switch LS1 is in a bidirectional on state or a positive on state, or the first high-side bidirectional switch HS1 is in a bidirectional on state or a negative on state, and the first low-side bidirectional switch LS1 is in a bidirectional on state or a negative on state.

[0076] Eighth zero vector I8: The second high-side bidirectional switch HS2 is in a bidirectional on state or a positive on state, and the second low-side bidirectional switch LS2 is in a bidirectional on state or a positive on state, or the second high-side bidirectional switch HS2 is in a bidirectional on state or a negative on state, and the second low-side bidirectional switch LS2 is in a bidirectional on state or a negative on state.

[0077] Ninth zero vector I9: The third high-side bidirectional switch HS3 is in a bidirectional on state or a positive on state, and the third low-side bidirectional switch LS3 is in a bidirectional on state or a positive on state, or the third high-side bidirectional switch HS3 is in a bidirectional on state or a negative on state, and the third low-side bidirectional switch LS3 is in a bidirectional on state or a negative on state.

[0078] The current reference vector Ir in sector n is approximated as a composite vector of the active vector and the zero vector Iz. Specifically, when x = n and y = x + 1, the approximation is as follows: However, when x = 6, y = 1. The current reference vector Ir in sector n is approximated as a composite vector of the x-th positive direction active vector Ix+, the y-th positive direction active vector Iy+, and the zero vector Iz. Alternatively, the current reference vector Ir in sector n is approximated as a composite vector of the x-th negative direction active vector Ix-, the y-th negative direction active vector Iy-, and the zero vector Iz. The zero vector Iz is the seventh zero vector I7 in sectors 1 and 4. The zero vector Iz is the ninth zero vector I9 in sectors 2 and 5. The zero vector Iz is the eighth zero vector I8 in sectors 3 and 6.

[0079] In principle, the first control unit 33 controls the on / off of each bidirectional switch TSW so that the reference vector Ir transitions between the above-mentioned active vector or zero vector Iz in a certain order depending on the magnitude relationship between the first voltage VA, the second voltage VB, and the third voltage VC.

[0080] Specifically, in sector na, the first control unit 33 controls the on / off of each bidirectional switch TSW so that the current vector follows the first vector sequence shown in (a) below. In sector nb, the first control unit 33 controls the on / off of each bidirectional switch TSW so that the current vector follows the second vector sequence shown in (b) below. (a) First vector sequence: The order is x-th positive direction active vector Ix+, y-th positive direction active vector Iy+, zero vector Iz, x-th negative direction active vector Ix-, y-th negative direction active vector Iy-, zero vector Iz. (b) Second vector sequence: The order is y-th positive direction active vector Iy+, x-th positive direction active vector Ix+, zero vector Iz, y-th negative direction active vector Iy-, x-th negative direction active vector Ix-, zero vector Iz.

[0081] Therefore, for example, when n=3, that is, in the sector 3a, the first control unit 33 controls the on / off of each bidirectional switch TSW so that the reference vector Ir transitions in the order of the third positive direction active vector I3+, the fourth positive direction active vector I4+, the eighth zero vector I8, the third negative direction active vector I3-, the fourth negative direction active vector I4-, and the eighth zero vector I8. In the sector 3b, the first control unit 33 controls the on / off of each bidirectional switch TSW so that the reference vector Ir transitions in the order of the fourth positive direction active vector I4+, the third positive direction active vector I3+, the eighth zero vector I8, the fourth negative direction active vector I4-, the third negative direction active vector I3-, and the eighth zero vector I8.

[0082] In the sector na, the first control unit 33 repeats the on / off control of each bidirectional switch TSW in the first vector sequence at a constant cycle Ts. The period of the cycle Ts is very short compared to each period of the sector na.

[0083] Furthermore, in the sector nb, the first control unit 33 repeats the on / off control of each bidirectional switch TSW in the second vector sequence with the same period Ts as the first vector sequence. The period Ts is very short compared to each period of the sector nb.

[0084] (4. Tap Switching Control) The following describes switching control that can be performed by the second control unit 46. First, the changes in the primary voltage Vp and the primary current Ip in one period Ts of the vector sequence will be described using the first vector sequence as an example.

[0085] 7 to 10, between time t0 and time t1, the reference vector Ir is the xth positive active vector Ix+. That is, the primary voltage Vp is positive. During this period, the direction of the primary current Ip changes from negative to positive.

[0086] Between time t1 and time t2, the reference vector Ir is the y-th positive active vector Iy+, i.e., the primary voltage Vp is positive. During this period, the direction of the primary current Ip is positive.

[0087] Between time t2 and time t3, the reference vector Ir is the zero vector Iz. That is, the primary voltage Vp is approximately zero. During this period, the direction of the primary current Ip is positive.

[0088] Between time t3 and time t4, the reference vector Ir is the x-th negative active vector Ix-, i.e., the primary voltage Vp is negative. During this period, the direction of the primary current Ip changes from positive to negative.

