Electric power conversion system

Abstract

To provide an electric power conversion system having an individual snubber circuit that is configured to make additional control thereto unnecessary and have robustness to fluctuation in output voltages and leak currents of a switching element so as to improve reliability thereof.SOLUTION: In a 3-level inverter circuit of a neutral point clamp type 3-level inverter circuit or an active clamp type 3-level inverter circuit, first to fourth resistances RT1-RT4 are connected in parallel to first to fourth switching elements T1-T4 respectively. The first to fourth resistances RT1-RT4 have the same resistance value. A fifth snubber resistance RS5 is connected between a connection point of a third snubber capacitor CS3 and a third snubber diode DS3 and a connection point of the first resistance RT1 and the second resistance RT2. A sixth snubber resistance RS6 is connected between a connection point of a second snubber capacitor CS2 and a second snubber diode DS2 and a connection point of the third resistance RT3 and the fourth resistance RT4.SELECTED DRAWING: Figure 9

Description

[Technical Field]

[0001] The present invention relates to a power conversion device having a discharge prevention type individual snubber circuit, and more particularly to an initial charging circuit for a snubber capacitor. [Background technology]

[0002] Patent Documents 1 to 4 are disclosed as prior art for comparison with the present invention. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent Application No. Hei 7-312872 [Patent Document 2] Japanese Patent Publication No. 2020-156163 [Patent Document 3] Japanese Patent Application Publication No. 11-55956 [Patent Document 4] Japanese Patent Application Publication No. 7-111784 Summary of the Invention [Problem to be solved by the invention]

[0004] Problems with the configuration of a power conversion device without an initial charging circuit are described in paragraphs 0050 to 0053 of Patent Document 1. Fig. 1 shows the circuit configuration of a power conversion device without an initial charging circuit and waveforms at the start of switching operation.

[0005] If switching element S2 or switching element S3 is turned on while snubber capacitor C1 is not charged to power supply voltage E, the voltage applied to snubber diode D3 will be greater than power supply voltage E. In the configuration of Figure 1, the difference between the DC voltage at t2 and the voltage of switching element S2 (2E - Vs2), or 3 / 2E, is applied. Therefore, if the element rating of snubber diode D3 is less than 3 / 2E, the element rating may be exceeded and it may be damaged.

[0006] In addition, immediately after operation starts, a resonant current flows through the wiring inductance and snubber capacitor C1, causing an overvoltage E1 to be applied. In the worst case, the overvoltage E1 can reach twice the power supply voltage E. If the ratings of the snubber capacitor C1, snubber diode D1, switching element S2, and switching element S3 are less than 2E but greater than E, the resonant current and overvoltage E1 may cause the ratings of the elements to be exceeded.

[0007] As a countermeasure, Patent Document 1 discloses the following prior arts 1 to 4. However, each of these has one of the following problems. -Requires additional control during startup. During standby, there is no robustness to snubber capacitor voltage fluctuations in response to output voltage fluctuations of the power conversion device (inverter) and voltage imbalances of the switching elements. -Variations in leakage current of switching elements and snubber capacitors cause overcharging of snubber capacitor voltage and switching elements.

[0008] [Prior art 1] Figure 2 shows the operating waveforms of Prior Art 1. As shown in paragraph 0057, by turning off switching elements S1 and S4 during the period from time t1 to t2 (time ta) and starting operation of the device, it is possible to suppress the overvoltage applied to switching element S3 when the device starts operating (time t2).

[0009] However, it can only be charged up to E / 2, and the snubber diode D3, snubber capacitor C1, switching element S2, and switching element S3 must be large and expensive elements that meet the withstand voltage of 3 / 2E, which causes the power conversion device (inverter) to become large and expensive.In addition, additional control is required to charge the snubber capacitor C1.

[0010] [Prior art 2] 3 shows the circuit configuration and operating waveforms at the start of switching operation of Prior Art 2. Paragraphs 0058 to 0062 show the configuration and operation in which a resistor R15 is connected across the snubber diode D3 that constitutes the snubber circuit of switching element S2, and a resistor R16 is also connected across the snubber diode D3 that constitutes the snubber circuit of switching element S3.

[0011] Prior Art 2 does not require any special control when starting operation, but it has the problem that when the voltage at AC output terminal OUT becomes 0 or 2E, snubber capacitor C1 is overcharged to 2E by resistor R15 or resistor R16. In addition, when switching element S2 or switching element S3 is turned on, the voltage 2E that has been overcharged to snubber capacitor C1 is applied to snubber diode D1. In other words, snubber capacitor C1 and snubber diode D1 require large, expensive elements that can withstand a voltage of 2E, which results in a problem of the power conversion device (inverter) becoming larger and more expensive.

[0012] [Prior art 3] 4 shows the circuit configuration of Prior Art 3. Paragraphs 0063 to 0066 describe a configuration in which high-impedance resistors R15 and R16 are connected in parallel to switching element S1 and switching element S4, respectively.

[0013] Prior Art 3 does not require any special control when starting operation, but it has the problem that when the voltage at AC output terminal OUT becomes 0 or 2E, snubber capacitor C1 is overcharged to 2E by resistor R15 or resistor R16. In addition, when switching element S2 or switching element S3 is turned on, the voltage 2E that has been overcharged to snubber capacitor C1 is applied to snubber diode D1. In other words, snubber capacitor C1 and snubber diode D1 require large, expensive elements that can withstand a voltage of 2E, which results in a problem of the power conversion device (inverter) becoming larger and more expensive.

[0014] [Prior art 4] Figure 5 shows the circuit configuration and operating waveforms of Prior Art 4. Paragraphs 0067 to 0081 explain a method in which switches SW1 and SW2 are connected in parallel to snubber diode D3, and snubber capacitor C1 is charged by appropriately turning on switches SW1 and SW2 in response to switching operations at the start of operation or after the start of operation.

[0015] Prior art 4 has a problem in that an additional switch, its drive circuit, and additional control are required to charge the snubber capacitor C1.

[0016] Furthermore, prior art snubber circuits other than Patent Document 1 include Prior Art 5 to Prior Art 7, each of which has problems.

[0017] [Prior art 5] Patent Document 4 discloses the circuit configuration of a power conversion device shown in Fig. 6. Prior Art 5 is a method of connecting a resistor in parallel to each of series-connected switching elements 3A to 3D to share voltage, as shown in Fig. 6.

