Power conversion device

Abstract

This invention provides a power conversion device. A control unit (50) controls a primary-side full-bridge circuit (30) and a secondary-side full-bridge circuit (40) such that one of the primary-side full-bridge circuit (30) and the secondary-side full-bridge circuit (40) applies a 2-level voltage to the transformer section (TS), while the other applies a 3-level voltage to the transformer section (TS). The control unit (50) controls the primary-side full-bridge circuit (30) and the secondary-side full-bridge circuit (40) to form a combination of a first phase difference, a second phase difference, the frequency of the 2-level voltage, and the frequency of the 3-level voltage, capable of outputting the required power, and to form a combination satisfying conditions 1 and 2.

Description

Technical Field

[0001] This disclosure relates to power conversion devices. Background Technology

[0002] The power conversion device disclosed in Patent Document 1 includes a transformer, a primary-side full-bridge circuit, a secondary-side full-bridge circuit, and a control unit. The transformer includes a primary-side winding and a secondary-side winding. The primary-side full-bridge circuit includes multiple primary-side switching elements. The secondary-side full-bridge circuit includes multiple secondary-side switching elements. The control unit controls the primary-side full-bridge circuit to apply a 3-level primary-side voltage to the primary-side winding. The control unit controls the secondary-side full-bridge circuit to apply a 3-level secondary-side voltage to the secondary-side winding. By controlling the phase difference between the primary-side voltage and the secondary-side voltage, the control unit achieves soft-switching operation of the primary-side and secondary-side switching elements.

[0003] Patent Document 1: International Publication No. 2020 / 003717

[0004] Power conversion devices seek to balance output power demand with soft switching operation. Summary of the Invention

[0005] One aspect of this disclosure relates to a power conversion device comprising: a transformer section having a primary winding, a secondary winding, and a reactor connected to at least one of the primary winding or the secondary winding; a primary-side full-bridge circuit connected to the primary winding and having a plurality of primary-side switching elements; a secondary-side full-bridge circuit connected to the secondary winding and having a plurality of secondary-side switching elements; and a control section configured to control the plurality of primary-side switching elements and the plurality of secondary-side switching elements. Condition 1 is the condition for performing soft-switching operation of the plurality of primary-side switching elements. Condition 2 is the condition for performing soft-switching operation of the plurality of secondary-side switching elements. Condition 1 is that, when at least one of the plurality of primary-side switching elements switches between ON and OFF, a primary-side conditional current value, which is the value of the current flowing through the primary winding at the diode connected in parallel with the primary-side switching element that changes from OFF to ON, is greater than or equal to the absolute value of the primary-side winding current threshold. Condition 2 is that, when at least one of the plurality of secondary-side switching elements switches between ON and OFF, the secondary-side conditional current value, which is the value of the current flowing through the secondary-side winding at the diode connected in parallel with the secondary-side switching element that changes from OFF to ON, is above the absolute value of the secondary-side winding current threshold. The control unit is configured to control the primary-side full-bridge circuit and the secondary-side full-bridge circuit such that one of the primary-side full-bridge circuit and the secondary-side full-bridge circuit applies a 2-level voltage to the transformer section, while the other applies a 3-level voltage to the transformer section. The 2-level voltage and the 3-level voltage are waveforms of the same frequency that are reversed every 180 degrees in phase. The difference between the first moment when the 3-level voltage rises from a low level to a medium level and the second moment when the 2-level voltage rises from a low level to a high level is the first phase difference. The difference between the first moment and the third moment when the 3-level voltage rises from a medium level to a high level is the second phase difference. The control unit is configured to control the primary-side full-bridge circuit and the secondary-side full-bridge circuit to achieve a combination of the first phase difference, the second phase difference, the frequency of the 2-level voltage, and the frequency of the 3-level voltage that can output the required power, and to satisfy the combination of condition 1 and condition 2. Attached Figure Description

[0006] Figure 1 This is a circuit diagram of the power conversion device according to the first embodiment.

[0007] Figure 2 It is a graph showing the relationship between the equivalent voltage ratio and the load mode.

[0008] Figure 3 It means Figure 2 The graph shows the relationship between the primary side voltage, secondary side voltage, primary side current, and secondary side current in one of the load modes, namely the first buck hysteresis phase mode.

[0009] Figure 4 It means Figure 2 The graph shows the relationship between the primary side voltage, secondary side voltage, primary side current, and secondary side current in one of the load modes, namely the first buck in-phase mode.

[0010] Figure 5 It means Figure 2 The graph shows the relationship between the primary side voltage, secondary side voltage, primary side current, and secondary side current in one of the load modes, namely the first buck lead phase mode.

[0011] Figure 6 yes Figure 1 The flowchart of the output control executed by the control unit.

[0012] Figure 7 It is a graph showing the relationship between power demand, frequency, and phase differences.

[0013] Figure 8 It means Figure 2 The diagram shows the primary and secondary voltages in one of the load modes, namely the second buck hysteresis phase mode.

[0014] Figure 9 It means Figure 2 The diagram shows the primary and secondary voltages in one of the load modes, namely the second buck in-phase mode.

[0015] Figure 10 It means Figure 2 The diagram shows the primary and secondary voltages in one of the load modes, namely the second buck lead phase mode. Detailed Implementation

[0016] [First Implementation Method]

[0017] The first embodiment of the power conversion device is described.

[0018] like Figure 1 As shown, the power system 100 includes a DC power supply 110, a load 120, and a power conversion device 10. The DC power supply 110 inputs DC power to the power conversion device 10. The DC power supply 110 is, for example, a battery or a power circuit. The power circuit is, for example, an AC / DC converter that converts AC power to DC power and outputs it. The load 120 is, for example, a secondary battery capable of charging and discharging DC power. The secondary battery is, for example, a lithium-ion battery or a lead-acid battery.

[0019] <Power Conversion Device>

[0020] The power conversion device 10 is a dual-active-bridge DC / DC converter. The power conversion device 10 is located between the DC power supply 110 and the load 120. The power conversion device 10 can convert the DC power input from the DC power supply 110 and output it to the load 120. The power conversion device 10 can also convert the DC power input from the load 120 and output it to the DC power supply 110. In the following description, the primary side is treated as the input and the secondary side as the output. That is, the power conversion device 10 converts the DC voltage input from the DC power supply 110 and outputs it to the load 120.