[0089] Between time t4 and time t5, the reference vector Ir is the y-th negative active vector Iy-, i.e., the primary voltage Vp is negative. During this period, the direction of the primary current Ip is negative.

[0090] Between time t5 and time t6, the reference vector Ir is the zero vector Iz. That is, the primary voltage Vp is approximately zero. During this period, the direction of the primary current Ip is negative.

[0091] 10 illustrates the period from time t3 to time t6 and the period from time t0' to time t3' in the next cycle of that period. The changes in the primary voltage Vp and the primary current Ip from time t0' to time t3' are similar to the changes in the primary voltage Vp and the primary current Ip from time t0 to time t3.

[0092] In the second vector sequence, the changes in the primary voltage Vp and the primary current Ip in one period Ts are similar to the changes in the primary voltage Vp and the primary current Ip in the first vector sequence.

[0093] The second control unit 46 switches the on / off states of the first bidirectional tap switch TS1 and the second bidirectional tap switch TS2 during a period in which the absolute value of the primary voltage Vp applied between the first terminal 44A and the second terminal 44B of the primary winding 44 is equal to or less than a predetermined threshold. In this embodiment, the predetermined threshold is 10 V. That is, the second control unit 46 switches the on / off states of the first bidirectional tap switch TS1 and the second bidirectional tap switch TS2 during a period in which the primary voltage Vp is equal to or greater than −10 V and equal to or less than 10 V. Hereinafter, the period in which the absolute value of the primary voltage Vp is equal to or less than the threshold will be referred to as the “period in which the primary voltage is substantially zero.” Specifically, the period in which the primary voltage Vp is substantially zero is the period in which the reference vector Ir is the zero vector Iz in the vector sequence. That is, these periods are the period from time t2 to time t3 and the period from time t5 to time t6.

[0094] The second control unit 46 is capable of executing tap switching control for the first bidirectional tap switch TS1 and the second bidirectional tap switch TS2. The tap switching control is control for switching between a state in which the first bidirectional tap switch TS1 is on in both directions and the second bidirectional tap switch TS2 is off, and a state in which the first bidirectional tap switch TS1 is off and the second bidirectional tap switch TS2 is on in both directions.

[0095] In the switching control by the second control unit 46, the combinations of the on and off states of the first switch element SW1, the second switch element SW2, the third switch element SW3, and the fourth switch element SW4 differ depending on whether the primary current Ip is flowing in the positive direction or the negative direction. That is, in the switching control, the switching pattern for each switch element executed by the second control unit 46 differs depending on the direction of the primary current Ip. Specifically, there are four switching patterns, a first switching pattern to a fourth switching pattern. The first switching pattern to the fourth switching pattern correspond to the first initial state to the fourth initial state of the second power conversion circuit 40, respectively. Note that the switching pattern here refers to a combination of the on and off states of each switch element.

[0096] (4-1. First Switching Pattern) The second control unit 46 can control each bidirectional tap switch according to the first switching pattern while the primary current Ip is flowing in the forward direction. Specifically, as shown in FIG. 7 , the second control unit 46 defines the state at time t2 as a first initial state and controls each bidirectional tap switch according to the first switching pattern. In the following description, three times within the period from time t2 to time t3 are defined as time SC1, time SC2, and time SC3, in order of closest to time t2. In addition, in the first initial state, the first bidirectional tap switch TS1 is in a bidirectional ON state, the second bidirectional tap switch TS2 is in an OFF state, and the primary current Ip flows in the forward direction.

[0097] The first switching pattern is composed of the following first to third controls. Specifically, at time SC1, the second control unit 46 executes first control, which switches the second switch element SW2 from the first initial state to the OFF state and switches the third switch element SW3 to the ON state. At time SC2, i.e., after the first control, the second control unit 46 executes second control, which switches the fourth switch element SW4 to the ON state. At time SC3, i.e., after the second control, the second control unit 46 executes third control, which switches the first switch element SW1 to the OFF state. Therefore, at time t3 when the third control is completed, the first bidirectional tap switch TS1 is in the OFF state and the second bidirectional tap switch TS2 is in the ON state in both directions.

[0098] (4-2. Second Switching Pattern) The second control unit 46 can control each bidirectional tap switch according to the second switching pattern while the primary current Ip is flowing in the negative direction. Specifically, as shown in FIG. 8 , the second control unit 46 defines the state at time t5 as a second initial state and controls each bidirectional tap switch according to the second switching pattern. In the following description, three times within the period from time t5 to time t6 are defined as time SC4, time SC5, and time SC6, in order of closest to time t5. In addition, in the second initial state, the first bidirectional tap switch TS1 is in a bidirectional ON state, the second bidirectional tap switch TS2 is in an OFF state, and the primary current Ip flows in the negative direction.

