Power Conversion Device

Abstract

To provide a power conversion device capable of suppressing reverse charge of a plurality of voltage-dividing capacitors while suppressing a circuit configuration from being complicated, in a power conversion device that includes a multilevel inverter circuit having a bi-directional switch circuit part connected with an intermediate potential point.SOLUTION: A power conversion device 100 comprises a power conversion unit 30 including a multilevel inverter circuit 30a of three or more levels. The multilevel inverter circuit 30a has: a switching element 31 connected with a positive electrode side; a switching element 32 connected with a negative electrode side; and a bi-directional switch circuit unit 35 connected between an intermediate potential point C1 and respective output sides of the switching element 31 and the switching element 32. A control unit 40 is configured to turn on a switching operation of the bi-directional switch circuit unit 35 in a case where a discharge circuit unit 10 makes capacitors 21 and 22 discharge their electric charges.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a power conversion device, and more particularly to a power conversion device including a discharge circuit unit. [Background technology]

[0002] Conventionally, an inverter (power conversion device) equipped with a discharge circuit is known (see, for example, Patent Document 1).

[0003] The inverter described in Patent Document 1 includes a three-level inverter circuit. In this inverter, a first voltage-dividing capacitor and a second voltage-dividing capacitor are connected in series between the positive and negative sides of a DC power supply. A three-level inverter circuit is connected to an intermediate potential point obtained by voltage division by the first and second voltage-dividing capacitors, a high potential point on the positive side of the DC power supply, and a low potential point on the negative side. Specifically, in this three-level inverter circuit, four switching elements are connected in series between the high potential point and the low potential point. A diode is connected in anti-parallel to each of the four switching elements. Two diodes connected in series are connected between a junction between the first and second of the four series-connected switching elements and a junction between the third and fourth of the four series-connected switching elements. The junction between these two diodes is connected to the intermediate potential point obtained by voltage division by the first and second voltage-dividing capacitors.

[0004] The inverter described in Patent Document 1 is also provided with a capacitor discharge circuit. This discharge circuit includes first and second discharge resistors, two discharge diodes, and a discharge switch. This discharge circuit is connected in parallel with two voltage-dividing capacitors between the positive and negative sides. In this discharge circuit, by closing the discharge switch, the first voltage-dividing capacitor is discharged by the first discharge resistor via one of the two discharge diodes, and the second voltage-dividing capacitor is discharged by the second discharge resistor via the other of the two discharge diodes. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2010-45975 Summary of the Invention [Problem to be solved by the invention]

[0006] Here, a three-level inverter, such as the inverter described in Patent Document 1, in which four switching elements are connected in series between the positive and negative poles of DC power and a diode is connected to an intermediate potential point (neutral point), is generally known as a diode-clamped three-level inverter circuit. Although not described in Patent Document 1, a T-type multilevel inverter circuit is known as a multilevel inverter circuit with three or more levels. In this T-type multilevel inverter circuit, an upper-arm switching element is connected to the positive pole of the input DC power, and a lower-arm switching element is connected to the negative pole of the input DC power. A bidirectional switch circuit is connected to the intermediate potential point, which is the neutral point. In the T-type multilevel inverter circuit, the switching operation of the bidirectional switch circuit connected to the intermediate potential point is controlled, thereby outputting the potential at the intermediate potential point in addition to the potentials of the positive and negative poles of the input DC power.

[0007] However, in a power conversion device including such a T-type multilevel inverter circuit, when discharging the charge of multiple voltage-dividing capacitors connected in series through multiple discharge resistors, as in the inverter described in Patent Document 1, multiple resistor elements are required, as well as diodes or switching elements to allow current to flow through each of the resistor elements, resulting in a complex circuit configuration. To avoid this complexity, a single discharge resistor element may be connected in parallel to the multiple voltage-dividing capacitors connected in series. In this case, if the voltage-dividing capacitors have different amounts of charge, discharging causes the output voltage of one voltage-dividing capacitor with a relatively small charge to reach zero first, while discharging from the other voltage-dividing capacitors with a relatively large charge continues. As a result, when discharging of one voltage-dividing capacitor with a relatively small charge is completed, discharging from the other voltage-dividing capacitors with a relatively large charge causes the one voltage-dividing capacitor with a relatively small charge to reverse charge. In consideration of the above points, it has been conventionally desirable to prevent a plurality of voltage-dividing capacitors from being reverse-charged while suppressing the complexity of the circuit configuration in a power conversion device including a multilevel inverter circuit in which a bidirectional switch circuit unit is connected to an intermediate potential point.

[0008] The present invention has been made to solve the above-mentioned problems, and one object of the present invention is to provide a power conversion device including a multilevel inverter circuit in which a bidirectional switch circuit unit is connected to an intermediate potential point, which is capable of preventing a plurality of voltage-dividing capacitors from being reverse-charged while suppressing an increase in the complexity of the circuit configuration. [Means for solving the problem]

[0009] In order to achieve the above object, a power conversion device according to one aspect of the present invention includes a power conversion unit including a plurality of voltage-dividing capacitors connected in series between a positive electrode side and a negative electrode side of input DC power and dividing a voltage of the input DC power, and a multilevel inverter circuit having three or more levels that performs power conversion operation to convert the DC power divided by the plurality of voltage-dividing capacitors into AC power, a discharge circuit unit that discharges electric charge in the plurality of voltage-dividing capacitors, and a control unit that controls the power conversion operation in the multilevel inverter circuit of the power conversion unit, wherein the multilevel inverter circuit of the power conversion unit has an upper-arm switching element connected to a positive electrode side, a lower-arm switching element connected to a negative electrode side, and a bidirectional switch circuit unit connected between an intermediate potential point formed by voltage division by the plurality of voltage-dividing capacitors and each output side of the upper-arm switching element and the lower-arm switching element, and the control unit is configured to turn on the switching operation of the bidirectional switch circuit unit when the discharge circuit unit is caused to discharge electric charge in the plurality of voltage-dividing capacitors.

[0010] In a power conversion device according to one aspect of the present invention, as described above, the control unit is configured to turn on the switching operation of the bidirectional switch circuit when discharging charge from the multiple voltage-dividing capacitors using the discharge circuit unit. By turning on the bidirectional switch circuit, a closed circuit can be formed so that a discharge current flows from the voltage-dividing capacitors to the discharge circuit unit via the bidirectional switch circuit. Therefore, when the multiple voltage-dividing capacitors have different amounts of charge, turning on the switching operation of the bidirectional switch circuit allows the discharge current to flow to the discharge circuit unit without passing through a voltage-dividing capacitor with a relatively small charge capacity among the multiple voltage-dividing capacitors, without providing multiple discharge resistor elements corresponding to each of the multiple voltage-dividing capacitors. Furthermore, because the charge from a voltage-dividing capacitor with a relatively large charge capacity can be discharged through the bidirectional switch circuit without passing through a voltage-dividing capacitor with a relatively small charge capacity, reverse charging of the voltage-dividing capacitor with a relatively small charge capacity can be prevented. As a result, in a power conversion device including a multilevel inverter circuit in which the bidirectional switch circuit is connected to an intermediate potential point, reverse charging of the multiple voltage-dividing capacitors can be prevented without increasing the complexity of the circuit configuration.