[0018] The voltage distribution of switching elements connected in series is determined by the leakage current of each element. If the leakage current varies greatly, the voltage imbalance causes voltage to concentrate on some elements, resulting in damage to the elements. In Patent Document 4, resistors that can pass a current large enough to compensate for the leakage current are connected in parallel to each of switching elements 3A to 3D. As a result, the voltage imbalance of the switching elements can be suppressed.

[0019] On the other hand, although a circuit configuration combined with a snubber circuit is disclosed, the problem of initial charging of the snubber capacitor is not taken into consideration. If the snubber circuit of Patent Document 1 is used in Prior Art 5, the snubber capacitor voltage will only be charged to 0 or E / 2, and large, expensive elements that satisfy a withstand voltage of 3 / 2E or more for the snubber diode D3, snubber capacitor C1, snubber diode D1, switching element S2, and switching element S3 are required, resulting in a problem of the power conversion device (inverter) becoming larger and more expensive.

[0020] [Prior art 6] Patent Document 2 discloses the circuit configuration of a power conversion device shown in Figure 7. Prior Art 6 is a flying capacitor type three-level inverter that uses individual snubber circuits, as shown in Figure 7. Snubber capacitors C2 and C3 are connected in parallel to flying capacitor FC, which is charged to Vdc / 2, via resistors R2 and R3, so they are always charged to Vdc / 2.

[0021] However, Prior Art 6 does not anticipate the simultaneous on-state of switching element Sb and switching element Sc. When a neutral-point clamped three-level inverter is used, if switching element Sb and switching element Sc are turned on simultaneously, the voltage of snubber capacitors C2 and C3 drops to zero via resistors R2 and R3. If the resistance values of resistors R2 and R3 are increased to suppress the voltage drop, the energy absorbed at turn-off cannot be discharged, causing the voltage of snubber capacitor C2 to remain elevated, resulting in the problem that the snubber circuit will no longer function.

[0022] [Prior art 7] Patent Document 3 discloses the circuit configuration of a power conversion device shown in Figure 8. In Prior Art 7, adding diodes Ds9 and Ds10 to a discharge prevention snubber circuit enables initial charging of snubber capacitors C2 and C3 without additional control. If there is no variation in the leakage current of switching elements S1 to S4, snubber capacitors C1 to C4 are initially charged to E1 or E2 regardless of the OUT voltage. However, when the leakage current of switching elements S1 and S4 is sufficiently large compared to the leakage current of switching elements S2 and S3, there is a problem in that snubber capacitors C2 and C3 may be overcharged to a voltage of up to E1 + E2.

[0023] Therefore, when configuring a circuit, the leakage current of the elements must be taken into consideration when arranging them.Alternatively, large, expensive elements that satisfy the maximum withstand voltage E1+E2 must be used for the snubber capacitors C2 and C3 and snubber diodes D2 and D3, which results in the problem of the power conversion device (inverter) becoming larger and more expensive.

[0024] In view of the above, it is an object of the present invention to provide a power conversion device having individual snubber circuits that does not require additional control and has improved reliability by employing a circuit configuration that is robust against output voltage fluctuations and leakage currents of switching elements. [Means for solving the problem]

[0025] The present invention has been devised in view of the above-mentioned problems of the related art, and one aspect thereof includes fifth and sixth capacitors connected in series between a P terminal and an N terminal, a fifth voltage dividing resistor connected in parallel with the fifth capacitor, a sixth voltage dividing resistor connected in parallel with the sixth capacitor, first to fourth switching elements connected in series between the P terminal and the N terminal, first to fourth resistors connected in parallel with the first to fourth switching elements, respectively, and having the same resistance value, a first snubber capacitor and a first snubber diode connected in series between one end and the other end of the first switching element, a second snubber capacitor and a second snubber diode connected in series between one end and the other end of the second switching element; a third snubber diode and a third snubber capacitor connected in series between one end and the other end of the third switching element; a fourth snubber diode and a fourth snubber capacitor connected in series between one end and the other end of the fourth switching element; first and fourth snubber resistors connected in series between a connection point between the first snubber capacitor and the first snubber diode and a connection point between the fourth snubber capacitor and the fourth snubber diode; a fifth and sixth diodes or fifth and sixth switching elements connected in series between a connection point of the resistor and a connection point of the third and fourth resistors; a fifth snubber resistor connected between a connection point of the third snubber capacitor and the third snubber diode and a connection point of the first and second resistors; a sixth snubber resistor connected between a connection point of the second snubber capacitor and the second snubber diode and a connection point of the third and fourth resistors; and a fifth switching element connected in series between a connection point of the third snubber diode, the third snubber capacitor, the fifth snubber resistor and the P terminal. a third snubber resistor, a sixth snubber diode, and a second snubber resistor connected in series between the N terminal and a connection point of the second snubber diode, the second snubber capacitor, and the sixth snubber resistor; a connection point of the fifth and sixth capacitors is a neutral point, a connection point of the first snubber resistor and the fourth snubber resistor is connected to the neutral point, a connection point of the fifth and sixth diodes or a connection point of the fifth and sixth switching elements is connected to the neutral point, and a connection point of the second and third switching elements is an AC output terminal.

[0026] In another aspect, the power supply may include fifth and sixth capacitors connected in series between a P terminal and an N terminal, a fifth voltage dividing resistor connected in parallel to the fifth capacitor, a sixth voltage dividing resistor connected in parallel to the sixth capacitor, first to fourth switching elements connected in series between the P terminal and the N terminal, first to fourth resistors connected in parallel to the first to fourth switching elements, respectively, and having the same resistance value, a first snubber capacitor and a first snubber diode connected in series between one end and the other end of the first switching element, a second snubber capacitor and a second snubber diode connected in series between one end and the other end of the second switching element, a third snubber diode and a third snubber capacitor connected in series between one end and the other end of the third switching element, a fourth snubber diode and a fourth snubber capacitor connected in series between one end and the other end of the fourth switching element, and a second snubber diode and a third snubber diode connected in series between the one end and the other end of the fourth switching element. a first snubber resistor, a fifth and sixth diode connected in series between the connection point of the first and second resistors and the connection point of the third and fourth resistors, or a fifth and sixth switching element; a third and fifth snubber resistor connected in series between the connection point of the third snubber capacitor and the third snubber diode and the connection point of the first and second resistors; a second and sixth snubber resistor connected between the connection point of the second snubber capacitor and the second snubber diode and the connection point of the third and fourth resistors; a fifth snubber diode connected between the connection point and the P terminal, and a sixth snubber diode connected between the connection point of the second and sixth snubber resistors and the N terminal, wherein the connection point of the fifth and sixth capacitors is a neutral point, the connection point of the first snubber resistor and the fourth snubber resistor is connected to the neutral point, the connection point of the fifth and sixth diodes or the connection point of the fifth and sixth switching elements is connected to the neutral point, and the connection point of the second and third switching elements is an AC output terminal.

