Power conversion device, capacitor device, control method, and computer program

Abstract

A power conversion device (1) includes: a conversion circuit (40) that includes a plurality of switching elements (42, 43) connected to a direct current power supply (10), that converts supplied direct current voltage to a predetermined voltage by executing switching processing, and that outputs the predetermined voltage to a load; a smoothing circuit (20) that includes a plurality of capacitor elements (21a, 21b, 21c, 21d) connected in parallel to the direct current power supply, and that smooths pulsation generated due to the converting by the conversion circuit (40); a voltage measuring device (30) that measures the voltage across each of the plurality of capacitor elements (21a, 21b, 21c, 21d); and a calculation circuit (61) that executes determination processing for determining if an imbalance in the current flowing to the plurality of capacitor elements (21a, 21b, 21c, 21d) has occurred. The determination processing includes: acquiring an effective value of an alternating current component of the voltage across each of the plurality of capacitor elements (21a, 21b, 12c, 21d) on the basis of the measured voltage across each of the plurality of capacitor elements (21a, 21b, 12c, 21d); and determining that an imbalance in the current flowing to the plurality of capacitor elements (21a, 21b, 12c, 21d) has occurred when an evaluation value based on a maximum value of the effective values exceeds a predetermined threshold.

Description

[Technical Field]

[0001] The present disclosure relates to a power conversion device, a capacitor device, a control method, and a computer program. [Background technology]

[0002] Conventionally, a technology has been considered in which an inverter is connected to a smoothing circuit including a reactor having an inductance component and a capacitor having a capacitance component, and the voltage output from a DC power supply to a load is smoothed. For example, Patent Document 1 discloses a control device for a power conversion device, which is characterized by including a voltage detection means for detecting the terminal voltage of the capacitor of the LC smoothing circuit, a filter means for extracting an oscillation component from the detected terminal voltage, and a switching frequency variation means for varying the switching frequency of the inverter based on the extracted oscillation component. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2009-60723 Summary of the Invention [Problem to be solved by the invention]

[0004] The technology disclosed in Patent Document 1 allows a control device to detect and suppress LC resonance caused by inductance and capacitance components. However, the technology disclosed in Patent Document 1 cannot detect LC resonance that may occur between capacitor elements when the capacitor is configured by connecting multiple capacitor elements in parallel. As a result, LC resonance may occur between the capacitor elements, causing excessive current to flow through the capacitor elements, resulting in a temperature rise due to heat generation, which may cause the capacitor to overheat or burn out, destabilize the device output, or stop functioning of the device.

[0005] An object of the present disclosure is to provide a power conversion device, a capacitor device, a control method, and a computer program that are capable of detecting the occurrence of LC resonance between capacitor elements connected in parallel to a DC power supply. [Means for solving the problem]

[0006] The power conversion device according to the present disclosure comprises: a conversion circuit having a plurality of switching elements connected to a DC power source, which converts a DC voltage supplied from the DC power source into a predetermined output voltage by performing a switching process that switches the plurality of switching elements at a predetermined switching frequency, and outputs the converted DC voltage to a load; a smoothing circuit having a plurality of capacitor elements connected in parallel to the DC power source, which smooths pulsations generated during conversion in the conversion circuit using the plurality of capacitor elements; a voltage measurement device that measures the voltages across each of the plurality of capacitor elements; and an arithmetic circuit that performs a determination process that determines whether an imbalance has occurred in the currents flowing through the plurality of capacitor elements. The determination process obtains the effective value of the AC component of the voltages across each of the plurality of capacitor elements based on the voltages across each of the plurality of capacitor elements measured by the voltage measurement device when the switching process is performed, and determines that an imbalance has occurred in the currents flowing through the plurality of capacitor elements when an evaluation value based on the maximum value of the effective values ​​exceeds a predetermined threshold.

[0007] The capacitor device according to the present disclosure is a capacitor device used in a power conversion device that includes: a conversion circuit having a plurality of switching elements connected to a DC power supply, and that converts a DC voltage supplied from the DC power supply into a predetermined output voltage by performing a switching process that switches the plurality of switching elements at a predetermined switching frequency, and outputs the converted voltage to a load; and a smoothing circuit having a plurality of capacitor elements connected in parallel to the DC power supply, and that smooths pulsations generated during conversion in the conversion circuit using the plurality of capacitor elements, and the capacitor device has a plurality of capacitor elements and a housing that mounts the plurality of capacitor elements, and each of the plurality of capacitor elements has a terminal that can detect the voltage across the plurality of capacitor elements and is accessible from outside the capacitor device.

[0008] The control method according to the present disclosure is a control method for determining an imbalance in currents flowing through a plurality of capacitor elements in a power conversion device having a conversion circuit having a plurality of switching elements connected to a DC power source, and converting a DC voltage supplied from the DC power source into a predetermined output voltage by performing a switching process that switches the plurality of switching elements at a predetermined switching frequency and outputs the converted voltage to a load, a smoothing circuit having a plurality of capacitor elements connected in parallel to the DC power source, and smoothing pulsations generated during conversion in the conversion circuit using the plurality of capacitor elements, and a voltage measurement device that measures the voltages across each of the plurality of capacitor elements, wherein the control method obtains an effective value of the AC component of the voltages across each of the plurality of capacitor elements based on the voltages across each of the plurality of capacitor elements measured by the voltage measurement device when the switching process is performed, and determines that an imbalance in currents flowing through the plurality of capacitor elements has occurred when an evaluation value based on the maximum value of the effective values ​​exceeds a predetermined threshold.

[0009] A computer program according to the present disclosure is a computer program for causing an arithmetic circuit to execute a control method according to the present disclosure. [Effects of the Invention]

[0010] According to the present disclosure, it is possible to provide a power conversion device, a capacitor device, a control method, and a computer program that are capable of detecting the occurrence of LC resonance between capacitor elements connected in parallel to a DC power supply. [Brief explanation of the drawings]

[0011] [Figure 1] 1 is a schematic circuit diagram of a configuration example of a power conversion device according to an embodiment; [Figure 2] 1 is a schematic circuit diagram of a configuration example of a smoothing circuit according to an embodiment; [Figure 3A] Graph showing frequency characteristics of impedance of a smoothing circuit according to an embodiment [Figure 3B]Graph showing frequency characteristics related to the magnitude of current flowing through each capacitor element of a smoothing circuit according to one embodiment. [Figure 4A] Graph showing frequency characteristics of impedance of another smoothing circuit according to an embodiment [Figure 4B] 10 is a graph showing frequency characteristics related to the magnitude of current flowing through each capacitor element of another smoothing circuit according to an embodiment. [Figure 5A] Graph showing frequency characteristics of impedance of another smoothing circuit according to an embodiment [Figure 5B] 10 is a graph showing frequency characteristics related to the magnitude of current flowing through each capacitor element of another smoothing circuit according to an embodiment. [Figure 6] 1 is a flowchart of a determination process executed by an arithmetic circuit of a power conversion device according to an embodiment; [Figure 7A] 1 is a graph showing the effective value of the current flowing through each capacitor element of a smoothing circuit according to an embodiment of the present invention; [Figure 7B] 10 is a graph showing the effective values ​​of AC components of voltages applied to each capacitor element of a smoothing circuit according to one embodiment; [Figure 7C] Graph showing the difference between the maximum and minimum values ​​of the effective values ​​shown in FIG. 7B [Figure 8A] 6 is a graph showing the effective values ​​of currents flowing through the capacitor elements of another smoothing circuit according to an embodiment. [Figure 8B] 10 is a graph showing the effective values ​​of AC components of voltages applied to each capacitor element of another smoothing circuit according to an embodiment of the present invention; [Figure 8C] A graph showing the difference between the maximum and minimum values ​​of the effective values ​​shown in FIG. 8B [Figure 9A] 6 is a graph showing the effective values ​​of currents flowing through the capacitor elements of another smoothing circuit according to an embodiment. [Figure 9B] 10 is a graph showing the effective values ​​of AC components of voltages applied to each capacitor element of another smoothing circuit according to an embodiment of the present invention; [Figure 9C] A graph showing the difference between the maximum and minimum values ​​of the effective values ​​shown in FIG. 9B [Figure 10A]Graph showing the difference between the maximum effective value shown in FIG. 8B and the calculated estimated effective value [Figure 10B] Graph showing the difference between the maximum effective value shown in FIG. 9B and the calculated estimated effective value [Figure 11A] FIG. 10 is a perspective view of an example of a capacitor device that can be used in a smoothing circuit of a power conversion device according to another embodiment. [Figure 11B] 11B is a perspective view of the capacitor device of FIG. 11A as seen from the bottom; [Figure 12A] FIG. 10 is a perspective view of an example of a capacitor device that can be used in a smoothing circuit of a power conversion device according to another embodiment. [Figure 12B] 12B is a perspective view of the capacitor device of FIG. 12A with a part of the housing removed. DETAILED DESCRIPTION OF THE INVENTION

[0012] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. However, the configurations described below are merely examples of the present disclosure, and the present disclosure is not limited to the following embodiments. Various modifications can be made depending on the design, etc., even if they are not within the scope of the technical concept of the present disclosure.

