Patent Publication Number: US-2023155367-A1

Title: Power supply control device, test method, and computer program

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is the U.S. national stage of PCT/JP2021/010289 filed on Mar. 15, 2021, which claims priority of Japanese Patent Application No. JP 2020-071837 filed on Apr. 13, 2020, the contents of which are incorporated herein. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to a power supply control device, a test method, and a computer program. 
     BACKGROUND 
     JP 2017-103963A discloses a vehicular power supply control device that controls the supply of power from a battery to a load. In this power supply control device, a switch is disposed in a current path of current flowing from the battery to the load. The supply of power from the battery to the load is controlled by turning the switch on or off. 
     In the power supply control device described in JP 2017-103963A, when the current flowing through the switch rises, the rising current flows through a resistor circuit. A resistor is included in the resistor circuit. When the voltage across both ends of the resistor circuit reaches a voltage greater than or equal to a predetermined voltage, the switch turns off, as the current flowing through the current path is large. As a result, overcurrent is prevented from flowing through the current path. 
     In a conventional power supply control device, such as that described in JP 2017-103963A, when the voltage across both ends of the resistor circuit reaches a voltage greater than or equal to a predetermined voltage, the switch turns off and a notifying circuit makes a notification. If the notifying circuit does not make a notification appropriately, it is highly likely that the switch has not turned off appropriately. If the switch has not turned off appropriately, overcurrent cannot be prevented from flowing through the current path. 
     In recent years, self-driving vehicles, in which the driving is handled by computers, are being developed. A person does not do the driving in a self-driving vehicle, and there is thus a need for a configuration that reliably prevents overcurrent from flowing. 
     Accordingly, an object is to provide a power supply control device, a test method, and a computer program capable of testing a notifying circuit that makes a notification when a voltage across both ends of a resistor circuit reaches a voltage greater than or equal to a predetermined voltage. 
     SUMMARY 
     A power supply control device according to one aspect of the present disclosure is a power supply control device that controls a supply of power by switching a switch on or off. The power supply control device includes: a resistor circuit in which a current that rises when a current flowing through the switch rises flows; a notifying circuit configured to make a notification when a voltage across both ends of the resistor circuit reaches a voltage greater than or equal to a predetermined voltage; an application circuit configured to apply a voltage greater than or equal to the predetermined voltage to the resistor circuit; and a processing unit configured to execute processing. The processing unit instructs the application circuit to apply the voltage to the resistor circuit, and determines whether or not the notifying circuit is making the notification after instructing the application circuit to apply the voltage. 
     A test method according to one aspect of the present disclosure is a test method, executed by a computer, for testing a notifying circuit that makes a notification when a voltage across both ends of a resistor circuit, in which a current that rises when a current flowing through the switch rises flows, reaches a voltage greater than or equal to a predetermined voltage. The test method includes: a step of instructing a voltage greater than or equal to the predetermined voltage to be applied to the resistor circuit; and a step of determining whether or not the notifying circuit is making the notification after instructing the voltage to be applied to the resistor circuit. 
     A computer program according to one aspect of the present disclosure is a computer program for causing a computer to test a notifying circuit that makes a notification when a voltage across both ends of a resistor circuit, in which a current that rises when a current flowing through the switch rises flows, reaches a voltage greater than or equal to a predetermined voltage. The computer program causes the computer to execute: a step of instructing a voltage greater than or equal to the predetermined voltage to be applied to the resistor circuit; and a step of determining whether or not the notifying circuit is making the notification after instructing the voltage to be applied to the resistor circuit. 
     Note that the present disclosure can be realized not only as a power supply control device including such characteristic processing units, but also as a test method that takes the characteristic processes as steps, a computer program that causes a computer to execute those steps, and so on. Additionally, the present disclosure can be realized as a semiconductor integrated circuit that implements some or all of the power supply control device, and as a power source system that includes the power supply control device. 
     Effects of the Present Disclosure 
     According to the present disclosure, a notifying circuit that makes a notification when a voltage across both ends of a resistor circuit reaches a voltage greater than or equal to a predetermined voltage can be tested. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a block diagram illustrating the primary configuration of a power source system according to a first embodiment. 
         FIG.  2    is a block diagram illustrating the primary configuration of a drive circuit. 
         FIG.  3    is a circuit diagram illustrating a latch circuit. 
         FIG.  4    is a block diagram illustrating the primary configuration of a microcomputer. 
         FIG.  5    is a descriptive diagram illustrating a range of a voltage across both ends, over which A/D conversion is performed. 
         FIG.  6    is a flowchart illustrating a sequence of on processing. 
         FIG.  7    is a flowchart illustrating a sequence of off processing. 
         FIG.  8    is a timing chart illustrating operations of a power supply control device. 
         FIG.  9    is a block diagram illustrating the primary configuration of a power source system according to a second embodiment. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     First, embodiments of the present disclosure will be described as examples. The embodiments described hereinafter may be at least partially combined as desired. 
     First Aspect 
     A power supply control device according to a first aspect of the present disclosure is a power supply control device that controls a supply of power by switching a switch on or off. The power supply control device includes: a resistor circuit in which a current that rises when a current flowing through the switch rises flows; a notifying circuit configured to make a notification when a voltage across both ends of the resistor circuit reaches a voltage greater than or equal to a predetermined voltage; an application circuit configured to apply a voltage greater than or equal to the predetermined voltage to the resistor circuit; and a processing unit configured to execute processing. The processing unit instructs the application circuit to apply the voltage to the resistor circuit, and determines whether or not the notifying circuit is making the notification after instructing the application circuit to apply the voltage. 
     Second Aspect 
     In the power supply control device according to a second aspect of the present disclosure, the notifying circuit makes the notification in response to a voltage greater than or equal to the predetermined voltage being applied to the resistor circuit, and the processing unit instructs the application circuit to stop applying the voltage after instructing the application circuit to apply the voltage, and after instructing the application circuit to stop applying the voltage, determines whether or not the notifying circuit is making the notification based on the voltage across both ends of the resistor circuit. 
     Third Aspect 
     The power supply control device according to a third aspect of the present disclosure further includes a switching unit configured to switch the switch off when the voltage across both ends of the resistor circuit reaches a voltage greater than or equal to the predetermined voltage, and the processing unit instructs the application circuit to apply the voltage to the resistor circuit when an off signal instructing the switch to turn off is input. 
     Fourth Aspect 
     In the power supply control device according to a fourth aspect of the present disclosure, the processing unit instructs the notifying circuit to stop applying the voltage when the notifying circuit is determined to be making the notification, and after instructing the notifying circuit to stop applying the voltage, determines whether or not the voltage across both ends of the resistor circuit is less than the predetermined voltage. 
     Fifth Aspect 
     A test method according to a fifth aspect of the present disclosure is a test method, executed by a computer, for testing a notifying circuit that makes a notification when a voltage across both ends of a resistor circuit, in which a current that rises when a current flowing through the switch rises flows, reaches a voltage greater than or equal to a predetermined voltage. The test method includes: a step of instructing a voltage greater than or equal to the predetermined voltage to be applied to the resistor circuit; and a step of determining whether or not the notifying circuit is making the notification after instructing the voltage to be applied to the resistor circuit. 
