Power conversion device and program

Abstract

A power conversion device (10) is provided with a transformer, a rectifier circuit, a snubber circuit (70), and a control unit (80). The snubber circuit (70) has a first snubber capacitor (74) and a snubber switch (76). After having switched the snubber switch (76) to the off state, the control unit (80) switches the snubber switch (76) to the on state after the direction of a secondary current flowing through the secondary winding of the transformer changes at least once from the positive direction to the negative direction and at least once from the negative direction to the positive direction.

Description

Power conversion device and program

[0001] The present disclosure relates to a power conversion device and a program.

[0002] The power conversion device includes a transformer, a rectifier circuit, a snubber circuit, and a control unit. The transformer has a primary winding and a secondary winding. The rectifier circuit is connected to the secondary winding. The rectifier circuit has a plurality of diodes. The snubber circuit is connected to the output side of the rectifier circuit. The snubber circuit has a snubber capacitor connected between a high-potential terminal and a low-potential terminal of the rectifier circuit. The snubber circuit has a semiconductor switch connected to the high-potential terminal of the snubber capacitor. The control unit is capable of switching the semiconductor switch on and off.

[0003] Japanese Patent Application Laid-Open No. 2015-70716

[0004] In the power conversion device described in Patent Document 1, the control unit switches on and off the semiconductor switch of the snubber circuit in synchronization with the timing at which a surge voltage occurs on the secondary side of the transformer. If the number of times the semiconductor switch is switched on and off increases, there is a risk that loss associated with the switching on and off of the semiconductor switch will increase.

[0005] In order to solve the above problems, the present disclosure provides a power conversion device comprising: a transformer having a primary winding and a secondary winding; a rectifier circuit connected to the secondary winding and having a high-potential output terminal and a low-potential output terminal; an external output terminal connected to the high-potential output terminal; a snubber circuit having a snubber capacitor connected between the high-potential output terminal and the low-potential output terminal, and a snubber switch connected between the high-potential terminal of the snubber capacitor and the external output terminal; and a control unit that controls the snubber switch, wherein when one direction of a current flowing through the secondary winding is defined as a positive direction and the opposite direction to the positive direction is defined as a negative direction, the control unit switches the snubber switch to an off state and then controls the snubber switch to an on state after the current flowing through the secondary winding has changed direction from the positive direction to the negative direction and from the negative direction to the positive direction at least once each.

[0006] The present disclosure also relates to a power conversion device including a transformer having a primary winding and a secondary winding, a rectifier circuit connected to the secondary winding and having a high-potential output terminal and a low-potential output terminal, an external output terminal connected to the high-potential output terminal, a snubber circuit having a snubber capacitor connected between the high-potential output terminal and the low-potential output terminal, and a snubber switch connected between the high-potential side terminal of the snubber capacitor and the external output terminal, and a control unit that controls the snubber switch, wherein, when one direction of a current flowing through the secondary winding is defined as a positive direction and the opposite direction to the positive direction is defined as a negative direction, the control unit switches the snubber switch to an off state, and then controls the snubber switch to an on state after the current flowing through the secondary winding changes direction from the positive direction to the negative direction and from the negative direction to the positive direction at least once each.

[0007] This reduces the number of times you have to turn the switch on and off.

[0008] Fig. 1 is a circuit diagram of a power conversion device. Fig. 2 is a circuit diagram of a switching circuit. Fig. 3 is a circuit diagram of a snubber circuit. Fig. 4 is a waveform diagram of the output voltage of a rectifier circuit, the voltage between terminals of a first snubber capacitor, and the current of a first current. Fig. 5 is a waveform diagram of the output voltage of the rectifier circuit and the voltage between terminals of a second snubber capacitor. Fig. 6 is a circuit diagram of a power conversion device in a modified example.

[0009] <One embodiment of a power conversion device and a program> An embodiment of a power conversion device and a program will be described below. Note that, although a plurality of electronic components may be collectively referred to as a single circuit configuration, this is merely a convenient way to collectively describe the objects. In other words, the classification and names of the circuit configuration are not limited to this. Furthermore, the term "terminal" includes a so-called node and simply means an electrical connection point.

[0010] 1 , the power conversion device 10 includes a first external input terminal 11A, a second external input terminal 11B, a third external input terminal 11C, a first external output terminal 12A, and a second external output terminal 12B. The power conversion device 10 also includes an input noise filter 20, a switching circuit 30, a transformer circuit 40, a snubber circuit 70, and an output noise filter 60.

[0011] The power conversion device 10 is a so-called three-phase insulated AC-DC converter. The power conversion device 10 converts three-phase AC power input to three external input terminals into DC power and outputs it from a pair of external output terminals. The presence of a transformer circuit 40 on the power path electrically insulates the external input terminal side from the external output terminal side.

[0012] The three external input terminals are connected to a three-phase AC power supply 90. The three-phase AC power supply 90 is a three-phase, three-wire commercial power system in which three AC power supplies are Y-connected. The three phases of the three-phase AC power input from the three-phase AC power supply 90 are input in a one-to-one relationship. An arbitrary load 100 can be connected between the first external output terminal 12A and the second external output terminal 12B. The load 100 is, for example, an electronic device driven by DC power.

[0013] The input noise filter 20 is a so-called low-pass filter and includes a first inductor L1, a second inductor L2, and a third inductor L3. The input noise filter 20 also includes a first capacitor C1, a second capacitor C2, and a third capacitor C3.

[0014] 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.

[0015] 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.

[0016] 2, the switching circuit 30 includes a plurality of input terminals, a pair of output terminals, and a plurality of bidirectional switches TSW. The switching circuit 30 is a circuit capable of converting a three-phase AC voltage into a single-phase AC voltage.

[0017] The multiple input terminals include a first input terminal 31A, a second input terminal 31B, and a third input terminal 31C. As shown in FIG. 1 , a second end of a first inductor L1 is connected to the first input terminal 31A. A second end of a second inductor L2 is connected to the second input terminal 31B. A second end of a third inductor L3 is connected to the third input terminal 31C. Therefore, three-phase AC power is input to the input terminals of the switching circuit 30 via the external input terminal and the input-side noise filter 20. The pair of output terminals includes a first output terminal 32A and a second output terminal 32B. Single-phase AC power converted by the multiple bidirectional switches TSW is output from the pair of output terminals.

[0018] 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 so that the anode terminals of the body diodes are connected to each other. That is, the switch elements constituting each bidirectional switch TSW are connected to each other so that the body diodes are in opposite directions. In other words, each bidirectional switch TSW has two switch elements whose source terminals are connected to each other.

[0019] 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.

[0020] The first high-side bidirectional switch HS1 connects the first input terminal 31A and the first output 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 input terminal 31A. The source terminal of the eleventh switch element S11 is connected to the source terminal of the twenty-first switch element S21. The drain terminal of the twenty-first switch element S21 is connected to the first output terminal 32A.

[0021] The first low-side bidirectional switch LS1 connects the first input terminal 31A and the second output 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 input terminal 31A. The source terminal of the 24th switch element S24 is connected to the source terminal of the 14th switch element S14. The drain terminal of the 14th switch element S14 is connected to the second output terminal 32B.

[0022] The second high-side bidirectional switch HS2 connects the second input terminal 31B and the first output 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 input terminal 31B. The source terminal of the thirteenth switch element S13 is connected to the source terminal of the twenty-third switch element S23. The drain terminal of the twenty-third switch element S23 is connected to the first output terminal 32A.

[0023] The second low-side bidirectional switch LS2 connects the second input terminal 31B and the second output 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 input 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 second output terminal 32B.

[0024] The third high-side bidirectional switch HS3 connects the third input terminal 31C and the first output 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 input terminal 31C. The source terminal of the fifteenth switch element S15 is connected to the source terminal of the twenty-fifth switch element S25. The drain terminal of the twenty-fifth switch element S25 is connected to the first output terminal 32A.

