Patent Publication Number: US-10770884-B2

Title: Power supply control apparatus

Description:
This application is the U.S. National Phase of PCT/JP2017/009473 filed Mar. 9, 2017, which claims priority to JP 2016-060699 filed Mar. 24, 2016, the entire disclosure of which is incorporated herein by reference. 
     BACKGROUND 
     The present disclosure relates to a power supply control apparatus for controlling power supply via a wire by turning ON or OFF a switch that is provided at a point along the wire. 
     Currently, vehicles are equipped with a power supply control apparatus (e.g. see JP 2015-53761A) for controlling power supply from a battery to a load. The power supply control apparatus described in JP 2015-53761A controls power supply from a battery to a load by turning ON or OFF a switch that is provided at a point along a wire that connects the battery to the load. 
     Furthermore, the switch is turned OFF if the current value of a current flowing through the wire is higher than or equal to a current threshold value. This configuration does not allow a current whose current value is higher than or equal to the current threshold value to flow through the wire, and it is accordingly possible to prevent significant performance deterioration of the wire due to an overcurrent. 
     SUMMARY 
     In a configuration in which power is supplied to a load whose resistance value is very small when a current starts to flow and increases as the current flowing time is longer, a large current temporarily flows through the wire when the switch is turned ON from OFF. This current is called “inrush current”. 
     If the current value of the aforementioned current threshold value is set to the current value of the inrush current or smaller, the switch turns ON and then returns to OFF immediately thereafter, and accordingly the load does not operate. To operate the load, the current threshold value needs to be set to a value higher than or equal to the current value of the inrush current. 
     The wire temperature is low when the switch is turned ON from OFF. For this reason, even if an inrush current flows through the wire, the wire performance does not significantly deteriorate. Accordingly, the current threshold value may also exceed the current value of the inrush current immediately after the switch is turned ON from OFF. 
     However, after an inrush current flows, a current has flown through the wire for at least a certain time period, and the wire temperature may exceed a certain temperature. For this reason, after an inrush current flows through the wire, if a current whose current value is the same as that of the inrush current flows through the wire, there is a possibility that the wire performance will significantly deteriorate. Accordingly, after an inrush current flows through the wire, the current threshold value needs to be set to a value smaller than or equal to the current value of the inrush current. 
     An exemplary aspect of the disclosure provides a power supply control apparatus capable of reliably preventing significant performance deterioration of a wire due to an overcurrent, while allowing an inrush current to flow through a wire immediately after a switch is turned ON from OFF. 
     A power supply control apparatus according to one aspect of the present disclosure includes a switching circuit that turns ON or OFF a switch provided at a point along a wire, wherein power supply via the wire is controlled by switching with the switching circuit; a current output circuit configured to output a current whose current value increases as a current value of a current flowing through the wire increases; a resistance circuit through which the current that the current output circuit outputs flows, wherein the resistance circuit includes: a first resistor; and a series circuit of a second resistor and a capacitor that is connected in parallel to the first resistor; and a voltage applying circuit configured to, if an end-to-end voltage value across the resistance circuit becomes higher than or equal to a predetermined voltage value, apply a voltage whose value is higher than the predetermined voltage value, wherein the switching circuit turns OFF the switch if the end-to-end voltage value becomes higher than or equal to the predetermined voltage value. 
     In this aspect of the present disclosure, the current value of the current flowing through the resistance circuit increases as the current value of the current flowing through the wire increases. If the end-to-end voltage value across the resistance circuit is changed from a voltage value lower than a predetermined voltage value to a voltage value higher than or equal to the predetermined voltage value, the switch provided at a point along a wire is turned OFF to stop power supply via the wire. In the resistance circuit, a series circuit of the second resistor and the capacitor is connected in parallel to the first resistor. 
     The current value of the current that flows through the wire when the end-to-end voltage value of the resistance circuit is the predetermined voltage value, that is, a current threshold value depends on the power that is stored in the capacitor. When no power is stored in the capacitor, the resistance value of the resistance circuit is a combined resistance value of the first resistor and the second resistor that are connected in parallel. The resistance value of the resistance circuit increases as power stored in the capacitor increases. The largest value of the resistance value of the resistance circuit corresponds to the resistance value of the first resistor. When power stored in the capacitor is small, the resistance value of the resistance circuit is small. Accordingly, the current value is large, which is calculated by dividing the predetermined voltage value by the resistance value of the resistance circuit, and the current threshold value is also large. When power stored in the capacitor is large, the resistance value of the resistance circuit is large. Accordingly, the current value is small, which is calculated by dividing the predetermined voltage value by the resistance value of the resistance circuit, and the current threshold value is also small. 
     If the switch turns ON from OFF, power stored in the capacitor is small, and the current threshold value is large. For this reason, an inrush current is allowed to flow through the wire immediately after the switch is turned ON from OFF. Furthermore, after the inrush current flows through the wire, power is stored in the capacitor, and the current threshold value is small. Accordingly, significant performance deterioration of the wire due to an overcurrent is reliably prevented. 
     The end-to-end voltage value across the resistance circuit increases as the current value of the current flowing through the wire increases. For this reason, it is possible to calculate relating to the current value of the current flowing through the wire based on the end-to-end voltage value. Furthermore, if the end-to-end voltage value is higher than a predetermined voltage value, a current whose current value is higher than or equal to the current threshold value flows through the wire, and thus enabling to detect that the switch is turned OFF. As mentioned above, based on the end-to-end voltage value, it is possible to perform calculation relating to the current value of the current flowing through the wire, and to detect an OFF state of the switch. 
     In the power supply control apparatus according to one aspect of the present disclosure, the switching circuit keeps the switch OFF until a predetermined time period elapses after the voltage applying circuit stops applying a voltage. 
     In this aspect of the present disclosure, because sufficient power is stored in the capacitor at a time when application of a voltage whose value is higher than the predetermined voltage value is stopped, the end-to-end voltage value across the resistance circuit is higher than or equal to the predetermined voltage value. The switch is kept OFF until the predetermined time period elapses after the application of the voltage whose value is higher than the predetermined voltage value is stopped, and the switch stands by until the end-to-end voltage value becomes lower than the predetermined voltage value by the capacitor discharging the power. Thus, although the current value of the current flowing through the wire is lower than the current threshold value, the switch is not erroneously turned OFF. 