[0013] (Embodiment) 1-1.Configuration example FIG. 1 is a schematic circuit diagram of an exemplary configuration of a power conversion device 1 according to an embodiment of the present disclosure. The power conversion device 1 includes a smoothing circuit 20, a voltage measurement device 30, a conversion circuit 40, and a control device 60. The control device 60 includes an arithmetic circuit 61 and a storage device 62. The smoothing circuit 20 includes a plurality of capacitor elements 21 connected in parallel to a DC power supply 10 (details will be described later). The smoothing circuit 20 smoothes pulsations generated during voltage conversion by the conversion circuit 40 using the plurality of capacitor elements 21. The conversion circuit 40 includes a plurality of switching elements 42 and 43 connected to the smoothing circuit 20 (details will be described later). The conversion circuit 40 converts a DC voltage supplied from the DC power supply 10 into a predetermined output voltage by performing a switching process that switches the plurality of switching elements 42 and 43 at a predetermined switching frequency, and outputs the converted voltage to a load 50. The voltage measurement device 30 measures the voltage across each of the plurality of capacitor elements.

[0014] The arithmetic circuit 61 executes a determination process to determine whether an imbalance has occurred in the currents flowing through the plurality of capacitor elements 21. Through the determination process, the arithmetic circuit 61 acquires an effective value of the AC component of the voltages across each of the plurality of capacitor elements 21 based on the voltages across each of the plurality of capacitor elements 21 measured by the voltage measurement device 30 during execution of the switching process. Then, through the determination process, the arithmetic circuit 61 determines that an imbalance has occurred in the currents flowing through the plurality of capacitor elements 21 when an evaluation value based on the maximum value of the effective values ​​exceeds a predetermined threshold.

[0015] By operating in this manner, the power conversion device 1 can detect the occurrence of LC resonance that may occur in the multiple capacitor elements 21 of the smoothing circuit 20. Furthermore, the power conversion device 1 according to the present disclosure does not need to include a filter for identifying in advance the frequency at which LC resonance may occur and extracting LC resonance at that frequency. Therefore, the power conversion device 1 according to the present disclosure can detect the occurrence of LC resonance even if an unexpected, accidental LC resonance occurs at an unexpected frequency. Upon detecting the occurrence of LC resonance, the power conversion device 1 can change the switching frequency executed in the switching process to reduce the current imbalance caused by the LC resonance. Furthermore, upon detecting the occurrence of LC resonance, the power conversion device 1 can reduce the output voltage output from the conversion circuit 40 to the load 50 to reduce the current imbalance caused by the LC resonance.

[0016] A power conversion device 1 according to an embodiment of the present disclosure will be described in more detail below. As shown in Fig. 1, the power conversion device 1 converts a DC voltage from a DC power supply 10 into a predetermined voltage and outputs it to a motor 50, which is a load. As described above, the power conversion device 1 includes a smoothing circuit 20, a voltage measurement device 30, a conversion circuit 40, and a control device 60. The power conversion device 1 is used, for example, in an inverter device for driving a motor mounted on an electric vehicle.

[0017] The DC power supply 10 is a power supply device that applies a DC voltage across the smoothing circuit 20 and the conversion circuit 40. As a result, the DC power supply 10 supplies a current to the conversion circuit 40 for driving the motor 50. The DC power supply 10 may be charged by a regenerative current from the motor 50, as described below. The DC power supply 10 may be, for example, a battery that supplies DC current. Alternatively, the DC power supply 10 may be a power supply device that includes an AC power supply and an AC-DC converter that converts AC voltage into DC voltage and supplies it to the conversion circuit 40.

[0018] The smoothing circuit 20 is charged with the DC voltage supplied from the DC power supply 10, supplies current to the conversion circuit 40, and smoothes pulsations generated in the conversion circuit 40. FIG. 2 is a schematic circuit diagram of an example configuration of the smoothing circuit 20 according to this embodiment. As shown in FIG. 2, the smoothing circuit 20 includes a plurality of capacitor elements 21 connected in parallel to the DC power supply 10. The smoothing circuit 20 according to this embodiment includes four capacitor elements 21a to 21d. The number of capacitor elements 21 is not limited to four and may be any number equal to or greater than two. When converting a voltage in the conversion circuit 40 (described later), for example, pulsations may occur in the voltage. The smoothing circuit 20 can smooth such pulsations generated by the conversion in the conversion circuit 40 using the plurality of capacitor elements 21a to 21d. Furthermore, the smoothing circuit 20 can smooth and suppress noise generated in the DC voltage due to a steep current flow when current is supplied from the DC power supply 10 to the motor 50. When a pulsating component exists in the DC voltage applied by the DC power supply 10, the smoothing circuit 20 can smooth the pulsating component. Furthermore, when a regenerative current flows due to the rotation of the motor 50, the smoothing circuit 20 can smooth the pulsating component that occurs in the DC voltage due to the regenerative current.

[0019] Voltage measurement device 30 is configured to measure the voltage across each of capacitor elements 21a to 21d. In this embodiment, voltage measurement device 30 has voltmeters 31a to 31d as described below (see FIG. 2). Voltage measurement device 30 outputs voltage information indicating the voltages measured by voltmeters 31a to 31d to control device 60.

[0020] The conversion circuit 40 converts the DC voltage supplied from the DC power supply 10 into a predetermined output voltage and outputs it to the motor 50, which is a load. The predetermined output voltage is, for example, a three-phase AC voltage. In this embodiment, the conversion circuit 40 is, for example, an inverter that converts the DC current supplied from the DC power supply 10 into a three-phase AC current and transmits it to the motor 50. The conversion circuit 40 may be able to convert the AC current into a DC current when a regenerative current generated by the rotation of the motor 50 flows.

[0021] The conversion circuit 40 has two or more legs connected in parallel between both ends of the DC power supply 10 and the smoothing circuit 20. In this embodiment, the conversion circuit 40 includes three legs 41A, 41B, and 41C. The leg 41A has a pair of switches, each consisting of an upstream switch 42A and a downstream switch 43A, connected in series. The leg 41B has a pair of switches, each consisting of an upstream switch 42B and a downstream switch 43B, connected in series. The leg 41C has a pair of switches, each consisting of an upstream switch 42C and a downstream switch 43C, connected in series. In the leg 41A, a connection point between the upstream switch 42A and the downstream switch 43A is connected to the motor 50 via a resistor 44A. In the leg 41B, a connection point between the upstream switch 42B and the downstream switch 43B is connected to the motor 50 via a resistor 44B. In the leg 41C, a connection point between the upstream switch 42C and the downstream switch 43C is connected to the motor 50 via a resistor 44C. The arithmetic circuit 61 can control the on / off of the six switches 42A to 42C and 43A to 43C. This allows the arithmetic circuit 61 to convert the DC voltage supplied from the DC power supply 10 into an AC voltage and supply it to the motor 50. For example, the arithmetic circuit 61 alternately turns on and off the upstream switches 42A to 42C and the downstream switches 43A to 43C in each of the legs 41A, 41B, and 41C with a predetermined phase difference (e.g., 120 degrees) between the three legs 41A, 41B, and 41C, thereby supplying a three-phase AC voltage to the motor 50. Each of the switches 42A to 42C and 43A to 43C may be configured, for example, with a transistor and a diode. Thus, the conversion circuit 40 has a plurality of switching elements 42 and 43 connected to the DC power supply 10 and the smoothing circuit 20. The conversion circuit 40 then converts the DC voltage supplied from the DC power supply 10 into a predetermined output voltage and outputs it to the motor 50 by the calculation circuit 61 executing a switching process in which the calculation circuit 61 switches the multiple switching elements 42, 43 at a predetermined switching frequency.

[0022] The motor 50 is, for example, a load circuit that rotates an arbitrary component based on current supplied from the DC power supply 10 via the conversion circuit 40. The load circuit is not limited to a circuit such as a rotary motor that rotates a component, but may be a linear motor that linearly moves a component or any load circuit that uses AC current. The motor 50 may generate regenerative current by being rotated by an external force when no current is being supplied, and supply the regenerative current to the DC power supply 10 via the conversion circuit 40.

[0023] The control device 60 is, for example, a computer that controls the operation of the power conversion device 1. The control device 60 includes an arithmetic circuit 61 and a storage device 62.

