Power conversion device

Abstract

A power conversion device that performs power conversion between a power supply and a load, the power conversion device having a pair of input terminals, a pair of output terminals, a reactor, a diode, a switch, a capacitor, a current sensor, and a control unit. The control unit is configured to execute a diagnosis process that diagnoses deterioration of the capacitor. The diagnosis process includes: a switching process that imparts an alternating component that varies at a predetermined frequency to a voltage applied from the power supply to a series circuit of the reactor and the capacitor by repeatedly switching the switch on and off; a determination process that determines a phase difference or a gain possessed by an alternating component that appears in a current measured by the current sensor with respect to the imparted alternating component; and a determination process that determines whether the phase difference or the gain determined by the determination process is within a normal range.

Description

TECHNICAL FIELD

[0001] The technology disclosed in this specification relates to a power conversion device. BACKGROUND

[0002] In Japanese Patent Application Publication No. 2019-153403, a power conversion device that performs power conversion between a fuel cell and a load is described. The power conversion device includes a pair of input terminals, a pair of output terminals, a reactor, a diode, a switch, a capacitor, a current sensor, a voltage sensor, and a control unit. The control unit can diagnose deterioration of the fuel cell based on an impedance of the fuel cell determined based on a voltage and a current of the fuel cell.

[0003] In the above-described power conversion device, the reactor, the diode, and the switch constitute a DC-DC converter. The DC-DC converter steps up direct-current power from a power source such as a fuel cell by the switch being turned on and off at high speed, and outputs from the pair of output terminals. The capacitor is connected between the pair of output terminals, and thus generation of a pulsating current is suppressed. SUMMARY

[0004] Generally, a capacitor deteriorates due to repeated charging and discharging. Due to the deterioration of the capacitor, the electrostatic capacity of the capacitor becomes small, and thus there is a possibility that the capacitor cannot sufficiently reduce a pulsating current. Therefore, in the design stage of the capacitor, a method of designing a capacitor that satisfies a required performance such as the number of times and the time of charging and discharging is adopted. However, for example, in a case where the environmental temperature actually experienced by the capacitor is different from the assumed temperature range, there is a possibility that the deterioration of the capacitor is accelerated. Therefore, depending on the environmental temperature experienced by the capacitor, there is a possibility that the capacitor will be greatly deteriorated in a shorter period than assumed, and then cannot sufficiently reduce a pulsating current. In order to avoid such a situation, a method of diagnosing the deterioration of the capacitor according to the actual state of the capacitor is sought.

[0005] The technology disclosed in this specification is embodied in a power conversion device. The power conversion device is a power conversion device that performs power conversion between a power supply and a load, and includes: a pair of input terminals connected to the power supply; a pair of output terminals connected to the load; an inductor connected at one end to one of the pair of input terminals; a diode connected at an anode to the other end of the inductor and connected at a cathode to one of the pair of output terminals; a switch connected at one end to the other end of the inductor and connected at the other end to the other of the pair of input terminals and to the other of the pair of output terminals; a capacitor connected between the pair of output terminals; a current sensor configured to measure a current flowing in the inductor; and a control unit configured to control an operation of the switch. The control unit is configured to execute a diagnosis process that diagnoses deterioration of the capacitor. The diagnosis process includes: a switching process that imparts, to a voltage applied from the power supply to a series circuit of the inductor and the capacitor, a first alternating-current component that varies at a predetermined frequency by repeating conduction and cutoff of the switch; a determination process that determines a phase difference or a gain possessed by a second alternating-current component that appears in a current measured by the current sensor with respect to the first alternating-current component imparted in the switching process; and a judgment process that judges whether or not the phase difference or the gain determined by the determination process is within a normal range.

