Power conversion device and air conditioner

Abstract

Provided is a highly reliable power conversion device. A power conversion device (100) comprises a converter circuit (10), a smoothing capacitor (30), an inverter circuit (60), and an overvoltage protection circuit (50), and also comprises a first control circuit (81) having a microcomputer, a second control circuit (82) configured from an analog electronic circuit, and a power supply circuit (70). The input side of the power supply circuit (70) is connected to a pair of DC lines (K1, K2). When the voltage of a smoothing capacitor (30) becomes equal to or greater than a first prescribed value, the first control circuit (81) switches the overvoltage protection circuit (50) to an ON state, and when the voltage of the smoothing capacitor (30) becomes equal to or greater than a second prescribed value without the microcomputer of the first control circuit (81) being normally driven, the second control circuit (82) switches the overvoltage protection circuit (50) to the ON state. The second prescribed value is greater than the first prescribed value.

Description

Power conversion device and air conditioner

[0001] The present disclosure relates to a power conversion device and the like.

[0002] For example, Patent Document 1 discloses a power conversion device that converts AC power into DC power and then converts the DC power back into a predetermined AC power. Patent Document 1 describes a power conversion device that includes an overvoltage protection circuit that has a resistor and a semiconductor element connected in series and protects an inverter circuit from overvoltage. The overvoltage protection circuit is controlled by a first control unit and a second control unit.

[0003] Patent No. 6844725

[0004] However, Patent Document 1 does not describe the drive power supply for the first control unit or the second control unit. For example, if a power outage occurs while driving a load, power may not be supplied to the first control unit or the second control unit, and as a result, the overvoltage protection circuit may also stop functioning. Furthermore, for example, if the microcomputer of the first control unit is reset due to noise, the overvoltage protection circuit may stop functioning. In terms of improving the reliability of the power conversion device, the technology described in Patent Document 1 leaves room for improvement.

[0005] Therefore, an object of the present disclosure is to provide a highly reliable power conversion device and the like.

[0006] In order to solve the above-mentioned problems, the power conversion device according to the present disclosure includes a converter circuit that converts an AC voltage applied from an AC power source into a DC voltage, a smoothing capacitor connected to a pair of DC lines on the output side of the converter circuit and smoothing the DC voltage on the output side of the converter circuit, an inverter circuit that converts the DC voltage of the smoothing capacitor into an AC voltage and applies the AC voltage to a load, and an overvoltage protection circuit connected to the pair of DC lines, as well as a first control circuit having a microcomputer, a second control circuit configured with an analog electronic circuit, and a power supply circuit that applies a predetermined drive voltage to each of the first control circuit and the second control circuit, the input side of the power supply circuit being connected to the pair of DC lines, and when the voltage of the smoothing capacitor becomes equal to or greater than a first predetermined value, the first control circuit switches the overvoltage protection circuit on, and when the microcomputer does not operate normally and the voltage of the smoothing capacitor becomes equal to or greater than a second predetermined value, the second control circuit switches the overvoltage protection circuit on, and the second predetermined value is greater than the first predetermined value.

[0007] According to the present disclosure, a highly reliable power conversion device and the like can be provided.

[0008] FIG. 1 is a configuration diagram of a power conversion device according to a first embodiment. FIG. 2 is a time chart including the operation of overvoltage protection when the first control circuit is functioning normally in the power conversion device according to the first embodiment. FIG. 3 is a time chart including the operation of overvoltage protection when the first control circuit is not functioning normally in the power conversion device according to the first embodiment. FIG. 4 is a time chart including the operation of overvoltage protection when the first control circuit is functioning normally in a power conversion device according to a modified example of the first embodiment. FIG. 5 is a configuration diagram of a power conversion device according to a second embodiment. FIG. 6 is a configuration diagram of a power conversion device according to a third embodiment. FIG. 7 is a configuration diagram of a power conversion device according to a fourth embodiment. FIG. 8 is a configuration diagram of an air conditioner according to a fifth embodiment.

[0009] First Embodiment Fig. 1 is a configuration diagram of a power conversion device 100 according to a first embodiment. The power conversion device 100 shown in Fig. 1 converts AC power supplied from an AC power source E1 into DC power, converts the DC power into a predetermined AC power, and outputs the converted AC power to a motor M1 (load). The motor M1 may be, for example, a permanent magnet synchronous motor or another type of motor. As shown in Fig. 1, the power conversion device 100 includes a converter circuit 10, a reactor 20, a smoothing capacitor 30, a DC voltage detection unit 40, an overvoltage protection circuit 50, an inverter circuit 60, a power supply circuit 70, and a control unit 80.

[0010] The converter circuit 10 is a power converter that converts AC voltage applied from a three-phase AC power source E1 into DC voltage (pulsating DC voltage). For example, a diode bridge circuit that performs full-wave rectification is used as the converter circuit 10, but the present invention is not limited thereto. Alternatively, a switching-type converter circuit may be used as the converter circuit 10. As shown in FIG. 1 , the output side of the converter circuit 10 is connected to an inverter circuit 60 via a positive DC line K1 and a negative DC line K2.

[0011] The reactor 20 and the smoothing capacitor 30 are elements that smooth the pulsating DC voltage applied from the converter circuit 10. As shown in Fig. 1, the reactor 20 is provided on the positive DC line K1. More specifically, the reactor 20 is provided on the positive DC line K1 between the converter circuit 10 and a connection point between the DC line K1 and the smoothing capacitor 30.

