Power supply cutoff device, motor drive device, and electric power steering device

Abstract

A power supply shut-off device according to the present disclosure comprises a power supply relay having serially connected first switching elements and second switching elements, a power supply relay driving circuit for inputting a driving signal into the power supply relay, a monitor for measuring an intermediate voltage, and an arithmetic unit. A first diode and a second diode are serially connected such that the same poles thereof are connected to a point where the intermediate voltage is measured by the monitor. The arithmetic unit determines whether or not the power relay is malfunctioning, on the basis of the intermediate voltage after the driving signal for stopping supply of an electric current has been input into the power supply relay.

Description

[Technical field]

[0001] The present disclosure relates to a power cut-off device, a motor drive device, and an electric power steering device. [Background technology]

[0002] Patent Document 1 discloses a device capable of determining whether an abnormality has occurred in a switching element in a power relay using a switching element. This device includes a power relay, a capacitor, a charging circuit, a computing device, a first monitor circuit, a second monitor circuit, and the like. The capacitor is connected between the power relay and a drive circuit. The charging circuit charges the capacitor in response to a command from the computing device. The second monitor circuit monitors the voltage of the capacitor. The computing device determines whether or not there is an abnormality in the power relay based on the voltage monitored by the second monitor circuit, and the like. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] International Publication No. 2014 / 184888 Summary of the Invention [Problem to be solved by the invention]

[0004] In the configuration of Patent Document 1, a charging circuit is used to charge a capacitor in order to determine whether or not there is an abnormality in the power relay. Therefore, even if the charging circuit is not required as an original function of the device, it is necessary to provide a charging circuit in order to determine whether or not there is an abnormality in the power relay. This can lead to an increase in the number of parts, and there is room for improvement.

[0005] In consideration of the above circumstances, the present disclosure aims to provide a power supply cut-off device, a motor drive device, and an electric power steering device that can suppress an increase in the number of parts required to determine whether or not there is an abnormality in a power supply relay. [Means for solving the problem]

[0006] One embodiment of a power supply cutoff device according to the present disclosure includes a power supply relay having a first switching element and a second switching element connected in series, and switching between supplying and stopping a current from a battery to an object to be powered, a power supply relay drive circuit that inputs a drive signal to the power supply relay to switch between supplying and stopping the current, a monitor that measures an intermediate voltage between the first switching element and the second switching element, and a calculation device capable of acquiring a measurement result of the intermediate voltage by the monitor, wherein the first switching element includes a first diode arranged in parallel to a path along which the first switching element passes a current from the battery to the object to be powered, and the second switching element includes a second diode arranged in parallel to the path along which the second switching element passes a current from the battery to the object to be powered, the first diode and the second diode are connected in series such that the same pole of each diode is connected to a measurement point of the intermediate voltage by the monitor, and the calculation device determines whether or not the power supply relay has a failure based on the intermediate voltage after the drive signal to stop the supply of current to the power supply relay is input.

[0007] One aspect of a motor drive device according to the present disclosure includes a motor which is the power supply object, and the power supply cutoff device.

[0008] One aspect of an electric power steering device according to the present disclosure includes the motor drive device. Effect of the Invention

[0009] According to the present disclosure, it is possible to provide a power supply cutoff device, a motor drive device, and an electric power steering device that are capable of suppressing an increase in the number of parts required to determine the presence or absence of an abnormality in a power supply relay. [Brief description of the drawings]

[0010] [Figure 1]1 is a schematic configuration diagram of an electric power steering device according to a first embodiment. [Diagram 2] FIG. 2 is a detailed circuit diagram of the periphery of a power relay in the first embodiment. [Diagram 3] 10(a) is a timing chart of the drive signal, and FIG. 10(b) is an example of the measurement results of the intermediate voltage. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. Note that the scope of the present disclosure is not limited to the following embodiments, and may be modified as desired within the scope of the technical concept of the present disclosure.

[0012] Embodiment 1 FIG. 1 is a schematic configuration diagram of an electric power steering device 20 according to a first embodiment. The electric power steering device 20 shown in FIG. 1 includes a motor 7 that is an object to be supplied with power, and a motor drive device 1 that drives the motor 7. The electric power steering device 20 is mounted on a vehicle, for example, for use. In the following description, a case in which the electric power steering device 20 is mounted on a vehicle will be described. However, the motor drive device 1 according to this embodiment may be used for purposes other than the electric power steering device 20 for a vehicle.

