Power supply circuit and power supply apparatus

A power supply circuit includes a first P-channel MOSFET and a first voltage application circuit. The first P-channel MOSFET is provided between an on-board power supply and a vehicular apparatus that is a power supply target, and is configured to switch a power-ON state in which electric power is supplied to the vehicular apparatus and a power-OFF state in which the supply of the electric power is interrupted. The first voltage application circuit is configured to apply a voltage having a potential lower than a potential of the on-board power supply to a gate terminal such that a state of the first P-channel MOSFET is switched to the power-ON state, and apply a voltage having a potential equal to the potential of the on-board power supply to the gate terminal such that the state of the first P-channel MOSFET is switched to the power-OFF state.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to Japanese Patent Application No. 2019-067944 filed on Mar. 29, 2019, incorporated herein by reference in its entirety.

BACKGROUND

1. Technical Field

The present disclosure relates to a power supply circuit and a power supply apparatus.

2. Description of Related Art

Japanese Unexamined Patent Application Publication No. 2015-23451 (JP 2015-23451 A) describes a power supply circuit configured to switch a power-ON state and a power-OFF state between a power supply and a power supply target. In the power-ON state, electric power is supplied to the power supply target. In the power-OFF state, the electric power supply is interrupted. The power supply circuit of JP 2015-23451 A includes a voltage application circuit configured to apply voltages to an N-channel metal-oxide-semiconductor field-effect-transistor (MOSFET) and to a gate terminal of the MOSFET. In this power supply circuit, when the N-channel MOSFET is switched ON to set the power-ON state, it is necessary to apply a voltage to the gate terminal such that a source terminal connected to the power supply has a lower potential due to characteristics of the N-channel MOSFET. That is, it is necessary to apply, to the gate terminal, a voltage having a potential higher than that of the source terminal connected to the power supply. Therefore, when the N-channel MOSFET is switched ON, the voltage application circuit applies, to the gate terminal, a voltage increased to be higher than that of the power supply through driving of a charge pump.

SUMMARY

For example, when electric power of an on-board power supply is supplied to a vehicular apparatus, it is necessary that various types of information stored in the vehicular apparatus be retained in the power-ON state. If the N-channel MOSFET is employed to switch the power-ON state and the power-OFF state as in JP 2015-23451 A, it is necessary that the driving of the charge pump be kept to keep the power-ON state. If the driving of the charge pump is kept for a long period, power consumption may increase.

The present disclosure can provide a power supply circuit and a power supply apparatus in which the increase in the power consumption can be suppressed.

A power supply circuit according to a first aspect of the present disclosure includes a first P-channel MOSFET and a first voltage application circuit. The first P-channel MOSFET is provided between an on-board power supply and a vehicular apparatus that is a power supply target, and is configured to switch a power-ON state in which electric power is supplied to the vehicular apparatus and a power-OFF state in which the supply of the electric power is interrupted. A source terminal of the first P-channel MOSFET is connected to the on-board power supply, and a drain terminal of the first P-channel MOSFET is connected to the vehicular apparatus. The first voltage application circuit is configured to apply a voltage having a potential lower than a potential of the on-board power supply to a gate terminal such that a state of the first P-channel MOSFET is switched to the power-ON state, and apply a voltage having a potential equal to the potential of the on-board power supply to the gate terminal such that the state of the first P-channel MOSFET is switched to the power-OFF state.

To switch the power-ON state and the power-OFF state by using the first P-channel MOSFET as in the configuration described above, it is only necessary that the voltage having a potential lower than the potential of the on-board power supply be applied to the gate terminal. Therefore, there is no need to use a booster circuit such as a charge pump, which is necessary when an N-channel MOSFET is used. Thus, even if the voltage is kept applied to the gate terminal of the first P-channel MOSFET to keep the power-ON state, the power consumption can be reduced because of no need to use the booster circuit such as the charge pump. Even if the power-ON state is kept for a long period, the increase in the power consumption can be suppressed as compared to the case where the N-channel MOSFET is used.

In the configuration described above, the first voltage application circuit may include a switching circuit configured to switch a state in which the gate terminal and a reference potential point of the on-board power supply are connected such that the state of the first P-channel MOSFET is switched to the power-ON state, and a state in which the gate terminal and the on-board power supply are connected such that the state of the first P-channel MOSFET is switched to the power-OFF state.