[0012] As described above, the smoothing capacitor 30 is an element that smoothes the DC voltage on the output side of the converter circuit 10, and is connected to the pair of DC lines K1, K2 on the output side of the converter circuit 10. Specifically, one end (one lead wire) of the smoothing capacitor 30 is connected to the positive DC line K1, and the other end (the other lead wire) is connected to the negative DC line K2. For example, a film capacitor may be used as the smoothing capacitor 30.

[0013] Although film capacitors have a relatively small capacitance, they are less expensive than electrolytic capacitors and have a longer lifespan. Furthermore, because film capacitors are smaller than electrolytic capacitors, it is possible to miniaturize the board (not shown) on which the circuits of the power conversion device 100 are mounted. However, because film capacitors have a small capacitance, the terminal voltage of the film capacitor is prone to fluctuate with changes in the amount of stored electricity. Therefore, in this embodiment, an overvoltage protection circuit 50 is provided to prevent the voltage of the smoothing capacitor 30 from becoming too high. The type of smoothing capacitor 30 is not limited to a film capacitor, and other types of capacitors may be used.

[0014] The DC voltage detection unit 40 detects the DC voltage Edc across the smoothing capacitor 30. In other words, the DC voltage detection unit 40 has a function of detecting the DC voltage Edc between the pair of DC lines K1 and K2. For example, the DC voltage across the smoothing capacitor 30 may be divided by a series connection of multiple resistor elements (not shown), and the DC voltage across the smoothing capacitor 30 may be detected based on the voltage division ratio or the voltage of a predetermined resistor element. The detection value of the DC voltage detection unit 40 is output to the first control circuit 81 and also to the non-inverting input terminal (+ terminal) of a comparator 82a of the second control circuit 82.

[0015] The overvoltage protection circuit 50 is a circuit for protecting the smoothing capacitor 30 from overvoltage, and is connected to the pair of DC lines K1, K2. For example, if a small-capacity film capacitor is used as the smoothing capacitor 30, a regenerative current may flow through the smoothing capacitor 30 immediately after the motor M1 is stopped, causing the voltage of the smoothing capacitor 30 to rise. Therefore, in the first embodiment, the overvoltage protection circuit 50 is connected in parallel to the smoothing capacitor 30 to prevent overvoltage from being applied to the smoothing capacitor 30.

[0016] As shown in FIG. 1 , the overvoltage protection circuit 50 includes a resistor 51 and a switch 52 connected in series. The resistor 51 consumes electrical energy associated with the regenerative current of the motor M1 and converts it into thermal energy. The switch 52 controls whether or not a current flows through the resistor 51. The switch 52 may be a switching element such as a metal-oxide-semiconductor field-effect transistor (MOSFET), an insulated gate bipolar transistor (IGBT), or a bipolar transistor. Alternatively, a mechanical switch may be used as the switch 52. As shown in FIG. 1 , one end of the series connection of the resistor 51 and the switch 52 is connected to a positive DC line K1, and the other end is connected to a negative DC line K2.

[0017] For example, when an ON signal is input from the logic operator 83 (OR circuit) of the control unit 80 to the switch 52, the switch 52 switches to the ON state, and a current (regenerative current of the motor M1) flows sequentially through the resistance element 51 and the switch 52. This converts electrical energy into thermal energy in the resistance element 51, thereby suppressing the current flowing through the smoothing capacitor 30. As a result, the voltage of the smoothing capacitor 30 can be prevented from exceeding a predetermined withstand voltage. Note that while an OFF signal is being input from the logic operator 83 to the switch 52, the switch 52 is maintained in the open state.

[0018] The inverter circuit 60 is a power converter that converts the DC voltage of the smoothing capacitor 30 into a predetermined AC voltage and applies this AC voltage to the motor M1 (load). Although not shown, the inverter circuit 60 is configured such that a first leg, a second leg, and a third leg, each of which is configured with a pair of switching elements connected in series, are connected in parallel to the smoothing capacitor 30. For example, a MOSFET or an IGBT is used as such a switching element. In the first leg, the connection point between the pair of switching elements is connected to the windings of the motor M1 via wiring. The same applies to the remaining second leg and third leg.

[0019] In addition, in order to prevent breakdown of the switching elements due to commutation in the inverter circuit 60, a free wheel diode (not shown) is connected in anti-parallel to each switching element. If the switching elements have a parasitic diode (not shown), this parasitic diode functions as the free wheel diode, so there is no need to provide a separate free wheel diode.

[0020] The power supply circuit 70 has a function of applying a predetermined drive voltage to each of the first control circuit 81 and the second control circuit 82 of the control unit 80. For example, a switching regulator is used as this power supply circuit 70. As shown in FIG. 1 , the input side of the power supply circuit 70 is connected to a pair of DC lines K1 and K2. Specifically, the input side of the power supply circuit 70 is connected to the positive DC line K1 via a line L1 and to the negative DC line K2 via another line L2. The power supply circuit 70 generates the predetermined drive voltage using the DC voltage Edc across the smoothing capacitor 30 as input power.