[0021] The power conversion device 10 has two primary side terminals 11 and 12 and two secondary side terminals 13 and 14. A DC power supply 110 is electrically connected to the primary side terminals 11 and 12. A load 120 is electrically connected to the secondary side terminals 13 and 14.

[0022] The power conversion device 10 includes a transformer section TS. The transformer section TS includes a transformer 20 and two reactors L1 and L2. The transformer 20 is an insulated type. The transformer 20 includes a magnetic core 21, a primary winding 22, and a secondary winding 23. The primary winding 22 and the secondary winding 23 are wound on the core 21. The transformer 20 is connected to the reactors L1 and L2. The reactors L1 and L2 can be components such as chokes, or they can be leakage inductance of the primary winding 22 and the secondary winding 23. Reactor L1 is connected to the primary winding 22. Reactor L2 is connected to the secondary winding 23. Reactors L1 and the primary winding 22 form a series connection 24. Reactors L2 and the secondary winding 23 form a series connection 25. Sometimes, reactor L1 is appropriately referred to as the first reactor L1, and reactor L2 is referred to as the second reactor L2.

[0023] The power conversion device 10 includes a primary-side full-bridge circuit 30. The primary-side full-bridge circuit 30 includes a first bridge arm 31 and a second bridge arm 32. The first bridge arm 31 and the second bridge arm 32 are connected in parallel to primary-side terminals 11 and 12. Thus, the primary-side full-bridge circuit 30 is electrically connected to a DC power supply 110 via the primary-side terminals 11 and 12. The first bridge arm 31 includes a first switching element Q1, a second switching element Q2, diodes D1 and D2, and capacitors C1 and C2. The first switching element Q1 and the second switching element Q2 are connected in series. The second bridge arm 32 includes a third switching element Q3, a fourth switching element Q4, diodes D3 and D4, and capacitors C3 and C4. The third switching element Q3 and the fourth switching element Q4 are connected in series. The first switching element Q1 and the third switching element Q3 constitute the upper arm. The second switching element Q2 and the fourth switching element Q4 constitute the lower arm.

[0024] The first switching element Q1, the second switching element Q2, the third switching element Q3, and the fourth switching element Q4 are multiple primary-side switching elements Q1 to Q4. The primary-side switching elements Q1 to Q4 are, for example, n-type MOSFETs (Metal Oxide Semiconductor Field Effect Transistors). The primary-side switching elements Q1 to Q4 can also be p-type MOSFETs, IGBTs (Insulated Gate Bipolar Transistors), or GaN-HEMTs (Gallium Nitride High Electron Mobility Transistors).

[0025] Diodes D1-D4 and capacitors C1-C4 are connected in parallel with primary-side switching elements Q1-Q4. Diodes D1-D4 can be parasitic diodes or components. Capacitors C1-C4 can be parasitic capacitances, components, or a combination of parasitic capacitances and components.

[0026] The connection point between the first switching element Q1 and the second switching element Q2 is connected to one end of the primary winding 22 via the first reactor L1, and the connection point between the third switching element Q3 and the fourth switching element Q4 is directly connected to the other end of the primary winding 22. That is, the primary full-bridge circuit 30 is connected to the transformer section TS.

[0027] The power conversion device 10 includes a primary-side capacitor 15. The primary-side capacitor 15 is disposed between the primary-side terminals 11 and 12 and the primary-side full-bridge circuit 30.

[0028] The power conversion device 10 includes a primary-side voltage sensor 37. The primary-side voltage sensor 37 detects the input voltage Vin from the DC power supply 110 to the primary-side full-bridge circuit 30.

[0029] The power conversion device 10 includes a secondary-side full-bridge circuit 40. The secondary-side full-bridge circuit 40 includes a third bridge arm 41 and a fourth bridge arm 42. The third bridge arm 41 and the fourth bridge arm 42 are connected in parallel to the secondary-side terminals 13 and 14. Thus, the secondary-side full-bridge circuit 40 is electrically connected to the load 120. The third bridge arm 41 includes a fifth switching element Q5, a sixth switching element Q6, diodes D5 and D6, and capacitors C5 and C6. The fifth switching element Q5 and the sixth switching element Q6 are connected in series. The fourth bridge arm 42 includes a seventh switching element Q7, an eighth switching element Q8, diodes D7 and D8, and capacitors C7 and C8. The seventh switching element Q7 and the eighth switching element Q8 are connected in series. The fifth switching element Q5 and the seventh switching element Q7 constitute the upper arm. The sixth switching element Q6 and the eighth switching element Q8 constitute the lower arm.

[0030] The fifth switching element Q5, the sixth switching element Q6, the seventh switching element Q7, and the eighth switching element Q8 are multiple secondary-side switching elements Q5 to Q8. These secondary-side switching elements Q5 to Q8 are, for example, n-type MOSFETs. Alternatively, they can be p-type MOSFETs, IGBTs, or GaN-HEMTs.

[0031] Diodes D5-D8 and capacitors C5-C8 are connected in parallel with secondary-side switching elements Q5-Q8, respectively. Diodes D5-D8 can be parasitic diodes or components. Capacitors C5-C8 can be parasitic capacitances, components, or a combination of parasitic capacitances and components.

[0032] The connection point between the fifth switching element Q5 and the sixth switching element Q6 is connected to one end of the secondary winding 23 via the second reactor L2, and the connection point between the seventh switching element Q7 and the eighth switching element Q8 is directly connected to the other end of the secondary winding 23. That is, the secondary full-bridge circuit 40 is connected to the transformer section TS.

[0033] The power conversion device 10 includes a secondary-side capacitor 16. The secondary-side capacitor 16 is disposed between the secondary-side terminals 13 and 14 and the secondary-side full-bridge circuit 40.

[0034] The output power of the secondary-side full-bridge circuit 40 is supplied to the load 120.

[0035] The power conversion device 10 includes a secondary-side voltage sensor 47. The secondary-side voltage sensor 47 detects the output voltage Vout of the power conversion device 10. The power conversion device 10 includes a control unit 50. The control unit 50 includes a processor and a storage unit. The processor is, for example, a CPU (Central Processing Unit), a GPU (Graphics Processing Unit), or a DSP (Digital Signal Processor). The storage unit includes RAM (Random Access Memory) and ROM (Read Only Memory). The storage unit is configured to store program code or instructions that cause the processor to perform processing. The storage unit, i.e., computer-readable medium, includes any available medium that can be accessed by a general-purpose or special-purpose computer. The control unit 50 may also be constructed from hardware circuits such as ASIC (Application Specific Integrated Circuit) or FPGA (Field Programmable Gate Array). As a processing circuit, the control unit 50 may include one or more hardware circuits such as processors, ASICs, or FPGAs, or combinations thereof, that operate according to a computer program.

