Electric circuit device

Abstract

[Problem] To provide an electric circuit device with which it is possible to quickly discharge a smoothing capacitor without using a low voltage power supply for a discharging circuit operation which discharges the smoothing capacitor, while keeping the loss at the time of discharging within a component rating range. [Solution] A discharging circuit 11 comprises a plurality of discharge resistors 13, 14, 18, 19, 21, 22 and a switch 16, 17 which switches a discharge resistor connected in parallel to a smoothing capacitor 6. The switch monitors the voltage of the smoothing capacitor and switches the discharge resistor connected in parallel to the smoothing capacitor in response to a drop in the voltage of the smoothing capacitor after an HV battery 8 is shut off, thereby limiting discharge-associated power consumption to within a predetermined range.

Description

Electrical circuit device

[0001] The present invention relates to an electrical circuit device equipped with a discharge circuit that discharges a smoothing capacitor when a high-voltage power supply is interrupted.

[0002] For example, a power converter that drives the motor of an electric compressor, which is part of a vehicle's air conditioning system, is connected to a high-voltage power supply (HV battery) via a connector (HV connector). If this connector becomes disconnected, there is a risk of accidents such as electric shock due to residual charge accumulated in the smoothing capacitor. Therefore, conventional designs have included a discharge circuit to discharge the residual charge from the smoothing capacitor when the high-voltage power supply is cut off, such as when the connector becomes disconnected.

[0003] Regarding the discharge of residual charge using this discharge circuit, conventional methods have been proposed, such as connecting a discharge resistor in parallel with a smoothing capacitor at all times, or using a switch with duty cycle control to supply current (discharge current) to the discharge resistor when the high-voltage power supply is cut off, such as when a connector is disconnected (see, for example, Patent Documents 1 and 2).

[0004] Patent No. 6466133 Patent No. 7385607

[0005] However, in all of the aforementioned patent documents, the discharge circuit is controlled by a circuit that operates on a low-voltage power supply mounted on the vehicle, which presents a problem as it will not operate if the low-voltage power supply is cut off. In particular, in the case of Patent Document 1, if the low-voltage power supply is cut off while a high-voltage power supply is connected (applied), a current exceeding the rating will flow through the additionally connected discharge resistor, and there is a risk that the loss (power consumption) will exceed the rating and cause thermal damage.

[0006] The present invention was made to solve the aforementioned conventional technical problems, and aims to provide an electrical circuit device that does not use a low-voltage power supply for the operation of the discharge circuit that discharges a smoothing capacitor, and that can discharge rapidly while keeping the loss during discharge within the component rating range.

[0007] The electrical circuit device of the present invention comprises a smoothing capacitor connected to a high-voltage power supply and a discharge circuit for discharging the smoothing capacitor when the high-voltage power supply is cut off. The discharge circuit comprises a plurality of discharge resistors and a switch for switching the discharge resistors connected in parallel to the smoothing capacitor. The switch monitors the voltage of the smoothing capacitor and, in accordance with the drop in the voltage of the smoothing capacitor after the high-voltage power supply is cut off, switches the discharge resistors connected in parallel to the smoothing capacitor, thereby suppressing the power consumption associated with discharge within a predetermined range.

[0008] The electrical circuit device of the second invention is characterized in that, in the above invention, the switch increases the number of discharge resistors connected in parallel to the smoothing capacitor as the voltage of the smoothing capacitor decreases.

[0009] The electrical circuit device of the third invention is characterized in that the switch in the present invention is a voltage monitoring IC that operates by being powered by a smoothing capacitor.

[0010] The electrical circuit device of the fourth invention is characterized in that, in the above invention, the discharge circuit comprises a discharge resistor that is always connected to a smoothing capacitor and a discharge resistor that is connected to the output terminal of a switch, and the switch is connected in parallel with the smoothing capacitor when the voltage value based on the voltage of the smoothing capacitor input to the input terminal of the switch falls below a predetermined threshold.