[0025] The third low-side bidirectional switch LS3 connects the third input terminal 31C and the second output 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 input terminal 31C. The source terminal of the 22nd switch element S22 is connected to the source terminal of the 12th switch element S12. The drain terminal of the 12th switch element S12 is connected to the second output terminal 32B.

[0026] As shown in FIG. 1 , the transformer circuit 40 includes a fourth inductor L4 and a transformer 41. The transformer 41 includes a primary winding 41A and a secondary winding 41B. A first end of the fourth inductor L4 is connected to the first output terminal 32A of the switching circuit 30. A first end of the primary winding 41A is connected to the second end of the fourth inductor L4. A second end of the primary winding 41A is connected to the second output terminal 32B of the switching circuit 30. Therefore, multiple switch elements are connected to the primary winding 41A of the transformer 41. The secondary winding 41B is connected to the first external output terminal 12A via the rectifier circuit 50 and the output noise filter 60. The primary winding 41A and the secondary winding 41B are electrically insulated from each other.

[0027] The rectifier circuit 50 has four semiconductor elements. The four semiconductor elements are a first switch element 51, a second switch element 52, a third switch element 53, and a fourth switch element 54. The first switch element 51 to the fourth switch element 54 are n-channel MOSFETs. The rectifier circuit 50 converts the AC voltage applied from the secondary winding 41B into a DC voltage through control of each switch element by a control unit 80 (described later).

[0028] The source terminal of the first switch element 51 is connected to a first end of the secondary winding 41B of the transformer 41. The drain terminal of the first switch element 51 is connected to a drain terminal of the third switch element 53. The source terminal of the third switch element 53 is connected to a second end of the secondary winding 41B and a drain terminal of the fourth switch element 54. The source terminal of the fourth switch element 54 is connected to the source terminal of the second switch element 52. The drain terminal of the second switch element 52 is connected to a first end of the secondary winding 41B and the source terminal of the first switch element 51.

[0029] The rectifier circuit 50 includes a high-potential output terminal 55A and a low-potential output terminal 55B. DC voltages are output from the high-potential output terminal 55A and the low-potential output terminal 55B of the rectifier circuit 50. In this embodiment, the high-potential output terminal 55A is a connection point between the drain terminal of the first switch element 51 and the drain terminal of the third switch element 53. The low-potential output terminal 55B is a connection point between the source terminal of the second switch element 52 and the source terminal of the fourth switch element 54. The high-potential output terminal 55A is connected to the first external output terminal 12A via the output-side noise filter 60. The low-potential output terminal 55B is connected to the second external output terminal 12B via the snubber circuit 70.

[0030] The output-side noise filter 60 is a so-called low-pass filter. The output-side noise filter 60 includes a fifth inductor L5 and a fourth capacitor C4. A first end of the fifth inductor L5 is connected to the high-potential output terminal 55A of the rectifier circuit 50. That is, a first end of the fifth inductor L5 is connected to the drain terminal of the first switch element 51 and the drain terminal of the third switch element 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.

[0031] The snubber circuit 70 includes a first connection terminal CT1, a second connection terminal CT2, a third connection terminal CT3, and a fourth connection terminal CT4. The first connection terminal CT1 is connected to the high-potential output terminal 55A of the rectifier circuit 50. The first connection terminal CT1 is connected to a first end of the fifth inductor L5. The second connection terminal CT2 is connected to the low-potential output terminal 55B of the rectifier circuit 50. The third connection terminal CT3 is connected to a second end of the fifth inductor L5, the first external output terminal 12A, and a first end of the fourth capacitor C4. The fourth connection terminal CT4 is connected to the second external output terminal 12B and a second end of the fourth capacitor C4.

[0032] 3 , the snubber circuit 70 includes a first diode 71, a second diode 72, and a third diode 73. The snubber circuit 70 also includes a first snubber capacitor 74, a second snubber capacitor 75, a snubber switch 76, and a snubber inductor 77.

[0033] The anode of the first diode 71 is connected to the first connection terminal CT1. The cathode of the first diode 71 is connected to a first end of the first snubber capacitor 74 and a first end of the second snubber capacitor 75. Therefore, the first diode 71 allows current to flow from the first connection terminal CT1 to the first snubber capacitor 74 and the second snubber capacitor 75. On the other hand, the first diode 71 does not allow current to flow from the first snubber capacitor 74 and the second snubber capacitor 75 to the first connection terminal CT1.

[0034] The second end of the first snubber capacitor 74 is connected to the second connection terminal CT2 and the fourth connection terminal CT4. Therefore, the first snubber capacitor 74 is connected between the high potential output terminal 55A and the low potential output terminal 55B of the rectifier circuit 50. The second end of the second snubber capacitor 75 is connected to the second connection terminal CT2 and the fourth connection terminal CT4. That is, the second snubber capacitor 75 is connected in parallel with the first snubber capacitor 74 between the high potential output terminal 55A and the low potential output terminal 55B. The capacitance of the first snubber capacitor 74 is larger than the capacitance of the second snubber capacitor 75.

[0035] The snubber switch 76 is an n-channel MOSFET. The drain terminal of the snubber switch 76 is connected to the cathode of the first diode 71, a first end of the first snubber capacitor 74, and a first end of the second snubber capacitor 75. The source terminal of the snubber switch 76 is connected to the third connection terminal CT3 via the snubber inductor 77. Therefore, the snubber switch 76 is connected between the high-potential terminal of the second snubber capacitor 75 and the first external output terminal 12A.

[0036] The cathode of the second diode 72 is connected to the source terminal of the snubber switch 76. The anode of the second diode 72 is connected to the second connection terminal CT2 and the fourth connection terminal CT4. That is, the second diode 72 allows current to flow from the second connection terminal CT2 and the fourth connection terminal CT4 to the snubber switch 76. On the other hand, the second diode 72 does not allow current to flow from the snubber switch 76 to the second connection terminal CT2 and the fourth connection terminal CT4. Therefore, the second diode 72 is a so-called freewheeling diode.

[0037] A first end of the snubber inductor 77 is connected to the source terminal of the snubber switch 76 and the cathode of the second diode 72. A second end of the snubber inductor 77 is connected to the anode of the third diode 73.

[0038] The cathode of the third diode 73 is connected to the third connection terminal CT3. Therefore, the third diode 73 allows current to flow from the snubber inductor 77 to the third connection terminal CT3. On the other hand, the third diode 73 does not allow current to flow from the third connection terminal CT3 to the snubber inductor 77.

[0039] 1, the power conversion device 10 includes a first substrate CB1 and a second substrate CB2. The first substrate CB1 and the second substrate CB2 are printed circuit boards. The first substrate CB1 and the second substrate CB2 are so-called parent-child substrates, with the first substrate CB1 serving as the parent substrate.

[0040] The first board CB1 is mounted with an input-side noise filter 20, a switching circuit 30, a transformer circuit 40, a rectifier circuit 50, and an output-side noise filter 60. Also, as shown in Fig. 3, the first board CB1 is mounted with a first diode 71 and a first snubber capacitor 74 of a snubber circuit 70.

[0041] The second board CB2 is mounted with the second snubber capacitor 75 and snubber switch 76 of the snubber circuit 70. The second board CB2 also is mounted with the control unit 80, second diode 72, third diode 73, and snubber inductor 77 (described later). Therefore, the rectifier circuit 50 and the first snubber capacitor 74 are mounted on the same first board CB1. Meanwhile, the second snubber capacitor 75 is mounted on a second board CB2 separate from the first board CB1. In other words, the first snubber capacitor 74 and the second snubber capacitor 75 are mounted on different boards.

