Power supply control device

Abstract

In the power supply control device (10), the power supply from the DC power supply (11) to the load (12) is controlled by switching the power supply FET (20) on or off. The current adjustment circuit (25) adjusts the current flowing through the resistor circuit (24) to the value obtained by dividing the voltage between the drain and source of the power supply FET (20) by the resistance value of the resistor circuit (24). The drive circuit (21) switches the power supply FET (20) off when the voltage across the detection resistor (26) exceeds a specified voltage. The on-resistance value of the power supply FET (20) varies according to the ambient temperature of the power supply FET (20). The resistance value of the resistor circuit (24) varies in the same direction as the on-resistance value according to the ambient temperature of the power supply FET (20).

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a power supply control device.

[0002] This application claims priority based on Japanese Application No. 2020-074813 filed on April 20, 2020, and all the written content described in the Japanese application is incorporated by reference. BACKGROUND

[0003] Patent Literature 1 discloses a power supply control device for a vehicle that controls power supply from a power supply to a load by switching a MOSFET (Metal Oxide Semiconductor Field Effect Transistor) that functions as a semiconductor switch to be on or off. In the power supply control device, a drain and a source of the MOSFET are arranged in a first current path of a current from the power supply to the load. In the power supply control device described in Patent Literature 1, a second current path of a current that flows from the power supply to a resistor is also provided. The current that flows in the second current path is adjusted to be a current that is proportional to a voltage between the drain and the source of the MOSFET.

[0004] The voltage between the drain and the source of the MOSFET is represented by a product of the current that flows via the first current path and an on-resistance value of the MOSFET. The on-resistance value is a resistance value of the drain and the source of the MOSFET when the MOSFET is on. The on-resistance value of the MOSFET varies depending on a temperature of the MOSFET.

[0005] In the power supply control device described in Patent Literature 1, regarding the voltage between the two ends of the resistor, an analog value is converted to a digital value. An arithmetic element calculates a current that flows via the MOSFET based on the voltage between the two ends of the resistor represented by the digital value and a surrounding temperature of the MOSFET that varies similarly to the temperature of the MOSFET. In a case where the current calculated by the arithmetic element is large, the MOSFET is switched to be off. Thereby, a case where an overcurrent flows via the MOSFET is prevented.

[0006] PRIOR ART DOCUMENTS

[0007] PATENT LITERATURE

[0008] Patent Literature 1: Japanese Patent Application Publication No. 2011-85470 SUMMARY

[0009] One aspect of the power supply control device disclosed herein controls power supply by switching a current-carrying semiconductor switch to be on or off. The power supply control device includes: a resistor circuit, one end of which is connected to an upstream end of the semiconductor switch; a current adjustment circuit that adjusts the current flowing through the resistor circuit to a value obtained by dividing the voltage across the semiconductor switch by the resistance value of the resistor circuit; a resistor disposed in the current path of the current flowing through the resistor circuit; and a switching circuit that switches the semiconductor switch to off when the voltage across the resistor exceeds a predetermined voltage. The on-resistance value of the semiconductor switch varies according to the ambient temperature of the semiconductor switch, and the resistance value of the resistor circuit varies in the same direction as the on-resistance value according to the ambient temperature. Attached Figure Description

[0010] Figure 1 This is a block diagram showing the main structural components of the power supply system in Implementation 1.

[0011] Figure 2 It is a time diagram used to illustrate the operation of the power supply control device.

[0012] Figure 3 It is a coordinate graph showing the relationship between the on-resistance of the power supply FET and the ambient temperature.

[0013] Figure 4 This is a circuit diagram of a resistive circuit.

[0014] Figure 5 This is an explanatory diagram illustrating the method for determining the resistance value of a series resistor.

[0015] Figure 6 This is a circuit diagram of the resistor circuit in Implementation Method 2.

[0016] Figure 7 It is a time diagram used to illustrate the operation of the power supply control device.

[0017] Figure 8 This is an illustration of the effect of parallel resistors.

[0018] Figure 9 This is a circuit diagram of the resistor circuit in implementation method 3.

[0019] Figure 10 This is a circuit diagram of the resistor circuit in implementation method 4. Detailed Implementation

[0020] [The problem this disclosure aims to solve]

[0021] Regarding Patent Document 1, calculating the current flowing through the MOSFET takes a long time. Therefore, the power supply control device described in Patent Document 1 has the problem that it cannot immediately disconnect the MOSFET when the current flowing through it is large.

[0022] Therefore, the objective is to provide a power supply control device that can switch the semiconductor switch to off at the appropriate time without calculating the current flowing through the semiconductor switch.

[0023] [The Effects of This Disclosure]

[0024] According to this disclosure, it is possible to switch a semiconductor switch to open at an appropriate time without calculating the current flowing through it.

[0025] [Description of embodiments of this disclosure]

[0026] First, embodiments of this disclosure will be described. At least some of the embodiments described below may be combined arbitrarily.

[0027] (1) One aspect of the power supply control device disclosed herein controls power supply by switching a current-carrying semiconductor switch to be on or off, wherein the power supply control device comprises: a resistor circuit, one end of which is connected to one end of the upstream side of the semiconductor switch; a current adjustment circuit that adjusts the current flowing through the resistor circuit to a value obtained by dividing the voltage between the two ends of the semiconductor switch by the resistance value of the resistor circuit; a resistor disposed in the current path of the current flowing through the resistor circuit; and a switching circuit that switches the semiconductor switch to be off when the voltage between the two ends of the resistor exceeds a predetermined voltage, wherein the on resistance value of the semiconductor switch varies according to the ambient temperature of the semiconductor switch, and the resistance value of the resistor circuit varies in the same direction as the on resistance value according to the ambient temperature.

[0028] In the above configuration, the current flowing through the resistor circuit is represented by (voltage across the semiconductor switch) / (resistance of the resistor circuit). The voltage across the semiconductor switch is represented by the product of the current flowing through the semiconductor switch and the switching resistance of the semiconductor switch. Depending on the ambient temperature of the semiconductor switch, the switching resistance of the semiconductor switch and the resistance of the resistor circuit change in the same direction. Therefore, even if the ambient temperature of the semiconductor switch changes, the current flowing through the resistor remains almost constant. The voltage across the resistor hardly changes with the ambient temperature of the semiconductor switch and is proportional to the current flowing through the semiconductor switch. When the voltage across the resistor exceeds a specified voltage, the current flowing through the semiconductor switch is large, switching the semiconductor switch off. Thus, without calculating the current flowing through the semiconductor switch, it is possible to switch the semiconductor switch off at the appropriate time.

[0029] (2) In one embodiment of the power supply control device disclosed herein, the resistor circuit has a second semiconductor switch, and when the switching unit switches the semiconductor switch to the on state, the second semiconductor switch is switched to the on state, and the on-resistance value of the second semiconductor switch changes in the same direction as the on-resistance value of the semiconductor switch according to the ambient temperature of the semiconductor switch.

[0030] In the above configuration, a second semiconductor switch is configured in the resistor circuit, and the on-resistance value of the second semiconductor switch changes in the same direction as the on-resistance value of the semiconductor switch depending on the ambient temperature of the semiconductor switch. Therefore, the resistance value of the resistor circuit changes in the same direction as the on-resistance value of the semiconductor switch depending on the ambient temperature of the semiconductor switch.

[0031] As a second semiconductor switch, for example, a switch with the same construction as the semiconductor switch can be used. Specifically, if the semiconductor switch is a MOSFET, the MOSFET can be used as the second semiconductor switch. In this case, a resistor circuit in which the resistance value varies with the switching resistance value of the semiconductor switch according to the ambient temperature can be easily implemented.