     Sixth Aspect 
     A computer program according to a sixth aspect of the present disclosure is a computer program for causing a computer to test a notifying circuit that makes a notification when a voltage across both ends of a resistor circuit, in which a current that rises when a current flowing through the switch rises flows, reaches a voltage greater than or equal to a predetermined voltage. The computer program causes the computer to execute: a step of instructing a voltage greater than or equal to the predetermined voltage to be applied to the resistor circuit; and a step of determining whether or not the notifying circuit is making the notification after instructing the voltage to be applied to the resistor circuit. 
     In the power supply control device, the test method, and the computer program according to the aspects described above, a voltage greater than or equal to the predetermined voltage is applied to the resistor circuit, and whether or not the notifying circuit is making a notification is determined. The notifying circuit is tested in this manner. 
     In the power supply control device according to the aspects described above, whether or not the notifying circuit is making a notification is determined based on whether or not the voltage across both ends of the resistor circuit is greater than or equal to the predetermined voltage after the application circuit has stopped applying the voltage. When the voltage across both ends of the resistor circuit is greater than or equal to the predetermined voltage, it is determined that the notification is being made. When the voltage across both ends of the resistor circuit is less than the predetermined voltage, it is determined that the notifying circuit is not making the notification. 
     In the power supply control device according to the aspects described above, when the current flowing through the switch rises, the voltage across both ends of the resistor circuit rises. The switch is switched off when the voltage across both ends of the resistor circuit reaches a voltage greater than or equal to the predetermined voltage. This prevents overcurrent from flowing through the switch. The notifying circuit is tested at a timing when an off signal is input, i.e., at a timing at which the switch is requested to switch off. If a fault has not occurred in the device, the switch switches off at the point in time when the application circuit applies a voltage greater than or equal to the predetermined voltage to the resistor circuit. 
     In the power supply control device according to the embodiments described above, whether or not the voltage across both ends of the resistor circuit is less than the predetermined voltage is determined after the application of the voltage to the resistor circuit by the notifying circuit is stopped. Through this, it is confirmed that the voltage across both ends of the resistor circuit has returned to a voltage less than the predetermined voltage. 
     Specific examples of the power source system according to embodiments of the present disclosure will be described hereinafter with reference to the drawings. Note that the present disclosure is not intended to be limited to these examples, and is defined instead by the scope of the appended claims. All changes that fall within the same essential spirit as the scope of the claims are intended to be included therein as well. 
     First Embodiment 
     Configuration of Power Source System 
       FIG.  1    is a block diagram illustrating the primary configuration of a power source system  1  according to a first embodiment. The power source system  1  can be favorably installed in a vehicle, and includes a power supply control device  10 , a DC power source  11 , and a load  12 . The DC power source  11  is, for example, a battery. The load  12  is an electrical device installed in the vehicle. 
     The power supply control device  10  has an N-channel FET (Field Effect Transistor)  20 , which functions as a switch, and a shunt resistor  21 . The positive electrode of the DC power source  11  is connected to the drain of the FET  20 . The source of the FET  20  is connected to one end of the shunt resistor  21 . The other end of the shunt resistor  21  is connected to one end of the load  12 . The negative electrode of the DC power source  11  and the other end of the load  12  are grounded. 
     When the FET  20  is on, the resistance value between the drain and the source of the FET  20  is sufficiently low, which enables current to flow through the drain and the source of the FET  20 . When the FET  20  is off, the resistance value between the drain and the source of the FET  20  is sufficiently high, and thus no current flows through the drain and the source of the FET  20 . 
     The power supply control device  10  switches the FET  20  on or off. When the FET  20  is switched on, current flows from the positive electrode of the DC power source  11  to the FET  20 , the shunt resistor  21 , the load  12 , and the negative electrode of the DC power source  11  in that order, and the power is thus supplied to the load  12 . The load  12  operates while the power is being supplied to the load  12 . When the FET  20  is switched off, the power supply to the load  12  stops and the load  12  stops operating. 
     As described above, the power supply control device  10  controls the supply of power from the DC power source  11  to the load  12  by switching the FET  20  on or off. 
     An on signal instructing the FET  20  to switch on and an off signal instructing the FET  20  to switch off are input to the power supply control device  10 . The power supply control device  10  switches the FET  20  on when the on signal is input. The power supply control device  10  switches the FET  20  off when the off signal is input. 
     Configuration of Power Supply Control Device  10   
     In addition to the FET  20  and the shunt resistor  21 , the power supply control device  10  includes a regulator  22 , a drive circuit  23 , a current output circuit  24 , a resistor circuit  25 , an application circuit  26 , and a microcomputer  27 . The resistor circuit  25  includes a detection resistor  30 . 
     The drain and the gate of the FET  20  are connected to the regulator  22  and the drive circuit  23 , respectively. One end and the other end of the shunt resistor  21  are connected separately to the current output circuit  24 . The current output circuit  24  is furthermore connected to one end of the detection resistor  30  of the resistor circuit  25 . The other end of the detection resistor  30  is grounded. One end and the other end of the detection resistor  30  correspond to one end and the other end of the resistor circuit  25 , respectively. A connection node between the current output circuit  24  and the detection resistor  30  is connected to the drive circuit  23 , the application circuit  26 , and the microcomputer  27 . Each of the regulator  22 , the drive circuit  23 , and the application circuit  26  is further connected to the microcomputer  27 . The microcomputer  27  is also grounded. 
     The FET  20  is on when the voltage at the gate of the FET  20 , a reference potential of which is a ground potential, is greater than or equal to a certain on voltage. The FET  20  is off when the voltage at the gate of the FET  20 , the reference potential of which is the ground potential, is less than a certain off voltage. The on voltage is greater than the off voltage. The off voltage is a positive voltage. The drive circuit  23  switches the FET  20  on by increasing the voltage at the gate of the FET  20 , the reference potential of which is the ground potential, to a voltage greater than or equal to the on voltage. The drive circuit  23  switches the FET  20  off by reducing the voltage at the gate of the FET  20 , the reference potential of which is the ground potential, to a voltage less than the off voltage. 
     mentioned above, when the FET  20  is on, current flows through the FET  20  and the shunt resistor  21  in that order. The current output circuit  24  outputs a current proportional to the current flowing through the shunt resistor  21  to the detection resistor  30  of the resistor circuit  25 . Current output by the current output circuit  24  flows through the detection resistor  30  of the resistor circuit  25 . In the following, the current flowing through the FET  20  will be denoted as “switch current”. The current flowing through the shunt resistor  21  substantially matches the switch current. Therefore, the current output from the current output circuit  24  is substantially equal to (switch current)/(predetermined number), and rises when the switch current rises. The predetermined number is a positive real number, e.g., 4000. 