[0042] As shown in FIG. 3 , among the wiring provided in the power conversion device 10, the wiring from the high-potential output terminal 55A of the rectifier circuit 50 to the first end of the first snubber capacitor 74 is referred to as the first wiring W1. Furthermore, among the wiring provided in the power conversion device 10, the wiring from the high-potential output terminal 55A of the rectifier circuit 50 to the first end of the second snubber capacitor 75 is referred to as the second wiring W2. The first end of each snubber capacitor is the high-potential side terminal of the snubber capacitor. As described above, the second snubber capacitor 75 is mounted on the second board CB2, which is separate from the first board CB1 on which the rectifier circuit 50 is mounted. Therefore, the length WL1 of the first wiring W1 is shorter than the length WL2 of the second wiring W2. Note that in FIG. 3 , the wiring and lengths from the third connection terminal CT3 to the first end of each snubber capacitor are virtually shown on the circuit diagram. The length of each wire is the physical length when tracing the wire on the substrate.

[0043] In this embodiment, the portion of each wiring from the high-potential output terminal 55A to the cathode of the first diode 71 of the snubber circuit 70 is common to all the wiring. In other words, each wiring branches off at a connection point on the cathode side of the first diode 71 and leads to the first end of each snubber capacitor. Therefore, the length from this connection point to the first end of the first snubber capacitor 74 is shorter than the length from this connection point to the first end of the second snubber capacitor 75.

[0044] Here, the Q factor (Quality Factor) for resonance caused by the capacitance of the first snubber capacitor 74 and the parasitic inductance of the first wiring W1 is defined as the first Q factor. The Q factor for resonance caused by the capacitance of the second snubber capacitor 75 and the parasitic inductance of the second wiring W2 is defined as the second Q factor. When each snubber capacitor and the wiring connected to the snubber capacitor are considered as a series resonant circuit, the Q factor is inversely proportional to the square root of the capacitance and proportional to the square root of the inductance. As described above, the capacitance of the first snubber capacitor 74 is greater than the capacitance of the second snubber capacitor 75. Furthermore, because the length WL1 of the first wiring W1 is shorter than the length WL2 of the second wiring W2, the parasitic inductance generated in the first wiring W1 is smaller than the parasitic inductance generated in the second wiring W2. In other words, the parasitic inductance between the high-potential output terminal 55A and the first snubber capacitor 74 is smaller than the parasitic inductance between the high-potential output terminal 55A and the second snubber capacitor 75. Therefore, the first Q value is smaller than the second Q value. In this embodiment, the first Q value is equal to or smaller than half of the second Q value. Therefore, the amplitude of the resonance generated by the first snubber capacitor 74 and the first wiring W1 is smaller than the amplitude of the resonance generated by the second snubber capacitor 75 and the second wiring W2.

[0045] (Regarding the Control Unit) As shown in Fig. 1, the power conversion device 10 includes a control unit 80. The control unit 80 has a storage device and an execution device, not shown. In other words, the control unit 80 is an MCU (Microcontroller Unit). The storage device of the control unit 80 stores various programs executed by the execution device. One of these programs is a program PG for switching the snubber switch 76 between an on state and an off state.

[0046] The execution device includes, for example, a CPU (Central Processing Unit), an MPU (Micro Processing Unit), an FPGA (Field Programmable Gate Array), a CPLD (Complex Programmable Logic Device), an ASIC (Application Specific Integrated Circuit), etc. Note that, hereinafter, control by the execution device of the control unit 80 will be simply referred to as control by the control unit 80.

[0047] As shown in FIG. 1 , the control unit 80 can detect the output voltage VRE of the rectifier circuit 50. Also, as shown in FIG. 3 , the control unit 80 can detect the value of the inter-terminal voltage VC1 of the first snubber capacitor 74. The control unit 80 can detect the value of the first current i1 that conducts the snubber switch 76 and the snubber inductor 77 when the snubber switch 76 is in the on state. As shown in FIG. 1 , the control unit 80 can detect the value of the second current i2 that flows between the high-potential output terminal 55A of the rectifier circuit 50 and the first external output terminal 12A. However, these detected values ​​may not be directly measured values, but may be theoretical values ​​calculated from measurements of other elements, connection points, etc. Furthermore, these detected values ​​may be replaced by measurements of other elements, connection points, etc. that are approximately the same values. In this embodiment, the value of the first current i1 is detected as the current that conducts the snubber inductor 77. The value of the second current i2 is detected as the current that conducts the fifth inductor L5. The unit of voltage is "V (volt)" and the value of current is "A (ampere)".

[0048] Furthermore, when power is supplied to the power conversion device 10 from the three-phase AC power supply 90, the control unit 80 executes a program stored in the storage device. In accordance with the program, the control unit 80 controls each bidirectional switch TSW of the switching circuit 30 and each switch element of the rectifier circuit 50 with a predetermined switching pattern. Specifically, the power conversion device 10 includes a gate drive circuit (not shown). The control unit 80 inputs a switching signal to an input terminal of the gate drive circuit. Then, the gate drive circuit outputs a gate drive voltage to the gate terminal of each switch element based on the switching signal. In other words, the control unit 80 controls the on / off of each switch element via the gate drive circuit.

[0049] 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 switch elements S11 to the sixteenth switch elements S16, respectively. The twenty-first switching signal SG21 to the twenty-sixth switching signal SG26 correspond one-to-one to the twenty-first switch elements S21 to the twenty-sixth switch elements S26, respectively. This allows the switching circuit 30 to convert three-phase AC power input to each input terminal into single-phase AC power. That is, a single-phase AC current is applied to the primary winding 41A of the transformer 41. When an AC voltage is applied to the primary winding 41A, a single-phase AC voltage is also generated in the secondary winding 41B due to electromagnetic induction. Hereinafter, the current flowing through the secondary winding 41B will be referred to as the "secondary current iT." Of the directions in which the secondary current iT flows, the direction from the second end to the first end of the secondary winding 41B is referred to as the positive direction. Of the directions in which the current flows, the direction from the first end to the second end of the secondary winding 41B is referred to as the negative direction. Note that in FIG. 1, only the positive direction of the secondary current iT is indicated by an arrow.

[0050] The switching signals include a first switching signal SG1, a second switching signal SG2, a third switching signal SG3, and a fourth switching signal SG4. The first to fourth switching signals SG1 to SG4 correspond one-to-one to the first to fourth switch elements 51 to 54 of the rectifier circuit 50, respectively.

[0051] When the secondary current iT flows in the positive direction, the control unit 80 switches the first switch element 51 and the fourth switch element 54 to the ON state. Furthermore, the control unit 80 switches the second switch element 52 and the third switch element 53 to the OFF state. At this time, the secondary current iT flows from the first end of the secondary winding 41B to the first external output terminal 12A via the first switch element 51.

[0052] Furthermore, when the secondary current iT flows in the negative direction, the control unit 80 switches the second switch element 52 and the third switch element 53 to the ON state. Furthermore, the control unit 80 switches the first switch element 51 and the fourth switch element 54 to the OFF state. At this time, the secondary current iT flows from the second end of the secondary winding 41B to the first external output terminal 12A via the third switch element 53. Therefore, the control unit 80 causes the rectifier circuit 50 to convert the AC power generated in the secondary winding 41B into DC power. As a result, while the power conversion device 10 is operating, the potential of the high-potential output terminal 55A of the rectifier circuit 50 is higher than the potential of the low-potential output terminal 55B.

[0053] (Regarding Control of Snubber Switch) Next, a description will be given of control of the snubber switch 76 by the control unit 80. The switching signals of the control unit 80 include a fifth switching signal SG5. The fifth switching signal SG5 corresponds to the snubber switch 76 included in the snubber circuit 70.

[0054] The control unit 80 executes switching control for the snubber switch 76 as defined in the program PG while the power conversion device 10 is operating. When the control unit 80 starts the switching control, it first controls the snubber switch 76 to the off state. The control unit 80 also starts detecting the voltage VC1 between the terminals of the first snubber capacitor 74.