According to the configuration described above, to switch the power-ON state and the power-OFF state, it is only necessary to provide the circuit configured to switch connection to a contact on the existing circuit. Therefore, there is no need to use a circuit configured to generate a dedicated voltage for switching to the respective states. Thus, complication of the voltage application circuit can be suppressed.

In the configuration described above, the power supply circuit may further include a second P-channel MOSFET and a second voltage application circuit. The second P-channel MOSFET is provided between the first P-channel MOSFET and the vehicular apparatus. A source terminal of the second P-channel MOSFET may be connected to the vehicular apparatus, and a drain terminal of the second P-channel MOSFET may be connected to the drain terminal of the first P-channel MOSFET. The second voltage application circuit may include a switching circuit configured to switch a state in which a gate terminal and the reference potential point of the on-board power supply are connected such that a state of the second P-channel MOSFET is switched to the power-ON state, and a state in which the gate terminal and the vehicular apparatus are connected such that the state of the second P-channel MOSFET is switched to the power-OFF state.

According to the configuration described above, in the power-ON state and the power-OFF state, current backflow from the power supply target to the on-board power supply can be suppressed by the second P-channel MOSFET. In the power-ON state, a voltage is applied to the gate terminal of the second P-channel MOSFET to suppress the current backflow, but similarly to the above, it is only necessary that the voltage having a potential lower than the potential of the on-board power supply be applied to the gate terminal. That is, there is no need to use the booster circuit such as the charge pump, which is necessary when the N-channel MOSFET is used. Thus, even if the voltage is kept applied to the gate terminal of the second P-channel MOSFET to suppress the current backflow in the power-ON state, the power consumption can be reduced because of no need to use the booster circuit such as the charge pump. Similarly to the above, when the power-ON state and the power-OFF state are switched for the second P-channel MOSFET, it is only necessary to switch connection to a contact on the existing circuit. Therefore, there is no need to use a circuit configured to generate a dedicated voltage for switching to the respective states. Thus, complication of the other voltage application circuit can be suppressed.

A power supply apparatus according to a second aspect of the present disclosure includes the power supply circuit described above, and a controller configured to control switching of the power-ON state and the power-OFF state. The vehicular apparatus is a steering system configured to apply power to a steering mechanism of a vehicle. The controller is configured to control the power supply circuit to keep the power-ON state irrespective of a state of a start switch of the vehicle.

According to the configuration described above, even if the power-ON state is kept for a long period, the increase in the power consumption can be suppressed as compared to the case where the N-channel MOSFET is used. Thus, it is possible to attain a power supply apparatus in which the increase in the power consumption is suppressed even if the power-ON state is kept irrespective of the state of the start switch of the vehicle.

According to the present disclosure, the increase in the power consumption can be suppressed.

DETAILED DESCRIPTION OF EMBODIMENTS

Description is given of an embodiment in which a power supply circuit and a power supply apparatus are applied to a steering system that is a vehicular apparatus. As illustrated inFIG. 1, a steering system1of this embodiment includes a steering mechanism2and an assist mechanism3. The steering mechanism2turns steered wheels16based on a driver's operation of a steering wheel10. The assist mechanism3includes a motor20configured to assist the driver's steering operation. The steering system1is a so-called electric power steering system configured to assist the driver's steering operation by applying a motor torque of the motor20to the steering mechanism2as a steering assist force.

The steering mechanism2includes a steering shaft12and a rack shaft14. The steering wheel10is fixed to one end of the steering shaft12, and a pinion gear11is provided at the other end of the steering shaft12. The rack shaft14is provided with a rack gear13that meshes with the pinion gear11. The pinion gear11and the rack gear13constitute a rack and pinion mechanism. Rotational motion of the steering shaft12is converted into reciprocating linear motion in an axial direction of the rack shaft14via the rack and pinion mechanism. The steering system1is mounted on a vehicle such that the axial direction of the rack shaft14is a vehicle width direction. The reciprocating linear motion of the rack shaft14is transmitted to the right and left steered wheels16via tie rods15coupled to respective ends of the rack shaft14. Thus, the steered angles of the steered wheels16are changed, and a traveling direction of the vehicle is changed.