[0021] As a result, even if a power outage occurs while the motor M1 is being driven, the power supply circuit 70 continues to operate using the DC voltage Edc of the smoothing capacitor 30 as input power. As a result, the control unit 80 can turn on the switch 52 as needed to prevent overvoltage from being applied to the smoothing capacitor 30.

[0022] If the power supply circuit 70 continues to operate during a power outage, the voltage of the smoothing capacitor 30 will gradually decrease, and it is possible that the voltage of the smoothing capacitor 30 will decrease to the point where it becomes difficult to operate the power supply circuit 70. In this case, the control unit 80 will be stopped, and the overvoltage protection circuit 50 will be switched off. However, since the voltage of the smoothing capacitor 30 has already decreased sufficiently, there is no particular risk of an overvoltage being applied to the smoothing capacitor 30.

[0023] The control unit 80 has the function of controlling various circuits. As shown in FIG. 1 , the control unit 80 includes a first control circuit 81, a second control circuit 82, and a logic operator 83. The first control circuit 81 is a circuit for controlling the inverter circuit 60 and the overvoltage protection circuit 50, and includes a microcomputer (hereinafter referred to as a MCU). For example, although not shown, the first control circuit 81 is configured to include electronic circuits such as a CPU (Central Processing Unit), ROM (Read Only Memory), RAM (Random Access Memory), and various interfaces, and reads out a program stored in the ROM and loads it into the RAM, causing the CPU to execute various processes.

[0024] The first control circuit 81 switches on and off each switching element (not shown) of the inverter circuit 60, for example, using a predetermined PWM (Pulse Width Modulation) signal. When the first control circuit 81 generates the PWM signal, it may appropriately use, in addition to the detection value of the DC voltage detector 40, the detected voltage value of the AC power supply E1, the detected current value on the DC side of the inverter circuit 60, the detected current value of each winding of the motor M1, etc. When the first control circuit 81 outputs the PWM signal to each switching element (not shown) of the inverter circuit 60, a predetermined AC voltage is applied to the three-phase windings of the motor M1, thereby driving the motor M1.

[0025] The first control circuit 81 also has a function of switching the switch 52 to the on state when the detection value of the DC voltage detection unit 40 (the voltage of the smoothing capacitor 30) becomes equal to or greater than a first predetermined value, thereby switching the overvoltage protection circuit 50 to the on state. Furthermore, after switching the switch 52 to the on state, if the detection value of the DC voltage detection unit 40 becomes less than the first predetermined value, the first control circuit 81 switches the switch 52 of the overvoltage protection circuit 50 to the off state. Note that the above-mentioned "first predetermined value" is a voltage threshold that serves as a criterion for determining whether the first control circuit 81 should switch the switch 52 on or off, and is set in advance.

[0026] The second control circuit 82 is configured as an analog electronic circuit (also referred to as a hardware circuit) and has the function of controlling the overvoltage protection circuit 50. Here, an "analog electronic circuit" refers to a circuit in which signals (voltage and current) are processed in an analog manner. An analog electronic circuit is configured with elements such as resistors, capacitors, coils, and transistors, and in the example of FIG. 1, a comparator 82a is used as the analog electronic circuit.

[0027] For example, when amplifying a voltage using the amplification function of a transistor, if the voltage on the input side of the transistor (for example, the voltage between the base and emitter) is continuously changed, the voltage on the output side (for example, the voltage between the base and collector) will also continuously change accordingly. In this way, characteristics in which the output signal changes continuously in response to continuous changes in the input signal are said to be "analog-like."

[0028] Note that circuits that process signals digitally are not considered analog electronic circuits. For example, a microcomputer that processes discrete signals consisting of "0" (low) and "1" (high) based on software is not considered an analog electronic circuit. Also, the second control circuit 82 does not include a microcomputer.

[0029] The comparator 82a is a circuit that compares the magnitude between the detection value of the DC voltage detection unit 40 and a second predetermined value. In the example of Fig. 1, a voltage corresponding to the detection value of the DC voltage detection unit 40 is applied to the non-inverting input terminal (+ terminal) of the comparator 82a, and a voltage corresponding to the second predetermined value is applied to the inverting input terminal (- terminal).

[0030] The "second predetermined value" is a voltage threshold value that serves as a criterion for determining whether the second control circuit 82 switches the state (on or off) of the switch 52, and is set in advance as a value greater than the first predetermined value. For example, when the detection value of the DC voltage detection unit 40 is equal to or greater than the second predetermined value, the comparator 82a outputs a signal of "1" (High) to the logic operator 83. When the detection value of the DC voltage detection unit 40 is less than the second predetermined value, the comparator 82a outputs a signal of "0" (Low) to the logic operator 83.

[0031] The logic operator 83 is a logical sum circuit (OR circuit) configured to output a signal of "1" (High) to the switch 52 when a signal of "1" (High) is input from at least one of the first control circuit 81 and the second control circuit 82. The input side of the logic operator 83 is connected to the output side of the first control circuit 81 and the output side of the second control circuit 82. The output side of the logic operator 83 is connected to the switch 52 of the overvoltage protection circuit 50.

[0032] As described above, the second control circuit 82 does not include a microcomputer. Therefore, even if the microcomputer in the first control circuit 81 is reset due to noise or the like and is unable to perform normal operation, the second control circuit 82 can switch the switch 52 on and off. As described above, in the first embodiment, when the first control circuit 81 is functioning normally, the first control circuit 81 controls the overvoltage protection circuit 50, and when the first control circuit 81 is not functioning normally, the second control circuit 82 controls the overvoltage protection circuit 50. As a result, even if the first control circuit 81 is reset due to noise or the like, the second control circuit 82 can still provide overvoltage protection for the smoothing capacitor 30.

