Power conversion device

Abstract

This power conversion device is provided with: a power converter circuit that is electrically connected between a first power source and a load device, and converts a form of power from the first power source to a form of power required by the load device; a control circuit that is electrically connected between a second power source and the power converter circuit, and passes, to the power converter circuit, a control signal for controlling operation of the power converter circuit; a smoothing capacitor electrically connected between the first power source and the power converter circuit; and a discharge circuit that is electrically connected to the smoothing capacitor and includes a resistance element for discharging power accumulated in the smoothing capacitor. If an electrical connection between the first power source and the load device is determined to have been interrupted, the discharge circuit discharges power accumulated in the smoothing capacitor to the resistance element, and also discharges to the control circuit.

Description

Power Conversion Device

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

[0002] Conventionally, there are known power conversion devices that supply power to a load device based on the output voltage of a high-voltage power supply (for example, Patent Documents 1 and 2). The power conversion device includes, for example, a power conversion circuit connected to the high-voltage power supply, a smoothing capacitor connected between the high-voltage power supply and the power conversion circuit, a discharge switching element and a discharge resistor for discharging residual charge in the smoothing capacitor in the event of an abnormality, and a control circuit electrically connected to the power conversion circuit. The control circuit controls the operation of the power conversion circuit based on the output voltage from the low-voltage power supply.

[0003] JP 2015-100241 A JP 2015-159684 A

[0004] In the event of an abnormality, the power conversion device described above may be electrically disconnected from the high-voltage power supply. In this case, the residual charge stored in the smoothing capacitor must be quickly discharged. Therefore, in the event of an abnormality, the discharge switching element is turned on, and the residual charge in the smoothing capacitor is discharged by the discharge resistor. However, in such a power conversion device, the number and size of the discharge resistors may increase depending on the magnitude of the residual charge in the smoothing capacitor. In this case, the circuit size may increase.

[0005] The present disclosure describes a power conversion device that can reduce the circuit size.

[0006] A power conversion device according to one aspect of the present disclosure includes: a power conversion circuit electrically connected between a first power source and a load device and configured to convert a mode of power from the first power source into a mode of power required by the load device; a control circuit electrically connected between a second power source and the power conversion circuit and configured to provide the power conversion circuit with a control signal that controls the operation of the power conversion circuit; a smoothing capacitor electrically connected between the first power source and the power conversion circuit; and a discharge circuit electrically connected to the smoothing capacitor and including a resistive element for discharging power stored in the smoothing capacitor. When the discharge circuit determines that the electrical connection between the first power source and the load device has been interrupted, the discharge circuit discharges the power stored in the smoothing capacitor to the resistive element and to the control circuit.

[0007] According to some aspects of the present disclosure, a power conversion device capable of reducing the circuit size is provided.

[0008] Fig. 1 is a circuit diagram showing a power conversion device of one embodiment. Fig. 2 is a circuit diagram explaining the driving operation of the power conversion device in normal operation. Fig. 3 is a circuit diagram explaining the driving operation of the power conversion device in abnormal operation. Fig. 4 is a circuit diagram showing a power conversion device of a comparative example.

[0009] A power conversion device according to one aspect of the present disclosure includes: a power conversion circuit electrically connected between a first power source and a load device and configured to convert a mode of power from the first power source into a mode of power required by the load device; a control circuit electrically connected between a second power source and the power conversion circuit and configured to provide the power conversion circuit with a control signal that controls the operation of the power conversion circuit; a smoothing capacitor electrically connected between the first power source and the power conversion circuit; and a discharge circuit electrically connected to the smoothing capacitor and including a resistive element for discharging power stored in the smoothing capacitor. When the discharge circuit determines that the electrical connection between the first power source and the load device has been interrupted, the discharge circuit discharges the power stored in the smoothing capacitor to the resistive element and to the control circuit.

[0010] In the above power conversion device, when the discharge circuit determines that the electrical connection between the first power source and the load device has been interrupted, it discharges the power stored in the smoothing capacitor to the resistive element and to the control circuit. In this case, the power stored in the smoothing capacitor can be used as a backup power source for the control circuit. This allows the power of the smoothing capacitor to be distributed and consumed by the resistive element and the control circuit. As a result, the power of the smoothing capacitor can be sufficiently discharged without increasing the number and size of the resistive elements, compared to when the power of the smoothing capacitor is discharged only through the resistive elements. Therefore, the above power conversion device allows for a smaller circuit size.

[0011] In some embodiments, the discharge circuit may include a selection circuit that selectively supplies the control circuit with either the power stored in the smoothing capacitor or the power provided by the second power source. In this case, even if the electrical connection between the first power source and the load device is interrupted, the power stored in the smoothing capacitor can be quickly supplied to the control circuit as backup power. In other words, the power stored in the smoothing capacitor can be used as backup power to continue operation of the control circuit. As a result, there is no need to separately provide a large-capacity electrolytic smoothing capacitor to continue operation of the control circuit, thereby reducing the possibility of the circuit size becoming excessively large.

[0012] In some embodiments, the discharge circuit may include an isolated power supply including an input terminal electrically connected to the smoothing capacitor and an output terminal electrically insulated from the input terminal, and transmitting power supplied from the smoothing capacitor to the output terminal at a voltage equal to or lower than the voltage indicated by the power provided by the second power source, and a selection circuit including a first input section electrically connected to the output terminal, a second input section electrically connected to the second power source, and an output section electrically connected to the control circuit, and selecting power indicating the higher voltage of the voltage applied to the first input section and the voltage applied to the second input section and supplying it to the control circuit. In this case, the power stored in the smoothing capacitor can be used as reserve power to continue operating the control circuit appropriately without interruption, while reducing the possibility of electrical noise from the first power source being transmitted to the control circuit.

[0013] In some embodiments, the selection circuit may include a first diode having an anode electrically connected to the first input section and a second diode having an anode electrically connected to the second input section. The cathode of the first diode and the cathode of the second diode may be electrically connected to the output section. One of the first diode and the second diode may be selectively brought into conduction with the output section so that power representing a higher voltage between a voltage applied to the anode of the first diode and a voltage applied to the anode of the second diode is supplied to the output section. In this case, compared to when the operation of the selection circuit is realized by switching control of multiple switching elements, output of a signal instructing switching control is not required, and therefore the operation of the selection circuit can be realized more easily.