[0011] The electrical circuit device of the fifth invention is characterized in that, in the above invention, the discharge circuit comprises a plurality of switches having a single input terminal and a single output terminal, or a plurality of switches having a plurality of input terminals and a plurality of output terminals, and a discharge resistor is connected to the output terminal of each switch.

[0012] The electrical circuit device of the sixth invention is characterized by comprising a charge-consuming element that generates power loss due to the current flowing through a discharge resistor connected to the output terminal of the switch in the fourth invention.

[0013] The seventh invention's electrical circuit device is characterized by supplying the voltage of the smoothing capacitor to the motor of an electric compressor mounted on a vehicle, as described in each of the above inventions.

[0014] According to the present invention, in an electrical circuit device comprising a smoothing capacitor connected to a high-voltage power supply and a discharge circuit for discharging the smoothing capacitor when the high-voltage power supply is cut off, the discharge circuit comprises a plurality of discharge resistors and a switch for switching between discharge resistors connected in parallel to the smoothing capacitor. The switch monitors the voltage of the smoothing capacitor and, in accordance with the decrease in the voltage of the smoothing capacitor after the high-voltage power supply is cut off, switches the discharge resistors connected in parallel to the smoothing capacitor, thereby suppressing the power consumption associated with discharge within a predetermined range. For example, as in the second invention, by increasing the number of discharge resistors connected in parallel to the smoothing capacitor as the voltage of the smoothing capacitor decreases using the switch, the number of discharge resistors through which the discharge current flows increases in accordance with the decrease in the voltage of the smoothing capacitor, making it possible to rapidly discharge the smoothing capacitor while keeping the discharge loss within the component rating range across the entire voltage range of the high-voltage power supply.

[0015] In particular, by configuring the switch as a voltage monitoring IC powered by a smoothing capacitor, as in the third invention, the smoothing capacitor can be safely and quickly discharged without using a low-voltage power supply for control.

[0016] In this case, as in the fourth invention, the discharge circuit is provided with a discharge resistor that is always connected to the smoothing capacitor and a discharge resistor that is connected to the output terminal of the switch, and when the voltage value based on the voltage of the smoothing capacitor input to the input terminal of the switch falls below a predetermined threshold, the switch connects the discharge resistor connected to the output of the switch in parallel with the smoothing capacitor.

[0017] Furthermore, the switch for the discharge circuit described above may consist of multiple switches having a single input terminal and a single output terminal, or multiple switches having multiple input terminals and multiple output terminals, as in the fifth invention, with the discharge resistors connected to the output terminals of the switches.

[0018] Furthermore, as in the sixth invention, by providing a charge-consuming element that generates power loss due to the current flowing through a discharge resistor connected to the output terminal of the switch, it becomes possible to shorten the discharge time of the smoothing capacitor by one phase.

[0019] Furthermore, the present invention is extremely effective when applied to an electric compressor mounted on a vehicle, as in the seventh invention.

[0020] This figure shows an example of an electrical circuit when the electrical circuit device of the present invention is applied to a power conversion device for an electric compressor. This figure shows the voltage change during discharge of the smoothing capacitor in Figure 1 and the change in losses (power consumption) in each component. This figure shows the voltage change during discharge of a conventional smoothing capacitor and the change in losses (power consumption) in each component.

[0021] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. Figure 1 is an electrical circuit diagram of an electric compressor that constitutes an air conditioning system of a vehicle to which the present invention is applied. In this embodiment, the power conversion device 1 for operating the motor 2 of the electric compressor is an embodiment of the electrical circuit device of the present invention.