[0099] The second switching pattern is composed of the following first to third controls. Specifically, at time SC4, the second control unit 46 executes first control, which switches the first switch element SW1 from the second initial state to the OFF state and switches the fourth switch element SW4 to the ON state. At time SC5, i.e., after the first control, the second control unit 46 executes second control, which switches the third switch element SW3 to the ON state. At time SC6, i.e., after the second control, the second control unit 46 executes third control, which switches the second switch element SW2 to the OFF state. Therefore, at time t6 when the third control is completed, the first bidirectional tap switch TS1 is in the OFF state and the second bidirectional tap switch TS2 is in the ON state in both directions.

[0100] The on / off states of the bidirectional tap switches in the initial states of the first and second switching patterns are the same as those after the third control ends for each switching pattern. However, the direction of the primary current Ip in each initial state is different. Therefore, the switching patterns in the tap switching control are different between the first switching pattern when the primary current Ip flows in the positive direction and the second switching pattern when the primary current Ip flows in the negative direction.

[0101] (4-3. Third Switching Pattern) The second control unit 46 can control each bidirectional tap switch according to the third switching pattern. Specifically, as shown in FIG. 9 , the second control unit 46 defines the state at time t2 as the third initial state and controls each bidirectional tap switch according to the third switching pattern. In the following description, two times within the period from time t2 to time t3 are defined as time SC7 and time SC8, in order of closest to time t2. A time within the period from time t5 to time t6 is defined as time SC9. In the third initial state, the first bidirectional tap switch TS1 is in the OFF state, the second bidirectional tap switch TS2 is in the ON state in both directions, and the primary current Ip flows in the positive direction.

[0102] The third switching pattern is composed of the following first to third controls. Specifically, at time SC7, the second control unit 46 executes the first control, which switches the fourth switch element SW4 from the third initial state to the OFF state and switches the first switch element SW1 to the ON state. At time SC8, i.e., after the first control, the second control unit 46 executes the second control, which switches the second switch element SW2 to the ON state. At time SC9, i.e., after the second control, the second control unit 46 executes the third control, which switches the third switch element SW3 to the OFF state. Therefore, in the fourth switching pattern, the second control unit 46 executes the first control and the second control during a period in which the primary current Ip flows in the positive direction, and executes the third control during a period in which the primary current Ip flows in the negative direction. Then, at time t3 when the third control is completed, the first bidirectional tap switch TS1 is in the OFF state and the second bidirectional tap switch TS2 is in the ON state in both directions.

[0103] (4-4. Fourth Switching Pattern) The second control unit 46 can control each bidirectional tap switch according to the fourth switching pattern while the primary current Ip is flowing in the negative direction. Specifically, as shown in FIG. 10 , the second control unit 46 defines the state at time t5 as the fourth initial state and controls each bidirectional tap switch according to the fourth switching pattern. In the following description, two times within the period from time t5 to time t6 are defined as time SC10 and time SC11, in order of proximity to time t5. A time within the period from time t2′ to time t3′ is defined as time SC12. In addition, in the fourth initial state, the first bidirectional tap switch TS1 is in the OFF state, the second bidirectional tap switch TS2 is in the ON state in both directions, and the primary current Ip flows in the negative direction.

[0104] The fourth switching pattern is composed of the following first to third controls. Specifically, at time SC10, the second control unit 46 executes the first control, which switches the third switch element SW3 from the fourth initial state to the OFF state and switches the second switch element SW2 to the ON state. At time SC11, i.e., after the first control, the second control unit 46 executes the second control, which switches the first switch element SW1 to the ON state. At time SC12, i.e., after the second control, the second control unit 46 executes the third control, which switches the fourth switch element SW4 to the OFF state. Therefore, in the fourth switching pattern, the second control unit 46 executes the first control and the second control during a period when the primary current Ip is flowing in the negative direction, and executes the third control during a period when the primary current Ip is flowing in the positive direction. Then, at time t3 when the third control is completed, the first bidirectional tap switch TS1 is in the OFF state and the second bidirectional tap switch TS2 is in the ON state in both directions.

[0105] The on / off states of the bidirectional tap switches in the initial states of the third and fourth switching patterns are the same as those after the third control ends for each switching pattern. However, the direction of the primary current Ip in each initial state is different. Therefore, the switching patterns in the tap switching control are different between the third switching pattern when the primary current Ip flows in the positive direction and the fourth switching pattern when the primary current Ip flows in the negative direction.

[0106] (5. Effects of the Present Embodiment) (1) In the above embodiment, the primary winding 44 of the transformer 43 has a tap 44C. The tap 44C is connected to the input terminal 41 via the second bidirectional tap switch TS2, thereby changing the effective turns ratio of the transformer 43. Changing the effective number of turns of the transformer 43 in this manner also makes it possible to accommodate changes in the magnitude of the input voltage. As a result, increases in switching loss and conduction loss can be suppressed compared to when changes in the magnitude of the input voltage are accommodated by adjusting the periods during which the switch elements in the first power conversion circuit 30 are in the on state and the off state, or when a converter for adjusting the voltage is added to the output terminal 42 side of the second power conversion circuit 40.