[0024] The arithmetic circuit 61 includes a general-purpose processor such as a CPU or MPU that executes a program to achieve a predetermined function. The arithmetic circuit 61 is configured to be able to communicate with the storage device 62 and performs various processes in the control device 60 by calling and executing arithmetic programs stored in the storage device 62. For example, the arithmetic circuit 61 may perform a switching process for switching the switches 42A to 42C and 43A to 43C on and off, and a determination process (described later). The arithmetic circuit 61 is not limited to a configuration in which hardware resources and software cooperate to achieve a predetermined function, but may also be a hardware circuit designed specifically to achieve a predetermined function. In other words, the arithmetic circuit 61 may be realized by various processors, such as a CPU, an MPU, a GPU, an FPGA, a DSP, or an ASIC. Such an arithmetic circuit 61 may be formed, for example, by a signal processing circuit that is a semiconductor integrated circuit.

[0025] The storage device 62 is a storage medium that can store various types of information. For example, the storage device 62 stores voltage information from the voltage measurement device 30, and the arithmetic circuit 61 can use the stored voltage information. The storage device 62 is realized, for example, by a memory such as a DRAM, an SRAM, or a flash memory, an HDD, an SSD, or other storage device, or by an appropriate combination of these. As described above, the storage device 62 stores programs for implementing various processes performed by the control device 60 using the arithmetic circuit 61. The storage device 62 may also store any parameters, such as a capacitance component, a resistance component, and an inductance component, of the smoothing circuit 20, which will be described later.

[0026] Next, the smoothing circuit 20 will be described in more detail. As described above, the smoothing circuit 20 includes four capacitor elements 21a to 21d. As shown in Fig. 2, the four capacitor elements 21a to 21d form a parallel circuit. The parallel circuit is connected in parallel to the DC power supply 10 via a connection point 23a on the first end side of the four capacitor elements 21a to 21d and a connection point 23b on the second end side of the four capacitor elements 21a to 21d.

[0027] Each of the capacitor elements 21a to 21d of the smoothing circuit 20 has a capacitance Ca to Cd [μF], resistance components Ra to Rd [mΩ] due to equivalent series resistance, and inductance components La1 to Ld1 [nH] due to equivalent series inductance. As can be seen from FIGS. 1 and 2, the smoothing circuit 20 has a first terminal 22a connected to the positive pole of the DC power supply 10 and a second terminal 22b connected to the negative pole. Each of the capacitor elements 21a to 21d is disposed between the first terminal 22a and the second terminal 22b. In this embodiment, the smoothing circuit 20 is configured such that the capacitor elements 21a to 21d are connected to the first terminal 22a and the second terminal 22b by the connection point 23a and the connection point 23b.

[0028] The smoothing circuit 20 has inductance components La2 to Ld2 [nH] formed by the circuits between the connection points 23a and 23b excluding the capacitor elements 21a to 21d, for the paths of the capacitor elements 21a to 21d, respectively. The smoothing circuit 20 also has an inductance component L3 formed by the circuits between the first terminal 22a and the connection point 23a and the circuits between the second terminal 22b and the connection point 23b. More specifically, the inductance component L3 represents the inductance component when the first terminal 22a is connected to the positive pole of the DC power supply 10 and the second terminal 22b is connected to the negative pole of the DC power supply 10.

[0029] In this manner, the circuits (also referred to herein as the paths of the capacitor elements 21a to 21d) associated with the capacitor elements 21a to 21d included in the smoothing circuit 20 have different impedance values. Here, for example, the path of the capacitor element 21a refers to the circuit extending from the connection point 23a through the capacitor element 21a to the connection point 23b.

[0030] As described above, the smoothing circuit 20 can smooth, by means of the plurality of capacitor elements 21a-21d, the pulsation component generated by conversion in the conversion circuit 40 when the DC power supply 10 supplies power to the motor 50. Furthermore, as described above, the smoothing circuit 20 is charged with the DC voltage supplied from the DC power supply 10. Furthermore, the smoothing circuit 20 supplies a current to the conversion circuit 40. At this time, a current flows through the smoothing circuit 20. Because the smoothing circuit 20 has the plurality of capacitor elements 21a-21d, the current is divided and flows among the plurality of capacitor elements 21a-21d. The proportion of the current shared by the plurality of capacitor elements 21a-21d is determined by the impedance values ​​described above.

[0031] Next, the current flowing through the plurality of capacitor elements 21a to 21d of the smoothing circuit 20 will be described using circuit examples 1, 2, and 3.

[0032] The smoothing circuit 20 of Circuit Example 1 has paths through the capacitor elements 21a to 21d with the parameters shown in Table 1 below. FIG. 3A is a graph showing frequency characteristics of the impedance of the smoothing circuit 20 of Circuit Example 1. The impedance is the impedance between the first terminal 22a and the second terminal 22b. FIG. 3B is a graph showing frequency characteristics of the magnitude of the current flowing through each of the capacitor elements 21a to 21d when a current having a constant effective value and an AC component of a predetermined frequency flows through the smoothing circuit 20 of Circuit Example 1. FIG. 3B shows frequency characteristics for a frequency range of 10 kHz to 100 kHz. FIGS. 4B and 5B, which will be described later, also show frequency characteristics for a frequency range of 10 kHz to 100 kHz. The predetermined frequency can be determined based on the switching frequency of the switching process executed by the arithmetic circuit 61. Hereinafter, the frequency of the AC component of the current flowing through each of the capacitor elements 21a to 21d will also be referred to as a ripple frequency. The inductance value of the inductance component L3 is 50 nH.

[0033] [Table 1]

[0034] As shown in FIG. 3A, the smoothing circuit 20 of Circuit Example 1 is configured to have a resonant frequency near 15 kHz. Furthermore, as shown in FIG. 3B, when a current having a constant effective value is passed through the smoothing circuit 20 of Circuit Example 1, currents of the same magnitude flow through the paths of the capacitor elements 21a to 21d, regardless of changes in ripple frequency. Therefore, when the paths of the capacitor elements 21a to 21d have the same impedance, as in the smoothing circuit 20 of this example, the same current flows through each path, and no current imbalance occurs. Therefore, the smoothing circuit 20 having paths with the parameters shown in Table 1 does not have a parallel resonant frequency, which is a frequency at which LC resonance can occur between the paths of the capacitor elements 21a to 21d.

[0035] The smoothing circuit 20 of Circuit Example 2 has paths through the capacitor elements 21a to 21d with the parameters shown in Table 2 below. FIG. 4A is a graph showing frequency characteristics of the impedance of the smoothing circuit 20 of Circuit Example 2. The impedance is the impedance between the first terminal 22a and the second terminal 22b. The parameters of Circuit Example 2 are changed from the parameters of Circuit Example 1 only in the inductance values ​​of the inductance components Lb2 to Ld2. That is, the parameters of Circuit Example 2 are changed from the parameters of Circuit Example 1 in the inductance values ​​based on the paths through the capacitor elements 21b to 21d. FIG. 4B is a graph showing frequency characteristics of the magnitude of the current flowing through the capacitor elements 21a to 21d when a current having a constant effective value and an AC component of a predetermined frequency flows through the smoothing circuit 20 of Circuit Example 2. The inductance value of the inductance component L3 is 50 nH.

[0036] [Table 2]

[0037] As shown in FIG. 4A, the smoothing circuit 20 of Circuit Example 2 is configured to have a resonant frequency around 15 kHz. Furthermore, compared to the frequency characteristics shown in FIG. 3A, the frequency characteristics shown in FIG. 4A have a resonant point around 20 kHz. This resonant point corresponds to the parallel resonant frequency, which is a frequency at which LC resonance can occur. Furthermore, as shown in FIG. 4B, when a current having a constant effective value is passed through the smoothing circuit 20 of Circuit Example 2, different current magnitudes flow through the paths of the capacitor elements 21a to 21d based on the impedance of each path. In FIG. 4B, the magnitude of the current flowing through the path of capacitor element 21a is indicated by a solid line. In FIG. 4B, the magnitude of the current flowing through the path of capacitor element 21b is indicated by a dashed line. In FIG. 4B, the magnitude of the current flowing through the path of capacitor element 21c is indicated by a thick solid line. In FIG. 4B, the magnitude of the current flowing through the path of capacitor element 21d is indicated by a dashed line.