A torque sensor17is attached to the steering shaft12to measure a steering torque TR applied to the steering shaft12through an operation of the steering wheel10. The torque sensor17of this embodiment detects a torsion amount of a torsion bar that constitutes the steering shaft12, and measures the steering torque TR based on the torsion amount.

The assist mechanism3includes a motor20and a speed reducer21for steering assist. The motor20is coupled to the steering shaft12via the speed reducer21. The speed reducer21reduces the speed of rotation of the motor20, and transmits a rotational force obtained through the speed reduction to the steering shaft12. A three-phase brushless motor is employed as the motor20of this embodiment. A worm gear mechanism is employed as the speed reducer21of this embodiment.

The steering system1includes a steering control apparatus30and a power supply apparatus40. The steering control apparatus30includes an inverter, which is a publicly-known circuit including two switching elements in each phase of the motor20(U phase, V phase, and W phase). When the steering system1is mounted on the vehicle, the power supply apparatus40is connected to an on-board power supply50, and the steering control apparatus30is connected to the on-board power supply50via the power supply apparatus40. The power supply apparatus40is provided between the on-board power supply50and the steering control apparatus30, which is a component of the steering system1that is a power supply target. The steering control apparatus30assists the driver's steering operation by controlling an operation of the motor20through electric power supply from the on-board power supply50. The steering control apparatus30includes a steering controller31and a memory32. The steering controller31executes various types of arithmetic processing for calculating, for example, a control amount for controlling the operation of the motor20. The memory32stores programs and data for the various types of arithmetic processing. The torque sensor17and a vehicle speed sensor18are connected to the steering controller31. The vehicle speed sensor18detects a traveling speed VS of the vehicle. To control the steering assist force, the steering controller31determines, based on the steering torque TR and the traveling speed VS, a steering assist force corresponding to a target steering assist force that is a target value of the steering assist force. The steering controller31controls the operation of the motor20through control of the inverter to generate the steering assist force corresponding to the target steering assist force.

Next, the electrical configuration of the power supply apparatus40is described. As illustrated inFIG. 2, the power supply apparatus40includes a power supply circuit41and a power supply controller42.

The power supply circuit41has a function of switching a power-ON state and a power-OFF state between the on-board power supply50and the steering control apparatus30. In the power-ON state, electric power is supplied to the steering control apparatus30. In the power-OFF state, the electric power supply is interrupted. A power supply voltage of the on-board power supply50is input to the power supply circuit41as an input voltage Vin. The power supply circuit41outputs the input voltage Vin as an output voltage Vout to be supplied to the steering control apparatus30. In this embodiment, the power supply voltage of the on-board power supply50, that is, the input voltage Vin is, for example, 12 volts (V). The voltage based on electric power to be supplied to the steering control apparatus30, that is, the output voltage Vout is substantially equal to the input voltage Vin. For example, the output voltage Vout is 12 V.

The power supply controller42has a function of controlling the switching of the power-ON state and the power-OFF state of the power supply circuit41. The power supply voltage of the on-board power supply50is input to the power supply controller42as the input voltage Vin. The power supply controller42controls the switching of the power-ON state and the power-OFF state of the power supply circuit41based on the input voltage Vin. In this embodiment, the power supply controller42is an example of a controller.

The power supply controller42outputs a control voltage VC to switch the power-ON state and the power-OFF state of the power supply circuit41. Specifically, when the electric power steering system has no abnormality in its function and is normally operable to apply the steering assist force to the steering mechanism2, the power supply controller42outputs a power-ON switching control voltage VC to make switching to the power-ON state such that the steering control apparatus30executes control on the application of the steering assist force. When the electric power steering system has an abnormality in its function and is not normally operable to apply the steering assist force to the steering mechanism2, the power supply controller42outputs a power-OFF switching control voltage VC to make switching to the power-OFF state such that the steering control apparatus30stops the control on the application of the steering assist force. In this embodiment, the power-ON switching control voltage VC is a low-level signal having a potential lower than that of the power-OFF switching control voltage VC. That is, the power-OFF switching control voltage VC is a high-level signal having a potential higher than that of the power-ON switching control voltage VC. The power supply controller42determines whether an abnormality occurs in the function of the electric power steering system based on, for example, an input of an abnormality signal from the steering control apparatus30. The abnormality signal indicates whether an abnormality occurs in the function of the electric power steering system.