[0107] (2) In the above embodiment, each bidirectional tap switch is on / off controlled during a period when the primary voltage Vp is substantially zero. If on / off control is performed when a voltage is applied to each bidirectional tap switch, an overvoltage may be applied to the switch element of the bidirectional tap switch, or an overcurrent may flow. Therefore, by performing on / off control during a period when the primary voltage Vp is substantially zero, it is possible to prevent breakdown of the switch element due to the overvoltage or overcurrent and reduce switching loss.

[0108] (3) In the above embodiment, each bidirectional tap switch is configured with two MOSFETs. This allows for relatively high-speed switching. In addition, because power loss is relatively small, this is suitable as a switch element for configuring a bidirectional tap switch.

[0109] (4) In the above embodiment, the second control unit 46 can control each bidirectional tap switch according to the first switching pattern. In the first switching pattern, at time SC1, the voltage applied between the drain terminal and the source terminal of the third switch element SW3 is approximately zero. That is, the first control is so-called zero-voltage switching. Also, at time SC2, the voltage applied between the drain terminal and the source terminal of the fourth switch element SW4 is approximately zero. That is, the second control is also zero-voltage switching. Furthermore, at time SC3, almost no current flows between the drain terminal and the source terminal of the first switch element SW1. That is, the third control is zero-current switching. These soft switching operations hardly generate switching losses. Furthermore, because the load on the switch elements when switching on and off is small, the possibility of the switch elements failing is low.

[0110] In addition, the first switching pattern executes the first to third controls during a period in which the primary current Ip flows in the positive direction. For example, if the third control is not executed after the second control and the primary current Ip flows in the negative direction, there is a risk that a current will flow between the first terminal 44A and the tap 44C of the primary winding 44 via each bidirectional tap switch. In other words, there is a risk that a current will flow through an unintended path in the primary winding 44. Therefore, according to this embodiment, it is possible to prevent a current from flowing through such an unintended path.

[0111] (5) In the above embodiment, the second control unit 46 can control each bidirectional tap switch according to a second switching pattern. In the second switching pattern, the first control and the second control are zero-voltage switching. The third control is zero-current switching. Therefore, even in the second initial state, the second control unit 46 can switch the on / off state of each bidirectional tap switch by soft switching.

[0112] Furthermore, the second switching pattern executes the first to third controls during a period in which the primary current Ip flows in the negative direction. For example, if the third control is not executed after the second control and the primary current Ip flows in the positive direction, there is a risk that a current will flow between the first terminal 44A and the tap 44C of the primary winding 44 via each bidirectional tap switch. In other words, there is a risk that a current will flow through an unintended path in the primary winding 44. Therefore, according to this embodiment, it is possible to prevent a current from flowing through such an unintended path.

[0113] (6) In the above embodiment, the second control unit 46 can control each bidirectional tap switch according to the third switching pattern. In the third switching pattern, the first control and the second control are zero-voltage switching. Therefore, the second control unit 46 can switch the on / off state of each bidirectional tap switch even in the third initial state. Furthermore, the switching loss of the first switch element SW1 and the second switch element SW2 can be reduced compared to when control according to the third switching pattern is not performed.

[0114] (7) In the above embodiment, in the third switching pattern, the second control unit 46 can execute the third control during the period when the primary current Ip is flowing in the negative direction. After the second control in the third switching pattern, and during the period when the primary current Ip is flowing in the negative direction within the same cycle, no current flows through the second bidirectional tap switch TS2. In other words, the third control is zero-current switching. Therefore, by executing the third control during the period when the primary current Ip is flowing in the negative direction, switching loss can be reduced. Furthermore, the possibility of failure of the switch element can be reduced.

[0115] (8) In the above embodiment, the second control unit 46 can control each bidirectional tap switch according to the fourth switching pattern. In the fourth switching pattern, the first control and the second control are zero-voltage switching. Therefore, the second control unit 46 can switch the on / off state of each bidirectional tap switch even in the fourth initial state. Furthermore, the switching loss of the first switch element SW1 and the second switch element SW2 can be reduced compared to when control according to the fourth switching pattern is not performed.

[0116] (9) In the above embodiment, in the fourth switching pattern, the second control unit 46 can execute the third control during the period when the primary current Ip is flowing in the positive direction. After the second control in the fourth switching pattern, when the primary current Ip is flowing in the positive direction within the next cycle, no current flows through the second bidirectional tap switch TS2. In other words, the third control is zero-current switching. Therefore, by executing the third control during the period when the primary current Ip is flowing in the positive direction, switching loss can be reduced. Furthermore, the possibility of failure of the switch element can be reduced.