[0038] As can be seen from FIG. 4B, currents of different magnitudes flow through the paths of capacitor elements 21a to 21d depending on the ripple frequency. For example, as can be seen from FIG. 4B, the largest current flows through each of capacitor elements 21b and 21c when a current having an AC component with a ripple frequency of approximately 20 kHz flows through smoothing circuit 20 in the frequency range of 10 kHz to 100 kHz. That is, the largest current flows through each of capacitor elements 21b and 21c when arithmetic circuit 61 performs switching processing at a predetermined frequency, causing a current having an AC component with a ripple frequency of approximately 20 kHz to flow through smoothing circuit 20. Furthermore, the largest current flows through capacitor element 21a when a current having an AC component with a ripple frequency higher than 20 kHz flows through smoothing circuit 20. The largest current flows through capacitor element 21d when a current having an AC component with a ripple frequency lower than 20 kHz flows through smoothing circuit 20. Therefore, when the paths of the capacitor elements 21a to 21d have different impedance values, as in the smoothing circuit 20 of the circuit example 2, a current flows through each path based on the impedance, causing an imbalance in current between the paths.

[0039] The smoothing circuit 20 of Circuit Example 3 has paths through the capacitor elements 21a to 21d with the parameters shown in Table 3 below. FIG. 5A is a graph showing frequency characteristics of the impedance of the smoothing circuit 20 of Circuit Example 3. The impedance is the impedance between the first terminal 22a and the second terminal 22b. The parameters of Circuit Example 3 are changed from the parameters of Circuit Example 1 in that only the inductance values ​​of the inductance components Lc2 and Ld2 are changed. That is, the parameters of Circuit Example 3 are changed from the parameters of Circuit Example 1 in that the inductance values ​​based on the paths through the capacitor elements 21c and 21d are changed. FIG. 5B is a graph showing frequency characteristics of the magnitude of the current flowing through the capacitor elements 21a to 21d when a current having a constant effective value and an AC component of a predetermined frequency flows through the smoothing circuit 20 of Circuit Example 3. The inductance value of the inductance component L3 is 50 nH.

[0040] [Table 3]

[0041] As shown in FIG. 5A, the smoothing circuit 20 of Circuit Example 3 is configured to have a resonant frequency around 15 kHz. Furthermore, compared to the frequency characteristics shown in FIG. 3A, the frequency characteristics shown in FIG. 5A have a resonant point around 20 kHz. This resonant point corresponds to the parallel resonant frequency, which is a frequency at which LC resonance can occur. Furthermore, as shown in FIG. 5B, when a current having a constant effective value is passed through the smoothing circuit 20 of Circuit Example 3, currents of different magnitudes flow through the paths of the capacitor elements 21a to 21d based on the impedance of each path. In FIG. 5B, the magnitude of the current flowing through the path of capacitor element 21a is indicated by a solid line. The solid line also indicates the magnitude of the current flowing through the path of capacitor element 21b. In FIG. 5B, the magnitude of the current flowing through the path of capacitor element 21c is indicated by a dashed line. The dashed line also indicates the magnitude of the current flowing through the path of capacitor element 21d.

[0042] As can be seen from FIG. 5B, currents of different magnitudes flow through the paths of capacitor elements 21a-21d depending on the ripple frequency. For example, as can be seen from FIG. 5B, the largest current flows through the paths of capacitor elements 21a-21d when a current having an AC component with a ripple frequency of approximately 20 kHz flows through smoothing circuit 20 in the frequency range of 10 kHz to 100 kHz. The smallest current flows through the paths of capacitor elements 21a and 21b when a current having an AC component with a ripple frequency lower than 20 kHz flows through smoothing circuit 20. The smallest current flows through the paths of capacitor elements 21c and 21d when a current having an AC component with a ripple frequency higher than 20 kHz flows through smoothing circuit 20. Therefore, when the paths of capacitor elements 21a and 21b and the paths of capacitor elements 21c and 21d have different impedances, as in the case of smoothing circuit 20 in this example, currents flow through each path based on the impedance, causing a current imbalance between the paths.

[0043] In this way, the amount of current flowing through multiple paths varies based on the ripple frequency, potentially causing excessive current to flow through a specific capacitor element. This can lead to current imbalances between the paths. When excessive current flows through a specific capacitor element, the temperature of that capacitor element rises due to Joule heat, causing the capacitor to overheat and burn out, potentially resulting in unstable output or malfunction of the device in which the capacitor is installed. Such current imbalances can occur, for example, when LC resonance occurs due to the capacitance components of each capacitor element 21a-21d in the smoothing circuit 20 and the inductance components of the paths connecting the capacitor elements 21a-21d. Such LC resonance can occur in a parallel circuit formed by the capacitor elements 21a-21d in the smoothing circuit 20. LC resonance is also called parallel resonance.

[0044] The power conversion device 1 according to this embodiment can detect the occurrence of the current imbalance. More specifically, the arithmetic circuit 61 of the control device 60 of the power conversion device 1 can determine whether or not a current imbalance has occurred, based on the voltages across each of the capacitor elements 21a to 21d detected by the voltage measurement device 30. Hereinafter, the process performed by the arithmetic circuit 61 to determine whether or not a current imbalance has occurred will be described.

[0045] 6 is a flowchart of a determination process executed by the arithmetic circuit 61 of the control device 60 included in the power conversion device 1 according to this embodiment. First, the arithmetic circuit 61 acquires an effective value of an AC component of the voltage across each of the plurality of capacitor elements 21a to 21d based on the voltages across the plurality of capacitor elements 21a to 21d measured by the voltage measurement device 30 (S10). The arithmetic circuit 61 may, for example, store each acquired effective value in the storage device 62. The arithmetic circuit 61 may store the effective value in the storage device 62 in association with the switching frequency of the switching process when the effective value was acquired. The arithmetic circuit 61 may also store the effective value in the storage device 62 in association with the ripple frequency.

[0046] Next, the arithmetic circuit 61 calculates an evaluation value based on the maximum value of the acquired effective values ​​(S11). For example, the arithmetic circuit 61 may determine the maximum value of the acquired effective values ​​and the minimum value of the acquired effective values, calculate the difference between the maximum value and the minimum value, and use the difference as the evaluation value. The arithmetic circuit 61 may also calculate the difference between the maximum value and a reference value and use the difference as the evaluation value. The reference value is, for example, the effective value of a voltage estimated by the voltage measurement device 30 as the AC component of the voltage across each of the multiple capacitor elements 21a-21d under conditions in which parallel resonance does not occur in the smoothing circuit 20. Hereinafter, such an effective value of the voltage is referred to as an estimated effective value. Conditions in which parallel resonance does not occur in the smoothing circuit 20 include the impedance parameters of the paths of the capacitor elements 21a-21d being identical, as in the smoothing circuit 20 of Circuit Example 1 described above. The estimated effective value may be calculated on the assumption that the impedance values ​​of the multiple paths, each including the multiple capacitor elements 21a to 21d in the smoothing circuit 20, relative to the DC power supply are the same as the smallest value of the impedance values.

[0047] The arithmetic circuit 61 determines whether or not the evaluation value is greater than a predetermined threshold Vth (S12). If the evaluation value is equal to or less than the predetermined threshold Vth (S12: NO), the arithmetic circuit 61 determines that no current imbalance has occurred. If the evaluation value is greater than the predetermined threshold Vth (S12: YES), the arithmetic circuit 61 determines that a current imbalance has occurred (S13). In this way, the arithmetic circuit 61 of the power conversion device 1 according to this embodiment can execute a determination process to determine whether or not a current imbalance has occurred in each path of the plurality of capacitor elements 21a to 21d in the smoothing circuit 20.

[0048] 1-2.Example (First Example) A first example of a process for determining a current imbalance by the arithmetic circuit 61 will be described. FIG. 7A is a graph showing the effective values ​​of the currents flowing through the capacitor elements 21a to 21d when the arithmetic circuit 61 performs a switching process at a predetermined switching frequency in a power conversion device 1 having a smoothing circuit 20 of Circuit Example 1. The horizontal axis of the graph represents the switching frequency × 2, which is generally the main frequency component of the ripple, and in this example, the range is set to 10 kHz to 30 kHz. Such current values ​​can be calculated, for example, by performing a simulation using a circuit simulating the power conversion device 1 having the smoothing circuit 20 of Circuit Example 1. The same applies to the voltage values ​​shown in FIG. 7B. Furthermore, for other examples described below, the current values ​​and voltage values ​​can be calculated in a similar manner by performing a simulation using a circuit simulating the power conversion device 1 having the smoothing circuit 20 of Circuit Example 2 or Circuit Example 3. As can be seen from FIG. 7A, the effective values ​​of the currents flowing through the capacitor elements 21a to 21d are the same in a power conversion device 1 having the smoothing circuit 20 of Circuit Example 1. Furthermore, even if the ripple frequency changes, the effective value of the current flowing through each of the capacitor elements 21a to 21d does not change significantly.