[0055] As described above, while the power conversion device 10 is operating, an AC current flows through the secondary winding 41B of the transformer 41. That is, the secondary current iT flowing through the secondary winding 41B alternately changes direction from positive to negative and from negative to positive. This direction change generates a surge voltage in the output voltage VRE of the rectifier circuit 50, as shown in FIG. 4 . This is due to the influence of the inductance of the secondary winding 41B of the transformer 41 and the parasitic capacitance of each switch element of the rectifier circuit 50. Note that in FIG. 4 , the voltage waveform at the first end and the voltage waveform at the second end of the secondary winding 41B are superimposed to represent the output voltage VRE of the rectifier circuit 50. Each voltage waveform is a square wave with a surge voltage added to it. Therefore, if the theoretical value of the output voltage VRE excluding the surge voltage is defined as the "ideal voltage," the output voltage VRE of the rectifier circuit 50 momentarily exceeds the ideal voltage by a large amount. For example, in this embodiment, the ideal voltage of the output voltage VRE is approximately 60 V. The peak value of the output voltage VRE due to the application of a surge voltage falls within a range of approximately 71 V to 79 V. The "peak value" refers to the instantaneous maximum value of the output voltage VRE due to the surge voltage generated by one change in direction of the secondary current iT. In the following, the maximum peak value among multiple peak values ​​will be referred to as the "maximum peak value" of the output voltage VRE. In other words, the maximum peak value of the output voltage VRE is the upper limit of the range of values ​​that the peak value can take. That is, in this embodiment, the maximum peak value of the output voltage VRE is approximately 79 V.

[0056] Therefore, when the snubber switch 76 is in the off state and the output voltage VRE of the rectifier circuit 50 is greater than the inter-terminal voltage VC1 of the first snubber capacitor 74, energy of the output voltage VRE corresponding to a potential difference that exceeds the voltage value at the first end of the first snubber capacitor 74 is charged to the first snubber capacitor 74. On the other hand, energy of the output voltage VRE corresponding to a voltage value excluding the potential difference is output from the first external output terminal 12A to the load 100.

[0057] For example, suppose the inter-terminal voltage VC1 of the first snubber capacitor 74 is approximately 68 V. At this time, energy corresponding to the peak value of the output voltage VRE of the rectifier circuit 50, in other words, the potential difference between the peak of the surge voltage and the inter-terminal voltage VC1 of approximately 68 V, is charged to the first snubber capacitor 74. That is, most of the surge energy is stored in the first snubber capacitor 74. Meanwhile, energy corresponding to a voltage value of the output voltage VRE equal to or less than approximately 68 V is supplied from the first external output terminal 12A to the load 100. However, as described above, there are inductance and resistance components due to the wiring between the first wiring W1 and the first snubber capacitor 74. Therefore, noise due to the LCR series resonant circuit is superimposed on the inter-terminal voltage VC1 of the first snubber capacitor 74, in other words, on the detected value of the inter-terminal voltage VC1 of the first snubber capacitor 74. Therefore, because energy loss occurs due to resonance and the inter-terminal voltage VC1 is not a constant value, not all of the energy of the potential difference is charged to the first snubber capacitor 74. Note that energy is also stored in the second snubber capacitor 75 in a similar manner.

[0058] The control unit 80 switches the snubber switch 76 to the ON state when the charging voltage of the first snubber capacitor 74, in other words, the inter-terminal voltage VC1, becomes equal to or greater than a first threshold TH1. The control unit 80 then controls the snubber switch 76 to maintain the ON state until the inter-terminal voltage VC1 of the first snubber capacitor 74 becomes equal to or less than a second threshold TH2. In other words, when the snubber switch 76 is in the ON state, the control unit 80 controls the snubber switch 76 to maintain the ON state until the inter-terminal voltage VC1 of the first snubber capacitor 74 becomes equal to or less than the second threshold TH2. The first threshold TH1 is set to a value smaller than the maximum peak value of the output voltage VRE of the rectifier circuit 50. The second threshold TH2 is set to a value larger than the ideal voltage of the output voltage VRE of the rectifier circuit 50 and smaller than the first threshold TH1. In this embodiment, the first threshold TH1 is 70 V. The second threshold TH2 is 65V.

[0059] When the control unit 80 switches the snubber switch 76 to the on state, the electrostatic energy stored in the first snubber capacitor 74 and the second snubber capacitor 75 is discharged to the third connection terminal CT3 through the snubber switch 76, the snubber inductor 77, and the third diode 73. As the electrostatic energy stored in the first snubber capacitor 74 is discharged, the voltage VC1 across the first snubber capacitor 74 decreases.

[0060] The control unit 80 switches the snubber switch 76 to the OFF state when the inter-terminal voltage VC1 of the first snubber capacitor 74 becomes equal to or lower than the second threshold value TH2. The control unit 80 then controls the snubber switch 76 to maintain the OFF state until the inter-terminal voltage VC1 of the first snubber capacitor 74 becomes equal to or higher than the first threshold value TH1. In other words, when the snubber switch 76 is in the OFF state, the control unit 80 controls the snubber switch 76 to maintain the OFF state until the inter-terminal voltage VC1 of the first snubber capacitor 74 becomes equal to or higher than the first threshold value TH1. When the snubber switch 76 is switched to the OFF state, electrostatic energy is again accumulated in the first snubber capacitor 74 and the second snubber capacitor 75.

[0061] The difference between the first threshold TH1 and the second threshold TH2 is set to a value significantly larger than the change in the inter-terminal voltage VC1 of the first snubber capacitor 74 caused by a single change in direction of the secondary current iT. Therefore, surge energy is charged into the first snubber capacitor 74 at least twice between the time the snubber switch 76 is turned off and the time it is turned on again. In other words, after turning the snubber switch 76 off, the control unit 80 turns the snubber switch 76 on after the secondary current iT changes direction from positive to negative at least once and from negative to positive at least once. Therefore, as shown in FIG. 4 , energy is charged into the first snubber capacitor 74 so that the voltage waveform of the inter-terminal voltage VC1 has a two- or more-step waveform. For example, in this embodiment, the surge voltage caused by a single change in direction of the secondary current iT increases the inter-terminal voltage VC1 of the first snubber capacitor 74 by approximately 1 V to 0.5 V. Since the difference between the first threshold value TH1 and the second threshold value TH2 is 5 V, the surge energy is charged into the first snubber capacitor 74 approximately five times from the time the snubber switch 76 is switched off until the time it is next switched on.

[0062] (Regarding Threshold Control) Next, the control of the first threshold TH1 and the second threshold TH2 by the control unit 80 will be described. The control unit 80 executes a threshold setting process for setting the first threshold TH1 and the second threshold TH2 as part of the switching control for the snubber switch 76 defined in the program PG. This threshold setting process is executed in parallel with the process related to the on / off switching of the snubber switch 76 described above. When executing the threshold setting process, the control unit 80 first sets the first threshold TH1 and the second threshold TH2 to initial values. For example, in this embodiment, the initial value of the first threshold TH1 is 70 V as described above. The initial value of the second threshold TH2 is 65 V.

[0063] Furthermore, after executing the threshold setting process, the control unit 80 starts detecting the first current i1 and the second current i2. Hereinafter, the sum of the value of the first current i1 and the value of the second current i2 will be referred to as the "supply current value."