[0022] In Figure 1, the power converter 1, an embodiment of the electrical circuit device of the present invention, comprises a control unit 3 configured with a microcomputer equipped with a microprocessor, an inverter circuit 4 for driving the motor 2, a smoothing capacitor 6, and a discharge circuit 11 for discharging the residual charge of the smoothing capacitor 6. It is connected to an HV battery 8 (DC 950V), which serves as a high-voltage power source mounted on the vehicle, via a connector 7. Note that 9 is an LV battery, which serves as a low-voltage power source for the control unit 3, and is also assumed to be mounted on the vehicle.

[0023] The inverter circuit 4 consists of a plurality of switching elements whose input node is connected between the power line 12 of the HV battery 8 (a 950V high-voltage power supply in this embodiment) and the ground GND (ground line). The switching elements are turned ON / OFF by the gate signal of the control unit 3, thereby switching the voltage (DC power) of the smoothing capacitor 6 to convert it into three-phase AC (AC power) and supplying it to the motor 2.

[0024] The smoothing capacitor 6 is a filter that smooths the DC power input from the HV battery 8, and is connected between the power line 12 and ground GND in the inverter circuit 4 and connector 7, that is, in electrical terms, between the inverter circuit 4 and the HV battery 8.

[0025] The discharge circuit 11 consists of a first discharge resistor 13 consisting of multiple resistors 13A connected in series, a second discharge resistor 14 consisting of multiple resistors 14A also connected in series, a first switch 16 composed of a voltage monitoring IC (reset IC), a second switch 17 also composed of a voltage monitoring IC, a third discharge resistor 18 and a fourth discharge resistor 19 with one end connected to the two output terminals 16A and 16B of the first switch 16, a fifth discharge resistor 21 and a sixth discharge resistor 22 with one end connected to the two output terminals 17A and 17B of the second switch 17, a switching element 23 as a charge consumption element composed of a MOSFET, a Zener diode 24, and the like.

[0026] One end of the first discharge resistor 13 is connected to the power line 12 between the smoothing capacitor 6 and the inverter circuit 4, and a Zener diode 24 is connected in series between the other end of the first discharge resistor 13 and ground GND. One end of the second discharge resistor 14 is connected to the power line 12 between the first discharge resistor 13 and the inverter circuit 4, and a series circuit of five voltage divider resistors 14B, 14C, 14D, 14E, and 14F, which constitute part of the second discharge resistor 14, is connected between the other end of the second discharge resistor 14 and ground GND.

[0027] In this embodiment, the voltage divider resistor 14B is connected to resistor 14A, and the voltage divider resistor 14F is connected to ground GND. The terminal voltage of the voltage divider resistor 14E is connected to one of the two input terminals 16C and 16D of the first switch 16 via connection circuit X1, and the terminal voltage of the voltage divider resistor 14D is connected to the other input terminal 16D of the first switch 16 via connection circuit X2.

[0028] Furthermore, the terminal voltage of the voltage divider resistor 14C is connected to one of the two input terminals 17C and 17D of the second switch 17 via the connection circuit X3, and the terminal voltage of the voltage divider resistor 14B is connected to the other input terminal 17D of the second switch 17 via the connection circuit X4.

[0029] In this embodiment, the threshold values ​​of the input terminals 16C, 16D, 17C, and 17D of each switch 16 and 17 are the same. When the terminal voltage of the voltage divider resistor 14E falls below the threshold value of input terminal 16C of the first switch 16, the output terminal 16A becomes "L" (ground GND), and when the terminal voltage of the voltage divider resistor 14D falls below the threshold value of input terminal 16D of the first switch 16, the output terminal 16B becomes "L" (ground GND). Similarly, when the terminal voltage of the voltage divider resistor 14C falls below the threshold value of input terminal 17C of the second switch 17, the output terminal 17A becomes "L" (ground GND), and when the terminal voltage of the voltage divider resistor 14B falls below the threshold value of input terminal 17D of the second switch 17, the output terminal 17B becomes "L" (ground GND).