[0036] The control unit 50 converts the input voltage Vin into the output voltage Vout by controlling multiple primary-side switching elements Q1 to Q4 and multiple secondary-side switching elements Q5 to Q8.

[0037] In this embodiment, the control unit 50 performs 3-level control on one of the primary-side full-bridge circuit 30 and the secondary-side full-bridge circuit 40, and performs 2-level control on the other. Thus, one of the primary-side full-bridge circuit 30 and the secondary-side full-bridge circuit 40 applies a 2-level voltage to the transformer section TS, while the other applies a 3-level voltage to the transformer section TS. In this embodiment, the control unit 50 performs 3-level control on the primary-side full-bridge circuit 30 and 2-level control on the secondary-side full-bridge circuit 40.

[0038] Three-level control switches the voltage applied to the transformer section TS (i.e., the series connection 24 formed by the primary winding 22 and the first reactor L1) between three levels: positive, negative, or zero. Two-level control switches the voltage applied to the transformer section TS (i.e., the series connection 25 formed by the secondary winding 23 and the second reactor L2) between two levels: positive and negative. Appropriately, in three-level control, a positive voltage applied to the series connection 24 is called a high level, a zero voltage applied to the series connection 24 is called a medium level, and a negative voltage applied to the series connection 24 is called a low level. In two-level control, a positive voltage applied to the series connection 25 is called a high level, and a negative voltage applied to the series connection 25 is called a low level. The voltage applied to the series connection 24 is called the primary side voltage V1, and the voltage applied to the series connection 25 is called the secondary side voltage V2. The primary-side voltage V1 is the potential difference between the connection points of the first switching element Q1 and the second switching element Q2, and between the connection points of the third switching element Q3 and the fourth switching element Q4. The secondary-side voltage V2 is the potential difference between the connection points of the fifth switching element Q5 and the sixth switching element Q6, and between the seventh switching element Q7 and the eighth switching element Q8. In this embodiment, the primary-side voltage V1 is a 3-level voltage. The secondary-side voltage V2 is a 2-level voltage. Figure 1 The direction of the middle arrow is set to positive for the primary side voltage V1 and the secondary side voltage V2. Under normal operation without changing the output, the primary side voltage V1 and the secondary side voltage V2 are waveforms of the same frequency that are reversed every 180 degrees.

[0039] In the case of performing 3-level control of the primary-side full-bridge circuit 30, the control unit 50 independently controls the first bridge arm 31 and the second bridge arm 32. The switching operation modes of the primary-side full-bridge circuit 30 include the first mode to the fourth mode.

[0040] The first mode is a switching action mode in which the first switching element Q1 is ON (connected, the same below), the second switching element Q2 is OFF (disconnected, the same below), the third switching element Q3 is OFF, and the fourth switching element Q4 is ON.

[0041] The second mode is a switching action mode in which the first switching element Q1 is ON, the second switching element Q2 is OFF, the third switching element Q3 is ON, and the fourth switching element Q4 is OFF.

[0042] The third mode is a switching action mode in which the first switching element Q1 is OFF, the second switching element Q2 is ON, the third switching element Q3 is ON, and the fourth switching element Q4 is OFF.

[0043] The fourth mode is a switching action mode in which the first switching element Q1 is OFF, the second switching element Q2 is ON, the third switching element Q3 is OFF, and the fourth switching element Q4 is ON.

[0044] In the case of performing 2-level control of the secondary-side full-bridge circuit 40, the control unit 50 controls the third bridge arm 41 and the fourth bridge arm 42 in a coordinated manner. The control unit 50 simultaneously turns on the fifth switching element Q5 and the eighth switching element Q8. The control unit 50 simultaneously turns on the sixth switching element Q6 and the seventh switching element Q7. The switching operation modes of the secondary-side full-bridge circuit 40 include the fifth mode and the sixth mode.

[0045] The fifth mode is a switching action mode in which the fifth switching element Q5 is ON, the sixth switching element Q6 is OFF, the seventh switching element Q7 is OFF, and the eighth switching element Q8 is ON.

[0046] The sixth mode is a switching action mode in which the fifth switching element Q5 is OFF, the sixth switching element Q6 is ON, the seventh switching element Q7 is ON, and the eighth switching element Q8 is OFF.

[0047] The control unit 50 outputs the output voltage Vout from the secondary full-bridge circuit 40 by combining any one of the first to fourth modes of the primary full-bridge circuit 30 with any one of the fifth and sixth modes of the secondary full-bridge circuit 40.

[0048] Here, the primary winding 22 of transformer 20 has N1 turns, and the secondary winding 23 has N2 turns. The ratio of the input voltage Vin to the primary full-bridge circuit 30 to the output voltage Vout from the secondary full-bridge circuit 40 is the voltage ratio. The equivalent voltage ratio when the turns ratio of transformer 20 is converted to 1:1 is (Vout×N1) / (Vin×N2). The control unit 50 has a first buck mode and a first boost mode as load modes. The first buck mode is a load mode that makes the equivalent voltage ratio less than 1. The first boost mode is a load mode that makes the equivalent voltage ratio greater than 1.

[0049] For example, in the case of transformer 20 with a winding ratio of 1:2, input voltage Vin = 200V, and output voltage Vout = 400V, if the winding ratio is converted to 1:1, then the equivalent voltage ratio = (Vout × 1) / (Vin × 2) = 1. Therefore, in the case of transformer 20 with a winding ratio of 1:2 and input voltage Vin = 200V, if the output voltage Vout is less than 400V, it is the first buck mode; if it is greater than 400V, it is the first boost mode. Furthermore, unless otherwise specified, the following explanation will use a winding ratio of 1:1.

[0050] <First Blood Pressure Reduction Mode>

[0051] like Figure 2 As shown, the first buck mode includes a first buck lag phase mode, a first buck in-phase mode, and a first buck lead phase mode.

[0052] like Figure 3 As shown, the first buck hysteresis phase mode is as follows: after the primary side voltage V1 rises from a low level to a medium level, the secondary side voltage V2 rises from a low level to a high level, and then the primary side voltage V1 rises from a medium level to a high level. This first buck hysteresis phase mode is the first buck mode when outputting a small amount of power based on the power demand from the load 120.