[0014] In some embodiments, the discharge circuit may include a normally-open switching element electrically connected to the smoothing capacitor and in an off state except for a period during which an on signal indicating an on state is received. The switching element may be turned on based on the on signal when it is determined that the electrical connection between the first power source and the load device has been interrupted. The power conversion device described above is configured to operate the control circuit without interruption by using power from the smoothing capacitor as a backup power source for the control circuit. However, in the event of an abnormality, it is conceivable that the supply of power to the control circuit may also be stopped. In such a situation, if a normally-closed switching element is used, when the supply of power to the control circuit is interrupted, the switching element is automatically turned on, and the discharge of power from the smoothing capacitor begins. In this case, if the timing of interruption of the power supply from the first power source to the power conversion device is delayed relative to the timing of discharging power from the smoothing capacitor, it is conceivable that the power from the first power source may place a heavy load on the resistance element. In contrast, when a normally open switching element is used, the switching element is automatically turned off when the power supply to the control circuit is cut off, thereby reducing the possibility of the power from the first power source placing a heavy load on the resistance element, regardless of the timing when the power supply from the first power source to the power conversion device is cut off.

[0015] In some embodiments, the power conversion device may include an input connection electrically connected to the first power source. A contactor may be provided between the first power source and the input connection, switching the electrical connection between the first power source and the input connection. If a normally closed switching element is used, it is possible to incorporate the contactor inside the power conversion device and strictly manage the timing of switching the contactor's open / close state using a control circuit to prevent a delay in the timing of cutting off the power supply from the first power source to the power conversion device relative to the timing at which the smoothing capacitor starts discharging. However, such a contactor tends to be relatively large, and incorporating it inside the power conversion device may result in an increase in the size of the power conversion device. On the other hand, if a normally open switching element is used, as described above, even if the power supply to the control circuit is cut off, the possibility of a high load being placed on the resistive element can be reduced regardless of the timing at which the power supply from the first power source to the power conversion device is cut off. This eliminates the need to strictly manage the timing of switching the contactor's open / close state using a control circuit. Therefore, by arranging a large contactor outside the power conversion device as in the above configuration, the circuit size of the power conversion device can be more reliably reduced.

[0016] In some embodiments, the control circuit may determine that the electrical connection between the first power source and the load device has been interrupted when the charging voltage of the smoothing capacitor exceeds a predetermined threshold, and may provide an ON signal to the switching element. In this case, even if the contactor is disposed outside the power conversion device, the control circuit can confirm the open / closed state of the contactor based on a change in the charging voltage of the smoothing capacitor, making it easy to determine whether the electrical connection between the first power source and the load device has been interrupted.

[0017] In some embodiments, the control circuit may provide a signal to the switching element at predetermined intervals. The control circuit may determine that the electrical connection between the first power source and the load device has been interrupted when a difference between the charging voltage of the smoothing capacitor when the signal is provided and the charging voltage of the smoothing capacitor when the next signal is provided exceeds a predetermined threshold, and may provide an ON signal to the switching element. In this case, even if the contactor is located outside the power conversion device, the control circuit can confirm the open / closed state of the contactor based on a change in the difference between the charging voltage of the smoothing capacitor, making it easy to determine whether the electrical connection between the first power source and the load device has been interrupted.

[0018] In some aspects, the load device may be a motor including a rotor and a stator. When the control circuit determines that the electrical connection between the first power source and the load device has been interrupted, the control circuit may control the power conversion circuit to maintain the q-axis current of the motor's d-axis current and q-axis current at a negative value and to maintain the d-axis current at a non-zero value. When the electrical connection between the first power source and the load device is interrupted, it is necessary to quickly decelerate the rotation of the motor to reduce the rotational energy generated in the motor. Here, the q-axis current of the motor contributes to the generation of torque in the rotor. On the other hand, the d-axis current of the motor generates magnetic flux in the stator but does not contribute to the generation of torque in the rotor. Therefore, by maintaining the q-axis current at a negative value, the control circuit can generate torque in the motor rotor in a direction opposite to the rotational direction of the rotor, thereby braking the rotor. The rotational energy of the motor is stored as a voltage in the smoothing capacitor. In response, the control circuit controls the d-axis current of the motor to flow through the motor by maintaining the d-axis current of the motor at a non-zero value. As a result, hysteresis loss and eddy current loss (so-called iron loss) occur in the stator, causing the stator coil to heat up, allowing the power of the smoothing capacitor to be consumed by the motor. Therefore, with the above configuration, the power of the smoothing capacitor can be discharged while the motor is decelerating. This eliminates the need to increase the size of the resistance element to the extent that it can consume the rotational energy of the motor, thereby more reliably reducing the circuit size.

[0019] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. In the description of the drawings, the same elements are designated by the same reference numerals, and duplicated description will be omitted.

[0020] A power conversion device 1 according to one embodiment will be described with reference to Fig. 1 . The power conversion device 1 converts power received from a high-voltage power supply HV (first power supply) into power required by a load device M. The high-voltage power supply HV is a DC power supply that provides high-voltage (e.g., 800 V) DC power to the power conversion device 1. The power conversion device 1 is, for example, an inverter that converts the DC power from the high-voltage power supply HV into three-phase AC power. The load device M is, for example, a three-phase AC motor including a rotor and a stator. The three-phase AC motor is driven by receiving the three-phase AC power from the power conversion device 1.

[0021] The load device M is used as a power source for driving the impeller. Therefore, the power conversion device 1 may be employed as an electrical component of an electric compressor or an electric blower. The load device M may be a generator that receives external power and generates electric power. The power conversion device 1 may be a converter that converts AC power to DC power, or may be one that converts the DC power of the first mode into the DC power of the second mode.

[0022] In the following description, unless otherwise specified, when a first element is "connected" to a second element, it means that the first element is "electrically connected" to the second element. "Electrically connected" means that the first element and the second element are connected to each other in a state where signals can be transmitted and power can be supplied between the first element and the second element. Therefore, "electrically connected" includes both a case where the first element and the second element are directly connected to each other by wiring, and a case where the first element and the second element are indirectly connected via a third element. The first element means any element that forms the power conversion device 1. The second element means an element that forms the power conversion device 1 and is different from the first element. The third element means an element that forms the power conversion device 1 and is different from the first element and the second element.

[0023] The power conversion device 1 includes a first input connection portion 11 (input connection portion), a second input connection portion 12, a third input connection portion 13, a first output connection portion 21, a second output connection portion 22, and a third output connection portion 23.

[0024] The first input connection part 11 is connected to a positive terminal T1 of the high-voltage power supply HV. The second input connection part 12 is connected to a negative terminal T2 of the high-voltage power supply HV. A contactor 2 is provided between the positive terminal T1 and the first input connection part 11. The power conversion device 1 is connected to the high-voltage power supply HV via the contactor 2. The contactor 2 is, for example, a high-voltage relay. The contactor 2 is arranged outside the power conversion device 1 and switches between electrical open and closed states between the high-voltage power supply HV and the power conversion device 1 based on instructions from a higher-level system.