In this embodiment, when the electric power steering system has no abnormality in its function and is normally operable to apply the steering assist force to the steering mechanism2, the power supply controller42outputs the power-ON switching control voltage VC irrespective of whether ignition is ON or OFF as a state of a start switch of the vehicle. That is, the on-board power supply50keeps supplying electric power to the steering control apparatus30, that is, the memory32when the ignition is not only ON but also OFF. Thus, when the ignition is not only ON but also OFF, the memory32can retain various types of information stored for the various types of arithmetic processing to be executed by the steering controller31.

The configuration of the power supply circuit41is described in detail. As illustrated inFIG. 2, the power supply circuit41includes a first P-channel MOSFET (PMOS1), a second P-channel MOSFET (PMOS2) different from the PMOS1, a first voltage application circuit71, and a second voltage application circuit81different from the first voltage application circuit71. The first voltage application circuit71has a function of switching switch states of the PMOS1. The second voltage application circuit81has a function of switching switch states of the PMOS2. The PMOS1and the PMOS2of this embodiment are P-channel MOSFETs having source terminals72and82associated with P-type semiconductor layers, drain terminals73and83associated with P-type semiconductor layers, and gate terminals74and84associated with N-type semiconductor layers, respectively.

As illustrated inFIG. 3, the P-channel MOSFET has the following characteristics. When a potential of a gate terminal G is lower than a potential of a source terminal S and a potential difference between those terminals is equal to or larger than a preset threshold such as 2 V, the P-channel MOSFET is turned ON such that a current is conductive between the source terminal S and a drain terminal D. This is because the potential difference between the source terminal S and the gate terminal G is larger than the threshold and holes are gathered near the surface of an N-type semiconductor layer Ng (represented by “N” inFIG. 3) on an insulating layer Z side to provide a P-type semiconductor layer Pg (represented by “P” inFIG. 3) serving as an inversion layer. In this case, a P-type semiconductor layer Ps (represented by “P” inFIG. 3) associated with the source terminal S and a P-type semiconductor layer Pd (represented by “P” inFIG. 3) associated with the drain terminal D are electrically connected through the P-type semiconductor layer Pg. Thus, a current is conductive between the source terminal S and the drain terminal D.

Further, the P-channel MOSFET has the following characteristics. When the potential of the source terminal S is closer to the potential of the gate terminal G and the potential difference between those terminals is smaller than the threshold, the P-channel MOSFET is turned OFF such that no current is conductive between the source terminal S and the drain terminal D. This is because the potential difference between the source terminal S and the gate terminal G is smaller than the threshold and the N-type semiconductor layer Ng associated with the gate terminal G electrically interrupts the P-type semiconductor layer Ps associated with the source terminal S and the P-type semiconductor layer Pd associated with the drain terminal D such that no current is conductive between the source terminal S and the drain terminal D.

Returning to the description with reference toFIG. 2, the source terminal72of the PMOS1is connected to a high-potential side of the on-board power supply50, and the drain terminal73of the PMOS1is connected to the steering control apparatus30via the PMOS2. The gate terminal74of the PMOS1is connected to the first voltage application circuit71. A contact C1on a connection line L1connecting the PMOS1and the high-potential side of the on-board power supply50is connected to the first voltage application circuit71.

The drain terminal83of the PMOS2is connected to the drain terminal73of the PMOS1, and the source terminal82of the PMOS2is connected to the steering control apparatus30. The gate terminal84of the PMOS2is connected to the second voltage application circuit81. A contact C2on a connection line L2connecting the PMOS2and the steering control apparatus30is connected to the second voltage application circuit81.

In this embodiment, the PMOS1and the PMOS2are connected in series by connecting their drain terminals73and83such that electric power can be supplied from the on-board power supply50to the steering control apparatus30. In this case, directions of a parasitic diode D1provided in the PMOS1and a parasitic diode D2provided in the PMOS2are opposite to each other such that current flows are blocked from the source terminals72and82to the drain terminals73and83, respectively.

Next, the structures of the first voltage application circuit71and the second voltage application circuit81are described in more detail. As illustrated inFIG. 4, the first voltage application circuit71includes a switching circuit constituted by a combination of a transistor TR1that is an NPN bipolar transistor and a transistor TR2that is a PNP bipolar transistor.