     In the power supply control apparatus according to one aspect of the present disclosure, the switching circuit keeps the switch OFF until the end-to-end voltage value becomes lower than a second predetermined voltage value that is lower than the predetermined voltage value after the voltage applying circuit stops applying a voltage. 
     In this aspect of the present disclosure, because sufficient power is stored in the capacitor at a time when application of a voltage whose value is higher than the predetermined voltage value is stopped, the end-to-end voltage value across the resistance circuit is higher than or equal to the predetermined voltage value. The switch is kept OFF until the end-to-end voltage value becomes lower than the second predetermined voltage value that is lower than the predetermined voltage value after the application of the voltage whose value is higher than the predetermined voltage value is stopped. Accordingly, although the current value of the current flowing through the wire is lower than the current threshold value, the switch is not erroneously turned OFF. 
     The power supply control apparatus according to one aspect of the present disclosure includes an input configured to receive a switching signal that instructs turning ON or OFF of the switch, the switching circuit is configured to turn the switch ON or OFF in accordance with an instruction of a switching signal that is input to the input, the switching circuit is configured to turn OFF the switch regardless of the instruction of the switching signal if the end-to-end voltage value becomes higher than or equal to the predetermined voltage value, and the voltage applying circuit is configured to stop applying the voltage if the instruction of the switching signal is changed from turning ON to turning OFF. 
     In this aspect of the present disclosure, normally, the switch is turned ON or OFF based on the switching signal that is input to the input. If the end-to-end voltage value across the resistance circuit becomes higher than or equal to the predetermined voltage value from a voltage value lower than the predetermined voltage value, the switch is turned OFF regardless of any instruction of the switching signal that is input to the input, and a voltage whose value is higher than the predetermined voltage value is applied across the resistance circuit. If the instruction of the switching signal is changed from turning ON to turning OFF, the application of this voltage is stopped. 
     The power supply control apparatus according to one aspect of the present disclosure includes a signal output circuit configured to output a notification signal indicating stop of the power supply if the end-to-end voltage value is higher than the predetermined voltage value. 
     In this aspect of the present disclosure, if the end-to-end voltage value becomes higher than or equal to the predetermined voltage value, a notification signal is output for notifying stop of power supply via the wire. 
     According to the present disclosure, it is possible to reliably prevent performance deterioration of a wire due to an overcurrent, while allowing an inrush current to flow through the wire immediately after the switch is turned ON from OFF. Furthermore, based on an end-to-end voltage value across the resistance circuit, it is possible to perform calculation relating to the current value of the current flowing through the wire, and to detect an OFF state of the switch. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram showing a configuration of essential parts of a power supply system according to Embodiment 1. 
         FIG. 2  is a diagram illustrating an effect of a resistance circuit. 
         FIG. 3  is a timing chart illustrating the operation of a power supply control apparatus. 
         FIG. 4  is a block diagram showing a configuration of essential parts of a power supply system according to Embodiment 2. 
         FIG. 5  is a timing chart illustrating the operation of a power supply control apparatus. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Hereinafter, the present disclosure will be described in detail based on the drawings showing the embodiments. 
     Embodiment 1 
       FIG. 1  is a block diagram showing a configuration of essential parts of a power supply system  1  according to Embodiment 1. The power supply system  1  is favorably mounted in a vehicle, and includes a power supply control apparatus  10 , a wire  11 , a battery  12 , and a load  13 . The power supply control apparatus  10  is provided at a point along the wire  11 . One end of the wire  11  is connected to a positive electrode of the battery  12 . The other end of the wire  11  is connected to one end of the load  13 . A negative electrode of the battery  12  and the other end of the load  13  are grounded. A communication line L 1  is further connected to the power supply control apparatus  10 . 
     The load  13  is an electrical equipment mounted in a vehicle such as a lamp. The resistance value of the load  13  is very small at a time when a current starts to flow, and increases as the current flowing time period becomes long. Power is supplied to the load  13  from a battery  12  via the wire  11 . Power supply from the battery  12  to the load  13  via the wire  11  is controlled by the power supply control apparatus  10 . If power is supplied from the battery  12  to the load  13 , the load  13  operates, whereas if power supply from the battery  12  to the load  13  is stopped, the load  13  stops operating. 
     An instruction signal that instructs operation of the load  13  or stop of operation of the load  13  is input to the power supply control apparatus  10  via the communication line L 1 . If an instruction signal that instructs stop of the operation of the load  13  is input to the power supply control apparatus  10 , the power supply control apparatus  10  stops supplying power from the battery  12  to the load  13 . As a result, the load  13  stops operating. Also, if an instruction signal that instructs operation of the load  13  is input to the power supply control apparatus  10 , the power supply control apparatus  10  cancels the stop of supplying power from the battery  12  to the load  13  via the wire  11 . As a result, power is supplied to the load  13 , and the load  13  operates. 
     The power supply control apparatus  10  performs a calculation relating to the wire current value of the current flowing through the wire  11 . Depending on the current value of the current flowing through the wire  11  or the calculation result, the power supply control apparatus  10  stops supplying power from the battery  12  to the load  13  regardless of any instruction of the instruction signal. At this time, the power supply control apparatus  10  outputs, via the communication line L 1 , a notification signal indicating that power supply via the wire  11  is stopped regardless of the instruction of the instruction signal. 
     The power supply control apparatus  10  includes a switch  20 , a switching unit  21  (switching circuit), an AND circuit  22 , a voltage output unit  23 , a microcomputer  24 , an inverter  25 , a protection unit  26 , a notification unit  27 , a current output unit  28  (current output circuit), a comparator  29 , a DC power source  30 , and a resistance circuit  31 . The switch  20  is an N-channel FET (Field Effect Transistor). The resistance circuit  31  includes a capacitor C 1 , a first resistor R 1 , and a second resistor R 2 . 
     Each of the switching unit  21 , the inverter  25 , and the notification unit  27  has one input terminal and one output terminal. Each of the AND circuit  22 , the voltage output unit  23 , and the protection unit  26  has two input terminals and one output terminal. The current output unit  28  has one output terminal. The comparator  29  has a plus terminal, a minus terminal, and an output terminal. 
