LATCH CIRCUIT AND POWER SUPPLY CONTROL DEVICE

In a first switch of a latch circuit, when the voltage between an emitter (input end) and a base (control end) has increased to a threshold voltage or more, the first switch is switched from off to on. The current that flows through a first resistor and a second resistor in that order is input to a collector (input end) of a second switch and a comparator switch. The second switch is switched on when the first switch is switched on. When the second switch or the comparator switch (third switch) is on, the voltage across the first resistor is the threshold voltage or more. A voltage is input to the emitter of the first switch from a microcomputer. A voltage is output from the collector of the second switch to a driving circuit.

TECHNICAL FIELD

The present disclosure relates to a latch circuit and a power supply control device.

BACKGROUND

In JP 2002-290223A, a latch circuit that is used in a vehicle to fix an output voltage is disclosed. A high-level voltage or a low-level voltage is input to the latch circuit. The latch circuit usually outputs the low-level voltage. When the voltage input to the latch circuit is switched from the low-level voltage to the high-level voltage, the latch circuit switches the output voltage from the low-level voltage to the high-level voltage. Thereafter, the latch circuit fixes the output voltage to the high-level voltage irrespective of the input voltage.

A specific configuration of a latch circuit in which one or more switches are used is not disclosed in JP 2002-290223A. In order to realize a small-sized latch circuit that can be easily installed in a vehicle, it is preferable to reduce the number of switches included in the latch circuit.

Therefore, the present disclosure aims to provide a latch circuit and a power supply control device in which the number of switches needed to fix an output voltage to a predetermined voltage is small.

SUMMARY

A latch circuit according to one aspect of the present disclosure is a latch circuit that fixes an output voltage to a predetermined voltage when a predetermined condition is satisfied. The latch circuit includes: a first switch that has an input end to which a current is input, an output end from which a current is output, and a control end, and is switched from off to on when the voltage between the input end and the control end has increased to a threshold voltage or more; a first resistor that is connected between the input end and the control end; a second resistor whose one end is connected to the control end; a second switch that has a second input end to which a resistor current that flows through the first resistor and second resistor in that order is input, and is switched on when the first switch is switched on; and a third switch to which the resistor current is input. When the second switch or third switch is on, the voltage across the first resistor is the threshold voltage or more, a voltage is input to the input end of the first switch, and a voltage is output from the second input end of the second switch.

A power supply control device according to one aspect of the present disclosure is a power supply control device that controls power supply through a power supply switch. The power supply control device includes: a voltage adjusting unit that is configured to output a voltage, and adjust the voltage to be output (output voltage); a latch circuit configured to output a voltage according to an output voltage of the voltage adjusting unit until a predetermined condition is satisfied, and fix its output voltage to a predetermined voltage when a predetermined condition is satisfied; and a switching circuit configured to switch the power supply switch on or off according to the output voltage of the latch circuit. The latch circuit includes: a first switch that has an input end to which a current is input, an output end from which a current is output, and a control end, and is switched from off to on when the voltage between the input end and the control end has increased to a threshold voltage or more; a first resistor that is connected between the input end and the control end; a second resistor whose one end is connected to the control end; a second switch that has a second input end to which a resistor current that flows through the first resistor and second resistor in that order is input, and is switched on when the first switch is switched on; and a third switch to which the resistor current is input. When the second switch or third switch is on, the voltage across the first resistor increases. The output voltage of the voltage adjusting unit is input to the input end of the first switch. A voltage is output to the switching circuit from the second input end of the second switch.

Advantageous Effects

According to the present disclosure, the number of switches needed to fix an output voltage to a predetermined voltage is small.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

First, modes for carrying out the disclosure will be enumerated and described. At least some of the modes described below may be combined as necessary.

A latch circuit according to one aspect of the present disclosure is a latch circuit that fixes an output voltage to a predetermined voltage when a predetermined condition is satisfied. The latch circuit includes: a first switch that has an input end to which a current is input, an output end from which a current is output, and a control end, and is switched from off to on when the voltage between the input end and the control end has increased to a threshold voltage or more; a first resistor that is connected between the input end and the control end; a second resistor whose one end is connected to the control end; a second switch that has a second input end to which a resistor current that flows through the first resistor and second resistor in that order is input, and is switched on when the first switch is switched on; and a third switch to which the resistor current is input. When the second switch or third switch is on, the voltage across the first resistor is the threshold voltage or more. A voltage is input to the input end of the first switch. A voltage is output from the second input end of the second switch.

In the aspect above, when the first switch, second switch, and third switch are off, a voltage input to the input end of the first switch is output through the first resistor and second resistor. One end of the second switch from which a current is output is denoted as a second output end. Assume that the third switch is switched on in a state in which the voltage at the input end of the first switch relative to the potential at the second output end (the potential at the second output end being the reference potential) is a fixed voltage or more. When the third switch is on, the voltage across the first resistor is the threshold voltage or more, and the first switch is on. When the first switch is switched on, the second switch is switched on.

When the first switch is on, the second switch is on. When the second switch is on, the first switch is on. Therefore, even when the third switch is switched from on to off, the second switch is kept on, as long as the voltage at the input end of the first switch relative to the potential at the second output end of the second switch is the fixed voltage or more. As a result, the output voltage is fixed to the predetermined voltage. The predetermined voltage is a voltage at the second output end of the second switch. The number of switches needed to fix the output voltage to the predetermined voltage is three, which is a small number.

The latch circuit according to one aspect of the present disclosure further includes a third resistor and a fourth resistor. The second switch further has a second output end from which a current is output, and a second control end. The second switch is switched from off to on when the voltage between the second output end and second control end has increased to a second threshold voltage or more. The third resistor is connected between the output end of the first switch and the second control end of the second switch. The fourth resistor is connected between the second control end and second output end of the second switch. A current flows through the first switch, third resistor, and fourth resistor in that order.

In the aspect above, in the case where the voltage at the input end of the first switch relative to the potential at the second output end of the second switch is the fixed voltage, when the first switch is on, a current flows through the first switch, third resistor, and fourth resistor in that order. Accordingly, there is a voltage drop at the fourth resistor. When the first switch is switched on, the voltage between the second control end and second output end of the second switch increases to the second threshold voltage or more. As a result, the second switch is switched on.

A power supply control device according to one aspect of the present disclosure is a power supply control device that controls power supply through a power supply switch. The power supply control device includes: a voltage adjusting unit that is configured to output a voltage, and adjust the voltage to be output (output voltage); a latch circuit configured to output a voltage according to an output voltage of the voltage adjusting unit until a predetermined condition is satisfied, and fix its output voltage to a predetermined voltage when a predetermined condition is satisfied; and a switching circuit configured to switch the power supply switch on or off according to the output voltage of the latch circuit. The latch circuit includes: a first switch that has an input end to which a current is input, an output end from which a current is output, and a control end, and is switched from off to on when the voltage between the input end and the control end has increased to a threshold voltage or more; a first resistor that is connected between the input end and the control end; a second resistor whose one end is connected to the control end; a second switch that has a second input end to which a resistor current that flows through the first resistor and second resistor in that order is input, and is switched on when the first switch is switched on; and a third switch to which the resistor current is input. When the second switch or third switch is on, the voltage across the first resistor increases. The output voltage of the voltage adjusting unit is input to the input end of the first switch. A voltage is output to the switching circuit from the second input end of the second switch.