[0030] Furthermore, the terminal voltage of the Zener diode 24 is connected to the gate terminal of the source follower switching element 23, and the drain terminal of the switching element 23 is connected to the power line 12 between the second discharge resistor 14 and the inverter circuit 4. In addition, a resistor 26 is connected between the source terminal of the switching element 23 and ground GND.

[0031] Furthermore, power supply circuits 27 and 28 for supplying power to each of the switches 16 and 17 are connected between the source terminal of the switching element 23 and the resistor 26, respectively. In addition, the other ends of the third discharge resistor 18 and the fourth discharge resistor 19, and the other ends of the fifth discharge resistor 21 and the sixth discharge resistor 22 are also connected between the source terminal of the switching element 23 and the resistor 26.

[0032] With the above configuration, the discharge operation of the residual charge of the smoothing capacitor 6 will now be explained with reference to Figures 1 to 3. During normal operation with the connector 7 connected, the control unit 3 converts the voltage of the smoothing capacitor 6 (DC voltage) into a three-phase AC voltage and applies it to the motor 2 by turning each switching element of the inverter circuit 4 ON / OFF.

[0033] Furthermore, during normal operation, current I flows through the first discharge resistor 13. R1 As a result, current I flows through the second discharge resistor 14 (including each voltage divider resistor 14B to 14F). R2 A current flows through the first discharge resistor 13. R1 As the current flows through the Zener diode 24, a voltage of 15V DC is generated at the terminals of the Zener diode 24 in this embodiment. This voltage is input as the gate voltage to the gate terminal of the switching element 23, causing the switching element 23 to conduct.

[0034] Here, the source terminal of the switching element 23 is constantly at approximately the same potential (15V) as the gate voltage generated by the Zener diode 24 by the source follower. When the switching element 23 is conducting, a voltage equal to the difference between the voltage of the power supply line 12 and the voltage of the source terminal (approximately the voltage generated by the Zener diode 24) is always applied between the drain and source.

[0035] The constant voltage at this source terminal is applied to the third discharge resistor 18, the fourth discharge resistor 19, the fifth discharge resistor 21, and the sixth discharge resistor 22, which are connected to the output terminals 16A, 16B, 17A, and 17B of the first switch 16 and the second switch 17, respectively. At the same time, the power supply circuits 17 and 18 also supply power to each of the switches 16 and 17 themselves as operating power.

[0036] However, when the connector 7 is connected and the voltage of the smoothing capacitor 6 (the voltage of the power supply line 12) is 950 V, the terminal voltage of the voltage dividing resistor 14E input to the input terminal 16C of the first switch 16 becomes equal to or higher than the threshold value of the input terminal 16C, so the output terminal 16A becomes "H" (15 V). Similarly, the terminal voltage of the voltage dividing resistor 14D input to the input terminal 16D of the first switch 16 also becomes equal to or higher than the threshold value of the input terminal 16D, so the output terminal 16B becomes "H".

[0037] Also, the terminal voltage of the voltage dividing resistor 14C input to the input terminal 17C of the second switch 17 becomes equal to or higher than the threshold value of the input terminal 17C, so the output terminal 17A becomes "H", and the terminal voltage of the voltage dividing resistor 14B input to the input terminal 17D of the second switch 17 also becomes equal to or higher than the threshold value of the input terminal 17D, so the output terminal 17B becomes "H". Therefore, the third to sixth discharge resistors 18, 19, 21, 22 are not connected in parallel with the smoothing capacitor 6, and no current flows.

[0038] Next, a case where the power supply from the HV battery (high-voltage power supply) 8 is cut off due to the disconnection of the connector 7 or the like will be described. In FIGS. 2 and 3, L1 represents the voltage of the smoothing capacitor 6 (HV voltage), L2 represents the loss generated in the entire discharge circuit 11 between the power supply line 12 and the ground GND, and L3 represents the change in the loss generated in the switching element (MOSFET) 23.