[0053] In the load mode of this embodiment, the difference between the first moment T1 when the primary side voltage V1 rises from a low level to a medium level and the second moment T2 when the secondary side voltage V2 rises from a low level to a high level is the first phase difference θ1. The difference between the first moment T1 and the third moment T3 when the primary side voltage V1 rises from a medium level to a high level is the second phase difference θ2.

[0054] Furthermore, as mentioned above, under normal operation without altering the output, the primary-side voltage V1 and the secondary-side voltage V2 are waveforms of the same frequency that invert every 180 degrees of phase. Therefore, the moment when the primary-side voltage V1 drops from a high level to a medium level is also the first moment T1, the moment when the secondary-side voltage V2 drops from a high level to a low level is also the second moment T2, and the moment when the primary-side voltage V1 drops from a medium level to a low level is also the third moment T3.

[0055] like Figure 4 As shown, the first buck in-phase mode is as follows: after raising the primary-side voltage V1 from a low level to a medium level, simultaneously raising the primary-side voltage V1 from a medium level to a high level and raising the secondary-side voltage V2 from a low level to a high level are performed. In the first buck in-phase mode, the second time T2 and the third time T3 are the same time. Therefore, the first phase difference θ1 and the second phase difference θ2 are the same value. That is, the difference between the first phase difference θ1 and the second phase difference θ2 is 0. The first buck in-phase mode is the first buck mode that outputs medium power based on the power demand from the load 120. Medium power is the output power with a maximum value greater than the minimum power.

[0056] like Figure 5As shown, the first buck lead phase mode is as follows: after the primary side voltage V1 rises from a low level to a medium level, the primary side voltage V1 rises from a medium level to a high level, and then the secondary side voltage V2 rises from a low level to a high level. The first buck lead phase mode is the first buck mode when a large power is output based on the power demand from load 120. Large power is the output power whose maximum value is greater than the medium power.

[0057] The first buck lagging phase mode is an example of a light load mode. The first buck leading phase mode is an example of a heavy load mode. The first buck in-phase mode is the load mode during the switch between light and heavy load modes, and is an example of a medium load mode. In the first buck mode, when the difference between the first phase difference θ1 and the second phase difference θ2 is 0, the switch occurs between the first buck lagging phase mode and the first buck leading phase mode.

[0058] <First Boost Mode>

[0059] like Figure 2 As shown, the first boost mode includes a first boost lag phase mode, a first boost in-phase mode, and a first boost lead phase mode. Except for the relationship between the secondary-side voltage V2 and the primary-side voltage V1, which differs from the first buck mode, the first boost mode is the same as the first buck mode, and therefore the diagram is omitted. Specifically, the first boost mode differs from the first buck mode in that the high level of the secondary-side voltage V2 is higher than the high level of the primary-side voltage V1. Furthermore, it differs from the first buck mode in that the low level of the secondary-side voltage V2 is lower than the low level of the primary-side voltage V1.

[0060] The first boost hysteresis phase mode is as follows: after the primary side voltage V1 rises from a low level to a medium level, the secondary side voltage V2 rises from a low level to a high level, and then the primary side voltage V1 rises from a medium level to a high level. This first boost hysteresis phase mode is the first boost mode when outputting a small amount of power based on the power demand from load 120.

[0061] The first boost in-phase mode is as follows: after raising the primary-side voltage V1 from a low level to a medium level, simultaneously raising the primary-side voltage V1 from a medium level to a high level and raising the secondary-side voltage V2 from a low level to a high level. This first boost in-phase mode is the first boost mode that outputs medium power based on the power demand from the load 120.

[0062] The first boost lead phase mode is as follows: after the primary side voltage V1 rises from a low level to a medium level, the primary side voltage V1 rises from a medium level to a high level, and then the secondary side voltage V2 rises from a low level to a high level. This first boost lead phase mode is the first boost mode when outputting large power based on the power demand from load 120.

[0063] The first boost lagging phase mode is an example of a light load mode. The first boost leading phase mode is an example of a heavy load mode. The first boost in-phase mode is the load mode during switching between light and heavy load modes, and is an example of a medium load mode. In the first boost mode, when the difference between the first phase difference θ1 and the second phase difference θ2 is 0, switching occurs between the first boost lagging phase mode and the first boost leading phase mode.

[0064] <Output Control>

[0065] The control unit 50 performs output control. Output control is performed in all modes: light load mode, medium load mode, and heavy load mode. Output control is achieved by controlling the first phase difference θ1, the second phase difference θ2, the frequency of the primary side voltage V1, and the frequency of the secondary side voltage V2 to output the required power. The frequency of the primary side voltage V1 and the frequency of the secondary side voltage V2 are the same. In the following description, the frequencies of the primary side voltage V1 and the secondary side voltage V2 are sometimes referred to as frequencies.

[0066] like Figure 6 As shown, in step S1, the control unit 50 derives the soft-switching operation region. The soft-switching operation region is set by the primary side current I1 and the secondary side current I2. The primary side current I1 is the current flowing through the primary side winding 22. The secondary side current I2 is the current flowing through the secondary side winding 23. Figure 1 The direction of the middle arrow represents the positive direction of the primary side current I1 and the secondary side current I2. The direction of the primary side current I1 flowing from the connection point between the first switching element Q1 and the second switching element Q2 towards the connection point between the third switching element Q3 and the fourth switching element Q4 is the positive direction of the primary side current I1. The direction of the secondary side current I2 flowing from the connection point between the seventh switching element Q7 and the eighth switching element Q8 towards the connection point between the fifth switching element Q5 and the sixth switching element Q6 is the positive direction of the secondary side current I2.

[0067] The condition for soft-switching operation of multiple primary-side switching elements Q1 to Q4 is that, when the primary-side switching elements Q1 to Q4 switch between ON and OFF, the value of the primary-side current I1 flowing in the forward direction at the diode connected in parallel with the primary-side switching element Q1 to Q4 that switches from OFF to ON (i.e., the primary-side conditional current value) is greater than or equal to the absolute value of the primary-side winding current threshold TI1. Hereinafter, this condition is referred to as condition 1. The region satisfying this condition is the soft-switching operation region of the primary-side current I1. When the primary-side switching elements Q1 to Q4 switch between ON and OFF, it refers to the time point at which at least one of the primary-side switching elements Q1 to Q4 changes from ON to OFF, or from OFF to ON. This is the same as the first time point T1 or the third time point T3. The first time point T1 is the moment when the primary-side voltage V1 rises from a low level to a medium level, or the moment when the primary-side voltage V1 falls from a high level to a medium level. The third moment T3 is the moment when the primary side voltage V1 rises from the medium level to the high level, or the moment when the primary side voltage V1 falls from the medium level to the low level.