[0027] In one aspect, the power supply comprises a seventh snubber diode and a fifth snubber capacitor connected in series between the cathode and anode of the fifth diode or between one end and the other end of the fifth switching element, a sixth snubber capacitor and an eighth snubber diode connected in series between the cathode and anode of the sixth diode or between one end and the other end of the sixth switching element, a seventh snubber resistor connected between the P terminal and a connection point between the seventh snubber diode and the fifth snubber capacitor, and an eighth snubber resistor connected between the N terminal and a connection point between the sixth snubber capacitor and the eighth snubber diode.

[0028] In one aspect, the resistance values of the first to fourth resistors and the resistance values of the first to fourth switching elements when they are off satisfy the relationship of the following formula (1).

[0029]

number

[0030] RT1: Resistance value of the first resistor RT2: Resistance value of the second resistor RT3: Resistance value of the third resistor RT4: Resistance value of the fourth resistor Roff1: Resistance value of the first switching element when it is off Roff2: Resistance value of the second switching element when it is off Roff3: Resistance value of the third switching element when it is off Roff4: Resistance value of the fourth switching element when it is off.

[0031] In one aspect, the resistance values of the first to fourth resistors and the resistance values of the first to fourth switching elements when they are off satisfy the relationship of the following formula (2).

[0032]

number

[0033] RT1: Resistance value of the first resistor RT2: Resistance value of the second resistor RT3: Resistance value of the third resistor RT4: Resistance value of the fourth resistor Roff1: Resistance value of the first switching element when it is off Roff2: Resistance value of the second switching element when it is off Roff3: Resistance value of the third switching element when it is off Roff4: Resistance value of the fourth switching element when it is off.

[0034] In one aspect, the limit on-time during which the second and third switching elements are kept on simultaneously satisfies the following formula (4).

[0035]

number

[0036] Vtm: Absolute maximum rated voltage of the first to fourth switching elements, the first to fourth snubber capacitors, and the first to sixth snubber diodes Vdc: Voltage between P and N terminals ΔVcsn: Voltage drop across the second and third snubber capacitors when the second and third switching elements are kept on simultaneously t0m: Maximum ON time for keeping the second and third switching elements ON simultaneously τ: Time constant of the fifth snubber resistor and the third snubber capacitor, and the sixth snubber resistor and the second snubber capacitor. [Effects of the Invention]

[0037] According to the present invention, it is possible to provide a power conversion device having individual snubber circuits that does not require additional control and has improved reliability by using a circuit configuration that is robust against output voltage fluctuations and leakage currents of switching elements. [Brief explanation of the drawings]

[0038] [Figure 1]10 is a diagram showing the circuit configuration of a power conversion device without an initial charging circuit and the operating waveforms at the start of switching operation. [Figure 2] FIG. 10 is a diagram showing the operating waveforms of the prior art 1. [Figure 3] FIG. 10 is a diagram showing the circuit configuration and operation waveforms of prior art 2. [Figure 4] FIG. 10 is a diagram showing the circuit configuration of Prior Art 3. [Figure 5] FIG. 10 is a diagram showing the circuit configuration of Prior Art 4 and the operating waveforms at the start of switching operation. [Figure 6] FIG. 10 is a diagram showing the circuit configuration of Prior Art 5. [Figure 7] FIG. 10 is a diagram showing the circuit configuration of Prior Art 6. [Figure 8] FIG. 10 is a diagram showing the circuit configuration of Prior Art 7. [Figure 9] FIG. 1 is a diagram showing a circuit configuration of a first embodiment. [Figure 10] FIG. 2 is a diagram showing the circuit configuration and voltages of each part of the first embodiment. [Figure 11] A diagram showing the charging paths of CS2 and CS3. [Figure 12] FIG. 10 is a diagram showing an equivalent circuit in a standby state. [Figure 13] FIG. 10 is a diagram showing the voltage duty of each element relative to the output voltage Vacn. [Figure 14] FIG. 10 is a diagram showing the snubber capacitor voltage when connected to the grid. [Figure 15] FIG. 2 is a diagram showing voltage waveforms of a switching element and a snubber capacitor. [Figure 16] A diagram showing the snubber capacitor discharge path when T2 and T3 are on. [Figure 17] FIG. 3 is a diagram showing operating waveforms in the power conversion device. [Figure 18] FIG. 10 is a diagram showing a circuit configuration of a second embodiment. [Figure 19] FIG. 10 is a diagram showing a circuit configuration of a third embodiment. [Figure 20] FIG. 10 is a diagram showing a circuit configuration of a fourth embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0039] Hereinafter, first to fourth embodiments of the power conversion device of the present invention will be described in detail with reference to Figures 9 to 20. Each embodiment has a configuration in which a capacitor, a resistor, and a diode are additionally connected to the prior art, and no special control is performed.

[0040] Although the switching elements in the first to fourth embodiments are indicated by IGBT symbols, other switching elements such as MOSFETs and GTOs can also be used. Furthermore, although neutral point clamped three-level inverter circuits are shown in Figures 9, 18, 19, and 20, they can also be applied to active clamped three-level inverter circuits in which the fifth and sixth diodes D5 and D6 are replaced with fifth and sixth switching elements.

[0041] [Embodiment 1] First, the circuit configuration of the power conversion device in the first embodiment will be described with reference to Fig. 9. Fifth and sixth capacitors C5 and C6 are connected in series between the P terminal and the N terminal. The connection point of the fifth and sixth capacitors C5 and C6 is the neutral point M. A fifth voltage dividing resistor R5 is connected in parallel to the fifth capacitor C5, and a sixth voltage dividing resistor R6 is connected in parallel to the sixth capacitor C6.

[0042] Furthermore, first to fourth switching elements T1 to T4 are connected in series between the P terminal and the N terminal. First to fourth resistors RT1 to RT4 are connected in parallel to the first to fourth switching elements T1 to T4, respectively. The connection point between the first resistor RT1 and the second resistor RT2 is designated as P1, the connection point between the second resistor RT2 and the third resistor RT3 is designated as AC output terminal AC, and the connection point between the third resistor RT3 and the fourth resistor RT4 is designated as N1.