[0049] 7B is a graph showing the effective values ​​of the AC components of the voltages applied to the capacitor elements 21a-21d when the arithmetic circuit 61 performs switching at the predetermined switching frequency in the power conversion device 1 having the smoothing circuit 20 of Circuit Example 1. That is, FIG. 7B is a graph showing the effective values ​​of the AC components of the voltages applied to the capacitor elements 21a-21d at ripple frequencies ranging from 10 kHz to 30 kHz. As can be seen from FIG. 7B, the effective values ​​of the AC components of the voltages applied to the capacitor elements 21a-21d are the same in the power conversion device 1 having the smoothing circuit 20 of Circuit Example 1. It can be seen that, in the frequency range from 10 kHz to 30 kHz, the effective values ​​of the AC components of the voltages applied to the capacitor elements 21a-21d are greatest at 10 kHz and tend to decrease as the ripple frequency increases.

[0050] 7C is a graph showing the difference between the maximum and minimum effective values ​​of the AC components of the voltages applied to the capacitor elements 21a-21d shown in FIG. 7B. As is clear from FIG. 7C, the difference between the maximum and minimum effective values ​​of the AC components of the voltages applied to the capacitor elements 21a-21d is zero at any frequency. Therefore, in the power conversion device 1 including the smoothing circuit 20 of Circuit Example 1, when the switching process is performed at the predetermined switching frequency, no current imbalance occurs between the paths of the capacitor elements 21a-21d. For example, when the predetermined threshold Vth is 0.3 V, the calculation circuit 61 determines through the determination process that no current imbalance occurs because the difference is zero as described above.

[0051] (Second Example) A second example of the process for determining current imbalance by the arithmetic circuit 61 will be described. FIG. 8A is a graph showing the effective values ​​of currents flowing through paths including capacitor elements 21a to 21d when the arithmetic circuit 61 performs switching at the predetermined switching frequency in a power conversion device 1 having a smoothing circuit 20 of circuit example 2. The horizontal axis of the graph indicates twice the switching frequency, which is generally the main frequency component of the ripple, and in this example, the range is set to 10 kHz to 30 kHz. In FIG. 8A, the current flowing through capacitor element 21a is indicated by a solid line. In FIG. 8A, the current flowing through capacitor element 21b is indicated by a dashed line. In FIG. 8A, the current flowing through capacitor element 21c is indicated by a dashed-dotted line. In FIG. 8A, the current flowing through capacitor element 21d is indicated by a two-dot-dash line. The types of lines corresponding to the paths are the same as those in the voltage graph shown in FIG. 8B, which will be described later.

[0052] As can be seen from FIG. 8A, in the power conversion device 1 having the smoothing circuit 20 of circuit example 2, the effective value of the current flowing through each of the capacitor elements 21a to 21d is different. The effective value of the current flowing through each of the capacitor elements 21a to 21d also differs depending on the ripple frequency. For example, when the arithmetic circuit 61 performs a switching process at a frequency that includes a ripple frequency of 14 kHz, the effective value of the current is large in the capacitor element 21a (approximately 50 A) and the capacitor element 21b (approximately 45 A). When the arithmetic circuit 61 performs a switching process at a frequency that includes a ripple frequency of 20 kHz, the effective value of the current is large in the capacitor element 21b (approximately 57 A). In this way, the capacitor element through which a large current flows and the effective value of the current change depending on the ripple frequency.

[0053] Fig. 8B is a graph showing the effective values ​​of the AC components of the voltages applied to the capacitor elements 21a to 21d when the arithmetic circuit 61 performs switching processing at the predetermined switching frequency in the power conversion device 1 having the smoothing circuit 20 of circuit example 2. That is, Fig. 8B is a graph showing the effective values ​​of the AC components of the voltages applied to the capacitor elements 21a to 21d at ripple frequencies in the range of 10 kHz to 30 kHz.

[0054] As can be seen from FIG. 8B, in the power conversion device 1 having the smoothing circuit 20 of Circuit Example 2, the effective values ​​of the AC components of the voltages applied to the capacitor elements 21a to 21d are different. Furthermore, the effective values ​​of the AC components of the voltages applied to the capacitor elements 21a to 21d also vary depending on the ripple frequency. As can be seen from FIGS. 8A and 8B, when the current value is large in FIG. 8A, the voltage value is generally also large in FIG. 8B. For example, when the arithmetic circuit 61 performs a switching process at a frequency that includes a ripple frequency of 14 kHz, the effective values ​​of the AC components of the voltages applied to the capacitor elements 21a and 21b are large (approximately 2.4 V and approximately 2.1 V, respectively). Furthermore, when the arithmetic circuit 61 performs a switching process at a frequency that includes a ripple frequency of 20 kHz, the effective value of the AC component of the voltage applied to the capacitor element 21b is large (approximately 3.3 V). Furthermore, it can be seen that in the frequency range of 10 kHz to 30 kHz, the minimum effective value of the AC component of the voltage applied to each of the capacitor elements 21a to 21d is greatest at 10 kHz, and tends to decrease as the ripple frequency increases.

[0055] 8C is a graph showing the difference between the maximum and minimum values ​​of the effective values ​​of the AC components of the voltages applied to the capacitor elements 21a-21d shown in FIG. 8B. As is clear from FIG. 8C, this difference is largest (approximately 1.8 V) when the arithmetic circuit 61 performs switching at frequencies that include a ripple frequency of 20 kHz. As described above, the smoothing circuit 20 of Circuit Example 2 has parameters for the paths of the capacitor elements 21a-21d that cause LC resonance at 20 kHz. Therefore, it can be seen that the ripple frequencies at which this difference is large correspond to the ripple frequencies at which LC resonance occurs.

[0056] 8C, the difference becomes larger when the arithmetic circuit 61 performs a switching process at a frequency that includes the ripple frequency of 14 kHz. Therefore, when the arithmetic circuit 61 performs a switching process at a frequency that includes the ripple frequency of 14 kHz, it is understood that an imbalance in current occurs in each path of the smoothing circuit 20.

[0057] In a simulation of a circuit simulating the power conversion device 1, the current waveforms and voltage waveforms of the capacitor elements 21a to 21d are waveforms that include harmonics. Therefore, the effective value is a value obtained by combining the harmonic components. The LC resonance caused by the ripple frequency of 14 kHz indicates that the harmonic components of the switching frequency are resonating around 20 kHz. Here, the switching frequency is half the ripple frequency, i.e., approximately 7 kHz. Therefore, according to the power conversion device 1 of this embodiment, by setting an appropriate threshold value Vth, it is possible to detect the LC resonance caused by the harmonic components of the switching frequency.

[0058] For example, when the predetermined threshold Vth is approximately 0.4 V, the arithmetic circuit 61 executes a switching process and the ripple frequency is any of 12 kHz, 14 kHz, 18 kHz, 20 kHz, and 22 kHz, the difference exceeds the predetermined threshold Vth. Therefore, when the arithmetic circuit 61 executes a switching process at a switching frequency that includes the above-mentioned ripple frequencies, the arithmetic circuit 61 can determine, through a determination process, that a current imbalance has occurred in each path of the smoothing circuit 20.

[0059] (Third Example) A third example of the process for determining current imbalance by the arithmetic circuit 61 will be described. FIG. 9A is a graph showing the effective values ​​of currents flowing through paths including capacitor elements 21a to 21d when the arithmetic circuit 61 executes switching processing at the predetermined switching frequency in a power conversion device 1 having a smoothing circuit 20 of circuit example 3. The horizontal axis of the graph indicates twice the switching frequency, which is generally the main frequency component of the ripple, and in this case, the range is set to 10 kHz to 30 kHz. In FIG. 9A, the currents flowing through capacitor elements 21a and 21b are indicated by solid lines. In FIG. 9A, the currents flowing through capacitor elements 21c and 21d are indicated by dashed lines.

[0060] As can be seen from FIG. 9A, in the power conversion device 1 including the smoothing circuit 20 of the circuit example 3, the effective values ​​of the currents flowing through the capacitor elements 21a and 21b are different from those flowing through the capacitor elements 21c and 21d. The effective values ​​of the currents flowing through the capacitor elements 21a to 21d also vary depending on the ripple frequency. For example, when the arithmetic circuit 61 performs a switching process at a frequency that includes a ripple frequency of 14 kHz, the effective values ​​of the currents are large in the capacitor elements 21a and 21b. When the arithmetic circuit 61 performs a switching process at a frequency that includes a ripple frequency of 20 kHz, the effective values ​​of the currents are large in each of the capacitor elements 21a to 21d. More specifically, the effective value of the current flowing through the capacitor elements 21a and 21b (approximately 75 A) is larger than the effective value of the current flowing through the capacitor elements 21c and 21d (approximately 62 A). Thus, the capacitor element through which a large current flows and the effective value of the current vary depending on the ripple frequency.