[0117] (10) In the above embodiment, the primary winding 44 has the tap 44C, not the secondary winding 45. The secondary voltage generated in the secondary winding 45 is generated by electromagnetic induction in the transformer 43. The phase of the secondary voltage lags the phase of the input voltage and the primary voltage Vp. Therefore, if the secondary winding 45 had a switch for switching the tap and turns ratio, it would be relatively difficult to synchronize the timing of switching the switch with the input voltage. Therefore, by having the tap 44C in the primary winding 44, soft switching can be easily performed in synchronization with changes in the input voltage during switching control.

[0118] Furthermore, the magnitude of the current flowing through the secondary winding 45 may be larger than the magnitude of the current flowing through the primary winding 44. If a switch element included in one of the bidirectional tap switches is turned off while a current is flowing through that switch element, the larger the current flowing through that switch, the larger the switching loss and the higher the possibility of the switch failing. Therefore, by providing the tap 44C in the primary winding 44, it is possible to prevent the switching loss from increasing. It is also possible to reduce the possibility of the switch failing.

[0119] <Modifications> The above embodiment and the following modifications can be implemented in combination with each other within the scope of technical compatibility.

[0120] (Modifications to the Configuration of the Power Converter) The configuration of the power converter 10 is not limited to the example of the above embodiment. For example, the power converter 10 does not have to include one or more selected from the input low-pass filter 20, the first power conversion circuit 30, the rectifier circuit 50, and the output low-pass filter 60.

[0121] The three-phase AC power supply 80 connected to the three external input terminals 11 is not limited to a three-phase three-wire system. The three-phase AC power supply 80 may be a three-phase four-wire system or a delta-connected three-phase three-wire system. The configuration of the power conversion device 10 may be changed as appropriate depending on the type of the three-phase AC power supply 80.

[0122] When the three-phase AC power supply 80 is delta-connected, a plurality of capacitors may be provided, instead of the input-side low-pass filter 20, connected between the lines of each phase to which the first voltage VA, the second voltage VB, and the third voltage VC are input.

[0123] The first control unit 33 of the first power conversion circuit 30 and the second control unit 46 of the second power conversion circuit 40 are not limited to being configured as separate chips. That is, the second control unit 46 may be capable of outputting the eleventh switching signal SG11 to the sixteenth switching signal SG16 and the twenty-first switching signal SG21 to the twenty-sixth switching signal SG26. Regardless of the configuration of each control unit, the control circuit capable of outputting the first switching signal SG1 to the fourth switching signal SG4 can be said to be the second control unit 46 of the second power conversion circuit 40.

[0124] The switch elements constituting the first bidirectional tap switch TS1 are not limited to those described in the above embodiment. For example, the two switch elements of the bidirectional switch TSW may be P-channel MOSFETs. In this case, the drain terminals of the two switch elements of the bidirectional switch TSW are connected to each other. This also applies to the second bidirectional tap switch TS2 and the bidirectional switch TSW of the first power conversion circuit 30.

[0125] The two switch elements included in each of the first bidirectional tap switch TS1 and the second bidirectional tap switch TS2 may be transistors capable of allowing current to flow in both the forward and reverse directions. In this case, the two switch elements are connected in series with their source terminals connected to each other. Specifically, the switch elements may be gallium nitride high electron mobility transistors (GaN-High Electron Mobility Transistors, GaN-HEMTs) or the like. More specifically, the first bidirectional tap switch TS1 may include a first switch element SW1 having a drain terminal connected to the first input terminal 41A and a second switch element SW2 having a drain terminal connected to the first terminal 44A of the primary winding 44 and a source terminal connected to the source terminal of the first switch element SW1. The second bidirectional tap switch TS2 may also include a third switch element SW3 having a drain terminal connected to the first input terminal 41A, and a fourth switch element SW4 having a drain terminal connected to the tap 44C of the primary winding 44 and a source terminal connected to the source terminal of the third switch element SW3.

[0126] The same applies to the bidirectional switch TSW of the first power conversion circuit 30. The second power conversion circuit 40 does not need to include the fourth inductor L4. In this case, the leakage inductance of the transformer 43 can be used for resonance instead of the fourth inductor L4.

[0127] The configuration of the tap 44C of the primary winding 44 is not limited to the example in the above embodiment. For example, the position of the tap 44C does not have to be located in the middle of the primary winding 44, as long as it is between the first terminal 44A and the second terminal 44B. Furthermore, the primary winding 44 may have two or more terminals similar to the tap 44C, and the second power conversion circuit 40 may have bidirectional tap switches corresponding to each tap 44C.

