Switch driving device

Abstract

To provide a switch driving device in which, in the occurrence of short-circuit of an electric load 4, the failure of a MOS transistor 11 due to a load current Ix is suppressed.SOLUTION: When it is determined that an ECU control signal to turn off a MOS transistor 11 is received, a control circuit 22 sets an over-current detection threshold used in a comparator 24 to be lower than that when it is determined that an ECU control signal to turn on the MOS transistor is received. Therefore, even when the MOS transistor 11 is softly turned off with an electric load 4 short-circuited, the control circuit 22 determines that a load current Ix is higher than the over-current detection threshold and controls the MOS transistor 11 to cut off a dynamic clamp.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to a switch driver. [Background technology]

[0002] A switch driver has been proposed that includes a control circuit that detects a short circuit in a MOS transistor based on the voltage between the drain terminal and the source terminal of the MOS transistor (see, for example, patent document 1). When the control circuit detects a short circuit in the MOS transistor, it disconnects the drain terminal and the source terminal of the MOS transistor. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent No. 6593454 Summary of the Invention [Problem to be solved by the invention]

[0004] The inventors have investigated a switch driver in which the control circuit controls the load current flowing from the power supply to the electrical load through the MOS transistor by PWM-controlling the MOS transistor disposed between the power supply and the electrical load. For example, when the control circuit receives an OFF command from an external device to turn off the MOS transistor during PWM control in order to suppress the generation of electrical noise, it is conceivable that the control circuit would soft-off the MOS transistor, shutting it off slowly.

[0005] On the other hand, in the above-mentioned switch driver, if the electrical load is short-circuited and the load current flowing through the MOS transistor exceeds the threshold and becomes an overcurrent, the control circuit may turn off the MOS transistor to prevent the overcurrent from flowing through the MOS transistor. However, if the load current is below the threshold and before it reaches the overcurrent state, the control circuit may receive an off command from an external device and soft-off the MOS transistor. In this case, the soft-off operation consumes a large amount of energy in the MOS transistor based on the load current, which may cause the MOS transistor to malfunction.

[0006] In view of the above, an object of the present disclosure is to provide a switch driver that prevents a field effect transistor from being damaged by a load current when an electrical load is short-circuited. [Means for solving the problem]

[0007] According to one aspect of the present disclosure, a switch driver includes: a field-effect transistor (11) disposed between the power supply device (3) and the electrical load (4), which connects the power supply device and the electrical load when turned on and disconnects the power supply device and the electrical load when turned off; a control unit (22) that controls a load current flowing from the power supply device through the electric load to ground by turning on and off the field effect transistor based on a command given from an external device (2); a current determination unit (24) that determines whether or not the load current is equal to or greater than a threshold; a threshold setting unit (22, 26, 27, 30) that, when it is determined that a command to turn off the field effect transistor has been given from the external device, sets the threshold to a value smaller than that when it is determined that a command to turn on the field effect transistor has been given from the external device; When the current determination unit determines that the load current is equal to or greater than the threshold value, the control unit controls the field effect transistor to disconnect the power supply device from the electrical load at a faster speed than when the field effect transistor is turned off to control the load current.

[0008] Therefore, when it is determined that a command to turn off the field-effect transistor has been given from an external device, the current determination unit appropriately determines that the load current is equal to or greater than the threshold value if the electrical load is short-circuited. Accordingly, the control unit controls the field-effect transistor to disconnect the power supply device from the electrical load at a faster speed than when the field-effect transistor is turned off to control the load current. Therefore, when the electrical load is short-circuited, the energy consumed by the field-effect transistor due to the load current can be reduced. This makes it possible to provide a switch driver that prevents the field-effect transistor from failing due to the load current when the electrical load is short-circuited.

[0009] According to another aspect of the present disclosure, a switch driver includes: a field-effect transistor (11) disposed between the power supply device (3) and the electrical load (4), which connects the power supply device and the electrical load when turned on and disconnects the power supply device and the electrical load when turned off; a control unit (22) that controls a load current (Ix) flowing from the power supply device through the electric load to ground by turning on and off the field effect transistor based on a command output from an external device (2); a current determination unit (24) that determines whether or not the load current is equal to or greater than a threshold; a temperature determination unit (44) that determines whether the temperature of the field effect transistor is equal to or higher than a predetermined temperature; a threshold setting unit (22, 26, 27, 30) that sets the threshold to a smaller value when the temperature determination unit determines that the temperature of the field effect transistor is equal to or higher than a predetermined temperature, compared to when the temperature determination unit determines that the temperature of the field effect transistor is lower than the predetermined temperature; When the current determination unit determines that the load current is equal to or greater than the threshold value, the control unit controls the field effect transistor to disconnect the power supply device from the electrical load at a faster speed than when the field effect transistor is turned off to control the load current.

[0010] Therefore, when the temperature determination unit determines that the temperature of the field-effect transistor is equal to or higher than a predetermined temperature, the current determination unit appropriately determines that the load current is equal to or higher than a threshold value when the electrical load is short-circuited. Accordingly, the control unit controls the field-effect transistor to disconnect the power supply device from the electrical load at a faster speed than when the field-effect transistor is turned off to control the load current. Therefore, when the electrical load is short-circuited, the energy consumed by the field-effect transistor due to the load current can be reduced. This makes it possible to provide a switch driver that prevents the field-effect transistor from failing due to the load current when the electrical load is short-circuited. The reference symbols in parentheses attached to each component indicate an example of the correspondence between the component and the specific components described in the embodiments described below. [Brief explanation of the drawings]

[0011] [Figure 1] 1 is an electric circuit diagram showing an electric circuit configuration of a switch driving device for a vehicle according to a first embodiment of the present disclosure. [Figure 2] 3 is a flowchart showing the operation of the vehicle switch drive device and the electronic control device in the first embodiment of FIG. 1. [Figure 3] 2 is a timing chart showing an ECU control signal, a gate-source terminal voltage, a drain-source terminal voltage, an overcurrent detection threshold, a state of a MOS transistor, a VGS detection signal, and a state of a MOS transistor in the vehicle switch drive device of FIG. 1. [Figure 4] 2 is a timing chart showing an ECU control signal, a drain-source terminal voltage, a load current, power consumed by a MOS transistor, and energy consumed by a MOS transistor in the vehicle switch drive device of FIG. 1. [Figure 5] 10 is a timing chart showing an ECU control signal, a drain-source terminal voltage, a load current, power consumed by a MOS transistor, and energy consumed by a MOS transistor in a comparative vehicle switch drive device. [Figure 6]FIG. 6 is an electric circuit diagram showing an electric circuit configuration of a switch driving device for a vehicle according to a second embodiment of the present disclosure. [Figure 7] 10 is a timing chart showing an ECU control signal, a gate-source terminal voltage, a drain-source terminal voltage, an overcurrent detection threshold, a state of a MOS transistor, a VDS detection signal, a VGS detection signal, and a state of a MOS transistor in the second embodiment. [Figure 8] FIG. 10 is an electric circuit diagram showing an electric circuit configuration of a switch driving device for a vehicle according to a third embodiment of the present disclosure. [Figure 9] 10 is a timing chart showing an ECU control signal, a gate-source terminal voltage, a drain-source terminal voltage, an overcurrent detection threshold, a state of a MOS transistor, a MOS detection temperature signal, a VGS detection signal, and a state of a MOS transistor in the third embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0012] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. In the following embodiments, identical or equivalent parts are denoted by the same reference numerals in the drawings to simplify the description.

[0013] (First embodiment) Next, a first embodiment of a vehicle switch drive device 1 in which the switch drive device of the present disclosure is applied to an automobile will be described with reference to Figures 1, 2, and 3. The vehicle switch drive device 1 of this embodiment controls a MOS transistor 11 with PWM control based on a command from an electronic control device (i.e., an external device) 2, thereby controlling a load current Ix that flows from the positive terminal of an on-board power supply device 3 to an electrical load 4. PWM is an abbreviation for Pulse Width Modulation.

[0014] Specifically, as shown in Fig. 1, the vehicle switch drive device 1 includes a switch assembly 10 and a drive circuit 20. The switch assembly 10 includes a MOS transistor 11 and a MOS transistor 12. Hereinafter, the MOS transistor 11 and the MOS transistor 12 will also be collectively referred to as the MOS transistors 11, 12. The MOS transistors 11, 12 are disposed between the positive terminal of the vehicle-mounted power supply device 3 and the electrical load 4, respectively.

[0015] When the MOS transistor 11 is turned on, it connects the positive terminal of the in-vehicle power supply device 3 to the electrical load 4. When the MOS transistor 11 is turned off, it disconnects the positive terminal of the in-vehicle power supply device 3 from the electrical load 4. A drain terminal 11d of the MOS transistor 11 is connected to the positive terminal of the in-vehicle power supply device 3. A source terminal 11s of the MOS transistor 11 is connected to the electrical load 4. The drain terminal 11d is an input terminal through which a load current Ix flows. The source terminal 11s is an output terminal that passes the load current Ix flowing in the drain terminal 11d to the electrical load 4.

[0016] The gate terminals of the MOS transistors 11 and 12 are connected together to form a common connection terminal 13. The gate terminal is a control terminal to which a gate signal output from the output terminal of the gate driver 21 is applied. The common connection terminal 13 is connected to the output terminal of the gate driver 21. In this embodiment, the electric load 4 is disposed between the source terminal 11s of the MOS transistor 11 and ground. Examples of the electric load 4 include an on-board electric heater, an on-board lighting fixture, and an on-board electric motor. The MOS transistor 12 is disposed between the positive terminal of the on-board power supply device 3 and the electric load 4.