In the aspect above, the latch circuit operates as described above. Therefore, when the first switch, second switch, and third switch are off, the output voltage of the voltage adjusting unit is output through the first resistor and second resistor of the latch circuit. The switching circuit switches the power supply switch on or off according to the output voltage of the voltage adjusting unit. In the case where the voltage at the input end of the first switch relative to the potential at the second output end of the second switch is the fixed voltage or more, when the third switch is switched on, the output voltage of the latch circuit is fixed to the predetermined voltage. As a result, the switching circuit fixes the state of the power supply switch to a state corresponding to the predetermined voltage. The number of switches needed to fix the output voltage of the latch circuit to the predetermined voltage is three, which is a small number.

In the power supply control device according to one aspect of the present disclosure, the latch circuit includes a second switching circuit configured to switch the third switch on or off, the second switching circuit switches the third switch on when the temperature difference between a wire temperature of a wire disposed on a current path of a current flowing through the power supply switch and an ambient temperature in the vicinity of the wire has increased to a predetermined temperature difference or more, and the predetermined voltage is a voltage to instruct to switch the power supply switch off.

In the aspect above, when the temperature difference regarding the wire has increased to the predetermined temperature difference or more, the third switch is switched on, and the output voltage of the latch circuit is fixed to the predetermined voltage. As a result, the power supply switch is kept off. The wire temperature of the wire is prevented from increasing to an abnormal temperature.

The power supply control device according to one aspect of the present disclosure further includes: a second switching circuit configured to switch the third switch on or off: a current output circuit configured to increase a current to be output as the current flowing through the power supply switch increases; and a temperature difference circuit configured to increase a voltage to be output as the temperature difference between a wire temperature of a wire disposed on a current path of a current flowing through the power supply switch and an ambient temperature in the vicinity of the wire increases. The output voltage of the temperature difference circuit is generated based on an output current of the current output circuit. The second switching circuit switches the third switch on when the output voltage of the temperature difference circuit has increased to a given voltage or more. The predetermined voltage is a voltage to instruct to switch the power supply switch off.

In the aspect above, when the temperature difference regarding the wire has increased to the predetermined temperature difference or more, the output voltage of the temperature difference circuit increases to the given voltage or more. Therefore, when the temperature difference regarding the wire has increased to the predetermined temperature difference or more, the third switch is switched on, and the output voltage of the latch circuit is fixed to the predetermined voltage. As a result, the power supply switch is kept off. The wire temperature of the wire is prevented from increasing to an abnormal temperature.

Specific examples of a power supply system according to embodiments of the present disclosure will be described below with reference to the drawings. Note that the present disclosure is not limited to these illustrative examples and is defined by the claims, and all changes that come within the meaning and range of equivalency of the claims are intended to be embraced therein.

Configuration of Power Supply System

FIG.1is a block diagram illustrating the main configuration of a power supply system1in Embodiment 1. The power supply system1is installed in a vehicle M. The power supply system1includes a DC power supply10, a load11, and a power supply control device12. The DC power supply10is a battery, for example. The load11is an electric device. When power is supplied to the load11, the load11starts operating. When power supply to the load11stops, the load11stops operating.

The power supply control device12includes a power supply switch20. The power supply switch20is an N-channel FET. FET is an abbreviation of “Field Effect Transistor”. When the power supply switch20is on, the resistance between the drain and source of the power supply switch20is sufficiently small. As a result, a current can flow through the drain and source thereof. When the power supply switch20is off, the resistance between the drain and source of the power supply switch20is sufficiently large. As a result, no current will flow through the drain and source thereof.

The drain of the power supply switch20is connected to a positive electrode of the DC power supply10. The source of the power supply switch20is connected to one end of the load11through a wire W. The negative electrode of the DC power supply10and the other end of the load11are grounded. Grounding is realized by the connection to a body of the vehicle M, for example.

When the power supply switch20is switched from off to on, a current flows from the positive electrode of the DC power supply10to the power supply switch20, the wire W, and the load11in that order, and power is supplied to the load11. Accordingly, the load11starts operating. When the power supply switch20is switched from on to off, power supply from the DC power supply10to the load11stops, and the load11stops operating. The power supply control device12controls power supply from the DC power supply10to the load11through the power supply switch20by switching the power supply switch20on or off.

Configuration of Power Supply Control Device12

The power supply control device12includes a driving circuit21, a latch circuit22, a device resistor23, a temperature detection circuit24, and a microcomputer25, in addition to the power supply switch20. The driving circuit21includes a voltage output end (output end) and a voltage input end (input end). The gate of the power supply switch20is connected to the output end of the driving circuit21. The input end of the driving circuit21is connected to the latch circuit22and one end of the device resistor23. The other end of the device resistor23is grounded. The latch circuit22is further connected to the temperature detection circuit24and the microcomputer25.

FIG.2is a diagram illustrating the arrangement of a plurality of constituent units included in the power supply control device12. The power supply control device12further includes a switch circuit board Bs and a control circuit board Bc. The power supply switch20, the driving circuit21, the latch circuit22, the device resistor23, and the temperature detection circuit24are disposed on the switch circuit board Bs. The arrangement of a circuit indicates the arrangement of one or more circuit elements constituting the circuit. The microcomputer25is disposed on the control circuit board Bc. The microcomputer25is an integrated circuit device, for example. The microcomputer25is connected to the latch circuit22through a cable F.

As shown inFIG.1, the temperature detection circuit24includes a thermistor30and a detection resistor31. The thermistor30is an NTC (negative temperature coefficient) thermistor. Therefore, as the temperature of the thermistor30increases, the resistance of the thermistor30decreases. One end of the thermistor30is connected to one end of the detection resistor31. A fixed voltage Vc is applied to the other end of the thermistor30. The fixed voltage Vc is a voltage relative to the ground potential (the ground potential being the reference potential). The other end of the detection resistor31is grounded. The connection node between the thermistor30and the detection resistor31is connected to the latch circuit22.

The fixed voltage Vc is generated by a regulator, which is not illustrated, for example. The regulator generates the fixed voltage Vc by stepping down the output voltage of the DC power supply10. The output voltage of the DC power supply10is a voltage that fluctuates in a range between 8 V to 12 V, for example. The fixed voltage Vc is 5 V, for example. The regulator continuously generates the fixed voltage Vc by adjusting the stepping down amount.