[0013] The motor drive device 1 includes a power relay 2, a monitor 3, a computing device 4, a motor drive circuit, a capacitor 6, a resistor 9, and a power relay drive circuit 21. The power relay 2 and the computing device 4 configure a power cutoff device 1a. In other words, the motor drive device 1 includes the power cutoff device 1a. The power cutoff device 1a may include components other than the power relay 2 and the computing device 4. The power cutoff device 1a according to this embodiment may also be used to control a power supply object other than the motor 7.

[0014] The arithmetic device 4 is connected to an ignition switch 22 mounted on the vehicle. The ignition switch 22 emits an ignition signal 12 based on the operation of the vehicle user. The emitted ignition signal 12 is input to the arithmetic device 4. The arithmetic device 4 controls the motor drive circuit 5 and the power supply relay 2. The arithmetic device 4 is a CPU (Central Processing Unit) or the like. The monitor 3 is used to diagnose the state of the power supply relay 2. The monitor 3 is capable of measuring an intermediate voltage Vs, which will be described later. The monitor 3 is connected to the arithmetic device 4. The arithmetic device 4 is capable of acquiring the measurement result of the intermediate voltage Vs by the monitor 3.

[0015] The motor drive circuit 5 drives the motor 7 based on a command from the arithmetic device 4. The power supply relay 2 supplies or cuts off power to the motor drive circuit 5 for driving the motor 7. The motor drive circuit 5 has a bridge circuit (not shown). The bridge circuit is composed of a high-side switching element and a low-side switching element for supplying current to each of the coil groups of the motor 7. The motor drive circuit 5 also has a motor relay switching element capable of cutting off the current to the motor 7.

[0016] The power relay drive circuit 21 generates a drive signal 11 based on a command signal output from the arithmetic unit 4. The power relay drive circuit 21 inputs the drive signal 11 to the power relay 2. The power relay drive circuit 21 can provide an ON command and an OFF command to the power relay 2 using the drive signal 11. The ON command is a command to the power relay 2 to supply current from the battery 8 to the motor drive circuit 5. The OFF command is a command to the power relay 2 not to supply current from the battery 8 to the motor drive circuit 5. In other words, the power relay 2 to which the ON command is given supplies current to the motor drive circuit 5, and the power relay 2 to which the OFF command is given cuts off the supply of current to the motor drive circuit 5. The battery 8 is mounted on the vehicle and functions as a power source for the motor drive device 1.

[0017] The power relay 2 is disposed between the positive terminal of the capacitor 6 and the positive terminal of the battery 8. As shown in FIG. 2, the power relay 2 has a first switching element 2A, a second switching element 2B, and a relay internal resistor 10. The first switching element 2A and the second switching element 2B are connected in series. In this embodiment, MOSFETs (metal-oxide-semiconductor field-effect transistors) are used as the first switching element 2A and the second switching element 2B. However, other types of elements may be used as the first switching element 2A and the second switching element 2B.

[0018] The first switching element 2A has a first gate electrode 2Ag and a first drain electrode 2Ad. The second switching element 2B has a second gate electrode 2Bg and a second drain electrode 2Bd. The first switching element 2A and the second switching element 2B have a common source electrode 2s. In this embodiment, the voltage at the source electrode 2s is called an intermediate voltage Vs. The intermediate voltage Vs is measured by the monitor 3. In other words, the source electrode 2s is a measurement point of the intermediate voltage Vs by the monitor 3. The first switching element 2A incorporates a first diode 2A1, and the second switching element 2B incorporates a second diode 2B1. The first gate electrode 2Ag and the second gate electrode 2Bg are each connected to a power relay drive circuit 21. The first drain electrode 2Ad is connected to a battery 8. The second drain electrode 2Bd is connected to a resistor 9 and a capacitor 6.