[0011] In the power conversion device according to the above aspect, the control unit is preferably configured to stop the power conversion operation of the power conversion unit, cause the discharge circuit unit to discharge the charge of the multiple voltage-dividing capacitors, and turn on the switching operation of the bidirectional switch circuit unit when a difference between the output voltages of the multiple voltage-dividing capacitors is greater than a predetermined stop threshold. With this configuration, when a difference between the charge amounts (output voltages) of the multiple voltage-dividing capacitors is greater than the predetermined stop threshold, the control unit can turn on the switching operation of the bidirectional switch circuit unit so as to discharge the charge stored in each of the multiple voltage-dividing capacitors. Therefore, when there is variation in the charge amounts of the multiple voltage-dividing capacitors, the control process by the control unit can effectively prevent the multiple voltage-dividing capacitors from being reversely charged while effectively suppressing the complexity of the circuit configuration.

[0012] In the power conversion device according to the above aspect, the control unit is preferably configured to turn on the switching operation of the bidirectional switch circuit unit when the output voltage of any of the plurality of voltage-dividing capacitors becomes smaller than a predetermined discharge completion threshold by discharging the charge of the plurality of voltage-dividing capacitors using the discharge circuit unit. With this configuration, the period during which discharge current flows from the plurality of voltage-dividing capacitors to the bidirectional switch circuit unit can be shortened compared to when the switching operation of the bidirectional switch circuit unit is turned on simultaneously with the start of discharge by the discharge circuit unit. Therefore, the burden on the bidirectional switch circuit unit can be reduced when discharging the plurality of voltage-dividing capacitors.

[0013] In the power conversion device according to the above aspect, the bidirectional switch circuit preferably includes a pair of intermediate potential switching elements connected in parallel or in series to have opposite polarities, and the control unit is configured to turn on the switching operation of both of the pair of intermediate potential switching elements when the discharge circuit discharges the charge of the multiple voltage-dividing capacitors. Here, the direction of the discharge current flowing through the bidirectional switch circuit differs depending on which of the multiple voltage-dividing capacitors has a relatively large charge amount. Therefore, by configuring the control unit to turn on the switching operation of both of the pair of intermediate potential switching elements when the discharge circuit discharges the charge of the multiple voltage-dividing capacitors, it is possible to reduce the complexity of the control process compared to when the control unit controls the switching operation of each of the pair of intermediate potential switching elements of the bidirectional switch circuit in accordance with the direction of the discharge current flowing through the bidirectional switch circuit based on the output voltage of each of the multiple voltage-dividing capacitors. As a result, it is possible to reduce the complexity of the control process caused by preventing the multiple voltage-dividing capacitors from being reversely charged.

[0014] In the power conversion device according to the above aspect, the discharge circuit unit preferably includes a discharge resistor element connected in parallel with the plurality of voltage-dividing capacitors connected in series between the positive and negative sides of the input DC power, and a discharge switch unit that switches the conduction of the discharge circuit unit, and the control unit is configured to turn on the switching operation of the bidirectional switch circuit unit when turning on the discharge switch unit to discharge the charge of the plurality of voltage-dividing capacitors by the discharge circuit unit. With this configuration, the charge of the plurality of voltage-dividing capacitors can be discharged by the discharge resistor element provided in common to the plurality of voltage-dividing capacitors connected in series. Therefore, the discharge circuit unit has a simple circuit configuration including the discharge resistor element and the discharge switch unit common to the plurality of voltage-dividing capacitors, making it possible to prevent reverse charging of the plurality of voltage-dividing capacitors while minimizing the complexity of the circuit configuration.

[0015] In the power conversion device according to the above aspect, preferably, a plurality of bidirectional switch circuits are provided so as to be connected to a plurality of intermediate potential points obtained by voltage division by a plurality of voltage-dividing capacitors, and the control unit is configured to turn on the switching operation of the plurality of bidirectional switch circuits when the discharge circuit unit discharges the charges of the plurality of voltage-dividing capacitors based on the output voltage of each of the plurality of voltage-dividing capacitors. With this configuration, even in a case where the power conversion device includes a multilevel inverter circuit having more than three levels and having a plurality of intermediate potential points, turning on the switching operation of the plurality of bidirectional switch circuits can prevent the plurality of voltage-dividing capacitors from being back-charged. Therefore, even in a case where the power conversion unit includes a multilevel inverter circuit having more than three levels, it is possible to prevent the plurality of voltage-dividing capacitors from being back-charged while suppressing the complexity of the circuit configuration. [Effects of the Invention]

[0016] According to the present invention, as described above, in a power conversion device including a multilevel inverter circuit in which a bidirectional switch circuit unit is connected to an intermediate potential point, it is possible to prevent a plurality of voltage-dividing capacitors from being reverse-charged while suppressing an increase in the complexity of the circuit configuration. [Brief explanation of the drawings]

[0017] [Figure 1] 1 is a block diagram showing the overall configuration of a power conversion device according to a first embodiment. [Figure 2] 1 is a circuit diagram showing a power conversion device according to a first embodiment. [Figure 3] 10A and 10B are diagrams for explaining the discharge of electric charge from a positive-side capacitor by a discharge circuit section. [Figure 4] 10 is a diagram for explaining the discharge of electric charge from a negative-side capacitor by a discharge circuit unit. FIG. [Figure 5] FIG. 3 is a flowchart illustrating a discharge control method for the power conversion device according to the first embodiment. [Figure 6] FIG. 4 is a block diagram showing the overall configuration of a power conversion device according to a second embodiment of the present invention. [Figure 7] FIG. 4 is a circuit diagram showing a power conversion device according to a second embodiment. [Figure 8] 10A and 10B are circuit diagrams showing the configuration of a bidirectional switch circuit unit according to first and second modified examples, where FIG. 10A is a circuit diagram showing the configuration of a bidirectional switch circuit unit according to a first modified example, and FIG. 10B is a circuit diagram showing the configuration of a bidirectional switch circuit unit according to a second modified example. [Figure 9] FIG. 10 is a circuit diagram showing the configuration of a power conversion device according to a third modified example. DETAILED DESCRIPTION OF THE INVENTION

[0018] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, specific embodiments of the present invention will be described with reference to the accompanying drawings.

[0019] [First embodiment] The configuration of a power conversion device 100 according to a first embodiment of the present invention will be described with reference to FIGS.

[0020] (Configuration of power conversion device) As shown in FIGS. 1 and 2, a power conversion device 100 converts DC power input from a DC power supply 101 into AC power and outputs the converted AC power to a load 102. The power conversion device 100 is, for example, a power supply device for driving a motor mounted on a railway vehicle. That is, the load 102 is a motor that provides power for driving the vehicle. The load 102 operates using three-phase (U-phase, V-phase, and W-phase) AC power output from the power conversion device 100. Specifically, the power conversion device 100 converts the DC power input from the DC power supply 101 into three-phase AC power. Note that in the power conversion device 100, components for outputting the U-phase, V-phase, and W-phase AC power are connected in parallel. In the following description and drawings, only the component for outputting the U-phase AC power of the power conversion device 100 will be described using the drawings. The components for outputting the V-phase and W-phase AC power are similar to the components for outputting the U-phase AC power, and therefore will not be described here.

