Power Conversion Device

Abstract

Reducing harmonics. [Solution] The power conversion device includes a transformer, a primary-side bridge circuit having twice the number of first legs as the transformers, a secondary-side bridge circuit having twice the number of second legs as the transformers, and a control unit. The control unit applies a primary-side voltage having a first voltage waveform W1 to the primary side of the transformer and a secondary-side voltage having a second voltage waveform W2 to the secondary side of the transformer so as to reduce nth-order harmonics generated by switching. The first voltage waveform W1 is a waveform in which the primary-side voltage switches between three levels: low level, middle level, and high level. The second voltage waveform W2 is a waveform in which the secondary-side voltage switches between three levels: low level, middle level, and high level. The period Pe1 during which the first voltage waveform W1 is at a high level and the period Pe3 during which the second voltage waveform W2 is at a high level are 2π / n.

Description

[Technical Field]

[0001] The present disclosure relates to a power conversion device. [Background technology]

[0002] BACKGROUND ART Conventionally, in a DAB DC / DC converter, a technique is known in which a low-pass filter is inserted into a current detection signal used for control to remove harmonic components of the signal (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2022-110235 Summary of the Invention [Problem to be solved by the invention]

[0004] However, with the technology disclosed in Patent Document 1, although it is possible to remove harmonic components from signals used for control, it is difficult to remove harmonic components from the main circuit portion. [Means for solving the problem]

[0005] The power conversion device that solves the above problem includes a transformer having a primary winding, a secondary winding, and a reactor connected to at least one of the primary winding and the secondary winding, and a primary bridge circuit having twice the number of first legs each having two primary switching elements connected in series to the transformer, the primary bridge circuit being connected to the primary winding, and a secondary bridge circuit having twice the number of second legs each having two secondary switching elements connected in series to the transformer, the secondary bridge circuit being connected to the secondary winding, and a reactor connected to the primary switching elements and the secondary switching elements. and a control unit that controls a switching element, wherein the control unit applies a primary-side voltage having a first voltage waveform to the primary side of the transformer and applies a secondary-side voltage having a second voltage waveform to the secondary side of the transformer so as to reduce n-th order harmonics generated by switching, wherein the first voltage waveform is a waveform in which the primary-side voltage switches between three levels: low level, middle level, and high level, and the second voltage waveform is a waveform in which the secondary-side voltage switches between three levels: low level, middle level, and high level, and the period in which the first voltage waveform is at a high level and the period in which the second voltage waveform is at a high level are 2π / n.

[0006] The control unit controls the first voltage waveform and the second voltage waveform so that the period during which they are at a high level is 2π / n, thereby reducing harmonics caused by switching, thereby reducing n-th order harmonics.

[0007] The power conversion device may include a plurality of the transformers. In the power conversion device, the control unit may change n in accordance with switching frequencies of the primary side switching element and the secondary side switching element. [Effects of the Invention]

[0008] According to the present invention, harmonics can be reduced. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a circuit diagram of a power conversion device. [Figure 2] FIG. 2 is a diagram showing the relationship between the AU-phase voltage, the AW-phase voltage, the primary side voltage, the BU-phase voltage, the BW-phase voltage, and the secondary side voltage and the phase. DETAILED DESCRIPTION OF THE INVENTION

[0010] An embodiment of a power conversion device will be described. 1, the power conversion device 10 is configured to perform bidirectional power conversion between, for example, a first DC power supply E1 and a second DC power supply E2. In the following description, a voltage E dA A voltage E dB Suppose that the following occurs.

[0011] <Configuration of power conversion device> The power conversion device 10 includes at least one transformer 11a, 11b. The power conversion device 10 of this embodiment includes two transformers 11a, 11b. The two transformers 11a, 11b include a first transformer 11a and a second transformer 11b. The power conversion device 10 is a four-phase power conversion device.

[0012] The first transformer 11a has a primary winding 12a, a secondary winding 15a, and reactors 13a, 14a, 16a, and 17a connected to at least one of the primary winding 12a and the secondary winding 15a. In this embodiment, the reactors 13a and 14a are connected to both ends of the primary winding 12a. The reactors may be connected to only one end of the primary winding 12a. The reactors 16a and 17a are connected to both ends of the secondary winding 15a. The reactors may be connected to only one end of the secondary winding 15a. The reactors 13a, 14a, 16a, and 17a may be elements such as choke coils or leakage inductances of the primary winding 12a and the secondary winding 15a.

