Diagnostic circuit for parallel-structure MOSFET including MUX and diagnostic method using the same

Abstract

The present invention relates to a circuit and a diagnostic method for diagnosing individual abnormalities in a plurality of internal FETs provided inside a MOSFET provided between a secondary battery pack and an electric vehicle equipped with a secondary battery pack, and the presence or absence of an abnormality is determined by measuring the voltage across each internal MOSFET while turning each internal MOSFET on and off individually and comparing the measured voltage with a diagnostic table.

Description

[Technical field]

[0001] This invention claims the benefit of priority to Korean Patent Application No. 2019-0150213 filed on November 21, 2019, Korean Patent Application No. 2019-0150214 filed on November 21, 2019, and Korean Patent Application No. 2020-0155900 filed on November 19, 2020, and all contents disclosed in the documents of these Korean patent applications are incorporated by reference into this specification.

[0002] The present invention relates to a parallel-structure MOSFET diagnostic circuit including a MUX (multiplexer) and a diagnostic method using the same. Specifically, the present invention relates to a circuit for diagnosing a failure of a specific MOSFET among multiple MOSFETs connected in parallel used for power supply of an automobile using a MUX, and a method for diagnosing a failure of a specific MOSFET among multiple MOSFETs connected in parallel using the same. [Background technology]

[0003] The demand for lithium secondary batteries is expected to increase significantly with the development of electric vehicles (EVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (Plug-In HEVs). Secondary batteries applied to electric vehicles are usually used in the form of an assembly of multiple secondary battery unit cells. Secondary battery packs with a multi-module structure in which multiple secondary battery unit cells are connected in series / parallel are commonly used.

[0004] The secondary battery pack is provided as a separate device from the electric vehicle, and a relay for electrical on / off connection is provided between the secondary battery pack and the electric vehicle.

[0005] The voltages supplied by secondary battery packs for automobiles are usually 12 V, 48 V, and 400 V. In the case of 400 V, mechanical relays are used due to physical limitations, but below that, they are rapidly being replaced by FETs, which are an electrical method.

[0006] Mechanical relays have the advantage of being able to handle high voltages and large currents, being able to withstand high heat, and being inexpensive. However, they generate noise when touching the contacts when turning them on and off, and their lifespan is limited by sparks that occur when touching the contacts. Mechanical relays also have the disadvantage of being vulnerable to physical shocks due to their internal mechanical components.

[0007] When a car suddenly stops or is involved in a collision, the car itself is subjected to a large amount of gravitational acceleration. This force can damage mechanical relays, and this damage and short circuit can cause secondary accidents such as fires.

[0008] MOSFET (Metal Oxide Semiconductor Field Effect Transistor) is a typical electrical relay. Compared to mechanical relays, MOSFET has the disadvantage of generating a large amount of heat due to its relatively low capacitance and material resistance (RDS). The temperature can sometimes rise to over 200°C due to the resistance.

[0009] Despite the heat generation issue and being more expensive than mechanical relays, MOSFETs are expected to replace mechanical relays in many cars due to their almost permanent lifespan and the fact that they are hardly damaged by physical shocks from the car. The issue of noise caused by mechanical relays is also something that requires continuous improvement, so electronic relays are expected to be prioritized and quickly adopted in expensive vehicles.

[0010] If the problems of capacitance and heat could be solved, MOSFETs would replace mechanical relays in electric vehicles, except for high-voltage vehicles. Even though there is the problem of oxidation due to moisture and oxygen, various research projects are being conducted, such as removing the FET case and directly connecting it to a metal collector to block contact with the outside air, or increasing the area for heat transfer.

[0011] Currently, some vehicles use multiple MOSFETs in parallel due to issues such as capacity. Multiple MOSFETs are controlled by a single driver. From the actual external configuration, a car only has one MOSFET, but in reality, it is often made up of six internal FETs or internal MOSFETs (hereinafter referred to as 'internal FETs') in a 2S3P (three sets of two in series connected in parallel). Depending on the required capacity, the number of internal FETs can be increased to 6, 10, or 12.

[0012] As described above, the internal FETs are connected in series and parallel, and currently there is no circuit or method for determining which part of the internal FETs is abnormal.