[0021] The power conversion device 100 includes a discharge circuit unit 10, a capacitor 21, a capacitor 22, a power conversion unit 30, and a control unit 40. In the first embodiment, the power conversion unit 30 includes a three-level multilevel inverter circuit 30a. The capacitors 21 and 22 are examples of "multiple voltage dividing capacitors" in the claims.

[0022] The discharge circuit section 10 is connected between the positive and negative sides of DC power input from the DC power supply 101. That is, the discharge circuit section 10 is connected between a connection point P on the positive side and a connection point N on the negative side. The discharge circuit section 10 also has a resistance element 11 and a switch section 12. In the discharge circuit section 10, the resistance element 11 and the switch section 12 are connected in series between the positive and negative sides. The resistance element 11 is an example of a "discharge resistance element" in the claims. The switch section 12 is an example of a "discharge switch section" in the claims.

[0023] In the first embodiment, the resistance element 11 is connected in parallel with a plurality of capacitors 21 and 22 that are connected in series with each other between the positive and negative sides of the DC power input from the DC power supply 101. The switch unit 12 switches between on and off based on a signal from the control unit 40, thereby switching the conduction of the discharge circuit unit 10. The switch unit 12 is, for example, a switching element such as an IGBT (Insulated Gate Bipolar Transistor). The switch unit 12 may also be a mechanical relay element.

[0024] In the first embodiment, the discharge circuit section 10 is provided to discharge the electric charges of the capacitors 21 and 22. The discharge of the electric charges of the capacitors 21 and 22 by the discharge circuit section 10 will be described in detail later.

[0025] In the first embodiment, capacitor 21 and capacitor 22 are connected in series with each other between the positive and negative sides of DC power input from DC power supply 101. In detail, capacitor 21 has one end connected to connection point P on the positive side and the other end connected to intermediate potential point C1. Capacitor 22 has one end connected to intermediate potential point C1 and the other end connected to connection point N on the negative side. Capacitor 21 and capacitor 22 are, for example, electrolytic capacitors.

[0026] In the first embodiment, the capacitors 21 and 22 divide the voltage of the input DC power. Specifically, the capacitors 21 and 22 have approximately the same capacitance. The output voltages of the capacitors 21 and 22 are approximately the same. That is, the intermediate potential point C1, which is the connection point between the capacitors 21 and 22, is a neutral point from which a voltage (potential) approximately half the magnitude of the input DC power is output. That is, the power conversion device 100 of the first embodiment is configured to have three output potentials: the potential of the connection point N, which is the potential on the negative side of the DC power supply 101, the potential of the intermediate potential point C1, and the potential of the connection point P, which is the potential on the positive side of the DC power supply 101. The capacitors 21 and 22 also function as smoothing capacitors that smooth the input DC power.

[0027] In the first embodiment, the multilevel inverter circuit 30a performs a power conversion operation of converting DC power divided by the capacitors 21 and 22 into AC power. The multilevel inverter circuit 30a includes a switching element 31, a switching element 32, a diode 33, a diode 34, and a bidirectional switch circuit unit 35. The switching element 31 is an example of an "upper-arm switching element" in the claims. The switching element 32 is an example of a "lower-arm switching element" in the claims.

[0028] In the first embodiment, the switching element 31 is connected between a connection point P on the positive electrode side and a connection point O on the output side. The switching element 32 is connected between a connection point N on the negative electrode side and a connection point O on the output side. The switching elements 31 and 32 are MOSFETs (metal-oxide-semiconductor field-effect transistors). Specifically, the drain terminal of the switching element 31 is connected to the connection point P, and the source terminal is connected to the connection point O. The drain terminal of the switching element 32 is connected to the connection point O, and the source terminal is connected to the connection point P. A gate signal is input to each gate terminal of the switching element 31 and the switching element 32 from a gate driver unit (not shown) under the control of the control unit 40. That is, the switching operations of the switching elements 31 and 32 are controlled by the control unit 40.

[0029] Furthermore, diodes 33 and 34 are connected in anti-parallel to switching element 31 and switching element 32, respectively. Specifically, the cathode side of diode 33 is connected to the drain terminal (connection point P side) of switching element 31, and the anode side is connected to the source terminal (connection point O side) of switching element 31. The cathode side of diode 34 is connected to the drain terminal (connection point O side) of switching element 32, and the anode side is connected to the source terminal (connection point N side) of switching element 32. Diodes 33 and 34 are provided as freewheeling diodes for switching elements 31 and 32. Note that, because MOSFETs have parasitic diodes (body diodes), it is not necessary to provide anti-parallel diodes 33 and 34 for switching elements 31 and 32, respectively.

[0030] In the first embodiment, the bidirectional switch circuit 35 is connected between an intermediate potential point C1, which is voltage-divided by the capacitors 21 and 22, and a connection point O, which is the output side of each of the switching elements 31 and 32. The bidirectional switch circuit 35 performs a switching operation under the control of the control unit 40. That is, the bidirectional switch circuit 35 is configured to switch between an ON state in which a current flows in both directions between the intermediate potential point C1 and the connection point O, and an OFF state in which a current is cut off between the intermediate potential point C1 and the connection point O.

[0031] Specifically, the bidirectional switch circuit unit 35 has a switching element 36, a switching element 37, a diode 38, and a diode 39. The switching elements 36 and 37 are an example of the "pair of intermediate potential switching elements" in the claims.

[0032] In the first embodiment, the switching elements 36 and 37 of the bidirectional switch circuit 35 are connected in series to have opposite polarities. The switching elements 36 and 37 are, for example, MOSFETs. Specifically, the drain terminal of the switching element 36 is connected to the intermediate potential point C1. The drain terminal of the switching element 37 is connected to the connection point O. The source terminals of the switching elements 36 and 37 are connected to each other. Similar to the switching elements 31 and 32, the switching operations of the switching elements 36 and 37 are controlled by the control unit 40 when gate signals are input to their gate terminals from a gate driver unit (not shown) based on the control of the control unit 40.

[0033] Similarly to the switching elements 31 and 32, diodes 38 and 39 are connected in anti-parallel to the switching elements 36 and 37, respectively. Specifically, the cathode of diode 38 is connected to the drain terminal (near midpoint C1) of switching element 36, and the anode is connected to the source terminal (near switching element 37) of switching element 36. The cathode of diode 39 is connected to the drain terminal (near node O) of switching element 37, and the anode is connected to the source terminal (near switching element 36) of switching element 37. Similar to diodes 33 and 34, diodes 38 and 39 are provided as freewheeling diodes for switching elements 36 and 37. Similarly, because MOSFETs have parasitic diodes (body diodes), it is not necessary to provide anti-parallel diodes 38 and 39 for switching elements 36 and 37, respectively.