[0025] When the contactor 2 is in the ON state, high-voltage DC power of, for example, 800 V is provided from the high-voltage power supply HV to the power conversion device 1. The supply of DC power from the high-voltage power supply HV to the power conversion device 1 is cut off when the contactor 2 is in the OFF state. The third input connection part 13 is connected to a terminal T3 of a low-voltage power supply LV (second power supply). The low-voltage power supply LV is a DC power supply that provides DC power exhibiting a lower voltage (for example, 24 V) than the DC power provided by the high-voltage power supply HV.

[0026] The first output connection portion 21, the second output connection portion 22, and the third output connection portion 23 are connected to the U-phase coil, the V-phase coil, and the W-phase coil, respectively, of the load device M. The first output connection portion 21 supplies U-phase AC power to the U-phase coil of the load device M. The second output connection portion 22 supplies V-phase AC power to the V-phase coil of the load device M. The third output connection portion 23 supplies W-phase AC power to the W-phase coil of the load device M.

[0027] The power conversion device 1 further includes a first DC bus L1 , a second DC bus L2 , a power conversion circuit 10 , a smoothing capacitor C, a discharge circuit 20 , and a control circuit 30 .

[0028] The power conversion circuit 10 is an inverter circuit including a plurality of switching elements 15, and is connected between the high-voltage power supply HV and the load device M. The high-voltage power supply HV and the power conversion circuit 10 are connected by a first DC bus L1 and a second DC bus L2. The first DC bus L1 extends from a positive terminal T1 of the high-voltage power supply HV through a first input connection part 11 to the power conversion circuit 10. The second DC bus L2 extends from a negative terminal T2 of the high-voltage power supply HV through a second input connection part 12 to the power conversion circuit 10.

[0029] The power conversion circuit 10 converts DC power from a high-voltage power supply HV into three-phase AC power by switching on and off a plurality of switching elements 15. The switching elements 15 may be semiconductor switches such as insulated gate bipolar transistors (IGBTs), metal oxide semiconductor field effect transistors (MOSFETs), or bipolar transistors. Control signals φ31A and φ31B are supplied to gates (control terminals) of the switching elements 15 from a control circuit 30. Each switching element 15 switches between an ON state and an OFF state in response to the control signals φ31A and φ31B from the control circuit 30.

[0030] The smoothing capacitor C is connected between the high-voltage power supply HV and the power conversion circuit 10. The smoothing capacitor C is disposed between the first DC bus L1 and the second DC bus L2 and is connected in parallel to the power conversion circuit 10. The smoothing capacitor C stabilizes the voltage applied between the first DC bus L1 and the second DC bus L2. The positive terminal Ca of the smoothing capacitor C is connected to the positive terminal T1 of the high-voltage power supply HV via the first DC bus L1. The negative terminal Cb of the smoothing capacitor C is connected to the negative terminal T2 of the high-voltage power supply HV via the second DC bus L2. When the contactor 2 is in the on state, DC power from the high-voltage power supply HV is stored in the smoothing capacitor C. The voltage of the stored power in the smoothing capacitor C is the same as the voltage of the DC power provided by the high-voltage power supply HV.

[0031] The discharge circuit 20 is connected between the high-voltage power supply HV and the power conversion circuit 10. The discharge circuit 20 forcibly discharges the stored power stored in the smoothing capacitor C in the event of an abnormality. For example, an abnormality may occur when an external shock or the like is received and the contactor 2 is turned off in response to a request from a higher-level system, thereby cutting off the electrical connection between the high-voltage power supply HV and the power conversion device 1. In this case, to ensure safety, it is necessary to quickly discharge the stored power stored in the smoothing capacitor C. The discharge circuit 20 is a circuit for quickly discharging the stored power in the smoothing capacitor C. The discharge circuit 20 includes, for example, a first discharge circuit 40 and a second discharge circuit 50.

[0032] The first discharge circuit 40 is connected between the smoothing capacitor C and the power conversion circuit 10. The first discharge circuit 40 is disposed between the first DC bus L1 and the second DC bus L2, and is connected in parallel to the smoothing capacitor C and the power conversion circuit 10. The first discharge circuit 40 includes a discharge switching element 41 (switching element), a first discharge resistor 42 (resistive element), and a second discharge resistor 43. The discharge switching element 41 and the first discharge resistor 42 are connected in series with each other between the first DC bus L1 and the second DC bus L2. The second discharge resistor 43 is connected in parallel to the first discharge resistor 42 and the discharge switching element 41.

[0033] A first end of the first discharge resistor 42 is connected to the positive terminal Ca of the smoothing capacitor C via the first DC bus L1. A second end of the first discharge resistor 42 is connected to the first end of the discharge switching element 41. A second end of the discharge switching element 41 is connected to the negative terminal Cb of the smoothing capacitor C via the second DC bus L2. A first end of the second discharge resistor 43 is connected to the positive terminal Ca of the smoothing capacitor C via the first DC bus L1. A second end of the second discharge resistor 43 is connected to the negative terminal Cb of the smoothing capacitor C via the second DC bus L2. Each of the first discharge resistor 42 and the second discharge resistor 43 does not need to be an element mounted on a circuit board and may be, for example, a cement resistor with a harness or a lead resistor.

[0034] The discharge switching element 41 is an element capable of switching electrical open / close between both ends thereof. The discharge switching element 41 is, for example, a normally open switching element that is in an off state except for a period during which an on signal φ32 indicating an on state is provided. The discharge switching element 41 is in an on state during a period during which the on signal φ32 is provided, and in an off state during a period during which the on signal φ32 is not provided. The discharge switching element 41 may be, for example, a semiconductor switch such as a GaN device, or a contact relay (i.e., a mechanical relay). During a period during which the discharge switching element 41 is in an on state, the stored power of the smoothing capacitor C is consumed in the first discharge resistor 42 and the second discharge resistor 43.

[0035] The second discharge circuit 50 is arranged on the opposite side of the smoothing capacitor C from the first discharge circuit 40. The second discharge circuit 50 is electrically connected to the smoothing capacitor C, the low-voltage power supply LV, and the control circuit 30. The second discharge circuit 50 discharges the power stored in the smoothing capacitor C to the control circuit 30. The second discharge circuit 50 includes a first connection line L3, a second connection line L4, an insulated power supply 51, and a selection circuit 55. The first connection line L3 connects the low-voltage power supply LV and the control circuit 30. The second connection line L4 connects the first DC bus L1 and the first connection line L3. The second connection line L4 is arranged between the first input connection portion 11, the second input connection portion 12, and the smoothing capacitor C.