[0006] In the power conversion device described above, the inductor, the diode, and the switch constitute a DC-DC converter, and the capacitor for suppressing a pulsating current is connected on a high-voltage side of the DC-DC converter. In this circuit configuration, there is an LC series circuit in which the inductor and the capacitor are connected in series with respect to the power supply. The control unit executes a diagnosis process that diagnoses deterioration of the capacitor by using this LC series circuit. In the diagnosis process, an alternating-current component (hereinafter referred to as an input alternating-current component) that varies at a predetermined frequency is imparted to a voltage applied from the power supply to the LC series circuit, and an alternating-current component (hereinafter referred to as an output alternating-current component) that appears in a current flowing in the inductor is observed. A phase difference and a gain corresponding to a resonance frequency of the LC series circuit are generated between the input alternating-current component and the output alternating-current component, and the resonance frequency of the LC series circuit varies depending on an electrostatic capacitance of the capacitor. Therefore, it is possible to judge a change in the electrostatic capacitance of the capacitor, that is, whether or not the capacitor is deteriorated, from the phase difference or the gain between the input alternating-current component and the output alternating-current component.

[0007] Thus, for example, even in a case where the capacitor deteriorates earlier than expected due to a difference between an environmental temperature experienced by the capacitor and an expected temperature range, or the like, the control unit can diagnose deterioration of the capacitor from an actual state of the capacitor. In a case where it is diagnosed that the capacitor is deteriorated, for example, it is possible to take appropriate measures such as replacing the capacitor. Thus, in the power conversion device, it is possible to maintain a function of the capacitor that reduces a pulsating current. Attached Figure Description

[0008] The features, advantages, and technical and industrial significance of exemplary embodiments of the present invention will now be described with reference to the accompanying drawings, in which the same reference numerals denote the same elements, wherein:

[0009] Figure 1 It is a diagram illustrating the structure of a power conversion device.

[0010] Figure 2 The diagram above shows an example of an input AC component assigned to an LC series circuit. The diagram below shows an example of the output AC component that appears in an LC series circuit when an input AC component is assigned.

[0011] Figure 3 This is a diagram illustrating an example of the phase difference θ between the input AC component and the output AC component, as well as the gain G, before capacitor degradation has progressed.

[0012] Figure 4 This is a diagram illustrating an example of the phase difference θ between the input and output AC components and the gain G under conditions of capacitor degradation. Furthermore, in Figure 4 In the middle, as a comparative example, a dashed line is used to represent... Figure 3 The phase difference θ and the gain G.

[0013] Figure 5 This is a flowchart illustrating an example of diagnostic processing performed by the control unit. Detailed Implementation

[0014] In one embodiment of this technology, the control unit may determine the phase difference during the determination process. In this case, the control unit may also determine whether the determined phase difference is within the normal range during the judgment process. That is, capacitor degradation may be diagnosed based on the phase difference determined during the determination process. However, as another embodiment, the gain may be determined during the determination process, and capacitor degradation may be diagnosed based on the gain.

[0015] In one embodiment of this technology, the power conversion device may further include a pair of relays disposed between a pair of output terminals and the load. In this case, the control unit may also perform an open-circuit operation to disconnect the pair of relays before performing diagnostic processing. With this structure, the influence caused by the presence of the load can be eliminated, and the diagnostic processing can be performed more accurately. Furthermore, it is possible to prevent pulsating current caused by switching operations from flowing into the load. Thus, for example, load degradation can be avoided.

[0016] In one embodiment of this technology, the diagnostic process may further include a notification process, in which a predetermined notification action is executed if the control unit determines, during the determination process, that the phase difference or gain is outside the normal range. Based on this structure, for example, users or managers of power conversion devices can identify capacitor deterioration and thus take appropriate measures such as replacing the capacitor.

[0017] In one embodiment of this technology, the power source can also be a fuel cell. Furthermore, the power source is not necessarily limited to a fuel cell; it can also be a generator such as an engine generator, a solar power device, or a secondary battery such as a lithium-ion battery.

[0018] In one embodiment of this technology, the power conversion device can also be mounted on a vehicle. However, as other embodiments, the power conversion device can also be mounted on mobile bodies such as ships and aircraft, or on stationary power generation systems.

[0019] In the above embodiments, diagnostic processing can also be performed when the vehicle is parked. When the vehicle is parked, there is no need for output from the power conversion device to the load. Therefore, the control unit can perform an open-circuit operation that disconnects a pair of relays before performing diagnostic processing.