[0032] (3) In one embodiment of the power supply control device of this disclosure, the resistor circuit has a series resistor connected in series with the second semiconductor switch.

[0033] In the above configuration, when the ratio of the on-resistance of the semiconductor switch to the resistance of the resistive circuit is constant regardless of the ambient temperature of the semiconductor switch, the current threshold related to the current flowing through the semiconductor switch is also constant regardless of the ambient temperature of the semiconductor switch. In this case, the semiconductor switch can be switched off at an appropriate time. It is assumed that the ratio of the on-resistance of the semiconductor switch to the on-resistance of the second semiconductor switch is not constant regardless of the ambient temperature. Even in this case, by connecting a series resistor in series with the second semiconductor switch, a structure in which the ratio of the on-resistance of the semiconductor switch to the resistance of the resistive circuit is constant regardless of the ambient temperature of the semiconductor switch can be achieved.

[0034] (4) In one embodiment of the power supply control device of this disclosure, the resistor circuit has a parallel resistor connected in parallel with the series circuit of the second semiconductor switch and the series resistor.

[0035] In the above configuration, it is assumed that the ratio of the on-resistance of the semiconductor switch to the on-resistance of the second semiconductor switch is not constant regardless of the ambient temperature. Even in this case, by connecting a series resistor in series with the second semiconductor switch and a parallel resistor in parallel with the series circuit, it is possible to achieve a structure in which the ratio of the on-resistance of the semiconductor switch to the resistance of the resistive circuit is constant regardless of the ambient temperature of the semiconductor switch.

[0036] (5) In one embodiment of the power supply control device of this disclosure, the resistor circuit has a parallel resistor connected in parallel with the second semiconductor switch.

[0037] In the above configuration, a parallel resistor is connected in parallel with the second semiconductor switch. Therefore, the resistance value decreases from the on-state resistance of the second semiconductor switch to the resistance value of the parallel circuit of the second semiconductor switch and the parallel resistor.

[0038] (6) In one embodiment of the power supply control device disclosed herein, the current adjustment circuit includes: a variable resistor; and a resistance adjustment section that adjusts the resistance value of the variable resistor in such a manner that the voltage at one end of the downstream side of the semiconductor switch and the resistor circuit is consistent.

[0039] In the above configuration, the resistance value of the variable resistor is adjusted so that the voltage at one downstream end of the semiconductor switch and the resistor circuit are the same. Consequently, the current flowing through the resistor circuit is adjusted to the value obtained by dividing the voltage across the semiconductor switch by the resistance value of the resistor circuit.

[0040] [Details of the embodiments of this disclosure]

[0041] Hereinafter, specific examples of power supply systems according to embodiments of the present disclosure will be described with reference to the accompanying drawings. It should be noted that the present invention is not limited to these examples, but is disclosed in the claims and is intended to include all modifications within the meaning and scope equivalent to the claims.

[0042] (Implementation Method 1)

[0043] <Structure of Power Supply System>

[0044] Figure 1 This is a block diagram showing the main structural components of the power system 1 in Embodiment 1. The power system 1 is preferably mounted in a vehicle and includes a power supply control device 10, a DC power supply 11, and a load 12. The DC power supply 11 is, for example, a battery. The load 12 is an electrical device mounted in the vehicle.

[0045] The power supply control device 10 has a power supply FET 20 for controlling the power supply from the DC power supply 11 to the load 12. The power supply FET 20 is an N-channel MOSFET and functions as a semiconductor switch. The drain and source of the power supply FET 20 are connected to the positive terminal of the DC power supply 11 and one end of the load 12, respectively. The negative terminal of the DC power supply 11 and the other end of the load 12 are grounded.

[0046] Regarding the power supply FET 20, when it is in the ON state, the resistance between its drain and source is small, and current may flow through both the drain and source. The ON resistance of the power supply FET 20 is the resistance between its drain and source when it is OFF. When the power supply FET 20 is OFF, current flows from the positive terminal of the DC power supply 11 through the power supply FET 20, the load 12, and then through the negative terminal of the DC power supply 11, supplying power to the load 12. With power supplied to the load 12, the load 12 operates.

[0047] Regarding the power supply FET 20, when it is in the off state, the resistance between its drain and source is large enough that current will not flow through the drain and source. When the power supply FET 20 is off, no power is supplied to the load 12. When the power supply FET 20 switches to off, power supply to the load 12 stops, and the load 12 stops operating.

[0048] The power supply control device 10 controls the power supply from the DC power supply 11 to the load 12 by switching the power supply FET 20 on or off.

[0049] <Structure of Power Supply Control Device 10>

[0050] In addition to the power supply FET 20, the power supply control device 10 also includes a drive circuit 21, a microcomputer (hereinafter referred to as a microcomputer) 22, a comparator 23, a resistor circuit 24, a current adjustment circuit 25, and a detection resistor 26. The resistor circuit 24 has an input terminal for the input current, an output terminal for the output current, and a control terminal. The current adjustment circuit 25 includes an adjustment FET 30 for adjusting the current and a differential amplifier 31. The comparator 23 and the differential amplifier 31 each have a positive terminal, a negative terminal, and an output terminal. The adjustment FET 30 is a P-channel FET (Field Effect Transistor).

[0051] The gate of the power supply FET 20 is connected to the drive circuit 21. The drive circuit 21 is also connected to the output terminals of the microcomputer 22 and the comparator 23. The drain and gate of the power supply FET 20 are connected to the input and control terminals of the resistor circuit 24, respectively. The output terminal of the resistor circuit 24 is connected to the source of the adjustment FET 30 in the current adjustment circuit 25. The drain of the adjustment FET 30 is connected to one end of the sensing resistor 26. The other end of the sensing resistor 26 is grounded. In the current adjustment circuit 25, the source and gate of the adjustment FET 30 are connected to the negative terminal and the output terminal of the differential amplifier 31, respectively. The positive terminal of the differential amplifier 31 is connected to the source of the power supply FET 20. One end of the sensing resistor 26 is also connected to the negative terminal of the comparator 23.

[0052] The drive circuit 21 outputs a voltage. This output voltage is applied to the gate of the power supply FET 20 and the control terminal of the resistor circuit 24. The reference potential for the output voltage of the drive circuit 21 is ground. When the output voltage of the drive circuit 21 is a voltage higher than or equal to the first turn-on voltage, the power supply FET 20 is turned on. Regarding the state of the resistor circuit 24, there are two states: an on state where current can flow through the resistor circuit 24 and an off state where current does not flow through the resistor circuit 24. When the output voltage of the drive circuit 21 is a voltage higher than or equal to the second turn-on voltage, the resistor circuit 24 is turned on.

[0053] When the output voltage of the drive circuit 21 is less than the first disconnect voltage, the power supply FET 20 is disconnected. When the output voltage of the drive circuit 21 is less than the second disconnect voltage, the resistor circuit 24 is disconnected. The first turn-on voltage exceeds the first disconnect voltage. The first disconnect voltage is a positive voltage. Similarly, the second turn-on voltage exceeds the second disconnect voltage. The second disconnect voltage is a positive voltage.