[0017] The MOS transistor 12 and the MOS transistor 11 form a current mirror circuit. The MOS transistor 12 passes a detection current Is proportional to a load current Ix between its drain terminal and source terminal based on the output voltage of the in-vehicle power supply device 3. Here, the load current Ix is a current flowing between the drain terminal 11d and source terminal 11s of the MOS transistor 11. The drain terminals of the MOS transistors 11 and 12 and the drain terminal of the MOS transistor 29 are connected in common to form a common connection terminal 14.

[0018] The common connection terminal 14 is connected to the positive terminal of the on-board power supply device 3 by a harness 5. The harness 5 is arranged between the positive terminal of the on-board power supply device 3 and the common connection terminal 14. The harness 5 has inductive reluctance and forms a current path through which a load current Ix and a detection current Is flow from the positive terminal of the on-board power supply device 3 to the drain terminals of the MOS transistors 11 and 12. The source terminal of the MOS transistor 12 is connected to the electric load 4 via a resistive element 25. The source terminal of the MOS transistor 12 and the resistive element 25 are connected together to form the common connection terminal 15.

[0019] The source terminal of the MOS transistor 11 and the resistive element 25 are connected together to form a common connection terminal 16. The common connection terminal 16 and the electrical load 4 are connected by a harness 6. The harness 6 is disposed between the common connection terminal 16 and the electrical load 4. The harness 6 forms a current path for passing a load current Ix and a detection current Is from the common connection terminal 16 to the electrical load 4. The harness 6 has inductive reluctance.

[0020] The MOS transistors 11 and 12 are, for example, n-channel metal oxide semiconductor field effect transistors. MOS is an abbreviation for metal oxide semiconductor. The drive circuit 20 includes a gate driver 21, a control circuit 22, comparators 23 and 24, resistors 25, 26, and 27, and a constant voltage circuit 28. The drive circuit 20 also includes MOS transistors 29 and 30, a current mirror circuit 31, a constant current source 32, and Zener diodes 33 and 34.

[0021] The gate driver 21 is composed of an electronic circuit including transistors, pull-up resistor elements, etc. The gate driver 21 is controlled by a control circuit 22 and outputs gate signals for turning on and off (e.g., soft-off) the MOS transistors 11 and 12 from an output terminal to the gate terminals of the MOS transistors 11 and 12. The control circuit 22 is composed of a central processing unit, memory, etc. As a control unit, the control circuit 22 controls each of the MOS transistors 29 and 30 and the gate driver 21 based on a command signal from the electronic control device 2 and output signals from comparators 23 and 24.

[0022] The comparator 23, as a voltage determination unit, compares the output voltage Vgm output from the negative terminal of the constant voltage circuit 28 with the output voltage Vs output from the common connection terminal 16, and outputs a VGS detection signal as a result to the control circuit 22. The non-inverting input terminal of the comparator 23 is connected to the common connection terminal 13 via the constant voltage circuit 28. The constant voltage circuit 28 is a constant voltage power supply that supplies a voltage obtained by stepping down the output voltage Vg output from the common connection terminal 13 to the non-inverting input terminal of the comparator 23 as the output voltage Vgm.

[0023] The constant voltage is the inter-terminal voltage generated between the positive and negative terminals of the constant voltage circuit 28. The positive terminal of the constant voltage circuit 28 is connected to the common connection terminal 13. The negative terminal of the constant voltage circuit 28 is connected to the non-inverting input terminal of the comparator 23. The inverting input terminal of the comparator 23 is connected to the common connection terminal 16. In this embodiment, the comparator 23 is used to determine whether the inter-terminal voltage VGS of the MOS transistor 11 is equal to or lower than a threshold value Sgs. The threshold value Sgs is set by the inter-terminal voltage between the positive and negative terminals of the constant voltage circuit 28.

[0024] The comparator 24 serves as a current determination unit, comparing the output voltage Va output from the common connection terminal 15 with the output voltage Vb output from the common connection terminal 17, and outputs an overcurrent determination signal representing the comparison result to the control circuit 22. The common connection terminal 15 is a common connection terminal to which the source terminal of the MOS transistor 12 and the resistance element 25 are commonly connected. The resistance element 25 serves to output the output voltage Va based on the detection current Is flowing through the MOS transistor 12 from the common connection terminal 15. The resistance element 25, together with the MOS transistor 12, constitutes a current detection unit that detects the load current Ix.

[0025] The common connection terminal 17 is a common connection terminal to which the collector terminal of the transistor 31a and the resistor element 26 are commonly connected. The resistor elements 26 and 27 are connected in series between the collector terminal of the transistor 31a and the common connection terminal 16. The resistor elements 26 and 27 serve to output an output voltage Vb from the common connection terminal 17 based on a reference current Ih output from the emitter terminal of the transistor 31a.

[0026] The non-inverting input terminal of the comparator 24 is connected to the common connection terminal 15. The inverting input terminal of the comparator 24 is connected to the common connection terminal 17. The comparator 24 of this embodiment determines whether the detection current Is flowing through the MOS transistor 12 is equal to or greater than a threshold. That is, the comparator 24 uses the detection current Is to determine whether the load current Ix is equal to or greater than the overcurrent detection threshold.

[0027] The MOS transistor 30 is controlled by the control circuit 22 to short-circuit or open the common connection terminal 16 and the common connection terminal 18. The drain terminal of the MOS transistor 30 is connected to the common connection terminal 18. The common connection terminal 18 is a common connection terminal to which the resistance elements 26 and 27 are commonly connected. The source terminal of the MOS transistor 30 is connected to the common connection terminal 16.

[0028] The MOS transistor 30 serves to short-circuit one terminal and the other terminal of the resistor element 26. The MOS transistor 30, together with the resistor elements 26 and 27, constitutes a threshold setting unit that sets the overcurrent detection threshold used in the comparator 24. The current mirror circuit 31 includes transistors 31a and 31b. The transistor 31b passes a reference current Ih, which is proportional to a constant current It that flows between the emitter terminal and collector terminal of the transistor 31a, from the collector terminal to the resistor elements 26 and 27.

[0029] The emitter terminals of the transistors 31a and 31b are connected to the positive terminal of the on-board power supply device 7. The collector terminal of the transistor 31a is connected to ground via a constant current source 32. The constant current source 32 passes a constant current It between the emitter terminal and collector terminal of the transistor 31a. The base terminals of the transistors 31a and 31b are connected to the collector terminal of the transistor 31a. In this embodiment, PNP bipolar transistors are used as the transistors 31a and 31b.

[0030] As will be described later, the Zener diodes 33 and 34 constitute a clamp circuit that keeps constant the voltage between the drain terminals and gate terminals of the MOS transistors 11 and 12 when the MOS transistor 11 is in a cut-off operation. The Zener diodes 33 and 34 are connected in series between the common connection terminals 13 and 14. The Zener diodes 33 and 34 prevent the MOS transistors 11 and 12 from breaking down due to back electromotive force generated based on magnetic energy stored in the harness 5.

[0031] The Zener diode 33 is a constant voltage generating element that generates a constant voltage (i.e., Zener voltage) based on a back electromotive current that flows from the common connection terminal 14 side to the output side of the gate driver 21. The cathode terminal of the Zener diode 33 is connected to the source terminal of the MOS transistor 29. The drain terminal of the MOS transistor 29 is connected to the common connection terminal 14. The anode terminals of the Zener diodes 33 and 34 are commonly connected to each other.

[0032] The Zener diode 34 is a constant voltage generating element that generates a constant voltage (i.e., forward voltage) based on the back electromotive current that flows from the common connection terminal 14 side to the output side of the gate driver 21. The cathode terminal of the Zener diode 34 is connected to the common connection terminal 13. The Zener diode 34 generates a constant forward voltage based on the back electromotive current that flows from the common connection terminal 14 side to the output side of the gate driver 21. The MOS transistor 29 is a switch that is controlled by the control circuit 22 to open or close the common connection terminal 14 and the cathode terminal of the Zener diode 33. The drain terminal of the MOS transistor 29 is connected to the common connection terminal 14. The source terminal of the MOS transistor 29 is connected to the cathode terminal of the diode 33. A p-channel metal oxide semiconductor field effect transistor is used as the MOS transistor 29 in this embodiment.

[0033] Next, the operation of the vehicle switch drive device 1 of this embodiment will be described with reference to Figures 1, 2, and 3. For convenience of explanation, the voltage between the gate terminal and source terminal of the MOS transistors 11 and 12 will be referred to as the inter-terminal voltage VGS. The voltage between the drain terminal and source terminal of the MOS transistors 11 and 12 will be referred to as the inter-terminal voltage VDS. The output signal output from the comparator 23 will be referred to as the VGS detection signal.

[0034] Fig. 2 is a flowchart showing the operations of the electronic control unit 2 and the control circuit 22. Fig. 3(a) is a timing chart showing changes in the signal level of the ECU control signal given from the electronic control unit 2 to the control circuit 22. Fig. 3(b) is a timing chart showing changes in the inter-terminal voltage VGS of the MOS transistor 11. Fig. 3(c) is a timing chart showing changes in the inter-terminal voltage VDS of the MOS transistor 11.