[0033] FIG. 2 is a time chart including the operation of overvoltage protection when the first control circuit is functioning normally (see also FIG. 1 as appropriate). The horizontal axis of each time chart in FIG. 2 represents time. The vertical axis of FIG. 2 represents, from top to bottom, the voltage of the smoothing capacitor 30, the state of the inverter circuit 60, the output signal of the first control circuit 81, the output signal of the second control circuit 82, and the output signal of the logic operator 83. In the example of FIG. 2, after the inverter circuit 60 switches to the OFF state at time t1 and the motor M1 is stopped, the voltage of the smoothing capacitor 30 rises. This is because a regenerative current is generated by the induced voltage of the motor M1 immediately after the motor M1 is stopped, and this regenerative current flows into the smoothing capacitor 30.

[0034] Then, at time t2, the voltage of the smoothing capacitor 30 reaches the first predetermined value Er1. In this case, the first control circuit 81 outputs an "ON" (High) signal to the input side of the logic operator 83. Note that, since the voltage of the smoothing capacitor 30 has not yet reached the second predetermined value Er2 at time t2, the second control circuit 82 outputs an "OFF" (Low) signal to the input side of the logic operator 83.

[0035] As a result, at time t2, the logic operator 83 outputs an "ON" (High) signal to the switch 52, switching the switch 52 to the ON state. As a result, the regenerative current of the motor M1 is consumed as heat energy by the resistance element 51, and the voltage of the smoothing capacitor 30 begins to decrease from time t2.

[0036] 2, the voltage of the smoothing capacitor 30 becomes lower than the first predetermined value Er1, so the output signal of the logic operator 83 switches to "OFF" (low), and as a result, the switch 52 also switches to the off state. As described above, since the first predetermined value is smaller than the second predetermined value, when the first control circuit 81 is functioning normally, the processing of the first control circuit 81 is reflected in the overvoltage protection circuit 50.

[0037] Fig. 3 is a time chart including the operation of overvoltage protection when the first control circuit is not functioning normally (see also Fig. 1 as appropriate). The vertical and horizontal axes in Fig. 3 are the same as those in Fig. 2. Fig. 3 shows a state in which the microcomputer of the first control circuit 81 is reset due to the influence of noise or the like, causing the first control circuit 81 to stop functioning normally. In this case, the voltage of the smoothing capacitor 30 continues to rise from time t11 when the motor M1 stops, and even when the voltage of the smoothing capacitor 30 reaches the first predetermined value Er1, the output signal of the first control circuit 81 does not switch to "ON."

[0038] Then, at time t12, the voltage of the smoothing capacitor 30 reaches the second predetermined value Er2. In this case, the second control circuit 82 outputs an "ON" (High) signal to the input side of the logic operator 83. Meanwhile, the first control circuit 81, which is in a reset state, outputs an "OFF" (Low) signal to the input side of the logic operator 83. As a result, at time t12, the logic operator 83 inputs an "ON" (High) signal to the switch 52, switching the switch 52 to the on state. As a result, the regenerative current of the motor M1 is consumed as thermal energy by the resistance element 51, and the voltage of the smoothing capacitor 30 begins to decrease from time t12.

[0039] At time t13 in FIG. 3 , the voltage of the smoothing capacitor 30 falls below the second predetermined value Er2, causing the output signal of the logic operator 83 to switch to "OFF" (low), which in turn switches the switch 52 to the off state. Thus, if the microcomputer in the first control circuit 81 malfunctions and the voltage of the smoothing capacitor 30 exceeds the second predetermined value Er2, the second control circuit 82 switches the switch 52 to the on state, thereby switching the overvoltage protection circuit 50 to the on state. For example, the phenomenon in which the microcomputer in the first control circuit 81 is reset due to noise is considered to be included in the occurrence of the microcomputer "not operating normally." Note that the second control circuit 82 is an analog electronic circuit including the comparator 82a, and therefore is not particularly susceptible to being reset due to noise, as is the case with a microcomputer.

[0040] <Effects> According to the first embodiment, even when a low-cost, long-life film capacitor is used as the smoothing capacitor 30, it is possible to prevent the voltage of the smoothing capacitor 30 from becoming too high due to the regenerative current immediately after the motor M1 stops.

[0041] Furthermore, even if a power outage occurs while the motor M1 is being driven, the DC voltage of the smoothing capacitor 30 is applied to the power supply circuit 70, so that the overvoltage protection circuit 50 can continue to be controlled by the first control circuit 81 and the second control circuit 82. In other words, application of an overvoltage to the smoothing capacitor 30 immediately after a power outage can be prevented, thereby improving the reliability of the power conversion device 100.

[0042] Furthermore, during normal operation when the microcomputer in the first control circuit 81 is not reset, the overvoltage protection circuit 50 is controlled by the first control circuit 81. Generally, microcomputers have relatively small variations in their characteristics and are capable of highly accurate control based on predetermined software. Therefore, the first control circuit 81 can control overvoltage protection at appropriate timing.