The fixed voltage Vc is divided by the thermistor30and the detection resistor31, and the resultant divided voltage is output to the latch circuit22as an output voltage of the temperature detection circuit24. The output voltage of the temperature detection circuit24is determined by the resistance ratio between the thermistor30and the detection resistor31. As the resistance of the thermistor30decreases, that is, as the temperature of the thermistor30increases, the output voltage of the temperature detection circuit24increases.

The thermistor30is disposed on the switch circuit board Bs. Therefore, as the temperature of the switch circuit board Bs increases, the temperature of the thermistor30increases. As the temperature of the thermistor30increases, the output voltage of the temperature detection circuit24increases. Therefore, as the temperature of the switch circuit board Bs increases, the output voltage of the temperature detection circuit24increases.

As described above, the temperature detection circuit24detects the temperature of the switch circuit board Bs, and adjusts the output voltage to a voltage according to the detected temperature.

The microcomputer25outputs a voltage to the latch circuit22. The microcomputer25switches its output voltage between the fixed voltage Vc and 0 V. The microcomputer25functions as a voltage adjusting unit for adjusting the output voltage. If the output voltage of the temperature detection circuit24is less than a fixed reference voltage, the latch circuit22outputs a voltage according to the output voltage of the microcomputer25to the driving circuit21. Here, as the output voltage of the microcomputer25increases, the output voltage of the latch circuit22increases. The output voltage of the latch circuit22is applied across the device resistor23. The reference voltage is larger than 0 V.

When the output voltage of the latch circuit22has increased to a fixed output threshold voltage or more, the driving circuit21switches the power supply switch20from off to on. Accordingly, a current flows through the power supply switch20and the wire W, and power is supplied to the load11. The output threshold voltage is larger than 0 V. When the output voltage of the latch circuit22has decreased below the output threshold voltage, the driving circuit21switches the power supply switch20from on to off. Accordingly, power supply to the load11through the power supply switch20and the wire W stops. The driving circuit21functions as a switching circuit.

When the gate voltage relative to the source potential is a fixed switch threshold voltage or more, the power supply switch20is on. When the gate voltage relative to the source potential is less than the switch threshold voltage, the power supply switch20is off. The switch threshold voltage is larger than 0 V.

When switching the power supply switch20on, the driving circuit21increases the gate voltage of the power supply switch20relative to the ground potential. Accordingly, the gate voltage of the power supply switch20relative to the source potential increases to the switch threshold voltage or more. When switching the power supply switch20off, the driving circuit21decreases the gate voltage of the power supply switch20relative to the ground potential. Accordingly, the gate voltage of the power supply switch20relative to the source potential decreases below the switch threshold voltage.

When the output voltage of the temperature detection circuit24is less than the reference voltage, if the output voltage of the microcomputer25is the fixed voltage Vc, the output voltage of the latch circuit22is the output threshold voltage or more. Therefore, the driving circuit21keeps the power supply switch20on. In a similar case, if the output voltage of the microcomputer25is 0 V, the output voltage of the latch circuit22is 0 V, and is less than the output threshold voltage. Therefore, the driving circuit21keeps the power supply switch20off.

As described above, when the power supply switch20is on, a current flows through the power supply switch20. When a current flows through the power supply switch20, the power supply switch20generates heat. When the heat generation amount per unit time is larger than the heat dissipation amount per unit time, the temperature of the power supply switch20increases. When the temperature of the power supply switch20increases, the temperature of the switch circuit board Bs increases. When the temperature of the switch circuit board Bs increases, the output voltage of the temperature detection circuit24increases.

When the power consumed in the power supply switch20increases, the heat generation amount of the power supply switch20increases. The power consumed in the power supply switch20is represented by the product of the square of the current flowing through the power supply switch20and the on-resistance of the power supply switch20. Therefore, as the current flowing through the power supply switch20increases, the heat generation amount increases.

When the output voltage of the microcomputer25is the fixed voltage Vc, if the output voltage of the temperature detection circuit24has increased to the reference voltage or more, the latch circuit22forcibly reduces its output voltage to 0 V. Accordingly, the driving circuit21forcibly switches the power supply switch20off. When the power supply switch20is switched off, the temperature of the power supply switch20decreases. When the temperature of the power supply switch20decreases, the temperature of the switch circuit board Bs decreases. When the temperature of the switch circuit board Bs decreases, the output voltage of the temperature detection circuit24decreases.

After forcibly reducing its output voltage to 0 V, the latch circuit22fixes the output voltage to 0 V (predetermined voltage), until the output voltage of the microcomputer25switches to 0 V. Therefore, the driving circuit21keeps the power supply switch20off in a period from when the latch circuit22has forcibly reduced the output voltage to 0 V until when the output voltage of the microcomputer25is switched to 0 V. Since the output threshold voltage is larger than 0 V as mentioned above, an output voltage of 0 V of the latch circuit22is the voltage to instruct to switch the power supply switch20off.

As described above, with the power supply control device12, the temperature of the switch circuit board Bs is prevented from increasing to an abnormal temperature. When the temperature of the switch circuit board Bs increases to an abnormal temperature, the circuit elements disposed on the switch circuit board Bs may not operate properly.

Note that the configuration of the temperature detection circuit24needs only be such that its output voltage increases when the temperature of the switch circuit board Bs increases. Therefore, the configuration of the temperature detection circuit24is not limited to a configuration in which the thermistor30and the detection resistor31are used. A detection resistor may be used in place of the thermistor30, and a PTC (positive temperature coefficient) thermistor may be used in place of the detection resistor31, for example. The resistance of a PTC thermistor increases as the temperature of the PTC thermistor increases.

Configuration of Latch Circuit22

FIG.3is a circuit diagram of the latch circuit22. The latch circuit22includes a first switch40, a second switch41, a first resistor42, a second resistor43, a third resistor44, a fourth resistor45, a comparator46, a first voltage-dividing resistor47, and a second voltage-dividing resistor48. The first switch40is a PNP bipolar transistor. The second switch41is an NPN bipolar transistor. The comparator46has a plus terminal and a minus terminal. The first switch40and the second switch41each have a collector, an emitter, and a base.

The emitter of the first switch40is connected to the microcomputer25. The first resistor42is connected between the base and emitter of the first switch40. One end of the second resistor43is also connected to the base of the first switch40. The other end of the second resistor43is connected to the collector of the second switch41. The third resistor44is connected between the collector of the first switch40and the base of the second switch41. The fourth resistor is connected between the base and emitter of the second switch41. The emitter of the second switch41is grounded. The collector of the second switch41is connected to the input end of the driving circuit21and the one end of the device resistor23.

In the latch circuit22, the output voltage of the microcomputer25is input to the emitter of the first switch40. The output voltage of the latch circuit22is output from the collector of the second switch41to the driving circuit21.