[0035] Fig. 3(d) is a timing chart showing changes in the overcurrent detection threshold indicated by the output voltage Vb applied from the common connection terminal 17 to the inverting input terminal of the comparator 24. Fig. 3(e) is a timing chart showing changes in the state of the MOS transistor 11. Fig. 3(f) is a timing chart showing changes in the signal level of the VGS detection signal output from the comparator 23. Fig. 3(g) is a timing chart showing changes in the on / off state of the MOS transistor 30.

[0036] First, the electronic control unit 2 PWM-controls the MOS transistor 11 by outputting an ECU control signal to the control circuit 22. This controls the load current Ix that flows from the positive terminal of the on-board power supply 3 through the MOS transistor 11 and the electrical load 4 to ground. For example, as shown in FIGS. 2 and 3(a), in step S100, the electronic control unit 2 outputs an ECU control signal with a high signal level to the control circuit 22 at timing t0 to instruct the MOS transistor 11 to be turned on.

[0037] At this time, in step S110, the control circuit 22 receives an ECU control signal having a high signal level from the electronic control unit 2. Accordingly, in step S120, the control circuit 22 drives the gate driver 21. As a result, the gate driver 21 outputs a gate control signal having a high signal level to the gate terminals of the MOS transistors 11 and 12. At this time, a current flows from the gate driver 21 to each of the gate terminals of the MOS transistors 11 and 12. Therefore, as shown in FIG. 3(b), due to the electrostatic capacitance between the gate terminals and source terminals of the MOS transistors 11 and 12, the inter-terminal voltage VGS of the MOS transistors 11 and 12 gradually increases over time between timing t0 and t1.

[0038] Thereafter, the voltage VGS across the MOS transistors 11 and 12 remains constant between timings t1 and t2. Here, the MOS transistors 11 and 12 each remain on between timings t0 and t1, as shown in FIG. 3(e). Thereafter, the MOS transistors 11 and 12 each enter a fully on state between timings t1 and t2. At this time, a load current Ix flows from the positive terminal of the on-board power supply device 3 to ground through the harness 5, the drain terminal 11d and source terminal 11s of the MOS transistor 11, the harness 6, and the electrical load 4.

[0039] Meanwhile, a detection current Is proportional to the load current Ix flows from the positive terminal of the on-board power supply device 3 to ground through the harness 5, the drain terminal and source terminal of the MOS transistor 12, the resistive element 25, the harness 6, and the electrical load 4. At this time, magnetic energy is stored in the harness 5 based on the load current Ix and the detection current Is. Furthermore, when the control circuit 22 receives an ECU control signal whose signal level is high from the electronic control device 2, in step S130, it turns off the MOS transistor 30 (i.e., the threshold switching SW) as shown in FIG. 3(g).

[0040] As a result, common connection terminals 16 and 18 are opened. At this time, constant current source 32 is activated, causing a constant current It to flow from the positive terminal of on-board power supply device 7 to ground through the emitter terminal and collector terminal of transistor 31b and constant current source 32. As a result, a constant reference current Ih proportional to the constant current It flows from the positive terminal of on-board power supply device 7 to ground through the emitter terminal and collector terminal of transistor 31a, resistor elements 26 and 27, harness 6, and electrical load 4.

[0041] Accordingly, a constant voltage determined by the resistance values ​​of the resistor elements 26 and 27 and the reference current Ih is generated between the common connection terminals 16 and 18. At this time, an output voltage Vb indicating the overcurrent detection threshold is applied from the common connection terminal 16 to the inverting input terminal of the comparator 24. At this time, the output voltage Vb is a voltage indicating the threshold Vth1 (i.e., the first threshold), as shown in FIG. 3(d). The output voltage Vb is a threshold for determining the magnitude of the output voltage Va (i.e., the detected current Is). The overcurrent detection threshold is a threshold for determining whether the load current Ix is an overcurrent.

[0042] At this time, as the detection current Is flows through the resistance element 25, a voltage determined by the detection current Is and the resistance value of the resistance element 25 is generated between the common connection terminals 15 and 16. Accordingly, an output voltage Va indicating the detection current Is is applied from the common connection terminal 15 to the non-inverting input terminal of the comparator 24. The output voltage Va increases as the detection current Is increases.

[0043] When the load current Ix is less than the threshold Vth1, the output voltage Va output from the common connection terminal 15 is lower than the output voltage Vb output from the common connection terminal 17. At this time, the comparator 24 determines that the load current Ix is less than the threshold Vth1 and is not an overcurrent, and makes a NO determination in step S140. Accordingly, the comparator 24 outputs an overcurrent determination signal having a low signal level to the control circuit 22. At this time, when the control circuit 22 receives an ECU control signal having a low signal level from the electronic control unit 2, it turns off the MOS transistor 29. As a result, the MOS transistor 29 opens the gap between the common connection terminal 14 and the Zener diode 33.

[0044] Next, in step S160, the electronic control unit 2 changes the signal level of the ECU control signal that is output to the control circuit 22 at timing t2 from high level (i.e., second level) to low level (i.e., first level). As a result, the electronic control unit 2 commands the control circuit 22 to turn off the MOS transistor 11. At this time, in step S170, the control circuit 22 receives the low-level ECU control signal from the electronic control unit 2.

[0045] 3(g), the control circuit 22 controls the transistor 30 to turn it on, thereby connecting the common connection terminals 16 and 18. At this time, the reference current Ih flows from the positive terminal of the on-board power supply device 7 through the emitter terminal and collector terminal of the transistor 31a, the resistive element 26, the drain terminal and source terminal of the MOS transistor 30, the harness 6, and the electrical load 4 to ground.

[0046] As a result, a constant voltage determined by the resistance value of the resistive element 26 and the reference current Ih is generated between the common connection terminals 16 and 18. Furthermore, an output voltage Vb indicating the overcurrent detection threshold is applied from the common connection terminal 16 to the inverting input terminal of the comparator 24. At this time, as shown in FIG. 3(d), the output voltage Vb is a voltage indicating a threshold Vth2 (i.e., a second threshold) determined by the resistance value of the resistive element 26 and the reference current Ih. Therefore, the overcurrent detection threshold used in the comparator 24 is switched from threshold Vth1 to threshold Vth2. Threshold Vth2 is a smaller value than threshold Vth1.

[0047] When the signal level of the ECU control signal is low, the control circuit 22, together with the MOS transistor 30 and the resistance elements 26 and 27, sets the overcurrent detection threshold to a value (i.e., threshold Vth2) that is smaller than when the signal level of the ECU control signal is high. For example, when the output voltage Va output from the common connection terminal 15 is lower than the output voltage Vb output from the common connection terminal 17, the comparator 24 determines that the load current Ix is less than threshold Vth2 and is not an overcurrent, and makes a NO determination in step S190.

[0048] Accordingly, in step S200, the control circuit 22 drives the gate driver 21 to soft-off the MOS transistor 11. To do this, the gate driver 21 changes the signal level of the gate control signal applied to the gate terminals of the MOS transistors 11 and 12 from high to low. At this time, current flows from the gate terminals of the MOS transistors 11 and 12 to ground through the gate driver 21.

[0049] Therefore, as shown in Fig. 3(b), the terminal voltage VGS of the MOS transistor 11 gradually decreases with time after timing t2 based on the capacitance between the gate terminal 11g and the source terminal 11s of the MOS transistor 11. Accordingly, the terminal voltage VDS of the MOS transistor 11 gradually increases with time after timing t2, as shown in Fig. 3(c).

[0050] Therefore, the MOS transistor 11 slowly disconnects the in-vehicle power supply device 3 from the electrical load 4. That is, as shown in FIG. 3(e), the MOS transistor 11 is turned off after timing t2. On the other hand, when the output voltage Va of the common connection terminal 15 becomes equal to or higher than the output voltage Vb of the common connection terminal 17, the comparator 24 determines that the load current Ix is equal to or higher than the threshold value Vth2, which indicates an overcurrent, and makes a YES determination in step S190.

[0051] Accordingly, the comparator 24 outputs a high-level overcurrent determination signal to the control circuit 22. Therefore, in step S210, the control circuit 22 controls the gate driver 21 and the MOS transistor 29 to perform dynamic clamp cutoff. At this time, the gate driver 21 is controlled by the control circuit 22 to change the signal level of the gate control signal applied to the gate terminals of the MOS transistors 11 and 12 from high to low more quickly than in the case of soft-off. Accordingly, the MOS transistors 11 and 12 are turned off.

[0052] In addition, the control circuit 22 turns on the MOS transistor 29. This connects the common connection terminal 14 and the cathode terminal of the Zener diode 33 via the MOS transistor 29. Therefore, a back-electromotive current based on magnetic energy flows from the harness 5 through the MOS transistor 29, the Zener diodes 33 and 34, and the gate driver 21 to ground. At this time, the back-electromotive current causes the inter-terminal voltage VGS of the MOS transistors 11 and 12 to rise for a short period of time. As a result, the MOS transistors 11 and 12 are each turned on for a short period of time. Furthermore, a constant voltage is applied between the drain terminals and gate terminals of the MOS transistors 11 and 12. This constant voltage is the sum of the Zener voltage of the Zener diode 33 generated based on the back-electromotive current and the forward voltage of the Zener diode 34.