[0013] The second transformer 11b has a primary winding 12b, a secondary winding 15b, and reactors 13b, 14b, 16b, and 17b connected to at least one of the primary winding 12b and the secondary winding 15b. In this embodiment, the reactors 13b and 14b are connected to both ends of the primary winding 12b. The reactors may be connected to only one end of the primary winding 12b. The reactors 16b and 17b are connected to both ends of the secondary winding 15b. The reactors may be connected to only one end of the secondary winding 15b. The reactors 13b, 14b, 16b, and 17b may be elements such as choke coils or leakage inductances of the primary winding 12b and the secondary winding 15b.

[0014] The power conversion device 10 includes a primary-side bridge circuit 20. The primary-side bridge circuit 20 includes leg pairs 21a and 21b, the number of which is the same as the number of transformers 11a and 11b. In this embodiment, the primary-side bridge circuit 20 includes two leg pairs 21a and 21b. The two leg pairs 21a and 21b include a first leg pair 21a and a second leg pair 21b.

[0015] The first leg pair 21a includes two first legs 22a and 23a. The first leg 22a includes two primary-side switching elements Q1a and Q2a connected in series. The first leg 23a includes two primary-side switching elements Q3a and Q4a connected in series.

[0016] The second leg pair 21b includes two first legs 22b, 23b. Therefore, the primary-side bridge circuit 20 includes twice the number of first legs 22a, 23a, 22b, 23b as the number of transformers 11a, 11b. The first leg 22b includes two primary-side switching elements Q1b, Q2b connected in series. The first leg 23b includes two primary-side switching elements Q3b, Q4b connected in series.

[0017] The first leg 22a corresponds to the AU phase, which is the U phase on the primary side. The first leg 22b corresponds to the AV phase, which is the V phase on the primary side. The first leg 23a corresponds to the AW phase, which is the W phase on the primary side. The first leg 23b corresponds to the AX phase, which is the X phase on the primary side.

[0018] The primary bridge circuit 20 is connected to primary windings 12a and 12b. A first end of the primary winding 12a is connected to a connection point P1a between the two primary switching elements Q1a and Q2a. In this embodiment, a reactor 13a is located between the connection point P1a and the first end of the primary winding 12a. Therefore, the first end of the primary winding 12a is connected to the connection point P1a via the reactor 13a. A second end of the primary winding 12a is connected to a connection point P2a between the two primary switching elements Q3a and Q4a. In this embodiment, a reactor 14a is located between the connection point P2a and the second end of the primary winding 12a. Therefore, the second end of the primary winding 12a is connected to the connection point P2a via the reactor 14a. In this manner, the first leg pair 21a is provided corresponding to the first transformer 11a.

[0019] A first end of the primary winding 12b is connected to a connection point P1b of the two primary switching elements Q1b, Q2b. In this embodiment, a reactor 13b is located between the connection point P1b and the first end of the primary winding 12b. Therefore, the first end of the primary winding 12b is connected to the connection point P1b via the reactor 13b. A second end of the primary winding 12b is connected to a connection point P2b of the two primary switching elements Q3b, Q4b. In this embodiment, a reactor 14b is located between the connection point P2b and the second end of the primary winding 12b. Therefore, the second end of the primary winding 12b is connected to the connection point P2b via the reactor 14b. In this manner, the second leg pair 21b is provided corresponding to the second transformer 11b.

[0020] The power conversion device 10 includes a secondary-side bridge circuit 30. The secondary-side bridge circuit 30 includes leg pairs 31a and 31b, the number of which is the same as the number of transformers 11a and 11b. In this embodiment, the secondary-side bridge circuit 30 includes two leg pairs 31a and 31b. The two leg pairs 31a and 31b include a first leg pair 31a and a second leg pair 31b.

[0021] The first leg pair 31a includes two second legs 32a and 33a. The second leg 32a includes two secondary-side switching elements Q5a and Q6a connected in series. The second leg 33a includes two secondary-side switching elements Q7a and Q8a connected in series.

[0022] The second leg pair 31b includes two second legs 32b and 33b. Therefore, the secondary-side bridge circuit 30 includes twice the number of second legs 32a, 33a, 32b, and 33b as the number of transformers 11a and 11b. The second leg 32b includes two secondary-side switching elements Q5b and Q6b connected in series. The second leg 33b includes two secondary-side switching elements Q7b and Q8b connected in series.