[0036] The insulated power supply 51 is disposed on the second connection line L4. An input terminal 51a of the insulated power supply 51 is connected to the positive terminal Ca of the smoothing capacitor C via the second connection line L4 and the first DC bus L1. An output terminal 51b of the insulated power supply 51 is connected to the control circuit 30 via the second connection line L4 and the first connection line L3. The output terminal 51b is electrically insulated from the input terminal 51a. While the discharge switching element 41 is in the on state, the input terminal 51a is supplied with the power stored in the smoothing capacitor C. The insulated power supply 51 transmits the voltage of the power stored in the smoothing capacitor C, supplied to the input terminal 51a, from the output terminal 51b to the control circuit 30 in an insulated manner.

[0037] The insulated power supply 51 includes at least a transformer 54. The transformer 54 includes a first coil 52 and a second coil 53. The first coil 52 is connected to the positive terminal Ca of the smoothing capacitor C via an input terminal 51a. The second coil 53 is connected to the control circuit 30 via an output terminal 51b. The second coil 53 is electrically insulated from the first coil 52. The first coil 52 and the second coil 53 are conductors wound around the same core. The first coil 52 and the second coil 53 are magnetically coupled to each other via the core and can transfer power between them. The transformer 54 reduces the voltage of the stored power supplied to the first coil 52 and transmits it to the second coil 53. The voltage of the stored power transmitted to the second coil 53 is equal to or lower than the voltage of the DC power provided by the low-voltage power supply LV (e.g., 24 V or lower).

[0038] The selection circuit 55 is disposed at the intersection of the first connection line L3 and the second connection line L4. The selection circuit 55 is connected between the isolated power supply 51 and the control circuit 30. The selection circuit 55 is connected between the low-voltage power supply LV and the control circuit 30. The selection circuit 55 selectively supplies to the control circuit 30 either the power provided by the smoothing capacitor C or the DC power provided by the low-voltage power supply LV. The selection circuit 55 includes a first input unit 55a, a second input unit 55b, and an output unit 55c.

[0039] The first input section 55a is connected to the output terminal 51b of the insulated power supply 51. The first input section 55a is supplied with stored power from the output terminal 51b. The second input section 55b is connected to the low-voltage power supply LV. The second input section 55b is supplied with low-voltage DC power from the low-voltage power supply LV. The output section 55c is connected to the control circuit 30 without passing through any other capacitors. In other words, no other capacitors are interposed between the selection circuit 55 and the control circuit 30.

[0040] The selection circuit 55 is a so-called diode OR circuit. The selection circuit 55 includes a first diode 56, a second diode 57, and a connection portion 55d. The connection portion 55d is an intersection of the first connection line L3 and the second connection line L4 and is connected to the control circuit 30. The first diode 56 is connected between the first input portion 55a and the connection portion 55d. The anode of the first diode 56 is connected to the first input portion 55a. The cathode of the first diode 56 is connected to the connection portion 55d. The second diode 57 is connected between the second input portion 55b and the connection portion 55d. The anode of the second diode 57 is connected to the second input portion 55b. The cathode of the second diode 57 is connected to the connection portion 55d.

[0041] The selection circuit 55 selects and outputs power that indicates the higher voltage between the voltage at the anode of the first diode 56 and the voltage at the anode of the second diode 57. For example, when the voltage applied to the anode of the second diode 57 is higher than the voltage of the power applied to the anode of the first diode 56, the second diode 57 and the output unit 55c are conductive. In this case, the selection circuit 55 supplies DC power from the low-voltage power supply LV to the control circuit 30 via the output unit 55c.

[0042] On the other hand, when the voltage applied to the anode of the first diode 56 is higher than the voltage applied to the anode of the second diode 57 (for example, when the low-voltage power supply LV is electrically disconnected from the power conversion device 1), the first diode 56 and the output port 55c become conductive. In this case, the selection circuit 55 supplies the stored power from the smoothing capacitor C to the control circuit 30 via the output port 55c. In this way, the selection circuit 55 gives priority to the power indicating the higher voltage between the voltage applied to the first input port 55a and the voltage applied to the second input port 55b and supplies it to the control circuit 30.

[0043] The control circuit 30 is connected to the power conversion circuit 10 and the first discharge circuit 40. The control circuit 30 is connected to the smoothing capacitor C and the low-voltage power supply LV via the second discharge circuit 50. The control circuit 30 is a computer including hardware such as a CPU (Central Processing Unit) and memories such as RAM (Random Access Memory) and ROM (Read Only Memory). The control circuit 30 operates each piece of hardware under the control of the CPU based on a computer program stored in the memory, thereby realizing the functions of the control circuit 30. The control circuit 30 includes, as functional elements, a power conversion control unit 31 and a discharge control unit 32.

[0044] The power conversion control unit 31 outputs control signals φ31A and φ31B to each switching element 15 in the power conversion circuit 10 to control driving of each switching element 15. The control signals φ31A and φ31B are, for example, PWM (Pulse Width Modulation) signals. The power conversion control unit 31 controls the on / off switching of each switching element 15 by outputting the control signals φ31A and φ31B to each switching element 15. As will be described later, the power conversion control unit 31 outputs the control signal φ31A to each switching element 15 in a normal state, and outputs the control signal φ31B to each switching element 15 in an abnormal state.

[0045] The discharge control unit 32 outputs an ON signal φ32 to the discharge switching element 41 of the first discharge circuit 40 to instruct the discharge switching element 41 to be in the ON state. The discharge control unit 32 provides the ON signal φ32 to the discharge switching element 41 in the event of an abnormality. When the contactor 2 is turned OFF and the high-voltage power supply HV and the power conversion device 1 are electrically disconnected, the discharge control unit 32 determines that the supply of power from the high-voltage power supply HV to the power conversion device 1 has been abnormally stopped and provides the ON signal φ32 to the discharge switching element 41. Here, if the contactor 2 is disposed external to the power conversion device 1, the discharge control unit 32 cannot directly check the open / closed state of the contactor 2. Therefore, the discharge control unit 32 indirectly checks the open / closed state of the contactor 2 based on the voltage of the stored power in the smoothing capacitor C inside the power conversion device 1. For example, the discharge control unit 32 checks whether the voltage of the stored power in the smoothing capacitor C (i.e., the charging voltage) exceeds a predetermined threshold (e.g., 800 V). The discharge control unit 32 may provide a test signal to the discharge switching element 41 at predetermined time intervals to check the open / closed state of the contactor 2. In this case, the discharge control unit 32 may check whether the difference in the charging voltage of the smoothing capacitor C before and after the signal exceeds a predetermined threshold value (e.g., 10 V). In other words, the discharge control unit 32 may check whether the difference between the charging voltage of the smoothing capacitor C when the signal is provided and the charging voltage of the smoothing capacitor C when the next signal is provided exceeds a predetermined threshold value.