     Current output from the current output circuit  24  flows through the detection resistor  30  of the resistor circuit  25 . The voltage across both ends of the detection resistor  30 , i.e., the resistor circuit  25 , is expressed as (current output from current output circuit  24 )·(resistance value of detection resistor  30 ). In the following, the voltage across both ends of the resistor circuit  25  will be denoted as a “voltage across both ends”. As mentioned above, the current output from the current output circuit  24  is substantially equal to (switch current)/(predetermined number). Therefore, the voltage across both ends of the resistor circuit  25  is substantially equal to (switch current)·(resistance value of detection resistor  30 )/(predetermined number), and is higher the greater the switch current is. 
     A voltage at the connection node between the current output circuit  24  and the detection resistor  30  is output to the drive circuit  23  and the microcomputer  27 . Here, the reference potential for the voltage at the connection node is the ground potential. Therefore, the voltage at the connection node between the current output circuit  24  and the detection resistor  30  is the voltage across both ends of the resistor circuit  25 . 
     The regulator  22  generates a constant voltage Vc by stepping down the output voltage of the DC power source  11 , a reference potential of which is the ground potential, and applies the generated constant voltage Vc to the microcomputer  27 . As a result, current flows from the positive electrode of the DC power source  11  to the regulator  22 , the microcomputer  27 , and the negative electrode of the DC power source  11  in that order, and power is supplied to microcomputer  27 . The constant voltage Vc is 5.0 V, 3.3 V, or the like. 
     The microcomputer  27  outputs the voltage to the drive circuit  23 . The reference potential of the output voltage of the microcomputer  27  is the ground potential. The microcomputer  27  switches the output voltage output to the drive circuit  23  to a high-level voltage or a low-level voltage. The high-level voltage is higher than the low-level voltage. The high-level voltage corresponds to the constant voltage Vc, for example. The low-level voltage is, for example, zero V. The drive circuit  23  switches the FET  20  on or off based on the output voltage of the microcomputer  27  and the voltage across both ends of the resistor circuit  25 . 
     If the microcomputer  27  switches the output voltage from the low-level voltage to the high-level voltage while the voltage across both ends of the resistor circuit  25  is less than a certain reference voltage, the drive circuit  23  switches the FET  20  on. If the microcomputer  27  switches the output voltage from the high-level voltage to the low-level voltage while the voltage across both ends of the resistor circuit  25  is less than the reference voltage, the drive circuit  23  switches the FET  20  off. 
     When the voltage across both ends of the resistor circuit  25  reaches a voltage greater than or equal to the reference voltage while the output voltage of the microcomputer  27  is the high-level voltage, the drive circuit  23  switches the FET  20  off and applies a voltage to the resistor circuit  25 . The voltage applied by the drive circuit  23  to the resistor circuit  25  will be denoted as a “notification voltage”. The notification voltage is a voltage greater than or equal to the reference voltage. A reference potential of the notification voltage is the ground potential. 
     By applying the notification voltage to the resistor circuit  25 , the drive circuit  23  notifies the microcomputer  27  that the voltage across both ends of the resistor circuit  25  has reached a voltage greater than or equal to the reference voltage. When the microcomputer  27  switches the output voltage from the high-level voltage to the low-level voltage, the drive circuit  23  stops applying the notification voltage to the resistor circuit  25  while keeping the FET  20  off. 
     When the output voltage of the microcomputer  27  is the low-level voltage, the drive circuit  23  keeps the FET  20  off regardless of the voltage across both ends of the resistor circuit  25 . 
     The application circuit  26  applies the voltage to the resistor circuit  25  and stops applying the voltage to the resistor circuit  25  according to instructions from the microcomputer  27 . The voltage applied by the application circuit  26  to the resistor circuit  25  will be denoted as an “applied voltage”. The applied voltage is a voltage greater than or equal to the reference voltage. A reference potential of the applied voltage is the ground potential. 
     The on signal and the off signal are input to the microcomputer  27 . When the on signal is input, the microcomputer  27  switches the output voltage being output to the drive circuit  23  from the low-level voltage to the high-level voltage. As a result, the drive circuit  23  switches the FET  20  on. As mentioned above, when the FET  20  is on, current flows through the FET  20 , the shunt resistor  21 , and the load  12  in that order, and power is supplied to the load  12 . The current output circuit  24  outputs a current proportional to the current flowing through the shunt resistor  21  to the detection resistor  30  of the resistor circuit  25 . 
     When the off signal is input, the microcomputer  27  instructs the application circuit  26  to apply a voltage. As a result, the application circuit  26  applies a voltage greater than or equal to the reference voltage to both ends of the resistor circuit. As a result, the voltage across both ends of the resistor circuit  25  reaches a voltage greater than or equal to the reference voltage, and thus the drive circuit  23  switches the FET  20  off. As mentioned above, when the voltage across both ends of the resistor circuit  25  has reached a voltage greater than or equal to the reference voltage, the drive circuit  23  makes a notification by applying the notification voltage to the resistor circuit  25 . 
     The microcomputer  27  instructs the application circuit  26  to apply the voltage, and then instructs the application circuit  26  to stop applying the voltage. As a result, the application circuit  26  stops applying a voltage greater than or equal to the reference voltage. If no fault has occurred in the power supply control device  10 , the drive circuit  23  continues to apply the voltage greater than or equal to the reference voltage after the application circuit  26  has stopped applying voltage. After instructing the application circuit  26  to stop applying the voltage, the microcomputer  27  determines whether or not the drive circuit  23  is making a notification based on the voltage across both ends of the resistor circuit  25 . In this manner, the microcomputer  27  tests a notification function of the drive circuit  23 . 
     The microcomputer  27  switches the output voltage to the low-level voltage after determining whether or not the drive circuit  23  is making a notification. As a result, the drive circuit  23  stops applying the notification voltage to the resistor circuit  25 . After switching the output voltage to the low-level voltage, the microcomputer  27  determines whether or not the voltage across both ends of the resistor circuit  25  is less than the reference voltage. Through this, the microcomputer  27  can confirm that the voltage across both ends of the resistor circuit  25  has returned to a voltage less than the reference voltage. 
     Configuration of Application Circuit  26   
     The application circuit  26  includes a transistor  40 , a diode  41 , and circuit resistors  42  and  43 . The transistor  40  is a PNP-type bipolar transistor and functions as a switch. When the transistor  40  is on, a resistance value between the emitter and the collector of the transistor  40  is sufficiently low, and current can therefore flow through the emitter and the collector of the transistor  40 . When the transistor  40  is off, the resistance value between the emitter and the collector of the transistor  40  is sufficiently high, and therefore no current flows through the emitter and the collector of the transistor  40 . 
     The cathode of the diode  41  is connected to a connection node between the current output circuit  24  and the detection resistor  30 . The anode of the diode  41  is connected to the collector of the transistor  40 . The circuit resistor  42  is connected between the emitter and the base of the transistor  40 . One end of the circuit resistor  43  is connected to the base of the transistor  40 . The other end of the circuit resistor  43  is connected to the microcomputer  27 . Like the microcomputer  27 , the constant voltage Vc is applied to the emitter of the transistor  40 . 
     The configuration of applying the constant voltage Vc to the emitter of the transistor  40  may be realized by the regulator  22  applying the constant voltage Vc. 