The comparator46includes a comparator switch50. One end of the comparator switch50is connected to the collector of the second switch41. The other end of the comparator switch50is grounded. One end of the first voltage-dividing resistor47is connected to one end of the second voltage-dividing resistor48. The fixed voltage Vc is applied to the other end of the first voltage-dividing resistor47. The other end of the second voltage-dividing resistor48is grounded. The connection node between the first voltage-dividing resistor47and the second voltage-dividing resistor48is connected to the plus terminal of the comparator46. The minus terminal of the comparator46is connected to the temperature detection circuit24.

If a current flows from the microcomputer25through the first resistor42and the second resistor43in that order, when the voltage between the emitter and base of the first switch40has increased to a fixed threshold voltage or more, the first switch40is switched from off to on. The threshold voltage is larger than 0 V. When the first switch40is on, the resistance between the emitter and collector of the first switch40is sufficiently small, and a current can flow through the emitter and collector.

In a similar case, when the voltage between the emitter and base of the first switch40has decreased below the threshold voltage, the first switch40is switched from on to off. When the first switch40is off, the resistance between the emitter and collector of the first switch40is sufficiently large, and no current will flow through the emitter and collector.

If a current flows from the microcomputer25through the first switch40, the third resistor44, and the fourth resistor45in that order, when the voltage between the emitter and base of the second switch41has increased to a fixed second threshold voltage or more, the second switch41is switched from off to on. The second threshold voltage is larger than 0 V. When the second switch41is on, the resistance between the emitter and collector of the second switch41is sufficiently small, and a current can flow through the collector and emitter.

In a similar case, when the voltage between the emitter and base of the second switch41has decreased below the second threshold voltage, the second switch41is switched from on to off. When the second switch41is off, the resistance between the emitter and collector of the second switch41is sufficiently large, and no current will flow through the collector and emitter.

The first voltage-dividing resistor47and second voltage-dividing resistor48divide the fixed voltage Vc, and output the divided voltage to the plus terminal of the comparator46as a reference voltage Vr. The reference voltage Vr is determined by the resistance ratio between the first voltage-dividing resistor47and second voltage-dividing resistor48. The resistances of the first voltage-dividing resistor47and second voltage-dividing resistor48are fixed values, and therefore the reference voltage Vr is a fixed voltage. The reference voltage Vr is smaller than the fixed voltage Vc. The temperature detection circuit24outputs the output voltage to the minus terminal of the comparator46.

When the output voltage of the temperature detection circuit24has increased to the reference voltage Vr or more, the comparator46switches the comparator switch50on. When the output voltage of the temperature detection circuit24has decreased below the reference voltage Vr, the comparator46switches the comparator switch50off.

An NPN bipolar transistor or an N-channel FET may be used as the comparator switch50, for example. If an NPN bipolar transistor is used as the comparator switch50, the collector of the comparator switch50is connected to the collector of the second switch41. The emitter of the comparator switch50is grounded. If an N-channel FET is used as the comparator switch50, the drain of the comparator switch50is connected to the collector of the second switch41. The source of the comparator switch50is grounded.

Operation of Latch Circuit22

FIG.4is a timing chart illustrating the operation of the latch circuit22. InFIG.4, changes in the output voltages of the microcomputer25, latch circuit22, and temperature detection circuit24are shown. InFIG.4, changes in the state of the first switch40, second switch41, power supply switch20, and comparator switch50are also shown. On and off of the switches are shown as the states thereof. The horizontal axes of seven charts inFIG.4show time. Vo indicates the output threshold. Vr indicates the reference voltage.

When the output voltage of the microcomputer25is 0 V, no current will flow through the first resistor42. Therefore, there is no voltage drop at the first resistor42. As a result, the voltage between the emitter and base of the first switch40is 0 V, and is less than the threshold voltage. The first switch40is off. When the first switch40is off, no current will flow through the fourth resistor45. Therefore, there is no voltage drop at the fourth resistor45. As a result, the voltage between the base and emitter of the second switch41is 0 V, and is less than the second threshold voltage. The second switch41is thus also off.

When the output voltage of the microcomputer25is 0 V, the output voltage of the latch circuit22is 0 V, and is less than the output threshold voltage Vo. Therefore, the driving circuit21keeps the power supply switch20off. When the output voltage of the temperature detection circuit24is less than the reference voltage Vr, the comparator46keeps the comparator switch50off.

When the microcomputer25switches the output voltage from 0 V to the fixed voltage Vc, in a state in which the first switch40, second switch41, and comparator switch50are off, a current flows from the microcomputer25.

FIG.5is a diagram illustrating a current flow when the microcomputer25switches the output voltage to the fixed voltage Vc. When the microcomputer25switches the output voltage from 0 V to the fixed voltage Vc, a current flows from the microcomputer25through the first resistor42, the second resistor43, and the device resistor23in that order, as shown inFIG.5. Therefore, there is a voltage drop at the first resistor42. The voltage across the first resistor42is proportional to the current flowing through the first resistor42.

The sum of resistances of the first resistor42, second resistor43, and device resistor23is sufficiently large. As a result, the current flowing through the first resistor42is very small. Therefore, the voltage across the first resistor42, that is the voltage between the emitter and base of the first switch40is less than the threshold voltage. The first switch40is off.

When a current flows through the first resistor42, second resistor43, and device resistor23in that order, a resistor circuit constituted by the first resistor42and second resistor43, and the device resistor23divides the fixed voltage Vc. The divided voltage is output to the driving circuit21as the output voltage of the latch circuit22. Here, the output voltage of the latch circuit22is determined by the ratio between the sum of resistances of the first resistor42and second resistor43, and the resistance of the device resistor23. The resistance of the device resistor23is sufficiently larger than the sum of resistances of the first resistor42and second resistor43. Therefore, a circuit voltage Vu close to the fixed voltage Vc is output to the driving circuit21, as the output voltage of the latch circuit22.

The circuit voltage Vu is equal to or larger than the output threshold voltage Vo. Therefore, when the output voltage of the latch circuit22increases from 0 V to the circuit voltage Vu, the driving circuit21switches the power supply switch20on. Accordingly, a current flows through the power supply switch20, and the power supply switch20generates heat. In a period from when a current starts flowing through the power supply switch20until when the heat generation amount of the power supply switch20becomes equal to the heat dissipation amount thereof, the temperature of the power supply switch20increases. When the temperature of the power supply switch20increases, the temperature of the switch circuit board Bs increases. When the temperature of the switch circuit board Bs increases, the output voltage of the temperature detection circuit24increases, as shown inFIG.4.

When the microcomputer25keeps the output voltage to the fixed voltage Vc, when the power supply system1is in a normal state, the temperature of the power supply switch20stabilizes at a low temperature. As a result, the output voltage of the temperature detection circuit24is less than the reference voltage Vr. The comparator switch50is kept off.

When the microcomputer25reduces the output voltage from the fixed voltage Vc to 0 V, the current flow through the first resistor42, second resistor43, and device resistor23stops. Accordingly, the voltage across the first resistor42decreases to 0 V. Therefore, the voltage between the emitter and base of the first switch40decreases to 0 V, and the first switch40is kept off. When the first switch40is off, the second switch41is off, as mentioned above.