[0034] The control unit 40 controls the power conversion operation of the multilevel inverter circuit 30a of the power conversion unit 30. For example, the control unit 40 is a microcomputer (microcontroller) including a central processing unit (CPU) and a storage device such as a flash memory. The control unit 40 controls the power conversion operation of the multilevel inverter circuit 30a by executing a program stored in the storage device based on a speed command value input from a vehicle-side control device (not shown). Specifically, the control unit 40 outputs AC power by controlling the switching operations of switching elements 31, 32, 36, and 37 via a gate driver unit (not shown). The control unit 40 controls the power conversion operation of each of the multilevel inverter circuits 30a for the U, V, and W phases.

[0035] Furthermore, the control unit 40 acquires the output voltages of the capacitors 21 and 22. Specifically, the control unit 40 acquires the output voltages of the capacitors 21 and 22 based on the output from a voltmeter (not shown). Then, the control unit 40 controls the power conversion operation of the multilevel inverter circuit 30a so that the output voltages of the capacitors 21 and 22 are approximately equal in magnitude.

[0036] <Discharge control by the control unit> In the first embodiment, the control unit 40 stops the power conversion operation of the power conversion unit 30 (multilevel inverter circuit 30a) when the difference between the output voltages of the capacitors 21 and 22 is greater than a predetermined stop threshold value that is set in advance. The stop threshold value is stored in advance in a storage device of the control unit 40. The stop threshold value is set based on the withstand voltages of the switching elements 31, 32, 36, and 37 so that the switching elements 31, 32, 36, and 37 do not perform switching operations when a voltage exceeding the withstand voltage is applied.

[0037] In the first embodiment, the control unit 40 is configured to stop the operation of the power conversion unit 30 when the difference between the output voltages of the capacitors 21 and 22 is greater than a predetermined stop threshold, and then cause the discharge circuit unit 10 to discharge the charges of the capacitors 21 and 22. Specifically, the control unit 40 turns on the switch unit 12 of the discharge circuit unit 10, thereby turning on the conduction of the discharge circuit unit 10. Then, the control unit 40 causes the discharge circuit unit 10 to discharge the charges stored in the capacitors 21 and 22 by causing a current (discharge current) from the capacitors 21 and 22 to flow through the resistance element 11. Note that before turning on the conduction of the discharge circuit unit 10, the control unit 40 disconnects the DC power supply 101 from the power conversion device 100 using a switch (not shown).

[0038] Furthermore, in the first embodiment, the control unit 40 is configured to turn on the switching operation of the bidirectional switch circuit unit 35 by turning on the switching operations of both switching elements 36 and 37 when discharging the charges of the capacitors 21 and 22 using the discharge circuit unit 10. Specifically, the control unit 40 is configured to turn on the switching operation of the bidirectional switch circuit unit 35 when discharging the charges of the capacitors 21 and 22 using the discharge circuit unit 10 causes the output voltage of either of the capacitors 21 and 22 to become smaller than a predetermined discharge completion threshold value that is stored in advance. The discharge completion threshold value is a threshold value of the output voltage that indicates that discharging has been completed in each of the capacitors 21 and 22, and is set to a magnitude of approximately zero.

[0039] For example, if the output voltage of capacitor 21 on the positive side (connection point P side) of capacitors 21 and 22 is larger (has a larger amount of charge stored therein) than capacitor 22 on the negative side (connection point N side), then due to discharging by discharge circuit unit 10, capacitor 22, which has a smaller amount of charge stored therein, will complete discharging before capacitor 21. That is, the output voltage of capacitor 22 will become smaller than the predetermined discharge completion threshold value before capacitor 21 does. The control unit 40 turns on the switching operation of bidirectional switch circuit unit 35 based on the fact that the output voltage of capacitor 22 is smaller than the discharge completion threshold value.

[0040] 3, a circuit is formed in which a discharge current flows from capacitor 21 through connection point P, discharge circuit section 10, connection point N, diode 34, connection point O, bidirectional switch circuit section 35, and intermediate potential point C1 in this order. As a result, the discharge current that discharges the charge from capacitor 21 is discharged by discharge circuit section 10 without flowing into capacitor 22, which has already completed discharging, so as to reverse charge it.

[0041] 4, when the output voltage of the negative-side capacitor 22 is higher than that of the positive-side capacitor 21 and the charge stored in the capacitor 22 is greater than that of the capacitor 21, the discharge of the capacitor 21 is completed before the discharge of the capacitor 22. In this case, the switching operation of the bidirectional switch circuit 35 is turned on, thereby forming a circuit through which a discharge current flows from the capacitor 22 in the following order: intermediate potential point C1, bidirectional switch circuit 35, node O, diode 33, node P, discharge circuit 10, and node N. Then, just as in the case where the output voltage of the capacitor 21 is higher than that of the capacitor 22, the discharge current that discharges the charge from the capacitor 22 is discharged by the discharge circuit 10 without flowing into the capacitor 21, which has already completed discharging.

[0042] (Discharge control method for power conversion device according to first embodiment) Next, a discharge control method for the power conversion device 100 by the control unit 40 of the first embodiment will be described with reference to Fig. 5. This discharge control method is executed by a control process by the control unit 40. Furthermore, the control process of this discharge control method is executed at predetermined determination intervals (calculation cycles) during a period in which the operation of the power conversion unit 30 continues.

[0043] First, in step S1, the output voltages of the capacitors 21 and 22 are obtained. Specifically, the output voltages (voltages across both ends) of the capacitors 21 and 22 are measured by a voltmeter (not shown). Then, the output voltages of the capacitors 21 and 22 are obtained based on the outputs from the voltmeters.

[0044] Next, in step S2, it is determined whether the difference in the output voltages of capacitors 21 and 22 is greater than a predetermined stop threshold. If it is determined that the difference in the output voltages of capacitors 21 and 22 is greater than the stop threshold, the process proceeds to step S3. If it is not determined that the difference in the output voltages of capacitors 21 and 22 is greater than the stop threshold, the control process of the discharge control method is terminated.

[0045] In step S3, the power conversion operation by the multilevel inverter circuit 30a of the power conversion unit 30 is stopped. Specifically, the switching operations of the switching elements 31, 32, 36, and 37 are stopped (turned off). That is, the gate signals input from the gate driver unit (not shown) to the gate terminals of the switching elements 31, 32, 36, and 37 are stopped. Then, the conduction in each of the switching elements 31, 32, 36, and 37 is interrupted.

[0046] Next, in step S4, the conduction of the discharge circuit section 10 is turned on. Specifically, the switch section 12 of the discharge circuit section 10 is turned on. This starts the discharge of the electric charge stored in the capacitors 21 and 22. The electric charge in the capacitors 21 and 22 is output as a discharge current and converted into thermal energy by the resistance element 11 of the discharge circuit section 10, thereby discharging.

[0047] Next, in step S5, it is determined whether the output voltage of either of capacitors 21 and 22 is smaller than a predetermined discharge completion threshold. If it is determined that the output voltage is smaller than the discharge completion threshold, the process proceeds to step S6. If it is not determined that the output voltage is smaller than the discharge completion threshold, the control process in step S5 is repeated at predetermined calculation intervals.