[0054] The drive circuit 21 switches the power supply FET 20 and resistor circuit 24 to ON by adjusting the output voltage to a voltage higher than the first ON voltage and the second ON voltage. The drive circuit 21 switches the power supply FET 20 and resistor circuit 24 to OFF by adjusting the output voltage to a voltage lower than the first OFF voltage and the second OFF voltage. When the power supply FET 20 is ON, as described above, current flows through the power supply FET 20, supplying power from the DC power supply 11 to the load 12. At this time, the current flows through the power supply FET 20 in the order of its drain and source. Therefore, in the current path flowing through the power supply FET 20, the drain and source of the power supply FET 20 are respectively one end on the upstream side and one end on the downstream side. When the power supply FET 20 is OFF, as described above, power supply to the load 12 stops. Hereinafter, the current flowing through the power supply FET 20 will be described as the switching current.

[0055] When resistor circuit 24 is on, current flows from the positive terminal of DC power supply 11 through resistor circuit 24, the adjustment FET 30 of current adjustment circuit 25, and the sensing resistor 26 in that order. Within resistor circuit 24, current flows through the input and output terminals in that order. Therefore, in the current path of the current flowing through resistor circuit 24, the input and output terminals are respectively upstream and downstream. Hereinafter, the current flowing through resistor circuit 24 will be referred to as the resistive current. In the current path of the resistive current, the adjustment FET 30 of current adjustment circuit 25 is located downstream of resistor circuit 24, and the sensing resistor 26 is located downstream of adjustment FET 30 of current adjustment circuit 25. When resistor circuit 24 is off, the flow of current through resistor circuit 24 stops.

[0056] The microprocessor 22 and comparator 23 output high-level and low-level voltages to the drive circuit 21, respectively. The reference potentials for the high-level and low-level voltages are ground potentials. The high-level voltage is higher than the low-level voltage. Based on the output voltages of the microprocessor 22 and comparator 23, the drive circuit 21 switches the power supply FET 20 and resistor circuit 24 to be on or off.

[0057] When the load 12 is operating, the microprocessor 22 switches the output voltage to a high level. When the load 12 stops operating, the microprocessor 22 switches the output voltage to a low level. A predetermined voltage is applied to the positive terminal of the comparator 23. The predetermined voltage is a positive voltage, for example, generated by stepping down the output voltage of the DC power supply 11 through a regulator. The reference potential of the predetermined voltage is ground potential. A voltage is applied to one end of the detection resistor 26, whose reference potential is ground potential, i.e., the voltage between the two ends of the detection resistor 26, to the negative terminal of the comparator 23. Hereinafter, the voltage between the two ends of the detection resistor 26 will be referred to as the detection voltage. When the detection voltage is below the predetermined voltage, the comparator 23 switches the output voltage from a low level to a high level. When the detection voltage exceeds the predetermined voltage, the comparator 23 switches the output voltage from a high level to a low level.

[0058] Within the current adjustment circuit 25, the differential amplifier 31 outputs a voltage with a reference potential equal to ground. The output voltage of the differential amplifier 31 is applied to the gate of the adjustment FET 30. The higher the output voltage of the differential amplifier 31, the greater the resistance between the drain and source of the adjustment FET 30. Conversely, the lower the output voltage of the differential amplifier 31, the smaller the resistance between the drain and source of the adjustment FET 30. The differential amplifier 31 adjusts the resistance between the drain and source of the adjustment FET 30 by adjusting its output voltage. The adjustment FET 30 and the differential amplifier 31 function as a variable resistor and a resistance adjustment section, respectively.

[0059] Hereinafter, the voltage at the source of the power supply FET20 will be recorded as the switching voltage. The voltage at the output of the resistor circuit 24 will be recorded as the resistor voltage. The reference potential for both the switching voltage and the resistor voltage is ground potential. The differential amplifier 31 will adjust its output voltage to be higher when the differential voltage calculated by subtracting the resistor voltage from the switching voltage is higher.

[0060] When the switching voltage rises to a level higher than the resistor voltage, the differential amplifier 31 causes its output voltage to rise. At this time, the larger the difference between the switching voltage and the resistor voltage, the greater the rise in the output voltage of the differential amplifier 31. Due to the rise in the output voltage of the differential amplifier 31, the resistance between the drain and source of the adjusting FET 30 increases, and the current flowing through the resistor circuit 24 decreases. As a result, the magnitude of the voltage drop generated in the resistor circuit 24 decreases, and the resistor voltage rises.

[0061] When the switching voltage drops to a level lower than the resistor voltage, the differential amplifier 31 causes its output voltage to decrease. At this time, the larger the absolute value of the difference between the switching voltage and the resistor voltage, the greater the drop in the output voltage of the differential amplifier 31. Due to the decrease in the output voltage of the differential amplifier 31, the resistance between the drain and source of the adjusting FET 30 decreases, and the current flowing through the resistor circuit 24 increases. As a result, the magnitude of the voltage drop generated in the resistor circuit 24 increases, and the resistor voltage decreases.

[0062] As described above, the differential amplifier 31 adjusts the resistance value between the drain and source of the adjustment FET 30 in a manner that makes the switching voltage and the resistor voltage consistent. The voltage across the DC power supply 11 is denoted as Vb. The on-resistance value of the power supply FET 20 is denoted as Ra. The switching current flowing through the power supply FET 20 is denoted as Ia. The resistance value of the resistor circuit 24 when it is on is denoted as Rt. The resistor current flowing through the resistor circuit 24 is denoted as Is. When the power supply FET 20 is on, the switching voltage is represented by (Vb - Ra·Ia). "·" indicates a product. When the resistor circuit 24 is on, the resistor voltage is represented by (Vb - Rt·Is).

[0063] Since the switching voltage is the same as the resistor voltage, the following equation holds true when the power supply FET20 and the resistor circuit 24 are turned on.

[0064] Vb-Ra·Ia=Vb-Rt·Is

[0065] By expanding this expression, we can obtain the following expression.

[0066] Is=Ra·Ia / Rt

[0067] Therefore, when the resistor circuit 24 is turned on, the differential amplifier 31 adjusts the resistor current Is to (Ra·Ia / Rt). Ra·Ia is the voltage between the two ends of the power supply FET 20 when the power supply FET 20 is turned on.

[0068] Let Rs and Vd represent the resistance value of the sensing resistor 26 and the sensing voltage, respectively. The sensing voltage Vd is represented by Rs·Is, and the resistor current Is is represented by (Ra·Ia / Rt), therefore the following formula holds.

[0069] Vd=Rs·Ra·Ia / Rt

[0070] The specified voltage applied to the positive terminal of comparator 23 is denoted as Vr. The output voltage of comparator 23 is a high-level voltage when Vr ≥ Vd. When Vr ≥ Vd, the following formula can be obtained by substituting (Rs·Ra·Ia / Rt) into Vd.

[0071] Vr ≥ Rs·Ra·Ia / Rt

[0072] The following formula can be obtained by expanding this formula.

[0073] Ia ≤ (Rt·Vr) / (Rs·Ra)

[0074] When the output voltage of comparator 23 satisfies Vr < Vd, it is a low-level voltage. When Vr < Vd, substitute (Rs·Ra·Ia / Rt) into Vd and expand the formula. Thus, the following formula can be obtained.

[0075] Ia > (Rt·Vr) / (Rs·Ra)

[0076] Define the current threshold Ith as follows.

[0077] Ith = (Rt·Vr) / (Rs·Ra)

[0078] Comparator 23 switches the output voltage to a high-level voltage when the switching current Ia becomes a current below the current threshold Ith, and switches the output voltage to a low-level voltage when the switching current Ia exceeds the current threshold Ith.