[0043] A first snubber capacitor CS1 and a first snubber diode DS1 are connected in series between one end and the other end of the first switching element T1. A second snubber capacitor CS2 and a second snubber diode DS2 are connected in series between one end and the other end of the second switching element T2. A third snubber diode DS3 and a third snubber capacitor CS3 are connected in series between one end and the other end of the third switching element T3. A fourth snubber diode DS4 and a fourth snubber capacitor CS4 are connected in series between one end and the other end of the fourth switching element T4. Here, the first to fourth snubber diodes DS1 to DS4 have one end side (upper side in FIG. 9) of the first to fourth switching elements T1 to T4 as the anode and the other end side (lower side in FIG. 9) as the cathode.

[0044] First and fourth snubber resistors RS1 and RS4 are connected in series between the connection point between the first snubber capacitor CS1 and the first snubber diode DS1 and the connection point between the fourth snubber capacitor CS4 and the fourth snubber diode DS4. The connection point between the first snubber resistor RS1 and the fourth snubber resistor RS4 is connected to the neutral point M.

[0045] A fifth diode D5 and a sixth diode D6 are connected in series between P1 and N1. The junction of the fifth and sixth diodes D5 and D6 is connected to the neutral point M. Here, the fifth diode D5 and the sixth diode D6 have their anodes on the N1 side and their cathodes on the P1 side. A fifth snubber resistor RS5 is connected between P1 and the junction of the third snubber capacitor CS3 and the third snubber diode DS3. A sixth snubber resistor RS6 is connected between N1 and the junction of the second snubber capacitor CS2 and the second snubber diode DS2.

[0046] The fifth snubber diode DS5 and the third snubber resistor RS3 are connected in series between the junction of the third snubber diode DS3, the third snubber capacitor CS3, and the fifth snubber resistor RS5 and the P terminal. The fifth snubber diode DS5 has its anode connected to the junction of the third snubber diode DS3, the third snubber capacitor CS3, and the fifth snubber resistor RS5, and its cathode connected to the P terminal. The sixth snubber diode DS6 and the second snubber resistor RS2 are connected in series between the junction of the second snubber diode DS2, the second snubber capacitor CS2, and the sixth snubber resistor RS6 and the N terminal. The sixth snubber diode DS6 has its cathode connected to the junction of the second snubber diode DS2, the second snubber capacitor CS2, and the sixth snubber resistor RS6, and its anode connected to the N terminal.

[0047] The first to fourth switching elements T1 to T4, the fifth and sixth diodes D5 and D6, the fifth and sixth capacitors C5 and C6, and the fifth and sixth voltage dividing resistors R5 and R6 constitute a neutral point clamped three-level inverter. If the fifth and sixth diodes D5 and D6 are used as the fifth and sixth switching elements, an active clamped three-level inverter circuit is formed.

[0048] The first to fourth snubber resistors RS1 to RS4, the first to fourth snubber capacitors CS1 to CS4, and the first to sixth snubber diodes DS1 to DS6 form discharge-preventing individual snubber circuits, and are connected to the first to fourth switching elements T1 to T4, respectively, for the purpose of reducing losses in the snubber circuits.

[0049] The first to fourth resistors RT1 to RT4 are provided for the purposes of initial charging of the first to fourth snubber capacitors CS1 to CS4 and controlling the voltages applied to the first to fourth switching elements T1 to T4 when the first to fourth switching elements T1 to T4 are turned off. The first to fourth resistors RT1 to RT4 have the same resistance value.

[0050] The sixth snubber resistor RS6 bridges the anode side of the second snubber diode DS2 and the N1 side to charge the second snubber capacitor CS2, and the fifth snubber resistor RS5 bridges the cathode side of the third snubber diode DS3 and the P1 side to charge the third snubber capacitor CS3.

[0051] The fifth voltage dividing resistor R5 is equal to the sixth voltage dividing resistor R6. The resistance values of the fifth and sixth snubber resistors RS5 and RS6 and the first to fourth resistors RT1 to RT4 are designed as follows.

[0052] Fifth and sixth snubber resistors RS5 and RS6: The time constant with respect to the capacitance of the second and third snubber capacitors CS2 and CS3 must be sufficiently larger than the system cycle and the switching carrier cycle.

[0053] First to fourth resistors RT1 to RT4: Designed so that the voltage distribution of the first to fourth switching elements T1 to T4 is equal during standby. The resistance value is set to a range that is sufficiently large compared to the impedance on the system side.

[0054] Furthermore, the resistance values when the first to fourth switching elements T1 to T4 are off are Roff1, Roff2, Roff3, and Roff4, respectively, and the first to fourth resistors RT1 to RT4 have values that satisfy the relationship of the following equation (1).

[0055]

number

[0056] If the leakage currents of the first to fourth switching elements T1 to T4 (the minute currents that flow through the switching elements when they are off) are not constant and vary from element to element, the variations in the leakage currents of the first to fourth switching elements T1 to T4 will cause a voltage imbalance between the P terminal and the N terminal. To suppress this voltage imbalance, the first to fourth resistors RT1 to RT4 are set to values that satisfy the relationship in equation (2) below.

[0057]

number

[0058] 10 and 11 show a circuit for explaining operation and the snubber capacitor charging path during standby, respectively. As shown in Fig. 10, the voltage between the P terminal and the N terminal is Vdc, the voltage of the fifth capacitor C5 is E1, the voltage of the sixth capacitor C6 is E2, the voltages of the first to fourth snubber capacitors CS1 to CS4 are vcs1 to vcs4, and the voltages of the first to fourth resistors RT1 to RT4 are vT1 to vT4.

[0059] As shown in FIG. 11, the second snubber capacitor CS2 is charged through the route RT1→CS2→RS6→RT4, and the third snubber capacitor CS3 is charged through the route RT1→RS5→CS3→RT4.

[0060] FIG. 12 shows an equivalent circuit during standby (first to fourth switching elements T1 to T4 are off) focusing on the initial charging of the second snubber capacitor CS2 and the third snubber capacitor CS3.

[0061] The voltages of the first to fourth switching elements T1 to T4 are divided to Vdc / 4. If the currents flowing through the first to fourth resistors RT1 to RT4 are sufficiently large compared to the leakage currents of the first to fourth switching elements T1 to T4, the effect of variations in the leakage currents of the switching elements on voltage sharing can be ignored.

[0062] For simplicity, the following explanation will ignore the effects of leakage current. The voltages across the second snubber capacitor CS2 and the third snubber capacitor CS3 are equal to the sum of the voltages applied to the second resistor RT2 and the third resistor RT3. Since RT1 = RT2 = RT3 = RT4, the voltage vcs2 across the second snubber capacitor CS2 and the voltage vcs3 across the third snubber capacitor CS3 fluctuate during switching operation (see FIG. 15), but are charged to approximately Vdc / 2 and are not affected by the output voltage. Therefore, it can be said that this embodiment 1 is robust against output voltage fluctuations.