[0053] At this time, as the MOS transistor 11 turns on, a back electromotive force (back electromotive force) based on magnetic energy flows from the harness 5 through the drain terminal 11d and source terminal 11s of the MOS transistor 11, the harness 6, and the electrical load 4 to ground. Similarly, as the MOS transistor 12 turns on, a back electromotive force based on magnetic energy flows from the harness 5 through the drain terminal and source terminal of the MOS transistor 12, the harness 6, and the electrical load 4 to ground. As a result, the back electromotive force generated in the harness 5 based on the magnetic energy is absorbed to ground through the MOS transistors 11 and 12. As a result, the value of the load current Ix flowing between the drain terminals and source terminals of the MOS transistors 11 and 12 rapidly decreases. As a result, the MOS transistors 11 and 12 can interrupt the connection between their drain terminals and source terminals more quickly than in the soft-off mode described above. Thereafter, the inter-terminal voltage VGS of the MOS transistors 11 and 12 decreases. As a result, the MOS transistors 11 and 12 are turned off. In addition, the control circuit 22 turns off the MOS transistor 29.

[0054] Furthermore, in step S140 described above, when the load current Ix becomes equal to or greater than the threshold value Vth1, the output voltage Va of the common connection terminal 15 becomes equal to or greater than the output voltage Vb of the common connection terminal 17. At this time, the comparator 24 determines that the load current Ix is an overcurrent and makes a YES determination. Accordingly, the comparator 24 outputs an overcurrent determination signal with a high signal level to the control circuit 22. Then, in the next step S150, the control circuit 22 controls the gate driver 21 and the MOS transistor 29 to perform dynamic clamp cutoff, as in step S210 described above. Therefore, the MOS transistors 11 and 12 each cut off the connection between their drain terminals and source terminals more quickly than in the soft-off mode described above.

[0055] In this way, the electronic control unit 2, the control circuit 22, and the MOS transistors 11, 12, and 29 repeat the operations of steps S100 to S210. Therefore, the electronic control unit 2 sets the signal level of the ECU control signal provided to the control circuit 22 to a high level or a low level, thereby causing the control circuit 22 to turn the MOS transistor 11 on or off. At this time, the electronic control unit 2 sets the sum of the high-level period and the low-level period of the ECU control signal as a fixed period, and changes the ratio between the high-level period and the low-level period. Therefore, the ratio between the on-period and the off-period of the MOS transistor 11 changes. Accordingly, the current value of the load current Ix flowing from the positive terminal of the on-board power supply device 3 to the electrical load 4 can be controlled.

[0056] The high-level period is a period during which the signal level of the ECU control signal is high. The low-level period is a period during which the signal level of the ECU control signal is low. The on-period is a period during which the MOS transistor 11 is on. The off-period is a period during which the MOS transistor 11 is off. Furthermore, as the MOS transistor 11 turns off, from timing t2 onwards, a current flows from the gate terminal of the MOS transistor 11 to ground via the gate driver 21. At this time, due to the electrostatic capacitance between the gate terminal and source terminal of the MOS transistor 11, the inter-terminal voltage VGS of the MOS transistor 11 gradually decreases over time, as shown in FIG. 3(b).

[0057] After that, at timing t3, when the inter-terminal voltage VGS of the MOS transistor 11 becomes less than the threshold value Sgs, the output voltage Vgm of the negative terminal of the constant voltage circuit 28 becomes smaller than the output voltage Vs of the common connection terminal 16. Accordingly, the signal level of the VGS detection signal provided from the comparator 23 to the control circuit 22 changes from high to low. At this time, the control circuit 22 controls the transistor 30 to turn off the MOS transistor 30.

[0058] As a result, the common connection terminals 16, 18 are opened. Accordingly, a constant voltage determined by the resistance values ​​of the resistive elements 26, 27 and the reference current Ih is generated between the common connection terminals 16, 18. As a result, an output voltage Vb indicating a threshold value Vth1 as an overcurrent detection threshold value is applied from the common connection terminal 16 to the inverting input terminal of the comparator 24. As a result, the overcurrent detection threshold value used by the comparator 24 is changed from the threshold value Vth2 to the threshold value Vth1.

[0059] Next, a specific example in which the electric load 4 is short-circuited by the short-circuit path 8 in the vehicle switch drive device 1 of this embodiment will be described with reference to Figures 1, 4, and 5. Figures 4(a) to 4(e) show a specific example in this embodiment in which dynamic clamping disconnection is performed using the MOS transistor 29 in a state in which the electric load 4 is short-circuited by the short-circuit path 8. Note that, for convenience of explanation, the state in which the electric load 4 is short-circuited by the short-circuit path 8 will be referred to as the short-circuit state hereinafter. Furthermore, the state in which the electric load 4 is not short-circuited by the short-circuit path 8 will be referred to as the normal state.

[0060] 4(a) is a timing chart showing changes in the signal level of an ECU control signal provided from the electronic control device 2 to the control circuit 22. FIG. 4(b) is a timing chart showing changes in the inter-terminal voltage VDS of the MOS transistor 11. FIG. 4(c) is a timing chart showing changes in the load current Ix flowing through the MOS transistor 11. FIG. 4(d) is a timing chart showing changes in the power WDS consumed between the drain terminal 11d and source terminal 11s of the MOS transistor 11. FIG. 4(e) is a timing chart showing changes in the energy Eas consumed between the drain terminal 11d and source terminal 11s of the MOS transistor 11.

[0061] 4(a) to 4(e), a specific example in which the control circuit 22 controls the MOS transistor 29 to perform dynamic clamp cutoff in this embodiment will be described below. First, when the electrical load 4 is in a normal state, as shown in FIG. 4(a), upon receiving an ECU control signal whose signal level is a high level from the electronic control unit 2, the control circuit 22 drives the gate driver 21 to turn on the MOS transistor 11.

[0062] 4(b), the inter-terminal voltage VDS of the MOS transistor 11 drops. At this time, as shown in FIG. 4(c), a load current Ix flows from the positive terminal of the on-board power supply device 3 to ground through the harness 5, the drain terminal 11d and source terminal 11s of the MOS transistor 11, the harness 6, and the electrical load 4. Thereafter, the electrical load 4 changes from a normal state to a short-circuit state.

[0063] As a result, a load current Ix flows from the positive terminal of the in-vehicle power supply device 3 to ground through the harness 5, the drain terminal 11d and source terminal 11s of the MOS transistor 11, the harness 6, and the short-circuit path 8. As a result, the current value Ix of the load current increases. Thereafter, as shown in FIG. 4(c), the current value of the load current Ix becomes equal to or greater than the threshold value Vth2. At this time, the output voltage Va of the common connection terminal 15 becomes greater than the output voltage Vb of the common connection terminal 17.

[0064] As a result, the signal level of the overcurrent determination signal output from the comparator 24 to the control circuit 22 changes from low to high. Accordingly, the control circuit 22 controls the gate driver 21 and the MOS transistor 29 to perform dynamic clamp cutoff. Specifically, the control circuit 22 turns on the MOS transistor 29 and controls the gate driver 21 to turn off the MOS transistor 11. As a result, a back-electromotive current flows from the harness 5 to ground through the MOS transistor 29, Zener diodes 33 and 34, and the gate driver 21 due to magnetic energy. Accordingly, the inter-terminal voltage VGS of the MOS transistors 11 and 12 rises for a short period of time due to the back-electromotive current. Therefore, the MOS transistors 11 and 12 are each turned on for a short period of time.

[0065] At this time, a back electromotive force (i.e., load current Ix) flows from the harness 5 to the ground through the drain and source terminals of the MOS transistors 11 and 12, the harness 6, and the short-circuit path 8 due to the magnetic energy. The current value of the back electromotive force (i.e., load current Ix) drops sharply after dynamic clamp cutoff, as shown in FIG. 4(c). At this time, the voltage VDS across the terminals of the MOS transistor 11 is clamped to a constant voltage value by the Zener diodes 33 and 34, as shown in FIG. 4(b). The constant voltage value is determined by the forward voltage of the Zener diode 34 and the Zener voltage of the Zener diode 33. After that, the MOS transistors 11 and 12 are both turned off.

[0066] Furthermore, as shown in Figure 4(d), the power WDS consumed by the MOS transistor 11 increases after dynamic clamping shutdown, reaches a constant value, and then decreases. Therefore, as shown in Figure 4(e), the energy Eas consumed by the MOS transistor 11 increases after dynamic clamping shutdown, reaches a constant value, and then remains constant. Therefore, the energy Eas consumed by the MOS transistor 11 becomes equal to or less than the threshold Eth, and breakdown of the MOS transistor 11 can be suppressed based on the load current Ix.

[0067] Next, in comparison with the present embodiment, a specific example in which the control circuit 22 controls the MOS transistor 29 to soft-off when the electrical load 4 is in a short-circuit state will be described with reference to Figures 5(a) to 5(e). Figures 5(a) to 5(e) show a specific example in which the control circuit 22 controls the MOS transistor 29 to soft-off when the electrical load 4 is in a short-circuit state. Figure 5(a) is a timing chart showing changes in the signal level of the ECU control signal given from the electronic control unit 2 to the control circuit 22.

[0068] 5(b) is a timing chart showing changes in the inter-terminal voltage VDS of the MOS transistor 11. FIG. 5(c) is a timing chart showing changes in the load current Ix flowing through the MOS transistor 11. FIG. 5(d) is a timing chart showing changes in the power WDS consumed between the drain terminal 11d and source terminal 11s of the MOS transistor 11. FIG. 5(e) is a timing chart showing changes in the energy Eas consumed between the drain terminal 11d and source terminal 11s of the MOS transistor 11.