[0079] When the power supply FET 20 and the resistor circuit 24 are off, the current does not flow through the load 12 and does not flow through the detection resistor 26. Therefore, the switching voltage and the resistor voltage are 0V, and thus the differential amplifier 31 stops adjusting the output voltage. When the resistor circuit 24 is off, the current does not flow through the detection resistor 26, so the detection voltage is 0V. Since the specified voltage is a positive voltage, 0V is a voltage below the specified voltage Vr. Therefore, when the power supply FET 20 and the resistor circuit 24 are off, comparator 23 outputs a high-level voltage.

[0080] <Operation of the power supply control device 10>

[0081] Figure 2 is a timing diagram for explaining the operation of the power supply control device 10. In Figure 2 shows the changes in the output voltage of the microcomputer 22, the state of the power supply FET 20, the output voltage of the comparator 23, and the switching current. The horizontal axis of the above changes represents time. In Figure 2 high-level voltage and low-level voltage are represented by H and L respectively. When the output voltage of the microcomputer 22 is a low-level voltage, the drive circuit 21 keeps the power supply FET 20 off, so the switching current is 0A. When the power supply FET 20 is off, the resistor circuit 24 is also off, so the output voltage of the comparator 23 is a high-level voltage.

[0082] When the microprocessor 22 switches the output voltage from a low level to a high level, the drive circuit 21 switches the power supply FET 20 to the on position, regardless of the output voltage of the comparator 23. As a result, the switching current flows through the power supply FET 20, and the switching current increases from 0A. When the power supply system 1 is functioning normally, the switching current is below the current threshold Ith when the power supply FET 20 is on.

[0083] As previously stated, when the drive circuit 21 switches the power supply FET 20 to the on state, the resistor circuit 24 also switches to the on state. Therefore, when the power supply FET 20 is on, a detection voltage proportional to the switching current is applied to the comparator 23. Here, the switching current is below the current threshold Ith, so the comparator 23 continuously outputs a high-level voltage.

[0084] When the microprocessor 22 switches the output voltage from a high level to a low level, the drive circuit 21 switches the power supply FET 20 off, regardless of the output voltage of the comparator 23. As a result, the switching current drops to 0A. As mentioned earlier, when the drive circuit 21 switches the power supply FET 20 off, the resistor circuit 24 also switches off. Therefore, the detection voltage drops to 0V. The comparator 23 continues to output a high level voltage.

[0085] As previously stated, when the microprocessor 22 switches the output voltage from a low level to a high level, the drive circuit 21 switches the power supply FET 20 to the on position, and the switching current flows through the power supply FET 20. When the power supply FET 20 is on, the comparator 23 outputs a high-level voltage when the switching current is below the current threshold Ith. A fault occurs in the power supply system 1, assumed to be an increase in the switching current. For example, a short circuit across the load 12.

[0086] When the switching current exceeds the current threshold Ith, comparator 23 switches the output voltage from a high level to a low level. When the output voltage of comparator 23 switches from a high level to a low level while the output voltage of microprocessor 22 is already high, drive circuit 21 switches the power supply FET 20 off. As a result, the switching current drops to 0A. With drive circuit 21 switching the power supply FET 20 off, resistor circuit 24 also switches off, thus the detected voltage drops to 0V. Consequently, comparator 23 switches the output voltage from a low level to a high level. Drive circuit 21 keeps power supply FET 20 off until the output voltage of microprocessor 22 switches from a low level to a high level. Drive circuit 21 functions as a switching circuit.

[0087] As described above, the drive circuit 21 switches the power supply FET 20 off when the switching current exceeds the current threshold Ith, thus preventing overcurrent from flowing through the power supply FET 20. In the event of overcurrent flowing through the power supply FET 20, the temperature of the power supply FET 20 may rise to an abnormal level. At this point, a malfunction may occur in the power supply FET 20.

[0088] <Temperature dependence of the on-resistance of power supply FET20>

[0089] Figure 3 This is a graph showing the relationship between the on-resistance Ra of the power supply FET 20 and the ambient temperature. When switching current flows through the power supply FET 20, the FET 20 heats up, and its temperature rises. The ambient temperature of the power supply FET 20 changes in the same way as its temperature. Therefore, the on-resistance Ra of the power supply FET 20 varies according to its temperature, i.e., the ambient temperature. Figure 3 As shown, the higher the ambient temperature of the power supply FET20, the higher the on-resistance Ra of the power supply FET20.

[0090] As previously stated, the current threshold Ith is represented by (Rt·Vr) / (Rs·Ra). Here, Rt, Vr, and Rs are the resistance values ​​of the resistor circuit 24, the specified voltage, and the resistance value of the sensing resistor 26, respectively. The current threshold Ith is preferably constant regardless of the temperature of the power supply FET 20, i.e., the ambient temperature of the power supply FET 20. However, the on-resistance Ra of the power supply FET 20 varies depending on the ambient temperature of the power supply FET 20.

[0091] The resistance value Rt of resistor circuit 24 changes in the same direction as the on-resistance value of power supply FET 20 according to the ambient temperature of power supply FET 20. Since the ambient temperature of power supply FET 20 increases, it is assumed that the on-resistance value of power supply FET 20 increases to twice its original value. In this case, if the resistance value Rt of resistor circuit 24 increases to twice its original value, the current threshold Ith will not change.

[0092] To maintain a constant target current Ig, the current threshold Ith needs to satisfy the following equation.

[0093] Rt / Ra=Rs·Ig / Vr

[0094] Here, the resistance value Rs of the sensing resistor 26 is constant regardless of the ambient temperature of the power supply FET 20, therefore (Rs·Ig / Vr) is constant. As for the resistor circuit 24, it is sufficient to design a circuit where the ratio of the resistance value Rt of the resistor circuit 24 to the on-resistance value Ra of the power supply FET 20 remains constant even if the ambient temperature of the power supply FET 20 varies. Thus, the target current Ig, which maintains a constant current threshold Ith regardless of the ambient temperature of the power supply FET 20, is achieved.

[0095] <Structure of Resistor Circuit 24>

[0096] Figure 4 This is the circuit diagram of resistor circuit 24. Resistor circuit 24 includes a FET 40 and a series resistor 41. FET 40 is an N-channel MOSFET, functioning as a second semiconductor switch. The drain and gate of FET 40 are connected to the drain and gate of power supply FET 20, respectively. One end of series resistor 41 is connected to the source of FET 40. The other end of series resistor 41 is connected to the source of adjustment FET 30 in current adjustment circuit 25. The drain and gate of FET 40 are the input and control terminals of resistor circuit 24, respectively. The other end of series resistor 41 is the output terminal of resistor circuit 24.

[0097] As described above, series resistor 41 is connected in series with circuit FET 40.

[0098] When the output voltage of the drive circuit 21 is above the second turn-on voltage, the FET 40 is turned on. Regarding the FET 40, when it is turned on, the resistance between the drain and source is small, and current may flow through both the drain and source. Turning on the FET 40 refers to the turning on of the resistor circuit 24. The resistance value of the FET 40 when it is turned on is the resistance value between the drain and source of the FET 40 when it is not turned on. When the FET 40 is turned on, current flows from the positive terminal of the DC power supply 11 in the order of FET 40, series resistor 41, adjusting FET 30, and sensing resistor 26.

[0099] When the output voltage of the drive circuit 21 is less than the second cut-off voltage, the FET 40 is off. Regarding the FET 40, when it is off, the resistance between the drain and source is sufficiently large, and current does not flow through the drain and source. The off state of the FET 40 refers to the disconnection of the resistor circuit 24. When the FET 40 is off, current does not flow through the resistor circuit 24.