     In the transistor  40 , if the voltage at the base, where the reference potential is the potential of the emitter, is less than a certain voltage threshold, the transistor  40  is on. The voltage threshold is a negative voltage. In the transistor  40 , if the voltage at the base, where the reference potential is the potential of the emitter, is greater than or equal to the voltage threshold, the transistor  40  is off. 
     The microcomputer  27  adjusts the voltage at the other end of the circuit resistor  43  of the application circuit  26 . In the following, the voltage at the other end of the circuit resistor  43  will be denoted as a “resistor voltage”. A reference potential of the resistor voltage is the ground potential. The transistor  40  is switched on or off by the microcomputer  27 . The microcomputer  27  reduces the resistor voltage to a sufficiently low voltage, e.g., zero V. As a result, current flows from the emitter of the transistor  40  to the circuit resistors  42  and  43  in that order, and a voltage drop occurs in the circuit resistor  42 . At this time, the current flowing through the circuit resistor  42  is large, and thus in the transistor  40 , the voltage at the base, where the reference potential is the potential of the emitter, is less than the voltage threshold. The transistor  40  is switched on as a result. 
     The microcomputer  27  raises the resistor voltage to a sufficiently high voltage, e.g., the constant voltage Vc. This causes the current flowing through the circuit resistor  42  to drop to zero A or a value near zero A. At this time, in the transistor  40 , the voltage at the base, where the reference potential is the potential of the emitter, rises to zero V or a value near zero V, which is greater than or equal to the voltage threshold. Therefore, the transistor  40  is switched off. 
     The microcomputer  27  switches the transistor  40  on or off in this manner. 
     When the transistor  40  is switched from off to on, current flows through the transistor  40 , the diode  41 , and the resistor circuit  25  in that order, and the application circuit  26  applies the voltage to the resistor circuit  25 . In the diode  41 , the range of the voltage drop that occurs in the diode  41  when current flows in the order of the anode and the cathode will be denoted as “forward voltage”. The applied voltage applied by the application circuit  26  to the resistor circuit  25  is expressed as (constant voltage Vc)—(forward voltage). As mentioned above, the applied voltage is greater than or equal to the reference voltage. 
     When the transistor  40  is switched from on to off, the flow of current through the transistor  40  and the diode  41  stops, and the application circuit  26  stops applying voltage to the resistor circuit  25 . 
     As described above, the microcomputer  27  instructs the application circuit  26  to apply the voltage to the resistor circuit  25  by reducing the resistor voltage to a sufficiently low voltage. As a result, the transistor  40  is switched on and the application circuit  26  applies a voltage greater than or equal to the reference voltage to the resistor circuit  25 . The microcomputer  27  instructs the application circuit  26  to stop applying the voltage to the resistor circuit  25  by increasing the resistor voltage to a sufficiently high voltage. As a result, the transistor  40  is switched off and the application circuit  26  stops applying the voltage to the resistor circuit  25 . 
     Configuration of Drive Circuit  23   
       FIG.  2    is a block diagram illustrating the primary configuration of the drive circuit  23 . The drive circuit  23  includes a drive unit  50 , a comparator  51 , and a latch circuit  52 . The comparator  51  has a plus terminal, a minus terminal, and an output terminal. The gate of the FET  20  is connected to the drive unit  50 . The drive unit  50  is further connected to the microcomputer  27 . A connection node between the drive unit  50  and the microcomputer  27  is connected to the latch circuit  52 . The output terminal of the comparator  51  is connected to the drive unit  50  and the latch circuit  52 . The minus terminal of the comparator  51  and the latch circuit  52  are connected to a connection node between the current output circuit  24  and the resistor circuit  25 . A reference voltage Vr is input to the plus terminal of the comparator  51 . The reference voltage Vr is generated, for example, by a regulator (not shown) stepping down the output voltage of the DC power source  11 . 
     The drive unit  50  switches the FET  20  on or off by adjusting the voltage at the gate of FET  20 , for which the reference potential is the ground potential, as described above. The comparator  51  outputs a voltage from the output terminal to the drive unit  50  and the latch circuit  52 . The reference potential of the output voltage from the comparator  51  is the ground potential. The comparator  51  switches the output voltage to the high-level voltage or the low-level voltage. 
     The comparator  51  is outputting the high-level voltage to the drive unit  50  and the latch circuit  52  when the voltage across both ends of the resistor circuit  25  is less than the reference voltage Vr. The comparator  51  switches the output voltage from the high-level voltage to the low-level voltage when the voltage across both ends of the resistor circuit  25  reaches a voltage that is greater than or equal to the reference voltage Vr. The comparator  51  switches the output voltage from the low-level voltage to the high-level voltage when the voltage across both ends of the resistor circuit  25  reaches a voltage that is less than the reference voltage Vr. The reference voltage Vr corresponds to a predetermined voltage. 
     The output voltage of the microcomputer  27  is output to the drive unit  50  and the latch circuit  52 . The latch circuit  52  applies the notification voltage to the resistor circuit  25  when the output voltage of the comparator  51  switches from the high-level voltage to the low-level voltage while the output voltage of the microcomputer  27  is the high-level voltage. A notification is made as a result. As mentioned above, the notification voltage is a voltage greater than or equal to the reference voltage Vr. The latch circuit  52  functions as a notifying circuit. While the latch circuit  52  is applying the notification voltage to the resistor circuit  25 , the output voltage of the comparator  51  is fixed at the low-level voltage. When the output voltage of the microcomputer  27  switches from the high-level voltage to the low-level voltage, the latch circuit  52  stops applying the notification voltage to the resistor circuit  25 . The latch circuit  52  continues to stop the application of the notification voltage to the resistor circuit  25  until the output voltage of the comparator  51  switches from the high-level voltage to the low-level voltage while the output voltage of the microcomputer  27  is the high-level voltage. 
     The drive unit  50  switches the FET  20  on when the microcomputer  27  switches the output voltage from the low-level voltage to the high-level voltage while the comparator  51  is outputting the high-level voltage. The comparator  51  outputting the high-level voltage means that the voltage across both ends of the resistor circuit  25  is less than the reference voltage Vr. The drive unit  50  switches the FET  20  off when the microcomputer  27  switches the output voltage from the high-level voltage to the low-level voltage while the comparator  51  is outputting the high-level voltage. 
     The drive unit  50  switches the FET  20  off when the output voltage of the comparator  51  switches from the high-level voltage to the low-level voltage while the output voltage of the microcomputer  27  is the high-level voltage. The drive unit  50  therefore functions as a switching unit. As mentioned above, when the switch current rises, the voltage across both ends of the resistor circuit  25  rises. When the voltage across both ends of the resistor circuit  25  reaches a voltage greater than or equal to the reference voltage Vr, the output voltage of the comparator  51  switches from the high-level voltage to the low-level voltage, and the drive unit  50  switches the FET  20  off. This prevents overcurrent from flowing through the FET  20 . 
     As mentioned above, when the output voltage of the comparator  51  switches from the high-level voltage to the low-level voltage while the output voltage of the microcomputer  27  is the high-level voltage, the latch circuit  52  fixes the output voltage of the comparator  51  to the low-level voltage by applying the notification voltage to the resistor circuit  25 . When the output voltage of the microcomputer  27  switches to the low-level voltage, the latch circuit  52  stops applying the notification voltage and cancels the fixing. 