[0019] The first switching element 2A and the second switching element 2B connect the battery 8 and the motor drive circuit 5 when the drive signal 11, which is an ON command, is provided to each of the gate electrodes 2Ag, 2Bg. Hereinafter, this state is referred to as the "ON state" of the first switching element 2A or the second switching element 2B. Also, when the drive signal 11, which is an OFF command, is provided to each of the gate electrodes 2Ag, 2Bg, the first switching element 2A and the second switching element 2B cut off the connection between the battery 8 and the motor drive circuit 5. Hereinafter, this state is referred to as the "OFF state" of the first switching element 2A or the second switching element 2B.

[0020] Here, the battery 8 is not necessarily connected correctly to the motor drive device 1. More specifically, the battery 8 may be connected to the motor drive device 1 so that the polarity of the battery 8 is in an inappropriate direction. In this specification, the connection state of the battery 8 in which the polarity is in an appropriate direction is referred to as a "normal connection state." Moreover, the connection state of the battery 8 in which the polarity is inappropriate is referred to as a "reverse connection state."

[0021] The first diode 2A1 of the first switching element 2A is provided in parallel to a path (path between the first drain electrode 2Ad and the source electrode 2s) through which the first switching element 2A passes a current from the battery 8 to the motor 7. The second diode 2B1 of the second switching element 2B is provided in parallel to a path (path between the source electrode 2s and the second drain electrode 2Bd) through which the second switching element 2B passes a current from the battery 8 to the motor 7. The first diode 2A1 is provided so as to cut off between the battery 8 and the motor drive circuit 5 if the first switching element 2A is in an off state when the battery 8 is in a positive connection state. More specifically, the cathode side of the first diode 2A1 is connected to the battery 8, and the anode side is connected to the source electrode 2s. On the other hand, the second diode 2B1 is provided so as to cut off between the battery 8 and the motor drive circuit 5 if the second switching element 2B is in an off state when the battery 8 is in a reverse connection state. More specifically, the cathode side of the second diode 2B1 is connected to the resistor 9, the capacitor 6, etc., and the anode side is connected to the source electrode 2s.

[0022] In other words, whether the battery 8 is in a forward connection state or a reverse connection state, when the drive signal 11 which is an ON command is not provided to each of the gate electrodes 2Ag, 2Bg, the connection between the battery 8 and the motor drive circuit 5 is cut off. Therefore, when the battery 8 is in a reverse connection state, it is possible to prevent current from flowing between the battery 8 and the motor drive circuit 5 even if the drive signal 11 which is an ON command is not input. This makes it possible to protect the motor drive circuit 5.

[0023] A first end of the resistor 10 in the relay is connected to the source electrode 2s. A second end of the resistor 10 in the relay is connected to the first gate electrode 2Ag and the second gate electrode 2Bg. When a drive signal 11, which is an ON command, is not input to the power relay 2, the resistor 10 in the relay has the role of discharging the charge stored between the first gate electrode 2Ag and the source electrode 2s and the charge stored between the second gate electrode 2Bg and the source electrode 2s.

[0024] As shown in Fig. 1, the capacitor 6 is connected between the positive input terminal 5a and the negative input terminal 5b of the motor drive circuit 5. The capacitor 6 discharges when the current flowing from the battery 8 to the motor drive circuit 5 is insufficient, and absorbs the current ripple of the motor drive circuit 5. In order to perform this role, it is preferable that the capacitance of the capacitor 6 is large. For example, an aluminum electrolytic capacitor can be used as the capacitor 6.

[0025] The resistor 9 is connected in parallel with the capacitor 6. The resistor 9 has the role of discharging the charge accumulated in the capacitor 6 when the power relay 2 is turned off. The resistor 9 may also be used as a resistor for stabilizing the voltage to be measured when monitoring the voltage downstream of the power relay 2. When the resistor 9 is used for such an application, a second monitor (not shown) is provided for monitoring the voltage between the resistor 9 and the power relay 2. In this specification, the side from which the current flows toward the positive input terminal 5a of the motor drive circuit 5 as viewed from the positive terminal of the battery 8 is referred to as the "downstream side."

[0026] Here, the motor drive device 1 according to the present embodiment is configured to be able to determine the presence or absence of a failure in the power relay 2 after completion of the operation of the motor drive device 1. More specifically, the arithmetic device 4 can determine that the power relay 2 continues to supply current from the battery 8 to the motor drive circuit 5 despite the drive signal 11 being an OFF command. The following description will be given with reference to FIG. 3. FIG. 3(a) is a timing chart showing the states of the ON command and the OFF command in the drive signal 11. FIG. 3(b) is a graph showing how the intermediate voltage Vs changes in response to the drive signal 11 switching from an ON command to an OFF command. The vertical axis of FIG. 3(b) is voltage, and "Vb" represents the voltage of the battery 8. The graph of FIG. 3(b) is created based on the value of the intermediate voltage Vs measured by the monitor 3.