[0069] First, the electrical load 4 changes from a normal state to a short-circuit state. As a result, a load current Ix flows from the positive terminal of the on-board power supply device 3 to ground through the harness 5, the drain terminal 11d and source terminal 11s of the MOS transistor 11, the harness 6, and the short-circuit path 8. When the electrical load 4 enters a short-circuit state, the current value of the load current Ix increases over time but is maintained below the threshold value Vth1.

[0070] At this time, the control circuit 22 soft-offs the MOS transistor 11 based on the ECU control signal provided from the electronic control unit 2. As a result, the inter-terminal voltage VDS of the MOS transistor 11 gradually increases over time, as shown in Fig. 5(b). Meanwhile, the current value of the load current Ix gradually decreases over time while remaining below the threshold Vth1, as shown in Fig. 5(c).

[0071] As shown in FIG. 5(d), the power WDS consumed by the MOS transistor 11 increases after the soft-off starts, reaches a certain value, and then decreases. Therefore, as shown in FIG. 5(e), the energy Eas consumed by the MOS transistor 11 increases after the soft-off starts and exceeds the threshold Eth. This causes the MOS transistor 11 to malfunction due to the load current ix. In contrast, in this embodiment, as described above, the comparator 24 uses a threshold Vth2, which is lower than the threshold Vth1, as the overcurrent detection threshold. This makes it possible to appropriately determine whether the load current Ix is an overcurrent, thereby preventing malfunction of the MOS transistor 11.

[0072] According to the present embodiment described above, the vehicle switch drive device 1 includes a MOS transistor 11 arranged between the vehicle-mounted power supply device 3 and the electrical load 4. When the MOS transistor 11 is turned on, it connects the vehicle-mounted power supply device 3 and the electrical load 4, and when it is turned off, it disconnects the vehicle-mounted power supply device 3 from the electrical load 4. The vehicle switch drive device 1 includes a control circuit 22, a comparator 24, and a MOS transistor 29.

[0073] The control circuit 22 performs PWM control on the MOS transistor 11 based on an ECU control signal output from the electronic control unit 2. In PWM control, the control circuit 22 turns on or off (specifically, soft-off) the MOS transistor 11 to control the load current Ix flowing from the in-vehicle power supply device 3 to the electrical load 4. The MOS transistor 12, together with the resistance element 25, detects a detection current Is proportional to the load current Ix. The comparator 24 determines whether the load current Ix is an overcurrent by determining whether the load current Ix is equal to or greater than an overcurrent detection threshold based on the detection current Is.

[0074] When the control circuit 22 determines that the load current Ix is equal to or greater than the overcurrent detection threshold and that the load current Ix is an overcurrent, it controls the MOS transistor 11 to perform dynamic clamp cutoff. On the other hand, when the control circuit 22 determines that it has received an ECU control signal to turn the MOS transistor 11 off, it sets the overcurrent detection threshold to a lower value (i.e., threshold Vth2) than when it has determined that it has received an ECU control signal to turn the MOS transistor on.

[0075] Therefore, even when the MOS transistor 11 is soft-off because the electrical load 4 is short-circuited, the control circuit 22 determines that the load current Ix is greater than the overcurrent detection threshold and controls the MOS transistor 11 to perform dynamic clamp cutoff. As a result, the MOS transistor 11 cuts off the connection between the in-vehicle power supply device 3 and the electrical load 4 at a faster speed than when the MOS transistor 11 is soft-off.

[0076] For this reason, in this embodiment, it is possible to reduce the energy Eas consumed in the MOS transistor 11. Therefore, it is possible to provide a vehicle switch drive device 1 that prevents the MOS transistor 11 from breaking down due to the load current Ix when the electrical load 4 is short-circuited. In this embodiment configured as described above, the following operational effects (a) and (b) can be obtained.

[0077] (a) The electronic control unit 2 sets the signal level of the ECU control signal to a low level so that the control circuit 22 turns off the MOS transistor 11. On the other hand, the electronic control unit 2 sets the signal level of the ECU control signal to a high level so that the control circuit 22 turns on the MOS transistor 11. When the signal level of the ECU control signal changes from a high level to a low level, the control circuit 22 determines that an OFF command for turning off the MOS transistor 11 has been given from the electronic control unit 2. Therefore, the control circuit 22 can appropriately determine that an OFF command for turning off the MOS transistor 11 has been given from the electronic control unit 2.

[0078] (b) The vehicle switch drive device 1 includes a gate driver 21 that is controlled by a control circuit 22 and outputs a gate signal from an output terminal to turn on and off the MOS transistor 11. The voltage between the gate terminal and the source terminal of the MOS transistor 11 is defined as an inter-terminal voltage VGS. A state in which the inter-terminal voltage VGS is equal to or greater than a threshold Sgs is defined as a first state, and a state in which the inter-terminal voltage VGS is less than the threshold Sgs is defined as a second state. When the control circuit 22 determines that the inter-terminal voltage VGS has changed from the first state to the second state, it changes the overcurrent detection threshold used by the comparator 24 from the threshold Vth2 to the threshold Vth1. Therefore, the overcurrent detection threshold used by the comparator 24 can be appropriately changed from the threshold Vth2 to the threshold Vth1.

[0079] (Second embodiment) In the first embodiment, an example has been described in which the control circuit 22 changes the overcurrent detection threshold used by the comparator 24 from the threshold Vth1 to the threshold Vth2 when the signal level of the ECU control signal changes from a high level to a low level. However, instead of this, an example will be described in which the control circuit 22 changes the overcurrent detection threshold from the threshold Vth1 to the threshold Vth2 when the inter-terminal voltage VDS changes from a state in which it is less than the threshold Sds to a state in which it is equal to or greater than the threshold Sds, with reference to FIG. 6 etc.

[0080] 6 is an electrical circuit diagram of the vehicle switch drive device 1 of this embodiment. The vehicle switch drive device 1 of this embodiment is similar to the vehicle switch drive device 1 of FIG. 1 in that a constant voltage power supply 40 and a comparator 41 are added. In FIG. 6, the same reference numerals as in FIG. 1 indicate the same components, and their explanations will be omitted. The positive terminal of the constant voltage power supply 40 is connected to the common connection terminal 14. The negative terminal of the constant voltage power supply 40 is connected to the non-inverting input terminal of the comparator 41.

[0081] Therefore, the constant-voltage power supply 40 outputs the voltage dropped from the output voltage of the common connection terminal 14 as the inter-terminal voltage of the constant-voltage power supply 40 as output voltage Vdn to the non-inverting input terminal of the comparator 41. The constant-voltage power supply 40 is a constant-voltage circuit that generates a constant voltage between the positive terminal and the negative terminal. The inter-terminal voltage is a constant voltage generated between the positive terminal and the negative terminal of the constant-voltage power supply 40.

[0082] The inverting input terminal of the comparator 41 is connected to the common connection terminal 16. The comparator 41 compares the output voltage Vdn of the constant-voltage power supply 40 with the output voltage Vs of the common connection terminal 16, and outputs a VDS detection signal indicating the result to the control circuit 22. This allows the comparator 41 to determine whether the inter-terminal voltage VDS is greater than the threshold value Sds. The threshold value Sds is a voltage threshold set by the inter-terminal voltage of the constant-voltage power supply 40. The VDS detection signal is used as a trigger for the control circuit 22 to turn on the MOS transistor 30 so that the overcurrent detection threshold can be changed from threshold value Vth1 to threshold value Vth2.

[0083] Next, the operation of the vehicle switch drive device 1 of this embodiment will be described with reference to Figures 7(a) to 7(h). Figure 7(a) is a timing chart showing changes in the signal level of an ECU control signal given from the electronic control device 2 to the control circuit 22. Figure 7(b) is a timing chart showing changes in the inter-terminal voltage VGS of the MOS transistor 11. Figure 7(c) is a timing chart showing changes in the inter-terminal voltage VDS of the MOS transistor 11.

[0084] 7(d) is a timing chart showing changes in the overcurrent detection threshold indicated by the output voltage Vb applied from the common connection terminal 17 to the inverting input terminal of the comparator 24. FIG. 7(e) is a timing chart showing changes in the state of the MOS transistor 11. FIG. 7(f) is a timing chart showing changes in the signal level of the VDS detection signal output from the comparator 23. FIG. 7(g) is a timing chart showing changes in the signal level of the VGS detection signal output from the comparator 23. FIG. 7(h) is a timing chart showing changes in the on / off state of the MOS transistor 30.

[0085] The vehicle switch drive device 1 of this embodiment differs from the vehicle switch drive device 1 of the first embodiment in the trigger that causes the control circuit 22 to change the overcurrent detection threshold from threshold Vth1 to threshold Vth2. The trigger that causes the control circuit 22 to turn on the MOS transistor 30 in the vehicle switch drive device 1 of this embodiment will be described below. As shown in FIG. 7(a), at timing t2, the signal level of the ECU control signal provided from the electronic control unit 2 to the control circuit 22 changes from high to low. Accordingly, the control circuit 22 drives the gate driver 21 to soft-off the MOS transistor 11. Therefore, the gate driver 21 changes the signal level of the gate control signal provided to the gate terminals of the MOS transistors 11 and 12 from high to low.