[0039] When the power supply from the HV battery 8 is cut off, discharge starts from the smoothing capacitor 6, and current I R1 flows through the first discharge resistor 13, and current I R2 flows through the second discharge resistor 14, so the voltage of the smoothing capacitor 6 decreases from 950 V as shown by L1 in FIG. 2 (period (1) in FIG. 2). Therefore, the discharge current I of the smoothing capacitor 6 in period (1) of FIG. 2 is I = I R1 + I R2 and becomes.

[0040] In period (1) of FIG. 2, as the voltage of the smoothing capacitor 6 decreases, the overall generated loss L2 also gradually decreases, but no discharge current flows through the switching element 23, so the generated loss L3 does not occur.

[0041] On the other hand, when the voltage of the smoothing capacitor 6 further decreases, the terminal voltage of the voltage dividing resistor 16E also decreases, and eventually falls below the threshold value of the input terminal 16C of the first switch 16 (period (2) in FIG. 2). Then, since the output terminal 16A of the first switch 16 becomes "L", the third discharge resistor 18 is connected in parallel with the smoothing capacitor 6. As a result, the current I1 flows through the third discharge resistor 18, and thus the discharge current I of the smoothing capacitor 6 in the period (2) of FIG. 2 is I = I R1 + I R2 + I1.

[0042] In the period (2) of FIG. 2, since the current I1 flows through the third discharge resistor 18, losses occur in the switching element 23 (L3). Thereby, although the overall loss L2 also increases, it gradually decreases as the voltage of the smoothing capacitor 6 decreases.

[0043] When the voltage of the smoothing capacitor 6 further decreases due to such discharge, the terminal voltage of the voltage dividing resistor 16D decreases and falls below the threshold value of the input terminal 16D of the first switch 16 (period (3) in FIG. 2). Then, since the output terminal 16B of the first switch 16 also becomes "L", the fourth discharge resistor 19 is connected in parallel with the smoothing capacitor 6. As a result, the current I2 also flows through the fourth discharge resistor 19, and thus the discharge current I of the smoothing capacitor 6 in the period (3) of FIG. 2 is I = I R1 + I R2 + I1 + I2.

[0044] In the period (3) of FIG. 2, since the current I2 also flows through the fourth discharge resistor 19, the losses generated in the switching element 23 become even larger (L3). Thereby, although the overall loss L2 also increases, it gradually decreases as the voltage of the smoothing capacitor 6 decreases.

[0045] From there, the voltage across the smoothing capacitor 6 drops further, and the terminal voltage of the voltage divider resistor 16C decreases until it falls below the threshold of the input terminal 17C of the second switch 17 (period (4) in Figure 2). At this point, the output terminal 17A of the second switch 17 becomes "L", and the fifth discharge resistor 21 is connected in parallel with the smoothing capacitor 6. As a result, a current I3 flows through the fifth discharge resistor 19, and the discharge current I of the smoothing capacitor 6 during this period (4) in Figure 2 is I = I R1 +I R2 This becomes +I1+I2+I3.

[0046] During period (4) in Figure 2, current I3 also flows through the fifth discharge resistor 21, so the loss generated in the switching element 23 becomes even larger (L3). As a result, the overall loss L2 also increases, but it gradually decreases as the voltage across the smoothing capacitor 6 decreases.

[0047] Subsequently, as the voltage across the smoothing capacitor 6 decreases further, the terminal voltage of the voltage divider resistor 16B drops and falls below the threshold of the input terminal 17D of the second switch 17 (period (5) in Figure 2), the output terminal 17B of the second switch 17 also becomes "L", so the sixth discharge resistor 22 is connected in parallel with the smoothing capacitor 6. As a result, a current I4 flows through the sixth discharge resistor 22, and the discharge current I of the smoothing capacitor 6 during this period (5) in Figure 2 is I = I R1 +I R2 This becomes +I1+I2+I3+I4.