[0048] In step S6, the switching operation of the bidirectional switch circuit unit 35 is turned on. Specifically, the switching operations of the switching elements 36 and 37 are turned on. That is, a gate driver unit (not shown) outputs a gate signal to each gate terminal of the switching elements 36 and 37 to turn on the switching operation.

[0049] Then, the control process in the method for controlling the discharge of the electric charges of the capacitors 21 and 22 is terminated. For example, when both of the output voltages of the capacitors 21 and 22 become smaller than the discharge completion threshold, the discharge circuit unit 10 and the bidirectional switch circuit unit 35 are turned off, thereby terminating the control process. Alternatively, the control process may be terminated when a predetermined time has elapsed since the bidirectional switch circuit unit 35 was turned on.

[0050] [Effects of the first embodiment] In the first embodiment, the following effects can be obtained.

[0051] In the first embodiment, as described above, the control unit 40 is configured to turn on the switching operation of the bidirectional switch circuit unit 35 when discharging the charge of the capacitors 21 and 22 (plurality of voltage-dividing capacitors) using the discharge circuit unit 10. As a result, by making the bidirectional switch circuit unit 35 conductive, a closed circuit can be formed so that a discharge current flows from the capacitors 21 and 22 to the discharge circuit unit 10 via the bidirectional switch circuit unit 35. Therefore, when the amounts of charge stored in the capacitors 21 and 22 are different, by turning on the switching operation of the bidirectional switch circuit unit 35, a discharge current can be passed to the discharge circuit unit 10 without passing through one of the capacitors 21 and 22 with a relatively small charge amount, without providing multiple discharge resistance elements corresponding to each of the capacitors 21 and 22. Furthermore, because the charge from a capacitor with a relatively large charge amount can be discharged via the bidirectional switch circuit unit 35 without passing through a capacitor with a relatively small charge amount, reverse charging of the capacitor with a relatively small charge amount can be prevented. As a result, in the power conversion device 100 including the multilevel inverter circuit 30a in which the bidirectional switch circuit part 35 is connected to the intermediate potential point C1, it is possible to prevent the capacitors 21 and 22 from being reversely charged while suppressing the circuit configuration from becoming complicated.

[0052] Furthermore, in the first embodiment, as described above, the control unit 40 is configured to stop the power conversion operation of the power conversion unit 30 (multilevel inverter circuit 30a), cause the discharge circuit unit 10 to discharge the charge of the capacitors 21 and 22, and turn on the switching operation of the bidirectional switch circuit unit 35, when the difference between the output voltages of the capacitors 21 and 22 (multiple voltage-dividing capacitors) is greater than a predetermined stop threshold. As a result, when the difference between the charge amounts (output voltages) of the capacitors 21 and 22 is greater than the predetermined stop threshold, the control unit 40 can turn on the switching operation of the bidirectional switch circuit unit 35 so as to discharge the charge stored in each of the capacitors 21 and 22. Therefore, when there is a difference in the charge amounts of the capacitors 21 and 22, the control processing by the control unit 40 can effectively prevent the capacitors 21 and 22 from being reversely charged while effectively suppressing the complexity of the circuit configuration.

[0053] Furthermore, in the first embodiment, as described above, the control unit 40 is configured to turn on the switching operation of the bidirectional switch circuit unit 35 when the output voltage of either of the capacitors 21 and 22 becomes smaller than a predetermined discharge completion threshold by discharging the charge of the capacitors 21 and 22 (plurality of voltage dividing capacitors) using the discharge circuit unit 10. This makes it possible to shorten the period during which the discharge current from the capacitors 21 and 22 flows to the bidirectional switch circuit unit 35, compared to when the switching operation of the bidirectional switch circuit unit 35 is turned on at the same time as the start of discharge by the discharge circuit unit 10. Therefore, it is possible to reduce the load on the bidirectional switch circuit unit 35 when discharging the capacitors 21 and 22.

[0054] Furthermore, in the first embodiment, as described above, the bidirectional switch circuit 35 includes switching elements 36 and 37 (a pair of intermediate potential switching elements) connected in series to have opposite polarities to each other, and the control unit 40 is configured to turn on the switching operations of both switching elements 36 and 37 when the discharge circuit 10 is caused to discharge the charge of the capacitors 21 and 22 (multiple voltage-dividing capacitors). Here, the direction of the discharge current flowing through the bidirectional switch circuit 35 differs depending on which of the capacitors 21 and 22 has a relatively large charge amount. Therefore, by configuring the control unit 40 to turn on the switching operations of both switching elements 36 and 37 when the discharge circuit 10 is caused to discharge the charge of the capacitors 21 and 22, it is possible to prevent the control process from becoming more complex than when the control unit 40 controls the switching operations of the switching elements 36 and 37 of the bidirectional switch circuit 35 in accordance with the direction of the discharge current flowing through the bidirectional switch circuit 35 based on the output voltages of the capacitors 21 and 22. As a result, it is possible to prevent the control process from becoming more complex due to the prevention of reverse charging of the capacitors 21 and 22.

[0055] Furthermore, in the first embodiment, as described above, the discharge circuit section 10 includes a resistance element 11 (discharge resistance element) connected in parallel with the capacitors 21 and 22 (multiple voltage-dividing capacitors) connected in series between the positive and negative sides of the input DC power, and a switch section 12 (discharge switch section) that switches the conduction of the discharge circuit section 10. The control section 40 is configured to turn on the switching operation of the bidirectional switch circuit section 35 when turning on the switch section 12 to cause the discharge circuit section 10 to discharge the charges of the capacitors 21 and 22. This allows the charges of the capacitors 21 and 22 to be discharged by the resistance element 11 provided in common to the capacitors 21 and 22 connected in series. Therefore, the discharge circuit section 10 has a simple circuit configuration including the resistance element 11 and switch section 12 common to the capacitors 21 and 22, making it possible to prevent the capacitors 21 and 22 from being reversely charged while minimizing the complexity of the circuit configuration.

[0056] [Second embodiment] Next, the configuration of a power conversion device 200 according to a second embodiment will be described with reference to Figures 6 and 7. Unlike the first embodiment, which illustrates an example of a three-level inverter in which the potential between the positive and negative sides of the input DC power is divided into approximately half by two capacitors 21 and 22, the second embodiment illustrates an example of a five-level inverter in which the potential between the positive and negative sides of the input DC power is divided into approximately four equal parts. Note that the same components as those in the first embodiment are denoted by the same reference numerals, and their description will be omitted.

[0057] (Configuration of power conversion device according to second embodiment) 6 and 7, similar to the power conversion device 100 according to the first embodiment, the power conversion device 200 converts DC power input from a DC power supply 101 into three-phase (U-phase, V-phase, and W-phase) AC power and outputs the converted AC power to a load 102. Note that, similar to the power conversion device 100 according to the first embodiment, in the power conversion device 200, components for outputting AC power of the U-phase, V-phase, and W-phase are connected in parallel. In the following description and drawings, similar to the first embodiment, only the component for outputting U-phase AC power of the power conversion device 200 will be described using the drawings. The components for outputting V-phase and W-phase AC power are similar to the component for outputting U-phase AC power, and therefore description thereof will be omitted.

