阅读说明：本技术 故障诊断装置 (Fault diagnosis device ) 是由 今中佑树 于 2018-06-18 设计创作，主要内容包括：抑制放电能力的降低，并且诊断放电电阻的故障。一种使蓄电元件(31)放电的放电电路(60)的故障诊断装置，所述放电电路(60)包括由并联连接的多个电阻块(B)构成的电阻电路(61)，所述电阻块(B)由串联连接的多个放电电阻(Ra、Rb)构成，所述故障诊断装置在所述蓄电元件(31)的放电过程中，基于所述放电电阻(Ra、Rb)的连接点(P)的电压或电流，诊断所述电阻电路(61)的故障。(The reduction of the discharge capability is suppressed, and the failure of the discharge resistance is diagnosed. A failure diagnosis device for a discharge circuit (60) that discharges an electrical storage element (31), wherein the discharge circuit (60) includes a resistance circuit (61) that is configured from a plurality of resistance blocks (B) connected in parallel, the resistance blocks (B) being configured from a plurality of discharge resistances (Ra, Rb) connected in series, and wherein the failure diagnosis device diagnoses a failure of the resistance circuit (61) based on a voltage or a current at a connection point (P) of the discharge resistances (Ra, Rb) during discharge of the electrical storage element (31).)

1. A failure diagnosis device for a discharge circuit that discharges a power storage element,

the discharge circuit includes a resistance circuit composed of a plurality of resistance blocks connected in parallel,

the resistance block is composed of a plurality of discharge resistances connected in series,

the failure diagnosis device diagnoses a failure of the resistance circuit based on a voltage current at a connection point of the discharge resistor during a discharge process of the power storage element.

2. The failure diagnosis device according to claim 1, comprising:

the detection element detects the voltage difference between the resistor blocks of the connection point of the discharge resistor and outputs a detection signal; and

and a determination unit that determines whether or not the resistance circuit has a failure based on the detection signal output from the detection element.

3. The failure diagnostic device according to claim 2,

a plurality of the detection elements are provided corresponding to the plurality of the resistive patches,

the plurality of detection elements are commonly connected to the determination unit.

4. The fault diagnosis device according to claim 2 or claim 3,

the detection element is an optical insulating element that transmits the detection signal to the determination unit while being insulated from the resistance circuit.

5. The failure diagnostic device according to claim 4,

the electric storage element is used for a 48V-series battery.

6. The fault diagnosis device according to any one of claim 2 to claim 5,

the discharge circuit includes a series switch connected in series with the resistive circuit,

the determination unit determines whether or not the series switch has failed, based on a voltage at a connection point of the discharge resistor.

7. The fault diagnosis device according to any one of claim 1 to claim 6,

the electric storage elements are provided in plurality in series,

the discharge circuit is provided corresponding to each of the plurality of power storage elements, and is a balancer circuit that equalizes voltages of the plurality of power storage elements.

Technical Field

The present invention relates to a technique for detecting a failure of a discharge resistor.

Background

A battery formed by connecting secondary batteries in series has a balancer (balancer) function to equalize the remaining capacity between the secondary batteries. In general, the resistance of the balancer circuit is formed in a structure in which a plurality of discharge resistors are connected in series and parallel for the reason of heat dissipation and rated power. In the balancer circuit, when the discharge resistor fails in the short-circuit mode, power loss in the resistor connected in series with the short-circuited resistor becomes large. Therefore, when the balancer circuit is operated in a short-circuited state, a failure may occur due to heat generation. Further, when the discharge resistor fails in the open mode, the discharge capacity of the balancer circuit is reduced, and therefore, the voltages of the secondary batteries cannot be balanced.

In patent document 1, resistance voltage dividing circuits 4A and 4B are provided in parallel with a discharge resistor 11A, and a failure of the discharge resistors 11A and 11B is determined by comparing voltages when a switch 12 is turned on and when the switch is turned off.

Disclosure of Invention

Problems to be solved by the invention

In the method of patent document 1, the switch is closed at the time of failure detection, and therefore, the discharge is temporarily stopped. Therefore, the usable period of the discharge circuit is limited, and the discharge capability is degraded.

The present invention has been made in view of the above circumstances, and an object thereof is to suppress a decrease in discharge capacity and diagnose a failure of a discharge resistor.