[0023] The second leg 32a corresponds to the BU phase, which is the U phase on the secondary side. The second leg 32b corresponds to the BV phase, which is the V phase on the secondary side. The second leg 33a corresponds to the BW phase, which is the W phase on the secondary side. The second leg 33b corresponds to the BX phase, which is the X phase on the secondary side.

[0024] The secondary bridge circuit 30 is connected to secondary windings 15a and 15b. A first end of the secondary winding 15a is connected to a connection point P3a between the two secondary switching elements Q5a and Q6a. In this embodiment, a reactor 16a is located between the connection point P3a and the first end of the secondary winding 15a. Therefore, the first end of the secondary winding 15a is connected to the connection point P3a via the reactor 16a. A second end of the secondary winding 15a is connected to a connection point P4a between the two secondary switching elements Q7a and Q8a. In this embodiment, a reactor 17a is located between the connection point P4a and the second end of the secondary winding 15a. Therefore, the second end of the secondary winding 15a is connected to the connection point P4a via the reactor 17a. In this manner, the first leg pair 31a is provided corresponding to the first transformer 11a.

[0025] A first end of a secondary winding 15b is connected to a connection point P3b of the two secondary switching elements Q5b, Q6b. In this embodiment, a reactor 16b is located between the connection point P3b and the first end of the secondary winding 15b. Therefore, the first end of the secondary winding 15b is connected to the connection point P3b via the reactor 16b. A second end of the secondary winding 15b is connected to a connection point P4b of the two secondary switching elements Q7b, Q8b. In this embodiment, a reactor 17b is located between the connection point P4b and the second end of the secondary winding 15b. Therefore, the second end of the secondary winding 15b is connected to the connection point P4b via the reactor 17b. In this manner, a second leg pair 31b is provided corresponding to the second transformer 11b.

[0026] The power conversion device 10 includes a control unit 40. The control unit 40 includes a processor and a memory unit. The processor is, for example, a central processing unit (CPU), a graphics processing unit (GPU), or a digital signal processor (DSP). The memory unit includes a random access memory (RAM) and a read-only memory (ROM). The memory unit stores program code or instructions configured to cause the processor to execute processing. The memory unit, i.e., a computer-readable medium, includes any available medium accessible by a general-purpose or special-purpose computer. The control unit 40 may be configured by a hardware circuit such as an application-specific integrated circuit (ASIC) or a field-programmable gate array (FPGA). The control unit 40, which is a processing circuit, may include one or more processors operating according to a computer program, one or more hardware circuits such as ASICs or FPGAs, or a combination thereof.

[0027] The control unit 40 switches the switching elements Q1a to Q8a and Q1b to Q8b between ON and OFF. The switching frequency of the switching elements Q1a to Q8a and Q1b to Q8b is an arbitrary value, for example, between 10 kHz and 50 kHz. In the following explanation, as an example, a case where the switching elements Q1a to Q8a are controlled will be explained.

[0028] <Control performed by the control unit> The control unit 40 generates a primary side voltage V having a first voltage waveform on the primary side of the first transformer 11a. AUW The primary side of the first transformer 11a is between the connection points P1a and P2a. The primary side voltage V AUW is the potential difference between the connection points P1a and P2a. The first voltage waveform is the primary voltage V AUW This is the waveform.

[0029] The control unit 40 generates a secondary-side voltage V having a second voltage waveform on the secondary side of the first transformer 11a. BUW The secondary side of the first transformer 11a is between the connection points P3a and P4a. The secondary side voltage V BUW is the potential difference between the connection points P3a and P4a. The second voltage waveform is the secondary voltage V BUW This is the waveform.

[0030] 2 shows a first voltage waveform W1 and a second voltage waveform W2 having a period ranging from -π to π. The horizontal axis of the diagram represents the phase [rad], and the vertical axis represents the waveform magnitude [V]. The first voltage waveform W1 is centered at 0 [V] and has a positive voltage +E dA [V] and the negative voltage -E dA [V] between the primary voltage V AUW The primary voltage V AUW +E dA -E for high level, 0 for middle level, dA The first voltage waveform W1 is at a low level when the primary voltage V AUW The first voltage waveform W1 has three levels: low level, middle level, and high level. The low level and high level of the first voltage waveform W1 are inverted in phase every 180 degrees. The high level period Pe1 and the low level period Pe2 are the same. The high level period Pe1 is the period when the primary side voltage V AUW rises from the middle level to the high level, and the primary voltage V AUW The phase difference between the phase P2 where the primary voltage V AUW Phase P3 where the voltage falls from the middle level to the low level, and the primary voltage V AUW is the phase difference with respect to phase P4 at which the signal rises from the low level to the middle level.