In the transistor TR1, a base terminal TR1bis connected to the power supply controller42such that the control voltage VC output from the power supply controller42is divided through a voltage division resistor and a current based on the divided voltage flows into the base terminal TR1b. In the transistor TR1, an emitter terminal TR1eis connected to a reference potential point GND, and a collector terminal TR1cis connected to a base terminal TR2bof the transistor TR2and the contact C1. That is, the transistor TR1is connected to the on-board power supply50via the collector terminal TR1cand the contact C1.

When a potential difference between the base terminal TR1band the emitter terminal TR1eis equal to or larger than a preset threshold such as 0.5 V, the transistor TR1is turned ON such that a current is conductive between the collector terminal TR1cand the emitter terminal TR1e. In this case, a current based on electric power supply from the on-board power supply50, that is, a current based on the input voltage Vin of the power supply circuit41flows between the collector terminal TR1cand the emitter terminal TR1e.

When the potential difference between the base terminal TR1band the emitter terminal TR1eis smaller than the threshold set in the transistor TR1, the transistor TR1is turned OFF such that no current is conductive between the collector terminal TR1cand the emitter terminal TR1e. In this case, the current based on the electric power supply from the on-board power supply50, that is, the current based on the input voltage Vin of the power supply circuit41, does not flow between the collector terminal TR1cand the emitter terminal TR1e.

In this embodiment, the power-OFF switching voltage that is the high-level signal output from the power supply controller42is set to a value equal to or larger than the threshold set in the transistor TR1, but the power-ON switching voltage that is the low-level signal output from the power supply controller42is set to a value smaller than the threshold set in the transistor TR1.

In the transistor TR2, the base terminal TR2bis connected to the on-board power supply50such that, when the transistor TR1is ON, a current based on a voltage obtained by dividing the input voltage Vin by a voltage division resistor flows into the base terminal TR2b. The transistor TR2is connected such that, when the transistor TR1is OFF, a current based on the input voltage Vin flows into the base terminal TR2b. In the transistor TR2, an emitter terminal TR2eis connected to the contact C1, and a collector terminal TR2cis connected to the reference potential point GND and the gate terminal74of the PMOS1via voltage division resistors. That is, the transistor TR2is connected to the on-board power supply50via the emitter terminal TR2eand the contact C1, and is connected to the PMOS1the collector terminal TR2cand the gate terminal74.

The transistor TR2is configured such that, when the transistor TR1is ON, a potential difference between the base terminal TR2band the collector terminal TR2cis equal to or larger than a preset threshold such as 0.5 V, and the transistor TR2is turned ON such that a current is conductive between the collector terminal TR2cand the emitter terminal TR2e. In this case, the current based on the electric power supply from the on-board power supply50, that is, the current based on the input voltage Vin of the power supply circuit41flows between the collector terminal TR2cand the emitter terminal TR2e. Thus, the high-potential side of the on-board power supply50and the gate terminal74of the PMOS1are connected via the first voltage application circuit71.

The transistor TR2is configured such that, when the transistor TR1is OFF, the potential difference between the base terminal TR2band the collector terminal TR2cis not equal to or larger than the threshold set in the transistor TR2, and the transistor TR2is turned OFF such that no current is conductive between the collector terminal TR2cand the emitter terminal TR2e. In this case, the current based on the electric power supply from the on-board power supply50, that is, the current based on the input voltage Vin of the power supply circuit41does not flow between the collector terminal TR2cand the emitter terminal TR2e. Thus, the reference potential point GND and the gate terminal74of the PMOS1are connected via the first voltage application circuit71.

As illustrated inFIG. 5, the second voltage application circuit81includes a switching circuit constituted by a combination of a transistor TR3that is an NPN bipolar transistor and a transistor TR4that is a PNP bipolar transistor.

The transistor TR3has the same structure as that of the transistor TR1of the first voltage application circuit71, but differs from the transistor TR1in that a collector terminal TR3cis connected to the contact C2. That is, the transistor TR3is connected to the steering control apparatus30via the collector terminal TR3cand the contact C2. When the transistor TR3is turned ON such that a current is conductive between the collector terminal TR3cand an emitter terminal TR3e, a current based on the electric power supply from the on-board power supply50to the steering control apparatus30, that is, a current based on the output voltage Vout of the power supply circuit41flows between the collector terminal TR3cand the emitter terminal TR3e. When the transistor TR3is turned OFF such that no current is conductive between the collector terminal TR3cand the emitter terminal TR3e, the current based on the output voltage Vout of the power supply circuit41does not flow between the collector terminal TR3cand the emitter terminal TR3e.