[0013] If some of the internal FETs are damaged and remain in a continuously connected state (fail on), the current that would have been distributed and not flowing through the internal FETs will concentrate on the damaged internal FETs, causing overheating. This can cause the entire system to overheat. Overheating of the battery pack can be a very serious danger to the safety of the vehicle.

[0014] FIG. 1 is a circuit diagram of a MOSFET for an automobile according to the prior art. The power supplied from the battery pack BAT of an automobile is controlled by a single MOSFET when viewed from the outside. The part shown by the two-dot chain line in FIG. 1 corresponds to a single MOSFET when viewed from the outside. The internal FET shown in FIG. 1 is connected in 2S3P as an example, but this can be modified in various ways to other combinations.

[0015] FET1 to FET6 indicate six internal FETs, and the MOSFETs can be N-type or P-type as required. Although only N-type is shown in all the drawings disclosed in the present invention, it can be easily selected to replace it with P-type as required.

[0016] When no voltage is applied to the gates, FET1, FET2, FET3 and FET4, FET5, FET6 operate differently from each other. When voltage is applied to the gates, FET1, FET2, FET3 allow current to flow from the battery pack to the car (from left to right in Fig. 1), and when no voltage is applied to the gates, the diode allows current to flow from the car to the battery pack (from right to left in Fig. 1). FET4, FET5, FET6 are the exact opposite; when voltage is applied to the gates, current can flow from the car to the battery pack, and when no voltage is applied to the gates, the diode allows current to flow from the battery pack to the car.

[0017] When no voltage is applied to the gate, none of the FETs operate, but a separate diode or an internal parasitic diode allows current to flow in the reverse direction (from right to left in FIG. 1) through FET1, FET2, and FET3, and allows current to flow in the forward direction (from left to right in FIG. 1) through FET4, FET5, and FET6. That is, current can flow from V2, V3, and V4 to V1 through FET1, FET2, and FET3, respectively, and current can flow from V2, V3, and V4 to V5 through FET4, FET5, and FET6.

[0018] The conventional MOSFETs shown in Figure 1 are all controlled at once by a single driver. In Figure 1, V2, V3, and V4 are interconnected to distribute a large amount of current to multiple internal FETs when a large amount of current flows instantaneously, such as when the car accelerates or brakes suddenly. Conventional automotive MOSFETs are turned on and off simultaneously by a single driver, and there is no way to diagnose errors in an internal FET.

[0019] Patent Documents 1 to 3 have a common point in that they are all techniques for diagnosing faults in a plurality of FETs connected in parallel.

[0020] In terms of specific configuration, Patent Document 1 is complex in that it uses the resistance value of the individual detection resistor of the mirror FET provided in each FET, and requires additional maintenance and repair of the individual mirror FET. Patent Document 2 is similar in that it diagnoses a fault by measuring the change in the resistance value of the load side terminal, i.e., the final output, which is generated by controlling the on / off of multiple FETs, but Patent Document 2 does not use a separate power source for detection, so a separate current measurement means required for high voltage must be provided, and Patent Document 2 does not present a solution such as a specific circuit. Patent Document 3 is complex in that it individually controls the drive of all FETs and adds separate resistors for current detection to all FETs, and many parts are added to maintain it, and the failure of these diagnostic circuits themselves can be a problem.

[0021] As described above, there is no effective method for diagnosing whether or not there is an abnormality in an individual FET of a MOSFET in an electric vehicle equipped with a secondary battery pack, and the risk of accidents due to MOSFETs, the demand of which will continue to increase in the future, is increasing. [Prior art documents] [Patent documents]

[0022] [Patent Document 1] JP 2000-293201 A [Patent Document 2] Patent No. 5526965 [Patent Document 3] Korean Patent Publication No. 2016-0041495 Summary of the Invention [Problem to be solved by the invention]

[0023] The present invention has been made to solve the above problems, and an object of the present invention is to provide a circuit capable of diagnosing individual abnormalities of a plurality of internal FETs provided inside a MOSFET provided between a secondary battery pack and an electric vehicle having a secondary battery pack, and a method for diagnosing an abnormality of the internal FETs using the same.