[0046] When the discharge control unit 32 determines that the charging voltage of the smoothing capacitor C does not exceed the predetermined threshold, it determines that the contactor 2 is in the ON state and stops outputting the ON signal φ32 to the discharge switching element 41. When the discharge control unit 32 determines that the charging voltage of the smoothing capacitor C exceeds the predetermined threshold, it determines that the contactor 2 is in the OFF state and starts outputting the ON signal φ32 to the discharge switching element 41. During the period in which the charging voltage of the smoothing capacitor C exceeds the predetermined threshold, the discharge control unit 32 provides the ON signal φ32 to the discharge switching element 41, thereby maintaining the discharge switching element 41 in the ON state.

[0047] Furthermore, when the discharge control unit 32 determines that the contactor 2 is in the OFF state, it provides the power conversion control unit 31 with a d-axis discharge instruction signal φ33 instructing d-axis discharge control. Upon receiving the d-axis discharge instruction signal φ33, the power conversion control unit 31 provides the power conversion circuit 10 with a control signal φ31B instructing d-axis discharge control, thereby controlling the power conversion circuit 10 so that the q-axis current of the load device M becomes a negative value and the d-axis current of the load device M becomes a non-zero value. Here, the rotation direction of the rotor of the load device M is defined as positive, and the opposite direction is defined as negative. The d-axis and q-axis form a rotating coordinate system that rotates together with the rotor. The d-axis represents the direction of magnetic flux formed by the rotor's magnetic poles. The q-axis represents, for example, a direction 90 degrees phase-advanced from the d-axis. The q-axis current contributes to the generation of torque in the load device M. The d-axis current generates magnetic flux in the stator but does not contribute to the generation of torque in the load device M.

[0048] The discharge control unit 32 maintains the q-axis current at a negative value, thereby generating torque in the load device M in a direction opposite to the rotational direction of the load device M. This causes the discharge control unit 32 to brake the load device M. The rotational energy of the load device M is stored in the smoothing capacitor C. Meanwhile, the discharge control unit 32 maintains the q-axis current at a negative value while simultaneously maintaining the d-axis current of the load device M at a non-zero value (i.e., a value other than zero). This causes a d-axis current to flow through the load device M. The d-axis current functions as a reactive current that does not contribute to the torque of the load device M. Therefore, when the d-axis current flows through the load device M, hysteresis loss and eddy current loss (so-called iron loss) occur in the stator, and the heat generated by the stator coil consumes the stored power in the smoothing capacitor C. In this way, by performing d-axis discharge control, the power conversion control unit 31 can discharge the stored power in the smoothing capacitor C while braking the load device M.

[0049] The operation of the above-described power conversion device 1 will be described with reference to Fig. 2 and Fig. 3. First, the driving operation of the power conversion device 1 in a normal state will be described with reference to Fig. 2, and then the driving operation of the power conversion device 1 in an abnormal state will be described with reference to Fig. 3.

[0050] As shown in FIG. 2 , during normal driving operation, the contactor 2 is turned on, connecting the power conversion device 1 to the high-voltage power supply HV. As a result, high-voltage DC power (hereinafter referred to as "high-voltage power HV1") is supplied from the high-voltage power supply HV to the smoothing capacitor C, and stored as stored power CV. The voltage of the stored power CV is the same as the voltage of the high-voltage power HV1. The voltage of the high-voltage power HV1 is also applied to the input terminal 51 a of the insulated power supply 51. The output terminal 51 b of the insulated power supply 51 outputs power that is equal to or lower than the voltage of the power from the low-voltage power supply LV (hereinafter referred to as "low-voltage power LV1"). Therefore, power that is higher in voltage than the voltage of the low-voltage power LV1 is not supplied from the selection circuit 55 to the control circuit 30. When the voltage of the low-voltage power LV1 is higher than the voltage applied to the output terminal 51b of the insulated power supply 51, the power V30 supplied from the selection circuit 55 to the control circuit 30 corresponds to the low-voltage power LV1 from the low-voltage power supply LV.

[0051] The discharge control unit 32 of the control circuit 30 determines the open / closed state of the contactor 2 by checking the voltage of the stored power CV of the smoothing capacitor C. If the voltage of the stored power CV is the same as the voltage of the high-voltage power HV1, the discharge control unit 32 determines that the contactor 2 is in the ON state and does not perform discharge control to discharge the stored power CV. In other words, the discharge control unit 32 does not provide the ON signal φ32 that turns the discharge switching element 41 on to the discharge switching element 41, but rather maintains the discharge switching element 41 in the OFF state. If the discharge control unit 32 determines that the contactor 2 is in the ON state, it does not provide the d-axis discharge instruction signal φ33 that instructs d-axis discharge to the power conversion control unit 31. In this case, the power conversion control unit 31 performs normal power conversion control using the control signal φ31A.

[0052] As shown in FIG. 3 , in the event of an abnormality, the contactor 2 is turned off, electrically disconnecting the power conversion device 1 from the high-voltage power supply HV. In this case, the discharge control unit 32 checks the voltage of the stored power CV of the smoothing capacitor C, determines that the contactor 2 has turned off, and performs discharge control to discharge the stored power CV. For example, the discharge control unit 32 provides an ON signal φ32 to the discharge switching element 41 of the first discharge circuit 40, turning the discharge switching element 41 on. As a result, a portion CV1 of the stored power CV is consumed in the first discharge resistor 42. Another portion CV2 of the stored power CV is also consumed in the second discharge resistor 43. Another portion CV3 of the stored power CV of the smoothing capacitor C is supplied to the input terminal 51 a of the insulated power supply 51. A power CV4 exhibiting a voltage lower than the low-voltage power LV1 is output from the output terminal 51 b of the insulated power supply 51.

[0053] A first input 55a of the selection circuit 55 receives power CV4 from the output terminal 51b of the insulated power supply 51. A second input 55b of the selection circuit 55 receives low-voltage power LV1 from the low-voltage power supply LV. The selection circuit 55 supplies power V30, which indicates the higher voltage of power CV4 or low-voltage power LV1, to the control circuit 30 from an output 55c. Furthermore, the discharge control unit 32 provides a d-axis discharge instruction signal φ33, which instructs d-axis discharge control, to the power conversion control unit 31. Upon receiving the d-axis discharge instruction signal φ33, the power conversion control unit 31 controls the power conversion circuit 10 so that the q-axis current of the load device M becomes a negative value and the d-axis current of the load device M becomes a non-zero value. As a result, the power conversion control unit 31 brakes the load device M while discharging the stored power CV of the smoothing capacitor C.