[0020] Referring to the accompanying drawings, the power conversion device 10 will be described. The power conversion device 10 is a device disposed between the power source 12 and the load 14, performing power conversion between them. In this embodiment, the power conversion device 10 includes a DC-DC converter 20, which can boost the DC power from the power source 12 and output it to the load 14. Furthermore, other power conversion circuits, such as an inverter circuit, may be disposed between the DC-DC converter 20 and the load 14 as needed. The power conversion device 10 of this embodiment can be installed in vehicles such as hybrid electric vehicles, fuel cell vehicles, or electric vehicles. However, the technology disclosed in this embodiment is not limited to the power conversion device 10 installed in automobiles; it can be used for various purposes such as power conversion devices for mobile bodies such as ships and aircraft, and stationary power generation systems.

[0021] As an example, in the power conversion device 10 of this embodiment, the power source 12 can also be a fuel cell. However, the power source 12 is not necessarily limited to a fuel cell; it can also be a generator such as an engine generator or a solar power generation device, or a secondary battery such as a lithium-ion battery. Furthermore, in the power conversion device 10 of this embodiment, the load 14 can also be a three-phase motor, such as a motor for driving a car. In this case, a three-phase inverter that generates three-phase alternating current with U-phase, V-phase, and W-phase can be provided between the power conversion device 10 and the load 14.

[0022] like Figure 1As shown, the power conversion device 10 includes a pair of input terminals 16 connected to a power supply 12 and a pair of output terminals 18 connected to a load 14. The pair of input terminals 16 includes a positive input terminal 16a connected to the positive terminal of the power supply 12 and a negative input terminal 16b connected to the negative terminal of the power supply 12. The pair of output terminals 18 includes a positive output terminal 18a connected to the positive terminal of the load 14 and a negative output terminal 18b connected to the negative terminal of the load 14. The negative input terminal 16b and the negative output terminal 18b are interconnected and maintained at the same potential.

[0023] like Figure 1 As shown, the DC-DC converter 20 also includes a reactor 22, a diode 24, and a switch 26. One end of the reactor 22 is connected to the positive input terminal 16a, and the other end is connected to the anode of the diode 24. The cathode of the diode 24 is connected to the positive output terminal 18a. Therefore, the reactor 22 and the diode 24 are connected in series between the positive input terminal 16a connected to the power supply 12 and the positive output terminal 18a connected to the load 14. One end of the switch 26 is connected to the other end of the reactor 22 and the anode of the diode 24. The other end of the switch 26 is connected to the negative input terminal 16b and the negative output terminal 18b.

[0024] With the above structure, in the DC-DC converter 20, when switch 26 is turned on, a closed loop is formed connecting power supply 12 and reactor 22, and electrical energy is supplied from power supply 12 to reactor 22 and stored therein. Then, when switch 26 is turned off, the electrical energy stored in reactor 22, together with the electrical energy from power supply 12, is output to a pair of output terminals 18. Therefore, in the DC-DC converter 20, switch 26 is alternately turned on and off at a predetermined frequency (i.e., a predetermined timing), thereby boosting the DC power from power supply 12 and outputting it from the pair of output terminals 18 to load 14.

[0025] like Figure 1 As shown, the power conversion device 10 also includes a capacitor 28. The capacitor 28 is a so-called smoothing capacitor, used to smooth the DC power output from the DC-DC converter 20. The capacitor 28 is connected between a pair of output terminals 18. That is, the capacitor 28 is connected between the positive output terminal 18a and the negative output terminal 18b. Thus, the capacitor 28 can suppress pulsating currents that occur when the switch 26 is alternately turned on and off at a predetermined frequency.

[0026] like Figure 1As shown, the power conversion device 10 also includes a control unit 50. The control unit 50 is a control device that monitors and controls the operation of the power conversion device 10. The control unit 50 is communicatively connected to the switch 26 and configured to perform PWM (Pulse Width Modulation) control of the switch 26. In this PWM control, the switch 26 is turned on and off at a predetermined frequency, and the on-time (duty cycle) of the switch 26 is adjusted according to the demand from the load 14. By executing the PWM control of the switch 26, the control unit 50 can regulate the power supplied to the load 14 according to the demand from the load 14.