[0024] An object of the present invention is to provide a circuit and method capable of diagnosing individual abnormalities in both the MOSFET arranged on the battery pack side and the MOSFET arranged on the automobile side, out of two MOSFETs connected in series. [Means for solving the problem]

[0025] In order to achieve the above object, the present invention provides a MOSFET assembly in which a number of MOSFETs are connected in series and parallel to control a connection between a (+) terminal of a secondary battery pack that supplies power to a vehicle and a vehicle that receives power from the secondary battery pack, the MOSFET assembly including a MUX connected to each MOSFET gate of the MOSFET assembly for each channel.

[0026] In addition, the present invention may further include an additional parallel connection including a switch between the (+) terminal of the secondary battery pack and a vehicle receiving power from the secondary battery pack.

[0027] The present invention may further include a driver module which is an input terminal of the MUX, and a micro control unit which provides inputs corresponding to the total number of the channels for controlling the MUX.

[0028] The MOSFET assembly includes two MOSFETs connected in series, and two or more sets of the two MOSFETs connected in series are connected in parallel.

[0029] Of the two MOSFETs connected in series, the MOSFET located on the battery pack side operates to allow current to flow from the battery pack to the vehicle when a voltage is applied to its gate, and a diode is arranged to allow current to flow from the vehicle to the battery pack when no voltage is applied to its gate. Of the two MOSFETs connected in series, the MOSFET located on the vehicle side operates to allow current to flow from the vehicle to the battery pack when a voltage is applied to its gate, and a diode is arranged to allow current to flow from the battery pack to the vehicle when no voltage is applied to its gate.

[0030] The MOSFETs arranged on the battery pack or automobile side are identical MOSFETs arranged symmetrically.

[0031] The diode may be a separately connected diode or a parasitic diode disposed inside the MOSET.

[0032] The MOSFET assembly may be externally composed of a single MOSFET, and the MOSFET assembly may be controlled by a Battery Management System (BMS) of the secondary battery pack.

[0033] The present invention also provides a method for checking for an abnormality in an individual MOSFET using a MOSFET assembly, the method including measuring a voltage across the individual MOSFET while sequentially turning on / off each channel of the MUX.

[0034] The method may also include measuring the voltage across every individual MOSFET.

[0035] The present invention also provides a method for checking for an abnormality in an individual MOSFET, the MOSFET assembly including two MOSFETs connected in series and two or more sets of the two series-connected MOSFETs connected in parallel, the method including checking for an abnormality in a MOSFET disposed on a battery pack side among the two series-connected MOSFETs by using a current supplied from the battery pack.

[0036] The present invention also provides a method for checking for an abnormality in an individual MOSFET using a MOSFET assembly further including an additional parallel connection including a switch between a (+) terminal of a secondary battery pack and a vehicle receiving power from the secondary battery pack, the method including: 1) turning the switch to an OFF state; 2) measuring a voltage across the individual MOSFET while sequentially turning on / off only a channel that controls a MOSFET arranged on the battery pack side among the channels of the MUX; 3) turning the switch to an ON state; and 4) measuring a voltage across the individual MOSFET while sequentially turning on / off only a channel that controls a MOSFET arranged on the vehicle side among the channels of the MUX.

[0037] Steps 2) and 4) may include measuring the voltages across all of the individual MOSFETs.

[0038] The present invention provides a method for checking whether or not an individual MOSFET is abnormal by comparing all the voltage values ​​across all the MOSFETs.

[0039] The present invention can be provided in any possible combination of the above-mentioned inventions. [Brief description of the drawings]

[0040] [Figure 1] FIG. 1 is a connection circuit diagram of an automotive MOSFET according to the prior art. [Diagram 2] FIG. 1 is a partially modified connection circuit diagram of a conventional automotive MOSFET; [Diagram 3] 1 is a connection circuit diagram of an automotive MOSFET according to a first embodiment of the present invention; [Figure 4] FIG. 4 is a connection circuit diagram of an automotive MOSFET according to a second embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0041] Hereinafter, the present invention will be described in detail with reference to the accompanying drawings, in which a person having ordinary skill in the art to which the present invention pertains can easily carry out the present invention. However, in describing the operation principle of the preferred embodiment of the present invention in detail, detailed description of related well-known functions or configurations will be omitted if it is determined that such detailed description may unnecessarily obscure the gist of the present invention.

[0042] In addition, the same reference numerals are used throughout the drawings for parts having similar functions and actions. Throughout the specification, when a part is said to be connected to another part, this includes not only the case where the part is directly connected to another part, but also the case where the part is indirectly connected via another element between the two parts. In addition, when a part includes a certain component, it does not mean that the other component is excluded, but that the part may further include the other component, unless otherwise specified.