[0068] The condition for soft-switching of multiple secondary-side switching elements Q5 to Q8 is that, when the secondary-side switching elements Q5 to Q8 switch between ON and OFF, the value of the forward-flowing secondary-side current I2 (i.e., the secondary-side conditional current value) at the diode connected in parallel with the switching element of secondary-side switching elements Q5 to Q8 that changes from OFF to ON is greater than or equal to the absolute value of the secondary-side winding current threshold TI2. Hereinafter, this condition is referred to as condition 2. The region satisfying this condition is the soft-switching operation region of the secondary-side current I2. When the secondary-side switching elements Q5 to Q8 switch between ON and OFF, it refers to the time point at which at least one of the secondary-side switching elements Q5 to Q8 changes from ON to OFF, or from OFF to ON. This is the same as the time point at which the secondary-side voltage V2 rises from a low level to a high level, or falls from a high level to a low level, i.e., the second moment T2.

[0069] As mentioned above, since the primary side voltage V1 and the secondary side voltage V2 are waveforms of the same frequency that reverse every 180 degrees in phase, the rising and falling phases are simply positive and negative reversals, so only one of them needs to be considered. The following explanation focuses only on the rising phase.

[0070] The first step-down hysteresis phase mode will be used as an example for explanation. Figure 3As shown, at the first time T1, the switching operation mode of the primary-side full-bridge circuit 30 switches from the third mode to the second mode. Furthermore, at the third time T3, the switching operation mode of the primary-side full-bridge circuit 30 switches from the second mode to the first mode. At both the first time T1 and the third time T3, if condition 1 is satisfied, the soft-switching operation of the primary-side switching elements Q1 to Q4 is established. At the second time T2, the switching operation mode of the secondary-side full-bridge circuit 40 switches from the sixth mode to the fifth mode. At this time, if condition 2 is satisfied, the soft-switching operation of the secondary-side switching elements Q5 to Q8 is established.

[0071] The primary winding current threshold TI1 is defined by the following equation (1).

[0072] [Mathematical Expression 1]

[0073]

[0074] TI1: Primary winding current threshold

[0075] k1: coefficient,

[0076] V1: Primary side voltage,

[0077] L: Inductance of the first reactor L1

[0078] C01: The combined capacitance of all capacitors C1 to C4 connected in parallel with the switching elements Q1 to Q4 of the primary-side full-bridge circuit 30, corresponding to one switching element. When the capacitance of capacitor C1 is C11, the capacitance of capacitor C2 is C12, the capacitance of capacitor C3 is C13, and the capacitance of capacitor C4 is C14, C01 can be C01 = (C11 + C12 + C13 + C14) / 4. When Cx is the largest capacitance among C11 to C14, C01 can also be Cx. Furthermore, for the secondary-side winding current threshold TI2, each value is replaced with the secondary-side value and specified in the same manner.

[0079] This allows us to deduce the soft-switch action area.

[0080] Next, in step S2, the control unit 50 calculates the target current. The target current is a current value that can output the required power from the load 120 and satisfies conditions 1 and 2.

[0081] Among the primary-side switching elements Q1 to Q4, the fourth switching element Q4 changes from OFF to ON at the third time T3. The fourth switching element Q4 is the primary-side switching element that changes from OFF to ON when the switching operation mode changes from the second mode to the first mode.

[0082] To satisfy condition 1, the primary-side current I1 at the first time point T1 and the third time point T3 needs to be considered. However, it is only necessary to consider the primary-side current I1 at the time point T3, which is the time point with the smaller absolute value of the current under this load mode. Therefore, to satisfy condition 1, the value of the forward-flowing primary-side current I1 at the diode D4 connected in parallel with the fourth switching element Q4 (which changes from OFF to ON at the third time point T3) must be greater than or equal to the absolute value of the primary-side winding current threshold TI1, "|TI1|". The value of this primary-side current I1 is... Figure 1 The value of "-I1". Furthermore, as according to... Figure 3 As determined, the primary side current I1 is negative at the third time point T3, therefore, "-I1" becomes positive. Furthermore, whether the absolute value of the first time point T1 or the third time point T3 will decrease depends on the load mode.

[0083] In this load mode, to satisfy condition 2, the value of the forward-flowing secondary current I2 at diodes D5 and D8 connected in parallel with the fifth switching element Q5 and the eighth switching element Q8 (which change from OFF to ON at the second time T2) must be greater than or equal to the absolute value of the secondary winding current threshold TI2, "|TI2|". This secondary current I2 is... Figure 1 The value of "I2". The fifth switching element Q5 and the eighth switching element Q8 are secondary-side switching elements that change from OFF to ON when the switching operation mode changes from the sixth mode to the fifth mode. Furthermore, as... Figure 3 It can be seen that the secondary current I2 is positive at the second moment T2, so "I2" becomes positive.

[0084] In practical applications, for example, for condition 1, it can be achieved by comparing TI1', obtained by converting the primary winding current threshold TI1 to the secondary side, with the secondary side current I2 at the first moment T1, thus satisfying condition 1. Alternatively, the reverse can also be true. In this case, only one of the primary side current I1 and the secondary side current I2 needs to be considered. Alternatively, the inductance L of the first reactor L1 can be pre-set so that satisfying either condition 1 or condition 2 automatically satisfies the other.

[0085] This allows us to calculate the combination of the required power output from load 120 that satisfies conditions 1 and 2. Furthermore, if no combination exists that satisfies both the required power output and the target current of conditions 1 and 2, it means that soft-switching cannot be performed under this power demand and load mode.

[0086] Next, in step S3, the control unit 50 derives a combination of the first phase difference θ1, the second phase difference θ2, and the frequency based on the calculated target current. Here, the combination of the first phase difference θ1, the second phase difference θ2, and the frequency is derived by following the calculated target current at the first time T1 and the second time T2. As long as the first phase difference θ1 and the second phase difference θ2 are constant, the lower the frequency, the greater the output power. Since the higher the frequency, the shorter the period T, the greater the power demand, the longer the period T of the primary side voltage V1 and the secondary side voltage V2. As the period T increases, the primary side current I1 and the secondary side current I2 will increase. Based on these relationships, the control unit 50 derives a combination of the first phase difference θ1, the second phase difference θ2, and the frequency to satisfy conditions 1 and 2. That is, the combination of the first phase difference θ1, the second phase difference θ2, and the frequency is a combination that can output the power demanded by the load 120 and satisfies conditions 1 and 2.