     When the output voltage of the microcomputer  27  is the low-level voltage, the drive unit  50  keeps the FET  20  off regardless of the output voltage of the comparator  51 , i.e., the voltage across both ends of the resistor circuit  25 . 
       FIG.  3    is a circuit diagram illustrating the latch circuit  52 . The latch circuit  52  includes an inverter  60 , an OR circuit  61 , second circuit resistors  62  and  63 , a second transistor  64 , and a second diode  65 . The inverter  60  has an input terminal and an output terminal. The OR circuit  61  has a first input terminal, a second input terminal, and an output terminal. 
     The second transistor  64  is a PNP-type bipolar transistor and functions as a switch. When the second transistor  64  is on, the resistance value between the emitter and the collector of the second transistor  64  is sufficiently low, and current can therefore flow through the emitter and the collector of the second transistor  64 . When the second transistor  64  is off, the resistance value between the emitter and the collector of the second transistor  64  is sufficiently high, and therefore no current flows through the emitter and the collector of the second transistor  64 . 
     The input terminal of the inverter  60  is connected to a connection node between the microcomputer  27  and the drive unit  50 . The output terminal of the inverter  60  is connected to the first input terminal of the OR circuit  61 . The second input terminal of the OR circuit  61  is connected to the output terminal of the comparator  51 . The output terminal of the OR circuit  61  is connected to one end of the second circuit resistor  62 . The other end of the second circuit resistor  62  is connected to the base of the second transistor  64 . The second circuit resistor  63  is connected between the emitter and the base of the second transistor  64 . The collector of the second transistor  64  is connected to the anode of the second diode  65 . The cathode of the second diode  65  is connected to a connection node between the current output circuit  24  and the detection resistor  30  of the resistor circuit  25 . 
     Like the microcomputer  27  and the transistor  40 , the constant voltage Vc is applied to the emitter of the second transistor  64 . 
     The configuration of applying the constant voltage Vc to the emitter of the second transistor  64  may be realized by the regulator  22  applying the constant voltage Vc. 
     The inverter  60  outputs a voltage to the OR circuit  61 . The reference potential of the output voltage of the inverter  60  is the ground potential. The output voltage of the inverter  60  is a high-level voltage or a low-level voltage. The output voltage of the microcomputer  27  is input to the inverter  60 . The inverter  60  outputs the low-level voltage to the OR circuit  61  when the output voltage of the microcomputer  27  is the high-level voltage. The inverter  60  outputs the high-level voltage to the OR circuit  61  when the output voltage of the microcomputer  27  is the low-level voltage. 
     In the second transistor  64 , if the voltage at the base, where the reference potential is the potential of the emitter, is less than a certain second voltage threshold, the second transistor  64  is on. The second voltage threshold is a negative voltage. In the second transistor  64 , if the voltage at the base, where the reference potential is the potential of the emitter, is greater than or equal to the second voltage threshold, the second transistor  64  is off. 
     The OR circuit  61  adjusts the voltage at the output terminal. As a result, the second transistor  64  is switched on or off. The OR circuit  61  reduces the voltage at the output terminal to a sufficiently low voltage, e.g., zero V. As a result, current flows from the emitter of the second transistor  64  to the second circuit resistors  63  and  62  in that order, and a voltage drop occurs in the second circuit resistor  63 . At this time, the current flowing through the second circuit resistor  63  is large, and thus in the second transistor  64 , the voltage at the base, where the reference potential is the potential of the emitter, is less than the second voltage threshold. The second transistor  64  is switched on as a result. 
     The OR circuit  61  raises the voltage of the output terminal to a sufficiently high voltage, e.g., the constant voltage Vc. This causes the current flowing through the second circuit resistor  62  to drop to zero A or a value near zero A. At this time, in the second transistor  64 , the voltage at the base, where the reference potential is the potential of the emitter, is greater than or equal to the second voltage threshold, and thus the second transistor  64  is switched off. 
     The OR circuit  61  switches the second transistor  64  on or off in this manner. 
     The output voltages of the comparator  51  and the inverter  60  are input to the OR circuit  61 . The OR circuit  61  switches the second transistor  64  from off to on when the output voltage of the comparator  51  switches from the high-level voltage to the low-level voltage while the output voltage of the inverter  60  is the low-level voltage. The output voltage of the inverter  60  being the low-level voltage means that the output voltage of the microcomputer  27  is the high-level voltage. When the second transistor  64  is switched from off to on, current flows through the second transistor  64 , the second diode  65 , and the resistor circuit  25  in that order, and the latch circuit  52  applies the notification voltage to the resistor circuit  25 . 
     In the second diode  65 , the range of the voltage drop that occurs in the second diode  65  when current flows in the order of the anode and the cathode will be denoted as “second forward voltage”. The notification voltage applied by the latch circuit  52  to the resistor circuit  25  is expressed as (constant voltage Vc)—(second forward voltage). As mentioned above, the notification voltage is a voltage greater than or equal to the reference voltage. While the latch circuit  52  is applying the notification voltage to the resistor circuit  25 , the voltage across both ends of the resistor circuit  25  is greater than or equal to the reference voltage Vr, and thus the comparator  51  continues to output the low-level voltage. 
     In this state, when the inverter  60  switches the output voltage from the low-level voltage to the high-level voltage, i.e., when the microcomputer  27  switches the output voltage from the high-level voltage to the low-level voltage, the OR circuit  61  switches the second transistor  64  from on to off. When the second transistor  64  is switched from on to off, the flow of current through the second transistor  64  and the second diode  65  stops, and the latch circuit  52  stops applying voltage to the resistor circuit  25 . Then, the OR circuit  61  keeps the second transistor  64  off until the output voltage of the comparator  51  switches from the high-level voltage to the low-level voltage while the output voltage of the microcomputer  27  is the high-level voltage. 
     Configuration of Microcomputer  27   
       FIG.  4    is a block diagram illustrating the primary configuration of the microcomputer  27 . The microcomputer  27  includes an output unit  70 , an adjustment unit  71 , an A/D conversion unit  72 , an input unit  73 , a notification unit  74 , a storage unit  75 , and a control unit  76 . These are connected to an internal bus  77 . The output unit  70  is further connected to the drive unit  50  of the drive circuit  23  and the inverter  60  of the latch circuit  52  included in the drive circuit  23 . The adjustment unit  71  is further connected to the other end of the circuit resistor  43  of the application circuit  26 . The A/D conversion unit  72  is further connected to the connection node between the current output circuit  24  and the resistor circuit  25 . 
     The output unit  70  outputs a voltage to the drive unit  50  and the inverter  60 . The output voltage of the output unit  70  is the output voltage of the microcomputer  27  described above. The output unit  70  switches the output voltage to a high-level voltage or a low-level voltage in response to instructions from the control unit  76 . 
     The adjustment unit  71  switches the transistor  40  of the application circuit  26  on or off by adjusting the resistance voltage, i.e., the voltage at the other end of the circuit resistor  43 . The adjustment unit  71  performs the switching of the transistor  40  in response to instructions from the control unit  76 . 