[0064] When the supply current value is equal to or less than a first predetermined value, which is a predetermined percentage of the rated current of the power conversion device 10, the control unit 80 increases the first threshold value TH1 and the second threshold value TH2 compared to when the supply current value is greater than the first predetermined value. For example, the first predetermined value is 10% of the rated current of the power conversion device 10. Specifically, when the supply current value is equal to or less than the first predetermined value, the control unit 80 updates the first threshold value TH1 to a value obtained by adding a second predetermined value to the current first threshold value TH1. Furthermore, the control unit 80 updates the second threshold value TH2 to a value obtained by adding the second predetermined value to the current second threshold value TH2. The second predetermined value is a positive value and is set within a range in which the second threshold value TH2 is higher than the ideal voltage of the rectifier circuit 50. In this embodiment, the second predetermined value is 2.0 V. Therefore, for example, when the first threshold TH1 is 70.0 V and the second threshold TH2 is 65.0 V and the supply current value becomes less than the first predetermined value, the control unit 80 updates the first threshold TH1 to 72.0 V and the second threshold TH2 to 67.0 V.

[0065] On the other hand, when the supply current value is equal to or greater than a second predetermined value, which is a predetermined percentage of the rated current of the power conversion device 10, the control unit 80 reduces the first threshold value TH1 and the second threshold value TH2 compared to when the supply current value is less than the second predetermined value. For example, the third predetermined value is 90% of the rated current of the power conversion device 10. Specifically, when the supply current value is equal to or greater than the third predetermined value, the control unit 80 updates the first threshold value TH1 to a value obtained by subtracting a fourth predetermined value from the current first threshold value TH1. Furthermore, the control unit 80 updates the second threshold value TH2 to a value obtained by subtracting the fourth predetermined value from the current second threshold value TH2. The fourth predetermined value is a positive value and is set within a range in which the first threshold value TH1 is smaller than the maximum peak value of the output voltage VRE of the rectifier circuit 50. In this embodiment, the fourth predetermined value is 2.0 V. Therefore, for example, when the first threshold value TH1 is 70.0 V and the second threshold value TH2 is 65.0 V, if the supply current value becomes equal to or greater than the third predetermined value, the control unit 80 updates the first threshold value TH1 to 68.0 V and the second threshold value TH2 to 63.0 V. Note that if the supply current value is greater than the first predetermined value and less than the third predetermined value, the control unit 80 does not change the current first threshold value TH1 and second threshold value TH2 and maintains them as they are.

[0066] Even after updating the first threshold value TH1 and the second threshold value TH2 in this manner according to the supply current value and the rated current value of the power conversion device 10, the control unit 80 determines whether the supply current value is equal to or less than the first predetermined value and whether the supply current value is equal to or greater than the third predetermined value. In this embodiment, the control unit 80 can decrease each threshold value in two stages using the initial values ​​of the first threshold value TH1 and the second threshold value TH2 as a reference. Also, the control unit 80 can increase each threshold value in two stages using the initial values ​​as a reference.

[0067] Specifically, when the second predetermined value is added twice from a state in which the first threshold TH1 is 70.0 V, the value of the first threshold TH1 becomes 74.0 V. If the second predetermined value is further added to the first threshold TH1, the value of the first threshold TH1 may exceed the peak value of the output voltage VRE of the rectifier circuit 50. In other words, surge energy may not be charged to the first snubber capacitor 74. Therefore, when the first threshold TH1 is 74.0 V or higher, the control unit 80 does not increase the thresholds even if the supply current value falls below the first predetermined value. On the other hand, when the second threshold TH2 is 65 V and the fourth predetermined value is subtracted twice, the value of the second threshold TH2 becomes 61 V. In this case, if the fourth predetermined value is further subtracted from the second threshold TH2, the value of the second threshold TH2 may fall below the ideal voltage of the rectifier circuit 50. Therefore, when the second threshold TH2 is equal to or lower than 61 V, the control unit 80 does not reduce the thresholds even if the supply current value becomes equal to or higher than the third predetermined value.

[0068] (Operation of the Present Embodiment) According to the above embodiment, the snubber circuit 70 of the power conversion device 10 includes a first snubber capacitor 74, a second snubber capacitor 75, and a snubber switch 76. A surge voltage occurs in the rectifier circuit 50 each time the secondary current iT changes direction. Each snubber capacitor absorbs most of the surge energy resulting from this surge voltage. This prevents the surge voltage from causing an excessive voltage to be output from the first external output terminal 12A. Furthermore, the control unit 80 switches the snubber switch 76 to the on state after the first snubber capacitor 74 is charged multiple times. This allows the energy stored in each snubber capacitor to be supplied to the load 100 as regenerative energy. Furthermore, energy loss caused by switching the snubber switch 76 on and off is reduced compared to switching the snubber switch 76 on and off each time the snubber capacitor is charged.

[0069] Furthermore, according to the above embodiment, the control unit 80 of the power conversion device 10 switches the snubber switch 76 on and off based on the inter-terminal voltage VC1 of the first snubber capacitor 74. As described above, the second Q value is greater than the first Q value. Therefore, as shown in FIG. 5 , a larger noise is superimposed on the inter-terminal voltage VC2 of the second snubber capacitor 75 than on the first snubber capacitor 74. Therefore, if the control unit 80 controls the snubber switch 76 based on the inter-terminal voltage VC2 of the second snubber capacitor 75, after the snubber switch 76 is switched off, the inter-terminal voltage VC2 becomes equal to or greater than the first threshold TH1 sooner than the inter-terminal voltage VC1 of the first snubber capacitor 74. Furthermore, after the snubber switch 76 is switched on, the inter-terminal voltage VC2 becomes equal to or less than the second threshold TH2 sooner than the inter-terminal voltage VC1 of the first snubber capacitor 74. In such a case, the number of times the snubber switch 76 is switched on and off per unit time increases. Therefore, according to the control of this embodiment, the number of times the snubber switch 76 is switched on and off per unit time can be reduced compared to when control is based on the inter-terminal voltage VC2 of the second snubber capacitor 75. This reduces energy loss caused by switching the snubber switch 76 on and off.

[0070] Furthermore, according to the above embodiment, the first threshold value TH1 and the second threshold value TH2 are updated according to the ratio of the supply current value to the rated current of the power conversion device 10. Specifically, when the supply current value is equal to or less than the first predetermined value, i.e., sufficiently smaller than the rated current, a second predetermined value is added to each threshold value. When the first threshold value TH1 and the second threshold value TH2 are increased, the potential difference between the peak value of the output voltage VRE of the rectifier circuit 50 and the voltage value at the first end of the first snubber capacitor 74 decreases. Therefore, the amount of surge energy charged to the first snubber capacitor 74 per charge is reduced compared to before the threshold values ​​were increased. Therefore, the power output per unit time from the first external output terminal 12A increases without charging the snubber capacitors.

[0071] On the other hand, when the supply current value is equal to or greater than the third predetermined value, i.e., when the supply current value is close to the rated current, the fourth predetermined value is subtracted from each threshold value. When each threshold value is reduced, the potential difference between the peak value of the output voltage VRE of the rectifier circuit 50 and the voltage value at the first end of the first snubber capacitor 74 increases. Therefore, the amount of surge energy charged to the first snubber capacitor 74 per charge increases compared to before the threshold values ​​were reduced. Therefore, the power output per unit time from the first external output terminal 12A, without charging the snubber capacitors, decreases.

[0072] (Effects of the Present Embodiment) (1) According to the above embodiment, a surge voltage may occur when the direction of the secondary current iT flowing through the secondary winding 41B changes. In the above embodiment, the snubber switch 76 is switched on at least fewer times than the number of times that a surge voltage may occur. Therefore, compared to switching the snubber switch 76 every time a surge voltage may occur, loss associated with switching the snubber switch 76 can be reduced.

[0073] (2) According to the above embodiment, the control unit 80 controls the snubber switch 76 based on the first threshold value TH1, the second threshold value TH2, and the inter-terminal voltage VC1 of the first snubber capacitor 74. This allows for simpler control than when detecting the direction of the secondary current iT. Furthermore, for example, by changing the first threshold value TH1 and the second threshold value TH2, the amount of energy charged to each snubber capacitor per charge can be controlled. This means that design changes and the like can be easily implemented.