[0031] The control unit 40 controls the AU-phase voltage V by switching the primary side switching elements Q1a to Q4a ON and OFF. AU and the AW phase voltage V AW The AU phase voltage V AU and the AW phase voltage VAW The difference between the potential at the connection point P1a and the potential at the connection point P2a is the difference between the potential at the connection point P1a and the potential at the connection point P2a. AU and voltage V AW The difference between these is the primary voltage V AUW As shown in the waveform W11, when the primary side switching element Q1a is turned on and the primary side switching element Q2a is turned off, the voltage V AU is E dA When the primary side switching element Q1a is turned off and the primary side switching element Q2a is turned on, the voltage V AU As shown in waveform W12, when the primary side switching element Q3a is turned on and the primary side switching element Q4a is turned off, the voltage V AW is E dA When the primary side switching element Q3a is turned off and the primary side switching element Q4a is turned on, the voltage V AW becomes 0.

[0032] Voltage V AU E dA and voltage V AW If is 0, the primary voltage V AUW +E dA , that is, it becomes high level. Voltage V AU E dA and voltage V AW E dA In this case, the primary voltage V AUW becomes 0, i.e., the middle level. Voltage V AU is 0 and the voltage V AW If is 0, the primary voltage V AUW becomes 0, i.e., the middle level. Voltage V AU is 0 and the voltage V AW E dA In this case, the primary voltage V AUW -E dA , that is, it becomes a low level.

[0033] The second voltage waveform W2 is centered at 0 [V] and has a positive voltage +E dB and the negative voltage -E dB Between the secondary voltage V BUW The secondary voltage VBUW +E dB -E for high level, 0 for middle level, dB The second voltage waveform W2 is at a low level when the secondary voltage V BUW The second voltage waveform W2 has three levels: low level, middle level, and high level. The low level and high level of the second voltage waveform W2 are inverted every 180 degrees. The high level period Pe3 and the low level period Pe4 are the same. The high level period Pe3 is the period when the secondary side voltage V BUW Phase P5 where the secondary voltage V rises from the middle level to the high level BUW The low-level period Pe4 is the phase difference between the phase P6 where the secondary voltage V BUW Phase P7 where the voltage V falls from the middle level to the low level BUW is the phase difference with respect to phase P8 at which the signal rises from the low level to the middle level.

[0034] The control unit 40 controls the BU-phase voltage V by switching the secondary side switching elements Q5a to Q8a ON and OFF. BU and the BW phase voltage V BW The voltage V of the BU phase is controlled. BU and the BW phase voltage V BW The difference between the potential at the connection point P3a and the potential at the connection point P4a is the difference between the potential at the connection point P3a and the potential at the connection point P4a. BU and voltage V BW The difference between these is the secondary voltage V BUW As shown in the waveform W21, when the secondary-side switching element Q5a is turned on and the secondary-side switching element Q6a is turned off, the voltage V BU is E dB When the secondary side switching element Q5a is turned off and the secondary side switching element Q6a is turned on, the voltage V BU As shown in waveform W22, when the secondary side switching element Q7a is turned on and the secondary side switching element Q8a is turned off, the voltage V BW is E dB When the secondary side switching element Q7a is turned off and the secondary side switching element Q8a is turned on, the voltage VBW becomes 0.

[0035] Voltage V BU E dB and voltage V BW If is 0, the secondary voltage V BUW +E dB , that is, it becomes high level. Voltage V BU E dB and voltage V BW E dB In this case, the secondary voltage V BUW becomes 0, i.e., the middle level. Voltage V BU is 0 and the voltage V BW If is 0, the secondary voltage V BUW becomes 0, i.e., the middle level. Voltage V BU is 0 and the voltage V BW E dB In this case, the secondary voltage V BUW -E dB , that is, it becomes a low level.