[0128] When the primary winding 44 has two or more taps 44C, for example, the following configuration may be adopted in addition to the configuration of the second power conversion circuit 40 of the above embodiment. The tap 44C in the second power conversion circuit 40 of the above embodiment is defined as the first tap. The primary winding has a second tap between the first tap of the primary winding and the second terminal. The power conversion circuit includes a third bidirectional tap switch having a first end connected to the first input terminal and a second end connected to the second tap of the primary winding. In this case, the second control unit may perform the switching control of the above embodiment for two bidirectional tap switches, namely, the bidirectional tap switch to be turned on in both directions and the bidirectional tap switch to be turned off, among the first to third bidirectional tap switches.

[0129] The specific circuit configuration of the rectifier circuit 50 is not limited to the example in the above embodiment. For example, the rectifier circuit 50 may be a half-wave rectifier circuit. In the above embodiment, which of the three input terminals 41 of the second power conversion circuit 40 corresponds to the first connection terminal 31A, the second connection terminal 31B, and the third connection terminal 31C can be changed as appropriate.

[0130] (Modification of Tap Switching Control) The second control unit 46 does not have to switch the on / off states of the first bidirectional tap switch TS1 and the second bidirectional tap switch TS2 during a period in which the absolute value of the primary voltage Vp is equal to or less than a predetermined threshold. The above range does not have to be a period in which the primary voltage Vp is approximately zero, and the above range does not have to be specified. Even in this case, as long as the effective turns ratio of the transformer 43 can be changed by the tap 44C of the primary winding 44, at least the effect described in (1) can be obtained.

[0131] Furthermore, when determining whether the absolute value of the primary voltage Vp is equal to or less than the predetermined threshold, the second control unit 46 does not need to calculate the absolute value of the primary voltage Vp; it is sufficient that the absolute value of the primary voltage Vp is substantially equal to or less than the predetermined threshold. For example, the second control unit 46 may switch the on / off state of each bidirectional tap switch during a period when the primary voltage Vp is within a predetermined range of values ​​including zero. The predetermined range of values ​​may be, for example, from -10 V to 10 V.

[0132] The second control unit 46 may execute any one of the first to fourth switching patterns. In addition, in the tap switching control, the switching pattern may be the same when the primary current Ip flows in the positive direction and when it flows in the negative direction.

[0133] The second control unit 46 does not have to be able to perform switching control according to the first to fourth switching patterns. It is sufficient that the second control unit 46 can perform switching control to switch the on / off states of the first bidirectional tap switch TS1 and the second bidirectional tap switch TS2 according to any switching pattern. Even if soft switching is not performed, at least the effect described in (1) can be obtained as long as the effective turns ratio of the transformer 43 can be changed by the tap 44C of the primary winding 44.

[0134] In the third switching pattern, the third control does not have to be performed during the period when the primary current Ip is flowing in the negative direction. Even in this case, the first control and the second control are performed during the period when the primary current Ip is flowing in the positive direction, thereby reducing the switching loss of the first switch element SW1 and the second switch element SW2.

[0135] In the fourth switching pattern, the third control does not have to be performed during the period when the primary current Ip is flowing in the positive direction. Even in this case, the first control and the second control are performed during the period when the primary current Ip is flowing in the negative direction, thereby reducing the switching loss of the first switch element SW1 and the second switch element SW2.

[0136] <Supplementary Notes> The technical ideas that can be derived from the above embodiments and modified examples are described below. [1] A power conversion circuit comprising: a transformer having a primary winding having a first terminal and a second terminal, and a secondary winding, an input terminal, a first bidirectional switch having a first end connected to the input terminal and a second end connected to the first terminal of the primary winding, a second bidirectional switch having a first end and a second end, the first end connected to the input terminal, and a controller that controls the first bidirectional switch and the second bidirectional switch, wherein the primary winding has a tap between the first terminal and the second terminal of the primary winding, and the second end of the second bidirectional switch is connected to the tap.

[0137] [2] The power conversion circuit according to [1], wherein the control unit switches the on / off states of the first bidirectional switch and the second bidirectional switch during a period in which an absolute value of a voltage applied between the first terminal and the second terminal of the primary winding is equal to or less than a predetermined threshold.

[0138] [3] The power conversion circuit according to [1] or [2], wherein the first bidirectional switch has a first switch element, a cathode terminal of a body diode of which is connected to the input terminal, and a second switch element, a cathode terminal of which is connected to the first terminal of the primary winding, and the first switch element and the second switch element have anode terminals of their body diodes connected to each other; the second bidirectional switch has a third switch element, a cathode terminal of whose body diode is connected to the input terminal, and a fourth switch element, a cathode terminal of whose body diode is connected to the tap of the primary winding, and the third switch element and the fourth switch element have anode terminals of their body diodes connected to each other; and the control unit controls the first bidirectional switch by controlling on / off of the first switch element and the second switch element, and controls the second bidirectional switch by controlling the third switch element and the fourth switch element.