[0100] When the drive circuit 21 switches the power supply FET 20 to the on state, the circuit FET 40 also switches to the on state. When the drive circuit 21 switches the power supply FET 20 to the off state, the circuit FET 40 also switches to the off state.

[0101] The circuit FET 40 is, for example, positioned near the power supply FET 20. Therefore, the on-resistance of the circuit FET 40 varies depending on the ambient temperature of the power supply FET 20. As mentioned earlier, both the power supply FET 20 and the circuit FET 40 are N-channel MOSFETs, and their constructions are identical. Therefore, the on-resistance of the circuit FET 40 varies in the same direction as the on-resistance of the power supply FET 20 depending on the ambient temperature of the power supply FET 20. Therefore, if the on-resistance of the power supply FET 20 increases due to a change in the ambient temperature of the power supply FET 20, the on-resistance of the circuit FET 40 also increases.

[0102] Resistor circuit 24 has circuit FET 40, so the resistance value of resistor circuit 24 varies in the same direction as the on-resistance value of power supply FET 20 depending on the ambient temperature of power supply FET 20. As circuit FET 40, a FET with the same construction as power supply FET 20 is used, thereby making resistor circuit 24 easy to implement.

[0103] By connecting a series resistor 41 in series with the circuit FET 40, the resistance value of the resistor circuit 24 when the circuit FET 40 is turned on can be adjusted in such a way that the ratio of the resistance value of the resistor circuit 24 when the circuit FET 40 is turned on to the on resistance value of the power supply FET 20 is kept constant.

[0104] The on-resistance of FET40 and the resistance of series resistor 41 are denoted as Rr and Rc, respectively. The resistance Rt of circuit 24 when FET40 is on is represented by (Rr + Rc). The resistance Rc of series resistor 41 is constant regardless of the ambient temperature of power supply FET20.

[0105] <Parameter Design>

[0106] Figure 5 This is an explanatory diagram illustrating the method for determining the resistance value Rc of the series resistor 41. Here, an example of parameter design is described. Figure 5 The relationship between the on-resistance Ra of power supply FET 20, the on-resistance Rr of circuit FET 40, and the resistance Rt of resistor circuit 24 and the ambient temperature of power supply FET 20 are shown. As mentioned earlier, Rt is the resistance value of resistor circuit 24 when circuit FET 40 is on.

[0107] Within a preset ambient temperature range, a FET whose on-resistance value changes linearly is used as the power supply FET 20 and the circuit FET 40. Therefore, the formulas representing the relationship between the on-resistance value Ra of the power supply FET 20 and the ambient temperature, and the formulas representing the relationship between the on-resistance value Rr of the circuit FET 40 and the ambient temperature, can be expressed by linear formulas.

[0108] The ambient temperature of the power supply FET20 is denoted as T. The relationship between the on-resistance Ra of the power supply FET20 and the ambient temperature T is expressed by the following approximate formula.

[0109] Ra=Ca·T+Ba

[0110] Ca is a positive constant. Ba is a constant. The on-resistance Ra of the power supply FET20 is represented by a linear function of the ambient temperature T of the power supply FET20.

[0111] The relationship between the on-resistance Rr of the circuit FET40 and the ambient temperature T is expressed by the following approximate formula.

[0112] Rr=Cr·T+Br

[0113] Cr is a positive constant. Br is a constant. The on-resistance Rr of circuit FET40 is represented by a linear function of the ambient temperature T of power supply FET20.

[0114] The resistance value of series resistor 41 is recorded as Rc. The resistance value Rt of resistor circuit 24 is expressed by the following formula.

[0115] Rt=Rr+Rc

[0116] By substituting Rr=Cr·T+Br into this equation, we can obtain the following approximate equation.

[0117] Rt=Cr·T+Br+Rc

[0118] Therefore, the ratio of the resistance value Rt of the resistor circuit 24 to the on-resistance value Ra of the power supply FET 20 is expressed by the following approximate formula.

[0119] Rt / Ra=(Cr·T+Br+Rc) / (Ca·T+Ba)

[0120] By expanding this expression, we can obtain the following expression.

[0121] Rt / Ra=(Cr / Ca)·(Ca·T+(Ca·(Br+Rc) / Cr)) / ((Ca·T)+Ba)

[0122] As mentioned earlier, when the ratio of the resistance value Rt of the resistor circuit 24 to the on-resistance value Ra of the power supply FET 20 is constant, the current threshold is constant regardless of the ambient temperature T of the power supply FET 20 (see reference). Figure 3 Therefore, under the condition that the following equation is satisfied, the ratio of the resistance value Rt of the resistor circuit 24 to the on-resistance value Ra of the power supply FET 20 is constant.

[0123] Ca·(Br+Rc) / Cr=Ba

[0124] By expanding this expression, we can obtain the following expression.

[0125] Rc=(Cr·Ba / Ca)-Br

[0126] Therefore, by using a resistor with a resistance value of ((Cr·Ba / Ca)-Br) as the series resistor 41, a structure in which the current threshold is constant regardless of the ambient temperature T of the power supply FET 20 can be achieved. In this case, the following equation holds.

[0127] Rt / Ra=Cr / Ca

[0128] As mentioned above, the ratio of the resistance value Rt of the resistor circuit 24 to the on-resistance value Ra of the power supply FET 20 is also expressed by the following formula.

[0129] Rt / Ra=Rs·Ig / Vr

[0130] As mentioned earlier, Rs, Ig, and Vr are the resistance value, target current, and specified voltage of the detection resistor 26, respectively.

[0131] Therefore, after determining the resistance value Rc of the series resistor 41, the resistance value Rs of the detection resistor 26, the target current Ig, and the specified voltage Vr can be determined by satisfying the following formula.

[0132] Rs·Ig / Vr=Cr / Ca

[0133] <Effect of Power Supply Control Device 10>

[0134] The switching resistance of the power supply FET 20 and the resistance of the resistor circuit 24 change in the same direction depending on the ambient temperature of the power supply FET 20. Therefore, even if the ambient temperature of the power supply FET 20 changes, the current flowing through the sensing resistor 26 remains almost unchanged. The voltage across the sensing resistor 26 remains almost constant depending on the ambient temperature of the power supply FET 20 and is proportional to the switching current flowing through the power supply FET 20. If the voltage across the sensing resistor 26 exceeds a specified voltage, the switching current exceeds a current threshold, and the drive circuit 21 switches the power supply FET 20 off. In this way, without calculating the switching current based on the voltage across the sensing resistor 26, the drive circuit 21 can switch the power supply FET 20 off at the appropriate time.

[0135] The ratio of the on-resistance of the power supply FET 20 to the on-resistance of the circuit FET 40 is assumed to be constant regardless of the ambient temperature. Even in such a case, as in the power supply control device 10, by connecting the circuit FET 40 in series with the series resistor 41, it is possible to achieve a structure in which the ratio of the on-resistance of the power supply FET 20 to the resistance of the resistor circuit 24 is constant regardless of the ambient temperature of the power supply FET 20.

[0136] (Implementation Method 2)

[0137] In the resistor circuit 24 of the power supply control device 10 in Embodiment 1, a series resistor 41 is connected in series with the circuit FET 40. However, the structure of the resistor circuit 24 is not limited to this structure.

[0138] Hereinafter, regarding Embodiment 2, the differences from Embodiment 1 will be explained. Except for the structure described later, the other structures are the same as in Embodiment 1. Therefore, for structural parts common to Embodiment 1, the same reference numerals as in Embodiment 1 will be used, and their descriptions will be omitted.