[0048] During period (5) in Figure 2, current I4 also flows through the sixth discharge resistor 22, so the loss generated in the switching element 23 becomes even larger (L3). As a result, the overall loss L2 also increases, but as the voltage across the smoothing capacitor 6 decreases, each loss L2 and L3 gradually decreases, and eventually the discharge ends.

[0049] During the discharge operation described above, the total power loss L2 and the power loss L3 generated at the switching element 23 are suppressed to within the component rating range of 0 to approximately 5W, as shown in Figure 2. On the other hand, the power loss L3 generated at the switching element 23 increases as the voltage of the smoothing capacitor 6 decreases, so the discharge time is shortened.

[0050] Figure 3 shows, for comparison, the voltage across the smoothing capacitor 6 during discharge and the changes in the generated losses L2 and L3 when only the first discharge resistor 13 and the second discharge resistor 14 are connected. In this case, since the switching element 23 is absent, the generated loss L3 is zero. In this case, the discharge current I is I = I R1 +I R2 Therefore, even after 5 seconds have elapsed since the start of discharge, the voltage of the smoothing capacitor 6 remains above 300V. However, with the configuration as shown in the embodiment, as shown in Figure 2, the voltage of the smoothing capacitor 6 becomes approximately zero within 4 seconds.

[0051] Thus, in this invention, each switch 16, 17 monitors the voltage of the smoothing capacitor 6 using voltage divider resistors 14B to 14F, and switches the discharge resistors 18, 19, 21, and 22 connected in parallel to the smoothing capacitor 6 in accordance with the decrease in the voltage of the smoothing capacitor 6 after the HV battery (high-voltage power supply) 8 is cut off. In other words, in the embodiment, each switch 16, 17 increases the number of discharge resistors 13, 14, 18, 19, and 21 connected in parallel to the smoothing capacitor 6 as the voltage of the smoothing capacitor 6 decreases. As the voltage of the smoothing capacitor 6 decreases, the number of discharge resistors 13, 14, 18, 19, and 21 through which the discharge current I flows increases, making it possible to rapidly discharge the smoothing capacitor 6 while keeping the discharge loss within the component rating range across the entire voltage range of the HV battery 8.

[0052] In particular, by configuring each switch 16 and 17 as a voltage monitoring IC powered by the smoothing capacitor 6, as in the embodiment, the smoothing capacitor 6 can be safely and quickly discharged without using the control LV battery 9.

[0053] Furthermore, in this embodiment, a switching element (charge-consuming element) 23 is provided that generates power loss due to the current flowing through the discharge resistors 18, 19, 21, and 22 connected to the output terminals 16A, 16B, 17A, and 17B of each switch 16 and 17, making it possible to shorten the discharge time of the smoothing capacitor 6 by one phase.

[0054] Furthermore, the present invention is extremely effective when applied to a power conversion device 1 of an electric compressor mounted on a vehicle, as shown in the embodiment.

[0055] In this embodiment, the discharge circuit 11 is provided with first and second discharge resistors 13 and 14 that are always connected to the smoothing capacitor 6, and third to sixth discharge resistors 18, 19, 21, and 22 that are connected to the output terminals 16A, 16B, 17A, and 17B of each switch 16 and 17. Each switch 16 and 17 will switch when the terminal voltage of the voltage divider resistors 14E to 14B, based on the voltage of the smoothing capacitor 6 input to the input terminals 16C, 16D, 17C, and 17D, falls below a predetermined threshold (the same threshold). Although the discharge resistors 18, 19, 21, and 22 connected to the output terminals 16A, 16B, 17A, and 17B of switches 16 and 17 are connected in parallel to the smoothing capacitor 6, the threshold values ​​of the input terminals 16C, 16D, 17C, and 17D of each switch 16 and 17 may be set to different values, and for example, the number of discharge resistors connected in parallel to the smoothing capacitor 6 may be switched by inputting the terminal voltage of the voltage divider resistor 14E to all input terminals 16C, 16D, 17C, and 17D.