As shown inFIG.4, when the output voltage of the microcomputer25has decreased from the fixed voltage Vc to 0 V, the output voltage of the latch circuit22decreases from the circuit voltage Vu to 0 V. 0 V is less than the output threshold voltage Vo. When the output voltage of the latch circuit22has decreased below the output threshold voltage Vo, the driving circuit21switches the power supply switch20off. When the power supply switch20is off, heat generation in the power supply switch20stops. Accordingly, the temperature of the power supply switch20decreases. When the temperature of the power supply switch20decreases, the temperature of the switch circuit board Bs decreases. When the temperature of the switch circuit board Bs decreases, the output voltage of the temperature detection circuit24decreases. When the temperature of the switch circuit board Bs stabilizes, the output voltage of the temperature detection circuit24stabilizes. The comparator switch50is kept off.

As described above, in the latch circuit22, when the first switch40, second switch41, and comparator switch50are off, the output voltage of the microcomputer25is output to the driving circuit21via the first resistor42and second resistor43. The latch circuit22outputs a voltage according to the output voltage of the microcomputer25. When the microcomputer25increases its output voltage from 0 V to the fixed voltage Vc, the driving circuit21switches the power supply switch20from off to on. When the microcomputer25has decreased the output voltage from the fixed voltage Vc to 0 V, the driving circuit21switches the power supply switch20from on to off. While the power supply system1is in a normal state, the first switch40and second switch41are kept off.

When the output voltage of the microcomputer25is the fixed voltage Vc, in a state in which the first switch40and second switch41are off, the output voltage of the latch circuit22is the circuit voltage Vu, and the power supply switch20is on, as mentioned above. When the power supply system1is in a normal state, when the power supply switch20is on, the output voltage of the temperature detection circuit24stabilizes at a voltage less than the reference voltage Vr.

For example, when both ends of the load11are short-circuited, the state of the power supply system1transitions to an abnormal state. In this case, a large current flows through the power supply switch20, and the heat generation amount of the power supply switch20increases considerably. Accordingly, the temperature of the switch circuit board Bs increases considerably. When the temperature of the switch circuit board Bs increases considerably, the output voltage of the temperature detection circuit24increases considerably as well.

When the output voltage of the temperature detection circuit24has increased to the reference voltage Vr or more, in a state in which the output voltage of the microcomputer25is the fixed voltage Vc, the comparator46switches the comparator switch50from off to on. In this case, a current flows through the comparator switch50.

FIG.6is a diagram illustrating a current flow when the comparator switch50is on. When the comparator switch50is on, a current flows from the microcomputer25through the first resistor42, second resistor43, and comparator switch50in that order. Accordingly, there is a voltage drop at the first resistor42.

No current flows through the device resistor23. As a result, a large current flows through the second resistor43. Therefore, when the comparator switch50is on, the voltage across the first resistor42is the threshold voltage or more. When the voltage across the first resistor42is the threshold voltage or more, the voltage between the emitter and base of the first switch40is the threshold voltage or more, and the first switch40is on. Therefore, when the comparator46switches the comparator switch50from off to on, the first switch40is switched from off to on.

When the first switch40is on, a current flows from the microcomputer25through the first switch40, third resistor44, and fourth resistor45in that order. Accordingly, there is a voltage drop at the fourth resistor45. When the first switch40is switched on, the voltage between the base and emitter of the second switch41increases to the second threshold voltage or more. When the voltage between the base and emitter of the second switch41has increased to the second threshold voltage or more, the second switch41is switched from off to on. When the second switch41is switched from off to on, a current flows from the microcomputer25through the first resistor42, second resistor43, and second switch41in that order.

When the comparator switch50is on, a resistor current that flows through the first resistor42and second resistor43in that order is input to the comparator switch50. The comparator switch50functions as a third switch. The comparator46functions as a second switching circuit. When the first switch40is on, a current is input to the emitter of the first switch40from the microcomputer25, and a current is output from the collector of the first switch40to the third resistor44. The emitter, collector, and base of the first switch40respectively function as an input end, an output end, and a control end. When the second switch41is on, a resistor current that flows through the first resistor42and second resistor43in that order is input to the collector of the second switch41, and a current is output from the emitter of the second switch41. The collector, emitter, and base of the second switch41respectively function as a second input end, a second output end, and a second control end.

When the comparator switch50is switched from off to on, one end of the device resistor23on the driving circuit21side is grounded. Accordingly, the output voltage of the latch circuit22decreases from the circuit voltage Vu to 0 V. As a result, the driving circuit21forcibly switches the power supply switch20from on to off. When the power supply switch20is switched off, the power supply switch20stops generating heat, and therefore the temperature of the power supply switch20decreases. When the temperature of the power supply switch20decreases, the temperature of the switch circuit board Bs decreases. When the temperature of the switch circuit board Bs decreases, the output voltage of the temperature detection circuit24decreases below the reference voltage Vr. As a result, the comparator46switches the comparator switch50from on to off.

FIG.7is a diagram illustrating a current flow when the comparator switch50is switched from on to off. At a point in time at which the comparator switch50is switched off, the second switch41is on. When the second switch41is on, a current flows from the microcomputer25through the first resistor42, second resistor43, and second switch41in that order, as mentioned above. No current flows through the device resistor23. Therefore, when the second switch41is on, the voltage across the first resistor42is the threshold voltage or more. As a result, the voltage between the emitter and base of the first switch40is kept to be the threshold voltage or more. The first switch40is kept on.

As mentioned above, when the first switch40is on, the second switch41is on. When the second switch41is on, the one end of the device resistor23at the driving circuit21is grounded. Therefore, the output voltage of the latch circuit22is fixed to 0 V. As a result, even if the comparator switch50is switched off, the driving circuit21keeps the power supply switch20off, as long as the output voltage of the microcomputer25is the fixed voltage Vc.

As described above, the latch circuit22outputs a voltage according to the output voltage of the microcomputer25until the comparator switch50is switched on. In the case where the output voltage of the microcomputer25is the fixed voltage Vc, when the comparator switch50is switched on, the latch circuit22reduces the output voltage from the circuit voltage Vu to 0 V (predetermined voltage). Thereafter, the latch circuit22fixes the output voltage to 0 V (predetermined voltage), irrespective of the state of the comparator switch50. Switching the comparator switch50on corresponds to a predetermined condition.

When the power supply switch20has been switched off, the output voltage of the temperature detection circuit24decreases until the temperature of the switch circuit board Bs stabilizes, as mentioned above. When the microcomputer25has switched the output voltage from the fixed voltage Vc to 0 V, in a state in which the latch circuit22fixes the output voltage to 0 V, the current flow through the first resistor42and the current flow through the fourth resistor45stop.