[0063] 13 shows the voltages of each element when the output of the power conversion device is open, or when Vacn=0, Vdc / 2, and Vdc, where Vacn represents the voltage at the AC output terminal AC.

[0064] It is also robust against output voltage fluctuations. We simulated the snubber capacitor voltage (average value) when the AC output side of the power conversion device (AC output terminal AC in Fig. 1) was connected to the grid during standby. The results are shown in Fig. 14.

[0065] The horizontal axis is the ratio of Vdc to the effective value of the AC output voltage (modulation rate), and the further to the right it is, the greater the voltage swing at the AC output terminals. The snubber capacitor voltage (average value) is constant at approximately Vdc / 2, regardless of the modulation rate.

[0066] The fact that the snubber capacitor voltage average value is almost constant regardless of the modulation rate means that the power conversion device can be kept connected to the grid and there are no restrictions on the timing of starting operation, and it reduces the possibility of failure due to external factors such as grid abnormalities or failure of the parallel operation device.

[0067] Applications of power conversion devices include uninterruptible power supplies (UPS) and power conditioners (PCS) that are connected to the grid. These devices require high-speed control to respond to sudden load changes and power outages. Prior art 2 and prior art 3 require additional control to avoid exceeding the device rating or reducing the surge suppression effect due to overcharging of snubber capacitor C1. For example, the output is left open during standby and connected to the grid just before operation begins, starting operation at an output voltage that does not overcharge. As a result, a delay occurs in control.

[0068] In addition, it is expected that PCSs will frequently cycle between operation and standby due to load fluctuations. If the snubber capacitor is not initially charged, as in Prior Art 1 and Prior Art 5, a large inrush current will flow through the snubber capacitor each time operation starts, which could reduce the lifespan of the component and reduce reliability.

[0069] When the output voltage is clamped to Vdc or zero due to a system abnormality or a failure in the parallel operation device, the snubber capacitor in Prior Art 2 and Prior Art 3 is overcharged to Vdc. If an abnormality is not anticipated and the design is based on a withstand voltage below Vdc, immediate parallel-off is required in the event of an abnormality to prevent damage to the elements. When using large, expensive elements that meet a withstand voltage of Vdc or higher, the power conversion device becomes larger and more expensive.

[0070] As described above, by adopting the first embodiment, it is possible to avoid the problem that occurs when the preceding example is put into practical use (when the snubber capacitor is not charged to Vdc / 2).

[0071] Figure 15 shows example voltage waveforms of the fifth and sixth capacitors C5 and C6 of the power conversion device (inverter) during initial charging, standby, and switching operation. From initial charging to standby, the first through fourth switching elements T1 through T4 are off, so the voltages vcs1 through vcs4 of the first through fourth snubber capacitors CS1 through CS4 are charged to Vdc / 2. During switching operation, the time constant of the initial charging circuit (fifth and sixth snubber resistors RS5 and RS6, and first through fourth resistors RT1 through RT4) is sufficiently larger than the switching frequency, so the effect of the initial charging circuit can be ignored, and the device operates in the same way as a discharge-prevention individual snubber circuit.

[0072] On the other hand, if the second switching element T2 and the third switching element T3 continue to be on during operation, a discharge path is formed, causing the voltage vcs2 of the second snubber capacitor CS2 and the voltage vcs3 of the third snubber capacitor CS3 to drop. The discharge paths are shown in Figure 16. The second snubber capacitor CS2 is discharged via the path CS2 → T2 → T3 → RS6. The third snubber capacitor CS3 is discharged via the path CS3 → RS5 → T2 → T3.

[0073] The voltage drop ΔVcsn is determined by the simultaneous on-time of the second switching element T2 and the third switching element T3 and the time constant τ of the discharge paths of the fifth snubber resistor RS5 and the third snubber capacitor CS3, and the sixth snubber resistor RS6 and the second snubber capacitor CS2. The voltage drop ΔVcsn is expressed by the following equation (3), where the simultaneous on-time of the second switching element T2 and the third switching element T3 is t0.

[0074]

number

[0075] If the second switching element T2 or the third switching element T3 is turned on when the voltage vcs2 of the second snubber capacitor CS2 and the voltage vcs3 of the third snubber capacitor CS3 are less than Vdc / 2, a mismatch between E1, E2 and the voltages vcs3, vcs2 of the third and second snubber capacitors CS3, CS2 will cause an inrush current to flow through the snubber capacitors, causing the snubber capacitor voltage to exceed Vdc / 2. Therefore, care must be taken during operation to ensure that the second switching element T2 and the third switching element T3 do not remain on for long periods of time.

[0076] Since the maximum value of the snubber capacitor voltage due to the above-mentioned inrush current is less than Vdc because ΔVcsn>0, it can be suppressed to a lower voltage than in prior arts 2 and 3 (Vdc in embodiment 1 corresponds to 2E in prior arts 2 and 3).

[0077] The reason why the snubber capacitor maximum voltage can be suppressed to less than Vdc in this first embodiment is that the influence of leakage current during standby is eliminated and the voltage between P1 and N1 is constant at Vdc / 2.

[0078] The resistive voltage division of the first through fourth resistors RT1 through RT4 uniquely determines the potentials of the P terminal, P1, AC output terminal AC, and neutral terminal M, N1, and N terminal, even during standby, eliminating the effects of leakage current. Additionally, in a neutral-point clamped three-level inverter, if the effects of leakage current can be ignored, the voltage (vT2 + vT3) between P1 and N1 during standby remains constant at Vdc / 2 regardless of the output voltage, as shown in Figure 13. The second snubber capacitor CS2 and the third snubber capacitor CS3 have one terminal connected to P1 and the other to N1, respectively. If the other terminal is connected to N1 or P1, they are charged to Vdc / 2 during standby. During operation, the voltage between P1 and N1 fluctuates between Vdc / 2 and 0 due to switching activity. Therefore, connecting them through resistors prevents overdischarge of the second snubber capacitor CS2 and the third snubber capacitor CS3.

[0079] Assuming there is no loss in the inrush current path, the limit on-time t0m during which the second and third switching elements T2 and T3 remain on simultaneously due to the inrush current must satisfy the following equation (4) for the absolute maximum rated voltage Vtm of the elements (first to fourth switching elements T1 to T4, first to fourth snubber capacitors CS1 to CS4, and first to sixth snubber diodes DS1 to DS6).