[0056] Furthermore, although the embodiment used two switches 16 and 17, each having two output terminals 16A, 16B, 17A, and 17B and two input terminals 16C, 16D, 17C, and 17D, the embodiment is not limited to this. Similar switching operation can be achieved by configuring a switch from a voltage monitoring IC having a single input terminal and an output terminal, and providing four such switches.

[0057] Furthermore, in this embodiment, the third to sixth discharge resistors 18, 19, 21, and 22 are connected in four stages to the first and second discharge resistors 13 and 14 which are always connected to the smoothing capacitor 6. However, the invention is not limited to this configuration, and only the third discharge resistor 18, or only the third and fourth discharge resistors 18 and 19, or only the third to fifth discharge resistors 18, 19, and 21 may be connected in stages, or even more discharge resistors may be used to connect them in even more stages.

[0058] Furthermore, although a MOSFET-based switching element 23 was used as the charge-consuming element in this embodiment, it is not limited to that, and other switching elements such as IGBTs may be used, or any element other than a switching element may be used as long as it can consume the charge of the smoothing capacitor 6 only when it is discharging.

[0059] Furthermore, although the present invention was applied to a power conversion device 1 of an electric compressor mounted on a vehicle in the embodiment, the present invention is not limited to this and is effective for electrical circuit devices of various equipment having a smoothing capacitor.

[0060] GND Ground 1 Power converter (electrical circuit device) 2 Motor 3 Control unit 4 Inverter circuit 6 Smoothing capacitor 7 Connector 8 HV battery (high voltage power supply) 9 LV battery 11 Discharge circuit 12 Power line 13 First discharge resistor 14 Second discharge resistor 16 First switch 17 Second switch 18 Third discharge resistor 19 Fourth discharge resistor 21 Fifth discharge resistor 22 Sixth discharge resistor 23 Switching element (charge consumption element) 24 Zener diode 27, 28 Power supply circuit

Claims

1. An electrical circuit device comprising a smoothing capacitor connected to a high-voltage power supply and a discharge circuit for discharging the smoothing capacitor when the high-voltage power supply is interrupted, wherein the discharge circuit comprises a plurality of discharge resistors and a switch for switching the discharge resistors connected in parallel to the smoothing capacitor, and the switch monitors the voltage of the smoothing capacitor and, in accordance with the decrease in the voltage of the smoothing capacitor after the high-voltage power supply is interrupted, switches the discharge resistors connected in parallel to the smoothing capacitor, thereby suppressing power consumption associated with discharge within a predetermined range.

2. The electrical circuit device according to claim 1, characterized in that the switch increases the number of discharge resistors connected in parallel to the smoothing capacitor as the voltage of the smoothing capacitor decreases.

3. The electrical circuit device according to claim 1, characterized in that the switch is a voltage monitoring IC that operates by being powered by the smoothing capacitor.

4. The electrical circuit device according to claim 3, wherein the discharge circuit comprises a discharge resistor that is always connected to the smoothing capacitor and a discharge resistor connected to the output terminal of the switch, and the switch connects the discharge resistor connected to the output of the switch in parallel with the smoothing capacitor when the voltage value based on the voltage of the smoothing capacitor input to the input terminal of the switch falls below a predetermined threshold.

5. The electrical circuit device according to claim 4, wherein the discharge circuit comprises a plurality of switches having a single input terminal and a single output terminal, or a plurality of switches having a plurality of input terminals and a plurality of output terminals, and the discharge resistors are connected to the output terminals of the switches, respectively.

6. The electrical circuit device according to claim 4, further comprising a charge-consuming element that generates power loss due to the current flowing through a discharge resistor connected to the output terminal of the changeover switch.

7. The electrical circuit device according to any one of claims 1 to 6, characterized in that the voltage of the smoothing capacitor is supplied to the motor of an electric compressor mounted on the vehicle.

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