[0074] (3) According to the above embodiment, the control unit 80 switches the snubber switch 76 to the ON state when the inter-terminal voltage VC1 of the first snubber capacitor 74 becomes equal to or greater than the first threshold. The control unit 80 then controls the snubber switch 76 to maintain the ON state until the inter-terminal voltage VC1 becomes equal to or less than the second threshold. Furthermore, the control unit 80 switches the snubber switch 76 to the OFF state when the inter-terminal voltage VC1 of the first snubber capacitor 74 becomes equal to or less than the second threshold. The control unit 80 then controls the snubber switch 76 to maintain the OFF state until the inter-terminal voltage VC1 becomes equal to or greater than the first threshold. As a result, the energy charged in each snubber capacitor is supplied to the load 100 as regenerative energy. Furthermore, energy loss caused by switching the snubber switch 76 on and off is reduced compared to switching the snubber switch 76 on and off each time charging occurs.

[0075] (4) According to the above embodiment, the control unit 80 changes the first threshold value TH1 and the second threshold value TH2 in accordance with the ratio of the supply current value to the rated current of the power conversion device 10. In this way, the control unit 80 controls the amount of charge to be applied to each snubber capacitor at one time. In addition, the control unit 80 can control the power output from the first external output terminal 12A to the load 100.

[0076] (5) According to the above embodiment, the control unit 80 switches on the snubber switch 76 when the inter-terminal voltage VC1 of the first snubber capacitor 74 is equal to or greater than the first threshold TH1. The resonant noise superimposed on the inter-terminal voltage VC1 of the first snubber capacitor 74 is smaller than the resonant noise superimposed on the inter-terminal voltage VC2 of the second snubber capacitor 75. Therefore, it is unlikely that a large resonant noise will be superimposed on the inter-terminal voltage VC1 of the first snubber capacitor 74, causing the inter-terminal voltage VC1 to exceed the first threshold TH1. Therefore, the control unit 80 can switch the snubber switch 76 on and off less frequently than when controlling the snubber switch 76 based on the inter-terminal voltage VC2 of the second snubber capacitor 75, on which a relatively large resonant noise may be superimposed. In other words, the influence of parasitic inductance on the control of the snubber circuit 70 by the control unit 80 can be reduced.

[0077] (6) In the above embodiment, the length WL1 of the first wiring W1 is smaller than the length WL2 of the second wiring W2. That is, the parasitic inductance of the first wiring W1 is smaller than the parasitic inductance of the second wiring W2. This configuration is suitable for controlling the snubber switch 76 with the inter-terminal voltage VC1 of the first snubber capacitor 74.

[0078] (7) According to the above embodiment, the capacitance of the first snubber capacitor 74 is larger than the capacitance of the second snubber capacitor 75. As described above, the Q factor is inversely proportional to the square root of the capacitance. The smaller the Q factor, the smaller the voltage across each snubber capacitor due to resonance. Therefore, the smaller the capacitance of the first snubber capacitor 74, the smaller the voltage across the first snubber capacitor 74 can be.

[0079] (8) In the above embodiment, the first snubber capacitor 74 and the rectifier circuit 50 are mounted on the first board CB1. The second snubber capacitor 75 is mounted on the second board CB2. When the rectifier circuit 50 and the second snubber capacitor 75 are mounted on separate boards in this manner, the second wiring W2 is longer than the first wiring W1. That is, the parasitic inductance of the second wiring W2 is greater than the parasitic inductance of the first wiring W1. Therefore, this configuration is suitable for controlling the snubber switch 76 based on the inter-terminal voltage VC1 of the first snubber capacitor 74.

[0080] (9) In the above embodiment, the first Q value is equal to or smaller than half of the second Q value. As described above, the smaller these values ​​are, the smaller the noise that is added to the voltage between the terminals of each snubber capacitor due to resonance. Therefore, the number of times the snubber switch 76 is switched can be reduced to about half compared to when the snubber switch 76 is controlled based on the second snubber capacitor 75.

[0081] <Modifications> The above embodiment can be modified as follows: The above embodiment and the following modifications can be combined and implemented within the scope of technical compatibility.

[0082] The configuration of the power conversion device 10 is not limited to the example of the above embodiment. For example, the power conversion device 10 does not have to include one or more selected from the input noise filter 20, the switching circuit 30, and the output noise filter 60. Furthermore, the power conversion device 10 may include elements and circuits other than those exemplified in the above embodiment.

[0083] The three-phase AC power supply 90 connected to the three external input terminals is not limited to a three-phase three-wire type, but may be a three-phase four-wire type or a delta-connected three-phase three-wire type three-phase AC power supply 90. Similarly, the configuration of the power conversion device 10 may be changed as appropriate depending on the type of the three-phase AC power supply 90.

[0084] The switch elements constituting the bidirectional switch TSW, the switch elements constituting the rectifier circuit 50, and the snubber switch 76 are not limited to the examples in the above embodiment. For example, the two switch elements of the bidirectional switch TSW may be transistors capable of flowing current in both the forward and reverse directions. 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. Furthermore, the switch elements constituting the bidirectional switch TSW may be gallium nitride high electron mobility transistors (GaN-High Electron Mobility Transistors, GaN-HEMTs) or the like. In this case, the two switch elements are connected in series with their source terminals connected to each other.

[0085] The transformer circuit 40 does not need to include the fourth inductor L4. In this case, the leakage inductance of the transformer 41 can be used for resonance instead of the fourth inductor L4. 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 or the like. Furthermore, the rectifier circuit 50 may be a circuit consisting of four rectifier diodes.

[0086] The snubber circuit 70 may include a second diode 72, a snubber switch 76, a snubber inductor 77, and at least one of a first snubber capacitor 74 and a second snubber capacitor 75. For example, as shown in Fig. 6, the snubber circuit 70 does not need to include the second snubber capacitor 75. Even in this case, the first snubber capacitor 74 can absorb and regenerate surge energy.

[0087] In the above embodiment, the rectifier circuit 50, the first snubber capacitor 74, and the second snubber capacitor 75 may be mounted on a single substrate. Even in this case, the parasitic inductance of the first wiring W1 can be smaller than the parasitic inductance of the second wiring W2.

[0088] In the above embodiment, the length WL1 of the first wiring W1 may be longer than the length WL2 of the second wiring W2. Even in this case, the parasitic inductance of the first wiring W1 may be smaller than the parasitic inductance of the second wiring W2 depending on factors such as the material and cross-sectional area of ​​each wiring and the magnetic material present in the surrounding area. Furthermore, the parasitic inductance between the high-potential output terminal 55A and the first snubber capacitor 74 does not have to be smaller than the parasitic inductance between the high-potential output terminal 55A and the second snubber capacitor 75. In other words, the parasitic inductance of the first wiring W1 does not have to be smaller than the parasitic inductance of the second wiring W2.

[0089] In the above embodiment, the first wiring W1 and the second wiring W2 share a common length from the high potential output terminal 55A to the connection point on the cathode side of the first diode 71. Therefore, it is not necessary to measure the length from the high potential output terminal 55A to the first end of each snubber capacitor, and it is also possible to compare the lengths from the connection point to the first end of each snubber capacitor.

[0090] The capacitance of the first snubber capacitor 74 may be smaller than the capacitance of the second snubber capacitor 75. The first Q value may be larger than half the second Q value. Even in these cases, if the parasitic inductance of the first wiring W1 is smaller than the parasitic inductance of the second wiring W2, the first Q value is likely to be smaller than the second Q value.

[0091] The control unit 80 is not limited to being configured with one MCU, etc. For example, it may have an MCU, etc. mounted on the first substrate CB1 and controlling the switch elements that configure the switching circuit 30 and the rectifier circuit 50, and an MCU, etc. mounted on the second substrate CB2 as a chip separate from the MCU, that controls the snubber switch 76. Even in such a case, the multiple chips can be considered as one control unit 80.