[0027] like Figure 1 As shown, the power conversion device 10 also includes a current sensor 30. As an example, the current sensor 30 is disposed between the cathode and the positive output terminal 18a of the diode 24, and is capable of measuring the current output from the power supply 12 via the reactor 22 from the positive output terminal 18a. The current sensor 30 is communicatively connected to the control unit 50, configured so that the control unit 50 can obtain the measurement result measured by the current sensor 30. Furthermore, the specific structure and configuration of the current sensor 30 are not particularly limited. The current sensor 30 can be any sensor capable of measuring the current flowing through the reactor 22.

[0028] like Figure 1 As shown, the power conversion device 10 also includes a pair of relays 32. The pair of relays 32 can electrically connect and disconnect the power supply 12 and the load 14. The pair of relays 32 are located between a pair of output terminals 18 and the load 14. When the pair of relays 32 are on, the pair of output terminals 18 are electrically connected to the load 14, and the power supply 12 and the load 14 are electrically connected via a DC-DC converter 20. Conversely, when the pair of relays 32 are off, the pair of output terminals 18 are electrically disconnected from the load 14, and the power supply 12 and the load 14 are also electrically disconnected. The connection and disconnection of the pair of relays 32 are controlled by the control unit 50. However, as another embodiment, the connection and disconnection of the pair of relays 32 can be switched by other control devices or by the user instead of the control unit 50.

[0029] In the above structure, reactor 22 and capacitor 28 form an LC series circuit with respect to power supply 12. This LC series circuit has a resonant frequency fx determined by the reactance of reactor 22 and the capacitance of capacitor 28. Figure 2As shown, when an AC component is assigned to the voltage V(t) applied to an LC series circuit, an AC component also appears in the current I(t) flowing through the LC series circuit. Hereinafter, the AC component assigned to the voltage V(t) is sometimes referred to as the input AC component, and the AC component appearing in the current I(t) is referred to as the output AC component. The input AC component and the output AC component have the same frequency, but there is a phase difference θ between them.

[0030] like Figure 3 As shown, the phase difference θ between the input AC component and the output AC component varies according to the relationship between the frequency f of the input AC component and the resonant frequency fx of the LC series circuit. For example, when the frequency f of the input AC component is equal to the resonant frequency fx of the LC series circuit, the phase difference θ between the input AC component and the output AC component is zero. When the frequency f of the input AC component is less than the resonant frequency fx of the LC series circuit, the phase difference θ is positive. Moreover, when the frequency f of the input AC component is greater than the resonant frequency fx of the LC series circuit, the phase difference θ is negative.

[0031] As described above, the resonant frequency fx of the LC series circuit is determined by the reactance of reactor 22 and the capacitance of capacitor 28. Since the reactance of reactor 22 and the capacitance of capacitor 28 are known, when a predetermined frequency fa (e.g., 1 kHz) is applied to the LC series circuit as an input AC component, an assumed phase difference θa (e.g., 65°) is observed between the input AC component and the output AC component.

[0032] However, capacitor 28 deteriorates due to repeated charging and discharging. As capacitor 28 deteriorates, its electrostatic capacitance changes. Generally, the more the capacitor 28 deteriorates, the lower its electrostatic capacitance becomes, but there is no particular limitation on this. As the electrostatic capacitance of capacitor 28 changes, the resonant frequency fx of the LC series circuit also changes. That is, the resonant frequency fx of the LC series circuit changes as capacitor 28 deteriorates.

[0033] like Figure 4As shown, when the resonant frequency fx of the LC series circuit changes, the relationship between the frequency f of the input AC component and the phase difference θ that appears between the input AC component and the output AC component also changes. Therefore, when the capacitor 28 is deteriorating, if an input AC component of the predetermined frequency fa (e.g., 1 kHz) is applied to the LC series circuit, a phase difference θb (e.g., 72°) occurs in the measured output AC component, which is different from the expected phase difference θa. In other words, if an input AC component of the predetermined frequency fa is applied to the LC series circuit, and a phase difference θb occurs in the measured output AC component, which is different from the expected phase difference θa, it can be determined that the capacitor 28 is deteriorating. On the other hand, if the expected phase difference θa occurs in the detected output AC component, it can be determined that the capacitor 28 is not deteriorating.