     An analog value of the voltage across both ends of the resistor circuit  25  is input to the A/D conversion unit  72 . The A/D conversion unit  72  performs A/D conversion, i.e., conversion from an analog value to a digital value, on the voltage across both ends of the resistor circuit  25 . The control unit  76  obtains the digital value of the voltage across both ends of the resistor circuit  25  from the A/D conversion unit  72 . 
       FIG.  5    is a descriptive diagram illustrating a range of a voltage across both ends, over which A/D conversion is performed. A lower limit value of the range of the voltage across both ends, over which A/D conversion is performed, is zero V. An upper limit value of the range of the voltage across both ends, over which the A/D conversion is performed, is the constant voltage Vc applied to the microcomputer  27  by the regulator  22 . The reference voltage Vr applied to the plus terminal of the comparator  51  is greater than zero V and less than the constant voltage Vc. 
     In  FIG.  5   , Va represents the applied voltage applied by the application circuit  26  to the resistor circuit  25 . Vi represents the notification voltage applied by the latch circuit  52  of the drive circuit  23  to the resistor circuit  25 .  FIG.  5    illustrates an example where the forward voltage of the diode  41  matches the second forward voltage of the second diode  65 . The applied voltage Va and the notification voltage Vi are each greater than or equal to the reference voltage Vr and less than or equal to the constant voltage Vc. 
     As illustrated in  FIG.  4   , the on signal and the off signal are input to the input unit  73 . When a signal is input, the input unit  73  communicates the input signal to the control unit  76 . 
     The notification unit  74  makes a notification in response to an instruction from the control unit  76 . The control unit  76  causes the notification unit  74  to make the notification when the voltage across both ends of the resistor circuit  25  is fixed at a voltage greater than or equal to the reference voltage Vr, even if the latch circuit  52  has been instructed to stop the application of the notification voltage to the resistor circuit  25 . The notification is realized by lighting a lamp, transmitting a signal through a communication line (not shown), or the like. 
     The storage unit  75  is non-volatile memory. A computer program P is stored in the storage unit  75 . The control unit  76  includes a processing device that executes processing, e.g., a Central Processing Unit (CPUs), and functions as a processing unit. The control unit  76  executes on processing for switching the FET  20  on and off processing for switching the FET  20  off by executing the computer program P. 
     Note that the computer program P may be stored in a storage medium A so as to be readable by the processing device of the control unit  76 . In this case, the computer program P read out from the storage medium A by a readout device (not shown) is written into the storage unit  75 . The storage medium A is an optical disk, a flexible disk, a magnetic disk, a magneto-optical disk, semiconductor memory, or the like. The optical disk is a CD (Compact Disc)-ROM (Read Only Memory), DVD (Digital Versatile Disc)-ROM, a BD (Blu-ray) (registered trademark) Disc), or the like. The magnetic disk is a hard disk, for example. Additionally, the computer program P may be downloaded from a device (not shown) connected to a communication network (not shown), and the downloaded computer program P may be written into the storage unit  75 . 
     The number of processing devices included in the control unit  76  is not limited to one, and may be two or more. In this case, the plurality of processing devices may cooperatively execute the on processing, the off processing, and the like in accordance with the computer program P. 
     In addition to the computer program P, a value of a fault flag is stored in the storage unit  75 . The value of the fault flag indicates whether or not a notification is to be made by the latch circuit  52 . The value of the fault flag being zero indicates that the notification is to be made. The value of the fault flag being 1 indicates that the notification is not to be made. The value of the fault flag is changed by the control unit  76 . 
     On Processing 
       FIG.  6    is a flowchart illustrating a sequence of the on processing. The control unit  76  executes the on processing when the microcomputer  27  has been started up or the execution of the off processing has ended. The on processing is executed while the output voltage of the output unit  70  is the low-level voltage. When the output voltage of the output unit  70  is the low-level voltage, the drive unit  50  of the drive circuit  23  keeps the FET  20  off. When the FET  20  is off, no current flows through the FET  20 . Accordingly, the voltage across both ends of the resistor circuit  25  is less than the reference voltage, and the output voltage of the comparator  51  of the drive circuit  23  is the high-level voltage. 
     When the output voltage of the output unit  70  is the low-level voltage, the latch circuit  52  stops applying the notification voltage. At the point in time when the on processing is executed, the adjustment unit  71  causes the application circuit  26  to stop applying the applied voltage by keeping the resistor voltage, which is the voltage at the other end of the circuit resistor  43  of the application circuit  26 , at a high voltage. 
     In the on processing, first, the control unit  76  determines whether or not the on signal has been input to the input unit  73  (step S 1 ). If it is determined that the on signal has been input (S 1 : YES), the control unit  76  determines whether or not the value of the fault flag is zero (step S 2 ). The value of the fault flag is 1 or zero, and thus the value of the fault flag not being zero means that the value of the fault flag is 1. 
     If it is determined that the on signal is not being input (S 1 : NO), or that the value of the fault flag is not zero (S 2 : NO), the control unit  76  executes step S 1  again. While the value of the fault flag is zero, the control unit  76  stands by until the on signal is input to the input unit  73 . If it is determined that the value of the fault flag is zero (S 2 : YES), the control unit  76  instructs the output unit  70  to switch the output voltage from the low-level voltage to the high-level voltage (step S 3 ). 
     If a fault has not occurred in the power supply control device  10 , the comparator  51  is outputting the high-level voltage at the point in time when step S 3  is executed. When step S 3  is executed while no fault has occurred in the power supply control device  10 , the drive unit  50  switches the FET  20  on. The control unit  76  ends the on processing after executing step S 3 . 
     As described above, when the on signal is input while the value of the fault flag is zero, the drive unit  50  of the drive circuit  23  switches the FET  20  on, and power is supplied to the load  12  through the FET  20 . When the value of the fault flag is 1, the drive unit  50  does not switch the FET  20  on. 
     Off Processing 
       FIG.  7    is a flowchart illustrating a sequence of the off processing. The control unit  76  executes the off processing when the execution of the on processing has ended. The off processing is executed while the output voltage of the output unit  70  is the high-level voltage and the value of the fault flag is zero. At the point in time when the off processing is executed, the adjustment unit  71  causes the application circuit  26  to stop applying the applied voltage by keeping the resistor voltage, which is the voltage at the other end of the circuit resistor  43  of the application circuit  26 , at a high voltage. 
     If, after the execution of the on processing has ended, the switch current is being kept at a current less than a current threshold, the voltage across both ends of the resistor circuit  25  is less than the reference voltage, and thus the output voltage of the comparator  51  is the high-level voltage. When the output voltage of the comparator  51  is the high-level voltage, the latch circuit  52  stops applying the notification voltage. 
     If, after the execution of the on processing, the switch current has reached a current greater than or equal to the current threshold, the voltage across both ends of the resistor circuit  25  reaches a voltage greater than or equal to the reference voltage, and the comparator  51  switches the output voltage from the high-level voltage to the low-level voltage. As a result, the drive unit  50  switches the FET  20  off. When the output voltage of the comparator  51  is the low-level voltage, the latch circuit  52  continues to apply the notification voltage to the resistor circuit  25  as long as the output voltage of the output unit  70  is being kept at the high-level voltage. As a result, the voltage across both ends of the resistor circuit  25  is kept at a voltage greater than or equal to the reference voltage. 