[0036] The control unit 40 controls the power transferred from the primary side to the secondary side of the first transformer 11a by controlling the phase difference θ between the phase P1 at which the first voltage waveform W1 rises from a middle level to a high level and the phase P5 at which the second voltage waveform W2 rises from a middle level to a high level. In the example shown in FIG. 2, the phase P1 at which the first voltage waveform W1 rises from a middle level to a high level occurs earlier than the phase P5 at which the second voltage waveform W2 rises from a middle level to a high level. In this case, power is transferred from the primary side to the secondary side of the first transformer 11a. Power can also be transferred from the secondary side to the primary side of the first transformer 11a by delaying the phase P1 at which the first voltage waveform W1 rises from a middle level to a high level relative to the phase P5 at which the second voltage waveform W2 rises from a middle level to a high level. The phase difference θ is also the difference between the phase P3 at which the first voltage waveform W1 falls from a middle level to a low level and the phase P7 at which the second voltage waveform W2 falls from a middle level to a low level.

[0037] The control unit 40 controls the switching elements Q1a-Q8a so that the period Pe1 during which the first voltage waveform W1 is at a high level and the period Pe3 during which the second voltage waveform W2 is at a high level are 2π / n. This reduces n-th order harmonics generated by the switching of the switching elements Q1a-Q8a. n is a positive integer. The periods Pe1 and Pe3 are the same as the periods Pe2 and Pe4 during which the first voltage waveform W1 and the second voltage waveform W2 are at a low level. Therefore, it can be said that the control unit 40 controls the switching elements Q1a-Q8a so that the periods Pe2 and Pe4 during which the first voltage waveform W1 and the second voltage waveform W2 are at a low level are 2π / n.

[0038] During the period Pe1 when the first voltage waveform W1 is at a high level, the voltage V AU is 0 to E dA The phase P1 and voltage V AW is 0 to E dA The phase difference δ is with respect to the phase P2 where During the period Pe2 when the first voltage waveform W1 is at a low level, the voltage V AU E dA Phase P3 and voltage V AW E dA The phase difference δ is between the phase P1 and the phase P2, which becomes 0 from the phase P2.

[0039] During the period Pe3 when the second voltage waveform W2 is at a high level, the voltage V BU is 0 to E dB The phase P5 and voltage V BW is 0 to E dB The phase difference between the phase P1 and the phase P6 is δ. During the period Pe4 when the second voltage waveform W2 is at a low level, the voltage V BU E dB Phase P7 and voltage V BW E dB The phase difference δ is between the phase P1 and the phase P2.

[0040] When controlling switching elements Q1a-Q8a, control unit 40 first determines the phase difference θ based on the target value of output power. Then, control unit 40 controls switching elements Q1a-Q8a so that the determined phase difference θ is achieved and the phase difference δ is 2π / n. n may be determined in advance based on the harmonics to be reduced. For example, if third-order harmonics are to be reduced, the phase difference δ should be set to 2π / 3, and if fifth-order harmonics are to be reduced, the phase difference δ should be set to 2π / 5.

[0041] Primary voltage V AUW By expanding into a Fourier series, the following equation (1) can be obtained: Secondary voltage V BUW By expanding this into a Fourier series, we can obtain the following equation (2).

[0042]

number

[0043]

number

[0044]

number

[0045] In the above explanation, the case where switching elements Q1a to Q8a are switched is described, but similar control can be performed for switching elements Q1b to Q8b. In this case, the switching phase of switching elements Q1a to Q8a and switching elements Q1b to Q8b should be shifted by 180 degrees.

[0046] When controlling the switching elements Q1b to Q8b, the AV phase voltage V AV , AX phase voltage V AX , BV phase voltage V BV , and the BX phase voltage V BX Controls the AV phase voltage V AV and the AX phase voltage V AX The difference between the potential at the connection point P1b and the potential at the connection point P2b is the difference between the potential at the connection point P1b and the potential at the connection point P2b. AV and voltage V AX The difference between these is the primary voltage V AVX is applied to the primary side of the second transformer 11b.

[0047] BV phase voltage V BV and BX phase voltage V BX The difference between the potential at the connection point P3b and the potential at the connection point P4b is the difference between the potential at the connection point P3b and the potential at the connection point P4b. BV and voltage V BX The difference between these is the secondary voltage V BVX The control unit 40 applies the primary side voltage V AVX and secondary voltage V BVX is high level is controlled to be 2π / n.