[0034] like Figure 3 , Figure 4 As shown, the magnitude of the output AC component relative to the input AC component (i.e., the gain G) also varies depending on the relationship between the frequency f of the input AC component and the resonant frequency fx of the LC series circuit. For example, when the frequency f of the input AC component is equal to the resonant frequency fx of the LC series circuit, the gain G of the output AC component relative to the input AC component becomes the largest. Moreover, the further the frequency f of the input AC component is from the resonant frequency fx of the LC series circuit, the lower the gain G of the output AC component relative to the input AC component. Therefore, when an input AC component with a predetermined frequency fa is applied to the LC series circuit, if the measured output AC component has an amplitude different from the expected amplitude, it can be determined that the capacitor 28 is deteriorating. On the other hand, if the measured output AC component has the expected amplitude, it can be determined that the capacitor 28 is not deteriorating.

[0035] Based on the above understanding, in the power conversion device 10 of this embodiment, the control unit 50 is configured to perform diagnostic processing to diagnose the deterioration of the capacitor 28. Hereinafter, referring to... Figure 5 This section provides a detailed example of a diagnostic process performed by the control unit 50.

[0036] exist Figure 5In this process, when the control unit 50 determines that the power conversion device 10 is in a stable state ("Yes" in step S10), it proceeds to the processing after step S12. Here, a stable state for the power conversion device 10 means that there is no need for output from the power conversion device 10 to the load 14. For example, when the power conversion device 10 is installed in a vehicle, and the vehicle is parked, the control unit 50 determines that the power conversion device 10 is in a stable state and proceeds to the processing after step S12. On the other hand, when the control unit 50 determines that the power conversion device 10 is not in a stable state ("No" in step S10), the control unit 50 returns to the processing of step S10. That is, the control unit 50 repeatedly executes the processing of step S10 until it determines that the power conversion device 10 is in a stable state.

[0037] In step S12, before performing diagnostic processing after step S14, the control unit 50 performs an open-circuit process to disconnect a pair of relays 32. When the pair of relays 32 are disconnected, the load 14 is electrically disconnected from the power conversion device 10. This eliminates the influence caused by the presence of the load 14, allowing for more accurate diagnostic processing. In step S14, the control unit 50 performs a switching process to impart the aforementioned input AC component. In this switching process, by repeatedly turning the switch 26 on and off, an AC component (i.e., the input AC component) varying at a predetermined frequency fa is imparted to the voltage applied from the power supply 12 to the LC series circuit. In step S16, the control unit 50 performs a determination process, in which the phase difference θ or gain G of the AC component appearing in the current measured by the current sensor 30 relative to the AC component imparted in the switching process of step S14 (i.e., the output AC component) is determined. That is, in the determination process, the phase difference θ or gain G of the output AC component relative to the input AC component imparted in the switching process is determined.

[0038] In step S18, the control unit 50 performs a determination process, in which it determines whether the phase difference θ or gain G determined by the determination process is within the normal range. The normal range is determined through experimentation or simulation and is pre-stored in the control unit 50. The specific format of the normal range is not particularly limited; it can be a range defined by one or both of a predetermined lower limit and an upper limit.

[0039] While not specifically limited, for example, the normal range for the phase difference θ of the output AC component when an input AC component of a predetermined frequency fa is applied to the LC series circuit can be experimentally determined based on the various states of the capacitor 28's initial state and its deterioration development. Specifically, the phase difference θa from the initial state of the capacitor 28 (refer to...) Figure 3The phase difference θb (refer to) the phase difference θb under the deterioration state of capacitor 28 (refer to) Figure 4 The range up to this point is considered the normal range. Here, the initial state of capacitor 28 refers to the state where the degradation of capacitor 28 has not developed and capacitor 28 has the electrostatic capacitance of the design value.