[0054] The effects obtained by the power conversion device 1 of the present embodiment described above will be described together with the problems of the comparative example.

[0055] As shown in FIG. 4 , the power conversion device 100 of the comparative example includes a contactor 102, a smoothing capacitor C100, a power conversion circuit 110, a discharge circuit 120, and a control circuit 130. The discharge circuit 120 includes two resistance elements 142 and 143 and a discharge switching element 141. The discharge switching element 141 is a normally closed switching element different from the discharge switching element 41 of the present embodiment. During normal driving operation, the discharge switching element 141 receives an off signal from the control circuit 130 and is maintained in an off state. During driving operation during an abnormality, the discharge switching element 141 switches from an off state to an on state. As a result, when the power conversion device 100 is electrically disconnected from the high-voltage power supply HV, the power stored in the smoothing capacitor C100 is consumed by the two resistance elements 142 and 143. However, in such a discharge circuit 120, the number and size of the resistance elements 142, 143 may increase depending on the power stored in the smoothing capacitor C100, which may result in the size of the discharge circuit 120 becoming larger.

[0056] Furthermore, the power conversion apparatus 100 includes an electrolytic capacitor 150 between the low-voltage power supply LV and the control circuit 130. The electrolytic capacitor 150 can supply power to the control circuit 130 without interruption even when the power conversion apparatus 100 is electrically disconnected from the low-voltage power supply LV. This allows the control circuit 130 to continue operations, such as leaving a log to identify the cause of an abnormality. However, in order to continue operating the control circuit 130 in this manner, a large-capacity electrolytic capacitor 150 is required. A large-capacity electrolytic capacitor 150 can result in an increase in the size of the power conversion apparatus 100.

[0057] Furthermore, when a normally closed discharge switching element 141 is used as in the power conversion apparatus 100, the discharge switching element 141 is turned on when the control circuit 130 is electrically disconnected from the low-voltage power supply LV. In this case, the smoothing capacitor C100 automatically discharges power. If the timing at which the smoothing capacitor C100 discharges power is delayed relative to the timing at which the power supply from the high-voltage power supply HV to the power conversion apparatus 100 is cut off (i.e., the timing at which the contactor 102 is turned off), the high-voltage power may place a heavy load on the resistance elements 142 and 143. Therefore, to ensure that these timings are synchronized, the contactor 102 is typically incorporated within the power conversion apparatus 100, and the control circuit 130 directly manages the opening and closing of the contactor 102. This makes it possible to strictly manage the timing at which the contactor 102 switches between open and closed states in accordance with the timing at which the normally closed discharge switching element 141 switches between open and closed states. However, the contactor 102 tends to be relatively large, which may lead to an increase in the size of the power conversion device 100.

[0058] In contrast, in the power conversion device 1 of this embodiment, when the discharge circuit 20 determines that the electrical connection between the high-voltage power supply HV and the load device M has been interrupted, the discharge circuit 20 discharges the stored power CV stored in the smoothing capacitor C to the first discharge resistor 42 and the second discharge resistor 43, and also to the control circuit 30. In this case, the stored power CV stored in the smoothing capacitor C can be used as a backup power source for the control circuit 30. This allows the stored power CV of the smoothing capacitor C to be distributed and consumed by the first discharge resistor 42, the second discharge resistor 43, and the control circuit 30. As a result, compared to when the stored power CV of the smoothing capacitor C is discharged only through the first discharge resistor 42 and the second discharge resistor 43, the stored power CV of the smoothing capacitor C can be sufficiently discharged without increasing the number and size of the first discharge resistor 42 and the second discharge resistor 43. Therefore, the power conversion device 1 of this embodiment allows for a smaller circuit size.

[0059] As in the present embodiment, the discharge circuit 20 may include a selection circuit 55 that selectively supplies to the control circuit 30 either the stored power CV stored in the smoothing capacitor C or the low-voltage power LV1 provided by the low-voltage power supply LV. In this case, even if the electrical connection between the high-voltage power supply HV and the load device M is interrupted, the stored power CV stored in the smoothing capacitor C can be quickly supplied to the control circuit 30 as backup power. In other words, the stored power CV stored in the smoothing capacitor C can be used as backup power for continuing the operation of the control circuit 30. As a result, there is no need to separately provide a large-capacity electrolytic capacitor for continuing the operation of the control circuit 30, thereby reducing the possibility of the circuit size becoming excessively large.

[0060] As in the present embodiment, the discharge circuit 20 may include an insulated power supply 51 that transmits the power CV3 supplied from the smoothing capacitor C to the input terminal 51a to the output terminal 51b at a voltage equal to or lower than the voltage indicated by the low-voltage power LV1, and a selection circuit 55 that selects the power V30 indicating the higher voltage between the voltage applied to the first input port 55a and the voltage applied to the second input port 55b and supplies the selected power V30 to the control circuit 30. In this case, the possibility of electrical noise from the high-voltage power supply HV being transmitted to the control circuit 30 is reduced, and the stored power CV stored in the smoothing capacitor C can be used as reserve power to enable the control circuit 30 to continue operating properly without interruption.

[0061] As in the present embodiment, one of the first diode 56 and the second diode 57 may be selectively made conductive to the output section 55c so that the power V30 indicating the higher voltage of the voltage applied to the anode of the first diode 56 and the voltage applied to the anode of the second diode 57 is supplied to the output section 55c. In this case, compared to when the operation of the selection circuit 55 is realized by switching control of a plurality of switching elements, output of a signal instructing the switching control is not required, and therefore the operation of the selection circuit 55 can be realized more easily.

[0062] As in the present embodiment, the discharge circuit 20 may include a normally-open discharge switching element 41 that is electrically connected to the smoothing capacitor C and is in an OFF state except for a period during which an ON signal φ32 indicating an ON state is received. The discharge switching element 41 may be turned ON based on the ON signal φ32 when it is determined that the electrical connection between the high-voltage power supply HV and the load device M has been interrupted. The power conversion device 1 of the present embodiment is configured to use the stored power CV of the smoothing capacitor C as a backup power source for the control circuit 30, thereby enabling the control circuit 30 to operate without interruption. However, in the event of an abnormality, it is conceivable that the supply of power V30 to the control circuit 30 may also be stopped. In such a situation, if a normally-closed discharge switching element is used, when the supply of power V30 to the control circuit 30 is interrupted, the normally-closed discharge switching element is automatically turned ON, and the discharge of the stored power CV of the smoothing capacitor C begins. At this time, if the timing at which the supply of high-voltage power HV1 from the high-voltage power supply HV to the power conversion device 1 is cut off is delayed relative to the timing at which the stored power CV in the smoothing capacitor C is discharged, the high-voltage power HV1 from the high-voltage power supply HV may impose a high load on the first discharge resistor 42 and the second discharge resistor 43. In contrast, if a normally open type discharge switching element 41 is used, the discharge switching element 41 is automatically turned off when the supply of power V30 to the control circuit 30 is cut off, so that it is possible to reduce the possibility of the high-voltage power HV1 from the high-voltage power supply HV applying a high load to the first discharge resistor 42 and the second discharge resistor 43, regardless of the timing at which the supply of high-voltage power HV1 from the high-voltage power supply HV to the power conversion device 1 is cut off.

