Motor driving device and steering system

1. A motor driving device comprising:

an inverter connected to a motor of multiple phases via power supply lines to supply a voltage selectively to the multiple phases of the motor;

phase-opening switches provided in the power supply lines, respectively, for opening and closing a current flow path between the inverter and the motor;

a pull-up resistor circuit connected closer to the inverter than the phase-opening switches in the power supply lines to supply a predetermined voltage from a power supply to the power supply lines;

a short-circuit determination circuit configured to determine a presence or absence of a short-circuit failure of a phase-opening switch, which is to be subjected to a short-circuit failure determination as a target switch; and

a voltage detection circuit configured to detect a voltage of each power supply line, wherein the phase-opening switches are configured to supply a current from the inverter to each phase of the motor in an open state thereof;

the short-circuit determination circuit is configured to

(i) drive all of the phase-opening switches to open,

(ii) then drive the inverter to electrically connect one of the power supply lines, the one of the power supply lines including the target switch among the phase-opening switches, to a ground by driving a lower arm of the inverter that is connected to the one of the power supply lines to close, wherein the lower arm is connected to the one of the power supply lines including the target switch, and

(iii) determine that the target switch has the short-circuit failure in case that a target voltage detected by the voltage detection circuit is lower than the predetermined voltage, the target voltage indicating a voltage of a power supply line other than the one of the power supply lines including the target switch, the target voltage being taken at a node between an upper arm and a lower arm of the inverter, which are coupled to the power supply line other than the one of the power supply lines including the target switch.

2. The motor driving device according to claim 1 , wherein:

the power supply is a battery; and

the short-circuit determination circuit is configured to determine whether the target voltage is smaller than the predetermined voltage by comparing the target voltage with a predetermined threshold value; and

the short-circuit determination circuit includes a threshold value changing circuit which changes the predetermined threshold value to a smaller value as an inter-terminal voltage of the battery decreases.

3. The motor driving device according to claim 1 , wherein:

the power supply is a battery; and

the short-circuit determination circuit is configured to determine whether the target voltage is smaller than the predetermined voltage based on a ratio of the target voltage relative to an inter-terminal voltage of the battery.

4. The motor driving device according to claim 1 , further comprising:

an open-circuit failure determination circuit configured to determine whether an open-circuit failure is present in a lower arm of the inverter, the lower arm being connected to the one of the power supply lines including the target switch, wherein

the short-circuit determination circuit is configured to connect the one of the power supply lines including the target switch, by driving the lower arm to close for determining whether the open-circuit failure is present in the lower arm; and

the short-circuit determination circuit is configured to determine the short-circuit failure of the target switch on condition that the open-circuit failure determination circuit determines that the lower arm has no open-circuit failure.

5. The motor driving device according to claim 1 , wherein the voltage detection circuit includes:

an output switching circuit configured to select and output the voltage of the power supply line other than the one of the power supply lines including the target switch; and

a conversion circuit configured to provide the target voltage by A/D converting the voltage outputted from the output switching circuit.

6. A steering system comprising:

the motor driving device according to claim 1 ;

the motor driven by the motor driving device to assist a steering operation in a vehicle by a driving force of the motor, wherein:

the motor is a multiple winding type motor including a first phase coil group and a second phase coil group;

the inverter includes a first inverter and a second inverter, which are configured to supply voltages to the first phase coil group and the second phase coil group;

a first switch group provided as the phase-opening switches for connecting the first inverter and the first phase coil group;

a second switch group provided as the phase-opening switches for connecting the second inverter and the second phase coil group; and

a prohibition circuit configured to prohibit an operation of one inverter between the first inverter and the second inverter connected to one of the first switch group or the second switch group which is determined to have the short-circuit failure, when the short-circuit determination circuit determines that either one of the first switch group and the second switch group has the short-circuit failure.

7. The motor driving device according to claim 1 , wherein the voltage detection circuit comprises a multiplexer, the multiplexer electrically connected to each of the power supply lines, the multiplexer configured to select and output the voltage of the power supply line other than the one of the power supply lines including the target switch.

8. A motor driving device for a motor of multiple phases which are supplied with a voltage from a power supply through an inverter and power supply lines connecting the inverter and provided in correspondence to the multiple phases, the motor driving device comprising:

phase-opening switches provided in the power supply lines between the inverter and the motor, respectively;

a resistor circuit connecting the power supply lines to the power supply;

a voltage detection circuit connected to the power supply lines to detect a voltage of each of the power supply lines; and

a control circuit configured to determine a short-circuit failure of the phase-opening switches, wherein the control circuit is configured to:

open all of the phase-opening switches to interrupt electric connection of the motor to the inverter and the resistor circuit;

electrically connect only one power supply line among the power supply lines to a ground by driving a lower arm of the inverter that is connected to the one of the power supply lines to close, wherein the lower arm is connected to the one power supply line, and the one power supply line includes a target switch among the phase-opening switches;

compare a voltage of another power supply line among the power supply lines with a predetermined value; and

determine a short-circuit failure of one phase-opening switch provided in the one phase-opening switch when the voltage of the another power supply line is smaller than the predetermined value, the voltage of the another power supply line indicating a voltage of a power supply line other than the one power supply line, the voltage being taken at a node between an upper arm and a lower arm of the inverter, which are coupled to the another power supply line.

9. The motor driving device according to claim 8 , wherein the control circuit is configured further to:

compare the voltage of the one power supply line with a threshold value; and

determine an open-circuit failure indicating that the one power supply line is not connected to the ground when the voltage of the one power supply line is larger than the threshold value.

10. The motor driving device according to claim 9 , wherein:

the control circuit is configured to determine the short-circuit failure of the one phase-opening switch on condition of no determination of the open-circuit failure.

11. The motor driving device according to claim 8 , wherein:

the control circuit is configured to change the predetermined value in correspondence to an inter-terminal voltage of the power supply.

12. The motor driving device according to claim 8 , further comprising a multiplexer, the multiplexer electrically connected to each of the power supply lines, the multiplexer configured to select and output the voltage of the power supply line other than the one of the power supply lines including the target switch.