[0092] The specific values ​​of the ideal voltage, peak value range, and maximum peak value of the rectifier circuit 50 are not limited to those in the above embodiment. The specific values ​​of the first threshold value TH1, the second threshold value TH2, and the first to fourth predetermined values ​​may be determined according to these values. In particular, the first predetermined value may be less than 10% of the rated current of the power conversion device 10 or may be greater than 10%. The third predetermined value may be less than 90% of the rated current of the power conversion device 10 or may be greater than 90%.

[0093] When the snubber switch 76 is in the on state, the control unit 80 does not have to maintain the snubber switch 76 in the on state until the inter-terminal voltage VC1 of the first snubber capacitor 74 becomes equal to or less than the second threshold value. In other words, the control unit 80 may switch the snubber switch 76 to the off state based on a condition other than the inter-terminal voltage VC1 being equal to or less than the second threshold value.

[0094] Furthermore, when the snubber switch 76 is in the OFF state, the control unit 80 does not need to maintain the snubber switch 76 in the OFF state until the inter-terminal voltage VC1 of the first snubber capacitor 74 becomes equal to or greater than the first threshold value. In other words, the control unit 80 may switch the snubber switch 76 to the ON state based on a condition other than the inter-terminal voltage VC1 being equal to or greater than the first threshold value.

[0095] In the above embodiment, the first threshold value TH1 and the second threshold value TH2 are increased or decreased in two stages based on their initial values, but this is not limited to this. It is sufficient that at least the first threshold value TH1 is a value smaller than the maximum peak value of the output voltage VRE of the rectifier circuit 50, and that the second threshold value TH2 is a value higher than the ideal voltage of the rectifier circuit 50.

[0096] In the above embodiment, when changing the first threshold value TH1 and the second threshold value TH2, it is not necessary to change both threshold values ​​simultaneously. Depending on the conditions, only the first threshold value TH1 or only the second threshold value TH2 may be changed. This allows for more precise control of the surge energy charged to each snubber capacitor and the energy regenerated from the snubber capacitor.

[0097] The control unit 80 may increase the unit change amount when changing the first threshold value TH1 and the unit change amount when changing the second threshold value TH2 as the difference between the supply current value and the first predetermined value increases. The unit change amount here refers to the amount of change per change of each threshold value. For example, the smaller the supply current value is than the first predetermined value and the greater the difference between the supply current value and the first predetermined value, the greater the change amount of each threshold value. In other words, the smaller the supply current value is relative to the rated current of the power conversion device 10, the larger the threshold value may be. This reduces the amount of energy stored in each snubber capacitor per change. Consequently, the amount of power supplied per unit time from the first external output terminal 12A increases.

[0098] Similarly, the unit change amount for changing each threshold may be increased as the supply current value is equal to or greater than the third predetermined value and the difference between the supply current value and the third predetermined value increases. In other words, the threshold may be decreased as the supply current value is greater than the rated current. This increases the energy stored in each snubber capacitor per charge. As a result, the amount of power supplied per unit time from the first external output terminal 12A decreases.

[0099] For example, in the above embodiment, the control unit 80 determines whether the supply current value is equal to or less than a fifth predetermined value. The fifth predetermined value is a value smaller than the first predetermined value, e.g., 5% of the rated current of the power conversion device 10. If the supply current value is equal to or less than the fifth predetermined value, the control unit 80 may set the first threshold value TH1 to a value obtained by adding a sixth predetermined value to the current first threshold value TH1. Furthermore, the control unit 80 may set the second threshold value TH2 to a value obtained by adding the sixth predetermined value to the current second threshold value TH2. The sixth predetermined value is a positive value that is greater than the second predetermined value and is set within a range in which the second threshold value TH2 is higher than the ideal voltage of the rectifier circuit 50. For example, the sixth predetermined value is 4.0 V.

[0100] Alternatively, for example, the control unit 80 may determine whether the supply current value is equal to or less than a seventh predetermined value. The seventh predetermined value is greater than the third predetermined value, e.g., 95% of the rated current of the power conversion device 10. If the supply current value is equal to or greater than the seventh predetermined value, the control unit 80 may set the first threshold value TH1 to a value obtained by subtracting the eighth predetermined value from the current first threshold value TH1. Furthermore, the control unit 80 may set the second threshold value TH2 to a value obtained by subtracting the eighth predetermined value from the current second threshold value TH2. The eighth predetermined value is a positive value that is greater than the fourth predetermined value and is set within a range in which the first threshold value TH1 is lower than the maximum peak value of the rectifier circuit 50. For example, the eighth predetermined value is 4.0 V.

[0101] When the supply current value is equal to or less than the first predetermined value, the control unit 80 may not increase the first threshold value TH1 and the second threshold value TH2 compared to when the supply current value is greater than the first predetermined value. Furthermore, when the supply current value is equal to or greater than the third predetermined value, the control unit 80 may not decrease the first threshold value TH1 and the second threshold value TH2 compared to when the supply current value is less than the third predetermined value. Even in such cases, surge energy can be regenerated by the snubber capacitor, and losses associated with switching the snubber switch 76 can be easily reduced.

[0102] In the power conversion device described in Patent Document 1, parasitic inductance occurs in the wiring between the rectifier circuit and the snubber capacitor. This parasitic inductance may affect the control of the snubber circuit by the control unit. From this perspective, in the above embodiment, after the snubber switch 76 is switched off, it is not necessary to switch the snubber switch 76 on after the current flowing through the secondary winding 41B changes direction from positive to negative at least once and from negative to positive at least once. The snubber circuit 70 may include a first snubber capacitor 74 and a second snubber capacitor 75. The control unit 80 may switch the snubber switch 76 on when the voltage VC1 across the first snubber capacitor 74 is equal to or greater than a first threshold TH1, and switch the snubber switch 76 off when the voltage VC1 across the first snubber capacitor 74 is equal to or less than a second threshold TH2 that is smaller than the first threshold TH1.

[0103] <Supplementary Notes> The technical ideas that can be understood from the above embodiments and modified examples will be described below. [1] A power conversion device comprising: a transformer having a primary winding and a secondary winding; a rectifier circuit connected to the secondary winding and having a high-potential output terminal and a low-potential output terminal; an external output terminal connected to the high-potential output terminal; a snubber circuit having a snubber capacitor connected between the high-potential output terminal and the low-potential output terminal and a snubber switch connected between the high-potential terminal of the snubber capacitor and the external output terminal; and a control unit that controls the snubber switch, wherein, when one direction of a current flowing through the secondary winding is defined as a positive direction and the opposite direction to the positive direction is defined as a negative direction, the control unit switches the snubber switch to an off state and then controls the snubber switch to an on state after the current flowing through the secondary winding has changed direction from the positive direction to the negative direction and from the negative direction to the positive direction at least once each.

[0104] [2] The control unit of the power conversion device described in [1] switches the snubber switch to an on state when the terminal voltage of the snubber capacitor is equal to or greater than a first threshold, and controls the snubber switch to an off state when the terminal voltage of the snubber capacitor is equal to or less than a second threshold that is smaller than the first threshold.

[0105] [3] The control unit controls the snubber switch to maintain the on state when the snubber switch is on until the voltage between the terminals of the snubber capacitor becomes equal to or less than the second threshold, and controls the snubber switch to maintain the off state when the snubber switch is off until the voltage between the terminals of the snubber capacitor becomes equal to or greater than the first threshold. This is a power conversion device described in [2].

[0106] [4] The power conversion device according to [2], wherein when the control unit switches the snubber switch to the on state, the current conducted through the snubber switch is a first current, and the current flowing between the high-potential output terminal and the external output terminal is a second current, the control unit adds a second predetermined positive value to the first threshold value and the second threshold value if the sum of the value of the first current and the value of the second current is equal to or less than a first predetermined value, and subtracts a fourth predetermined positive value from the first threshold value and the second threshold value if the sum is equal to or greater than a third predetermined value.