[0063] As in the present embodiment, the power conversion device 1 may include a first input connection 11 electrically connected to the high-voltage power supply HV. A contactor 2 may be provided between the high-voltage power supply HV and the first input connection 11 to electrically open and close the high-voltage power supply HV and the input connection. If a normally closed discharge switching element is used, it is possible to incorporate the contactor 2 into the power conversion device 1 and strictly manage the timing of opening and closing the contactor 2 using a control circuit 30 in order to prevent a delay in the timing at which the supply of high-voltage power HV1 from the high-voltage power supply HV to the power conversion device 1 is cut off relative to the timing at which the discharge of the stored power CV in the smoothing capacitor C begins. However, since the contactor 2 tends to be relatively large, incorporating it into the power conversion device 1 may result in an increase in the size of the power conversion device 1. On the other hand, when a normally open type discharge switching element 41 is used, as described above, even if the supply of power V30 to the control circuit 30 is cut off, the possibility of a high load being applied to the first discharge resistor 42 and the second discharge resistor 43 can be reduced regardless of the timing at which the supply of high-voltage power HV1 from the high-voltage power supply HV to the power conversion device 1 is cut off, so there is no need to strictly manage the timing at which the contactor 2 is switched between open and closed by the control circuit 30. Therefore, by arranging the large contactor 2 outside the power conversion device 1, the circuit scale of the power conversion device 1 can be more reliably reduced.

[0064] As in the present embodiment, the control circuit 30 may determine that the electrical connection between the high-voltage power supply HV and the load device M has been interrupted when the charging voltage of the smoothing capacitor C exceeds a predetermined threshold, and may provide an on signal φ32 to the discharge switching element 41. The control circuit 30 may provide a test signal to the discharge switching element 41 at predetermined time intervals to check the open / closed state of the contactor 2, and may check whether the difference in the charging voltage of the smoothing capacitor C before and after the signal exceeds a predetermined threshold. In this case, even if the contactor 2 is located outside the power conversion device 1, the control circuit 30 can check the open / closed state of the contactor 2 based on changes in the charging voltage of the smoothing capacitor C, and therefore, can easily determine whether the electrical connection between the high-voltage power supply HV and the load device M has been interrupted.

[0065] As in this embodiment, the load device M may be a motor. When the control circuit 30 determines that the electrical connection between the high-voltage power supply HV and the load device M has been interrupted, the control circuit 30 may control the power conversion circuit 10 to maintain the q-axis current of the motor's d-axis current and q-axis current at a negative value and to maintain the d-axis current at a non-zero value. When the electrical connection between the high-voltage power supply HV and the load device M is interrupted, it is necessary to quickly decelerate the rotation of the motor to reduce the rotational energy generated in the motor. Here, the q-axis current of the motor contributes to the generation of torque in the rotor. The d-axis current of the motor generates magnetic flux in the stator but does not contribute to the generation of torque in the rotor. Therefore, by maintaining the q-axis current at a negative value, the control circuit 30 generates torque in the rotor in the direction opposite to the rotor's rotation direction. This allows the rotor to be braked. The rotational energy of the motor is stored as a voltage in the smoothing capacitor C. In response, the control circuit 30 controls the motor so that the d-axis current flows through the motor by maintaining the d-axis current at a non-zero value. As a result, hysteresis loss and eddy current loss (so-called iron loss) occur in the stator, and the stator coil generates heat, allowing the motor to consume the stored power CV of the smoothing capacitor C. Therefore, according to this embodiment, the stored power CV of the smoothing capacitor C can be discharged while decelerating the motor. This eliminates the need to increase the size of the first discharge resistor 42 and the second discharge resistor 43 to an extent that the rotational energy of the motor can be consumed, thereby more reliably reducing the circuit size.

[0066] The power conversion device of the present disclosure is not limited to the above-described embodiment and may be modified in various ways. For example, in the above-described embodiment, the discharge circuit includes two discharge resistors. The number and size of the discharge resistors are not limited to those in the above-described embodiment. The number of discharge resistors may be one, or may be three or more. In the above-described embodiment, the discharge circuit includes a normally open discharge switching element. However, the discharge circuit may include a normally closed discharge switching element instead of a normally open discharge switching element. In this case, when a single power conversion device is connected to a high-voltage power supply, the contactor may be incorporated inside the power conversion device. When multiple power conversion devices are connected to a high-voltage power supply, the contactor may be disposed external to the multiple power conversion devices or may be connected to each of the multiple power conversion devices.

[0067] In the above-described embodiment, the motor is decelerated by d-axis discharge. A short-circuit brake or a regenerative brake may be used as a technique for decelerating the motor. The selection circuit included in the power conversion device may be a circuit other than a diode-OR circuit. For example, the selection circuit may be configured with a plurality of switching elements. In this case, each switching element may be switched to select and supply to the control circuit power indicating a higher voltage between a voltage applied to a first input terminal of the selection circuit and a voltage applied to a second input terminal of the selection circuit.

[0068] <Additional Notes> The present disclosure is [1] "a power conversion device comprising: a power conversion circuit electrically connected between a first power source and a load device, and converting a form of power from the first power source into a form of power required by the load device; a control circuit electrically connected between a second power source and the power conversion circuit, and providing a control signal to the power conversion circuit that controls an operation of the power conversion circuit; a smoothing capacitor electrically connected between the first power source and the power conversion circuit; and a discharge circuit electrically connected to the smoothing capacitor and including a resistance element for discharging power stored in the smoothing capacitor, wherein the discharge circuit discharges the power stored in the smoothing capacitor to the resistance element and also to the control circuit when it is determined that the electrical connection between the first power source and the load device has been interrupted."

[0069] The present disclosure is [2] "The power conversion device described in [1], wherein the discharge circuit includes a selection circuit that selectively supplies one of the power stored in the smoothing capacitor and the power provided by the second power source to the control circuit."