[0048] In this way, the leg pairs 21a, 21b, 31a, 31b provided corresponding to each transformer 11a, 11b can be controlled in the same manner with the phases shifted. Therefore, even if the number of transformers is increased to three or more, the leg pairs corresponding to the increased transformers can be controlled in the same manner as in the embodiment.

[0049] [Effects of the embodiment] (1) To reduce harmonics caused by switching, the control unit 40 controls the period Pe1 during which the first voltage waveform W1 is at a high level and the period Pe3 during which the second voltage waveform W2 is at a high level to be 2π / n, thereby reducing n-th order harmonics.

[0050] (2) The power conversion device 10 includes a plurality of transformers 11a and 11b. As a result, a composite wave of the outputs of the plurality of transformers 11a and 11b is output. If there are two transformers 11a and 11b as in the embodiment, the secondary side voltage V BUW and secondary voltage V BVX The voltage output is the sum of the secondary voltage V BUW and secondary voltage V BVX Since the waveforms are out of phase with each other by 180 degrees, increasing the number of transformers 11a and 11b can suppress fluctuations in the amplitude of the output voltage, thereby suppressing noise.

[0051] [Example of change] The embodiment can be modified as follows: The embodiment and the following modifications can be combined with each other to the extent that they are not technically inconsistent.

[0052] The control unit 40 may change n depending on the switching frequencies of the primary-side switching elements Q1a-Q4a, Q1b-Q4b and the secondary-side switching elements Q5a-Q8a, Q5b-Q8b. For example, the switching frequencies of the switching elements Q1a-Q8a, Q1b-Q8b can be varied between 30 kHz and 50 kHz. Furthermore, control is performed so that harmonics of, for example, 150 kHz or higher are not generated.

[0053] When the switching frequency is 30 kHz, the control unit 40 sets n to 5 to reduce the fifth harmonic. When the switching frequency is 50 kHz, the control unit 40 sets n to 3 to reduce the third harmonic. When the switching frequency is 50 kHz, both the third and fifth harmonics are 150 kHz or higher, but since the higher the order of harmonics, the smaller the harmonics become, the control unit 40 reduces the third harmonic, which is the lowest order harmonics of those 150 kHz or higher.

[0054] In this way, by changing n according to the switching frequency, it is possible to reduce harmonics in accordance with the switching frequency. The number of transformers 11a and 11b may be one, or three or more. [Explanation of symbols]

[0055] Q1a to Q4a, Q1b to Q4b...primary side switching elements, Q5a to Q8a, Q5b to Q8b...secondary side switching elements, W1...first voltage waveform, W2...second voltage waveform, 10...power conversion device, 11a, 11b...transformer, 12a, 12b...primary side winding, 13a, 13b, 14a, 14b, 16a, 16b, 17a, 17b...reactor, 15a, 15b...secondary side winding, 20...primary side bridge circuit, 22a, 22b, 23a, 23b...first leg, 30...secondary side bridge circuit, 32a, 32b, 33a, 33b...second leg, 40...control unit.

Claims

1. a transformer having a primary winding, a secondary winding, and a reactor connected to at least one of the primary winding and the secondary winding; a primary bridge circuit including twice the number of first legs each having two primary switching elements connected in series with each other, the number of first legs being twice the number of the transformers, the primary bridge circuit being connected to the primary winding; a secondary bridge circuit including a second leg having two secondary switching elements connected in series with each other, the number of which is twice the number of the transformers, the secondary bridge circuit being connected to the secondary winding; a control unit that controls the primary side switching element and the secondary side switching element, the control unit applies a primary-side voltage having a first voltage waveform to a primary side of the transformer and applies a secondary-side voltage having a second voltage waveform to a secondary side of the transformer so as to reduce n-th order harmonics generated by switching; the first voltage waveform is a waveform in which the primary side voltage is switched between three levels: a low level, a middle level, and a high level; the second voltage waveform is a waveform in which the secondary side voltage is switched between three levels: a low level, a middle level, and a high level; A power conversion device, wherein a period during which the first voltage waveform is at a high level and a period during which the second voltage waveform is at a high level are 2π / n.

2. The power conversion device according to claim 1 , comprising a plurality of the transformers.

3. The power conversion device according to claim 1 , wherein the control unit changes n in accordance with switching frequencies of the primary-side switching element and the secondary-side switching element.

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