     The switch  20  is provided at a point along the wire  11 . The drain of the switch  20  is connected via the wire  11  to the positive electrode of the battery  12 . The source of the switch  20  is connected via the wire  11  to the one end of the load  13 . The gate of the switch  20  is connected to the output terminal of the switching unit  21 . The input terminal of the switching unit  21  is connected to the output terminal of the AND circuit  22 . One input terminal of the AND circuit  22  is connected to the output terminal of the voltage output unit  23 . One input terminal of the voltage output unit  23  is connected to the microcomputer  24 . 
     The other input terminal of the AND circuit  22  is connected to the output terminal of the inverter  25 . The input terminal of the inverter  25  is connected to the other input terminal of the voltage output unit  23 , the output terminal of the protection unit  26 , and the input terminal of the notification unit  27 . One input terminal of the protection unit  26  is connected to the one input terminal of the voltage output unit  23 . The output terminal of the notification unit  27  is connected to the microcomputer  24 , the output terminal of the current output unit  28 , the plus terminal of the comparator  29 , and one end of both the first resistor R 1  and the second resistor R 2  that the resistance circuit  31  has. The positive electrode of the DC power source  30  is connected to the minus terminal of the comparator  29 . The negative electrode of the DC power source  30  is grounded. The output terminal of the comparator  29  is connected to the other input terminal of the protection unit  26 . In the resistance circuit  31 , the other end of the second resistor R 2  is connected to one end of the capacitor C 1 . The other ends of the first resistor R 1  and the capacitor C 1  are grounded. The communication line L 1  is further connected to the microcomputer  24 . 
     As described above, in the resistance circuit  31 , a series circuit of the second resistor R 2  and the capacitor C 1  is connected in parallel to the first resistor R 1 . The one ends of the first resistor R 1  and the second resistor R 2  correspond to one end of the resistance circuit  31 , and the other ends of the first resistor R 1  and the capacitor C 1  correspond to the other terminal of the resistance circuit  31 . 
     Note, that it is sufficient that the capacitor C 1  and the second resistor R 2  are connected in series, and thus the connection of the capacitor C 1  and the second resistor R 2  may also be reversed. 
     Regarding the switch  20 , if a voltage value of the gate is higher than or equal to a fixed voltage value, a current can flow between the drain and source of the switch  20 . At this time, the switch  20  is ON. If a voltage value of the gate of the switch  20  is lower than the fixed voltage value, no current flows between the drain and source of the switch  20 . At this time, the switch  20  is OFF. 
     The switching unit  21  turns the switch  20  ON or OFF by adjusting a voltage value of the gate of the switch  20 . If the switching unit  21  turns ON the switch  20 , power is supplied from the battery  12  to the load  13  via the wire  11 , and the load  13  operates. If the switching unit  21  turns OFF the switch  20 , power supply from the battery  12  to the load  13  via the wire  11  is stopped, and the load  13  stops operating. 
     The microcomputer  24  outputs, to each of the one input terminals of the voltage output unit  23  and the protection unit  26 , a switching signal that instructs turning ON or turning OFF of the switch  20 . The switching signal is constituted by a high-level voltage and a low-level voltage. A high-level voltage of the switching signal indicates that turning ON of the switch  20  is instructed, whereas a low-level voltage of the switching signal indicates that turning OFF of the switch  20  is instructed. 
     The voltage output unit  23  and the protection unit  26  function as an input unit/input to which a switching signal is input. 
     A high-level voltage or a low-level voltage is input to the other input terminal of the voltage output unit  23  from the output terminal of the protection unit  26 . The voltage output unit  23  outputs, from the output terminal, a high-level voltage or a low-level voltage to the one input terminal of the AND circuit  22 . 
     In the time periods other than an initial time period in which a predetermined time has not yet elapsed after the voltage that the protection unit  26  outputs was switched from a high-level voltage to the low-level voltage, the voltage output unit  23  switches the voltage that it outputs to the AND circuit  22  to a high-level voltage or a low-level voltage according to the voltage of the switching signal. Accordingly, if the voltage of the switching signal is switched to a high-level voltage, the voltage output unit  23  switches the voltage that it outputs to the AND circuit  22  to a high-level voltage, if the voltage of the switching signal is switched to a low-level voltage, the voltage output unit  23  switches the voltage that it outputs to the AND circuit  22  to a low-level voltage. 
     If the voltage that the protection unit  26  outputs is switched from a high-level voltage to a low-level voltage, the voltage output unit  23  switches the voltage that it outputs to the AND circuit  22  to a low-level voltage. During the initial time period, the voltage output unit  23  continues to output a low-level voltage regardless of any instruction of the switching signal. 
     The power supply control apparatus  10  further includes, for example, a timer, and the voltage output unit  23  realizes a constitution in which a low-level voltage is continuously output during the initial time period using the timer. Specifically, if the voltage that the protection unit  26  outputs is switched from a high-level voltage to a low-level voltage, the voltage output unit  23  instructs the timer to start measuring time. The voltage output unit  23  continues to output a low-level voltage until the measured time becomes a predetermined time period or longer. If the measured time becomes the predetermined time period or longer, the voltage output unit  23  instructs the timer to end the time measuring, and switches the voltage that it outputs to the AND circuit  22  to a high-level voltage or a low-level voltage, depending on the voltage of the switching signal. 
     A high-level voltage or a low-level voltage is input to the other input terminal of the protection unit  26  from the output terminal of the comparator  29 . The protection unit  26  outputs, from the output terminal, a high-level voltage or a low-level voltage to the other input terminal of the voltage output unit  23 , the input terminal of the inverter  25 , and the input terminal of the notification unit  27 . 
     If the voltage that is input to the protection unit  26  from the comparator  29  is switched from a low-level voltage to a high-level voltage, the protection unit  26  switches the voltage that it outputs to the voltage output unit  23 , the inverter  25 , and the notification unit  27  to a high-level voltage. Also, if the voltage of the switching signal is switched from a high-level voltage to a low-level voltage, the protection unit  26  switches the voltage that it outputs to the voltage output unit  23 , the inverter  25 , and the notification unit  27  to a low-level voltage. 