[0061] Fig. 9B is a graph showing the effective values ​​of the AC components of the voltages applied to the capacitor elements 21a to 21d when the arithmetic circuit 61 performs switching processing at the predetermined switching frequency in the power conversion device 1 having the smoothing circuit 20 of Circuit Example 3. That is, Fig. 9B is a graph showing the effective values ​​of the AC components of the voltages applied to the capacitor elements 21a to 21d at ripple frequencies in the range of 10 kHz to 30 kHz.

[0062] As can be seen from FIG. 9B, in the power conversion device 1 having the smoothing circuit 20 of Circuit Example 3, the effective values ​​of the AC components of the voltages applied to the capacitor elements 21a to 21d are different. Furthermore, the effective values ​​of the AC components of the voltages applied to the capacitor elements 21a to 21d also vary depending on the ripple frequency. As can be seen from FIGS. 9A and 9B, when the current value is large in FIG. 9A, the voltage value is generally also large in FIG. 9B. For example, when the arithmetic circuit 61 performs switching processing at a frequency that includes a ripple frequency of 20 kHz, the effective value (approximately 4.3 V) of the AC components of the voltages applied to the capacitor elements 21a and 21b is large. Furthermore, the effective value (approximately 3.5 V) of the AC components of the voltages applied to the capacitor elements 21c and 21d is large. Furthermore, it can be seen that in the frequency range of 10 kHz to 30 kHz, the minimum effective value of the AC component of the voltage applied to each of the capacitor elements 21a to 21d is greatest at 10 kHz and tends to decrease as the frequency increases.

[0063] 9C is a graph showing the difference between the maximum and minimum values ​​of the effective values ​​of the AC components of the voltages applied to the capacitor elements 21a to 21d shown in FIG. 9B. As is clear from FIG. 9C, this difference is largest (approximately 0.8 V) when the arithmetic circuit 61 performs switching at frequencies that include a ripple frequency of 20 kHz. As described above, the smoothing circuit 20 of Circuit Example 3 has parameters for the paths of the capacitor elements 21a to 21d that cause LC resonance at 20 kHz. Therefore, it can be seen that the ripple frequencies at which this difference is large correspond to the ripple frequencies at which LC resonance occurs.

[0064] For example, when the predetermined threshold Vth is approximately 0.2 V, the arithmetic circuit 61 executes a switching process and the ripple frequency is any of 12 kHz, 18 kHz, 20 kHz, and 22 kHz, the difference exceeds the predetermined threshold Vth. Therefore, when the arithmetic circuit 61 executes a switching process at a switching frequency that includes the above-mentioned ripple frequencies, the arithmetic circuit 61 can determine, through a determination process, that a current imbalance has occurred in each path of the smoothing circuit 20.

[0065] (Fourth Example) A fourth example of the process of determining a current imbalance by the arithmetic circuit 61 will be described. In the fourth example, the arithmetic circuit 61 executes the determination process in the power conversion device 1 having the smoothing circuit 20 of the circuit example 2. The fourth example differs from the second example in the method of calculating an evaluation value based on the maximum value of the effective values ​​of the AC components of the voltages across each of the capacitor elements 21a to 21d.

[0066] In the second embodiment, the arithmetic circuit 61 uses the difference between the maximum and minimum of the effective values ​​as an evaluation value to determine whether a current imbalance has occurred. In contrast, in the fourth embodiment, the arithmetic circuit 61 uses the difference between the maximum of the effective values ​​and the estimated effective value as an evaluation value. In the fourth embodiment, the arithmetic circuit 61 calculates the estimated effective value by assuming that the impedance values ​​of the DC power supplies of the multiple paths, each including multiple capacitor elements 21a to 21d in the smoothing circuit 20, are the same as the minimum of the impedance values. This calculation method is also used in the fifth embodiment for the smoothing circuit 20 of Circuit Example 3, which will be described later.

[0067] In the smoothing circuits 20 of the above-described circuit examples 2 and 3, the path having the smallest impedance value is the path including the capacitor element 21a. Therefore, when calculating the estimated effective value in the smoothing circuit 20 of circuit example 2 or circuit example 3, the arithmetic circuit 61 can calculate the estimated effective value by assuming that the smoothing circuit 20 has the parameters of circuit example 1. That is, in the fourth embodiment, the estimated effective value is equivalent to the effective value of the AC component of the voltage shown in FIG. 7B.

[0068] 10A is a graph showing the difference between the maximum effective value of the AC component of the voltage applied to each of the capacitor elements 21a to 21d shown in FIG. 8B and the estimated effective value. As is clear from FIG. 10A, the difference is largest (approximately 1.9 V) when the arithmetic circuit 61 performs switching at frequencies that include a ripple frequency of 20 kHz. As described above, the smoothing circuit 20 of Circuit Example 2 has parameters for the paths of the capacitor elements 21a to 21d that cause LC resonance at a ripple frequency of 20 kHz. Therefore, it can be seen that the ripple frequency at which the difference is large corresponds to the ripple frequency at which LC resonance occurs.

[0069] 10A, the difference becomes larger when the arithmetic circuit 61 performs a switching process at a frequency that includes the ripple frequency of 14 kHz. Therefore, when the arithmetic circuit 61 performs a switching process at a frequency that includes the ripple frequency of 14 kHz, a current imbalance occurs in each path of the smoothing circuit 20. For example, when the predetermined threshold Vth is approximately 0.4 V, the arithmetic circuit 61 performs a switching process and the ripple frequency is 12 kHz, 14 kHz, 18 kHz, 20 kHz, or 22 kHz, the difference exceeds the predetermined threshold Vth. Therefore, when the arithmetic circuit 61 performs a switching process at a switching frequency that includes the above-mentioned ripple frequency, the arithmetic circuit 61 can determine, through a determination process, that a current imbalance occurs in each path of the smoothing circuit 20.

[0070] (Fifth Example) A fifth example of the current imbalance determination process by the arithmetic circuit 61 will be described. In the fifth example, the arithmetic circuit 61 performs the determination process in a power conversion device 1 having a smoothing circuit 20 of circuit example 3. The fifth example differs from the third example in the method of calculating an evaluation value based on the maximum value of the effective values ​​of the AC components of the voltages across each of the capacitor elements 21a to 21d. In the fifth example, similar to the fourth example, the arithmetic circuit 61 uses the difference between the maximum value of each effective value and the estimated effective value as the evaluation value. As described above, in the fifth example, the estimated effective value is equivalent to the effective value of the AC component of the voltage shown in FIG. 7B.

[0071] 10B is a graph showing the difference between the maximum effective value of the AC component of the voltage applied to each of the capacitor elements 21a-21d shown in FIG. 9B and the estimated effective value. As is clear from FIG. 10B, the difference is largest (approximately 2.9 V) when the arithmetic circuit 61 performs switching at a frequency that includes a ripple frequency of 20 kHz. As described above, the smoothing circuit 20 of Circuit Example 2 has parameters for the paths of the capacitor elements 21a-21d that cause LC resonance at a ripple frequency of 20 kHz. Therefore, it can be seen that the ripple frequency at which the difference is large corresponds to the ripple frequency at which LC resonance occurs.

[0072] 9C corresponding to the third embodiment and FIG. 10B corresponding to the fifth embodiment, the voltage difference is significantly different at a ripple frequency of 20 kHz. In the third embodiment, when the arithmetic circuit 61 performs a switching process at frequencies including the ripple frequency of 20 kHz, the current flowing through the path of capacitor elements 21a and 21b is larger than that at other ripple frequencies. Similarly, the current flowing through the path of capacitor elements 21c and 21d is larger than that at other ripple frequencies. Therefore, in the third embodiment, both the maximum and minimum values ​​of the effective value are large, and the difference between the maximum and minimum values ​​is small. Thus, when the power conversion device 1 operates at a switching frequency that can cause LC resonance, a small evaluation value (i.e., the difference between the maximum and minimum values) can be calculated, even if a large current flows through a specific path. In the fifth embodiment, when the arithmetic circuit 61 performs a switching process at frequencies including the ripple frequency of 20 kHz, the current flowing through the path of capacitor elements 21a and 21b is larger than that at other ripple frequencies. Similarly, the current flowing through the path of capacitor elements 21c and 21d is larger than that at other ripple frequencies. Therefore, according to the calculation method of the fifth embodiment, the evaluation value is the difference between the maximum effective value and the estimated effective value, and therefore may be larger than the evaluation value obtained by the calculation method of the third embodiment.