[0027] When the operation of the motor drive device 1 is completed, the calculation device 4 gives a command to the power relay drive circuit 21. Based on this command, the power relay drive circuit 21 outputs the drive signal 11, which is an OFF command, to the power relay 2. In FIGS. 3(a) and 3(b), at time t0, the state of the drive signal 11 switches from an ON command to an OFF command. In this embodiment, the calculation device 4 compares the intermediate voltage Vs measured by the monitor 3 with a first threshold voltage Vth1 and a second threshold voltage Vth2 at a first time t1 and a second time t2 to determine whether or not the power relay 2 has a fault.

[0028] The first time t1 is a point in time after the time t0, and the second time t2 is a point in time after the first time t1. The time interval between the time t0 and the first time t1 and the time interval between the first time t1 and the second time t2 are determined in advance so as to be able to determine whether or not the power supply relay 2 has a failure. The first threshold voltage Vth1 is a value that is greater than 0 V and less than the battery voltage Vb. The second threshold voltage Vth2 is a value that is greater than the first threshold voltage Vth1 and less than the battery voltage Vb. The specific numerical values ​​of the first threshold voltage Vth1 and the second threshold voltage Vth2 are determined in advance so as to be able to determine whether or not the power supply relay 2 has a failure.

[0029] The solid line in FIG. 3(b) shows the transition of the intermediate voltage Vs when neither the first switching element 2A nor the second switching element 2B has a failure. In the solid line in FIG. 3(b), after time t0, the intermediate voltage Vs drops at a constant slope. This slope is determined by a first time constant τ1 determined by a parasitic capacitance (not shown) between the gate and source of the first switching element 2A and the second switching element 2B and the resistor 10 in the relay. More specifically, the charge stored in the parasitic capacitance is discharged by the first time constant τ1, causing the intermediate voltage Vs to drop. For example, if the parasitic capacitance is 10 nF and the resistance value of the resistor 10 in the relay is 100 kΩ, the time constant τ1 is about 1 ms.

[0030] As shown in FIG. 2, the first diode 2A1 and the second diode 2B1 are connected in series to the source electrode 2s. The diodes 2A1 and 2B1 are parasitic diodes of the first switching element 2A and the second switching element 2B, respectively. The anodes of the first diode 2A1 and the second diode 2B1 are connected to the source electrode 2s. In other words, the first diode 2A1 and the second diode 2B1 are connected in series so that the same poles are connected to the source electrode 2s. For this reason, if no failure occurs in the first switching element 2A and the second switching element 2B, when the drive signal 11 is switched to an OFF command at time t0, no charge flows from the battery 8 and the capacitor 6 to the source electrode 2s. As the intermediate voltage Vs drops, the gate-source voltages of the first switching element 2A and the second switching element 2B drop. When the gate-source voltage falls below the inherent threshold value of each of the first switching element 2A and the second switching element 2B, the first switching element 2A and the second switching element 2B are turned off.

[0031] The calculation device 4 determines that the power relay 2 is normal (no fault occurs) when the value of the intermediate voltage Vs is equal to or less than the first threshold voltage Vth1 at the first time t1. For example, when the time constant τ1 is 1 ms, if no fault occurs in the power relay 2, the intermediate voltage Vs will be approximately 0 V one second after the time t0. Therefore, for example, the time interval between the time t0 and the first time t1 can be set to 1 second, and the value of the first threshold voltage Vth1 can be set to 1 V. In this case, when the intermediate voltage Vs is equal to or less than 1 V one second after (the first time t1) from the time (time t0) when the drive signal 11 is switched to an OFF command, the calculation device 4 can determine that the power relay 2 is normal.