[0043] Furthermore, even if the microcomputer of the first control circuit 81 is reset due to the influence of noise or the like, overvoltage protection control can be performed by the second control circuit 82. Thus, according to the first embodiment, even if an abnormal event such as a power outage or a reset of the microcomputer occurs, it is possible to prevent the smoothing capacitor 30 from becoming overvoltage.

[0044] Modification of First Embodiment In the first embodiment, the process of switching the switch 52 to the ON state based on the determination result of the first control circuit 81 (see FIG. 1 ) and then switching the switch 52 to the OFF state when the voltage of the smoothing capacitor 30 becomes less than the first predetermined value has been described. However, this is not limiting. For example, the process shown in the following FIG. 4 may be performed. Note that the configuration of the power conversion device 100 (see FIG. 1 ) is assumed to be the same as that of the first embodiment.

[0045] 4 is a time chart showing the operation of overvoltage protection when the first control circuit is functioning normally in a power conversion device according to a modification of the first embodiment. As shown in FIG. 4 , when the voltage of the smoothing capacitor 30 reaches a first predetermined value Er1 at time t2, the first control circuit 81 may output an "ON" (High) signal to the logic operator 83 for a first predetermined time Δt (the time from time t2 to t4). This causes the logic operator 83 to output an "ON" (High) signal to the switch 52 for the first predetermined time Δt. In this way, the first control circuit 81 returns the overvoltage protection circuit 50 to the OFF state when the first predetermined time Δt has elapsed since the first control circuit 81 switched the overvoltage protection circuit 50 to the ON state.

[0046] The aforementioned "first predetermined time Δt" is the time from when the voltage of the smoothing capacitor 30 reaches the first predetermined value Er1 and when the switch 52 is turned on to when it is turned back off, and is set in advance. Specifically, the length of the first predetermined time Δt is set so that the voltage of the smoothing capacitor 30 has dropped sufficiently (for example, to below the first predetermined value Er1) when the first predetermined time Δt has elapsed since the switch 52 was turned on.

[0047] The same applies to the case where the second control circuit 82 switches the switch 52 on and off. That is, the second control circuit 82 may return the overvoltage protection circuit 50 to the off state when a second predetermined time has elapsed since the overvoltage protection circuit 50 was switched to the on state. This type of processing also achieves the same effects as the first embodiment.

[0048] Second Embodiment In the second embodiment, the configuration of the overvoltage protection circuit 50A (see FIG. 5) is different from that of the first embodiment, but other configurations and processing contents are similar to those of the first embodiment. Therefore, only the parts that are different from the first embodiment will be described, and a description of the overlapping parts will be omitted.

[0049] FIG. 5 is a configuration diagram of a power conversion device 100A according to the second embodiment. As shown in FIG. 5, an overvoltage protection circuit 50A of the power conversion device 100A includes a capacitor 53 and a switch 52 connected in series. The capacitor 53 is an element that stores electric charge when the switch 52 is turned on. For example, an electrolytic capacitor is used as the capacitor 53. The capacitance of the capacitor 53 may be larger than the capacitance of the smoothing capacitor 30. This makes it easier for the regenerative current of the motor M1 to flow into the capacitor 53 rather than the smoothing capacitor 30.

[0050] 5, one end of the series connection of the capacitor 53 and the switch 52 is connected to the positive DC line K1 and the other end is connected to the negative DC line K2. More specifically, the positive electrode of the capacitor 53 is connected to the positive DC line K1, and the negative electrode of the capacitor 53 is connected to the negative DC line K2 via the switch 52.

[0051] Then, the first control circuit 81 or the second control circuit 82 switches the overvoltage protection circuit 50A to the on state by switching the switch 52 to the on state. For example, if a regenerative current is generated after the motor M1 is stopped and the switch 52 is switched to the on state, the regenerative current flows into each of the smoothing capacitor 30 and the capacitor 53 at a ratio corresponding to the ratio of the capacitances of the smoothing capacitor 30 and the capacitor 53. Therefore, by appropriately adjusting the capacitance of the capacitor 53 at the design stage, it is possible to prevent an overvoltage from being applied to the smoothing capacitor 30.

[0052] According to the second embodiment, by using the overvoltage protection circuit 50A including the capacitor 53 and the switch 52, it is possible to prevent an overvoltage from being applied to the smoothing capacitor 30. Furthermore, since there is no particular need to provide a high-resistance resistor element in the overvoltage protection circuit 50A to consume the regenerative current from the motor M1, procurement of electronic components is easier than in the first embodiment.

[0053] Third Embodiment In the third embodiment, the configuration of the overvoltage protection circuit 50B (see FIG. 6) is different from that of the first embodiment, but other configurations and processing contents are similar to those of the first embodiment. Therefore, only the parts that are different from the first embodiment will be described, and a description of the overlapping parts will be omitted.

[0054] FIG. 6 is a configuration diagram of a power conversion device 100B according to a third embodiment. As shown in FIG. 6, an overvoltage protection circuit 50B of the power conversion device 100B includes a capacitor 53, a switch 52, a first resistor element 54, and a second resistor element 55. The capacitor 53 is an element that stores electric charge when the switch 52 is turned on. For example, an electrolytic capacitor is used as the capacitor 53. In the example shown in FIG. 6, the positive electrode of the capacitor 53 is connected to the positive DC line K1 via the first resistor element 54. The negative electrode of the capacitor 53 is connected to the negative DC line K2.