[0107] [5] The control unit of the power conversion device described in [4] increases the unit change amount when changing the first threshold value and the unit change amount when changing the second threshold value when the total value is less than or equal to a first predetermined value and the difference between the total value and the first predetermined value is greater.

[0108] [6] A power conversion device comprising: a transformer having a primary winding and a secondary winding; a rectifier circuit connected to the secondary winding and having a high potential output terminal and a low potential output terminal; a first snubber capacitor and a second snubber capacitor connected in parallel between the high potential output terminal and the low potential output terminal; a snubber circuit connected to the high potential terminal of the second snubber capacitor and having a snubber switch; and a control unit for controlling the snubber switch, wherein a parasitic inductance between the high potential output terminal and the first snubber capacitor is smaller than a parasitic inductance between the high potential output terminal and the second snubber capacitor, and the control unit controls the snubber switch to an on state when a voltage between the terminals of the first snubber capacitor is equal to or greater than a first threshold, and controls the snubber switch to an off state when the voltage between the terminals of the first snubber capacitor is equal to or less than a second threshold that is smaller than the first threshold.

[0109] [7] The control unit controls the snubber switch to maintain the on state when the snubber switch is on until the terminal voltage of the first snubber capacitor becomes equal to or less than the second threshold, and controls the snubber switch to maintain the off state when the snubber switch is off until the terminal voltage of the first snubber capacitor becomes equal to or greater than the first threshold, in a power conversion circuit as described in [6].

[0110] [8] A power conversion device according to [6] or [7], wherein the length of the wiring from the high potential output terminal to the first snubber capacitor is shorter than the length of the wiring from the high potential output terminal to the second snubber capacitor.

[0111] [9] The power conversion device according to any one of [6] to [8], wherein the capacitance of the first snubber capacitor is larger than the capacitance of the second snubber capacitor.

[10] The power conversion device according to any one of [6] to [9], wherein the rectifier circuit and the first snubber capacitor are mounted on the same substrate, and the first snubber capacitor and the second snubber capacitor are mounted on different substrates.

[0112]

[11] A power conversion device described in any one of [6] to

[10] , wherein when the wiring from the high potential output terminal to the first snubber capacitor is defined as the first wiring and the wiring from the high potential output terminal to the second snubber capacitor is defined as the second wiring, the Q value of the first snubber capacitor and the first wiring is less than or equal to half the Q value of the second snubber capacitor and the second wiring.

[0113]

[12] A program applied to a power conversion device comprising: a transformer having a primary winding and a secondary winding; a rectifier circuit connected to the secondary winding and having a high-potential output terminal and a low-potential output terminal; an external output terminal connected to the high-potential output terminal; a snubber circuit having a snubber capacitor connected between the high-potential output terminal and the low-potential output terminal, and a snubber switch connected between the high-potential side terminal of the snubber capacitor and the external output terminal; and a control unit that controls the snubber switch, wherein, when one direction of the current flowing through the secondary winding is defined as a positive direction and the opposite direction to the positive direction is defined as a negative direction, the program causes the control unit to switch the snubber switch to an off state, and then controls the snubber switch to an on state after the current flowing through the secondary winding has changed direction from the positive direction to the negative direction and from the negative direction to the positive direction at least once each.

[0114]

[13] A program applicable to a power conversion device comprising: a transformer having a primary winding and a secondary winding; a rectifier circuit connected to the secondary winding and having a high potential output terminal and a low potential output terminal; a first snubber capacitor and a second snubber capacitor connected in parallel between the high potential output terminal and the low potential output terminal; a snubber circuit connected to the high potential terminal of the second snubber capacitor and having a snubber switch; and a control unit for controlling the snubber switch, wherein the parasitic inductance between the high potential output terminal and the first snubber capacitor is smaller than the parasitic inductance between the high potential output terminal and the second snubber capacitor, and the program causes the control unit to control the snubber switch to an on state when the terminal voltage of the first snubber capacitor is equal to or greater than a first threshold, and to control the snubber switch to an off state when the terminal voltage of the first snubber capacitor is equal to or less than a second threshold which is smaller than the first threshold.

[0115] REFERENCE SIGNS LIST 10...power conversion device 11A...first external input terminal 11B...second external input terminal 11C...third external input terminal 12A...first external output terminal 12B...second external output terminal 30...switching circuit 40...transformer circuit 41...transformer 41A...primary side winding 41B...secondary side winding 50...rectifier circuit 55A...high potential output terminal 55B...low potential output terminal VRE...output voltage 70...snubber circuit 71...first diode 72...second diode 73...third diode 74...first snubber capacitor 75...second snubber capacitor 76...snubber switch 77...snubber inductor VC1...inter-terminal voltage VC2...inter-terminal voltage W1...first wiring W2...second wiring 80...controller PG...program CB1...first board CB2...second board 90...Three-phase AC power supply 100...Load iT...Secondary current i1...First current i2...Second current

Claims

1. A power conversion device comprising: a transformer having a primary winding and a secondary winding; a rectifier circuit connected to the secondary winding and having a high potential output terminal and a low potential output terminal; an external output terminal connected to the high potential output terminal; a snubber circuit having a snubber capacitor connected between the high potential output terminal and the low potential output terminal and a snubber switch connected between the high potential side terminal of the snubber capacitor and the external output terminal; and a control unit for controlling the snubber switch, wherein when one direction of current flowing through the secondary winding is defined as a positive direction and the opposite direction to the positive direction is defined as a negative direction, after switching the snubber switch to an off state, the control unit controls the snubber switch to an on state after the current flowing through the secondary winding has changed direction from the positive direction to the negative direction and from the negative direction to the positive direction at least once each.

2. The power conversion device according to claim 1, wherein the control unit switches the snubber switch to an ON state when the voltage across the snubber capacitor is equal to or greater than a first threshold, and controls the snubber switch to an OFF state when the voltage across the snubber capacitor is equal to or less than a second threshold that is smaller than the first threshold.

3. The power conversion device according to claim 2, wherein the control unit controls the snubber switch so that, when the snubber switch is in an on state, the snubber switch remains on until the voltage between the terminals of the snubber capacitor becomes equal to or lower than the second threshold, and controls the snubber switch so that, when the snubber switch is in an off state, the snubber switch remains off until the voltage between the terminals of the snubber capacitor becomes equal to or higher than the first threshold.

4. The power conversion device according to claim 2, wherein when the control unit switches the snubber switch to an on state, a current conducted through the snubber switch is a first current, and a second current flowing between the high potential output terminal and the external output terminal is a second current, the control unit adds a second predetermined positive value to the first threshold value and the second threshold value if the sum of the value of the first current and the value of the second current is equal to or less than a first predetermined value, and subtracts a fourth predetermined positive value from the first threshold value and the second threshold value if the sum is equal to or greater than a third predetermined value.

5. The power conversion device according to claim 4, wherein the control unit increases the unit change amount when changing the first threshold value and the unit change amount when changing the second threshold value as the total value is equal to or less than a first predetermined value and the difference between the total value and the first predetermined value increases.

6. A program applied to a power conversion device comprising: a transformer having a primary winding and a secondary winding; a rectifier circuit connected to the secondary winding and having a high potential output terminal and a low potential output terminal; an external output terminal connected to the high potential output terminal; a snubber circuit having a snubber capacitor connected between the high potential output terminal and the low potential output terminal and a snubber switch connected between the high potential side terminal of the snubber capacitor and the external output terminal; and a control unit for controlling the snubber switch, wherein when one direction of the current flowing through the secondary winding is defined as a positive direction and the opposite direction to the positive direction is defined as a negative direction, the program causes the control unit to switch the snubber switch to an off state, and then control the snubber switch to an on state after the current flowing through the secondary winding has changed direction from the positive direction to the negative direction and from the negative direction to the positive direction at least once each.

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