[0058] The power conversion device 200 according to the second embodiment includes a discharge circuit unit 10, capacitors 221, 222, 223, and 224, a power conversion unit 230, and a control unit 240. The configuration of the discharge circuit unit 10 is the same as that of the first embodiment. In the second embodiment, the power conversion unit 230 includes a five-level multilevel inverter circuit 230a. The capacitors 221, 222, 223, and 224 are examples of "plurality of voltage dividing capacitors" in the claims.

[0059] In the second embodiment, the capacitors 221 to 224 are connected in series with each other between the positive and negative sides of the DC power input from the DC power supply 101. In detail, one end of the capacitor 221 is connected to the connection point P on the positive side, and the other end is connected to the intermediate potential point C201. Furthermore, one end of the capacitor 222 is connected to the intermediate potential point C201, and the other end is connected to the intermediate potential point C202. Furthermore, one end of the capacitor 223 is connected to the intermediate potential point C202, and the other end is connected to the intermediate potential point C203. Furthermore, one end of the capacitor 224 is connected to the intermediate potential point C203, and the other end is connected to the connection point N.

[0060] That is, in the second embodiment, the input DC power is divided into four stages by the capacitors 221 to 224. The capacitors 221 to 224 are electrolytic capacitors having approximately the same capacitance. In the second embodiment, a plurality of (three) intermediate potential points C201, C202, and C203 are provided so that the potential difference between the positive and negative sides of the input DC power is divided into approximately four equal parts. That is, the power conversion device 200 of the second embodiment is configured to have five output potential stages (five levels): the potential of the connection point N, which is the potential on the negative side of the DC power supply 101, the potential of the intermediate potential point C203, the potential of the intermediate potential point C202, the potential of the intermediate potential point C201, and the potential of the connection point P, which is the potential on the positive side of the DC power supply 101. Furthermore, as in the first embodiment, the capacitors 221 to 224 also function as smoothing capacitors that smooth the input DC power.

[0061] In the second embodiment, the multilevel inverter circuit 230a includes a switching element 231a, a switching element 231b, a switching element 232a, a switching element 232b, a diode 233a, a diode 233b, a diode 234a, a diode 234b, a bidirectional switch circuit unit 235a, a bidirectional switch circuit unit 235b, and a bidirectional switch circuit unit 235c. The switching elements 231a and 231b are an example of the "upper-arm switching elements" in the claims. The switching elements 232a and 232b are an example of the "lower-arm switching elements" in the claims.

[0062] The switching elements 231a, 231b, 232a, and 232b have the same configuration as the switching elements 31 and 32 of the first embodiment. The switching elements 231a, 231b, 232a, and 232b are connected in series in this order, with the drain terminal connected to the positive electrode side and the source terminal connected to the negative electrode side. Specifically, the drain terminal of the switching element 231a is connected to the connection point P, and the source terminal is connected to the connection point O201. The drain terminal of the switching element 231b is connected to the connection point O201, and the source terminal is connected to the connection point O202. The drain terminal of the switching element 232a is connected to the connection point O202, and the source terminal is connected to the connection point O203. The drain terminal of the switching element 232b is connected to the connection point O203, and the source terminal is connected to the connection point N.

[0063] The diodes 233a, 233b, 234a, and 234b are connected in antiparallel to the switching elements 231a, 231b, 232a, and 232b, respectively. That is, the diodes 233a, 233b, 234a, and 234b are provided as freewheeling diodes, similar to the diodes 33 and 34 in the first embodiment.

[0064] In the second embodiment, a plurality of (three) bidirectional switch circuits 235a, 235b, and 235c are provided so as to be connected to three intermediate potential points C201 to C203, respectively, which are voltage-divided by four capacitors 221 to 224. Specifically, one end of the bidirectional switch circuit 235a is connected to the intermediate potential point C201, and the other end is connected to the connection point O201. Furthermore, one end of the bidirectional switch circuit 235b is connected to the intermediate potential point C202, and the other end is connected to the connection point O202. Furthermore, one end of the bidirectional switch circuit 235c is connected to the intermediate potential point C203, and the other end is connected to the connection point O203. Note that the specific configuration of each of the bidirectional switch circuits 235a, 235b, and 235c is the same as the configuration of the bidirectional switch circuit 35 according to the first embodiment.

[0065] The control unit 240, like the control unit 40 of the first embodiment, controls the power conversion operation in the multilevel inverter circuit 230a of the power conversion unit 230. Also, like the control unit 40 of the first embodiment, the control unit 240 is a microcomputer including a CPU and a storage device such as a flash memory. The control unit 240 outputs AC power by controlling the switching operations of the switching elements 231a, 231b, 232a, and 232b and each of the switching elements of the bidirectional switch circuit units 235a to 235c.

[0066] Furthermore, the control unit 240 acquires the output voltages of the plurality of capacitors 221 to 224, and controls the power conversion operation of the multilevel inverter circuit 230a so that the output voltages of the plurality of capacitors 221 to 224 are approximately equal in magnitude.

[0067] In the second embodiment, the control unit 240 controls the discharge of the capacitors 221-224 of the power conversion device 200, similar to the control unit 40 in the first embodiment. Specifically, the control unit 240 stops the power conversion operation of the power conversion unit 230 (multilevel inverter circuit 230a) when the difference between the output voltages of the capacitors 221-224 is greater than a predetermined stop threshold value that has been set in advance. Then, the control unit 240 causes the discharge circuit unit 10 to discharge the electric charge of each of the plurality of capacitors 221-224.

[0068] In the second embodiment, the control unit 240 is configured to turn on the switching operations of the plurality of bidirectional switch circuit units 235a to 235c when the discharge circuit unit 10 discharges the electric charge of each of the plurality of capacitors 221 to 224 based on the output voltage of each of the plurality of capacitors 221 to 224.

[0069] Specifically, the control unit 240 is configured to turn on the switching operations of all three bidirectional switch circuits 235a to 235c when the output voltage of any one of the capacitors 221 to 224 becomes smaller than a predetermined discharge completion threshold value while the discharge circuit unit 10 is discharging the electric charge of the plurality of capacitors 221 to 224. The discharge completion threshold value is preset to approximately zero, as in the first embodiment.

[0070] The other configurations of the second embodiment are the same as those of the first embodiment.

[0071] [Effects of the second embodiment] In the second embodiment, the following effects can be obtained.

[0072] In the second embodiment, as described above, a plurality of bidirectional switch circuits 235a, 235b, and 235c are provided so as to be connected to a plurality of intermediate potential points C201, C202, and C203, respectively, which are voltage-divided by the capacitors 221, 222, 223, and 224 (a plurality of voltage-dividing capacitors), and the control unit 240 is configured to turn on the switching operation of the plurality of bidirectional switch circuits 235a to 235c when discharging the charge of the capacitors 221 to 224 using the discharge circuit unit 10, based on the output voltage of each of the capacitors 221 to 224. As a result, even in the case of a multilevel inverter circuit having more than three levels and provided with a plurality of intermediate potential points C201 to C203, it is possible to prevent the capacitors 221 to 224 from being reversely charged by turning on the switching operation of the plurality of bidirectional switch circuits 235a to 235c. Therefore, even when the power conversion unit 230 includes a multilevel inverter circuit 230a having a higher level than a three-level inverter, it is possible to prevent the capacitors 221 to 224 from being reversely charged while suppressing the circuit configuration from becoming complicated.