[0073] As described above, when calculating an evaluation value using the calculation methods according to the first to third embodiments, if the maximum and minimum values ​​of the effective values ​​of the AC components of multiple voltages change in the same manner due to LC resonance, the evaluation value cannot reflect the amount of change in the effective values. Therefore, the evaluation value decreases. Therefore, unless an appropriate threshold value Vth is set, the arithmetic circuit 61 may not be able to determine that a current imbalance has occurred through the determination process. Furthermore, the effective values ​​of the AC components of the voltages of the capacitor elements 21a to 21d increase or decrease as the output of the motor 50 increases or decreases. As the output of the motor 50 decreases, the effective values ​​of the AC components of the voltages of the capacitor elements 21a to 21d decrease overall. As each effective value decreases, the difference between the maximum and minimum values ​​of the effective values ​​decreases, and the arithmetic circuit 61 may need to correct or separately set the threshold value Vth depending on the output.

[0074] In contrast, when calculating an evaluation value using the calculation methods according to the fourth and fifth embodiments, the estimated effective value used to calculate the evaluation value is not affected by LC resonance and does not change in the same manner as the change in the maximum value due to LC resonance. Furthermore, as described above, when the output of the motor 50 decreases, the effective values ​​of the AC components of the voltages at the capacitor elements 21a to 21d decrease overall. Therefore, by subtracting the estimated effective value from the maximum effective value, the overall change in voltage due to the change in motor output can be offset. Therefore, when the calculation circuit 61 uses the calculation methods according to the fourth and fifth embodiments, the need to correct or set the threshold value Vth according to the output of the motor 50, as described above, is reduced. This allows the calculation circuit 61 to more easily determine whether a current imbalance has occurred.

[0075] For example, in the fifth embodiment, when the predetermined threshold Vth is about 0.4 V, the arithmetic circuit 61 executes the switching process and the ripple frequency is 20 kHz, the difference between the maximum value and the estimated effective value exceeds the predetermined threshold. Therefore, when the arithmetic circuit 61 executes the switching process at a switching frequency that includes the ripple frequency, the arithmetic circuit 61 can determine, through the determination process, that a current imbalance has occurred in each path of the smoothing circuit 20.

[0076] When the arithmetic circuit 61 of the control device 60 of the power conversion device 1 determines that a current imbalance has occurred, it can determine that LC resonance has occurred. Therefore, the arithmetic circuit 61 can adjust the operation of the power conversion device 1 to suppress the LC resonance. As described above, LC resonance can occur when a current having an AC component with a predetermined ripple frequency flows through the smoothing circuit 20. Therefore, for example, the arithmetic circuit 61 can change the ripple frequency and suppress the LC resonance by changing the switching frequency of the switching process. Furthermore, an increase in the amount of current flowing through the capacitor element 21 due to the LC resonance can cause an abnormality in the capacitor element 21. Therefore, the arithmetic circuit 61 may control the switching process to reduce the output voltage from the conversion circuit 40 to the motor 50 in order to reduce the amount of current flowing through the capacitor element 21. By changing the operation of the power conversion device 1 in this manner, the arithmetic circuit 61 can eliminate or reduce the current imbalance occurring among the multiple capacitor elements 21 of the smoothing circuit 20. Therefore, the power conversion device 1 according to the present disclosure can suppress overheating of the capacitors without stopping the operation of the motor 50.

[0077] With the above-described configuration, the power conversion device 1 can measure the voltage across the capacitor element 21 and determine whether a current imbalance has occurred using the arithmetic circuit 61. Therefore, the power conversion device 1 does not need to include a filter means or a frequency analysis means for extracting an oscillation component from the voltage across the capacitor element. Even if a filter means is included, a number of filter means corresponding to the expected frequencies must be provided in order to extract an oscillation component corresponding to the ripple frequency. Furthermore, when an oscillation component is extracted using a filter means, if an LC resonance occurs due to an oscillation component based on a frequency that is not expected in advance, the occurrence of the LC resonance cannot be detected. However, the power conversion device 1 according to the present disclosure does not need a filter means, thereby simplifying the configuration. Furthermore, even if an LC resonance occurs due to a frequency that is not expected in advance, the occurrence of the LC resonance can be detected.

[0078] A method of providing a current sensor inside the device to measure the magnitude of the current flowing through each capacitor element 21 is conceivable. However, when a capacitor device 25 as described below is applied to a smoothing circuit 20, providing a current sensor inside the capacitor device 25 is not appropriate from the viewpoints of structure, performance, or cost. The power conversion device 1 according to the present disclosure measures the magnitude of the current flowing through each capacitor element 21 by measuring the voltage across each capacitor element 21. Therefore, there is no need to provide a current sensor for measuring the current inside the device, and the magnitude of the current flowing through each capacitor element 21 can be easily measured.

[0079] The power conversion device 1 according to the present disclosure can detect a current imbalance and detect the occurrence of LC resonance even while the motor 50 is operating. For example, the arithmetic circuit 61 can detect the occurrence of LC resonance by monitoring the temperature of the capacitor elements 21. However, a certain period of time must pass after an overcurrent flows before a rise in the temperature of the capacitor elements 21 can be detected, resulting in a delay between the occurrence of LC resonance and its detection. Furthermore, because a temperature rise can occur due to factors other than an increase in the amount of current caused by LC resonance, it can be difficult to determine whether an abnormal mode has occurred. According to the power conversion device 1 according to the present disclosure, the arithmetic circuit 61 determines whether a current imbalance has occurred based on the magnitude of the current flowing through each capacitor element 21, allowing for detection without delay.

[0080] FIG. 11A is a perspective view of an example of a capacitor device 25A that can be used in a smoothing circuit 20 of a power conversion device 1 according to another embodiment. FIG. 11B is a perspective view of the capacitor device 25A of FIG. 11A as seen from the bottom. As shown in FIG. 11A, the capacitor device 25A has six capacitors 26 and a housing 27 in which the capacitors 26 are connected in parallel. The housing 27 may be a member, such as a plate-shaped member, on which the capacitors 26 can be arranged. The capacitor device 25A may thus be a so-called capacitor bank having a plurality of capacitors in the housing 27. Each of the six capacitors 26 houses a capacitor element 21. Each of the capacitors 26 may house a plurality of capacitor elements 21.

[0081] 11B, the capacitor device 25A has terminals 28A and 28B that can measure the voltage across each capacitor 26. The terminals 28A and 28B are provided so that they can be used from outside the capacitor device 25A. The capacitor device 25A functions as the smoothing circuit 20 in the power conversion device 1 by connecting terminals (not shown) to, for example, the first terminal 22a and the second terminal 22b. When the capacitor device 25A is connected to the first terminal 22a and the second terminal 22b, the six capacitors 26 are configured to be electrically connected in parallel between the first terminal 22a and the second terminal 22b.

[0082] FIG. 12A is a perspective view of an example of a capacitor device 25B including a smoothing circuit 20 of a power converter 1 according to yet another embodiment. FIG. 12B is a perspective view of the capacitor device 25B of FIG. 12A with a portion of the housing 27 removed. As shown in FIGS. 12A and 12B , the capacitor device 25B has five capacitor elements 21 and a housing 27 for connecting the capacitor elements 21 in parallel. The capacitor device 25B may be a so-called capacitor module having multiple capacitor elements within the housing 27. The capacitor device 25B has terminals 28A and 28B that can measure the voltage across each capacitor element 21. The terminals 28A and 28B are provided so as to be accessible from outside the capacitor device 25B. The capacitor device 25B functions as the smoothing circuit 20 in the power converter 1 by connecting terminals (not shown) to, for example, the first terminal 22a and the second terminal 22b. When the capacitor device 25B is connected to the first terminal 22a and the second terminal 22b, the five capacitor elements 21 are configured to be electrically connected in parallel between the first terminal 22a and the second terminal 22b.

[0083] With this configuration, the capacitor devices 25A and 25B can be applied as the smoothing circuit 20 of the power conversion device 1.

[0084] In the above examples, the arithmetic circuit 61 performs the determination process using the calculation methods according to the first to third embodiments or the calculation methods according to the fourth and fifth embodiments, but the present invention is not limited to this. For example, the arithmetic circuit 61 may calculate the evaluation value as the difference between the maximum value and the average value of the effective values ​​of the AC components of the voltages applied to the capacitor elements 21a to 21d. Alternatively, the arithmetic circuit 61 may obtain a deviation based on the effective values ​​of the AC components of the voltages of the capacitor elements 21a to 21d and use the deviation in the determination process.

[0085] In the above example, the difference in impedance regarding the paths of the plurality of capacitor elements 21a to 21d is caused by the difference in inductance, but is not limited to this. For example, the difference in impedance may be caused by the difference in capacitance of the capacitors.

[0086] In the above example, the smoothing circuit 20 is configured by connecting a plurality of capacitor elements 21a to 21d in parallel one by one, but this is not limited to this. For example, the smoothing circuit 20 may have a plurality of capacitor elements 21 connected in series in at least a portion of the parallel paths. Furthermore, the components connected in parallel are not limited to the capacitor elements 21, and a capacitor 26 having at least one capacitor element 21 may be connected in parallel.