[0055] The first resistor element 54 is an element for adjusting the magnitude of the current flowing into the capacitor 53 when the switch 52 is switched to the on state, and is connected in series to one side (positive electrode) of the capacitor 53. The second resistor element 55 is an element for consuming (converting into thermal energy) the charge stored in the capacitor 53, and is connected in parallel to the capacitor 53. The switch 52 is an element for switching whether or not to allow a current to flow through the first resistor element 54, and is connected in series to the other side (negative electrode) of the capacitor 53.

[0056] 6, one end of the series connection of the first resistor element 54, the capacitor 53, and the switch 52 is connected to the positive DC line K1 and the other end is connected to the negative DC line K2. The first control circuit 81 or the second control circuit 82 switches the switch 52 to the on state, thereby switching the overvoltage protection circuit 50b to the on state.

[0057] According to the third embodiment, the magnitude of the current flowing through the capacitor 53 when the switch 52 is switched to the on state can be adjusted by the first resistor element 54. That is, by appropriately adjusting the resistance value of the first resistor element 54 at the circuit design stage, it is possible to prevent an excessive current from flowing through the capacitor 53. In addition, the charge stored in the capacitor 53 can be consumed by the second resistor element 55.

[0058] Fourth Embodiment In the fourth embodiment, the configuration of the overvoltage protection circuit 50C (see FIG. 7) is different from that of the third embodiment (see FIG. 6), but other configurations and processing contents are similar to those of the third embodiment. Therefore, only the differences from the third embodiment will be described, and a description of overlapping portions will be omitted.

[0059] Fig. 7 is a configuration diagram of a power conversion device 100C according to the fourth embodiment. As shown in Fig. 7, an overvoltage protection circuit 50C of the power conversion device 100C includes a capacitor 53, a switch 52, a first resistor element 54, a second resistor element 55, a third resistor element 56, and diodes 57 and 58. Note that the connections of the capacitor 53, the first resistor element 54, and the second resistor element 55 are the same as those in the third embodiment (see Fig. 6), and therefore will not be described here.

[0060] 7 is a high-impedance resistor for stabilizing the voltage applied to the switch 52. One end of the third resistor 56 is connected to the negative electrode of the capacitor 53, and the other end is connected to the negative DC line K2. When a predetermined switching element is used for the switch 52, the diodes 57 and 58 are elements for forming a current path when a reverse voltage is applied to the switching element.

[0061] One of the diodes, 57, is connected in parallel with the switch 52 together with the third resistor element 56. The anode of the diode 57 is connected to the negative DC line K2. The cathode of the diode 57 is connected to the connection line between the third resistor element 56 and the capacitor 53. The other diode 58 has an anode connected to the negative DC line K2 via the switch 52 and a cathode connected to the positive DC line K1.

[0062] According to the fourth embodiment, the provision of the third resistor element 56 can stabilize the voltage applied to the switch 52. Furthermore, the provision of the diodes 57 and 58 can form a current path when a reverse voltage is applied to the switch 52 (switching element).

[0063] Fifth Embodiment In the fifth embodiment, an air conditioner W1 (see FIG. 8) including the power conversion device 100 (see FIG. 1) having the configuration described in the first embodiment will be described. Note that the configuration and processing content of the power conversion device 100 are the same as those in the first embodiment, and therefore description thereof will be omitted.

[0064] FIG. 8 is a configuration diagram of an air conditioner W1 according to a fifth embodiment. The solid arrows in FIG. 8 indicate the flow of refrigerant in the heating cycle. The dashed arrows in FIG. 8 indicate the flow of refrigerant in the cooling cycle. The air conditioner W1 is a device that performs air conditioning, such as cooling and heating. As shown in FIG. 8 , the air conditioner W1 includes, as components provided in the outdoor unit U1, a compressor 91, an outdoor heat exchanger 92, an outdoor fan 93, an expansion valve 94, and a four-way valve 95. The air conditioner W1 also includes, as components provided in the indoor unit U2, an indoor heat exchanger 96 and an indoor fan 97.

[0065] Although not shown in Fig. 8, the air conditioner W1 is equipped with the power conversion device 100 (see Fig. 1) described in the first embodiment. The power conversion device 100 is mounted on a circuit board (not shown) of the outdoor unit U1.

[0066] The compressor 91 is a device that compresses a low-temperature, low-pressure gas refrigerant and discharges it as a high-temperature, high-pressure gas refrigerant. Although not shown in Fig. 8, an accumulator for separating the refrigerant into gas and liquid is connected to the suction side of the compressor 91. A motor M1, which is a drive source of the compressor 91, is connected as a "load" to the output side of the inverter circuit 60 (see Fig. 1) of the power conversion device 100 (see Fig. 1).

[0067] The outdoor heat exchanger 92 is a heat exchanger in which heat is exchanged between the refrigerant flowing through its heat transfer tubes and the outside air sent in from the outdoor fan 93. The outdoor fan 93 is a fan that sends the outside air to the outdoor heat exchanger 92. The outdoor fan 93 has an outdoor fan motor 93a that serves as a drive source, and is installed near the outdoor heat exchanger 92.