[0139] <Structure of Resistor Circuit 24>

[0140] Figure 6 This is a circuit diagram of the resistor circuit 24 in Embodiment 2. In addition to the circuit FET 40 and the series resistor 41, the resistor circuit 24 in Embodiment 2 also includes a parallel resistor 42. The parallel resistor 42 is connected between the drain and source of the circuit FET 40. Thus, the parallel resistor 42 is connected in parallel with the circuit FET 40.

[0141] Therefore, in Embodiment 2, the resistor current flows through the resistor circuit 24 not only when the circuit FET 40 is on, but also when the circuit FET 40 is off. The resistance value of the parallel resistor 42 is recorded as Rp. The resistance value Rp of the parallel resistor 42 is constant regardless of the ambient temperature of the power supply FET 20. The resistance value Rt of the resistor circuit 24 is (Rr·Rp / (Rr+Rp))+Rc. As described in Embodiment 1, Rr and Rc are the on-resistance value of the circuit FET 40 and the resistance value of the series resistor 41, respectively. Rt is the resistance value of the resistor circuit 24 when the circuit FET 40 is on. The resistance value of the resistor circuit 24 when the circuit FET 40 is off is (Rp+Rc).

[0142] <Operation of Power Supply Control Device 10>

[0143] With the power supply FET 20 off, the switching voltage is 0V. Therefore, the differential amplifier 31 lowers the gate voltage of the adjustment FET 30 to reduce the resistor voltage. As a result, with the power supply FET 20 off, the resistance between the drain and source of the adjustment FET 30 is adjusted to a sufficiently small value. With the power supply FET 20 off, the circuit FET 40 is off, and the voltage across the DC power supply 11 is divided by the resistor circuit 24 and the sensing resistor 26. The divided voltage obtained by dividing the voltage across the DC power supply 11 is applied to the negative terminal of the comparator 23. The divided voltage when the circuit FET 40 is off exceeds the specified voltage Vr.

[0144] Figure 7 This is a timing diagram used to illustrate the operation of the power supply control device 10. Figure 7 Corresponding to Figure 2 .exist Figure 7 In, with Figure 2 Similarly, the shifts in the output voltage of microprocessor 22, the state of power supply FET 20, the output voltage of comparator 23, and the switching current are shown. The horizontal axis represents time. Figure 7 In this context, high-level voltage and low-level voltage are represented by H and L, respectively.

[0145] In Embodiment 2, when the power supply FET 20 is off, the circuit FET 40 is also off. With the circuit FET 40 off, the voltage divided by the resistor circuit 24 and the detection resistor 26 exceeds the specified voltage Vr, as previously described. Therefore, the output voltage of the comparator 23 is a low-level voltage. When the power supply FET 20 is on, the shift in the output voltage of the comparator 23 is the same as in Embodiment 1. The shift in the output voltage of the microcomputer 22 and the shift in the state of the power supply FET 20 are both the same as in Embodiment 1. Therefore, when the output voltage of the microcomputer 22 is high, and the switching current flowing through the power supply FET 20 exceeds the current threshold, the output voltage of the comparator 23 switches from high to low, and the drive circuit 21 switches the power supply FET 20 off.

[0146] <The effect of parallel resistor 42>

[0147] Figure 8 This is an illustration of the effect of the parallel resistor 42. Figure 8 The relationship between the on-resistance Rr of circuit FET40 and the ambient temperature of power supply FET20 is shown. Furthermore, Figure 8 The diagram shows the relationship between the resistance value of the parallel circuit of FET40 and parallel resistor42 and the ambient temperature of power supply FET20. Here, the resistance value of the parallel circuit is the resistance value when FET40 is turned on.

[0148] The resistance of the parallel circuit of FET40 and parallel resistor 42 is (Rr·Rp / (Rr+Rp)), which is lower than the on-resistance Rr of FET40. Therefore, by connecting parallel resistor 42 in parallel with FET40, the resistance decreases from Rr to (Rr·Rp / (Rr+Rp)). The on-resistance of FET40 when the ambient temperature of power supply FET20 is a first temperature T1 is recorded as Rr1. The on-resistance of FET40 when the ambient temperature of power supply FET20 is a second temperature T2 is recorded as Rr2. Similar to Embodiment 1, the higher the ambient temperature of power supply FET20, the greater the on-resistance Rr of FET40. The second temperature T2 is higher than the first temperature T1, therefore the on-resistance Rr2 is greater than the on-resistance Rr1.

[0149] When the ambient temperature around the power supply FET 20 is a first temperature T1, the resistance value of the parallel circuit is represented by (Rr1·Rp / (Rr1+Rp)). By connecting a parallel resistor 42 in parallel with the circuit FET 40, the resistance value decreases from Rr1 to (Rr1·Rp / (Rr1+Rp)). Similarly, when the ambient temperature around the power supply FET 20 is a second temperature T2, the resistance value of the parallel circuit is represented by (Rr2·Rp / (Rr2+Rp)). By connecting a parallel resistor 42 in parallel with the circuit FET 40, the resistance value decreases from Rr2 to (Rr2·Rp / (Rr2+Rp)).

[0150] The decrease in resistance when the ambient temperature of the power supply FET 20 is the second temperature T2 is greater than the decrease in resistance when the ambient temperature of the power supply FET 20 is the first temperature T1. For example, let the on-resistance values ​​Rr1, Rr2, and Rp be assumed to be 5 ohms, 10 ohms, and 5 ohms, respectively. When the ambient temperature of the power supply FET 20 is the second temperature T2, the resistance decreases from 10 ohms to 3.33 (=10 / 3) ohms, a decrease of 6.66 (=20 / 3) ohms. When the ambient temperature of the power supply FET 20 is the first temperature T1, the resistance decreases from 5 ohms to 2.5 ohms, a decrease of 2.5 ohms.

[0151] In Embodiment 2, the resistance value of the parallel circuit is separately described as Rr. Furthermore, within a predetermined ambient temperature range, the relationship between the resistance value of the parallel circuit and the ambient temperature of the power supply FET 20 is expressed by the following approximation.

[0152] Rr=Cr·T+Br

[0153] Here, T is the ambient temperature of the power supply FET20. Cr is a positive constant. Br is a constant.

[0154] Regarding the resistance value Rc of the series resistor 41, it is determined to be ((Cr·Ba / Ca)-Br) in the same manner as in Embodiment 1. This achieves a structure where the current threshold is constant regardless of the ambient temperature T of the power supply FET 20. In this case, the formula remains the same as in Embodiment 1.

[0155] Rt / Ra=Cr / Ca

[0156] As described in Embodiment 1, Ba and Ca are constants used in the approximation formula for the on-resistance value Ra of the power supply FET 20. Rt is the resistance value of the resistor circuit 24.

[0157] Comparing Embodiment 2 with Embodiment 1, by connecting a parallel resistor 42 in parallel with the circuit FET 40, the constant Cr can be reduced. As a result, the ratio of the resistance value Rt of the resistor circuit 24 to the on-resistance value Ra of the power supply FET 20 can be adjusted.

[0158] As described in Embodiment 1, the ratio of the resistance value Rt of the resistor circuit 24 to the on-resistance value Ra of the power supply FET 20 is also expressed by the following formula.

[0159] Rt / Ra=Rs·Ig / Vr

[0160] As described in Embodiment 1, Rs, Ig, and Vr are the resistance value of the detection resistor 26, the target current, and the specified voltage, respectively.

