Gate driver and power module equipped with same

1. A gate driver that drives a field-effect transistor on the basis of an input signal, comprising:

a comparator that compares an applied voltage applied between a drain and a source of the field-effect transistor to a reference voltage for detecting noise occurring between the drain and the source of the field-effect transistor; and

a gate voltage switching circuit that, if the input signal indicates an instruction to turn off the field-effect transistor, and the field-effect transistor is off, switches a voltage applied between a gate and the source of the field-effect transistor from a first voltage to a second voltage when the output of the comparator transitions from a state indicating that the applied voltage between the drain and the source is less than the reference voltage to a state indicating that the applied voltage between the drain and the source is equal to or greater than the reference voltage;

wherein, provided that Vth is a threshold voltage of the field-effect transistor, V1 is the first voltage, and V2 is the second voltage,

a relationship Vth>V1>V2 is satisfied, and additionally, the second voltage is a negative voltage.

2. The gate driver according to claim 1 , wherein

the gate voltage switching circuit additionally

if the input signal indicates the instruction to turn off the field-effect transistor, and the field-effect transistor is off, switches the voltage applied between the gate and the source of the field-effect transistor from the second voltage to a third voltage when the output of the comparator transitions from the state indicating that the applied voltage between the drain and the source is equal to or greater than the reference voltage to the state indicating that the applied voltage between the drain and the source is less than the reference voltage, and

provided that V3 is the third voltage,

a relationship Vth>V3>V2 is satisfied.

3. The gate driver according to claim 2 , wherein

the third voltage is equal to the first voltage, and

the gate voltage switching circuit includes

a first switch inserted on a line joining the gate of the field-effect transistor to a first power supply that generates the first voltage,

a second switch inserted on a line joining the gate of the field-effect transistor to a second power supply that generates the second voltage, and

a switch control circuit that, if the input signal indicates the instruction to turn off the field-effect transistor, and the field-effect transistor is off, turns on the first switch and turns off the second switch if the output of the comparator indicates that the applied voltage between the drain and the source is less than the reference voltage, and turns off the first switch and turns on the second switch if the output of the comparator indicates that the applied voltage between the drain and the source is equal to or greater than the reference voltage.

4. The gate driver according to claim 2 , wherein

the first voltage V1 and the third voltage V3 satisfy a relationship V3<V1, and

the gate voltage switching circuit includes

a first switch inserted on a line joining the gate and the source of the field-effect transistor,

a second switch inserted on a line joining the gate of the field-effect transistor to a second power supply that generates the second voltage, and

a switch control circuit that, if the input signal indicates the instruction to turn off the field-effect transistor, and the field-effect transistor is off, (a) if the output of the comparator indicates that the applied voltage between the drain and the source is less than the reference voltage, and in addition, the voltage between the gate and the source of the field-effect transistor is a more positive voltage than the first voltage, turns off the first switch and turns on the second switch, and if the output of the comparator indicates that the applied voltage between the drain and the source is less than the reference voltage, and in addition, the voltage between the gate and the source of the field-effect transistor is included in a range from the first voltage to the third voltage, turns off the first switch and turns off the second switch, (b) if the output of the comparator indicates that the applied voltage between the drain and the source is equal to or greater than the reference voltage, turns off the first switch and turns on the second switch, and (c) if the output of the comparator indicates that the applied voltage between the drain and the source is less than the reference voltage, and in addition, the voltage between the gate and the source of the field-effect transistor is included in a range from the third voltage to the second voltage, turns on the first switch and turns off the second switch.

5. The gate driver according to claim 1 , further comprising:

a period detecting unit that detects whether or not a predetermined period has elapsed since the output of the comparator transitioned from the state indicating that the applied voltage between the drain and the source of the field-effect transistor is less than the reference voltage to the state indicating that the applied voltage between the drain and the source is equal to or greater than the reference voltage;

wherein the gate voltage switching circuit additionally

if the input signal indicates the instruction to turn off the field-effect transistor, and the field-effect transistor is off, switches the voltage applied between the gate and the source of the field-effect transistor from the second voltage to a third voltage when a detection result of the period detecting unit transitions from a state in which the predetermined period has not elapsed to a state in which the predetermined period has elapsed, and

provided that V3 is the third voltage,

a relationship Vth>V3>V2 is satisfied.