[0068] The expansion valve 94 is a valve that reduces the pressure of the refrigerant condensed in the "condenser" (one of the outdoor heat exchanger 92 and the indoor heat exchanger 96). The refrigerant reduced in pressure by the expansion valve 94 is guided to the "evaporator" (the other of the outdoor heat exchanger 92 and the indoor heat exchanger 96). The indoor heat exchanger 96 is a heat exchanger that exchanges heat between the refrigerant flowing through its heat transfer tube (not shown) and indoor air (air in the air-conditioned room) sent in by an indoor fan 97. The indoor fan 97 is a fan that sends indoor air to the indoor heat exchanger 96. The indoor fan 97 is equipped with an indoor fan motor 97a that serves as a drive source, and is installed near the indoor heat exchanger 96.

[0069] The four-way valve 95 switches the refrigerant flow path depending on the operating mode of the air conditioner W1. For example, during cooling operation (see the dashed arrow in FIG. 8 ), the refrigerant circulates sequentially through the compressor 91, the outdoor heat exchanger 92 (condenser), the expansion valve 94, and the indoor heat exchanger 96 (evaporator). During heating operation (see the solid arrow in FIG. 8 ), the refrigerant circulates sequentially through the compressor 91, the indoor heat exchanger 96 (condenser), the expansion valve 94, and the outdoor heat exchanger 92 (evaporator). Air that has exchanged heat with the refrigerant flowing through the indoor heat exchanger 96 is then blown out of the indoor unit U2 into the air-conditioned room.

[0070] According to the fifth embodiment, the air conditioner W1 is provided with the power conversion device 100 (see FIG. 1) having the same configuration as that of the first embodiment, and therefore the reliability of the air conditioner W1 is improved.

[0071] <<Modifications>> The power conversion device 100 and the air conditioner W1 according to the present disclosure have been described above in various embodiments. However, they are not limited to these descriptions and various modifications can be made. For example, in the first embodiment (see FIG. 1 ), the voltage threshold (first predetermined value) when the first control circuit 81 (see FIG. 1 ) switches the overvoltage protection circuit 50 to the ON state is the same as the voltage threshold (first predetermined value) when the overvoltage protection circuit 50 is switched to the OFF state. However, this is not limited to this. That is, after the first control circuit 81 switches the overvoltage protection circuit 50 to the ON state, the overvoltage protection circuit 50 may be switched to the OFF state if the voltage of the smoothing capacitor 30 becomes less than a third predetermined value. The "third predetermined value" is a voltage threshold used as a criterion for determining whether the first control circuit 81 switches the overvoltage protection circuit 50 to the OFF state and is set to a value equal to or less than the "first predetermined value" described in the first embodiment. For example, by setting the third predetermined value smaller than the first predetermined value, on / off hunting of the switch 52 can be suppressed.

[0072] The same applies to the voltage threshold when the second control circuit 82 switches the overvoltage protection circuit 50 to the OFF state. That is, after the second control circuit 82 switches the overvoltage protection circuit 50 to the ON state, if the voltage of the smoothing capacitor 30 becomes less than a fourth predetermined value, the overvoltage protection circuit 50 may be switched to the OFF state. The "fourth predetermined value" is a voltage threshold that serves as a criterion for determining whether the second control circuit 82 switches the overvoltage protection circuit 50 to the OFF state, and is set to a value equal to or less than the "second predetermined value" described in the first embodiment. For example, by setting the fourth predetermined value smaller than the second predetermined value, on / off hunting of the switch 52 can be suppressed.

[0073] Furthermore, in the first embodiment, the case where there is one smoothing capacitor 30 (see FIG. 1) has been described, but this is not limiting. That is, the smoothing capacitor may be formed by a plurality of capacitors connected in series, parallel, or series-parallel. In this case, the "voltage of the smoothing capacitor" refers to the voltage across the smoothing capacitor (the DC voltage between the DC lines K1 and K2) when the capacitances of the plurality of capacitors are combined and regarded as a single smoothing capacitor. The same applies to the second to fifth embodiments.

[0074] Furthermore, the first control circuit 81 (see FIG. 1 ) described in the first embodiment may include a predetermined element in addition to a microcomputer. The same applies to the second to fifth embodiments. In the first embodiment, the first control circuit 81 switches the switch 52 to the on state when the detected value (the voltage of the smoothing capacitor 30) of the DC voltage detector 40 becomes equal to or greater than a first predetermined value after the motor M1 is stopped. However, this is not limited to this. For example, the first control circuit 81 may switch the switch 52 to the on state simultaneously with the stopping of the motor M1 or after a predetermined time has elapsed since the motor M1 was stopped. This allows the overvoltage protection operation to be initiated at an earlier timing, thereby improving reliability. Alternatively, for example, the first predetermined value may be set to a value equal to or greater than the range of the DC voltage during the operation of the motor M1 (during the operation of the inverter circuit 60). This shortens the time from the stopping of the motor M1 until the switch 52 is switched to the on state, thereby improving reliability during overvoltage protection. In the fifth embodiment (see FIG. 8 ), the power conversion device 100 is connected to the motor M1 of the compressor 91, but the present invention is not limited to this. For example, the power conversion device 100 may be connected to the outdoor fan motor 93 a. Furthermore, the power conversion device 100 may be connected to the motor M1 of the compressor 91, and the power conversion device 100 may be connected to the outdoor fan motor 93 a.