When the current flow through the first resistor42has stopped, the voltage between the emitter and base of the first switch40decreases to 0 V, and the first switch40is switched off. When the current flow through the fourth resistor45has stopped, the voltage between the base and emitter of the second switch41decreases to 0 V, and the second switch41is switched off. When the first switch40and the second switch41have been switched off, the forcible off performed by the driving circuit21is canceled. When the first switch40and second switch41are off, the latch circuit22outputs a voltage according to the output voltage of the microcomputer25, as mentioned above.

After the forcible off has been canceled, when the microcomputer25has switched the output voltage from 0 V to the fixed voltage Vc, the driving circuit21switches the power supply switch20from off to on.

Effects of Power Supply Control Device12

In the latch circuit22of the power supply control device12, the number of switches needed to fix the output voltage to 0 Vis three, which is a small number.

Modifications

The temperature detection circuit24needs only be configured such that the output voltage thereof indicates the temperature of the switch circuit board Bs. Therefore, the configuration of the temperature detection circuit24is not limited to a configuration in which the output voltage increases, as the temperature of the switch circuit board Bs increases. The configuration of the temperature detection circuit24may be such that the output voltage decreases, as the temperature of the switch circuit board Bs increases. A detection resistor31and a thermistor30may be used in place of the thermistor30and the detection resistor31, respectively, as a first example. A PTC thermistor may also be used as the thermistor30, as a second example. In a configuration in which the output voltage decreases, as the temperature of the switch circuit board Bs increases, the output voltage of the temperature detection circuit24is output to the plus terminal of the comparator46of the latch circuit22. The reference voltage Vr is output to the minus terminal of the comparator46. The configuration of the temperature detection circuit24is not limited to a configuration in which a thermistor is used.

In the power supply control device12in Embodiment 1, the temperature of the switch circuit board Bs is prevented from increasing to an abnormal temperature. However, the object whose temperature is prevented from increasing to an abnormal temperature may also be an object different from the switch circuit board Bs.

The differences between Embodiment 2 and Embodiment 1 will be described below. Structural features other than those described below are the same as in Embodiment 1. Therefore, constituent units that are the same as in Embodiment 1 are given the same reference numerals as in Embodiment 1, and their further description will be omitted.

Configuration of Power Supply Control Device12

FIG.8is a block diagram illustrating the main configuration of a power supply system1in Embodiment 2. When the power supply system1in Embodiment 2 is compared with the power supply system1in Embodiment 1, the configuration of the power supply control device12differs. The power supply control device12in Embodiment 2 includes a power supply switch20, a driving circuit21, a latch circuit22, a device resistor23, and a microcomputer25, similarly to Embodiment 1. The connection between these constituent units is similar to that in Embodiment 1.

The power supply control device12in Embodiment 2 includes a current output circuit26and a temperature difference circuit27, in place of the temperature detection circuit24. The current output circuit26is connected to a drain of the power supply switch20and the temperature difference circuit27. The temperature difference circuit27is further connected to the latch circuit22. The temperature difference circuit27is grounded. The current output circuit26and the temperature difference circuit27are disposed on a switch circuit board Bs.

As described in Embodiment 1, when the driving circuit21switches the power supply switch20on, a current flows from a positive electrode of a DC power supply10through the power supply switch20, a wire W, and a load11in that order. Therefore, the wire W is disposed on a current path of current flowing through the power supply switch20. The wire current flowing through the wire W is denoted as Ih. The wire current Ih is the same as the current flowing through the power supply switch20. The current output circuit26draws in current from the drain of the power supply switch20, and outputs the drawn-in current to the temperature difference circuit27. The output current output from the current output circuit26is denoted as Is.

The current output circuit26adjusts the output current Is such that the following formula (1) is satisfied.

Here, K is a constant. The constant K is 4000, for example. As shown in formula (1), the output current Is of the current output circuit26increases as the wire current Ih increases.

The temperature difference circuit27outputs a voltage to a minus terminal of the comparator46of the latch circuit22. A reference voltage Vr generated by dividing the fixed voltage Vc by a first voltage-dividing resistor47and a second voltage-dividing resistor48is input to a plus terminal of the comparator46.

When the power supply switch20is switched on, a wire current Ih flows, as mentioned above. When the wire current Ih flows, the wire W generates heat. The heat generation amount of the wire W increases, as the power consumed in the wire W increases. The power consumed in the wire W is represented by the product of the square of the wire current Ih and the resistance of the wire W. When the heat generation amount of the wire W per unit time exceeds the heat dissipation amount thereof per unit time, the wire temperature of the wire W increases.

When the power supply switch20is switched off, the wire current Ih decreases to 0 A, and the wire temperature of the wire W decreases. The temperature difference circuit27adjusts the output voltage according to the temperature difference between the wire temperature of the wire W and an ambient temperature in the vicinity of the wire W.

FIG.9is a timing chart illustrating the operation of the temperature difference circuit27. InFIG.9, a change in the state of the power supply switch20, a change in the temperature difference regarding the wire W, and a change in the output voltage of the temperature difference circuit27are shown. The horizontal axes of these charts show time.

The output voltage of the temperature difference circuit27is generated based on the output current of the current output circuit26. In the case where the power supply switch20is off, when the temperature difference regarding the wire W is zero degrees, the output voltage of the temperature difference circuit27is 0 V. The temperature difference being zero degrees indicates that the wire temperature is the same as the ambient temperature in the vicinity of the wire W. When the driving circuit21switches the power supply switch20on, a wire current Ih flows, and the temperature difference regarding the wire W increases. When the temperature difference increases, the output voltage of the temperature difference circuit27also increases.

When the power supply system1is in a normal state, the heat generation amount of the wire W is the same as the heat dissipation amount thereof, and the temperature difference regarding the wire W stabilizes. When the temperature difference stabilizes, the output voltage of the temperature difference circuit27stabilizes. When the driving circuit21switches the power supply switch20off, the temperature difference regarding the wire W decreases, as mentioned above. When the temperature difference regarding the wire W decreases, the output voltage of the temperature difference circuit27also decreases.

As described above, the output voltage of the temperature difference circuit27increases, as the temperature difference regarding the wire W increases. When the temperature difference regarding the wire W reaches a fixed reference temperature difference ΔTr, the output voltage of the temperature difference circuit27reaches the reference voltage Vr.

When the output voltage of the temperature difference circuit27is less than the reference voltage Vr, the latch circuit22adjusts the output voltage according to the output voltages of the microcomputer25and temperature difference circuit27. The driving circuit21switches the power supply switch20on or off according to the output voltage of the latch circuit22. When the output voltage of the temperature difference circuit27has increased to the reference voltage Vr or more, the driving circuit21forcibly switches the power supply switch20from on to off, as described below. Accordingly, the wire current Ih decreases to 0 A, and the wire temperature decreases.