[0087] The control unit 50 derives the combination in which the difference between the primary side current I1 and the primary side winding current threshold TI1 is minimized, and the difference between the secondary side current I2 and the secondary side winding current threshold TI2 is minimized, among the combinations of the first phase difference θ1, the second phase difference θ2 and the frequency.

[0088] Furthermore, the control unit 50 derives the combination of the first phase difference θ1, the second phase difference θ2, and the frequency in a manner that minimizes the current used in the soft-switching operation. As an example, the switching operation of the fifth switching element Q5 and the eighth switching element Q8 at the second time T2 will be explained. At the second time T2, the secondary side voltage V2 is negative, and the secondary side current I2 is positive. To perform the soft-switching operation, the charge stored in capacitors C5 and C8 needs to be discharged. As described in condition 2 above, when the secondary side current I2 above |TI2| flows in the forward direction of diodes D5 and D8, the charge stored in capacitors C5 and C8 is discharged, and diodes D5 and D8 turn on. That is, the voltage between the two terminals of the fifth switching element Q5 and the eighth switching element Q8 becomes zero. At this time, due to the switching operation mode switching, the fifth switching element Q5 and the eighth switching element Q8 change from OFF to ON. Thus, the soft-switching operation is achieved. On the other hand, the secondary side current I2 used in the soft-switching operation flows in the opposite direction to the current supplied to the load 120, which leads to increased losses. Therefore, the control unit 50 derives the combination of the first phase difference θ1, the second phase difference θ2, and the frequency in a manner that minimizes the current used in the soft-switching operation.

[0089] In step S4, the control unit 50 controls the primary-side full-bridge circuit 30 and the secondary-side full-bridge circuit 40 to form the first phase difference θ1, the second phase difference θ2, and the frequency derived in step S3.

[0090] The required power, the first phase difference θ1, the second phase difference θ2, and the frequency can be calculated each time, or they can be pre-calculated and stored in a mapping diagram, etc.

[0091] [Effect of the first implementation method]

[0092] The control unit 50 continuously changes the first phase difference θ1, the second phase difference θ2, and the frequency in all modes, including light load mode, medium load mode, and heavy load mode, according to the power demand.

[0093] like Figure 7 As shown, in light load mode, the frequency gradually decreases as the power demand increases. This allows for increased output power. Furthermore, in light load mode, the difference between the first phase difference θ1 and the second phase difference θ2 gradually decreases as the power demand increases. When the difference between the first phase difference θ1 and the second phase difference θ2 becomes 0, and the load mode transitions to heavy load mode, the frequency gradually decreases as the power demand increases. Conversely, in heavy load mode, the difference between the first phase difference θ1 and the second phase difference θ2 gradually increases as the power demand increases. Figure 7 In the example shown, the frequency changes in an arc shape in the light load mode and in a linear shape in the heavy load mode, but this may vary depending on the circuit structure of the power conversion device 10, etc.

[0094] [Effects of the First Embodiment]

[0095] (1-1) The control unit 50 controls the primary-side full-bridge circuit 30 and the secondary-side full-bridge circuit 40, such that the combination of the first phase difference θ1, the second phase difference θ2, and the frequency satisfies conditions 1 and 2. By satisfying conditions 1 and 2, soft-switching operations can be performed on the primary-side switching elements Q1 to Q4 and the secondary-side switching elements Q5 to Q8. In addition, there is a correlation between frequency and output power; the lower the frequency, the greater the output power. Therefore, by controlling the first phase difference θ1, the second phase difference θ2, and the frequency, both the required output power and the soft-switching operation can be achieved.

[0096] (1-2) The control unit 50 changes the frequency according to the power demand. The period T is determined by the frequency; the longer the period T, the longer the voltage is applied to the transformer section TS. Since the primary side current I1 and the secondary side current I2 flow through the transformer section TS during the period when the voltage is applied, if the frequency is fixed, the peak values ​​of the primary side current I1 and the secondary side current I2 may increase. In contrast, by making the frequency variable, the time when the voltage is applied to the transformer section TS can be adjusted, thus reducing the peak values ​​of the primary side current I1 and the secondary side current I2.

[0097] Furthermore, the lower the frequency, the greater the power output of the power conversion device 10. Therefore, compared to keeping the frequency constant, the maximum output power can be increased.

[0098] (1-3) By changing the frequency, the excitation current flowing through the transformer section TS will change. Therefore, by adjusting the excitation current, it is easier to perform soft switching operation, and if the excitation current is reduced, the resistance loss can be reduced.

[0099] (1-4) The control unit 50 performs control in all modes, including light load mode, medium load mode, and heavy load mode, to form a combination of the first phase difference θ1, the second phase difference θ2, and the frequency that can output the required power, and forms a combination of the first phase difference θ1, the second phase difference θ2, and the frequency that satisfies conditions 1 and 2. Thus, regardless of whether it is light load mode, medium load mode, or heavy load mode, it can perform soft switching operation while outputting the required power.

[0100] [Second Implementation]

[0101] A second embodiment of the power conversion device is described. The hardware structure of the power conversion device in the second embodiment is the same as that in the first embodiment.

[0102] The control unit 50 performs 2-level control on the primary-side full-bridge circuit 30 and 3-level control on the secondary-side full-bridge circuit 40. In this case, 3-level control switches the voltage applied to the series connector 25 between three levels: positive, negative, or 0. 2-level control switches the voltage applied to the series connector 24 between two levels: positive or negative. Appropriately, in 3-level control, a positive voltage applied to the series connector 25 is called a high level, a 0 voltage applied to the series connector 25 is called a medium level, and a negative voltage applied to the series connector 25 is called a low level. Similarly, in 2-level control, a positive voltage applied to the series connector 24 is called a high level, and a negative voltage applied to the series connector 24 is called a low level. The primary-side voltage V1 is a 2-level voltage. The secondary-side voltage V2 is a 3-level voltage.

[0103] In the case of performing 2-level control of the primary-side full-bridge circuit 30, the control unit 50 controls the first bridge arm 31 and the second bridge arm 32 in a coordinated manner. The control unit 50 simultaneously turns on the first switching element Q1 and the fourth switching element Q4. The control unit 50 simultaneously turns on the second switching element Q2 and the third switching element Q3. The switching operation modes of the primary-side full-bridge circuit 30 include a seventh mode and an eighth mode.