[0080]

number

[0081] As a countermeasure, if the second switching element T2 and the third switching element T3 continue to be on simultaneously, one method is to set the time constant τ to be sufficiently larger than the expected limit on-time t0m, perform switching operation within the limit on-time t0m, and output Vdc and zero voltage for only a short time. Methods for setting a large time constant τ include using large-capacity second and third snubber capacitors CS2 and CS3, or using large resistance values for the fifth and sixth snubber resistors RS5 and RS6.

[0082] Figure 17 shows an example of the operating waveforms of a power conversion device (inverter). This assumes that after initial charging, the inverter goes into stand-alone operation (no-load operation with the inverter disconnected from the grid) and then into grid-connected operation. During standby, the first through fourth switching elements T1 through T4 are turned off, so the voltages vcs1 through vcs4 of the first through fourth snubber capacitors CS1 through CS4 are charged to Vdc / 2. During periods when the output voltage is zero due to implementation constraints when the command value is small, such as immediately after the start of switching operation, the voltage vcs2 of the second snubber capacitor CS2 and the voltage vcs3 of the third snubber capacitor CS3 drop significantly.

[0083] Additionally, there is a period during which the second switching element T2 and the third switching element T3 each remain on for half a cycle. Because the snubber capacitors cannot be charged by switching operation and discharge occurs when the second and third switching elements T2 and T3 are simultaneously on, the voltage vcs2 of the second snubber capacitor CS2 and the voltage vcs3 of the third snubber capacitor CS3 decrease during this period. Therefore, the actual inverter operating conditions must be taken into account when designing the fifth snubber resistor RS5, the sixth snubber resistor RS6, and the limit on-time t0m, which is the upper limit of the time the second and third switching elements T2 and T3 are simultaneously on.

[0084] [effect] The first to fourth resistors RT1 to RT4 cause the output voltage to be near Vdc / 2 in standby mode when the output is open. In a device in which the neutral point M is grounded, the voltage to ground of the electrical components (between the charging part and ground) is near Vdc / 2. If the output voltage is indefinite, the voltage to ground in standby mode must be considered to be maximum Vdc as a worst-case scenario, which could result in a decrease in reliability due to insulation degradation or an increase in the size and cost of the power conversion device due to insulation countermeasures. In this first embodiment, the output voltage is near Vdc / 2, which can prevent the device from becoming larger and more expensive.

[0085] Since the rated voltage of the components can be designed to be less than Vdc, it may be possible to use inexpensive and small components.

[0086] Since the only additional component is a resistor, no additional control is required for initial charging (advantage over prior arts 1 and 4).

[0087] The currents of the first to fourth resistors RT1 to RT4 are designed to be sufficiently large compared to the leakage currents of the first to fourth switching elements T1 to T4, which reduces overcharging of the snubber capacitors due to variations in leakage current, making it less likely that overvoltage breakdown of the snubber capacitors will occur.

[0088] Before the start of operation, the sum of the voltages applied to the second resistor RT2 and the third resistor RT3 is always Vdc / 2, so the average charging voltage of the snubber capacitor is robust to the output voltage.

[0089] In Prior Art 2 and 3, the snubber capacitor is overcharged to 2E when the output voltage is 0 or 2E, so a large, expensive snubber capacitor with a high withstand voltage is required. On the other hand, in this embodiment 1, as described above, the maximum applied voltage to the snubber capacitor can be suppressed to less than 2E. This allows the snubber capacitor to be made smaller and less expensive.

[0090] Furthermore, in the first embodiment, the first to fourth resistors RT1 to RT4 and the snubber circuits are permanently installed, which allows for cost reduction compared to when they are removable.

[0091] [Embodiment 2] The circuit configuration of the power conversion device according to the second embodiment is shown in Fig. 18. The same parts as those in the first embodiment are given the same reference numerals and their explanation will be omitted.

[0092] In the second embodiment, unlike the first embodiment, the connections between the fifth snubber diode DS5 and the third snubber resistor RS3 and the sixth snubber diode DS6 and the second snubber resistor RS2 are reversed, and the fifth snubber resistor RS5 is connected between the fifth snubber diode DS5 and the third snubber resistor RS3, and the sixth snubber resistor RS6 is connected between the sixth snubber diode DS6 and the second snubber resistor RS2.

[0093] That is, third and fifth snubber resistors RS3 and RS5 are connected in series between the junction of the third snubber capacitor CS3 and the third snubber diode DS3 and P1. Second and sixth snubber resistors RS2 and RS6 are connected in series between the junction of the second snubber capacitor CS2 and the second snubber diode DS2 and N1. A fifth snubber diode DS5 is connected between the junction of the third snubber resistor RS3 and the fifth snubber resistor RS5 and the P terminal. Here, the anode of the fifth snubber diode DS5 is the junction of the third and fifth snubber resistors RS3 and RS5, and the cathode is the P terminal. A sixth snubber diode DS6 is connected between the junction of the second snubber resistor RS2 and the sixth snubber resistor RS6 and the N terminal. Here, the cathode of the sixth snubber diode DS6 is the junction of the second and sixth snubber resistors RS2 and RS6, and the anode is the N terminal.

[0094] A second snubber resistor RS2 is provided to connect the anode side of the second snubber diode DS2 to the cathode side of the sixth snubber diode DS6, and a third snubber resistor RS3 is provided to connect the cathode side of the third snubber diode DS3 to the anode side of the fifth snubber diode DS5, for the purposes of initial charging of the second snubber capacitor CS2 and the third snubber capacitor CS3, reducing losses in the fifth and sixth snubber resistors RS5 and RS6 (described below), and discharging the charge stored in the second and third snubber capacitors CS2 and CS3 during switching.

[0095] In addition, for the purpose of initial charging of the second snubber capacitor CS2 and the third snubber capacitor CS3, a sixth snubber resistor RS6 is provided which bridges the cathode side of the sixth snubber diode DS6 and the N1 side, and a fifth snubber resistor RS5 is provided which bridges the anode side of the fifth snubber diode DS5 and P1.

[0096] The resistance values of the fifth voltage dividing resistor R5=sixth voltage dividing resistor R6, the fifth and sixth snubber resistors RS5 and RS6, and the first to fourth resistors RT1 to RT4 are designed the same as in the first embodiment.

[0097] The equivalent circuits for the first to fourth snubber capacitors CS1 to CS4 and the first to fourth switching elements T1 to T4 during initial charging and standby are the same as those in the first embodiment, and therefore will not be described here.