[0086] At this time, a current flows from the gate terminal of the MOS transistor 11 to ground through the gate driver 21. As a result, as shown in FIG. 7(b), the terminal voltage VGS of the MOS transistor 11 gradually decreases over time after timing t2 based on the capacitance between the gate terminal and source terminal of the MOS transistor 11. On the other hand, as shown in FIG. 7(c), the terminal voltage VDS of the MOS transistor 11 gradually increases over time after timing t2.

[0087] After that, at timing t2a, when the inter-terminal voltage VDS becomes equal to or greater than the threshold value Sds, the output voltage Vdn of the constant-voltage power supply 40 becomes greater than the output voltage Vs of the common connection terminal 16. Accordingly, the comparator 41 changes the signal level of the VDS detection signal provided to the control circuit 22 from low to high. At this time, the control circuit 22 controls the transistor 30 to turn on, as shown in FIG. 7(h). As a result, the common connection terminals 16 and 18 are connected.

[0088] At this time, reference current Ih flows from the positive terminal of on-board power supply device 7 to ground through the emitter terminal and collector terminal of transistor 31a, resistor element 26, harness 6, and electrical load 4. As a result, a constant voltage determined by the resistance value of resistor element 26 and reference current Ih is generated between common connection terminals 16 and 18. Furthermore, output voltage Vb indicating the overcurrent detection threshold is applied from common connection terminal 16 to the inverting input terminal of comparator 24.

[0089] 7(d), the output voltage Vb is a voltage indicating the threshold value Vth2 determined from the resistance value of the resistive element 26 and the reference current Ih. Therefore, the overcurrent detection threshold used in the comparator 24 is switched from the threshold value Vth1 to the threshold value Vth2. The threshold value Vth2 is a lower value than the threshold value Vth1. Thereafter, as in the first embodiment, when the inter-terminal voltage VGS of the MOS transistor 11 becomes lower than the threshold value Sgs, the output voltage Vgm of the negative terminal of the constant voltage circuit 28 becomes lower than the output voltage Vs of the common connection terminal 16.

[0090] Accordingly, the signal level of the VGS detection signal provided from the comparator 23 to the control circuit 22 changes from high to low. At this time, the control circuit 22 turns off the MOS transistor 30. Accordingly, a constant voltage determined by the resistance values ​​of the resistive elements 26 and 27 and the reference current Ih is generated between the common connection terminals 16 and 18. Therefore, an output voltage Vb indicating a threshold value Vth1 as an overcurrent detection threshold value is provided from the common connection terminal 16 to the inverting input terminal of the comparator 24. As a result, the overcurrent detection threshold value used by the comparator 24 is changed from threshold value Vth2 to threshold value Vth1.

[0091] According to the present embodiment described above, the MOS transistor 11 includes a drain terminal 11d into which a load current Ix flows from the positive terminal of the on-board power supply device 3, and a source terminal 11s from which the load current Ix flowing to the drain terminal 11d flows to the electrical load 4. A first state is defined as a state in which the inter-terminal voltage VDS of the MOS transistor 11 is less than a threshold value Sds, and a second state is defined as a state in which the inter-terminal voltage VDS is equal to or greater than the threshold value Sds. When the inter-terminal voltage VDS changes from the first state to the second state, the control circuit 22 determines that an OFF command for turning off the field-effect transistor has been given from the electronic control device 2.

[0092] Therefore, the control circuit 22 can appropriately determine that an OFF command has been given from the electronic control device 2. Accordingly, the overcurrent detection threshold used in the comparator 24 can be appropriately switched from threshold value Vth1 to threshold value Vth2. Therefore, even when the MOS transistor 11 is soft-off because the electrical load 4 is in a short-circuit state, the control circuit 22 determines that the load current Ix is larger than the overcurrent detection threshold and controls the MOS transistor 11 to perform dynamic clamp cutoff.

[0093] As a result, the MOS transistor 11 disconnects the connection between the in-vehicle power supply device 3 and the electric load 4 at a higher speed than when the MOS transistor 11 is soft-off. This makes it possible to reduce the energy Eas consumed by the MOS transistor 11. Therefore, it is possible to provide a vehicle switch drive device 1 that prevents the MOS transistor 11 from breaking down due to the load current Ix when the electric load 4 is short-circuited.

[0094] (Third embodiment) In the first embodiment, an example has been described in which the control circuit 22 switches the overcurrent detection threshold from the threshold Vth1 to the threshold Vth2 when the signal level of the ECU control signal from the electronic control unit 2 changes from high level to low level. However, a third embodiment will be described with reference to FIG. 8 in which the control circuit 22 switches the overcurrent detection threshold from the threshold Vth1 to the threshold Vth2 when the temperature of the MOS transistor 11 becomes equal to or higher than a predetermined temperature.

[0095] Fig. 8 is an electrical circuit diagram of the vehicle switch driving device 1 of this embodiment. The vehicle switch driving device 1 of this embodiment is obtained by adding diodes 42a, 42b, 42c, and 42d, a constant current power supply 43, a comparator 44, and a constant voltage circuit 45 to the vehicle switch driving device 1 of Fig. 1. Here, in Fig. 8, the same reference numerals as in Fig. 1 indicate the same parts, and their explanations will be omitted.

[0096] The diodes 42a, 42b, 42c, and 42d are each connected in series between the constant current power supply 43 and ground. The anode terminals of the diodes 42a, 42b, 42c, and 42d are each connected to the constant current power supply 43. Specifically, the anode terminal of the diode 42a is connected to the constant current power supply 43, and the cathode terminal of the diode 42a is connected to the anode terminal of the diode 42b.

[0097] The cathode terminal of diode 42b is connected to the anode terminal of diode 42c. The cathode terminal of diode 42c is connected to the anode terminal of diode 42d. The cathode terminal of diode 42d is connected to ground. In this embodiment, diodes 42a, 42b, 42c, and 42d are each fixed to the MOS transistor 11. Therefore, heat is conducted from the MOS transistor 11 to diodes 42a, 42b, 42c, and 42d.

[0098] As a result, the temperatures of the diodes 42a, 42b, 42c, and 42d change due to heat transferred from the MOS transistor 11. Therefore, the forward voltages of the diodes 42a, 42b, 42c, and 42d decrease as the temperature of the MOS transistor 11 increases. The diodes 42a, 42b, 42c, and 42d form a temperature detection unit that detects the temperature of the MOS transistor 11. The diode 42a and the constant current power supply 43 are connected together to form a common connection terminal 46. The common connection terminal 46 outputs an output voltage Vj, which is the sum of the forward voltages of the diodes 42a, 42b, 42c, and 42d, to the inverting input terminal of the comparator 44.

[0099] The comparator 44 outputs a MOS temperature detection signal having a high signal level when the output voltage Sj of the constant voltage circuit 45 is equal to or greater than the output voltage Vj of the common connection terminal 46. The comparator 44 outputs a MOS temperature detection signal having a low signal level when the output voltage Vj of the common connection terminal 46 is less than the output voltage Sj of the constant voltage circuit 45. The comparator 44 constitutes a temperature determination unit that determines whether the temperature of the MOS transistor 11 is equal to or greater than a predetermined temperature.

[0100] The constant voltage circuit 45 outputs an output voltage Sj from its positive terminal to the non-inverting input terminal of the comparator 44. The output voltage Sj indicates the predetermined temperature used to determine the temperature of the MOS transistor 11. The negative terminal of the constant voltage circuit 45 is connected to ground. The MOS temperature detection signal is used as a trigger to switch the overcurrent detection threshold from threshold Vth1 to threshold Vth2. The constant current power supply 43 passes a constant current to ground through diodes 42a, 42b, 42c, and 42d.

[0101] Next, the operation of the vehicle switch drive device 1 of this embodiment will be described with reference to Figures 9(a) to 9(h). Figure 9(a) is a timing chart showing changes in the signal level of the ECU control signal given from the electronic control device 2 to the control circuit 22. Figure 9(b) is a timing chart showing changes in the inter-terminal voltage VGS of the MOS transistor 11. Figure 9(c) is a timing chart showing changes in the inter-terminal voltage VDS of the MOS transistor 11.

[0102] 9(d) is a timing chart showing changes in the overcurrent detection threshold indicated by the output voltage Vb applied from the common connection terminal 17 to the inverting input terminal of the comparator 24. FIG. 9(e) is a timing chart showing changes in the state of the MOS transistor 11. FIG. 9(f) is a timing chart showing changes in the signal level of the MOS detection temperature signal output from the comparator 44. FIG. 9(g) is a timing chart showing changes in the signal level of the VGS detection signal output from the comparator 23. FIG. 9(h) is a timing chart showing changes in the on / off state of the MOS transistor 30.

[0103] The vehicle switch driving device 1 of this embodiment and the vehicle switch driving device 1 of the first embodiment are different in the trigger that causes the control circuit 22 to change the overcurrent detection threshold from threshold Vth1 to threshold Vth2. The trigger that causes the control circuit 22 to turn on the MOS transistor 30 in the vehicle switch driving device 1 of this embodiment will be described below.

[0104] 9(a), at timing t0, the electronic control unit 2 outputs an ECU control signal having a high level to the control circuit 22 in order to instruct the MOS transistor 11 to be turned on. Accordingly, the control circuit 22 drives the gate driver 21. At this time, a current flows from the gate driver 21 to each of the gate terminals of the MOS transistors 11 and 12.