[0032] Next, the case where the first switching element 2A has failed will be described. The dashed-dotted line in Fig. 3(b) indicates the case where the first switching element 2A remains in the on state even after the drive signal 11 is switched to the off command at time t0. In this case, the insulation between the first drain electrode 2Ad and the source electrode 2s is not normal, and even after time t0, the intermediate voltage Vs becomes almost the same value as the battery voltage Vb. Therefore, at the first time t1, the intermediate voltage Vs becomes greater than the first threshold voltage Vth1. Also, although details will be described later, when the second switching element 2B has failed, the intermediate voltage Vs also becomes greater than the first threshold voltage Vth1. Therefore, as a result of comparing the intermediate voltage Vs at the first time t1 with the first threshold voltage Vth1, when Vs > Vth1, the arithmetic unit 4 determines that at least one of the first switching element 2A and the second switching element 2B or both of the first switching element 2A and the second switching element 2B have failed.

[0033] Note that the battery voltage Vb is not necessarily constant and can take various values. In the present embodiment, the lower limit value of the battery voltage Vb at which the motor drive device 1 can operate is represented as Vb_min. Then, the value of the first threshold voltage Vth1 is set so as to satisfy Vth1 < Vb_min. Thereby, it is possible to prevent the arithmetic unit 4 from making an incorrect determination that the first switching element 2A has failed even though it is normal.

[0034] Next, a case where the second switching element 2B is broken will be described. The dashed line in FIG. 3(b) shows a case where the second switching element 2B remains in the ON state even after the drive signal 11 is switched to the OFF command at time t0. In this case, the second drain electrode 2Bd and the source electrode 2s are not properly insulated from each other, and the capacitor 6 and the source electrode 2s are in a conductive state. If the voltage drop due to the existence of the second diode 2B1 is ignored, the intermediate voltage Vs and the capacitor voltage Vc become substantially the same. The capacitor voltage Vc is the voltage of the capacitor 6. After time t0, the charge stored in the capacitor 6 is discharged by the second time constant τ2 determined by the capacitance of the capacitor 6 and the resistance value of the resistor 9. Therefore, the capacitor voltage Vc and the intermediate voltage Vs also decrease with the second time constant τ2.

[0035] Capacitor 6 discharges when the current flowing from battery 8 to motor drive circuit 5 is insufficient, and serves to absorb current ripples in motor drive circuit 5. Furthermore, it is desirable for resistor 9 to have a small effect on the amount of charge stored in capacitor 6 during operation of motor drive device 1. For these reasons, for example, if capacitor 6 has a capacitance of 1000 uF and resistor 9 has a resistance value of 100 kΩ, then time constant τ2 will be approximately 10 s.

[0036] Consider the case where, under the above conditions, the capacitor voltage Vc is, for example, 10V, the first time t1 is set to 1 second after the time t0, and the first threshold voltage Vth1 is set to 1V. At this time, the capacitor voltage Vc at the first time t1 is greater than the first threshold voltage Vth1. If the second switching element 2B is faulty, the intermediate voltage Vs and the capacitor voltage Vc are approximately equal. Therefore, in this case, the intermediate voltage Vs is also greater than the first threshold voltage Vth1. Also, as described above, if the first switching element 2A is faulty, the intermediate voltage Vs at the first time t1 is greater than the first threshold voltage Vth1. Therefore, the calculation device 4 determines that at least one of the first switching element 2A and the second switching element 2B is faulty when Vs>Vth1 as a result of comparing the intermediate voltage Vs at the first time t1 with the first threshold voltage Vth1.

[0037] Here, the arithmetic device 4 according to the present embodiment uses the second threshold voltage Vth2 to determine whether a failure has occurred in the first switching element 2A or the second switching element 2B. When the first switching element 2A has a failure, the intermediate voltage Vs remains at the battery voltage Vb and does not change much, as shown by the two-dot chain line in FIG. 3(b). On the other hand, when the second switching element 2B has a failure, the intermediate voltage Vs decreases due to the time constant τ2, as shown by the one-dot chain line in FIG. 3(b). Therefore, the second threshold voltage Vth2 is set so that the intermediate voltage Vs falls below the second threshold voltage Vth2 at the second time t2 when the intermediate voltage Vs decreases due to the time constant τ2. This makes it possible to determine whether a failure has occurred in the first switching element 2A or the second switching element 2B, based on the result of comparing the intermediate voltage Vs and the second threshold voltage Vth2 at the second time t2. More specifically, when the intermediate voltage Vs at the second time t2 is lower than the second threshold voltage Vth2, the arithmetic device 4 determines that the second switching element 2B has a failure.