[0070] The present disclosure is [3] "the power conversion device according to [1], wherein the discharge circuit includes: an isolated power supply including an input terminal electrically connected to the smoothing capacitor and an output terminal electrically insulated from the input terminal, and which transmits the power supplied from the smoothing capacitor to the input terminal to the output terminal at a voltage equal to or lower than a voltage indicated by the power provided by the second power supply; and a selection circuit including a first input section electrically connected to the output terminal, a second input section electrically connected to the second power supply, and an output section electrically connected to the control circuit, which selects power indicating a higher voltage between a voltage applied to the first input section and a voltage applied to the second input section and supplies the selected power to the control circuit."

[0071] The present disclosure is [4] "the power conversion device described in [3], wherein the selection circuit includes a first diode having an anode electrically connected to the first input portion, and a second diode having an anode electrically connected to the second input portion, the cathode of the first diode and the cathode of the second diode are electrically connected to the output portion, and one of the first diode and the second diode is selectively made conductive to the output portion so that power indicating a higher voltage of a voltage applied to the anode of the first diode and a voltage applied to the anode of the second diode is supplied to the output portion."

[0072] The present disclosure is [5] "The power conversion device according to any one of [1] to [4], wherein the discharge circuit includes a normally open switching element that is electrically connected to the smoothing capacitor and is in an off state except for a period during which an on signal instructing an on state is given, and the switching element is turned to the on state based on the on signal when it is determined that the electrical connection between the first power source and the load device has been interrupted."

[0073] The present disclosure is [6] "The power conversion device described in [5], which includes an input connection part electrically connected to the first power source, and a contactor is provided between the first power source and the input connection part to switch between electrically opening and closing the first power source and the input connection part."

[0074] The present disclosure is [7] "The power conversion device described in [6], wherein the control circuit determines that the electrical connection between the first power source and the load device has been interrupted when the charging voltage of the smoothing capacitor exceeds a predetermined threshold, and provides the on signal to the switching element."

[0075] The present disclosure is [8] "The power conversion device described in [6], wherein the control circuit provides a signal to the switching element at predetermined intervals, and when a difference between a charging voltage of the smoothing capacitor when the signal is provided and a charging voltage of the smoothing capacitor when the next signal is provided exceeds a predetermined threshold, the control circuit determines that the electrical connection between the first power source and the load device has been interrupted, and provides the on signal to the switching element."

[0076] The present disclosure is [9] "The power conversion device according to any one of [1] to [8], wherein the load device is a motor including a rotor and a stator, and when the control circuit determines that the electrical connection between the first power supply and the load device has been interrupted, the control circuit controls the power conversion circuit so as to maintain the q-axis current of the d-axis current and the q-axis current of the motor at a negative value and to maintain the d-axis current at a non-zero value."

[0077] REFERENCE SIGNS LIST 1 Power conversion device 2 Contactor 10 Power conversion circuit 11 First input connection portion (input connection portion) 20 Discharge circuit 30 Control circuit 41 Discharge switching element (switching element) 42 First discharge resistor (resistance element) 51 Insulated power supply 51a Input terminal 51b Output terminal 55 Selection circuit 55a First input portion 55b Second input portion 55c Output portion 56 First diode 57 Second diode C Smoothing capacitor HV High voltage power supply (first power supply) LV Low voltage power supply (second power supply) M Load device φ31A, φ31B Control signal φ32 On signal

Claims

1. A power conversion device comprising: a power conversion circuit electrically connected between a first power source and a load device, for converting the form of power from the first power source into the form of power required by the load device; a control circuit electrically connected between a second power source and the power conversion circuit, for providing the power conversion circuit with a control signal that controls the operation of the power conversion circuit; a smoothing capacitor electrically connected between the first power source and the power conversion circuit; and a discharge circuit electrically connected to the smoothing capacitor and including a resistance element for discharging power stored in the smoothing capacitor, wherein the discharge circuit discharges the power stored in the smoothing capacitor to the resistance element and to the control circuit when it determines that the electrical connection between the first power source and the load device has been interrupted.

2. The power conversion device according to claim 1, wherein the discharge circuit includes a selection circuit that selectively supplies one of the power stored in the smoothing capacitor and the power provided by the second power source to the control circuit.

3. The power conversion device according to claim 1, wherein the discharge circuit comprises: an isolated power supply including an input terminal electrically connected to the smoothing capacitor and an output terminal electrically insulated from the input terminal, and which transmits the power supplied from the smoothing capacitor to the input terminal to the output terminal at a voltage equal to or lower than the voltage indicated by the power provided by the second power supply; and a selection circuit including a first input section electrically connected to the output terminal, a second input section electrically connected to the second power supply, and an output section electrically connected to the control circuit, which selects power indicating the higher voltage of the voltage applied to the first input section and the voltage applied to the second input section and supplies the selected power to the control circuit.

4. The power conversion device according to claim 3, wherein the selection circuit includes a first diode having an anode electrically connected to the first input portion and a second diode having an anode electrically connected to the second input portion, the cathode of the first diode and the cathode of the second diode being electrically connected to the output portion, and one of the first diode and the second diode being selectively brought into conduction with the output portion so that power representing a higher voltage of a voltage applied to the anode of the first diode and a voltage applied to the anode of the second diode is supplied to the output portion.

5. A power conversion device according to any one of claims 1 to 4, wherein the discharge circuit includes a normally open switching element that is electrically connected to the smoothing capacitor and is in an off state except for a period during which an on signal instructing an on state is given, and the switching element is turned to the on state based on the on signal when it is determined that the electrical connection between the first power source and the load device has been cut off.

6. The power conversion device according to claim 5, further comprising an input connection part electrically connected to said first power source, and a contactor is provided between said first power source and said input connection part for switching between electrical open and closed states between said first power source and said input connection part.

7. The power conversion device according to claim 6, wherein the control circuit determines that the electrical connection between the first power source and the load device has been interrupted when the charging voltage of the smoothing capacitor exceeds a predetermined threshold, and provides the on signal to the switching element.

8. The power conversion device according to claim 6, wherein the control circuit supplies a signal to the switching element at predetermined intervals, and when a difference between the charging voltage of the smoothing capacitor when the signal is supplied and the charging voltage of the smoothing capacitor when the next signal is supplied exceeds a predetermined threshold, the control circuit determines that the electrical connection between the first power source and the load device has been interrupted, and supplies the on signal to the switching element.

9. A power conversion device according to any one of claims 1 to 4, wherein the load device is a motor including a rotor and a stator, and when the control circuit determines that the electrical connection between the first power supply and the load device has been interrupted, the control circuit controls the power conversion circuit so as to maintain the q-axis current of the motor's d-axis current and q-axis current at a negative value and to maintain the d-axis current at a non-zero value.

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