Bidirectional switch and method for driving the same

1. A bidirectional switch for controlling between a conduction state in which a current flows at least in one direction between a first terminal and a second terminal and a cut-off state in which a current does not flow therebetween, the switch comprising:

a field-effect transistor including a first ohmic electrode and a second ohmic electrode of which one serves as a source electrode and the other serves as a drain electrode and a gate electrode formed between the first ohmic electrode and the second ohmic electrode, the first ohmic electrode being connected to the first terminal, the second ohmic electrode being connected to the second terminal; and

a control circuit for applying a bias voltage to the gate electrode to control between the conduction state and the cut-off state,

wherein the control circuit applies the bias voltage with reference to the first ohmic electrode when a potential of the second ohmic electrode is higher than the potential of the first ohmic electrode and applies the bias voltage with reference to the second ohmic electrode when the potential of the second ohmic electrode is lower than the potential of the first ohmic electrode.

2. The bidirectional switch of claim 1 , wherein the control circuit includes a first power supply,

electrically connects, when the potential of the second ohmic electrode is higher than the potential of the first ohmic electrode, the first power supply between the first ohmic electrode and the gate electrode to apply the bias voltage to the gate electrode, and

electrically connects, when the potential of the second ohmic electrode is lower than the potential of the first ohmic electrode, the first power supply between the second ohmic electrode and the gate electrode to apply the bias voltage to the gate electrode.

3. The bidirectional switch of claim 2 , wherein the control circuit includes a first switch connected between the first power supply and the first ohmic electrode, a second switch connected between the first power supply and the second ohmic electrode and a driving circuit for driving the first switch and the second switch, and

the driving circuit switches, on the basis of a result of comparison of the potential of the first ohmic electrode with the potential of the second ohmic electrode, between an ON state and an OFF state of each of the first switch and the second switch.

4. The bidirectional switch of claim 1 , wherein the control circuit includes a first power supply and a second power supply,

electrically connects, when the potential of the second ohmic electrode is higher than the potential of the first ohmic electrode, the first power supply between the first ohmic electrode and the gate electrode to apply the bias voltage to the gate electrode, and

electrically connects, when the potential of the second electrode is lower than the potential of the first ohmic electrode, the second power supply between the second ohmic electrode and the gate electrode to apply the bias voltage to the gate electrode.

5. The bidirectional switch of claim 4 , wherein each of the first power supply and the second power supply outputs a higher voltage than a threshold voltage of the field-effect transistor in the conduction state, and outputs a lower voltage than the threshold voltage of the field-effect transistor in the cut-off state.

6. The bidirectional switch of claim 4 , wherein the first power supply outputs a higher voltage than a threshold voltage of the field-effect transistor and the second power supply outputs a lower voltage than the threshold voltage of the field-effect transistor to make a current flow in one direction and cut off a current in the other direction between the first terminal and the second terminal.

7. The bidirectional switch of claim 4 , wherein the control circuit includes a first switch connected between the first power supply and the gate electrode, a second switch connected between the second power supply and the gate electrode and a driving circuit for driving the first switch and the second switch, and

the driving circuit switches, on the basis of a result of comparison of the potential of the first ohmic electrode with the potential of the second ohmic electrode, between an ON state and an OFF state of each of the first switch and the second switch.

8. The bidirectional switch of claim 7 , wherein the first switch and the second switch are a first photocoupler and a second photocoupler, respectively,

the driving circuit includes an differential amplifier for receiving a voltage corresponding to a voltage applied between the first ohmic electrode and the second ohmic electrode at an input terminal, and

the differential amplifier drives a light emitting diode of the first photocoupler and a light emitting diode of the second photocoupler.

9. The bidirectional switch of claim 7 , wherein the first switch and the second switch are a first photocoupler and a second photocoupler, respectively,

the driving circuit includes a first differential amplifier for receiving a voltage corresponding to a voltage applied between the first ohmic electrode and the second ohmic electrode at a non-inverting input terminal and a second differential amplifier for receiving a voltage corresponding to a voltage applied between the first ohmic electrode and the second ohmic electrode at an inverting input terminal, the first differential amplifier having an inverting input terminal connected to the first ohmic electrode, the second differential amplifier having a non-inverting input terminal connected to the first ohmic electrode,

the first differential amplifier drives the light emitting diode of the first photocoupler, and

the second differential amplifier drives the light emitting diode of the second photocoupler.

10. The bidirectional switch of claim 1 , wherein the field-effect transistor includes a semiconductor layer formed on a substrate,

the first ohmic electrode and the second ohmic electrode are formed on the semiconductor layer so as to be spaced apart from each other, and

the gate electrode is formed on the semiconductor layer so that a distance between the first ohmic electrode and the gate electrode and a distance between the second ohmic electrode and the gate electrode are equal.

11. A method for driving a bidirectional switch including a field-effect transistor having a first ohmic electrode and a second ohmic electrode of which one serves as a source electrode and the other serves as a drain electrode and a gate electrode formed between the first ohmic electrode and the second ohmic electrode, the method comprising the steps of:

comparing a potential of the second electrode with a potential of the first ohmic electrode; and

applying a bias voltage to a gate electrode of the field-effect transistor with reference to the first ohmic electrode when the potential of the second ohmic electrode is higher than the potential of the first ohmic electrode and applying the bias voltage to the gate electrode with reference to the second ohmic electrode when the potential of the second ohmic electrode is lower than the potential of the first ohmic electrode,

wherein the bias voltage is a higher voltage than a threshold voltage of the field-effect transistor in a conduction state in which a current flows at least in one direction between a first terminal and a second terminal and is a voltage lower than the threshold voltage in a cut-off state in which a current does not flow therebetween.