Means for solving the problems

A failure diagnosis device for a discharge circuit that discharges an electric storage element, the discharge circuit including a resistance circuit composed of a plurality of resistance blocks connected in parallel, the resistance blocks being composed of a plurality of discharge resistances connected in series, the failure diagnosis device diagnosing a failure of the resistance circuit based on a voltage or a current at a connection point of the discharge resistances during discharge of the electric storage element.

Effects of the invention

In this configuration, a failure of the resistance circuit can be detected during the operation of the discharge circuit. Therefore, it is not necessary to stop the discharge circuit during the failure detection, and therefore, the reduction of the discharge capability of the discharge circuit can be suppressed.

Drawings

Fig. 1 is a side view of an automobile according to embodiment 1.

Fig. 2 is a perspective view of the battery.

Fig. 3 is an exploded perspective view of the battery.

Fig. 4 is a block diagram showing an electrical structure of the battery.

Fig. 5 is a circuit diagram of a discharge circuit.

Fig. 6 is a graph obtained by summing the input voltages to the BM in the state of the discharge resistor.

Fig. 7 is a circuit diagram of a discharge circuit of embodiment 2.

Fig. 8 is a graph in which the input voltage Vin to the BM is summed up for each state of the transistor Q4.

Fig. 9 is a circuit diagram of a discharge circuit of embodiment 3.

Fig. 10 is a circuit diagram of a discharge circuit according to another embodiment.

Detailed Description

A failure diagnosis device for a discharge circuit that discharges an electric storage element, wherein the discharge circuit includes a resistance circuit composed of a plurality of resistance blocks connected in parallel, the resistance blocks being composed of a plurality of discharge resistances connected in series, and the failure diagnosis device diagnoses a failure of the resistance circuit based on a voltage or a current at a connection point of the discharge resistances during discharge of the electric storage element. In this configuration, a failure of the resistance circuit can be diagnosed during the operation of the discharge circuit. Therefore, it is not necessary to stop the discharge circuit for failure diagnosis, and therefore, a decrease in the discharge capability of the discharge circuit can be suppressed.

The failure diagnosis device may include: the detection element detects the voltage difference between the resistor blocks of the connection point of the discharge resistor and outputs a detection signal; and a determination unit that determines whether or not the resistance circuit has a failure based on a detection signal output from the detection element. In this configuration, the determination unit can diagnose a failure of the resistance circuit by monitoring only the presence or absence of the detection signal output from the detection element. Therefore, it is not necessary for the determination unit to have a high arithmetic function, and the failure diagnosis of the resistance circuit can be performed with a low-cost configuration

The plurality of detection elements may be provided corresponding to the plurality of resistance blocks, and the plurality of detection elements may be connected to the determination unit in common. In this configuration, only one detection line connecting the judgment section and each detection element is sufficient, and the circuit configuration is simple. In addition, the determination unit may monitor only one input when determining whether or not the resistance circuit has a failure, and thus the monitoring load is small.

The detection element is an optical insulating element that transmits the detection signal to the determination unit while being insulated from the resistance circuit. In this configuration, the determination unit can be protected from overvoltage. Further, since the determination unit is less likely to fail, the reliability of failure diagnosis of the resistance circuit is high.

The electricity storage element is for a 48V-series battery. The voltage of the 48V-series battery is 4 times that of the 12V-series battery, and the determination unit is likely to malfunction. By applying this technique, the determination unit can be effectively protected from overvoltage, and since a failure is less likely to occur in the determination unit, the reliability of failure diagnosis of the resistance circuit is high.

The discharge circuit may include a series switch connected in series with the resistor circuit, and the determination unit may determine whether or not the series switch has a failure based on a voltage value at a connection point of the discharge resistor. In this configuration, it is possible to detect not only a failure of the resistance circuit but also a failure of the series switch.

Preferably, the plurality of power storage elements are provided in series, and the discharge circuit is a balancer circuit provided corresponding to each of the plurality of power storage elements and configured to equalize voltages of the plurality of power storage elements. In this configuration, since it is not necessary to stop the discharge circuit for failure diagnosis, the balance capability of the balancer circuit (discharge circuit) becomes high.

< embodiment 1 >

1. Description of the storage batteries

Fig. 1 is a side view of an automobile, fig. 2 is a perspective view of a battery, fig. 3 is an exploded perspective view of the battery, and fig. 4 is a block diagram showing an electrical structure of the battery.