[0073] Other effects of the second embodiment are similar to those of the first embodiment.

[0074] [Variations] The embodiments disclosed herein should be considered to be illustrative and not restrictive in all respects. The scope of the present invention is defined by the claims rather than the above description of the embodiments, and further includes all modifications (variations) within the meaning and scope of the claims.

[0075] First and second modified examples For example, in the first and second embodiments described above, the bidirectional switch circuit unit 35 (235a to 235c) includes switching elements 36 and 37 (a pair of intermediate potential switching elements) connected in series to have mutually opposite polarities, but the present invention is not limited to this. In the present invention, the bidirectional switch circuit unit 35 may include switching elements 336 and 337, 436 and 437 (intermediate potential switching elements) connected in parallel to have mutually opposite polarities, as in the bidirectional switch circuit units 335 and 435 according to first and second modifications shown in Figures 8a and 8b.

[0076] Specifically, as shown in FIG. 8A, in bidirectional switch circuit unit 335, switching elements 336 and 337 are connected in parallel to have opposite polarities. These switching elements 336 and 337 are, for example, reverse-blocking IGBTs. A reverse-blocking IGBT is an IGBT that has a withstand voltage not only in the forward direction but also in the reverse direction. Also, as shown in FIG. 8B, in bidirectional switch circuit unit 435, switching elements 436 and 437, which are normal IGBTs rather than reverse-blocking IGBTs, are connected in parallel to have opposite polarities. In this case, in addition to freewheeling diodes connected in antiparallel to each of switching elements 436 and 437, forward diodes are also connected in series.

[0077] <Third Modification> In the first embodiment, the multilevel inverter circuit 30a is a three-level inverter circuit in which two capacitors 21 and 22 (voltage-dividing capacitors) are connected in series, but the present invention is not limited to this. In the present invention, four capacitors 521a, 521b, 522a, and 522b connected in series may be provided as the multiple voltage-dividing capacitors, as in a power conversion device 500 according to a third modification shown in FIG.

[0078] 9, in power conversion device 500, capacitors 521a and 521b are connected in series between node P, which is the high potential side (positive side) of the DC power, and intermediate potential node C501, and capacitors 522a and 522b are connected in series between node N, which is the low potential side (negative side) of the DC power, and intermediate potential node C501. Thus, by connecting capacitors 521a and 521b and capacitors 522a and 522b in series, it is possible to divide a DC voltage that exceeds the voltage (withstand voltage) that can be applied to a single capacitor. Capacitors 521a, 521b, 522a, and 522b have approximately the same capacitance. Furthermore, in the power conversion device 500, resistor elements 551a, 551b, 552a, and 552b are connected in parallel to the capacitors 521a, 521b, 522a, and 522b, respectively. The resistor elements 551a, 551b, 552a, and 552b have substantially equal resistance values. This prevents variations in the output voltages of the capacitors 521a, 521b, 522a, and 522b. The configuration of the multilevel inverter circuit 30a of the power conversion unit 30 is the same as that of the first embodiment. That is, the power conversion device 500 according to the third modification has a three-level multilevel inverter circuit 30a.

[0079] Other Modifications Furthermore, in the above-described first and second embodiments, when the difference in output voltages of the capacitors 21 and 22, 221 to 224 (multiple voltage-dividing capacitors) is greater than a predetermined stop threshold, the power conversion operation of the power conversion unit 30 (230) is stopped, the discharge circuit unit 10 discharges the charge of the capacitors 21 and 22, 221 to 224, and the switching operation of the bidirectional switch circuit unit 35 (235) is turned on. However, the present invention is not limited to this. In the present invention, even when the difference in output voltages of the multiple voltage-dividing capacitors is less than a predetermined stop threshold, when the power conversion operation of the power conversion unit is stopped, the discharge circuit unit may discharge the charge of the multiple voltage-dividing capacitors and the switching operation of the bidirectional switch circuit unit may be turned on. In other words, regardless of whether there is variation in the output voltages (charge amounts) of the multiple voltage-dividing capacitors, the switching operation of the bidirectional switch circuit unit may be turned on when the discharge circuit unit discharges the voltage-dividing capacitors. With this configuration, even if variations occur in the discharge speed while the multiple voltage dividing capacitors are being discharged, it is possible to prevent the multiple voltage dividing capacitors from being reversely charged.

[0080] Furthermore, in the above-described first and second embodiments, an example was shown in which the switching operation of the bidirectional switch circuit unit 35 (235) was turned on when the output voltage of any of the capacitors 21, 22, and 221 to 224 (multiple voltage-dividing capacitors) became smaller than a predetermined discharge completion threshold by discharging the charge of the capacitors 21, 22, and 221 to 224 using the discharge circuit unit 10, but the present invention is not limited to this. For example, the switching operation of the bidirectional switch circuit unit may be turned on at the time when the discharge circuit unit 10 starts discharging, regardless of the magnitude of the output voltage of the multiple voltage-dividing capacitors. Alternatively, the switching operation of the bidirectional switch circuit unit may be turned on after a predetermined time has elapsed since the discharge circuit unit 10 started discharging.

[0081] In the first and second embodiments, when the discharge circuit 10 discharges the charge of the capacitors 21 and 22, 221 to 224 (multiple voltage-dividing capacitors), the switching operations of both of the switching elements 36 and 37 (a pair of intermediate potential switching elements) included in the bidirectional switch circuit 35 (235a to 235c) are turned on, but the present invention is not limited to this. For example, the bidirectional switch circuit may be configured to turn on the switching operation of only one of the pair of intermediate potential switching elements included in the bidirectional switch circuit.

[0082] In this case, the direction of the current (discharge current) flowing in the bidirectional switch circuit is calculated based on the output voltages of the multiple voltage-dividing capacitors, and the switching operation of only one of the pair of intermediate potential switching elements is turned on to correspond to the calculated direction of the discharge current. When the pair of intermediate potential switching elements are connected in series, the other intermediate potential switching element that is not turned on allows the discharge current to flow through a freewheeling diode (or a parasitic diode) connected in antiparallel. However, when the discharge current flows only through the freewheeling diode on one side of the pair of intermediate potential switching elements, the voltage drop in the bidirectional switch circuit is larger than when both of the pair of intermediate potential switching elements are turned on. Therefore, it is preferable to turn on both of the pair of intermediate potential switching elements because reducing the voltage drop in the bidirectional switch circuit can reduce errors in the calculation of the discharge time measurement (lifetime estimation of the voltage-dividing capacitors).