[0098] [effect] The second embodiment achieves the same effects as the first embodiment. Furthermore, unlike the first embodiment, the voltage can be shared between the third snubber resistor RS3 and the fifth snubber resistor RS5, and between the second snubber resistor RS2 and the sixth snubber resistor RS6. This reduces the losses that occur in the fifth snubber resistor RS5 and the sixth snubber resistor RS6 during initial charging and switching. This allows the losses and sizes of the second snubber resistor RS2, the third snubber resistor RS3, the fifth snubber resistor RS5, and the sixth snubber resistor RS6 to be equalized. Equalizing the sizes of multiple snubber resistors can be advantageous for reducing the size of a power conversion device in some cases.

[0099] [Embodiment 3] The circuit configuration of a power conversion device according to the third embodiment is shown in Fig. 19. The same components as those in the first embodiment are assigned the same reference numerals, and a description thereof will be omitted. The third embodiment is configured such that the fifth diode D5 and the sixth diode D6 in the first embodiment are provided with discharge prevention snubber circuits. In the case of an active clamp three-level inverter circuit, discharge prevention snubber circuits are provided for the fifth and sixth switching elements provided in place of the fifth and sixth diodes D5 and D6.

[0100] Specifically, a seventh snubber diode DS7 and a fifth snubber capacitor CS5 are connected between the cathode and anode of the fifth diode D5, and a seventh snubber resistor RS7 is connected between the connection point of the seventh snubber diode DS7 and the fifth snubber capacitor CS5 and the P terminal.

[0101] Similarly, a sixth snubber capacitor CS6 and an eighth snubber diode DS8 are connected between the cathode and anode of the sixth diode D6. An eighth snubber resistor RS8 is connected between the N terminal and the connection point of the sixth snubber capacitor CS6 and the eighth snubber diode DS8. Here, the seventh and eighth snubber diodes DS7 and DS8 have their anodes connected to the cathodes of the fifth and sixth diodes D5 and D6, respectively.

[0102] The fifth and sixth snubber capacitors CS5, CS6, seventh and eighth snubber diodes DS7, DS8, and seventh and eighth snubber resistors RS7, RS8 are connected to the fifth and sixth diodes D5, D6 as individual discharge-preventing snubber circuits in order to reduce losses in the snubber circuits.

[0103] The resistance values of the fifth voltage dividing resistor R5=sixth voltage dividing resistor R6, the fifth and sixth snubber resistors RS5 and RS6, and the first to fourth resistors RT1, RT2, RT3 and RT4 are designed the same as in the first embodiment.

[0104] The equivalent circuits for the first to fourth snubber capacitors CS1 to CS4 and the first to fourth switching elements T1 to T4 during initial charging and standby are the same as those in the first embodiment, and therefore will not be described here.

[0105] [effect] The third embodiment achieves the same effects as the first and second embodiments. Moreover, unlike the first and second embodiments, the third embodiment can suppress surge voltages across the fifth diode D5 and the sixth diode D6 during switching. This suppresses overvoltage breakdown of the fifth diode D5 and the sixth diode D6, improving the reliability of the power conversion device.

[0106] [Embodiment 4] The circuit configuration of the power conversion device according to the fourth embodiment is shown in Fig. 20. The same components as those in the first to third embodiments are given the same reference numerals, and the description thereof will be omitted. The fourth embodiment is a combination of the second and third embodiments.

[0107] In the first embodiment, the connections of the fifth snubber diode DS5 and the third snubber resistor RS3 and the sixth snubber diode DS6 and the second snubber resistor RS2 are reversed, and the fifth snubber resistor RS5 is connected between the fifth snubber diode DS5 and the third snubber resistor RS3, and the sixth snubber resistor RS6 is connected between the sixth snubber diode DS6 and the second snubber resistor RS2.

[0108] That is, third and fifth snubber resistors RS3 and RS5 are connected in series between the junction of the third snubber capacitor CS3 and the third snubber diode DS3 and P1. Second and sixth snubber resistors RS2 and RS6 are connected in series between the junction of the second snubber capacitor CS2 and the second snubber diode DS2 and N1. A fifth snubber diode DS5 is connected between the junction of the third snubber resistor RS3 and the fifth snubber resistor RS5 and the P terminal. Here, the anode of the fifth snubber diode DS5 is the junction of the third and fifth snubber resistors RS3 and RS5, and the cathode is the P terminal. A sixth snubber diode DS6 is connected between the junction of the second snubber resistor RS2 and the sixth snubber resistor RS6 and the N terminal. Here, the cathode of the sixth snubber diode DS6 is the junction of the second and sixth snubber resistors RS2 and RS6, and the anode is the N terminal.

[0109] Furthermore, a discharge prevention type snubber circuit is provided for the fifth diode D5 and the sixth diode D6. In the case of an active clamp type three-level inverter circuit, a discharge prevention type snubber circuit is provided for the fifth and sixth switching elements provided in place of the fifth and sixth diodes D5 and D6.

[0110] Specifically, a seventh snubber diode DS7 and a fifth snubber capacitor CS5 are connected between the cathode and anode of the fifth diode D5, and a seventh snubber resistor RS7 is connected between the connection point of the seventh snubber diode DS7 and the fifth snubber capacitor CS5 and the P terminal.

[0111] Similarly, a sixth snubber capacitor CS6 and an eighth snubber diode DS8 are connected between the cathode and anode of the sixth diode D6. An eighth snubber resistor RS8 is connected between the N terminal and the connection point of the sixth snubber capacitor CS6 and the eighth snubber diode DS8. Here, the seventh and eighth snubber diodes DS7 and DS8 have their anodes connected to the cathodes of the fifth and sixth diodes D5 and D6, respectively.

[0112] The resistance values of the fifth voltage dividing resistor R5=sixth voltage dividing resistor R6, the fifth and sixth snubber resistors RS5 and RS6, and the first to fourth resistors RT1 to RT4 are designed the same as in the first embodiment.

[0113] The equivalent circuits for the first to fourth snubber capacitors CS1 to CS4 and the first to fourth switching elements T1 to T4 during initial charging and standby are the same as those in the first embodiment, and therefore will not be described here.

[0114] [effect] The fourth embodiment has the same effects as the first to third embodiments. Moreover, unlike the first, second, and third embodiments, it is possible to reduce the surge voltages across the fifth and sixth diodes D5 and D6 during initial charging and switching, and the losses that occur in the fifth and sixth snubber resistors RS5 and RS6.