As shown in fig. 1, the automobile 1 includes a battery (power storage device) 20. As shown in fig. 2, the battery 20 has a block-shaped battery case 21, and a battery pack 30 including a plurality of secondary batteries 31 and a control board 28 are housed in the battery case 21. In the following description, referring to fig. 2 and 3, the vertical direction of the battery case 21 when the battery case 21 is placed horizontally without being inclined with respect to the installation surface is referred to as the Y direction, the direction along the longitudinal direction of the battery case 21 is referred to as the X direction, and the depth direction of the battery case 21 is referred to as the Z direction.

As shown in fig. 3, battery case 21 includes a box-shaped case body 23 that is open at the top, a positioning member 24 that positions a plurality of secondary batteries 31, an intermediate lid 25 attached to the top of case body 23, and an upper lid 26 attached to the top of intermediate lid 25. As shown in fig. 3, in the case main body 23, a plurality of battery chambers 23A in which the respective secondary batteries 31 are individually housed are arranged in the X direction.

As shown in fig. 3, a plurality of bus bars 27 are arranged on the upper surface of the positioning member 24, the positioning member 24 is arranged above the plurality of secondary batteries 31, and the plurality of secondary batteries 31 are arranged in the case main body 23, whereby the plurality of secondary batteries 31 are positioned and connected in series by the plurality of bus bars 27.

As shown in fig. 2, the middle cap 25 has a substantially rectangular shape in plan view and has a step in the Y direction. A pair of terminal portions 22P and 22N are provided at both ends of the middle cover 25 in the X direction, and a harness terminal not shown is connected to the pair of terminal portions 22P and 22N. The pair of terminals 22P and 22N are made of metal such as lead alloy, for example, 22P is a positive-side terminal and 22N is a negative-side terminal.

As shown in fig. 3, the control board 28 is housed inside the middle cover 25, and the secondary battery 31 is connected to the control board 28 by attaching the middle cover 25 to the case main body 23.

Referring to fig. 4, an electrical structure of battery 20 is explained. The battery 20 includes a battery pack 30, a current sensor 41, a current cutoff device 37, and a battery management device (hereinafter, BM)50 that manages the battery pack 30.

Referring to fig. 4, an electrical structure of battery 20 is explained. The battery 20 includes a battery pack 30, a current cut-off device 37, a current sensor 41, a voltage detection unit 45, a temperature sensor 47, a discharge circuit 60, and a battery management device (hereinafter, BM)50 that manages the battery pack 30.

The assembled battery 30 is constituted by a plurality of secondary batteries (for example, lithium ion secondary batteries) 31 connected in series. The battery pack 30, the current sensor 41, and the current cut-off device 37 are connected in series via the current-carrying path 35. The current sensor 41 is disposed on the negative electrode side, the current cutoff device 37 is disposed on the positive electrode side, the current sensor 41 is connected to the negative electrode side terminal portion 22N, and the current cutoff device 37 is connected to the positive electrode side terminal portion 22P.

The cell voltage E of the secondary cell 31 is about 3.5[ V ], the total voltage of the assembled cell 30 is about 14V, and the voltage class of the secondary cell 20 is 12V series. The battery 20 is for starting the engine. As shown in fig. 4, a battery motor 15 for starting an engine mounted on the automobile 1 is connected to the battery 20, and the battery motor 15 is driven by receiving electric power supplied from the battery 20. In addition to the battery motor 15, a vehicle load (not shown) such as an electric component and an alternator 17 are connected to the battery 20. When the amount of power generated by the alternator 17 is larger than the amount of power consumed by the vehicle load, the battery 20 is charged by the alternator 17. When the amount of power generated by the alternator 17 is smaller than the amount of power consumed by the vehicle load, the battery 20 is discharged to compensate for the shortage.

The current sensor 41 is provided inside the battery case 21 and detects a current flowing through the secondary battery 31. The current sensor 41 is electrically connected to the BM50 through a signal line, and the output of the current sensor 41 is acquired by the BM 50.

The voltage detector 45 is provided inside the battery case 21, and detects the voltage of each secondary battery 31 and the total voltage of the assembled battery 30. The voltage detection unit 45 is electrically connected to the BM50 through a signal line, and the output of the voltage detection unit 45 is acquired by the BM 50.

The temperature sensor 47 is provided inside the battery case 21 and detects the temperature of the secondary battery 31. The temperature sensor 47 is electrically connected to the BM50 by a signal line, and the output of the temperature sensor 47 is acquired by the BM 50.

The current interrupting device 37 may be a contact switch (mechanical type) such as a relay, an FET (field effect transistor), or a semiconductor switch such as a transistor. The current interrupting device 37 is disposed in the current carrying path 35 of the assembled battery 30, and opens and closes the current carrying path 35 of the secondary battery 31.