[0040] Regarding the normal range of gain G, similarly, the gain G of the output AC component can be experimentally determined based on the initial state of capacitor 28 and various states of degradation development of capacitor 28 when an input AC component of a predetermined frequency fa is applied to the LC series circuit. Furthermore, the specific determination method is the same as that for the phase difference θ described above, and therefore is omitted.

[0041] In the determination process of step S18, if the control unit 50 determines that the phase difference θ or gain G is outside the normal range ("Yes" in step S18), it executes a notification process for performing a predetermined notification action (step S20). The predetermined notification action may include, for example, directly notifying the user or manager of the power conversion device 10 of the determination result. Furthermore, the predetermined notification action may also include indirectly notifying the user or manager of the power conversion device 10 of the determination result by performing a predetermined action indicating that the phase difference θ or gain G is outside the normal range (e.g., illuminating a notification light). Thus, the predetermined notification action can be used to notify the user or manager of the deterioration of the capacitor 28, and the specific method is not particularly limited.

[0042] If the phase difference θ or gain G is determined to be within the normal range in the determination process of step S18 ("No" in step S18), or if the process of step S20 has been executed, the control unit 50 ends the diagnostic process.

[0043] As described above, the control unit 50 can perform diagnostic processing to diagnose the deterioration of the capacitor 28 using an LC series circuit consisting of the reactor 22 and the capacitor 28. With this structure, for example, even if the capacitor 28 deteriorates earlier than expected due to a difference between the ambient temperature experienced by the capacitor 28 and the intended temperature range, the control unit 50 can diagnose the deterioration of the capacitor 28 based on its actual condition. If deterioration of the capacitor 28 is diagnosed, appropriate measures such as replacing the capacitor 28 can be taken. Thus, in the power conversion device 10, the function of the capacitor 28, which reduces pulsating current, can be maintained.

[0044] The above details several specific examples, but these are merely illustrative and do not limit the scope of the claims. The technology described in the claims includes examples obtained by various modifications and alterations of the specific examples illustrated above. The technical elements described in this specification or the accompanying drawings are technically useful individually or in combination.

Claims

1. A power conversion device for converting power between a power source and a load, the power conversion device comprising: A pair of input terminals are connected to the power supply. A pair of output terminals are connected to the load. A reactor, one end of which is connected to one of the pair of input terminals; The diode has its anode connected to the other end of the reactor and its cathode connected to one of the pair of output terminals. A switch, one end of which is connected to the other end of the reactor, and the other end of which is connected to the other side of the pair of input terminals and the other side of the pair of output terminals; A capacitor is connected between the pair of output terminals; A current sensor configured to measure the current flowing through the reactor; A pair of relays are disposed between the pair of output terminals and the load; as well as The control unit is configured to control the operation of the switch. The control unit is configured to perform diagnostic processing to diagnose the deterioration of the capacitor. The diagnostic process includes: Determine whether the power conversion device is in a stable state, wherein a stable state is a state in which no output is required from the power conversion device to the load; When it is determined that the power conversion device is in a stable state, an open-circuit process is executed to disconnect the pair of relays; The switching process, by repeatedly turning the switch on and off, imparts a predetermined frequency-varying input AC component to the voltage applied from the power source to the series circuit of the reactor and the capacitor. The process involves determining the phase difference or gain of the output AC component appearing in the current measured by the current sensor relative to the input AC component assigned in the switching process. The determination process determines whether the phase difference or the gain determined by the determination process is within the normal range; and If the phase difference or the gain is determined to be outside the normal range, it is diagnosed as a sign of capacitor degradation. Specifically, the range from the phase difference of the capacitor in its initial state to the phase difference in its deteriorated state is defined as the normal range, or the range from the gain of the capacitor in its initial state to the gain in its deteriorated state is defined as the normal range.

2. The power conversion device according to claim 1, wherein, The diagnostic process also includes a notification process, in which a predetermined notification action is executed if the control unit determines in the determination process that the phase difference or the gain is outside the normal range.

3. The power conversion device according to claim 1 or 2, wherein, The power source is a fuel cell.

4. The power conversion device according to claim 1 or 2, wherein, The power conversion device is mounted on the vehicle.

5. The power conversion device according to claim 4, wherein, The diagnostic process is performed when the vehicle is parked.

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