[0115] Although the present invention has been described in detail above only with respect to the specific examples, it will be apparent to those skilled in the art that various modifications and variations are possible within the scope of the technical concept of the present invention, and it is natural that such modifications and variations fall within the scope of the claims. [Explanation of symbols]

[0116] C5, C6...5th and 6th capacitors M…Neutral point R5, R6...5th and 6th voltage dividing resistors T1 to T4: First to fourth switching elements RT1~RT4...1st to 4th resistors CS1 to CS6: 1st to 6th snubber capacitors DS1 to DS8: 1st to 8th snubber diodes RS1 to RS8: 1st to 8th snubber resistors D5, D6...5th and 6th diodes AC…AC output terminal

Claims

1. fifth and sixth capacitors connected in series between the P terminal and the N terminal; a fifth voltage dividing resistor connected in parallel with the fifth capacitor; a sixth voltage dividing resistor connected in parallel with the sixth capacitor; first to fourth switching elements connected in series between the P terminal and the N terminal; first to fourth resistors connected in parallel to the first to fourth switching elements, respectively, and having the same resistance value; a first snubber capacitor and a first snubber diode connected in series between one end and the other end of the first switching element; a second snubber capacitor and a second snubber diode connected in series between one end and the other end of the second switching element; a third snubber diode and a third snubber capacitor connected in series between one end and the other end of the third switching element; a fourth snubber diode and a fourth snubber capacitor connected in series between one end and the other end of the fourth switching element; first and fourth snubber resistors connected in series between a connection point between the first snubber capacitor and the first snubber diode and a connection point between the fourth snubber capacitor and the fourth snubber diode; fifth and sixth diodes or fifth and sixth switching elements connected in series between the connection point of the first and second resistors and the connection point of the third and fourth resistors; a fifth snubber resistor connected between a connection point between the third snubber capacitor and the third snubber diode and a connection point between the first and second resistors; a sixth snubber resistor connected between a connection point of the second snubber capacitor and the second snubber diode and a connection point of the third and fourth resistors; a fifth snubber diode and a third snubber resistor connected in series between a connection point of the third snubber diode, the third snubber capacitor, and the fifth snubber resistor and the P terminal; a sixth snubber diode and a second snubber resistor connected in series between a connection point of the second snubber diode, the second snubber capacitor, and the sixth snubber resistor and the N terminal; Equipped with a connection point between the fifth and sixth capacitors as a neutral point, a connection point between the first snubber resistor and the fourth snubber resistor as a neutral point, a connection point between the fifth and sixth diodes or a connection point between the fifth and sixth switching elements as a neutral point, and a connection point between the second and third switching elements as an AC output terminal.

2. fifth and sixth capacitors connected in series between the P terminal and the N terminal; a fifth voltage dividing resistor connected in parallel with the fifth capacitor; a sixth voltage dividing resistor connected in parallel with the sixth capacitor; first to fourth switching elements connected in series between the P terminal and the N terminal; first to fourth resistors connected in parallel to the first to fourth switching elements, respectively, and having the same resistance value; a first snubber capacitor and a first snubber diode connected in series between one end and the other end of the first switching element; a second snubber capacitor and a second snubber diode connected in series between one end and the other end of the second switching element; a third snubber diode and a third snubber capacitor connected in series between one end and the other end of the third switching element; a fourth snubber diode and a fourth snubber capacitor connected in series between one end and the other end of the fourth switching element; first and fourth snubber resistors connected in series between a connection point between the first snubber capacitor and the first snubber diode and a connection point between the fourth snubber capacitor and the fourth snubber diode; fifth and sixth diodes or fifth and sixth switching elements connected in series between the connection point of the first and second resistors and the connection point of the third and fourth resistors; third and fifth snubber resistors connected in series between a connection point between the third snubber capacitor and the third snubber diode and a connection point between the first and second resistors; second and sixth snubber resistors connected between a connection point between the second snubber capacitor and the second snubber diode and a connection point between the third and fourth resistors; a fifth snubber diode connected between the connection point of the third and fifth snubber resistors and the P terminal; a sixth snubber diode connected between the connection point of the second and sixth snubber resistors and the N terminal; Equipped with a connection point between the fifth and sixth capacitors as a neutral point, a connection point between the first snubber resistor and the fourth snubber resistor as a neutral point, a connection point between the fifth and sixth diodes or a connection point between the fifth and sixth switching elements as a neutral point, and a connection point between the second and third switching elements as an AC output terminal.

3. a seventh snubber diode and a fifth snubber capacitor connected in series between the cathode and anode of the fifth diode or between one end and the other end of the fifth switching element; a sixth snubber capacitor and an eighth snubber diode connected in series between the cathode and anode of the sixth diode or between one end and the other end of the sixth switching element; a seventh snubber resistor connected between the P terminal and a connection point between the seventh snubber diode and the fifth snubber capacitor; an eighth snubber resistor connected between the N terminal and a connection point between the sixth snubber capacitor and the eighth snubber diode; 3. The power conversion device according to claim 1, further comprising:

4. The power conversion device according to claim 1 or 2, wherein the resistance values of the first to fourth resistors and the resistance values of the first to fourth switching elements when they are off are related by the following equation (1). [Equation 1] RT1: Resistance value of the first resistor RT2: Resistance value of the second resistor RT3: Resistance value of the third resistor RT4: Resistance value of the fourth resistor Roff1: Resistance value of the first switching element when it is off Roff2: Resistance value of the second switching element when it is off Roff3: Resistance value of the third switching element when it is off Roff4: Resistance value of the fourth switching element when it is off

5. The power conversion device according to claim 1 or 2, wherein the resistance values of the first to fourth resistors and the resistance values of the first to fourth switching elements when they are off are related by the following equation (2). [Equation 2] RT1: Resistance value of the first resistor RT2: Resistance value of the second resistor RT3: Resistance value of the third resistor RT4: Resistance value of the fourth resistor Roff1: Resistance value of the first switching element when it is off Roff2: Resistance value of the second switching element when it is off Roff3: Resistance value of the third switching element when it is off Roff4: Resistance value of the fourth switching element when it is off

6. 3. The power conversion device according to claim 1, wherein the limit on-time during which the second and third switching elements are kept on simultaneously satisfies the following formula (4): [Equation 4] Vtm: Absolute maximum rated voltage of the first to fourth switching elements, the first to fourth snubber capacitors, and the first to sixth snubber diodes Vdc: Voltage between P and N terminals ΔVcsn: voltage drop across the second and third snubber capacitors when the second and third switching elements are kept on simultaneously t0m: limit on-time for keeping the second and third switching elements on simultaneously τ: Time constant of the fifth snubber resistor and the third snubber capacitor, and the sixth snubber resistor and the second snubber capacitor

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