The BM50 includes a CPU51 having an arithmetic function, a memory 53 storing various information, a communication unit 55, and the like, and the BM50 is provided on the control board 28. A vehicle ECU (electronic control Unit) attached to the automobile 1 is connected to the communication Unit 55, and the BM50 can receive information about the vehicle such as the operating state of the engine from the vehicle ECU 100.

The BM50 monitors the current of the secondary battery 31 based on the output of the current sensor 41. The voltage of each secondary battery 31 and the total voltage of the assembled battery 30 are monitored based on the output of the voltage detection unit 45. The temperature of the secondary battery 31 is monitored based on the output of the temperature sensor 47.

When the voltage, current, or temperature of the secondary battery 31 is abnormal, the BM50 sends a command to the current cutoff device 37 to cut off the current, thereby protecting the battery 20.

As shown in the following equation (2), BM50 estimates the SOC (state of charge) of battery 20 based on the integrated value of current I detected by current sensor 41 with respect to time. The sign of the current is positive during charging, and negative during discharging.

(1) The formula represents the definition of SOC.

SOC＝Cr/Co×100……(1)

Co is the full charge capacity of the secondary battery, and Cr is the residual capacity of the secondary battery.

SOC＝SOCo+100×∫Idt/Co……(2)

SOCo is the initial value of SOC, and I is the current.

2. Discharging circuit 60 and fault diagnosis

The discharge circuit 60 is provided individually for each secondary battery 31. As shown in fig. 5, the discharge circuit 60 is composed of a resistor circuit 61 and a 4 th transistor Q4.

The resistance circuit 61 is formed of a plurality of resistance blocks B1 to B3 connected in parallel. In fig. 5, 3 resistance blocks B1 to B3 are connected in parallel. Each of the resistance blocks B1 to B3 is composed of two discharge resistors Ra and Rb connected in series. The two discharge resistors Ra and Rb have the same resistance value.

The reason why the resistor circuit 61 is formed by a plurality of resistors Ra and Rb connected in series and parallel is to increase the discharge capacity of the resistor circuit 61.

The 4 th transistor Q4 is an NPN transistor. The collector of the 4 th transistor Q4 is connected to the resistor circuit 61, and the emitter is connected to the negative electrode of the secondary battery 31. The base of the 4 th transistor Q4 is connected to the BM50 via a signal line.

When an operation signal is applied to the base of the 4 th transistor Q4 to turn on the 4 th transistor Q4, a current can flow through the resistance circuit 61 to discharge the secondary battery 31. The 4 th transistor Q4 is an example of the "series switch" of the present invention.

The BM50 monitors the voltage of each secondary battery 31 constituting the assembled battery 30 based on the output of the voltage detection unit 45. When the voltage difference of the secondary batteries 31 (for example, the difference between the highest voltage and the lowest voltage) exceeds a threshold value, the secondary batteries 31 having a high voltage are discharged using the discharge circuit 60, whereby the voltages of the secondary batteries 31 constituting the assembled battery 30 are equalized. Thus, the discharge circuit 60 is a balancer circuit that equalizes the voltages of the secondary batteries 31.

As shown in fig. 5, the battery 20 includes 3 transistors Q1 to Q3 corresponding to 3 resistance blocks B1 to B3. The 1 st transistor Q1 is a PNP transistor, and the emitter of the 1 st transistor Q1 is connected to the connection point P1 of the two discharge resistors Ra and Rb of the resistor block B1, and the base is connected to the connection point P2 of the two discharge resistors Ra and Rb of the resistor block B2.

The 2 nd transistor Q2 is a PNP transistor, and the emitter of the 2 nd transistor Q2 is connected to the connection point P2 of the two discharge resistors Ra and Rb of the resistor block B2, and the base is connected to the connection point P3 of the two discharge resistors Ra and Rb of the resistor block B3. The 3 rd transistor Q3 is a PNP transistor, and the emitter of the 3 rd transistor Q3 is connected to the connection point P3 between the two discharge resistors Ra and Rb of the resistor block B3, and the base is connected to the connection point P1 between the two discharge resistors Ra and Rb of the resistor block B1.

The collector of the 1 st transistor Q1, the collector of the 2 nd transistor Q2, and the collector of the 3 rd transistor Q3 are commonly connected to a detection line Lo.