[0043] The present invention will now be described in more detail.

[0044] FIG. 2 is a partially modified connection circuit diagram of a conventional automotive MOSFET.

[0045] In order to determine whether or not an individual MOSFET is abnormal, the present invention partially modifies the conventional MOSFET connection circuit diagram for automobiles. In Fig. 1, V2, V3, and V4 are connected, but in the present invention, these are shorted and two individual MOSFETs are connected in series, and the majority of these are connected in parallel.

[0046] 3 is a connection circuit diagram of a MOSFET for automobiles according to a first embodiment of the present invention, and FIG 4 is a connection circuit diagram of a MOSFET for automobiles according to a second embodiment of the present invention.

[0047] In this specification, including FIGS. 1 to 4, the term FET refers to a power supply MOSFET.

[0048] The present invention relates to a MOSFET assembly in which a number of MOSFETs FET1 to FET6 are connected in series and parallel for controlling a connection between a (+) terminal of a secondary battery pack that supplies power to a vehicle and a vehicle that receives power from the secondary battery pack. Here, the connection points of the vehicle are indicated as V6 and V7.

[0049] FET1 to FET6 indicate six internal FETs, and the MOSFETs can be N-type or P-type as required. Although only N-type is shown in all the drawings disclosed in the present invention, it can be easily selected to replace it with P-type as required.

[0050] When no voltage is applied to the gates, FET1, FET2, FET3 and FET4, FET5, FET6 operate differently from each other. When voltage is applied to the gates, FET1, FET2, FET3 allow current to flow from the battery pack to the car, and when no voltage is applied to the gates, the diode allows current to flow from the car to the battery pack. FET4, FET5, FET6 are the exact opposite; when voltage is applied to the gates, current can flow from the car to the battery pack, and when no voltage is applied to the gates, the diode allows current to flow from the battery pack to the car.

[0051] The six MUXes connected to the MOSFET gates of the MOSFET assemblies each have six channels CH1, CH2, CH3, CH4, CH5, and CH6, and each channel is controlled by a control signal (S0, S1, S2) transmitted from the microcontroller unit MCU.

[0052] A driver module, which is the input of the MUX, is also connected to the MUX.

[0053] MUX can further add CH7, and when no diagnosis is performed, all FETs can be controlled collectively by CH7 and the switch to operate normally. In Figure 3, FET1, FET2, and FET3 can be continuously ON by CH7. Here, the remaining CH1 to CH6 are OFF.

[0054] The MOSFET assembly includes two MOSFETs connected in series, and two or more sets of the two MOSFETs connected in series are connected in parallel.

[0055] Of the two MOSFETs connected in series, the MOSFETs FET1, FET2, and FET3 arranged on the battery pack side have diodes arranged therein that allow current to flow from the vehicle to the battery pack when no voltage is applied to their gates, and of the two MOSFETs connected in series, the MOSFETs FET4, FET5, and FET6 arranged on the vehicle side have diodes arranged therein that allow current to flow from the battery pack to the vehicle when no voltage is applied to their gates.

[0056] The diode may be a separately connected diode or a parasitic diode disposed inside the MOSET.

[0057] In a method for checking for an abnormality in an individual MOSFET using an automotive MOSFET assembly according to a first embodiment of the present invention, the method includes measuring a voltage across the individual MOSFET while sequentially turning on / off each channel of the MUX.

[0058] The method may include measuring the voltage across every individual MOSFET.

[0059] When the corresponding channel is turned on by the MUX, a signal is generated in the corresponding channel that is turned on, and the corresponding FET is operated. If CH1 is turned on, voltage is applied only to the gate of FET1. For example, if the battery pack voltage is 48V and FET1 is operating normally, when only FET1 is turned on, V1, V2, and V5 will be 48V, and V3 and V4 will be 0V. If FET1 is always on (fail on or short), V1, V2, and V5 will be 48V even if all FETs are turned off, and if FET1 is always off (fail off), only V1 will be 48V even if only FET1 is turned on.

[0060] Table 1 is the diagnostic table for FET1.

[0061] [Table 1]

[0062] In this manner, it is possible to determine whether or not there is an abnormality in FET1, FET2, and FET3. In the case of FET2 and FET3, the value corresponding to V2 is replaced by V3 and V4, respectively.