     The inverter  25  outputs, from the output terminal, a high-level voltage or a low-level voltage to the other input terminal of the AND circuit  22 . If the voltage that the protection unit  26  outputs is switched to a high-level voltage, the inverter  25  switches the voltage that it outputs to the AND circuit  22  to a low-level voltage, whereas if the voltage that the protection unit  26  outputs is switched to a low-level voltage, the inverter  25  switches the voltage that it outputs to the AND circuit  22  to a high-level voltage. 
     If at least one of the voltages output from the voltage output unit  23  and the inverter  25  is a low-level voltage, the AND circuit  22  outputs a low-level voltage. Furthermore, if both voltages output from the voltage output unit  23  and the inverter  25  are a high-level voltage, the AND circuit  22  outputs a high-level voltage. Accordingly, if the voltage that the inverter  25  outputs is a low-level voltage, that is, if the voltage that the protection unit  26  outputs is a high-level voltage, the AND circuit  22  outputs a low-level voltage regardless of the voltage that the voltage output unit  23  outputs. Similarly, if the voltage that the voltage output unit  23  outputs is a low-level voltage, the AND circuit  22  outputs a low-level voltage regardless of the voltage that the inverter  25  outputs, that is, the voltage that the protection unit  26  outputs. 
     Also, if the voltage that the inverter  25  outputs is a high-level voltage, that is, if the voltage that the protection unit  26  outputs is a low-level voltage, the AND circuit  22 , which operates as mentioned above, outputs the voltage that the voltage output unit  23  outputs to the input terminal of the switching unit  21  as it is from the output terminal. 
     If the voltage that the AND circuit  22  outputs is switched to a high-level voltage, the switching unit  21  turns ON the switch  20 , and if the voltage that the AND circuit  22  outputs is switched to a low-level voltage, the switching unit  21  turns OFF the switch  20 . 
     In the power supply control apparatus  10  configured as mentioned above, in a low-level period in which the protection unit  26  outputs a low-level voltage, during the time periods other than the above-mentioned initial period, the voltage output unit  23  outputs the voltage of the switching signal to the AND circuit  22  as it is, and the AND circuit  22  outputs the voltage that the voltage output unit  23  outputs to the switching unit  21  as it is. For this reason, the switching unit  21  turns the switch  20  ON or OFF in accordance with an instruction of the switching signal that the microcomputer  24  outputs to the voltage output unit  23  and the protection unit  26 . 
     During the initial time period, that is, during a time period in which a predetermined time has not yet elapsed after the voltage that the protection unit  26  outputs was switched from a high-level voltage to the low-level voltage, the voltage output unit  23  continues to output a low-level voltage as mentioned above, and the AND circuit  22  also continues to output a low-level voltage. Accordingly, during the initial time period, the switching unit  21  keeps the switch  20  OFF. 
     Note, that during the initial time period, the voltage that the protection unit  26  outputs is not switched from a low-level voltage to a high-level voltage. 
     If the switch  20  is OFF, the voltage that the comparator  29  outputs to the protection unit  26  is not switched from a low-level voltage to a high-level voltage. During a time period in which a predetermined time has not yet elapsed after the voltage that the protection unit  26  outputs was switched from a high-level voltage to the low-level voltage, the voltage output unit  23  continues to output a low-level voltage, and thus the switching unit  21  does not turn ON the switch  20 . Accordingly, the voltage that the protection unit  26  outputs is not switched to a high-level voltage during the initial time period. 
     If the voltage that the protection unit  26  outputs is switched from a low-level voltage to a high-level voltage, the AND circuit  22  outputs a low-level voltage, and the switching unit  21  turns OFF the switch  20 , regardless of the voltage that the voltage output unit  23  outputs, that is, the voltage of the switching signal. During a high-level period in which the protection unit  26  outputs a high-level voltage, the switching unit  21  keeps the switch  20  OFF. 
     The current output unit  28  is constituted using, for example, a current mirror circuit. The current output unit  28  outputs, to the resistance circuit  31 , a current whose current value is the wire current value of the current flowing the wire  11  divided by a predetermined number. The predetermined number may be, for example, 4000. Accordingly, the current value of the current that the current output unit  28  outputs increases as the wire current value increases. The current that the current output unit  28  outputs flows through the resistance circuit  31 . 
     An end-to-end voltage value of the voltage across the resistance circuit  31  is output to the microcomputer  24  and the plus terminal of the comparator  29 . The end-to-end voltage value is given by (wire current value)×(resistance value of resistance circuit  31 )/(predetermined number). 
     A reference voltage value Vr 1  is input to the minus terminal of the comparator  29  from the DC power source  30 . The reference voltage value Vr 1  is a fixed value and is predetermined. If the end-to-end voltage value is lower than the reference voltage value Vr 1 , the comparator  29  outputs, from the output terminal, a low-level voltage to the other input terminal of the protection unit  26 , whereas if the end-to-end voltage value is higher than or equal to the reference voltage value Vr 1 , the comparator  29  outputs, from the output terminal, a high-level voltage to the other input terminal of the protection unit  26 . 
     The end-to-end voltage value increases as the wire current value increases. If the end-to-end voltage value becomes higher than or equal to the reference voltage value Vr 1 , the comparator  29  switches the voltage that it outputs to the protection unit  26  from a low-level voltage to a high-level voltage. Thus, the protection unit  26  switches the voltage that it outputs to the voltage output unit  23 , the inverter  25 , and the notification unit  27  from a low-level voltage to a high-level voltage. As a result, the AND circuit  22  outputs a low-level voltage regardless of the voltage that the voltage output unit  23  outputs, and the switching unit  21  turns OFF the switch  20 . Accordingly, the wire current value and the current value that the current output unit  28  outputs become substantially 0 A. 
     As mentioned above, if the end-to-end voltage value becomes higher than or equal to the reference voltage value Vr 1 , the protection unit  26  causes the switching unit  21  to turn OFF the switch  20 , thus preventing an overcurrent from flowing through the wire  11 , and protecting the wire  11  from significant performance deterioration. 
     The fact that the end-to-end voltage value is higher than or equal to the reference voltage value Vr 1  means that the wire current value is higher than or equal to a current threshold value that is given by (reference voltage value Vr 1 )×(predetermined number)/(resistance value of the resistance circuit  31 ). The current threshold value is larger the smaller the resistance value of the resistance circuit  31  is. 