As mentioned above, the output voltage of the temperature difference circuit27being less than the reference voltage Vr indicates that the temperature difference regarding the wire W is less than the reference temperature difference ΔTr. The output voltage of the temperature difference circuit27being the reference voltage Vr or more indicates that the temperature difference regarding the wire W is the reference temperature difference ΔTr or more. Therefore, when the temperature difference regarding the wire W is less than the reference temperature difference ΔTr, the comparator46keeps the comparator switch50off. When the temperature difference regarding the wire W has increased to the reference temperature difference ΔTr or more, the comparator46switches the comparator switch50from off to on. When the temperature difference regarding the wire W has decreased below the reference temperature difference ΔTr, the comparator46switches the comparator switch50from on to off. The reference temperature difference ΔTr corresponds to a predetermined temperature difference.

Operation of Latch Circuit22

FIG.10is a timing chart illustrating the operation of the latch circuit22. Changes in output voltages of the microcomputer25, latch circuit22, and temperature difference circuit27are shown inFIG.10. Also, changes in the state of the first switch40, second switch41, power supply switch20, and comparator switch50are shown inFIG.10. On and off states of the switches are shown as the states thereof. The horizontal axes of seven charts inFIG.10show time. As described above in Embodiment 1, Vu and Vo respectively indicate the circuit voltage and the output threshold voltage.

The operation of the latch circuit22in Embodiment 2 is similar to the operation of the latch circuit22in Embodiment 1. The operation of the latch circuit22in Embodiment 2 can be described by replacing the temperature detection circuit24with the temperature difference circuit27in the description of the operation of the latch circuit22in Embodiment 1.

When the output voltage of the temperature difference circuit27is less than the reference voltage Vr, the comparator switch50is off. When the first switch40, second switch41, and comparator switch50are off, the latch circuit22outputs a voltage according to the output voltage of the microcomputer25to the driving circuit21. When the output voltage of the microcomputer25is 0 V, the output voltage of the latch circuit22is 0 V. When the output voltage of the latch circuit22is 0 V, the driving circuit21keeps the power supply switch20off.

When the microcomputer25has switched the output voltage from 0 V to the fixed voltage Vc, the output voltage of the latch circuit22increases from 0 V to the circuit voltage Vu, and the driving circuit21switches the power supply switch20from off to on. When the microcomputer25has switched the output voltage from the fixed voltage Vc to 0 V, the output voltage of the latch circuit22decreases from the circuit voltage Vu to 0 V, and the driving circuit21switches the power supply switch20from on to off.

In the case where the output voltage of the microcomputer25is the fixed voltage Vc, when the output voltage of the temperature difference circuit27has increased to the reference voltage Vr or more, the comparator46switches the comparator switch50off to on. Accordingly, the output voltage of the latch circuit22decreases from the fixed voltage Vc to 0 V, and the driving circuit21switches the power supply switch20from on to off. The reference voltage Vr corresponds to a predetermined voltage.

When the comparator46has switched the comparator switch50from off to on, the first switch40and second switch41are switched from off to on. When the power supply switch20is switched off, the wire current Ih decreases to [[0 V]] 0 A, and the wire temperature decreases. When the wire temperature decreases, the output voltage of the temperature difference circuit27decreases below the reference voltage Vr. Accordingly, the comparator46switches the comparator switch50from on to off.

At the point in time at which the comparator46switches the comparator switch50off, the first switch40and second switch41are on, and therefore the latch circuit22fixes the output voltage to 0 V (predetermined voltage), unless the output voltage of the microcomputer25decreases from the fixed voltage Vc to 0 V. When the output voltage of the latch circuit22is 0 V, the driving circuit21keeps the power supply switch20off, as mentioned above. When the microcomputer25has switched the output voltage from the fixed voltage Vc to 0 V, the first switch40and second switch41are switched from on to off, and the forcible off performed by the driving circuit21is canceled.

Effects of Power Supply Control Device12

In the power supply control device12in Embodiment2, when the temperature difference regarding the wire W has increased to the reference temperature difference ΔTr or more, the output voltage of the temperature difference circuit27increases to the reference voltage Vr or more. Therefore, when the temperature difference regarding the wire W has increased to the reference temperature difference ΔTr or more, the comparator switch50is switched on, and the output voltage of the latch circuit22is fixed to 0 V (predetermined voltage). As a result, the power supply switch20is kept off. The wire temperature of the wire W can be prevented from increasing to an abnormal temperature. When the wire temperature of the wire W increases to an abnormal temperature, the performance of the wire W may be degraded.

The power supply control device12in Embodiment 2 similarly achieves the effects achieved by the power supply control device12in Embodiment 1, other than the effects obtained by using the temperature detection circuit24.

Note that, in Embodiment 2, the driving circuit21may have a function of switching the power supply switch20off according to the temperature of the power supply switch20. In this configuration, a temperature detector, which is not illustrated, detects the temperature of the power supply switch20. When the temperature of the power supply switch20detected by the temperature detector has increased to a fixed temperature or more, the driving circuit21switches the power supply switch20off, irrespective of the output voltage of the latch circuit22. With this configuration, the temperature of the switch circuit board Bs can be prevented from increasing to an abnormal temperature.

Configuration of Temperature Difference Circuit27

The configuration of the temperature difference circuit27is not a known configuration. Therefore, the configuration of the temperature difference circuit27will be described in detail below. The temperature difference circuit27is a circuit in which a current flows (is conducted) similarly to the heat in a thermal circuit of the wire W. Therefore, first, the thermal circuit of the wire W will be described.

FIG.11is a thermal circuit diagram of the wire W. A cross section of the wire W is shown in a lower portion inFIG.11. As shown in the lower portion inFIG.11, in the wire W, an outer face of a bar-shaped conductor60through which a current flows is covered by an insulator61. The thermal circuit shown in the upper portion inFIG.11is a thermal circuit when the wire W includes a conductor60and an insulator61. The wire current Ih flows through the conductor60. When the wire current Ih flows, heat is generated in the conductor60.

The thermal circuit of the wire W includes a heat source70, a thermal resistor71, and a thermal capacitor72. The thermal resistor71and thermal capacitor72are connected in parallel to the heat source70. The heat source70outputs heat to one end of each of the thermal resistor71and thermal capacitor72. The temperature of the one end of each of the thermal resistor71and thermal capacitor72is the wire temperature of the wire W. The temperature of the other end of each of the thermal resistor71and thermal capacitor72is an ambient temperature in the vicinity of the wire W.

Some of the heat generated in the heat source70is discharged to the outside of the wire W through the thermal resistor71. The remaining heat generated in the heat source70is stored in the thermal capacitor72. The heat stored in the thermal capacitor72is discharged to the outside of the wire W through the thermal resistor71. The difference across the heat source70corresponds to the temperature difference between the wire temperature and the ambient temperature. In the example inFIG.11, the thermal resistor71is the thermal resistance of the insulator61.

The amount of heat output by the heat source70is denoted as Jw. The resistance of the thermal resistor71is denoted as Rt. The capacitance of the thermal capacitor72is denoted as Ct. The wire temperature, the ambient temperature, and the temperature difference are respectively denoted as Tw, Ta, and ΔT. The resistance of the wire W is denoted as Rw.