[0083] Furthermore, in the first and second embodiments, an example has been shown in which one resistance element 11 (discharge resistance element) is provided in the discharge circuit section 10, but the present invention is not limited to this. For example, a plurality of discharge resistance elements may be connected in series to the discharge circuit section 10. Even in this case, if a plurality of discharge resistance elements are provided separately corresponding to each of a plurality of voltage-dividing capacitors, a plurality of discharge switch sections or diodes or the like must be provided to separately discharge the charge of each of the plurality of voltage-dividing capacitors. Therefore, by configuring the plurality of voltage-dividing capacitors to be discharged collectively by a plurality of discharge resistance elements provided so as to be connected in parallel with the plurality of voltage-dividing capacitors connected in series with each other, it is possible to suppress the complexity of the circuit configuration.

[0084] In the second embodiment, the multilevel inverter circuit 230a is a five-level inverter circuit, but the present invention is not limited to this. For example, the multilevel inverter circuit in the power conversion unit may be a multilevel inverter circuit with more than five levels.

[0085] In the second embodiment, an example was shown in which the switching operations of all of the bidirectional switch circuits 235a to 235c are turned on when the output voltage of any one of the capacitors 221 to 224 (multiple voltage-dividing capacitors) is lower than a predetermined discharge completion threshold, but the present invention is not limited to this. For example, the switching operations of the multiple bidirectional switch circuits may be individually controlled according to the magnitude of the output voltage of each of the multiple voltage-dividing capacitors.

[0086] For example, when input DC power is divided into five stages (five levels) by connecting four voltage-dividing capacitors in series, three bidirectional switch circuits are provided. In this case, if only one voltage-dividing capacitor located on the highest potential side (positive side) is charged and the remaining three voltage-dividing capacitors have completed discharging, by turning on only the bidirectional switch circuit located on the highest potential side (positive side) of the three bidirectional switch circuits, the charged voltage-dividing capacitor can be discharged via the bidirectional switch circuit while suppressing backcharging of the voltage-dividing capacitor that has completed discharging. In this way, based on the output voltage of the multiple voltage-dividing capacitors, the switching operation of only some of the multiple bidirectional switch circuits may be turned on so that discharge current does not flow into voltage-dividing capacitors that have already completed discharging among the multiple voltage-dividing capacitors connected to each other in series. In this way, by flowing discharge current through only some of the multiple bidirectional switch circuits, backflow in the voltage-dividing capacitors can be suppressed, and therefore, more current than necessary can be prevented from flowing through the bidirectional switch circuit compared to when switching operation is turned on so that discharge current flows through all of the multiple bidirectional switch circuits. As a result, the load on the switching elements included in the bidirectional switch circuit section can be reduced.

[0087] In the first and second embodiments, the power conversion devices 100 and 200 are mounted on a railway vehicle, convert DC power input from a DC power supply 101 into three-phase AC power, and output the converted three-phase AC power to a load 102. However, the present invention is not limited to this. For example, the power conversion device may be an uninterruptible power supply that converts input AC power and outputs it. That is, the power conversion device may be configured to have a rectifier circuit that converts input AC power into DC power. The power conversion device may also be mounted on a vehicle such as an electric vehicle. The power conversion device may also be configured as a power conditioning system (PCS) used in a solar power generation system or the like.

[0088] In the first and second embodiments, the switching elements 36 and 37 (a pair of intermediate potential switching elements) connected in series in the bidirectional switch circuit unit 35 (235a to 235c) are MOSFETs, but the present invention is not limited to this. For example, the pair of intermediate potential switching elements in the bidirectional switch circuit unit may be IGBTs. Similarly, the switching elements 31, 231a, and 231b (upper arm switching elements) and the switching elements 32, 232a, and 232b (lower arm switching elements) included in the multilevel inverter circuit 30a (230a) may be configured using IGBTs instead of MOSFETs. [Explanation of symbols]

[0089] 10 Discharge circuit section 11 Resistive element (discharge resistive element) 12 Switch section (discharge switch section) 21, 22, 221, 222, 223, 224, 521a, 521b, 522a, 522b Capacitors (multiple voltage divider capacitors) 30, 230 Power conversion unit 30a, 230a multilevel inverter circuit 31, 231a, 231b Switching elements (upper arm switching elements) 32, 232a, 232b Switching elements (lower arm side switching elements) 35, 235a, 235b, 235c, 335, 435 Bidirectional switch circuit section 36, 37, 336, 337, 436, 437 Switching elements (pair of intermediate potential switching elements) 40, 240 Control unit 100, 200, 500 Power Converter

Claims

1. a plurality of voltage dividing capacitors connected in series between the positive and negative sides of the input DC power, for dividing the voltage of the input DC power; a power conversion unit including a multilevel inverter circuit with three or more levels that performs a power conversion operation of converting the DC power divided by the plurality of voltage dividing capacitors into AC power; a discharge circuit unit that discharges the charges of the plurality of voltage dividing capacitors; a control unit that controls a power conversion operation in the multilevel inverter circuit of the power conversion unit, The multilevel inverter circuit of the power conversion unit an upper arm switching element connected to a positive electrode side; a lower arm switching element connected to a negative electrode side; a bidirectional switch circuit unit connected between an intermediate potential point obtained by dividing the voltage by the plurality of voltage dividing capacitors and an output side of each of the upper arm switching element and the lower arm switching element, the control unit is configured to turn on a switching operation of the bidirectional switch circuit unit when the discharge circuit unit discharges electric charges from the plurality of voltage-dividing capacitors.

2. 2. The power conversion device according to claim 1, wherein, when a difference between the output voltages of the plurality of voltage-dividing capacitors is greater than a predetermined stop threshold, the control unit stops a power conversion operation of the power conversion unit, causes the discharge circuit unit to discharge charges in the plurality of voltage-dividing capacitors, and turns on a switching operation of the bidirectional switch circuit unit.

3. 3. The power conversion device according to claim 1, wherein the control unit is configured to turn on the switching operation of the bidirectional switch circuit unit when an output voltage of any of the plurality of voltage-divider capacitors becomes smaller than a predetermined discharge completion threshold by causing the discharge circuit unit to discharge charges in the plurality of voltage-divider capacitors.

4. the bidirectional switch circuit unit includes a pair of intermediate potential switching elements connected in parallel or in series to have opposite polarities to each other, The power conversion device according to any one of claims 1 to 3, wherein the control unit is configured to turn on switching operations of both of the pair of intermediate potential switching elements when the discharge circuit unit discharges the electric charges of the plurality of voltage dividing capacitors.

5. the discharge circuit unit includes a discharge resistance element connected in parallel with the plurality of voltage dividing capacitors connected in series between a positive electrode side and a negative electrode side of the input DC power, and a discharge switch unit that switches the conduction of the discharge circuit unit, The control unit is configured to turn on a switching operation of the bidirectional switch circuit unit when the discharge switch unit is turned on to discharge the electric charge of the plurality of voltage-dividing capacitors by the discharge circuit unit. The power conversion device according to any one of claims 1 to 4.

6. a plurality of the bidirectional switch circuits are provided so as to be connected to a plurality of intermediate potential points obtained by voltage division by the plurality of voltage dividing capacitors, 6. The power conversion device according to claim 1, wherein the control unit is configured to turn on switching operations of the plurality of bidirectional switch circuits when the discharge circuit unit discharges charges of the plurality of voltage-dividing capacitors based on the output voltages of the plurality of voltage-dividing capacitors.

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