Load control device for high-efficiency loads

1. A load control device for controlling power delivered from an AC power source to an electrical load, the load control device comprising:

a thyristor adapted to be electrically coupled between the AC power source and the electrical load, the thyristor having first and second main terminals through which current can be conducted to energize the electrical load and a gate terminal through which current can be conducted to render the thyristor conductive;

a gate coupling circuit electrically coupled between the first main terminal and the gate terminal of the thyristor to conduct current through the gate terminal of the thyristor;

a controllable switching circuit, separate from and independently controllable in relation to the gate coupling circuit, and electrically coupled between the first and second main terminals of the thyristor; and

a control circuit configured to render the gate coupling circuit conductive to conduct a pulse of current through the gate terminal of the thyristor to render the thyristor conductive at a firing time during a present half-cycle of the AC power source, the control circuit configured to control the gate coupling circuit to allow the gate coupling circuit to conduct at least one other pulse of current through the gate terminal of the thyristor again after the firing time until a transition time before an end of the present half-cycle;

wherein the control circuit is configured to render the gate coupling circuit non-conductive between approximately the transition time and the end of the present half-cycle to prevent the gate coupling circuit from conducting pulses of current through the gate terminal of the thyristor between approximately the transition time and the end of the present half-cycle, the control circuit configured to render the controllable switching circuit conductive to conduct current through the electrical load between approximately the transition time and the end of the present half-cycle.

2. The load control device of claim 1 , wherein the control circuit is configured to generate a first control signal for rendering the gate coupling circuit conductive and non-conductive and a second control signal for rendering the controllable switching circuit conductive and non-conductive.

3. The load control device of claim 2 , wherein the control circuit is configured to control the second control signal to render the controllable switching circuit non-conductive at approximately the end of the present half-cycle.

4. The load control device of claim 3 , wherein the control circuit is configured to control the second control signal to render the controllable switching circuit conductive before controlling the first control signal to render the gate coupling circuit non-conductive at approximately the transition time.

5. The load control device of claim 1 , wherein the controllable switching circuit comprises at least one MOS-gated transistor.

6. The load control device of claim 5 , wherein the controllable switching circuit comprises a single MOS-gated transistor in a full-wave rectifier bridge.

7. The load control device of claim 1 , wherein the gate coupling circuit comprises at least one MOS-gated transistor electrically coupled between the first main terminal of the thyristor and the gate terminal of thyristor.

8. The load control device of claim 7 , wherein the gate coupling circuit comprises one of a single MOS-gated transistor in a full-wave rectifier bridge and two MOS-gated transistors electrically coupled in anti-series connection.

9. The load control device of claim 1 , wherein the thyristor is able to commutate off after the transition time during the present half-cycle, and the control circuit is further configured to maintain the gate coupling circuit non-conductive until at least a beginning of a next half-cycle of the AC power source, thereby assuring that the thyristor remains non-conductive for a remainder of the present half-cycle after the thyristor commutates off.

10. A load control device for controlling power delivered from an AC power source to an electrical load, the load control device comprising:

a thyristor adapted to be electrically coupled between the AC power source and the electrical load, the thyristor having first and second main terminals through which current can be conducted to energize the electrical load and a gate terminal through which current can be conducted to render the thyristor conductive;

a gate coupling circuit electrically coupled between the first main terminal and the gate terminal of the thyristor to conduct current through the gate terminal of the thyristor;

a controllable switching circuit electrically coupled between the first and second main terminals of the thyristor; and

a control circuit configured to generate a first control signal for rendering the gate coupling circuit conductive and non-conductive and a second control signal for rendering the controllable switching circuit conductive and non-conductive, the control circuit configured to control the first control signal to render the gate coupling circuit conductive at a firing time during a present half-cycle of the AC power source to allow the gate coupling circuit to conduct a pulse of current through the gate terminal of the thyristor to render the thyristor conductive, the control circuit configured to control the first control signal to control the gate coupling circuit gate coupling circuit to allow the gate coupling circuit to conduct at least one other pulse of current through the gate terminal of the thyristor again after the firing time until a transition time before an end of the present half-cycle;

wherein the control circuit is configured to control the first control signal to render the gate coupling circuit non-conductive between approximately the transition time and the end of the present half-cycle, the control circuit being configured to control the second control signal to render the controllable switching circuit conductive to conduct current through the electrical load between approximately the transition time and the end of the present half-cycle.

11. The load control device of claim 10 , wherein the control circuit is configured to control the second control signal to render the controllable switching circuit non-conductive at approximately the end of the present half-cycle.

12. The load control device of claim 11 , wherein the control circuit is configured to control the first control signal to maintain the gate coupling circuit non-conductive after the transition time until at least a beginning of a next half-cycle of the AC power source.

13. The load control device of claim 12 , wherein the thyristor is able to commutate off after the transition time during the present half-cycle.

14. The load control device of claim 12 , wherein, after the thyristor commutates off, the thyristor remains non-conductive for a remainder of the present half-cycle.

15. The load control device of claim 10 , wherein the control circuit is configured to control the second control signal to render the controllable switching circuit conductive before controlling the first control signal to render the gate coupling circuit non-conductive at approximately the transition time.

16. The load control device of claim 15 , wherein the control circuit is configured to control the second control signal to render the controllable switching circuit conductive approximately ten microseconds before controlling the first control signal to render the gate coupling circuit non-conductive.

17. A method of controlling power delivered from an AC power source to an electrical load, the method comprising:

conducting current through first and second main terminals of a thyristor to energize the electrical load;

generating a first control signal for rendering a gate coupling circuit conductive and non-conductive, the gate coupling circuit electrically coupled between the first main terminal and a gate terminal of the thyristor to conduct current through the gate terminal of the thyristor;

generating a second control signal for rendering a controllable switching circuit conductive and non-conductive, the controllable switching circuit electrically coupled between the first and second main terminals of the thyristor;

controlling the first control signal to render the gate coupling circuit conductive at a firing time during a present half-cycle of the AC power source to allow the gate coupling circuit to conduct a pulse of current through the gate terminal of the thyristor and render the thyristor conductive;

controlling the first control signal to control the gate coupling circuit gate coupling circuit to allow the gate coupling circuit to conduct at least one other pulse of current through the gate terminal of the thyristor again after the firing time until a transition time before an end of the present half-cycle;

controlling the first control signal to render the gate coupling circuit non-conductive between approximately the transition time and the end of the present half-cycle; and

controlling the second control signal to render the controllable switching circuit conductive to conduct current through the electrical load between approximately the transition time and the end of the present half-cycle.

18. The method of claim 17 , further comprising:

controlling the second control signal to render the controllable switching circuit non-conductive at approximately the end of the present half-cycle.

19. The method of claim 18 , further comprising:

controlling the first control signal after the transition time to maintain the gate coupling circuit non-conductive until at least a beginning of a next half-cycle of the AC power source.

20. The method of claim 17 , further comprising:

controlling the second control signal to render the controllable switching circuit conductive before controlling the first control signal to render the gate coupling circuit non-conductive at approximately the transition time.