[0063] The MOSFET connection circuit diagram for an automobile according to the second embodiment of the present invention further includes an additional parallel connection including a switch SW1 between the (+) terminal of the secondary battery pack and the automobile receiving power from the secondary battery pack in addition to the first embodiment. The switch SW1 is connected only for diagnosis.

[0064] A second embodiment of the present invention is a method for checking an individual MOSFET for anomalies using an automotive MOSFET assembly, comprising:

[0065] 1) turning the switch to an OFF state;

[0066] 2) measuring a voltage across an individual MOSFET while sequentially turning on / off only a channel that controls a MOSFET disposed on the battery pack side among the channels of the MUX;

[0067] 3) turning the switch on; and

[0068] 4) A method for checking for an abnormality in an individual MOSFET, the method including a step of measuring a voltage across each individual MOSFET while sequentially turning on / off only a channel that controls a MOSFET arranged on the automobile side among the channels of the MUX.

[0069] Steps 2) and 4) may include measuring the voltages across all of the individual MOSFETs.

[0070] When the corresponding channel is turned on by the MUX, a signal is generated in the corresponding channel that is turned on, and the corresponding FET is operated. If CH1 is turned on, voltage is applied only to the gate of FET1. For example, if the battery pack voltage is 48V and FET1 is operating normally, when only FET1 is turned on, V1, V2, and V5 will be 48V, and V3 and V4 will be 0V. If FET1 is always on (fail on or short), V1, V2, and V5 will be 48V even if all FETs are turned off, and if FET1 is always off (fail off), only V1 will be 48V even if only FET1 is turned on.

[0071] In the opposite case, if SW1 is turned ON, a voltage of 48V is also applied to V5. If CH4 is turned ON, a voltage is applied only to the gate of FET4. For example, if the battery pack voltage is 48V and FET4 is operating normally, when only FET4 is turned ON, V1, V2, and V5 will be 48V, and V3 and V4 will be 0V. If FET4 is always in the ON state (fail on or short), V1, V2, and V5 will be 48V even if all FETs are turned OFF, and if FET4 is always in the OFF state (fail off), only V1 and V5 will be 48V even if only FET4 is turned ON.

[0072] Table 2 is a diagnostic table for FET1 and FET4.

[0073] [Table 2]

[0074] In this manner, it is possible to check for abnormalities in FET1, FET2, FET3, FET4, FET5, and FET6. In the cases of FET2, FET3, FET5, and FET6, the values ​​corresponding to V2 are replaced with V3 and V4, respectively.

[0075] Although Figures 3 and 4 are diagrammatically illustrated as two series connections and three sets of these connected in parallel, the solution principle of the present invention is similarly applied even if these are modified from two or more to a larger number, and these are also within the scope of the rights of the present invention.

[0076] Although certain parts of the present invention have been described in detail above, it will be apparent to those skilled in the art that such specific descriptions are merely preferred embodiments and do not limit the scope of the present invention. Various changes and modifications are possible within the scope of the scope and technical ideas of the present invention, and it goes without saying that such changes and modifications fall within the scope of the appended claims. [Industrial Applicability]

[0077] As described above, the present invention has an advantage that, in an electric vehicle having a secondary battery pack, individual abnormalities of a number of internal FETs provided inside a MOSFET provided between the secondary battery pack and the electric vehicle can be diagnosed, thereby providing a power system with a higher degree of safety than a conventional MOSFET assembly. [Explanation of symbols]

[0078] 10. Prior art automotive MOSFET connection circuit diagram 20 MOSFET connection circuit diagram for automobiles with partial improvements on conventional technology 30 is a circuit diagram showing a MOSFET connection circuit for an automobile according to a first embodiment of the present invention. 40 MOSFET connection circuit diagram for automobiles according to the second embodiment of the present invention BAT Secondary battery pack V1, V2, V3, V4, V5, V6, V7 voltage measurement points FET1, FET2, FET3, FET4, FET5, FET6 Internal FET DRIVER MCU Micro-control Unit CH1, CH2, CH3, CH4, CH5, CH6, CH7 MUX control channels CTRL Transmission signal input section of driver unit S0, S1, S3 Control signal input section of micro control unit