       FIG. 2  is a diagram illustrating an effect of the resistance circuit  31 .  FIG. 2  shows graphs of how the switch  20  is turned ON and OFF, the wire current value, and the current threshold value of the wire current value. In  FIG. 2 , the horizontal axis represents time. 
     The resistance value of the resistance circuit  31  depends on the power that is stored in the capacitor C 1 . If the switching unit  21  keeps the switch  20  OFF and no power is stored in the capacitor C 1 , the capacitor C 1  behaves like a conducting wire, and thus the resistance value of the resistance circuit  31  is the combined resistance value of the first resistor R 1  and the second resistor R 2 , which are connected in parallel, and has its smallest value. Accordingly, the current threshold value is the largest. 
     If the switching unit  21  turns the switch  20  from OFF to ON, a current flows from the battery  12  to the load  13  through the wire  11 , and a current is output from the current output unit  28  to the resistance circuit  31 . Accordingly, power is stored in the capacitor C 1  of the resistance circuit  31 , the resistance value of the resistance circuit  31  increases, and the current threshold value decreases. If the voltage value across the capacitor C 1  coincides with the voltage value across the first resistor R 1 , no power is further stored in the capacitor C 1 , all of the current that the current output unit  28  outputs flows through the first resistor R 1 . At this time, the resistance value of the resistance circuit  31  is the largest, and this maximum of the resistance value of the resistance circuit  31  is the resistance value of the first resistor R 1 . 
     As mentioned above, the resistance value of the load  13  is very small at a time when current starts to flow and increases as the time the current flows progresses. Accordingly, immediately after the switching unit  21  turns the switch  20  from OFF to ON, an inrush current flows through the wire  11 , and the current value of the current flowing through the wire  11  temporarily increases. Immediately after the switching unit  21  turns the switch  20  from OFF to ON, the current threshold value is sufficiently large. For this reason, the end-to-end voltage value across the resistance circuit  31  does not become higher than or equal to the reference voltage value Vr 1  even if an inrush current flows through the wire  11 , and thus the switching unit  21  does not turn OFF the switch  20 . Accordingly, flowing of an inrush current is allowed immediately after the switching unit  21  turns the switch  20  from OFF to ON. 
     As mentioned above, if the voltage value across the first resistor R 1  in the resistance circuit  31  coincides with the voltage value of the capacitor C 1 , a current flows through only the first resistor R 1 . At this time, the resistance value of the resistance circuit  31  coincides with the resistance value of the first resistor R 1  and becomes maximal. If the resistance value of the resistance circuit  31  coincides with the resistance value of the first resistor R 1 , the current threshold value is minimal and is lower than the current value of an inrush current. If the wire current value becomes higher than or equal to the current threshold value, the switching unit  21  turns OFF the switch  20 . 
     Let us assume that the resistance value of each of the first resistor R 1  and the second resistor R 2  is 4 kΩ, the reference voltage value Vr 1  is 5V, and the predetermined number is 1000, for example. If no power is stored in the capacitor C 1 , the resistance value of the resistance circuit  31  is the combined resistance value of the first resistor R 1  and the second resistor R 2  that are connected in parallel, that is, 2 kΩ. At this time, the current threshold value is 2.5 A (=5×1000/2000). If the voltage value across the first resistor R 1  coincides with the voltage value across the capacitor C 1 , the resistance value of the resistance circuit  31  is the resistance value of the first resistor R 1 , that is, 4 kΩ. At this time, the current threshold value is 1.25 A (=5×1000/4000). 
     Accordingly, if the switch  20  is turned from OFF to ON in a state where no power is stored in the capacitor C 1 , the current threshold value is gradually decreased from 2.5 A to 1.25 A as time elapses. After the current threshold value becomes 1.25 A, the current threshold value does not decrease and stays at 1.25 A as long as the capacitor C 1  does not discharge power by the switch  20  turning OFF. 
     As described above, after an inrush current flows through the wire  11 , the current threshold value is kept at the smallest value as long as the capacitor C 1  does not discharge power by the switch  20  turning OFF. Because the current threshold value decreases after an inrush current flows through the wire  11 , significant performance deterioration of the wire due to an overcurrent is reliably prevented. 
     If the switch  20  turns from ON to OFF in a state where the protection unit  26  outputs a low-level voltage, the wire current value becomes 0 A, and the capacitor C 1  discharges power. At this time, a current flows from the one end of the capacitor C 1  to the second resistor R 2  and the first resistor R 1  in this order, and returns to the other end of the capacitor C 1 . As the power that is stored in the capacitor C 1  decreases, the current threshold value increases. 
     As described above, if the voltage that the comparator  29  shown in  FIG. 1  outputs is switched from a low-level voltage to a high-level voltage, the voltage that the protection unit  26  outputs is switched from a low-level voltage to a high-level voltage, the switching unit  21  turns OFF the switch  20 , and the current value that the current output unit  28  outputs becomes substantially 0 A. 
     If the voltage that the protection unit  26  outputs is switched from a low-level voltage to a high-level voltage, the notification unit  27  applies a voltage whose value is higher than the reference voltage value Vr 1  across the resistance circuit  31 . At this time, the end-to-end voltage value substantially coincides with a voltage value Vn of the voltage that the notification unit  27  applies across the resistance circuit  31 . The voltage value Vn is a fixed value and is predetermined. Specifically, the voltage value Vn is higher than the largest value of the end-to-end voltage value across the resistance circuit  31  in a case where the notification unit  27  applies no voltage across the resistance circuit  31 . 
     Because the notification unit  27  is grounded the same as the resistance circuit  31 , a voltage whose voltage value is Vn can be applied across the resistance circuit  31 . The illustration of the grounding of the notification unit  27  is omitted. 
     If the end-to-end voltage value is higher than the reference voltage value Vr 1 , the microcomputer  24  detects that the switching unit  21  turns OFF the switch  20  regardless of the voltage of the switching signal. 
     By applying a voltage whose voltage value is the voltage value Vn across the resistance circuit  31 , the notification unit  27  gives notice that the switching unit  21  has turned OFF the switch  20  regardless of the voltage of the switching signal. 
     Note that while the protection unit  26  outputs a high-level voltage, the notification unit  27  applies a voltage across the resistance circuit  31 , and thus the capacitor C 1  discharges no power and the resistance value of the resistance circuit  31  is large. 