[0161] After determining the resistance value Rc of the series resistor 41, the resistance value Rs of the detection resistor 26, the target current Ig, and the specified voltage Vr are determined in such a way that Rt / Ra=Rs·Ig / Vr is satisfied. In Embodiment 2, the constant Cr can be adjusted, so the resistance value Rs of the detection resistor 26, the target current Ig, and the specified voltage Vr can be easily determined to the desired values.

[0162] <Effect of Power Supply Control Device 10>

[0163] The power supply control device 10 of Embodiment 2 also performs the same effect as the power supply control device 10 of Embodiment 1.

[0164] (Implementation Method 3)

[0165] In the resistor circuit 24 of Embodiment 2, a circuit FET40, i.e., a semiconductor switch, is used as the element whose resistance value changes according to the ambient temperature of the power supply FET20. However, the element whose resistance value changes according to the ambient temperature of the power supply FET20 is not limited to a semiconductor switch.

[0166] Hereinafter, regarding Embodiment 3, the differences from Embodiment 2 will be explained. Except for the structure described later, the other structures are common to Embodiment 2. Therefore, for structural parts common to Embodiment 2, the same reference numerals as in Embodiment 2 will be used, and their descriptions will be omitted.

[0167] Figure 9This is a circuit diagram of the resistor circuit 24 in Embodiment 3. The resistor circuit 24 in Embodiment 3, like that in Embodiment 2, has a series resistor 41 and a parallel resistor 42. The resistor circuit 24 in Embodiment 3 replaces the circuit FET 40 with a thermistor 50. The resistance value of the thermistor 50 increases as the temperature of the thermistor 50 rises. The thermistor 50 is located near the power supply FET 20. Therefore, as the temperature around the power supply FET 20 rises, the temperature of the thermistor 50 also rises. As a result, the resistance value of the thermistor 50 increases.

[0168] One end of the thermistor 50 is connected to the drain of the power supply FET 20. The other end of the thermistor 50 is connected to one end of the series resistor 41. A parallel resistor 42 is connected in parallel to the thermistor 50. The resistance value of the thermistor 50 corresponds to the on-resistance value of the circuit FET 40 in Embodiment 2. One end of the thermistor 50 is the input terminal of the resistor circuit 24. Similar to Embodiment 2, the other end of the series resistor 41 is the output terminal of the resistor circuit 24. In Embodiment 3, the resistor circuit 24 does not have a control terminal.

[0169] <Operation of Power Supply Control Device 10>

[0170] In Embodiment 3, the drive circuit 21 is not connected to the resistor circuit 24, and the resistance value of the resistor circuit 24 does not change according to the output voltage of the drive circuit 21. The resistance value of the thermistor 50 corresponds to the on-resistance value Rr of the circuit FET 40 in Embodiment 2. A thermistor whose resistance value changes linearly is used as the thermistor 50 within a preset ambient temperature range. The resistance value of the resistor circuit 24 corresponds to the resistance value Rt in Embodiment 2. When the power supply FET 20 is off, the resistance value between the drain and source of the FET 30 is adjusted to be sufficiently small, so the voltage between the two ends of the DC power supply 11 is divided by the resistor circuit 24 and the sensing resistor 26. The voltage divided by dividing the voltage between the two ends of the DC power supply 11 is applied to the negative terminal of the comparator 23. The voltage divided when the power supply FET 20 is off exceeds the specified voltage Vr. Therefore, the operation of the power supply control device 10 is the same as in Embodiment 2.

[0171] <Effect of Power Supply Control Device 10>

[0172] The power supply control device 10 in Embodiment 3 also performs the same effect as the power supply control device 10 in Embodiment 2.

[0173] (Implementation Method 4)

[0174] In the resistor circuit 24 of the power supply control device 10 in Embodiment 2, a parallel resistor 42 is connected in parallel with the circuit FET 40. However, the location where the parallel resistor 42 is connected is not limited to the drain and source of the circuit FET 40.

[0175] Hereinafter, regarding Embodiment 4, the differences from Embodiment 2 will be explained. Except for the structure described later, the other structures are common to Embodiment 2. Therefore, for structural parts common to Embodiment 2, the same reference numerals as in Embodiment 2 will be used, and their descriptions will be omitted.

[0176] Figure 10 This is a circuit diagram of the resistor circuit 24 in embodiment 4. For example... Figure 10 As shown, in the resistor circuit 24 of embodiment 4, a parallel resistor 42 is connected in parallel to the series circuit of the circuit FET 40 and the series resistor 41.

[0177] In Embodiment 4, similar to Embodiment 2, the resistor current flows through the resistor circuit 24 not only when the circuit FET 40 is turned on, but also when the circuit FET 40 is turned off. The resistance value of the series circuit when the circuit FET 40 is turned on is recorded as Rf. Here, the series circuit is a circuit in which the series resistor 41 is connected in series with the circuit FET 40. The resistance value Rf is represented by (Rr + Rc). As described in Embodiment 1, Rr and Rc are the on-resistance value of the circuit FET 40 and the resistance value of the series resistor 41, respectively.

[0178] The resistance value Rt of resistor circuit 24 is (Rf·Rp / (Rf+Rp)). As described in Embodiment 2, Rp is the resistance value of parallel resistor 42. Rt is the resistance value of resistor circuit 24 when circuit FET 40 is turned on. The resistance value of resistor circuit 24 when circuit FET 40 is turned off is Rp.

[0179] <Operation of Power Supply Control Device 10>

[0180] As described in Embodiment 2, when the power supply FET 20 is off, the resistance between the drain and source of the adjustment FET 30 is adjusted to a sufficiently small value. When the power supply FET 20 is off, the circuit FET 40 is off, and the voltage across the DC power supply 11 is divided by the resistor circuit 24 and the detection resistor 26. The divided voltage obtained by dividing the voltage across the DC power supply 11 is applied to the negative terminal of the comparator 23. The divided voltage when the circuit FET 40 is off exceeds the specified voltage Vr. Therefore, the operation of the power supply control device 10 is the same as in Embodiment 2 (see [reference]). Figure 7 ).

[0181] <Effect of series resistor 41>

[0182] like Figure 5 As shown, the resistance value Rf of the series circuit when the FET40 is turned on is represented by (Rr + Rc), which exceeds the on-resistance value Rr of the FET40. Here, the series circuit is a circuit in which the series resistor 41 is connected in series with the FET40. The larger the resistance value Rc of the series resistor 41, the larger the resistance value Rf of the series circuit. By adjusting the resistance value Rc of the series resistor 41, the minimum resistance value Rt of the resistance circuit 24 within a preset ambient temperature range can be adjusted.

[0183] <The effect of parallel resistor 42>

[0184] In the explanation of the effect of the parallel resistor 42 in Embodiment 2 (see...), Figure 8 In Embodiment 4, the effect of the parallel resistor 42 can be explained by replacing the on-resistance value Rr of the circuit FET 40 with the resistance value Rf of the series circuit of the circuit FET 40 and the series resistor 41. By adjusting the resistance value of the parallel resistor 42, the slope of the resistance value Rt of the resistance circuit 24, which rises according to the ambient temperature of the power supply FET 20, can be adjusted. As described in Embodiment 1, the resistance value Rt is the resistance value of the resistance circuit 24 when the circuit FET 40 is on.

[0185] <Parameter Design>

[0186] Here is an example of parameter design. The resistance values ​​Rc of series resistor 41 and Rp of parallel resistor 42 are determined such that the resistance value Rt of resistor circuit 24 satisfies the following equation, independent of the ambient temperature of power supply FET 20. D is a positive constant.