When the voltage differences between the emitter and the base of the 1 st to 3 rd transistors Q1 to Q3 are higher than the operating voltage (for example, 0.6V), the 1 st to 3 rd transistors Q1 to Q3 are turned on. The 1 st to 3 rd transistors Q1 to Q3 are examples of the "detection element" of the present invention.

While the secondary battery 31 is being discharged by the resistance circuit 61, the BM50 monitors the input voltage Vin from the detection line Lo, thereby diagnosing a failure of the resistance circuit 61. The BM50 is an example of the "determination unit" of the present invention.

Next, a method of diagnosing a failure of the resistor circuit 61 will be described in detail.

When the 4 th transistor Q4 is turned on, a discharge current flows from the secondary battery 31 to the resistor circuit 61. When the voltage of the secondary battery 31 is set to E [ V ], the voltages at the connection points P1 to P3 of the resistor blocks B1 to B3 are all E/2[ V ] when the discharge circuit 61 is normal.

Therefore, as shown in fig. 6, all of the transistors Q1 to Q3 are turned off, and the input voltage Vin to the input BM50 becomes zero [ V ].

Next, when the discharge resistance Ra of the resistance block B1 is open-circuited due to disconnection or the like, the voltage at the connection point P1 of the resistance block B1 becomes 0[ V ], and the voltages at the connection points P2 and P3 of the resistance blocks B2 and B3 become E/2[ V ]. This generates a positive voltage difference between the emitter and the base of the 3 rd transistor Q3, which is higher than the operating voltage of the transistor Q. Accordingly, the 3 rd transistor Q3 among the transistors Q1 to Q3 is turned on, and the input voltage Vin of the input BM50 becomes E/2[ V ]. That is, the detection signal of E/2[ V ] is input from the 3 rd transistor Q3 to the BM 50.

When the discharge resistor Ra of the resistance block B1 has a short-circuit fault, the voltage at the connection point P1 of the resistance block B1 becomes E [ V ], and the voltages at the connection points P2 and P3 of the resistance blocks B2 and B3 become E/2[ V ]. This generates a positive voltage difference between the emitter and the base of the 1 st transistor Q1, which is higher than the operating voltage of the transistor Q. Accordingly, the 1 st transistor Q1 among the transistors Q1 to Q3 is turned on, and the input voltage Vin of the input BM50 becomes E [ V ]. That is, the 1 st transistor Q1 inputs the detection signal of E [ V ] to the BM 50.

Next, when the discharge resistor Rb of the resistance block B1 is open-circuited due to disconnection or the like, the voltage at the connection point P1 of the resistance block B1 becomes E [ V ], and the voltages at the connection points P2 and P3 of the resistance blocks B2 and B3 become E/2[ V ]. This generates a positive voltage difference between the emitter and the base of the 1 st transistor Q1, which is higher than the operating voltage of the transistor Q. Accordingly, the 1 st transistor Q1 among the transistors Q1 to Q3 is turned on, and the input voltage Vin of the input BM50 becomes E [ V ]. That is, the 1 st transistor Q1 inputs the detection signal of E [ V ] to the BM 50.

When the discharge resistor Rb of the resistance block B1 is short-circuited, the voltage at the connection point P1 of the resistance block B1 becomes 0[ V ], and the voltages at the connection points P2 and P3 of the resistance blocks B2 and B3 become E/2[ V ]. This generates a positive voltage difference between the emitter and the base of the 3 rd transistor Q3, which is higher than the operating voltage of the transistor Q. Accordingly, the 3 rd transistor Q3 among the transistors Q1 to Q3 is turned on, and the input voltage Vin of the input BM50 becomes E/2[ V ]. That is, the detection signal of E/2[ V ] is input from the 3 rd transistor Q3 to the BM 50.

When one of the discharge resistors Ra and Rb of the resistor block B2 fails, one of the 1 st transistor Q1 and the 2 nd transistor Q2 is turned on, and the input voltage Vin of the input BM50 becomes E [ V ] or E/2[ V ]. That is, the detection signal of E [ V ] or E/2[ V ] is input to the BM50 from one of the 1 st transistor Q1 and the 2 nd transistor Q2.

When any one of the discharge resistors Ra and Rb of the resistor block B3 fails, one of the 2 nd transistor Q2 and the 3 rd transistor Q3 is turned on, and the input voltage Vin to the input BM50 becomes E [ V ] or E/2[ V ]. That is, the detection signal of E [ V ] or E/2[ V ] is input to the BM50 from one of the 2 nd transistor Q2 and the 3 rd transistor Q3.