     In the off processing, the control unit  76  determines whether or not the voltage across both ends of the resistor circuit  25  is greater than or equal to the reference voltage (step S 11 ). As a result, it is determined whether or not a notification is being made by the latch circuit  52 . The voltage across both ends being greater than or equal to the reference voltage means that the notification is being made. The voltage across both ends is obtained from the A/D conversion unit  72 . If it is determined that the voltage across both ends is less than the reference voltage (S 11 : NO), the control unit  76  determines whether or not the off signal has been input to the input unit  73  (step S 12 ). If it is determined that the off signal has not been input (S 12 : NO), the control unit  76  executes step S 11  again, and stands by until the voltage across both ends reaches a voltage greater than or equal to the reference voltage or until the off signal is input. 
     If it is determined that the off signal has been input (S 12 : YES), the control unit  76  instructs the application circuit  26  to apply the applied voltage to the resistor circuit  25  (step S 13 ). The control unit  76  instructs the application circuit  26  to apply the applied voltage by causing the adjustment unit  71  to reduce the resistor voltage, which is the voltage at the other end of the circuit resistor  43 . As mentioned earlier, when the resistor voltage drops, the transistor  40  switches on, and the application circuit  26  applies the applied voltage to the resistor circuit  25 . 
     If a fault has not occurred in the power supply control device  10 , when the applied voltage is applied, the voltage across both ends of the resistor circuit  25  reaches a voltage greater than or equal to the reference voltage. Through this, the drive unit  50  of the drive circuit  23  switches the FET  20  off, and the latch circuit  52  applies the notification voltage to the resistor circuit  25 . A notification is made as a result of the notification voltage being applied. 
     After executing step S 13 , the control unit  76  instructs the application circuit  26  to stop applying the applied voltage (step S 14 ). The control unit  76  instructs the application circuit  26  to stop applying the applied voltage by causing the adjustment unit  71  to raise the resistor voltage. As mentioned earlier, when the resistor voltage rises, the transistor  40  switches off, and the application circuit  26  stops applying the applied voltage. If a fault has not occurred in the power supply control device  10 , the latch circuit  52  continues applying the notification voltage even after the application circuit  26  has stopped applying the applied voltage, and thus the voltage across both ends of the resistor circuit  25  is kept at a voltage greater than or equal to the reference voltage. 
     After executing step S 14 , the control unit  76  determines whether or not the voltage across both ends of the resistor circuit  25  is greater than or equal to the reference voltage (step S 15 ). As a result, it is determined whether or not the latch circuit  52  is making a notification. The voltage across both ends being greater than or equal to the reference voltage means that the notification is being made. The voltage across both ends being less than the reference voltage means that the notification is not being made. The control unit  76  tests the latch circuit  52  by executing step S 15 . If the drive circuit  23  is a single integrated circuit, it is possible that there is a fault in the drive circuit  23  when the latch circuit  52  does not make a notification. By testing the latch circuit  52 , the possibility of a fault in the drive circuit  23  can be detected in advance. If it is determined that the voltage across both ends is less than the reference voltage (S 15 : NO), the control unit  76  changes the value of the fault flag to 1 (step S 16 ). 
     If it is determined that the voltage across both ends is greater than or equal to the reference voltage (S 11 : YES), the control unit  76  determines whether or not the off signal has been input to the input unit  73  (step S 17 ). If it is determined that the off signal has not been input (S 17 : NO), the control unit  76  executes step S 17  again, and stands by until the off signal is input. 
     If it is determined that the voltage across both ends is greater than or equal to the reference voltage (S 15 : YES), the control unit  76  instructs the output unit  70  to switch the output voltage from the high-level voltage to the low-level voltage (step S 18 ) after step S 16  has been executed or if it is determined that the off signal has been input (S 17 : YES). If the output voltage of the output unit  70  has switched to the low-level voltage while the latch circuit  52  is applying the notification voltage, the latch circuit  52  stops applying the notification voltage. Accordingly, instructing the output unit  70  to switch the output voltage to the low-level voltage corresponds to instructing the latch circuit  52  to stop applying the notification voltage. 
     If a fault has not occurred in the power supply control device  10 , the voltage across both ends of the resistor circuit  25  drops to a voltage less than the reference voltage, e.g., zero V, when the application of the notification voltage stops. The FET  20  is off at the point in time when step S 18  is executed. As such, the state of the FET  20  does not change in response to the control unit  76  executing step S 18 . 
     After executing step S 18 , the control unit  76  determines whether or not the voltage across both ends of the resistor circuit  25  is less than the reference voltage (step S 19 ). As a result, it is determined whether or not the voltage across both ends has dropped to a voltage less than the reference voltage. If it is determined that the voltage across both ends is greater than or equal to the reference voltage (S 19 : NO), the control unit  76  instructs the notification unit  74  to make the notification (step S 20 ). If it is determined that the voltage across both ends is less than the reference voltage (S 19 : YES), or after executing step S 20 , the control unit  76  ends the off processing. 
     Operations of Power Supply Control Device  10   
       FIG.  8    is a timing chart illustrating operations of the power supply control device  10 .  FIG.  8    illustrates transitions in the output voltage of the output unit  70 , the state of the FET  20 , the state of application by the application circuit  26 , the state of application by the latch circuit  52 , the voltage across both ends of the resistor circuit  25 , and the switch current. “H” and “L” represent the high-level voltage and the low-level voltage, respectively. Va, Vi, Vr, and Ith represent the applied voltage, the notification voltage, the reference voltage, and the current threshold, respectively.  FIG.  8    illustrates operations performed when no fault has occurred in the power supply control device  10 , and the applied voltage Va is the same as the notification voltage Vi. 
     As illustrated in  FIG.  8   , when the output voltage of the output unit  70  is the low-level voltage, the drive unit  50  keeps the FET  20  off, and the switch current is zero A. Because the switch current is zero A, the voltage across both ends of the resistor circuit  25  is zero V, and is therefore less than the reference voltage Vr. When the output voltage of the output unit  70  is the low-level voltage, the application circuit  26  and the latch circuit  52  are not applying the applied voltage and the notification voltage, respectively. 
     When the on signal is input to the input unit  73  of the microcomputer  27 , the control unit  76  instructs the output unit  70  to switch the output voltage from the low-level voltage to the high-level voltage. At this time, the switch current is zero A, and the voltage across both ends of the resistor circuit  25  is less than the reference voltage. Accordingly, the drive unit  50  of the drive circuit  23  switches the FET  20  on. When the FET  20  is switched on, the switch current flows through the FET  20 . As a result, the switch current rises to a current that exceeds zero A but is less than the current threshold Ith. Additionally, the voltage across both ends of the resistor circuit  25  rises to a voltage that exceeds zero V but is less than the reference voltage Vr. 