[0104] The seventh mode is a switching action mode in which the first switching element Q1 is ON, the second switching element Q2 is OFF, the third switching element Q3 is OFF, and the fourth switching element Q4 is ON.

[0105] The eighth mode is a switching action mode in which the first switching element Q1 is OFF, the second switching element Q2 is ON, the third switching element Q3 is ON, and the fourth switching element Q4 is OFF.

[0106] In the case of 3-level control of the secondary-side full-bridge circuit 40, the control unit 50 independently controls the third bridge arm 41 and the fourth bridge arm 42. The switching operation modes of the secondary-side full-bridge circuit 40 include modes nine to twelfth modes.

[0107] The ninth mode is a switching operation mode in which the fifth switching element Q5 is ON, the sixth switching element Q6 is OFF, the seventh switching element Q7 is OFF, and the eighth switching element Q8 is ON.

[0108] The tenth mode is a switching action mode in which the fifth switching element Q5 is ON, the sixth switching element Q6 is OFF, the seventh switching element Q7 is ON, and the eighth switching element Q8 is OFF.

[0109] The eleventh mode is a switching action mode in which the fifth switching element Q5 is OFF, the sixth switching element Q6 is ON, the seventh switching element Q7 is ON, and the eighth switching element Q8 is OFF.

[0110] The twelfth mode is a switching operation mode in which the fifth switching element Q5 is OFF, the sixth switching element Q6 is ON, the seventh switching element Q7 is OFF, and the eighth switching element Q8 is ON.

[0111] The control unit 50 outputs the output voltage Vout from the secondary full-bridge circuit 40 by combining any one of the seventh and eighth modes of the primary full-bridge circuit 30 with any one of the ninth to twelfth modes of the secondary full-bridge circuit 40.

[0112] The control unit 50 is equipped with a second buck mode and a second boost mode as load modes. The second buck mode is a load mode in which the equivalent voltage ratio is less than 1. The second boost mode is a load mode in which the equivalent voltage ratio is greater than 1.

[0113] <Second Blood Pressure Reduction Mode>

[0114] The second buck mode includes a second buck lag phase mode, a second buck in-phase mode, and a second buck lead phase mode.

[0115] like Figure 8As shown, the second buck hysteresis phase mode is as follows: after the secondary side voltage V2 rises from a low level to a medium level, the primary side voltage V1 rises from a low level to a high level, and then the secondary side voltage V2 rises from a medium level to a high level. The second buck hysteresis phase mode is the second buck mode when outputting a small amount of power based on the power demand from load 120.

[0116] In the load mode of the second embodiment, the difference between the first moment T11 when the secondary side voltage V2 rises from a low level to a medium level and the second moment T12 when the primary side voltage V1 rises from a low level to a high level is the first phase difference θ11. The difference between the first moment T11 and the third moment T13 when the secondary side voltage V2 rises from a medium level to a high level is the second phase difference θ12.

[0117] Under normal operation without altering the output, the primary-side voltage V1 and the secondary-side voltage V2 are waveforms of the same frequency that invert every 180 degrees. Therefore, the moment when the secondary-side voltage V2 drops from a high level to a medium level is the first moment T11, the moment when the primary-side voltage V1 drops from a high level to a low level is the second moment T12, and the moment when the secondary-side voltage V2 drops from a medium level to a low level is the third moment T13.

[0118] like Figure 9 As shown, the second buck in-phase mode is as follows: after simultaneously raising the secondary side voltage V2 from a low level to a medium level and raising the primary side voltage V1 from a low level to a high level, the secondary side voltage V2 is raised from the medium level to a high level. In the second buck in-phase mode, the first time T11 and the second time T12 are the same time. Therefore, the first phase difference θ11 is 0. The second buck in-phase mode is the second buck mode when outputting medium power according to the power demand from the load 120.

[0119] like Figure 10 As shown, the second buck lead phase mode is as follows: after the primary side voltage V1 rises from a low level to a high level, the secondary side voltage V2 rises from a low level to a medium level, and then the secondary side voltage V2 rises from a medium level to a high level. The second buck lead phase mode is the second buck mode when outputting a large power according to the power demand from load 120.

[0120] The second buck lag phase mode is an example of a light load mode. The second buck lead phase mode is an example of a heavy load mode. The second buck in-phase mode is the load mode during the switch between light and heavy load modes, and is an example of a medium load mode. In the second buck mode, when the first phase difference θ11 is 0, the switch occurs between the second buck lag phase mode and the second buck lead phase mode.

[0121] <Second Boost Mode>

[0122] The second boost mode includes a second boost lag phase mode, a second boost in-phase mode, and a second boost lead phase mode. Except for the relationship between the secondary-side voltage V2 and the primary-side voltage V1, which differs from the second buck mode, the second boost mode is the same as the second buck mode, and therefore, illustrations are omitted. Specifically, the second boost mode differs from the second buck mode in that it makes the high level of the secondary-side voltage V2 higher than the high level of the primary-side voltage V1. Furthermore, it differs from the second buck mode in that it makes the low level of the secondary-side voltage V2 lower than the low level of the primary-side voltage V1.

[0123] The second boost hysteresis phase mode is as follows: after the secondary side voltage V2 rises from a low level to a medium level, the primary side voltage V1 rises from a low level to a high level, and then the secondary side voltage V2 rises from a medium level to a high level. This second boost hysteresis phase mode is the second boost mode when outputting a small amount of power based on the power demand from load 120.

[0124] The second boost in-phase mode is as follows: after simultaneously raising the secondary voltage V2 from a low level to a medium level and raising the primary voltage V1 from a low level to a high level, the secondary voltage V2 is raised from the medium level to a high level. This second boost in-phase mode is the second boost mode that outputs medium power based on the power demand from load 120.

[0125] The second boost lead phase mode is as follows: after the primary side voltage V1 rises from a low level to a high level, the secondary side voltage V2 rises from a low level to a medium level, and then the secondary side voltage V2 rises from a medium level to a high level. This second boost lead phase mode is the second boost mode when a large power is output based on the power demand from load 120.

[0126] The second boost lagging phase mode is an example of a light load mode. The second boost leading phase mode is an example of a heavy load mode. The second boost in-phase mode is the load mode when switching between light and heavy load modes, and is an example of a medium load mode. In the second boost mode, when the first phase difference θ11 is 0, switching occurs between the second boost lagging phase mode and the second boost leading phase mode.