[0139] [4] The power conversion circuit according to [3], wherein the control unit is capable of executing switching control to switch from a state in which both the first switch element and the second switch element are in an on state and both the third switch element and the fourth switch element are in an off state to a state in which both the first switch element and the second switch element are in an off state and both the third switch element and the fourth switch element are in an on state, and in the switching control, the control unit controls so that a combination of the on state and the off state of the first switch element, the second switch element, the third switch element, and the fourth switch element differs between a case in which a current flows from the first terminal side to the second terminal side of the primary winding and a case in which a current flows from the second terminal side to the first terminal side.

[0140] [5] The power conversion circuit according to [3] or [4], wherein, when an initial state is one in which both the first switch element and the second switch element are in an on state, both the third switch element and the fourth switch element are in an off state, and a current is flowing from the first terminal to the second terminal of the primary winding, the control unit is capable of executing, in the initial state, the following during a period in which a current is flowing from the first terminal side to the second terminal side of the primary winding: a first control of switching the second switch element to an off state from the initial state and switching the third switch element to an on state after the first control; a second control of switching the fourth switch element to an on state after the second control; and a third control of switching the first switch element to an off state after the second control.

[0141] [6] The power conversion circuit according to any one of [3] to [5], wherein, when an initial state is one in which both the first switch element and the second switch element are in an on state, both the third switch element and the fourth switch element are in an off state, and a current is flowing from the second terminal to the first terminal of the primary winding, the control unit is capable of performing the following during a period in which a current is flowing from the second terminal side to the first terminal side of the primary winding in the initial state: a first control of switching the first switch element to an off state from the initial state and switching the fourth switch element to an on state after the first control; a second control of switching the third switch element to an on state after the second control; and a third control of switching the second switch element to an off state after the second control.

[0142] [7] The power conversion circuit according to any one of [3] to [6], wherein, when an initial state is one in which both the third switch element and the fourth switch element are in an on state, both the first switch element and the second switch element are in an off state, and a current is flowing from the tap of the primary winding to the second terminal, the control unit is capable of: in the initial state, performing a first control of switching the fourth switch element to an off state from the initial state and switching the first switch element to an on state; and, after the first control, performing a second control of switching the second switch element to an on state, during a period in which a current is flowing from the first terminal side to the second terminal side of the primary winding; and, after the second control, performing a third control of switching the third switch element to an off state.

[0143] [8] The power conversion circuit according to [7], wherein the control unit executes the third control in a state where a current flows from the second terminal to the first terminal of the primary winding.

[0144] [9] The power conversion circuit according to any one of [3] to [8], wherein, when an initial state is one in which both the third switch element and the fourth switch element are in an on state, both the first switch element and the second switch element are in an off state, and a current is flowing from the second terminal of the primary winding to the tap, the control unit is capable of: in the initial state, performing a first control of switching the third switch element to an off state from the initial state and switching the second switch element to an on state; and, after the first control, performing a second control of switching the first switch element to an on state, during a period in which a current is flowing from the second terminal side to the first terminal side of the primary winding; and, after the second control, performing a third control of switching the fourth switch element to an off state.

[0145]

[10] The power conversion circuit according to [9], wherein the control unit executes the third control in a state where a current flows from the first terminal to the second terminal of the primary winding.

[0146]

[11] The power conversion circuit according to [1] or [2], wherein the first bidirectional switch has a first switch element having a drain terminal connected to the input terminal and a second switch element having a drain terminal connected to the first terminal of the primary winding and a source terminal connected to a source terminal of the first switch element, the second bidirectional switch has a third switch element having a drain terminal connected to the input terminal and a fourth switch element having a drain terminal connected to the tap of the primary winding and a source terminal connected to a source terminal of the third switch element, the first switch element, the second switch element, the third switch element, and the fourth switch element are transistors that are capable of allowing a current to flow in a forward direction and a reverse direction, and the control unit controls the first bidirectional switch by controlling on / off of the first switch element and the second switch element, and controls the second bidirectional switch by controlling the third switch element and the fourth switch element.

[0147] REFERENCE SIGNS LIST 10... Power conversion device 40... Second power conversion circuit 41A... First input terminal 41B... Second input terminal 42A... First output terminal 42B... Second output terminal 43... Transformer 44... Primary winding 44A... First terminal 44B... Second terminal 44C... Tap 45... Secondary winding 46... Second control unit TS1... First bidirectional tap switch SW1... First switch element SW2... Second switch element TS2... Second bidirectional tap switch SW3... Third switch element SW4... Fourth switch element

Claims

1. A power conversion circuit comprising: a transformer having a primary winding having a first terminal and a second terminal, and a secondary winding; an input terminal; a first bidirectional switch having a first end connected to the input terminal and a second end connected to the first terminal of the primary winding; a second bidirectional switch having a first end and a second end, the first end connected to the input terminal; and a controller for controlling the first bidirectional switch and the second bidirectional switch, wherein the primary winding has a tap between the first and second terminals of the primary winding, and the second end of the second bidirectional switch is connected to the tap.