The transistor TR4has the same structure as that of the transistor TR2of the first voltage application circuit71, but differs from the transistor TR2in that an emitter terminal TR4eis connected to the contact C2. That is, the transistor TR4is connected to the steering control apparatus30via the emitter terminal TR4eand the contact C2.

When the transistor TR4is turned ON such that a current is conductive between a collector terminal TR4cand the emitter terminal TR4e, the current based on the output voltage Vout of the power supply circuit41flows between the collector terminal TR4cand the emitter terminal TR4e. Thus, a high-potential side of the steering control apparatus30and the gate terminal84of the PMOS2are connected via the second voltage application circuit81.

When the transistor TR4is turned OFF such that no current is conductive between the collector terminal TR4cand the emitter terminal TR4e, the current based on the output voltage Vout of the power supply circuit41does not flow between the collector terminal TR4cand the emitter terminal TR4e. Thus, the reference potential point GND and the gate terminal84of the PMOS2are connected via the second voltage application circuit81.

Next, description is given of an operation of the power supply circuit41when the power-ON state and the power-OFF state for the steering control apparatus30are switched. As illustrated inFIG. 4, when the control voltage VC is input from the power supply controller42to the first voltage application circuit71as the low-level signal, the transistor TR1is turned OFF. When the transistor TR1is OFF, the transistor TR2is turned OFF. Therefore, the gate terminal74and the reference potential point GND are connected as indicated by an arrow of a long dashed short dashed line inFIG. 4. Thus, the potential of the gate terminal74is switched to a potential of the reference potential point GND. In this case, the potential of the gate terminal74is lower than a potential of the source terminal72, and a potential difference between those terminals is 12 V that is equal to or larger than the threshold set in the PMOS1. Thus, the PMOS1is turned ON.

As illustrated inFIG. 5, when the control voltage VC is input from the power supply controller42to the second voltage application circuit81as the low-level signal, the transistor TR3is turned OFF. When the transistor TR3is OFF, the transistor TR4is turned OFF. Therefore, the gate terminal84and the reference potential point GND are connected as indicated by an arrow of a long dashed short dashed line inFIG. 5. Thus, the potential of the gate terminal84is switched to the potential of the reference potential point GND. In this case, the potential of the gate terminal84is lower than a potential of the source terminal82, and a potential difference between those terminals is 12 V that is equal to or larger than the threshold set in the PMOS2. Thus, the PMOS2is turned ON.

As described above, the power supply controller42switches the state of the power supply circuit41to the power-ON state by outputting the low-level signals to the first voltage application circuit71and the second voltage application circuit81and switching ON the PMOS1and the PMOS2.

As illustrated inFIG. 4, when the control voltage VC is input from the power supply controller42to the first voltage application circuit71as the high-level signal, the transistor TR1is turned ON. When the transistor TR1is ON, the transistor TR2is turned ON. Therefore, the contact C1and the gate terminal74are connected via the transistor TR2as indicated by an arrow of a long dashed double-short dashed line inFIG. 4. Thus, the potential of the gate terminal74is switched to a potential of the contact C1. In this case, the potential of the source terminal72is equal to the potential of the gate terminal74, and the potential difference between those terminals is 0 V that is smaller than the threshold set in the PMOS1. Thus, the PMOS1is turned OFF.

As illustrated inFIG. 5, when the control voltage VC is input from the power supply controller42to the second voltage application circuit81as the high-level signal, the transistor TR3is turned ON. When the transistor TR3is ON, the transistor TR4is turned ON. Therefore, the contact C2and the gate terminal84are connected via the transistor TR4as indicated by an arrow of a long dashed double-short dashed line inFIG. 5. Thus, the potential of the gate terminal84is switched to a potential of the contact C2. In this case, the potential of the source terminal82is equal to the potential of the gate terminal84, and the potential difference between those terminals is 0 V that is smaller than the threshold set in the PMOS2. Thus, the PMOS2is turned OFF.

As described above, the power supply controller42switches the state of the power supply circuit41to the power-OFF state by outputting the high-level signals to the first voltage application circuit71and the second voltage application circuit81and switching OFF the PMOS1and the PMOS2.