[0075] Furthermore, in the fifth embodiment (see FIG. 8 ), a configuration in which the air conditioner W1 includes a four-way valve 95 has been described, but this is not limiting. That is, the four-way valve 85 may be omitted as appropriate to configure an air conditioner dedicated to cooling or heating. Furthermore, the fifth embodiment (see FIG. 8 ) can be applied to various types of air conditioners, such as commercial air conditioners and multi-air conditioners for buildings, in addition to room air conditioners. Furthermore, each embodiment can also be applied to other devices such as water heaters and refrigerators. Furthermore, each embodiment can be combined as appropriate. For example, the first to fourth embodiments can be combined with the fifth embodiment.

[0076] Furthermore, each embodiment has been described in detail to clearly explain the present disclosure, and is not necessarily limited to having all of the described configurations. Furthermore, it is possible to add, delete, or replace part of the configuration of each embodiment with other configurations. Furthermore, the above-described mechanisms and configurations are those considered necessary for explanation, and do not necessarily represent all mechanisms and configurations of the product.

[0077] 10 Converter circuit 20 Reactor 30 Smoothing capacitor 40 DC voltage detection unit 50, 50A, 50B, 50C Overvoltage protection circuit 51 Resistance element 52 Switch 53 Capacitor 54 First resistance element 55 Second resistance element 56 Third resistance element 57, 58 Diode 60 Inverter circuit 70 Power supply circuit 80 Control unit 81 First control circuit 82 Second control circuit 82a Comparator 83 Logic operator 91 Compressor 92 Outdoor heat exchanger 93 Outdoor fan 93a Outdoor fan motor 94 Expansion valve 95 Four-way valve 96 Indoor heat exchanger 97 Indoor fan 97a Indoor fan motor 100, 100A, 100B, 100C Power conversion device E1 AC power supply K1 DC line (positive DC line) K2 DC line (negative DC line) M1 Motor (load) U1 Outdoor unit U2 Indoor unit W1 Air conditioner

Claims

1. A power conversion device comprising: a converter circuit that converts an AC voltage applied from an AC power source into a DC voltage; a smoothing capacitor connected to a pair of DC lines on the output side of the converter circuit to smooth the DC voltage on the output side of the converter circuit; an inverter circuit that converts the DC voltage of the smoothing capacitor into an AC voltage and applies the AC voltage to a load; an overvoltage protection circuit connected to the pair of DC lines; a first control circuit having a microcomputer; a second control circuit composed of an analog electronic circuit; and a power supply circuit that applies a predetermined drive voltage to each of the first control circuit and the second control circuit. The input side of the power supply circuit is connected to the pair of DC lines. When the voltage of the smoothing capacitor becomes equal to or higher than a first predetermined value, the first control circuit switches the overvoltage protection circuit to an on state. When the microcomputer fails to drive normally and the voltage of the smoothing capacitor becomes equal to or higher than a second predetermined value, the second control circuit switches the overvoltage protection circuit to an on state. The second predetermined value is larger than the first predetermined value.

2. The power conversion device according to claim 1, wherein the first control circuit switches the overvoltage protection circuit to an off state when the voltage of the smoothing capacitor becomes lower than a third predetermined value after switching the overvoltage protection circuit to an on state, and the third predetermined value is equal to or lower than the first predetermined value.

3. The power conversion device according to claim 1, wherein the second control circuit switches the overvoltage protection circuit to an off state when the voltage of the smoothing capacitor becomes lower than a fourth predetermined value after switching the overvoltage protection circuit to an on state, and the fourth predetermined value is equal to or lower than the second predetermined value.

4. The power conversion device according to claim 1, wherein the first control circuit returns the overvoltage protection circuit to an off state when a first predetermined time has elapsed after switching the overvoltage protection circuit to an on state.

5. The power conversion device according to claim 1, wherein the second control circuit returns the overvoltage protection circuit to an off state when a second predetermined time has elapsed after switching the overvoltage protection circuit to an on state.

6. The overvoltage protection circuit has a configuration in which a resistor element and a switch are connected in series. One end of the series connection of the resistor element and the switch is connected to the positive-side DC line, and the other end is connected to the negative-side DC line. The first control circuit or the second control circuit switches the switch to the on state to switch the overvoltage protection circuit to the on state. The power conversion device according to claim 1, characterized in that.

7. The overvoltage protection circuit has a configuration in which a capacitor and a switch are connected in series. One end of the series connection of the capacitor and the switch is connected to the positive-side DC line, and the other end is connected to the negative-side DC line. The first control circuit or the second control circuit switches the switch to the on state to switch the overvoltage protection circuit to the on state. The power conversion device according to claim 1, characterized in that.

8. The overvoltage protection circuit includes a capacitor, a first resistor element connected in series to one side of the capacitor, a second resistor element connected in parallel to the capacitor, and a switch connected in series to the other side of the capacitor. One end of the series connection of the first resistor element, the capacitor, and the switch is connected to the positive-side DC line, and the other end is connected to the negative-side DC line. The first control circuit or the second control circuit switches the switch to the on state to switch the overvoltage protection circuit to the on state. The power conversion device according to claim 1, characterized in that.

9. An air conditioner comprising the power conversion device according to any one of claims 1 to 8, and further comprising a compressor, an outdoor heat exchanger, an expansion valve, and an indoor heat exchanger, wherein a motor that is a drive source of the compressor is connected to the output side of the inverter circuit as the load.

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