6. The gate driver according to claim 5 , wherein

the first voltage and the third voltage are equal, and

the gate voltage switching circuit includes

a first switch inserted on a line joining the gate of the field-effect transistor to a first power supply that generates the first voltage,

a second switch inserted on a line joining the gate of the field-effect transistor to a second power supply that generates the second voltage, and

a switch control circuit that, if the input signal indicates the instruction to turn off the field-effect transistor, and the field-effect transistor is off, turns on the first switch and turns off the second switch if the output of the comparator indicates that the applied voltage between the drain and the source is less than the reference voltage, and turns off the first switch and turns on the second switch when the detection result of the period detecting unit transitions from the state in which the predetermined period has not elapsed to the state in which the predetermined period has elapsed.

7. The gate driver according to claim 5 , wherein

the first voltage V1 and the third voltage V3 satisfy a relationship V3<V1, and

the gate voltage switching circuit includes

a first switch inserted on a line joining the gate and the source of the field-effect transistor,

a second switch inserted on a line joining the gate of the field-effect transistor to a second power supply that generates the second voltage, and

a switch control circuit that, if the input signal indicates the instruction to turn off the field-effect transistor, and the field-effect transistor is off, (a) if the output of the comparator indicates that the applied voltage between the drain and the source is less than the reference voltage, and in addition, the voltage between the gate and the source of the field-effect transistor is a more positive voltage than the first voltage, turns off the first switch and turns on the second switch, and if the output of the comparator indicates that the applied voltage between the drain and the source is less than the reference voltage, and in addition, the voltage between the gate and the source of the field-effect transistor is included in a range from the first voltage to the third voltage, turns off the first switch and turns off the second switch, (b) if the detection result of the period detecting unit indicates the state in which the predetermined period has not elapsed, turns off the first switch and turns on the second switch, and (c) if the detection result of the period detecting unit indicates the state in which the predetermined period has elapsed, and in addition, the voltage between the gate and the source of the field-effect transistor is included in a range from the third voltage to the second voltage, turns on the first switch and turns off the second switch.

8. The gate driver according to claim 1 , further comprising:

a high-pass filter, inserted on a pathway that transmits the applied voltage between the drain and the source of the field-effect transistor from the field-effect transistor to the comparator, that transmits the applied voltage between the drain and the source of the field-effect transistor to the comparator only if a time rate of change of the applied voltage between the drain and the source of the field-effect transistor is equal to or greater than a predetermined value.

9. The gate driver according to claim 8 , further comprising:

a period detecting unit that detects whether or not a predetermined period has elapsed since the output of the comparator transitioned from the state indicating that the applied voltage between the drain and the source of the field-effect transistor is less than the reference voltage to the state indicating that the applied voltage between the drain and the source is equal to or greater than the reference voltage;

wherein the gate voltage switching circuit additionally

if the input signal indicates an instruction to turn off the field-effect transistor, and the field-effect transistor is off, switches the voltage applied between the gate and the source of the field-effect transistor from the second voltage to a third voltage when a detection result of the period detecting unit transitions from a state in which the predetermined period has not elapsed to a state in which the predetermined period has elapsed, and

provided that V3 is the third voltage,

a relationship Vth>V3>V2 is satisfied.

10. The gate driver according to claim 1 , further comprising:

a current detecting unit that detects a drain current of the field-effect transistor; and

a switch, inserted on a pathway that transmits the applied voltage between the drain and the source of the field-effect transistor from the field-effect transistor to the comparator, that transmits the applied voltage between the drain and the source of the field-effect transistor to the comparator if the field-effect transistor is off, and transmits a detection result of the current detecting unit to the comparator if the field-effect transistor is on.

11. A power module, comprising:

the gate driver according to claim 1 ; and

the field-effect transistor.

12. The power module according to claim 11 , wherein

the field-effect transistor is provided with a channel layer, and

the field-effect transistor functions as a diode that allows current to flow from the source to the drain via the channel layer when a negative voltage is applied between the drain and the source while a negative voltage is being applied between the gate and the source.