[0187] Rt / Ra = D

[0188] As described in Embodiment 1, Ra is represented by (Ca·T+Ba). As described in Embodiment 1, T is the ambient temperature of the power supply FET 20. Ca is a positive constant. Ba is a constant. The resistance values ​​Rc of the series resistor 41 and Rp of the parallel resistor 42 are determined such that the resistance value Rt of the resistor circuit 24 satisfies the following formula.

[0189] Rt=D·(Ca·T+Ba)

[0190] As described in Embodiment 1, Rt / Ra is represented by (Rs·Ig / Vr). Rs, Ig, and Vr are the resistance value of the sensing resistor 26, the target current, and the specified voltage, respectively. The resistance value Rs, the target current Ig, and the specified voltage Vr of the sensing resistor 26 can be determined as long as the following formula is satisfied.

[0191] Rs·Ig / Vr=D

[0192] <Effect of Power Supply Control Device 10>

[0193] Similar to Embodiment 2, the on-resistance value of the power supply FET 20 and the resistance value of the resistor circuit 24 change in the same direction according to the ambient temperature of the power supply FET 20, so the drive circuit 21 can switch the power supply FET 20 to off at the appropriate time.

[0194] It is assumed that the ratio of the on-resistance of the power supply FET 20 to the on-resistance of the circuit FET 40 is not constant regardless of the ambient temperature. Even in this case, as in the power supply control device 10, by connecting a series resistor 41 in series with the circuit FET 40 and a parallel resistor 42 in parallel with the series circuit, it is possible to achieve a structure in which the ratio of the on-resistance of the power supply FET 20 to the resistance of the resistor circuit 24 is constant regardless of the ambient temperature of the power supply FET 20.

[0195] <Variation Example>

[0196] In the resistor circuit 24 of Embodiment 2, a parallel resistor 42 may also be connected in parallel with the series circuit of the circuit FET 40 and a resistor (not shown). In this case, a series resistor 41 is connected in series in the series circuit. Similarly, in the resistor circuit 24 of Embodiment 3, a parallel resistor 42 may also be connected in parallel with the series circuit of the thermistor 50 and a resistor (not shown). In this case, a series resistor 41 is also connected in series in the series circuit. In Embodiment 4, similar to Embodiment 3, the thermistor 50 may be used instead of the circuit FET 40.

[0197] In Embodiment 1, when the ratio of the on-resistance value of the circuit FET40 to the on-resistance value of the power supply FET20 is constant regardless of the ambient temperature of the power supply FET20, the resistor circuit 24 may not have a series resistor 41. In this case, the source of the circuit FET40 is the output terminal of the resistor circuit 24, which is connected to the source of the adjustment FET30 of the current adjustment circuit 25.

[0198] In Embodiment 1, similar to Embodiment 3, thermistor 50 can be used instead of circuit FET 40.

[0199] In embodiment 2, when the ratio of the resistance value of the parallel circuit of FET40 and parallel resistor 42 to the on-resistance value of power supply FET20 is constant regardless of the ambient temperature of power supply FET20, resistor circuit 24 may not have series resistor 41. Here, the resistance value of the parallel circuit of FET40 and parallel resistor 42 is the resistance value when FET40 is on. When resistor circuit 24 does not have series resistor 41, the source of FET40 is the output terminal of resistor circuit 24 and is connected to the source of adjustment FET30 of current adjustment circuit 25.

[0200] Similarly, in Embodiment 3, if the ratio of the resistance value of the parallel circuit of the thermistor 50 and the parallel resistor 42 to the on-resistance value of the power supply FET 20 is constant regardless of the ambient temperature of the power supply FET 20, the resistor circuit 24 may not have a series resistor 41. In this case, the other end of the thermistor 50 is connected to the source of the adjustment FET 30 of the current adjustment circuit 25.

[0201] In embodiments 1, 2, and 4, the switch in the resistor circuit 24 can be any semiconductor switch whose switching resistance value changes in the same direction as the switching resistance value of the power supply FET 20 depending on the ambient temperature of the power supply FET 20. Therefore, the switch in the resistor circuit 24 in embodiments 1, 2, and 4 is not limited to the circuit FET 40, i.e., an N-channel MOSFET, but can also be an N-channel FET, a P-channel FET, or an IGBT (Insulated Gate Bipolar Transistor), which are different from MOSFETs.

[0202] In embodiments 1 to 4, the switch connected between the DC power supply 11 and the load 12 can be any semiconductor switch, and is not limited to the power supply FET 20, i.e., an N-channel MOSFET. The switch connected between the DC power supply 11 and the load 12 can also be, for example, an IGBT.

[0203] In embodiments 1 to 4, the variable resistor in the current adjustment circuit 25 is not limited to the adjustment FET 30, i.e., a P-channel FET. The variable resistor can also be, for example, a PNP type bipolar transistor. In this case, the emitter, collector, and base of the bipolar transistor correspond to the source, drain, and gate of the adjustment FET 30, respectively.

[0204] The disclosed embodiments 1 to 4 should be considered illustrative in all respects rather than limiting. The scope of the invention is defined not by the foregoing but by the claims, and is intended to include all modifications within the meaning and scope equivalent to the claims.

[0205] Label Explanation

[0206] 1 Power System

[0207] 10 Power supply control device

[0208] 11 DC Power Supply

[0209] 12 Load

[0210] 20 Power Supply FET (Semiconductor Switch)

[0211] 21. Drive circuit (switching circuit)

[0212] 22 microcomputers

[0213] 23 Comparator

[0214] 24 Resistor Circuit

[0215] 25 Current Adjustment Circuit

[0216] 26. Detection resistor

[0217] 30. Adjusting the FET (Variable Resistor)

[0218] 31 Differential Amplifier (Resistance Adjustment Section)

[0219] 40 Circuit FET (Second Semiconductor Switch)

[0220] 41 Series resistor

[0221] 42 Parallel resistors

[0222] 50 Thermistor.

Claims

1. A power supply control device that controls a power supply by switching a semiconductor switch through which a current flows to be on or off, wherein the power supply control device comprises: a resistance circuit that has one end connected to an upstream side of the semiconductor switch; a current adjustment circuit that adjusts a current flowing through the resistance circuit to a value obtained by dividing a voltage across the semiconductor switch by a resistance value of the resistance circuit; a resistance that is arranged in a current path of the current flowing through the resistance circuit; and a switching circuit that switches the semiconductor switch to be off in a case where a voltage across the resistance exceeds a prescribed voltage, the on-resistance value of the semiconductor switch varies according to a surrounding temperature of the semiconductor switch, the resistance value of the resistance circuit varies in the same direction as the on-resistance value according to the surrounding temperature, the resistance circuit has a second semiconductor switch, the switching circuit switches the second semiconductor switch to be on in a case where the semiconductor switch is switched to be on, and the on-resistance value of the second semiconductor switch varies in the same direction as the on-resistance value of the semiconductor switch according to the surrounding temperature of the semiconductor switch.

2. The power supply control device according to claim 1, wherein the resistance circuit has a series resistance connected in series with the second semiconductor switch.

3. The power supply control device according to claim 1, wherein the resistance circuit has a parallel resistance connected in parallel with a series circuit of the second semiconductor switch and the series resistance.

4. The power supply control device according to claim 1 or 2, wherein the resistance circuit has a parallel resistance connected in parallel with the second semiconductor switch.

5. The power supply control device according to any one of claims 1 to 3, wherein the current adjustment circuit has: a variable resistor; and a resistance adjustment section that adjusts the resistance value of the variable resistor in a manner that the voltage of the downstream side of the semiconductor switch and the resistance circuit is uniform. ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

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