[0087] (Summary of the embodiment) The excitation circuit, vibration device, and vehicle according to the present embodiment described above may be configured as follows.

[0088] (Aspect 1) A power conversion device (1) includes a conversion circuit (40) having a plurality of switching elements (42, 43) connected to a DC power source (10), and converting a DC voltage supplied from the DC power source (10) into a predetermined output voltage by performing a switching process of switching the plurality of switching elements (42, 43) at a predetermined switching frequency, and outputting the predetermined output voltage to a load (50); a smoothing circuit (20) having a plurality of capacitor elements (21) connected in parallel to the DC power source (10), and smoothing pulsation components generated by conversion in the conversion circuit (40) by the plurality of capacitor elements (21); The switching control system includes a voltage measurement device (30) that measures the voltage across each of the capacitor elements (21), and an arithmetic circuit (61) that executes a determination process to determine whether an imbalance has occurred in the currents flowing through the plurality of capacitor elements (21). The determination process acquires an effective value of an AC component of the voltage across each of the plurality of capacitor elements (21) based on the voltage across each of the plurality of capacitor elements (21) measured by the voltage measurement device (30) during execution of the switching process, and determines that an imbalance has occurred in the currents flowing through the plurality of capacitor elements (21) when an evaluation value based on the maximum value of the effective values ​​exceeds a predetermined threshold.

[0089] (Embodiment 2) In the power conversion device (1) of embodiment 1, the evaluation value may be the difference between the maximum value and the minimum value of the effective values.

[0090] (Embodiment 3) In the power conversion device (1) of embodiment 1, the evaluation value may be the difference between the maximum value and a reference value, and the reference value may be an estimated effective value of the AC component of the voltage across each of the plurality of capacitor elements (21) under conditions where parallel resonance does not occur in the smoothing circuit (20).

[0091] (Aspect 4) In the power conversion device (1) of aspect 3, the estimated effective value may be calculated by assuming that the impedance values ​​of the multiple paths, each including the multiple capacitor elements (21) in the smoothing circuit (20), with respect to the DC power supply (10) are equal to the smallest value among the impedance values.

[0092] (Aspect 5) In the power conversion device (1) of any one of aspects 1 to 4, when it is determined by the determination process that an imbalance has occurred, the arithmetic circuit (61) may change the predetermined switching frequency so as to reduce the imbalance.

[0093] (Aspect 6) In the power conversion device (1) of any one of aspects 1 to 4, when the arithmetic circuit (61) determines that an imbalance has occurred through the determination process, the arithmetic circuit (61) may reduce the output voltage from the conversion circuit (40) to the load (50) so as to reduce the imbalance.

[0094] (Aspect 7) A capacitor device (25) is used in a power conversion device (1) including: a conversion circuit (40) having a plurality of switching elements (42, 43) connected to a DC power source (10), which converts a DC voltage supplied from the DC power source (10) into a predetermined output voltage by performing a switching process of switching the plurality of switching elements (42, 43) at a predetermined switching frequency, and outputs the output voltage to a load (50); and a smoothing circuit (20) having a plurality of capacitor elements (21) connected in parallel to the DC power source (10), which smooths pulsation components generated by conversion in the conversion circuit (40) using the plurality of capacitor elements (21). The capacitor device (25) has a plurality of capacitor elements (21) and a housing (27) in which the plurality of capacitor elements (21) are mounted, and each of the plurality of capacitor elements (21) has terminals (28A, 28B) that can detect voltages across the plurality of capacitor elements (21) and are accessible from outside the capacitor device (25).

[0095] (Embodiment 8) A control method includes a converter circuit (40) having a plurality of switching elements (42, 43) connected to a DC power supply (10), and converting a DC voltage supplied from the DC power supply (10) into a predetermined output voltage by performing a switching process of switching the plurality of switching elements (42, 43) at a predetermined switching frequency, and outputting the predetermined output voltage to a load (50); a smoothing circuit (20) having a plurality of capacitor elements (21) connected in parallel to the DC power supply (10), and smoothing pulsation components generated by conversion in the converter circuit (40) with the plurality of capacitor elements (21); The control method for determining imbalance in currents flowing through a plurality of capacitor elements (21) in a power conversion device (1) having a voltage measurement device (30) that measures the voltages across each of the plurality of capacitor elements (21), the method obtaining an effective value of an AC component of the voltages across each of the plurality of capacitor elements (21) based on the voltages across each of the plurality of capacitor elements (21) measured by the voltage measurement device (30) during execution of a switching process, and determining that an imbalance in currents flowing through the plurality of capacitor elements (21) has occurred when an evaluation value based on the maximum value of the effective values ​​exceeds a predetermined threshold.

[0096] (Embodiment 9) A computer program can cause a computer to execute the control method of embodiment 8.

[0097] The power conversion device, capacitor device, control method, and computer program described in the present disclosure are realized by cooperation between hardware resources, such as a processor and memory, and software resources (computer programs). [Industrial Applicability]

[0098] According to the present disclosure, it is possible to provide a power conversion device, a capacitor device, a control method, and a computer program that can detect the occurrence of LC resonance between capacitor elements connected in parallel to a DC power supply, and therefore the present disclosure can be suitably used in this type of industrial field. [Explanation of symbols]

[0099] 1 Power conversion device 10 DC power supply 20 Smoothing circuit 21, 21a, 21b, 21c, 21d Capacitor elements 25A, 25B capacitor device 27 Case 28A, 28B terminals 30 Voltage measuring device 40 Conversion circuit 50 motor 60 Control device 61 Arithmetic circuit

Claims

1. a conversion circuit having a plurality of switching elements connected to a DC power supply, which performs a switching process of switching the plurality of switching elements at a predetermined switching frequency to convert a DC voltage supplied from the DC power supply into a predetermined output voltage and outputs the predetermined output voltage to a load; a smoothing circuit having a plurality of capacitor elements connected in parallel to the DC power supply, smoothing pulsation components generated by conversion in the conversion circuit by the plurality of capacitor elements; a voltage measurement device that individually measures the voltage across each of the plurality of capacitor elements; an arithmetic circuit that executes a determination process to determine whether an imbalance has occurred in the currents flowing through the plurality of capacitor elements; Equipped with The determination process includes: acquiring an effective value of an AC component of the voltage across each of the plurality of capacitor elements based on the voltage across each of the plurality of capacitor elements measured by the voltage measurement device during execution of the switching process; When an evaluation value based on the maximum value of the effective values ​​exceeds a predetermined threshold, it is determined that an imbalance in currents flowing through the plurality of capacitor elements has occurred. Power conversion device.

2. The power conversion device according to claim 1 , wherein the evaluation value is a difference between the maximum value and a minimum value of the effective values.

3. the evaluation value is the difference between the maximum value and a reference value, The power conversion device according to claim 1 , wherein the reference value is an estimated effective value of an AC component of a voltage across each of the plurality of capacitor elements under conditions where no parallel resonance occurs in the smoothing circuit.

4. 4. The power conversion device according to claim 3, wherein the estimated effective value is calculated on the assumption that each of impedance values ​​of a plurality of paths each including the plurality of capacitor elements in the smoothing circuit with respect to the DC power supply is equal to a smallest value among the impedance values.

5. The power conversion device according to claim 1 , wherein, when the determination process determines that the imbalance occurs, the arithmetic circuit changes the predetermined switching frequency so as to reduce the imbalance.

6. 5. The power conversion device according to claim 1, wherein, when the determination process determines that the imbalance has occurred, the arithmetic circuit reduces the output voltage from the conversion circuit to the load so as to reduce the imbalance.

7. a conversion circuit having a plurality of switching elements connected to a DC power supply, which performs a switching process of switching the plurality of switching elements at a predetermined switching frequency to convert a DC voltage supplied from the DC power supply into a predetermined output voltage and outputs the predetermined output voltage to a load; a smoothing circuit having a plurality of capacitor elements connected in parallel to the DC power supply, smoothing pulsation components generated by conversion in the conversion circuit by the plurality of capacitor elements; a voltage measurement device that individually measures the voltage across each of the plurality of capacitor elements; A control method for determining an imbalance in currents flowing through the plurality of capacitor elements in a power conversion device having the acquiring an effective value of an AC component of the voltage across each of the plurality of capacitor elements based on the voltage across each of the plurality of capacitor elements measured by the voltage measurement device during execution of the switching process; When an evaluation value based on the maximum value of the effective values ​​exceeds a predetermined threshold, it is determined that an imbalance in currents flowing through the plurality of capacitor elements has occurred. Control method.

8. A computer program for causing an arithmetic circuit to execute the control method according to claim 7.

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