     If the notification unit  27  applies no voltage across the resistance circuit  31 , the microcomputer  24  performs a calculation relating to the wire current value, based on the end-to-end voltage value. Specifically, the microcomputer  24  calculates, for example, the wire current value or a temperature value of the wire  11 . 
     As mentioned above, based on the end-to-end voltage value, the microcomputer  24  can perform the aforementioned detection of OFF of the switch  20  and calculation relating to the wire current value. 
     An instruction signal is input to the microcomputer  24  via the communication line L 1 . The microcomputer  24  switches the voltage of the switching signal to a high-level voltage or a low-level voltage, depending on the instruction of the input instruction signal and a calculation result. 
     The microcomputer  24  determines, based on the calculation result, whether the switch  20  is to be turned OFF regardless of the instruction of the instruction signal. If it determines that the switch  20  is to be turned OFF regardless of the instruction of the instruction signal, the microcomputer  24  switches the voltage of the switching signal to a low-level voltage. As a result, both the voltage output unit  23  and the AND circuit  22  output a low-level voltage, and the switching unit  21  turns OFF the switch  20 . 
     If it determines that the switch  20  is not to be turned OFF regardless of the instruction of the instruction signal, the microcomputer  24  switches the voltage of the switching signal to a high-level voltage when the instruction signal instructs the operation of the load  13 , whereas the microcomputer  24  switches the voltage of the switching signal to a low-level voltage when the instruction signal instructs stop of the operation of the load  13 . 
     If the microcomputer  24  determines that the switch  20  is to be turned OFF regardless of the instruction of the instruction signal, or if it determines that the end-to-end voltage value is higher than the reference voltage value Vr 1 , the microcomputer  24  outputs, via the communication line L 1 , a notification signal that indicates that power supply via the wire  11  is stopped regardless of the instruction of the instruction signal. The microcomputer  24  functions as a signal output unit/signal output circuit. 
     The end-to-end voltage value is input to one of the terminals of the microcomputer  24 , and the microcomputer  24  performs a calculation relating to the wire current value and outputs a notification signal based on the end-to-end voltage value that is input to a shared terminal. 
     The notification signal is input to an apparatus (not shown) that is connected to the communication line L 1 . If a notification signal is input to this apparatus, this apparatus gives notice that power supply via the wire  11  is stopped regardless of an instruction signal by lighting a lamp, displaying a message, or the like. 
     Note that the notification signal may also include information indicating that the agent that stops power supply regardless of any instruction of the instruction signal is the microcomputer  24  or the protection unit  26 . A lighting pattern of the lamp or contents of the message may also be changed based on this information. 
       FIG. 3  is a timing chart illustrating the operation of the power supply control apparatus  10 .  FIG. 3  shows the graphs of the voltage of the switching signal, the voltage that the voltage output unit  23  outputs, how the switch  20  is turned ON and OFF, the voltage that the comparator  29  outputs, the voltage that the protection unit  26  outputs, whether the notification unit  27  applies a voltage across the resistance circuit  31 , and the end-to-end voltage value. The horizontal axis of each graph represents time. 
     In  FIG. 3 , a high-level voltage is indicated by “H” and a low-level voltage is indicated by “L”. The fact that the notification unit  27  applies a voltage across the resistance circuit  31  is indicated by “Y”, and the fact that the notification unit  27  applies no voltage across the resistance circuit  31  is indicated by “N”. 
     If the switching signal has a high-level voltage in a state where the protection unit  26  outputs a low-level voltage, the voltage output unit  23  outputs a high-level voltage, and the switching unit  21  turns ON the switch. If the end-to-end voltage value is lower than the reference voltage value Vr 1 , because the comparator  29  outputs a low-level voltage, the protection unit  26  continues to output a low-level voltage, and the notification unit  27  outputs no voltage. 
     If the wire current value increases due to an abnormality, the end-to-end voltage value also increases. If the end-to-end voltage becomes a voltage value higher than or equal to the reference voltage value Vr 1  from a voltage value lower than the reference voltage value Vr 1 , the voltage that the comparator  29  outputs is switched from a low-level voltage to a high-level voltage, and the voltage that the protection unit  26  outputs is also switched from a low-level voltage to a high-level voltage. Accordingly, the AND circuit  22  outputs a low-level voltage regardless of the voltage of the switching signal and the voltage that the voltage output unit  23  outputs, and the switching unit  21  turns OFF the switch  20  regardless of any instruction given by the switching signal that the microcomputer  24  outputs to the voltage output unit  23  and the protection unit  26 . Accordingly, the wire current value and the current value that the current output unit  28  outputs become substantially 0 A. 
     If the voltage that the protection unit  26  is switched from a low-level voltage to a high-level voltage, the notification unit  27  applies a voltage across the resistance circuit  31 . As a result, the end-to-end voltage value increases from the reference voltage value Vr 1  up to the voltage value Vn that the notification unit  27  applies across the resistance circuit  31 . Accordingly, the voltage that the comparator  29  outputs is kept at a high-level voltage even after the switch  20  turns OFF and the wire current value becomes substantially 0 A. As mentioned above, if the end-to-end voltage value is higher than the reference voltage value Vr 1 , the microcomputer  24  outputs a notification signal via the communication line L 1 . The notification unit  27  functions as a voltage applying unit/voltage applying circuit. 
     The protection unit  26  continues to output a high-level voltage until an instruction signal indicating stop of the operation of the load  13  is input to the microcomputer  24  and the voltage of the switching signal is switched from a high-level voltage to a low-level voltage. For this reason, the switch  20  is kept OFF, the notification unit  27  continues to output a voltage, and the end-to-end voltage value is kept at the voltage value Vn. Because the protection unit  26  outputs a high-level voltage and the switching signal has a high-level voltage, the voltage output unit  23  outputs a high-level voltage. 
     If the voltage of the switching signal that the microcomputer  24  outputs to the voltage output unit  23  and the protection unit  26  is switched from a high-level voltage to a low-level voltage, as mentioned above, the voltage that the protection unit  26  outputs is switched from a high-level voltage to a low-level voltage. As a result, the voltage that the voltage output unit  23  outputs is switched from a high-level voltage to a low-level voltage, and also the notification unit  27  stops applying a voltage across the resistance circuit  31 . Because the voltage output unit  23  outputs a low-level voltage, the switch  20  is still OFF. 