[0038] The greater the time interval between time t0 and the second time t2, the greater the difference in the vertical axis between the dashed line and the dashed line in FIG. 3(b), making it easier to determine whether the first switching element 2A or the second switching element 2B is faulty. However, since the arithmetic unit 4 determines whether a fault has occurred based on the value of the intermediate voltage Vs at the second time t2, the greater the time interval between time t0 and the second time t2, the longer the determination takes. Therefore, the second time t2 should be set to a time that allows for the determination of a fault and is as close to time t0 as possible. In addition, it is preferable that the specific value of the second threshold voltage Vth2 is set in consideration of the time constant τ2 determined by the capacitance of the capacitor 6 and the resistance value of the resistor 9, the first time t1, the second time t2, and the like.

[0039] Note that the specific values ​​of the first time t1, the second time t2, the first threshold voltage Vth1, the second threshold voltage Vth2, the capacitance of the capacitor 6, the resistance value of the resistor 9, etc., are merely examples. Each value can be changed as appropriate, taking into account the constants and temperature characteristic variations of each component.

[0040] As described above, the power supply cutoff device 1a according to the present disclosure includes a power supply relay 2 having a first switching element 2A and a second switching element 2B connected in series, which switches between supplying and stopping a current from a battery 8 to a power supply object (motor 7), a power supply relay drive circuit 21 which inputs a drive signal 11 for switching between supplying and stopping a current to the power supply relay 2, a monitor 3 which measures an intermediate voltage Vs between the first switching element 2A and the second switching element 2B, and a calculation device 4 which is capable of acquiring a measurement result of the intermediate voltage Vs by the monitor 3. The first switching element 2A is a first switching element. The switching element 2A includes a first diode 2A1 arranged in parallel to the path along which the second switching element 2B passes a current from the battery 8 to the object to be powered, and the second switching element 2B includes a second diode 2B1 arranged in parallel to the path along which the second switching element 2B passes a current from the battery 8 to the object to be powered, and the first diode 2A1 and the second diode 2B1 are connected in series such that the same pole is connected to the point at which the intermediate voltage Vs is measured by the monitor 3, and the computing device 4 determines whether or not the power relay 2 has a fault based on the intermediate voltage Vs after a drive signal 11 for stopping the supply of current is input to the power relay 2.

[0041] According to the power supply cutoff device 1a having the above configuration, it is possible to determine whether or not the power supply relay 2 has a failure without providing a charging circuit for charging the capacitor 6. Therefore, it is possible to suppress an increase in the number of parts required to determine whether or not the power supply relay 2 has a failure.

[0042] Specifically, the calculation device 4 determines whether or not the power relay 2 has a fault by comparing the intermediate voltage Vs at the first time t1 after a drive signal 11 that stops the supply of current to the power relay 2 is input with a first threshold voltage Vth1.

[0043] Furthermore, when the intermediate voltage Vs at the first time t1 is equal to or lower than the first threshold voltage Vth1, the arithmetic device 4 determines that the first switching element 2A and the second switching element 2B are normal.

[0044] Furthermore, when the intermediate voltage Vs at the first time t1 is greater than the first threshold voltage Vth1, the arithmetic device 4 determines that a failure has occurred in the first switching element 2A or the second switching element 2B.

[0045] In addition, the calculation device 4 determines whether a failure has occurred in the first switching element 2A or the second switching element 2B based on a comparison between the intermediate voltage Vs at a second time t2 after the first time t1 and a second threshold voltage Vth2 that is higher than the first threshold voltage Vth1.

[0046] In addition, the battery 8, the first switching element 2A, the second switching element 2B, and the power supply object are connected in this order, and at a second time t2, when the intermediate voltage Vs is greater than the second threshold voltage Vth2, the calculation device 4 determines that a fault has occurred in the first switching element 2A.

[0047] In addition, one terminal of the capacitor 6 is connected between the second switching element 2B and the power supply object, and at a second time t2 after the first time t1, when the intermediate voltage Vs is smaller than the second threshold voltage Vth2, the calculation device 4 determines that a fault has occurred in the second switching element 2B.