Claims

1. A MOSFET assembly in which a number of MOSFETs are connected in series and parallel to control a connection between a terminal of a secondary battery pack for supplying power to a vehicle and the vehicle receiving power from the secondary battery pack, a MUX connected to each MOSFET gate of the MOSFET assembly for each channel; an additional parallel connection including a switch connected between a terminal of the secondary battery pack and a vehicle receiving power from the secondary battery pack, the switch being connected only for diagnosing an abnormality in the MOSFET; A driver module which is an input of the MUX and controls the MOSFET; a microcontrol unit providing inputs corresponding to the total number of channels for controlling the MUX; All the MOSFETs are collectively controlled via the MUX in response to a control signal transmitted from the microcontrol unit. MOSFET assembly.

2. 2. The MOSFET assembly according to claim 1, wherein the MOSFET assembly comprises two MOSFETs connected in series, and two or more sets of the two MOSFETs connected in series are connected in parallel.

3. The MOSFET disposed on the secondary battery pack side among the two MOSFETs connected in series is configured to operate so that a current flows from the secondary battery pack to the vehicle when a voltage is applied to the gate, and is configured with a diode to allow a current to flow from the vehicle to the secondary battery pack when no voltage is applied to the gate; 3. The MOSFET assembly according to claim 2, wherein the MOSFET disposed on the automobile side among the two MOSFETs connected in series is configured to operate so that a current flows from the automobile to the secondary battery pack when a voltage is applied to its gate, and a diode is disposed to allow a current to flow from the secondary battery pack to the automobile when no voltage is applied to its gate.

4. 4. The MOSFET assembly of claim 3, wherein the diode is a separately connected diode or a parasitic diode disposed within the MOSFET.

5. 2. The MOSFET assembly of claim 1, wherein the MOSFET assembly outwardly comprises a single MOSFET.

6. The MOSFET assembly according to claim 1 , wherein the MOSFET assembly is controlled by a BMS (Battery Management System) of the secondary battery pack.

7. 13. A method for checking for anomalies in individual MOSFETs using the MOSFET assembly of claim 1, comprising the steps of: A method of identifying anomalies in individual MOSFETs, comprising measuring a voltage across the individual MOSFETs while sequentially turning on and off each channel of the MUX.

8. A MOSFET assembly in which a number of MOSFETs are connected in series and parallel to control a connection between a terminal of a secondary battery pack that supplies power to an automobile and the automobile that receives power from the secondary battery pack, a MUX connected to each MOSFET gate of the MOSFET assembly for each channel; an additional parallel connection including a switch connected between a terminal of the secondary battery pack and a vehicle receiving power from the secondary battery pack, the switch being connected only for diagnosing an abnormality in the MOSFET; A driver module which is an input of the MUX and controls the MOSFET; a microcontrol unit providing inputs corresponding to the total number of channels for controlling the MUX; All of the MOSFETs are collectively controlled via the MUX in response to a control signal transmitted from the microcontrol unit, and a method for checking an abnormality of an individual MOSFET using the MOSFET assembly, comprising: 1) turning the switch to an OFF state; 2) measuring a voltage across an individual MOSFET while sequentially turning on / off only a channel that controls a MOSFET disposed on the secondary battery pack side among the channels of the MUX; 3) turning the switch on; and 4) measuring the voltage across each individual MOSFET while sequentially turning on / off only the channels of the MUX that control the MOSFETs arranged on the vehicle side; A method for identifying anomalies in individual MOSFETs, comprising:

9. 8. A method for identifying anomalies in individual MOSFETs as recited in claim 7, including the step of measuring the voltage across every individual MOSFET.

10. 9. The method of claim 8, further comprising the step of measuring the voltage across all of the individual MOSFETs in steps 2) and 4).

11. 9. The method for checking for an abnormality in an individual MOSFET according to claim 7, further comprising the step of checking for the presence or absence of an abnormality in the individual MOSFET by comparing all voltage values ​​across all of the MOSFETs.

12. The MOSFET assembly includes two MOSFETs connected in series, and two or more sets of the two MOSFETs connected in series are connected in parallel, 8. The method for checking for an abnormality in an individual MOSFET according to claim 7, further comprising checking for an abnormality in a MOSFET disposed on a secondary battery pack side among the two MOSFETs connected in series, using a current supplied from the secondary battery pack.

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