     Until a predetermined time period has elapsed after the voltage that the protection unit  26  outputs was switched to a low-level voltage, that is, after the notification unit  27  stopped applying a voltage, the voltage output unit  23  continues to output a low-level voltage, and the switching unit  21  kept the switch  20  OFF regardless of any instruction of the switching signal. 
     As shown in  FIG. 3 , even if the switching signal has a high-level voltage until the predetermined time period has elapsed, the voltage output unit  23  outputs a low-level voltage, and the switch  20  is kept OFF. 
     Because the switch  20  is kept OFF until the predetermined time period has elapsed after the notification unit  27  stopped applying a voltage, no current flows through the wire  11 , and the current output unit  28  outputs no current. Accordingly, in the resistance circuit  31 , the capacitor C 1  starts to discharge power after the notification unit  27  stopped applying a voltage, and thus the end-to-end voltage value gradually decreases from the voltage value Vn. The predetermined time period is set to a time period that is longer than the time period for which the end-to-end voltage value Vn is lower than the reference voltage value Vr 1  by the capacitor C 1  discharging power. 
     If the end-to-end voltage value becomes lower than the reference voltage value Vr 1 , the comparator  29  switches the voltage that it outputs to the protection unit  26  from a high-level voltage to a low-level voltage. 
     When the predetermined time period has elapsed, the voltage output unit  23 , the protection unit  26 , and the comparator  29  output a low-level voltage, the switch  20  is OFF, the notification unit  27  stops applying a voltage, and the end-to-end voltage value is lower than the reference voltage value Vr 1 . 
     Accordingly, although the wire current value is lower than the current threshold value, the end-to-end voltage value is higher than or equal to the reference voltage value Vr 1 , and thus the switching unit  21  does not erroneously turn OFF the switch  20 . 
     After the predetermined time period has elapsed, the voltage output unit  23  switches the voltage that it outputs to the AND circuit  22  according to the voltage of the switching signal, and the switching unit  21  turns the switch  20  ON or OFF according to the instruction of the switching signal. As shown in  FIG. 3 , if the voltage of the switching signal is switched to a high-level voltage, the voltage that the voltage output unit  23  outputs to the AND circuit  22  is switched to a high-level voltage, and the switch  20  turns ON. As a result, the end-to-end voltage value across the resistance circuit  31  gradually increases. If the voltage of the switching signal is switched to a low-level voltage, the voltage that the voltage output unit  23  outputs to the AND circuit  22  is switched to a low-level voltage, and the switch  20  turns OFF. As a result, the end-to-end voltage value across the resistance circuit  31  gradually decreases. 
     Embodiment 2 
     In Embodiment 1, the end-to-end voltage value is decreased to a voltage value lower than the reference voltage value Vr 1  by keeping the switch  20  OFF until the predetermined time has elapsed after the notification unit  27  has stopped applying a voltage. The configuration in which the end-to-end voltage value is decreased to a voltage value lower that the reference voltage value Vr 1  is not limited to a configuration in which the switch  20  is kept OFF until a predetermined time has elapsed. 
     Hereinafter, aspects of Embodiment 2 that are different from those of Embodiment 1 will be described. Because structures other than those described below are the same as those of Embodiment 1, the same reference numerals as those in Embodiment 1 are attached to the constituent units that are the same as Embodiment 1, and their further description is omitted. 
       FIG. 4  is a block diagram showing a configuration of essential parts of a power supply system  1  according to Embodiment 2. Compared with the power supply system  1  in Embodiment 1, the power supply system  1  according to Embodiment 2 differs in the configuration of the power supply control apparatus  10 . 
     In addition to constituent units of the power supply control apparatus  10  according to Embodiment 1, the power supply control apparatus  10  according to Embodiment 2 further has a comparator  40  and a DC power source  41 . Similar to the comparator  29 , the comparator  40  has a plus terminal, a minus terminal, and an output terminal. A voltage output unit  23  according to Embodiment 2 has three input terminals and one output terminal. 
     Similar to Embodiment 1, first and second input terminals of the voltage output unit  23  are respectively connected to the microcomputer  24  and the output terminal of the protection unit  26 . A third input terminal of the voltage output unit  23  is connected to the output terminal of the comparator  40 . The plus terminal of the comparator  40  is connected to the one end of both the first resistor R 1  and the second resistor R 2  of the resistance circuit  31 . The minus terminal of the comparator  40  is connected to a positive electrode of the DC power source  41 . A negative electrode of the DC power source  41  is grounded. 
     An end-to-end voltage value across the resistance circuit  31  is input to the plus terminal of the comparator  40 . A second reference voltage value Vr 2  is input to the minus terminal of the comparator  40  from the DC power source  41 . The second reference voltage value Vr 2  is a fixed value, which is predetermined. The second reference voltage value Vr 2  is lower than the reference voltage value Vr 1 , and is higher than the end-to-end voltage value in a state where the wire current value is in a steady state. 
     If the end-to-end voltage value is lower than the second reference voltage value Vr 2 , the comparator  40  outputs, from the output terminal, a low-level voltage to the third input terminal of the voltage output unit  23 , whereas if the end-to-end voltage value is higher than or equal to the second reference voltage value Vr 2 , the comparator  40  outputs, from the output terminal, a high-level voltage to the third input terminal of the voltage output unit  23 . 
     As mentioned above, the second reference voltage value Vr 2  is lower than the reference voltage value Vr 1 . Accordingly, if the comparator  29  outputs a high-level voltage, the comparator  40  also outputs a high-level voltage. As mentioned in Embodiment 1, if the voltage that the comparator  29  outputs is switched from a low-level voltage to a high-level voltage, the voltage that the protection unit  26  outputs is switched from a low-level voltage to a high-level voltage. Also, if the voltage of the switching signal is switched from a high-level voltage to a low-level voltage, the voltage that the protection unit  26  outputs is switched from a high-level voltage to a low-level voltage. 