     When the off signal is input to the input unit  73 , the control unit  76  instructs the application circuit  26  to apply the applied voltage Va to the resistor circuit  25 . Through this, the voltage across both ends of the resistor circuit  25  rises to the applied voltage Va, which is greater than or equal to the reference voltage Vr. As a result, the drive unit  50  switches the FET  20  off, and the latch circuit  52  applies the notification voltage Vi to the resistor circuit  25 . When the FET  20  is switched off, the switch current drops to zero A. 
     If the applied voltage Va and the notification voltage Vi are different from each other, the voltage across both ends is the same as the higher of the applied voltage Va and the notification voltage Vi while the application circuit  26  and the latch circuit  52  are applying their respective voltages. 
     After the application circuit  26  applies the applied voltage Va, the control unit  76  instructs the application circuit  26  to stop applying the applied voltage Va. Even if the application of the applied voltage Va has stopped, the latch circuit  52  is applying the notification voltage Vi, and thus the voltage across both ends of the resistor circuit  25  is kept at a voltage greater than or equal to the reference voltage Vr. After instructing the application circuit  26  to stop applying the applied voltage Va, the control unit  76  determines whether or not the voltage across both ends of the resistor circuit  25  is greater than or equal to the reference voltage Vr. As a result, the control unit  76  determines whether or not the latch circuit  52  is making a notification. If it is determined that the voltage across both ends is less than the reference voltage Vr, the control unit  76  changes the value of the fault flag to zero from  1 , assuming that a notification is not being made. 
     After determining whether or not a notification is being made, the control unit  76  instructs the output unit  70  to switch the output voltage from the high-level voltage to the low-level voltage. As a result, the latch circuit  52  stops applying the notification voltage Vi. At this time, the switch current is zero A, and thus the voltage across both ends of the resistor circuit  25  drops to zero V. Zero Vis a voltage less than the reference voltage Vr. After the output unit  70  has switched the output voltage to the low-level voltage, the control unit  76  determines whether or not the voltage across both ends has dropped to a voltage less than the reference voltage Vr. Through this, it is confirmed that the voltage across both ends of the resistor circuit  25  has returned to a voltage less than the reference voltage Vr. When the voltage across both ends is greater than or equal to the reference voltage Vr, the control unit  76  causes the notification unit  74  to make a notification. 
     As described thus far, in the power supply control device  10 , the latch circuit  52  is tested at a timing when the off signal is input, i.e., at a timing when the FET  20  is requested to switch off. If no fault is occurring in the power supply control device  10 , the FET  20  is switched off at the point in time when the application circuit  26  applies the applied voltage to the resistor circuit  25 . 
     Variations on First Embodiment 
     It is sufficient for the FET  20  to function as a switch. Accordingly, a P-channel FET, a bipolar transistor, a relay contact, or the like may be used instead of the FET  20 . The resistor circuit  25  may be any circuit that includes the detection resistor  30 . Accordingly, the resistor circuit  25  may be a circuit in which a circuit device, e.g., a capacitor, is connected to the detection resistor  30  in parallel. The voltage applied to the emitter of the transistor  40  of the application circuit  26  may be any voltage higher than the reference voltage Vr. Accordingly, the voltage applied to the emitter of the transistor  40  may be different from the constant voltage Vc generated by the regulator  22 . It is sufficient for the transistor  40  included in the application circuit  26  to function as a switch. Accordingly, a P-channel tFET, a relay contact, or the like may be used instead of the transistor  40 . 
     The application circuit  26  may be any circuit that applies the applied voltage to the resistor circuit  25  in response to an instruction from the microcomputer  27 , i.e., from the control unit  76 . Accordingly, the application circuit  26  may be a circuit including a processing device, e.g., a CPU. In this case, the processing device of the application circuit  26  instructs the applied voltage to be applied to the resistor circuit  25  in response to an instruction from the control unit  76 . 
     The voltage applied to the emitter of the second transistor  64  of the latch circuit  52  may be any voltage higher than the reference voltage Vr. Accordingly, the voltage applied to the emitter of the second transistor  64  may be different from the constant voltage Vc generated by the regulator  22 . It is sufficient for the second transistor  64  included in the latch circuit  52  to function as a switch. Accordingly, a P-channel FET, a relay contact, or the like may be used instead of the second transistor  64 . 
     The latch circuit  52  may be any circuit that applies the notification voltage and stops the application of the notification voltage based on the output voltage of the output unit  70  included in the microcomputer  27  and the output voltage of the comparator  51 . Accordingly, the application circuit  26  may be a circuit including a processing device, e.g., a CPU. In this case, the processing device of the application circuit  26  determines whether or not to apply the notification voltage, whether or not to stop the application of the notification voltage, and the like based on the output voltages of the output unit  70  and the comparator  51 . 
     The timing at which the latch circuit  52  is tested is not limited to the timing at which the off signal is input to the input unit  73  of the microcomputer  27 . The control unit  76  of the microcomputer  27  may test the latch circuit  52  periodically. The timing at which the latch circuit  52  stops applying the notification voltage is not limited to the timing at which the output voltage of the output unit  70  switches from the high-level voltage to the low-level voltage, and may be, for example, a timing at which the set amount of time has passed after the notification voltage is applied. 
     The notification made by the latch circuit  52  is not limited to a notification realized by applying the notification voltage, and may, for example, be a notification realized by outputting a signal indicating that the voltage across both ends of the resistor circuit  25  has reached a voltage greater than or equal to the reference voltage. 
     Second Embodiment 
     In the first embodiment, the power supply control device  10  is disposed upstream from the load  12  in the current path of the current flowing from the positive electrode to the negative electrode of the DC power source  11 . However, the location where the power supply control device  10  is disposed is not limited to being upstream from the load  12 . 
     Points of the second embodiment that are different from the first embodiment will be described hereinafter. The rest of the configuration, aside from the points described below, is the same as in the first embodiment. As such, constituent elements that are the same as in the first embodiment will be given the same reference signs as in the first embodiment, and will not be described. 
     Configuration of Power Source System  1   
       FIG.  9    is a block diagram illustrating the primary configuration of a power source system  1  according to the second embodiment. Comparing the power source system  1  in the second embodiment with the power source system  1  in the first embodiment, the power supply control device  10  is disposed in a different location. In the power source system  1  of the second embodiment, the power supply control device  10  is disposed downstream from the load  12  in the path of the current flowing from the positive electrode to the negative electrode of the DC power source  11 . 
     The positive electrode of the DC power source  11  is connected to one end of the load  12 . If the power supply control device  10  includes the FET  20 , the drain of the FET  20  is connected to the other end of the load  12 . Similar to the first embodiment, the source of the FET  20  is connected to one end of the shunt resistor  21 . The other end of the shunt resistor  21  is grounded. Similar to the first embodiment, the negative electrode of the DC power source  11  is grounded. 
     The effects achieved by the power supply control device  10  in the second embodiment are similar to those achieved by the power supply control device  10  in the first embodiment. 
     The first and second embodiments disclosed here are intended to be in all ways exemplary and in no ways limiting. The scope of the present disclosure is defined not by the foregoing descriptions but by the scope of the claims, and is intended to include all changes equivalent in meaning to and falling within the scope of the claims.