[0048] By employing these determination methods, it is possible to determine not only whether or not a failure has occurred in the power relay 2, but also whether a failure has occurred in the first switching element 2A or the second switching element 2B.

[0049] The technical scope of the present disclosure is not limited to the above-described embodiment, and various modifications can be made without departing from the spirit of the present disclosure.

[0050] For example, although the number of capacitors 6 is one in FIG. 1, a plurality of capacitors 6 may be provided in parallel. When a plurality of capacitors 6 are provided, the time constant τ2 takes a value according to the number of capacitors 6. Therefore, in this case, it is preferable to determine the second threshold voltage Vth2 in consideration of the time constant τ2 corresponding to the number of capacitors 6. Furthermore, there is a possibility that some of the provided capacitors 6 may fail. Since the time constant τ2 changes according to the number of failed capacitors 6, it is also possible to detect that some of the capacitors 6 have failed by monitoring the intermediate voltage Vs.

[0051] In the above embodiment, the first switching element 2A and the second switching element 2B are switched between the on and off states by a common drive signal 11. However, the first switching element 2A and the second switching element 2B may be individually switched between the on and off states by providing independent drive signals 11 to them. In this case as well, it is possible to detect failures in both the first switching element 2A and the second switching element 2B, and to determine which of the elements has a failure.

[0052] Also, the diodes 2A1 and 2B1 in the above embodiment are parasitic diodes of the MOSFETs which are the switching elements 2A and 2B, respectively. However, the switching elements 2A and 2B do not have to be MOSFETs, and the diodes 2A1 and 2B1 do not have to be parasitic diodes of the MOSFETs.

[0053] In addition, the above-described embodiments and modifications may be combined as appropriate. [Explanation of symbols]

[0054] REFERENCE SIGNS LIST 1...motor drive device 1a...power supply cutoff device 2...power supply relay 2A...first switching element 2A1...first diode 2B...second switching element 2B1...second diode 3...monitor 4...arithmetic unit 6...capacitor 7...motor 8...battery 20...electric power steering device 21...power supply relay drive circuit t1...first time t2...second time Vs...intermediate voltage Vth1...first threshold voltage Vth2...second threshold voltage

Claims

1. a power supply relay having a first switching element and a second switching element connected in series, for switching between supplying and stopping a current from the battery to a power supply object; a power relay drive circuit that inputs a drive signal to the power relay to switch between supplying and stopping the current; a monitor for measuring an intermediate voltage between the first switching element and the second switching element; a calculation device capable of acquiring a measurement result of the intermediate voltage by the monitor, the first switching element includes a first diode provided in parallel with a path through which the first switching element passes a current from the battery to the power supply object; the second switching element includes a second diode provided in parallel with a path through which the second switching element passes a current from the battery to the power supply target, the first diode and the second diode are connected in series such that the same poles are connected to a point where the intermediate voltage is measured by the monitor; the arithmetic device determines whether or not the power supply relay has a failure by comparing the intermediate voltage at a first time point after the drive signal for stopping the supply of current is input to the power supply relay with a first threshold voltage; the arithmetic device determines that the first switching element and the second switching element are normal when the intermediate voltage at the first time is equal to or lower than the first threshold voltage; the arithmetic device determines that a failure has occurred in the first switching element or the second switching element when the intermediate voltage at the first time is greater than the first threshold voltage; The computing device determines whether a failure has occurred in the first switching element or the second switching element based on a comparison between the intermediate voltage at a second time that is later than the first time and a second threshold voltage that is greater than the first threshold voltage.

2. the battery, the first switching element, the second switching element, and the power supply object are connected in this order; 2 . The power cutoff device according to claim 1 , wherein, when the intermediate voltage is greater than the second threshold voltage at the second time, the arithmetic unit determines that a failure has occurred in the first switching element.

3. the battery, the first switching element, the second switching element, and the power supply object are connected in this order; one terminal of a capacitor is connected between the second switching element and the power supply object; 3. The power cutoff device according to claim 1, wherein the arithmetic unit determines that a failure has occurred in the second switching element when the intermediate voltage is lower than the second threshold voltage at the second time.

4. A motor as the power supply object; A motor drive device comprising: a power supply cutoff device according to any one of claims 1 to 3.

5. An electric power steering device comprising the motor drive device according to claim 4.

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