[0105] Therefore, as shown in FIG. 9(b), the terminal voltage VGS of the MOS transistor 11 gradually increases over time between timing t0 and timing t1 due to the capacitance between the gate terminal and the source terminal of the MOS transistor 11. Thereafter, the terminal voltage VGS of the MOS transistor 11 maintains a constant level until timing t2. Here, the MOS transistor 11 maintains an on state between timing t0 and timing t1, as shown in FIG. 9(e). Thereafter, the MOS transistor 11 enters a fully on state between timing t1 and timing t2.

[0106] Between timing t0 and timing t2, energy based on the inter-terminal voltage VDS and the load current Ix is consumed in the MOS transistor 11. This causes heat to be generated in the MOS transistor 11. The heat generated in the MOS transistor 11 is transferred to the diodes 42a, 42b, 42c, and 42d. As a result, the temperatures of the diodes 42a, 42b, 42c, and 42d increase. As a result, the forward voltages of the diodes 42a, 42b, 42c, and 42d decrease.

[0107] Therefore, as the temperature of the MOS transistor 11 rises, the output voltage Vj of the common connection terminal 46 decreases. Thereafter, at timing t1b, when the temperature of the MOS transistor 11 reaches or exceeds a predetermined temperature, the output voltage Vj of the common connection terminal 46 becomes less than the output voltage Sj of the positive terminal of the constant voltage circuit 45. In other words, the output voltage Sj of the positive terminal of the constant voltage circuit 45 becomes equal to or greater than the output voltage Vj of the common connection terminal 46. Therefore, the comparator 44 changes the signal level of the MOS temperature detection signal provided to the control circuit 22 from low to high, as shown in FIG. 9(f).

[0108] At this time, the control circuit 22 controls the transistor 30 to turn it on, as shown in FIG. 9(h). This connects the common connection terminals 16 and 18. Accordingly, a constant voltage determined by the resistance value of the resistive element 26 and the reference current Ih is generated between the common connection terminals 16 and 18. Therefore, an output voltage Vb indicating a threshold value Vth2 as an overcurrent detection threshold value is applied from the common connection terminal 16 to the inverting input terminal of the comparator 24. As a result, the overcurrent detection threshold value used by the comparator 24 is switched from the threshold value Vth1 to the threshold value Vth2.

[0109] Thereafter, similarly to the first embodiment, at timing t2, the electronic control unit 2 changes the signal level of the ECU control signal that it outputs to the control circuit 22 from high level (i.e., second level) to low level (i.e., first level). At this time, the control circuit 22 receives the low-level ECU control signal from the electronic control unit 2. For example, when the output voltage Va output from the common connection terminal 15 is lower than the output voltage Vb output from the common connection terminal 17, the comparator 24 determines that the load current Ix is less than the threshold value Vth2 and is not an overcurrent. Accordingly, similarly to the first embodiment, the control circuit 22 drives the gate driver 21 to soft-off the MOS transistor 11.

[0110] As a result, the MOS transistor 11 slowly disconnects the in-vehicle power supply device 3 from the electrical load 4. That is, after timing t2, the MOS transistor 11 is turned off. As a result, the MOS transistor 11 no longer consumes energy based on the inter-terminal voltage VDS and the load current Ix. As a result, heat is no longer generated in the MOS transistor 11. Heat is no longer transferred from the MOS transistor 11 to the diodes 42a, 42b, 42c, and 42d. Therefore, the temperatures of the diodes 42a, 42b, 42c, and 42d decrease. As a result, the forward voltages of the diodes 42a, 42b, 42c, and 42d increase.

[0111] Therefore, as the temperature of the MOS transistor 11 decreases, the output voltage Vj of the common connection terminal 46 increases. Thereafter, at timing t3a, when the temperature of the MOS transistor 11 falls below a predetermined temperature, the output voltage Vj of the common connection terminal 46 becomes equal to or greater than the output voltage Sj of the positive terminal of the constant voltage circuit 45. In other words, the output voltage Sj of the positive terminal of the constant voltage circuit 45 becomes less than the output voltage Vj of the common connection terminal 46. Therefore, as shown in FIG. 9(f), the comparator 44 changes the signal level of the MOS temperature detection signal provided to the control circuit 22 from high to low.

[0112] At this time, the control circuit 22 controls the transistor 30 to turn it off, as shown in FIG. 9(h). This opens the gap between the common connection terminals 16 and 18. Accordingly, a constant voltage determined by the resistance values ​​of the resistive elements 26 and 27 and the reference current Ih is generated between the common connection terminals 16 and 18. Therefore, an output voltage Vb indicating a threshold value Vth1 as an overcurrent detection threshold value is applied from the common connection terminal 16 to the inverting input terminal of the comparator 24. As a result, the overcurrent detection threshold value used by the comparator 24 is switched from the threshold value Vth2 to the threshold value Vth1.

[0113] According to the present embodiment described above, in the vehicle switch drive device 1, the diodes 42a, 42b, 42c, and 42d are fixed to the MOS transistor 11. The diodes 42a, 42b, 42c, and 42d are connected in series between the constant current power supply 43 and ground, and generate a forward voltage based on a constant current flowing from the constant current power supply 43. The forward voltages of the diodes 42a, 42b, 42c, and 42d decrease as the temperature of the MOS transistor 11 increases.

[0114] As a result, diodes 42a, 42b, 42c, and 42d detect the temperature of MOS transistor 11. A common connection terminal 46 between the anode terminal of diode 42a and constant current power supply 43 outputs an output voltage Vj indicating the temperature of MOS transistor 11. When the temperature of MOS transistor 11 reaches or exceeds a predetermined temperature St, the output voltage Vj of common connection terminal 46 becomes less than the output voltage Sj of constant voltage circuit 45. Accordingly, comparator 44 changes the signal level of the MOS temperature detection signal provided to control circuit 22 from low to high.

[0115] At this time, the control circuit 22 turns on the transistor 30 to change the output voltage Vb supplied from the common connection terminal 17 to the non-inverting input terminal of the comparator 24. This changes the overcurrent detection threshold used by the comparator 24 from threshold Vth1 to threshold Vth2. In other words, when the temperature of the MOS transistor 11 is equal to or higher than the predetermined temperature St, the overcurrent detection threshold used by the comparator 24 is changed to a smaller value than when the temperature of the MOS transistor 11 is lower than the predetermined temperature St.

[0116] Therefore, when the electric load 4 is in a short-circuit state and the MOS transistor 11 is soft-off, the control circuit 22 appropriately determines that the load current Ix is an overcurrent and controls the gate driver 21 and the MOS transistor 29 to perform dynamic clamp cutoff. This allows the MOS transistor 29 to cut off the connection between the in-vehicle power supply device 3 and the electric load 4 at a higher speed than when the MOS transistor 11 is soft-off. Therefore, when the electric load 4 is in a short-circuit state, the energy consumed by the MOS transistor 11 due to the load current Ix can be reduced. This makes it possible to provide a vehicle switch drive device 1 that prevents the MOS transistor 11 from breaking down due to the load current Ix.

[0117] (Other embodiments) (1) In the first, second, and third embodiments, an example has been described in which a metal oxide semiconductor field effect transistor is used as the MOS transistor 11 disposed between the on-board power supply device 3 and the electrical load 4. However, instead of this, a junction field effect transistor may be used as the MOS transistor 11. Similarly, a junction field effect transistor may be used as the MOS transistor 12.

[0118] (2) In the first, second, and third embodiments, an n-channel metal oxide semiconductor field effect transistor is used as the MOS transistor 29. However, instead of this, a p-channel metal oxide semiconductor field effect transistor may be used as the MOS transistor 29. Similarly, a p-channel metal oxide semiconductor field effect transistor may be used as the MOS transistor 30.

[0119] (3) In the first, second, and third embodiments, examples have been described in which a field-effect transistor is used as the MOS transistor 29. However, instead of this, various semiconductor elements other than a field-effect transistor, such as a bipolar transistor, may be used as the MOS transistor 29. Similarly, various semiconductor elements other than a field-effect transistor, such as a bipolar transistor, may be used as the MOS transistor 30.

[0120] (4) In the first, second, and third embodiments, an example was described in which the Zener diode 33 that generates a Zener voltage and the Zener diode 34 that generates a forward voltage were used as the constant voltage generating element. However, instead of this, the Zener diode 34 that generates a forward voltage may be omitted as the constant voltage generating element. Alternatively, a plurality of Zener diodes 33 that generate a Zener voltage may be used as the constant voltage generating element.

[0121] (5) In the first, second, and third embodiments, examples have been described in which the diodes 42a, 42b, 42c, and 42d are used as temperature detectors for detecting the temperature of the MOS transistor 11. However, semiconductor elements other than the diodes 42a, 42b, 42c, and 42d that are capable of changing the output voltage depending on the temperature may also be used. Alternatively, a temperature detector such as a thermistor may be used as the temperature detector for detecting the temperature of the MOS transistor 11.

[0122] (6) In the first, second, and third embodiments, an example was described in which the switch drive device according to the present disclosure is applied to an automobile. However, instead of this, the switch drive device according to the present disclosure may be applied to moving objects other than automobiles, such as trains and airplanes. Alternatively, the switch drive device according to the present disclosure may be applied to various types of electrical equipment, such as machine tools, household electrical appliances, and industrial power equipment.