As described above, the BM50 monitors the input voltage Vin while the secondary battery 61 is being discharged by the discharge circuit 60, and can diagnose a failure of the discharge resistors Ra and Rb.

Specifically, when the input voltage Vin is zero [ V ], the resistor circuit 61 is determined to be normal, that is, all of the discharge resistors Ra and Rb constituting the resistor circuit 61 are normal. On the other hand, when the input voltage Vin is E [ V ] or E/2[ V ], it can be determined that the resistor circuit 61 is defective. That is, it can be determined that one of the discharge resistors Ra and Rb of the discharge blocks B1 to B3 constituting the resistor circuit 61 has failed.

In this configuration, the BM50 diagnoses a failure of the resistor circuit 61 based on the detection signals (E [ V ] or H-level voltage signals of E/2[ V ]) output from the 1 st to 3 rd transistors Q1 to Q3. The 3 transistors Q1 to Q3 and BM50 constitute a failure diagnosis device for performing failure diagnosis of the resistance circuit 61.

3. Description of the effects

In this configuration, a failure of the resistance circuit 61 can be diagnosed during the operation of the discharge circuit 60. Therefore, it is not necessary to stop the discharge circuit 60 in the failure diagnosis process, and therefore, the discharge capability of the discharge circuit 60 can be suppressed from being degraded.

In the present configuration, the BM50 monitors only the input voltage Vin, thereby being able to diagnose a failure of the discharge circuit 61. Therefore, the BM50 does not need to have a high arithmetic function, and can diagnose a failure of the resistor circuit 61 with a low-cost configuration.

In this configuration, the circuit configuration is simple because only one detection line Lo connecting the BM50 and the transistors Q1 to Q3 is required. When determining whether or not the resistor circuit 61 has a failure, the BM50 may monitor only one input voltage Vin, and thus the monitoring load is small.

In the present configuration, since the discharge circuit 60 has a high discharge capability, it is effective when the battery 20 is used in a state in which the battery is easily overcharged by charging the battery with a relatively high current, for example, when the battery is charged by a rapid charger. That is, since the discharge circuit 60 has a high discharge capacity, the voltage of the secondary battery 31 can be reduced in a short time, and overcharge of the secondary battery 31 can be suppressed.

< embodiment 2 >

As shown in fig. 7, embodiment 2 is configured to add a 5 th transistor Q5 to the circuit configuration of embodiment 1. A 5 th transistor Q5 is provided for each resistance circuit 61. The 5 th transistor Q5 is an NPN transistor, and the emitter of the 5 th transistor Q5 is connected to the negative electrode of the secondary battery 31, and the collector thereof is connected to the base of the 3 rd transistor Q3. The base is connected to BM50 via a signal line.

When an operation signal is input from the BM50 to the base of the 5 th transistor Q5, the 5 th transistor Q5 is turned on, and the base voltage of the 3 rd transistor Q3 drops. Thereby, the 3 rd transistor Q3 is switched from off to on, and therefore, the voltage at the connection point P3 of the resistor block B3 is input to the BM50 via the 3 rd transistor Q3.

The BM50 determines whether or not the 4 th transistor Q4 has failed, based on the voltage at the connection point P3 when the 4 th transistor Q4 is turned on and turned off. Specifically, when the operation signal of off is applied from the BM50 to the 4 th transistor Q4 and the operation signal of on is applied to the 5 th transistor Q5, as shown in fig. 8, when the 4 th transistor Q4 operates normally (when it is off), the input voltage Vin input to the BM50 becomes E [ V ]. On the other hand, in the case where the 4 th transistor Q4 has a short-circuit failure, the input voltage Vin input to the BM50 becomes E/2[ V ]. Therefore, the BM50 can determine the short circuit failure of the 4 th transistor Q4 according to the value of the input voltage Vin.

When the BM50 applies the on operation signal to the 4 th transistor Q4 and the on operation signal to the 5 th transistor Q5, as shown in fig. 8, the input voltage Vin to the BM50 becomes E/2[ V ] when the 4 th transistor Q4 normally operates (turns on). On the other hand, in the case where the 4 th transistor Q4 has an open failure, the input voltage Vin input to the BM50 becomes E [ V ]. Therefore, the BM50 can determine the open failure of the 4 th transistor Q4 according to the value of the input voltage Vin.

In embodiment 3, it is possible to determine not only a failure of the resistor circuit 61 but also a failure of the 4 th transistor Q4 as a series switch.

< embodiment 3 >