When heat is generated in the wire W, the temperature difference ΔT is represented by the following Formula (2).

Here, t is a period in which the wire W generates heat, that is a current conduction period in which a current flows through the wire W. “·” indicates multiplication. Also, the amount of heat Jw is represented by the following Formula (3).

The amount of heat Jw depends on the wire current Ih.

The reference temperature difference is denoted as ΔTr, as mentioned above. The wire current Ih when the temperature difference ΔT is the reference temperature difference ΔTr is denoted as If.

The wire current If is represented by the following Formula (4) using Formulas (2) and (3).

FIG.12is a circuit diagram of the temperature difference circuit27. The temperature difference circuit27includes a first circuit resistor80, a second circuit resistor81, and a capacitor82. One end of the first circuit resistor80is connected to the current output circuit26. The other end of the first circuit resistor80is grounded. The one end of the first circuit resistor80is connected to one end of the second circuit resistor81. The other end of the second circuit resistor81is connected to one end of the capacitor82. The other end of the capacitor82is grounded. The one end of the capacitor82is connected to the minus terminal of the comparator46of the latch circuit22.

Some of the output current of the current output circuit26flows through the first circuit resistor80. The remaining output current of the current output circuit26flows in the capacitor82through the second circuit resistor81. Accordingly, power is stored in the capacitor82. When power is stored in the capacitor82, a current flows from the one end of the capacitor82to the second circuit resistor81and first circuit resistor80in that order, and the capacitor82discharges.

As described above, the current flow in the temperature difference circuit27is similar to the heat conduction in the thermal circuit of the wire W. The temperature difference circuit27corresponds to the thermal circuit shown in the upper portion inFIG.11.

The voltage across the capacitor82is output to the minus terminal of the comparator46as an output voltage of the temperature difference circuit27. The comparator46switches the comparator switch50on or off based on the result of comparison between the output voltage of the temperature difference circuit27and the reference voltage Vr, as mentioned above.

The voltage across the capacitor82(output voltage of the temperature difference circuit27) is denoted as Vd. The resistances of the first circuit resistor80and second circuit resistor81are respectively denoted as R1and R2. The capacitance of the capacitor82is denoted as C1. When a wire current flows through the wire W, that is, when the wire W is generating heat, the voltage Vd across the capacitor82is represented by the following Formula (5). Here, t is the aforementioned current conduction period.

In Formula (5), K is a constant, as described in Embodiment 1. The wire current Ih when the voltage Vd across the capacitor82is the reference voltage Vr is denoted as Ir. The wire current Ir is represented by the following Formula (6).

The method of determining the constant will be described next. The resistance Rw of the wire W and the capacitance Ct of the thermal capacitor72that depend on the structure of the wire W are determined in advance. The reference temperature difference ΔTr is set such that, even if the ambient temperature in the vicinity of the wire W is the maximal temperature, the wire W will not emit smoke at this temperature difference. For example, assume that the maximal temperature of the ambient temperature is 80 degrees. In a configuration in which the wire W is caused to stop generating heat when the wire temperature reaches 100 degrees, the reference temperature difference ΔTr is set to 20 degrees.

The constant K, reference voltage Vr, resistances R1and R2, and capacitance C1are determined based on Formulas (4) and (6) such that, with respect to any current conduction period t, the wire current Ir is substantially the same as the wire current If. Specifically, with respect to any current conduction period t, the constant K, reference voltage Vr, resistances R1and R2, and capacitance C1are determined such that the following Formulas (7) and (8) are substantially satisfied.

Note that a square root is used in the left side of Formula (8). The square root is not used in the right side of Formula (8). Therefore, it is impossible to determine the resistances R1and R2and capacitance C1that satisfy Formula (8) throughout the entire current conduction period t. Therefore, the resistances R1and R2and capacitance C1are determined such that the difference in value between the left side and the right side of Formula (8) for any current conduction period t is a fixed preset value or less. Regarding Formula (7), the constant K, reference voltage Vr, and resistance R1are determined such that the values of left and right sides are the same.

When the plurality of constants of the temperature difference circuit27are determined as described above, the current flow in the temperature difference circuit27becomes similar to the heat conduction in the thermal circuit of the wire W. When the wire W generates heat in the thermal circuit of the wire W, heat is stored in the thermal capacitor72. At this moment, the capacitor82is charged in the temperature difference circuit27. In the thermal circuit of the wire W, heat is dissipated from the thermal capacitor72to the outside of the wire W. Accordingly, the wire W dissipates heat. When the amount of heat generated by the wire W is the same as the amount of heat dissipated from the thermal capacitor72, the amount of heat stored in the thermal capacitor72is kept at a fixed value. At this moment, the voltage across the capacitor82is also kept at a fixed voltage. When the wire W dissipates heat, the capacitor82is discharged.

The voltage across the capacitor82, that is, the output voltage of the temperature difference circuit27is generated based on the output current of the current output circuit26, and increases as the temperature difference regarding the wire W increases. The output voltage of the temperature difference circuit27indicates the temperature difference regarding the wire W.

The temperature difference circuit27is an electric circuit corresponding to a thermal circuit of the single-stage Cauer model. The temperature difference circuit27is not limited to this electric circuit. The temperature difference circuit27may also be an electric circuit corresponding to the thermal circuit of the Foster model, multi-stage Cauer model, or the like.

Modifications of Embodiments 1 and 2

In Embodiments 1 and 2, the circuit board on which the microcomputer25is disposed is not limited to the control circuit board Bc. The microcomputer25may also be disposed on the switch circuit board Bs, for example. Also, the predetermined voltage is not limited to 0 V, and need only be a voltage to instruct to switch the power supply switch20off. Also, the power supply switch20need only be a switch that the driving circuit21can switch on or off. Therefore, the power supply switch20is not limited to an N-channel FET, and may also be a P-channel FET, a bipolar transistor, or a switch such as a relay contact.

The first switch40need only be a switch that is switched on or off according to the voltage between an input end to which a current is input and a control end. Therefore, the first switch40is not limited to a PNP bipolar transistor, and may also be a P-channel FET. The second switch41need only be a switch that is switched on when the first switch40is switched on. Therefore, the second switch41is not limited to an NPN bipolar transistor, and may also be an N-channel FET or a switch such as a relay contact. The source, drain, and gate of an FET respectively correspond to an emitter, a collector, and a base of a bipolar transistor.

The device that includes the latch circuit22is not limited to the power supply control device12. A device that includes a switch that is switched on or off according to the output voltage of the latch circuit22is a first example of the device that includes the latch circuit22. A device that includes an electric device that performs operations depending on the output voltage of the latch circuit22is a second example of the device that includes the latch circuit22.

Embodiments 1 and 2 disclosed herein are illustrative in all aspects and should not be considered restrictive. The scope of the present disclosure is indicated by the scope of claims. not the above-mentioned meaning. and is intended to include all modifications within the meaning and scope equivalent to the scope of claims.