[0123] (7) In the first, second, and third embodiments, the inter-terminal voltage VGS of the MOS transistor 11 gradually increases over time between times t0 and t1 due to the capacitance between the gate and source terminals of the MOS transistor 11. However, instead of this, the gate driver 21 can control the current flowing from the gate driver 21 itself to the gate terminal of the MOS transistor 11, thereby gradually increasing the inter-terminal voltage VGS over time between times t0 and t1. The inter-terminal voltage VGS may also be gradually increased over time between times t0 and t1 due to the current control by the gate driver 21 and the capacitance between the gate and source terminals of the MOS transistor 11. The same applies to the inter-terminal voltage VGS of the MOS transistor 12.

[0124] (8) In the first, second, and third embodiments, the inter-terminal voltage VGS of the MOS transistor 11 gradually decreases over time after timing t2 based on the capacitance between the gate terminal and source terminal of the MOS transistor 11. However, instead of this, the gate driver 21 can control the current flowing from the gate terminal of the MOS transistor 11 to the gate driver 21 itself, thereby gradually decreasing the inter-terminal voltage VGS over time after timing t2. The inter-terminal voltage VGS may also be gradually decreased over time after timing t2 based on the current control by the gate driver 21 and the capacitance between the gate terminal and source terminal of the MOS transistor 11. The same applies to the inter-terminal voltage VGS of the MOS transistor 12.

[0125] (9) Note that the present disclosure is not limited to the above-described embodiments and can be modified as appropriate within the scope of the claims. Furthermore, the above-described embodiments are not unrelated to each other and can be combined as appropriate unless the combination is clearly impossible. It goes without saying that, in each of the above-described embodiments, the elements constituting the embodiments are not necessarily essential unless specifically stated as essential or clearly considered essential in principle. Furthermore, in each of the above-described embodiments, when numerical values ​​such as the number, values, amounts, and ranges of components of the embodiments are mentioned, they are not limited to the specific numbers unless specifically stated as essential or clearly limited to a specific number in principle. Furthermore, in each of the above-described embodiments, when the shape, positional relationship, etc. of components, etc. are mentioned, they are not limited to the shape, positional relationship, etc., unless specifically stated or limited to a specific shape, positional relationship, etc. in principle. [Explanation of symbols]

[0126] 1. Vehicle switch drive device 2. Electronic control device 3 On-vehicle power supply 6 Electrical Load 11 MOS transistor 12 MOS transistors 21 Gate Driver 22 Control circuit 25 Resistive element 26 Resistive element 27 Resistive element 30 MOS transistors

Claims

1. a field-effect transistor (11) disposed between the power supply device (3) and the electrical load (4), which connects the power supply device and the electrical load when turned on and disconnects the power supply device and the electrical load when turned off; a control unit (22) that controls a load current flowing from the power supply device to ground through the electric load by turning on and off the field effect transistor based on a command given from an external device (2); a current determination unit (24) that determines whether the load current is equal to or greater than a threshold; a threshold setting unit (22, 26, 27, 30) that, when it is determined that the command to turn off the field effect transistor has been given from the external device, sets the threshold to a value smaller than that when it is determined that the command to turn on the field effect transistor has been given from the external device; When the current determination unit determines that the load current is equal to or greater than the threshold value, the control unit controls the field effect transistor to disconnect the power supply device from the electrical load at a faster speed than when the field effect transistor is turned off to control the load current.

2. the external device sets a signal level of a control signal to be provided to the control unit to a first level in order to turn off the field effect transistor by the control unit; Furthermore, the external device sets the signal level of the control signal to be provided to the control unit to a second level in order to turn on the field effect transistor by the control unit, 2. The switch driving device according to claim 1, wherein the threshold setting unit determines that the command to turn off the field effect transistor has been received from the external device when the signal level of the control signal changes from the second level to the first level.

3. The field effect transistor includes an input terminal (11d) into which the load current flows from the power supply device, and an output terminal (11s) through which the current flowing through the input terminal flows to the electrical load, In the field effect transistor, a voltage between the input terminal and the output terminal is defined as a terminal voltage (VDS), a state in which the terminal voltage is less than a voltage threshold (Sds) is defined as a first state, and a state in which the terminal voltage is equal to or greater than the voltage threshold is defined as a second state.

2. The switch drive device according to claim 1, wherein the threshold setting unit determines that the command to turn off the field effect transistor by the control unit has been received from the external device when the inter-terminal voltage changes from the first state to the second state.

4. a gate driver (21) controlled by the control unit to output a gate signal for turning on and off the field effect transistor; 2. The switch drive device according to claim 1, wherein the field effect transistor is an n-channel field effect transistor having an input terminal (11g) through which the load current flows from the power supply device, an output terminal (11s) through which the load current flowing in the input terminal flows to the electrical load, and a control terminal (11g) to which a gate signal given from the gate driver is input.

5. a current path (5) having inductive reluctance is arranged between the power supply device and the input terminal of the field effect transistor, for allowing the load current to flow from the power supply device to the input terminal; When a terminal to which the current path and the input terminal are commonly connected is defined as a common connection terminal (14), a constant voltage generating element (33, 34) disposed between the common connection terminal and the gate driver, for generating a constant voltage based on a current flowing from the common connection terminal side to the gate driver side; a switch (29) controlled by the control unit to connect or disconnect the common connection terminal and the constant voltage generating element; magnetic energy is stored in the current path based on the load current; the control unit controls the switch to connect the common connection terminal and the constant voltage generating element in order to disconnect the power supply device from the electrical load at high speed, and controls the gate driver to turn off the field effect transistor; When the field effect transistor is turned off, a back electromotive current flows from the current path based on the magnetic energy to ground through the switch, the constant voltage generating element, and the gate driver; 5. The switch drive device according to claim 4, wherein the field effect transistor is turned on based on a voltage generated between the control terminal and the output terminal based on the back electromotive force, and the back electromotive force flows from the current path to ground through the input terminal, the output terminal, and the electrical load of the field effect transistor.

6. In the field effect transistor, a voltage between the control terminal and the output terminal is defined as an inter-terminal voltage (VGS), a state in which the inter-terminal voltage is equal to or greater than a threshold value (Sgs) is defined as a first state, and a state in which the inter-terminal voltage is less than the threshold value is defined as a second state; when the control unit determines that the command to turn on the field effect transistor has been received from the external device, the threshold value used in the current determination unit is set to a first threshold value (Vth1); When the control unit determines that the command to turn off the field effect transistor has been received from the external device, the threshold value used in the current determination unit is set to a second threshold value (Vth2), a voltage determination unit (23) that determines whether the inter-terminal voltage has changed from the first state to the second state; 5. The switch drive device according to claim 4, wherein when the voltage determination unit determines that the inter-terminal voltage has changed from the first state to the second state, the threshold setting unit changes the threshold used in the control unit from the second threshold to the first threshold.

7. a field-effect transistor (11) disposed between the power supply device (3) and the electrical load (4), which connects the power supply device and the electrical load when turned on and disconnects the power supply device and the electrical load when turned off; a control unit (22) that controls a load current (Ix) flowing from the power supply device to ground through the electric load by turning on and off the field effect transistor based on a command output from an external device (2); a current determination unit (24) that determines whether the load current is equal to or greater than a threshold; a temperature determination unit (44) that determines whether the temperature of the field effect transistor is equal to or higher than a predetermined temperature; a threshold setting unit (22, 26, 27, 30) that sets the threshold to a smaller value when the temperature determination unit determines that the temperature of the field effect transistor is equal to or higher than the predetermined temperature, compared to when the temperature determination unit determines that the temperature of the field effect transistor is lower than the predetermined temperature, When the current determination unit determines that the load current is equal to or greater than the threshold value, the control unit controls the field effect transistor to disconnect the power supply device from the electrical load at a faster speed than when the field effect transistor is turned off to control the load current.

8. a gate driver (21) controlled by the control unit to output a gate signal for turning on and off the field effect transistor; 8. The switch drive device according to claim 7, wherein the field effect transistor is an n-channel field effect transistor having an input terminal (11g) through which the load current flows from the power supply device, an output terminal (11s) through which the load current flowing in the input terminal flows to the electrical load, and a control terminal (11g) to which a gate signal given from the gate driver is input.

9. a current path (5) having inductive reluctance is arranged between the power supply device and the field effect transistor, and causing the load current to flow from the power supply device to the field effect transistor; When a terminal to which the current path and the input terminal are commonly connected is defined as a common connection terminal (14), a constant voltage generating element (33, 34) disposed between the common connection terminal and the gate driver, for generating a constant voltage based on a current flowing from the common connection terminal side to the gate driver side; a switch (29) controlled by the control unit to connect or disconnect the common connection terminal and the constant voltage generating element; magnetic energy is stored in the current path based on the load current; the control unit controls the switch to connect the common connection terminal and the constant voltage generating element in order to disconnect the power supply device from the electrical load at high speed, and controls the gate driver to turn off the field effect transistor; When the field effect transistor is turned off, a back electromotive current flows from the current path based on the magnetic energy to ground through the switch, the constant voltage generating element, and the gate driver; 9. The switch drive device according to claim 8, wherein the field effect transistor is turned on based on a voltage generated between the control terminal and the output terminal based on the back electromotive force, and the back electromotive force flows from the current path to ground through the input terminal, the output terminal, and the electrical load of the field effect transistor.

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