2. The power conversion circuit according to claim 1, wherein the control unit switches the on / off states of the first bidirectional switch and the second bidirectional switch during a period in which an absolute value of a voltage applied between the first terminal and the second terminal of the primary winding is equal to or less than a predetermined threshold value.

3. The power conversion circuit according to claim 1 or 2, wherein the first bidirectional switch has a first switch element, a cathode terminal of a body diode connected to the input terminal, and a second switch element, a cathode terminal of the body diode connected to the first terminal of the primary winding, and the first switch element and the second switch element have anode terminals of their body diodes connected to each other; the second bidirectional switch has a third switch element, a cathode terminal of whose body diode is connected to the input terminal, and a fourth switch element, a cathode terminal of whose body diode is connected to the tap of the primary winding, and the third switch element and the fourth switch element have anode terminals of their body diodes connected to each other; and the control unit controls the first bidirectional switch by controlling on and off of the first switch element and the second switch element, and controls the second bidirectional switch by controlling the third switch element and the fourth switch element.

4. The power conversion circuit according to claim 3, wherein the control unit is capable of executing switching control to switch from a state in which the first switch element and the second switch element are both in an on state and the third switch element and the fourth switch element are both in an off state to a state in which the first switch element and the second switch element are both in an off state and both the third switch element and the fourth switch element are in an on state, and in the switching control, the control unit performs control such that a combination of the on state and the off state of the first switch element, the second switch element, the third switch element, and the fourth switch element differs between a case in which a current flows from the first terminal side of the primary winding to the second terminal side and a case in which a current flows from the second terminal side to the first terminal side.

5. The power conversion circuit according to claim 3, wherein, when an initial state is one in which the first switch element and the second switch element are both in an on state, the third switch element and the fourth switch element are both in an off state, and a current is flowing from the first terminal to the second terminal of the primary winding, the control unit is capable of executing, in the initial state, a first control for switching the second switch element to an off state from the initial state and switching the third switch element to an on state, after the first control, a second control for switching the fourth switch element to an on state, and after the second control, a third control for switching the first switch element to an off state, during a period in which a current is flowing from the first terminal side to the second terminal side of the primary winding.

6. The power conversion circuit according to claim 3, wherein, when an initial state is one in which the first switch element and the second switch element are both in an on state, the third switch element and the fourth switch element are both in an off state, and a current is flowing from the second terminal of the primary winding to the first terminal, the control unit is capable of executing, in the initial state, a first control for switching the first switch element to an off state from the initial state and switching the fourth switch element to an on state, after the first control, a second control for switching the third switch element to an on state, and after the second control, a third control for switching the second switch element to an off state, during a period in which a current is flowing from the second terminal side of the primary winding to the first terminal side.

7. The power conversion circuit according to claim 3, wherein, when an initial state is one in which both the third switch element and the fourth switch element are in an on state, both the first switch element and the second switch element are in an off state, and a current is flowing from the tap of the primary winding to the second terminal, the control unit is capable of: in the initial state, a first control for switching the fourth switch element to an off state from the initial state and switching the first switch element to an on state; and, after the first control, a second control for switching the second switch element to an on state, during a period in which a current is flowing from the first terminal side to the second terminal side of the primary winding; and, after the second control, a third control for switching the third switch element to an off state.

8. The power conversion circuit according to claim 7, wherein the control unit executes the third control in a state in which a current flows from the second terminal to the first terminal of the primary winding.

9. The power conversion circuit according to claim 3, wherein, when an initial state is one in which both the third switch element and the fourth switch element are in an on state, both the first switch element and the second switch element are in an off state, and a current is flowing from the second terminal of the primary winding to the tap, the control unit is capable of: in the initial state, a first control for switching the third switch element to an off state from the initial state and switching the second switch element to an on state; and, after the first control, a second control for switching the first switch element to an on state, during a period in which a current is flowing from the second terminal side to the first terminal side of the primary winding; and, after the second control, a third control for switching the fourth switch element to an off state.

10. The power conversion circuit according to claim 9, wherein the control unit executes the third control in a state in which a current flows from the first terminal to the second terminal of the primary winding.

11. The power conversion circuit according to claim 1 or 2, wherein the first bidirectional switch has a first switch element having a drain terminal connected to the input terminal and a second switch element having a drain terminal connected to the first terminal of the primary winding and a source terminal connected to a source terminal of the first switch element, the second bidirectional switch has a third switch element having a drain terminal connected to the input terminal and a fourth switch element having a drain terminal connected to the tap of the primary winding and a source terminal connected to a source terminal of the third switch element, the first switch element, the second switch element, the third switch element and the fourth switch element are transistors capable of allowing a current to flow in a forward direction and a current to flow in a reverse direction, and the control unit controls the first bidirectional switch by controlling on and off of the first switch element and the second switch element, and controls the second bidirectional switch by controlling the third switch element and the fourth switch element.

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