[0127] In the second embodiment, the output control performed by the control unit 50 is the same as in the first embodiment. Therefore, in the second embodiment, the same effects as in the first embodiment can be obtained.

[0128] [Example of Change]

[0129] The implementation method can be modified as follows. The implementation method and the following modifications can be combined with each other to implement the method within the scope of technical inconsistency.

[0130] In various embodiments, while a lower frequency results in greater output power, it is also possible to increase the frequency, conversely. Therefore, since the possible range of values ​​for the first and second phase differences satisfying conditions 1 and 2 can be expanded, the output power can also be increased by adjusting the first and second phase differences.

[0131] In various implementations, it is also possible to, for example Figure 7 As shown, an upper and lower limit are set for the frequency. The region where the frequency exceeds the upper limit is a low-load region. The region where the frequency is below the lower limit is a heavy-load region. In this embodiment, the frequency is increased in the low-load region where the frequency exceeds the upper limit, and decreased in the heavy-load region where the frequency is below the lower limit. Alternatively, for example, the control unit 50 may fix the frequency at the upper limit in the low-load region where the frequency exceeds the upper limit. Alternatively, the control unit 50 may fix the frequency at the lower limit in the heavy-load region where the frequency exceeds the lower limit. The control unit 50 varies the frequency between the upper and lower limits. Specifically, when deriving the combination of the first phase difference θ1, the second phase difference θ2, and the frequency in step S3, the control unit 50 derives the frequency between the upper and lower limits. The upper limit is set based on the loss or AC resistance when the switching elements Q1 to Q8 are disconnected. For example, the upper limit is set to avoid an increase in loss due to the increase in frequency. The lower limit is set, for example, to prevent magnetic saturation of the transformer 20.

[0132] By setting upper and lower limits for the frequency, it is possible to suppress the increase in losses due to higher frequency or the magnetic saturation of transformer 20 due to lower frequency.

[0133] In various embodiments, the control unit 50 can change the circuits in the primary-side full-bridge circuit 30 and the secondary-side full-bridge circuit 40 that are subject to 3-level control in both light-load and heavy-load modes. For example, the control unit 50 can perform 3-level control on the primary-side full-bridge circuit 30 in light-load mode and 3-level control on the secondary-side full-bridge circuit 40 in heavy-load mode. In this case, 2-level control is performed on the one of the primary-side full-bridge circuit 30 and the secondary-side full-bridge circuit 40 that is not subject to 3-level control.

[0134] Alternatively, in various embodiments, the transformer section TS may include one of a first reactor L1 connected to the primary winding 22 and a second reactor L2 connected to the secondary winding 23. In the case where the transformer section TS only includes the first reactor L1, the secondary voltage V2 is the voltage applied to the secondary winding 23. In the case where the transformer section TS only includes the second reactor L2, the primary voltage V1 is the voltage applied to the primary winding 22.

[0135] Alternatively, in each embodiment, the control unit 50 performs output control of the embodiment in at least one of the light load mode, medium load mode, and heavy load mode. That is, the control unit 50 only needs to be able to balance the output power demand and soft switching operation in at least one of the light load mode, medium load mode, and heavy load mode.

Claims

1. A power conversion device, characterized in that, have: The transformer section includes a primary winding, a secondary winding, and a reactor connected to at least one of the primary winding or the secondary winding. The primary-side full-bridge circuit is a circuit connected to the primary-side winding and has multiple primary-side switching elements; The secondary-side full-bridge circuit is a circuit connected to the secondary-side winding and has multiple secondary-side switching elements; as well as The control unit is configured to control the plurality of primary-side switching elements and the plurality of secondary-side switching elements. Condition 1 is the condition for performing soft switching operations on the plurality of primary-side switching elements, and condition 2 is the condition for performing soft switching operations on the plurality of secondary-side switching elements. Condition 1 is that, when at least one of the plurality of primary-side switching elements switches between ON and OFF, the primary-side conditional current value, which is the value of the current flowing through the primary-side winding at the diode connected in parallel with the primary-side switching element that changes from OFF to ON, is greater than or equal to the absolute value of the primary-side winding current threshold. Condition 2 is that, when at least one of the plurality of secondary-side switching elements switches between ON and OFF, the secondary-side conditional current value, which is the value of the current flowing through the secondary-side winding, is above the absolute value of the secondary-side winding current threshold at the diode connected in parallel with the secondary-side switching element that changes from OFF to ON. The control unit is configured to control the primary-side full-bridge circuit and the secondary-side full-bridge circuit such that one of the primary-side full-bridge circuit and the secondary-side full-bridge circuit applies a level 2 voltage to the transformer section, while the other applies a level 3 voltage to the transformer section. The voltage at level 2 and the voltage at level 3 are waveforms of the same frequency that are reversed every 180 degrees in phase. The difference between the first moment when the voltage of the 3-level rises from low to medium and the second moment when the voltage of the 2-level rises from low to high is the first phase difference; the difference between the first moment and the third moment when the voltage of the 3-level rises from medium to high is the second phase difference. The control unit is configured to control the primary-side full-bridge circuit and the secondary-side full-bridge circuit to achieve a combination of the first phase difference, the second phase difference, the frequency of the 2-level voltage, and the frequency of the 3-level voltage that can output the required power, and to achieve the combination that satisfies the conditions 1 and 2.

2. The power conversion device according to claim 1, characterized in that, The control unit includes: a light load mode; a medium load mode, wherein the maximum output power is greater than that of the light load mode; and a heavy load mode, wherein the maximum output power is greater than that of the medium load mode. The control unit is configured to control the primary-side full-bridge circuit and the secondary-side full-bridge circuit in all modes, including the light load mode, the medium load mode, and the heavy load mode, so as to realize a combination of the first phase difference, the second phase difference, the frequency of the 2-level voltage, and the frequency of the 3-level voltage that can output the required power, and to realize the combination that satisfies the conditions 1 and 2.

3. The power conversion device according to claim 1 or 2, characterized in that, Upper and lower limits are set for the frequency of the two-level voltage and the frequency of the three-level voltage. The control unit is configured to vary the frequency of the 2-level voltage and the frequency of the 3-level voltage between the upper limit and the lower limit.

4. The power conversion device according to claim 1 or 2, characterized in that, The control unit is configured such that the greater the power demand, the lower the frequency of the 2-level voltage and the frequency of the 3-level voltage.

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