Effects of this embodiment are described below. (1) To switch the power-ON state and the power-OFF state of the power supply circuit41of this embodiment by using the PMOS1and the PMOS2, it is only necessary that voltages having potentials lower than that of the on-board power supply50be applied to the gate terminals74and84. Therefore, there is no need to use a booster circuit such as a charge pump, which is necessary when N-channel MOSFETs are used. Thus, even if the voltages are kept applied to the gate terminals74and84of the PMOS1and the PMOS2to keep the power-ON state, power consumption can be reduced because of no need to use the booster circuit such as the charge pump. Even if the power-ON state is kept for a long period, an increase in the power consumption can be suppressed as compared to the case where the N-channel MOSFETs are used.

(2) To switch the power-ON state and the power-OFF state, the power supply circuit41of this embodiment only needs to have the circuits configured to switch connection to the reference potential point GND and connection to the contacts C1and C2on the existing circuits. Therefore, there is no need to use circuits configured to generate dedicated voltages for switching to the respective states. Thus, complication of the voltage application circuits71and81can be suppressed.

(3) In the power-ON state and the power-OFF state of the power supply circuit41of this embodiment, current backflow from the power supply target to the on-board power supply50can be suppressed by the PMOS2different from the PMOS1. In the power-ON state, a voltage is applied to the gate terminal84of the PMOS2to suppress the current backflow, but similarly to the above, it is only necessary that a voltage having a potential lower than that of the on-board power supply50be applied to the gate terminal84. That is, there is no need to use the booster circuit such as the charge pump, which is necessary in the case of N-channel MOSFETs. Thus, even if the voltage is kept applied to the gate terminal84of the PMOS2to suppress the current backflow in the power-ON state, the power consumption can be reduced because of no need to use the booster circuit such as the charge pump. Similarly to the above, when the power-ON state and the power-OFF state are switched for the PMOS2, it is only necessary to provide the circuit configured to switch connection to the reference potential point GND and connection to the contact C2on the existing circuit. Therefore, there is no need to use the circuit configured to generate a dedicated voltage for switching to the respective states. Thus, the complication of the second voltage application circuit81can be suppressed.

(4) In the power supply apparatus40of this embodiment, even if the power-ON state is kept for a long period, the increase in the power consumption can be suppressed as compared to the case where the N-channel MOSFETs are used. Thus, it is possible to attain a power supply apparatus40in which the increase in the power consumption is suppressed even if the power-ON state is kept irrespective of the state of the start switch of the vehicle.

The embodiment described above may be modified as follows. The following other embodiments may be combined without causing any technical contradiction. The PMOS2is provided to suppress the current backflow from the power supply target to the on-board power supply50, but the PMOS2need not be provided. In this case, there is no need to provide even the second voltage application circuit81configured to switch ON and OFF of the PMOS2.

When the PMOS1is switched ON, the potential of the gate terminal74of the PMOS1is equal to the potential of the reference potential point GND, but it is only necessary to apply a voltage to the gate terminal74of the PMOS1such that the potential difference between the gate terminal74and the source terminal72of the PMOS1is equal to or larger than the threshold. The same applies to the PMOS2.

When the PMOS1is switched OFF, the potential of the gate terminal74of the PMOS1is the potential of the on-board power supply50, but it is only necessary to apply a voltage to the gate terminal74of the PMOS1such that the potential difference between the gate terminal74and the source terminal72of the PMOS1is smaller than the threshold. The same applies to the PMOS2.

The electric power supply to the inverter of the steering control apparatus30may be configured to be interrupted after the start switch is turned OFF until the start switch is turned ON next time.

In the embodiment described above, the steering system1to which the power supply apparatus40is applied is the electric power steering system in which the motor20is coupled to the steering shaft12via the speed reducer21, but may be an electric power steering system in which the motor20is coupled to the rack shaft14via the speed reducer21. Further, the steering system1is not limited to the electric power steering system to which the power supply apparatus40is applied. For example, the power supply apparatus40may be applied to a steer-by-wire type steering system.

The power supply target of the power supply apparatus40may be other vehicular apparatuses such as an air bag apparatus. For example, the vehicular apparatus that is the power supply target of the power supply apparatus40may be an unmanned transport vehicle.