     Similar to Embodiment 1, if the voltage that the protection unit  26  outputs is switched from a high-level voltage to a low-level voltage, the voltage output unit  23  switches the voltage that it outputs to the AND circuit  22  to a low-level voltage. The voltage output unit  23  continues to output a low-level voltage regardless of any instruction of the switching signal until the voltage that the comparator  40  outputs is switched to a low-level voltage after the voltage that the protection unit  26  outputs was switched to a low-level voltage. 
     In the time periods other than the time period until the voltage that the comparator  40  outputs is switched to a low-level voltage after the voltage that the protection unit  26  outputs was switched to a low-level voltage, the voltage output unit  23  switches the voltage that it outputs to the AND circuit  22  to a high-level voltage or a low-level voltage depending on the voltage of the switching signal. 
       FIG. 5  is a timing chart illustrating the operation of the power supply control apparatus  10 .  FIG. 5  shows the graphs of the voltage of the switching signal, the voltage that the voltage output unit  23  outputs, how the switch  20  is turned ON and OFF, voltages that the comparators  29  and  40  output, the voltage that the protection unit  26  outputs, whether the notification unit  27  applies a voltage across the resistance circuit  31 , and the end-to-end voltage value. The horizontal axis of each graph represents time. 
     Also in  FIG. 5 , similar to  FIG. 3 , a high-level voltage is indicated by “H” and a low-level voltage is indicated by “L”. The fact that the notification unit  27  applies a voltage across the resistance circuit  31  is indicated by “Y”, and the fact that the notification unit  27  applies no voltage across the resistance circuit  31  is indicated by “N”. 
     Compared with the operation of the power supply control apparatus  10  according to Embodiment 1 shown in  FIG. 3 , the operation of the power supply control apparatus  10  according to Embodiment 2 shown in  FIG. 5  differs in that the operation of the voltage output unit  23  is different in a case where the voltage that the protection unit  26  outputs is switched from a high-level voltage to a low-level voltage. 
     If the end-to-end voltage value becomes a voltage value higher than or equal to the second reference voltage value Vr 2  from a voltage value lower than the second reference voltage value Vr 2 , the comparator  40  switches the voltage that it outputs to the voltage output unit  23  from a low-level voltage to a high-level voltage. As shown in  FIG. 5 , at a time when the voltage that the protection unit  26  outputs is switched from a high-level voltage to a low-level voltage, that is, at a time when the end-to-end voltage value becomes higher than or equal to the reference voltage value Vr 1 , the comparator  40  outputs a high-level voltage. 
     If the voltage that the protection unit  26  outputs is switched from a high-level voltage to a low-level voltage, the voltage that the voltage output unit  23  outputs is switched from a high-level voltage to a low-level voltage, and the notification unit  27  stops applying a voltage across the resistance circuit  31 . Because the voltage output unit  23  outputs a low-level voltage, the switch  20  is still OFF. 
     Until the voltage that the comparator  40  outputs is switched to a low-level voltage after the voltage that the protection unit  26  outputs was switched to a low-level voltage, that is, after the notification unit  27  has stopped applying a voltage, the voltage output unit  23  continues to output a low-level voltage, and the switching unit  21  keeps the switch  20  OFF regardless of any instruction of the switching signal. 
     As shown in  FIG. 5 , even if the switching signal has a high-level voltage, the voltage output unit  23  outputs a low-level voltage and the switch  20  is kept OFF until the voltage that the comparator  40  outputs is switched to a low-level voltage. 
     If the notification unit  27  stops applying a voltage, and while the switch  20  is OFF, because no current flows through the wire  11  and the current output unit  28  outputs no current, in the resistance circuit  31 , the capacitor C 1  discharges power and the end-to-end voltage value gradually decreases from the voltage value Vn. 
     After the end-to-end voltage value becomes lower than the second reference voltage value Vr 2  and the voltage that the comparator  40  outputs is switched to a low-level voltage, the voltage output unit  23  switches the voltage that it outputs to the AND circuit  22  according to the voltage of the switching signal, and the switching unit  21  turns the switch  20  ON or OFF according to the instruction of the switching signal. The operation shown in  FIG. 5  that the power supply control apparatus  10  according to Embodiment 2 performs after the voltage that the comparator  40  outputs was switched to a low-level voltage is similar to the operation shown in  FIG. 3  that the power supply control apparatus  10  according to Embodiment 1 performs after the predetermined time period has elapsed. 
     As mentioned above, the second reference voltage value Vr 2  is lower than the reference voltage value Vr 1 . Accordingly, at a time when the voltage that the comparator  40  outputs is switched from a high-level voltage to a low-level voltage, the end-to-end voltage value is certainly lower than the reference voltage value Vr 1 . 
     Therefore, although the wire current value is lower than the current threshold value, the end-to-end voltage value is higher than or equal to the reference voltage value Vr 1 , and thus the switching unit  21  does not erroneously turn OFF the switch  20 . 
     In the power supply control apparatus  10  according to Embodiment 2, structures other than the structure by which the end-to-end voltage value is decreased to a voltage value lower than the reference voltage value Vr 1  are similar to those of the power supply control apparatus  10  according to Embodiment 1. Therefore, the power supply control apparatus  10  according to Embodiment 2 provides similar effects as the power supply control apparatus  10  according to Embodiment 1, other than the effect that can be provided by the switch  20  being kept OFF until the predetermined time period has elapsed after the notification unit  27  stopped applying a voltage. 
     Note, that in Embodiment 2, the second reference voltage value Vr 2  may also be lower than or equal to the end-to-end voltage value if the wire current value is in a steady state. It is sufficient that the second reference voltage value Vr 2  is a voltage value at which the wire current value does not exceed the current threshold value by an inrush current even if the switch  20  turns ON at the same time when keeping OFF of the switch  20 , which is performed by the switching unit  21  regardless of any instruction of the switching signal, is canceled. In other words, it is sufficient that the current threshold value of the wire current value if the end-to-end voltage value is the second reference voltage value Vr 2  is larger than the largest value of the current value of the inrush current. 
     Note, that in Embodiments 1 and 2, the switch  20  is not limited to an N-channel FET, and may also be, for example, a P-channel FET, bipolar transistor or a relay contact. 
     Embodiments 1 and 2 disclosed above are examples in all aspects, and should be considered to be non-restrictive. The scope of the present disclosure is defined not by the above-mentioned explanations but by the claims, and is intended to include all modifications within the meanings and scope equivalent to the claims.