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 configured to receive first and second drive voltages, the gate coupling circuit comprising two MOS-gated transistors electrically coupled in anti-series connection, the MOS-gated transistors electrically coupled to conduct current through the gate terminal of the thyristor in response to the first and second drive voltages; and

a control circuit configured to generate the first and second drive voltages for individually controlling the MOS-gated transistors of the gate coupling circuit, the control circuit being configured to control the gate coupling circuit to cause the MOS-gated transistors to conduct a pulse of current through the gate terminal of the thyristor at a firing time during a present half-cycle of the AC power source to render the thyristor conductive, the control circuit being further configured to control the gate coupling circuit to allow the MOS-gated transistors to conduct at least one other pulse of current through the gate terminal of the thyristor after the firing time during the present half-cycle.

2. The load control device of claim 1 , wherein the control circuit is configured to render both of the MOS-gated transistors conductive at the firing time.

3. The load control device of claim 2 , wherein the control circuit is configured to render a first one of the MOS-gated transistors non-conductive prior to an end of the present half-cycle, and to render a second one of the MOS-gated transistors non-conductive after the end of the present half-cycle.

4. The load control device of claim 3 , wherein the control circuit is configured to prevent the gate coupling circuit from conducting any further pulses of gate current through the gate terminal of the thyristor between a second time and the end of the present half-cycle to allow the thyristor to become non-conductive and remain non-conductive until the end of the present half-cycle.

5. 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 configured to receive first and second drive voltages, the gate coupling circuit comprising two MOS-gated transistors electrically coupled in anti-series connection, the MOS-gated transistors electrically coupled to conduct current through the gate terminal of the thyristor in response to the first and second drive voltages; and

a control circuit configured to generate the first and second drive voltages for individually controlling the MOS-gated transistors of the gate coupling circuit, the control circuit being configured to control the gate coupling circuit to cause the MOS-gated transistors to conduct a pulse of current through the gate terminal of the thyristor at a firing time during a present half-cycle of the AC power source to render the thyristor conductive, the control circuit being further configured to control the gate coupling circuit to allow the MOS-gated transistors to conduct at least one other pulse of current through the gate terminal of the thyristor after the firing time during the present half-cycle;

wherein the control circuit is configured to render both of the MOS-gated transistors conductive at the firing time;

wherein the control circuit is configured to render a first one of the MOS-gated transistors non-conductive prior to an end of the present half-cycle, and to render a second one of the MOS-gated transistors non-conductive after the end of the present half-cycle;

wherein the control circuit is configured to prevent the gate coupling circuit from conducting any further pulses of gate current through the gate terminal of the thyristor between a second time and the end of the present half-cycle to allow the thyristor to become non-conductive and remain non-conductive until the end of the present half-cycle; and

wherein the control circuit is configured to control the gate coupling circuit to prevent the thyristor from becoming conductive until the control circuit controls the MOS-gated transistors to render the thyristor conductive during a next half-cycle.

6. The load control device of claim 3 , wherein the first one of the MOS-gated transistors is able to block current at a beginning of a next half-cycle, and the second one of the MOS-gated transistors is able to conduct current until the end of the present half-cycle.

7. The load control device of claim 2 , wherein the gate coupling circuit comprises two control inputs for receiving the respective drive voltages from the control circuit, the gate coupling circuit being configured to conduct through each of the control inputs an amount of current appropriate to charge an input capacitance of a gate of the respective MOS-gated transistor when the MOS-gated transistors are rendered conductive at the firing time.

8. The load control device of claim 1 , further comprising:

a controllable switching circuit electrically coupled in series between the gate coupling circuit and the gate terminal of the thyristor and configured to conduct current through the gate terminal of the thyristor;

wherein the gate coupling circuit is coupled between the first main terminal of the thyristor and the controllable switching circuit, the control circuit being configured to render the controllable switching circuit conductive and to control the gate coupling circuit to conduct the pulse of current through the gate terminal of the thyristor at the firing time to render the thyristor conductive, the control circuit being further configured to render the controllable switching circuit non-conductive before an end of the present half-cycle to prevent the gate coupling circuit from thereafter conducting pulses of current through the gate terminal of the thyristor.

9. The load control device of claim 8 , wherein the thyristor is able to commutate off after the control circuit renders the controllable switching circuit non-conductive, the control circuit being further configured to maintain the controllable switching 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. The load control device of claim 9 , wherein the control circuit is configured to control the gate coupling circuit to provide a path for current to be conducted through the electrical load after the controllable switching circuit is rendered non-conductive during the present half-cycle.

11. The load control device of claim 8 , wherein the control circuit is configured to control the gate coupling circuit to allow the gate coupling circuit to conduct the at least one other pulse of current between the firing time and a second time that occurs after the firing time and before the end of the present half-cycle, the control circuit configured to render the controllable switching circuit non-conductive at the second time in order to prevent any further pulses of current from being thereafter conducted through the gate terminal of the thyristor during the present half-cycle.

12. The load control device of claim 11 , wherein the gate coupling circuit is electrically coupled to conduct current through the electrical load when the thyristor becomes non-conductive after the second time.

13. The load control device of claim 1 , wherein the MOS-gated transistors comprise MOSFETs or IGBTs.

14. A load control device for controlling an amount of 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 configured to receive first and second drive voltages, the gate coupling circuit comprising two MOS-gated transistors electrically coupled in anti-series connection, the MOS-gated transistors electrically coupled to conduct current through the gate terminal of the thyristor in response to the first and second drive voltages; and

a control circuit configured to generate the first and second drive voltages for controlling the MOS-gated transistors of the gate coupling circuit, the control circuit being configured to control the gate coupling circuit to cause the MOS-gated transistors to conduct a pulse of current through the gate terminal of the thyristor at a firing time during a present half-cycle of the AC power source to render the thyristor conductive, the control circuit being further configured to control the gate coupling circuit to allow the MOS-gated transistors to conduct at least one other pulse of current through the gate terminal of the thyristor after the firing time during the present half-cycle;

wherein the control circuit is configured to render a first one of the MOS-gated transistors non-conductive prior to the end of the present half-cycle, and to render a second one of the MOS-gated transistors non-conductive after the end of the present half-cycle.

15. The load control device of claim 14 , further comprising:

a controllable switching circuit electrically coupled in series between the gate coupling circuit and the gate terminal of the thyristor and configured to conduct current through the gate terminal of the thyristor;

wherein the gate coupling circuit is coupled between the first main terminal of the thyristor and the controllable switching circuit, the control circuit being configured to render the controllable switching circuit conductive and to control the gate coupling circuit to conduct the pulse of current through the gate terminal of the thyristor at the firing time to render the thyristor conductive, the control circuit being further configured to render the controllable switching circuit non-conductive before an end of the present half-cycle to prevent the gate coupling circuit from thereafter conducting pulses of current through the gate terminal of the thyristor.

16. The load control device of claim 15 , wherein the control circuit is configured to control the gate coupling circuit to provide a path for current to be conducted through the electrical load after the controllable switching circuit is rendered non-conductive during the present half-cycle.

17. The load control device of claim 14 , wherein the control circuit is configured to render both of the MOS-gated transistors conductive at the firing time.

18. The load control device of claim 14 , wherein the configuration of the control circuit is such that the first one of the MOS-gated transistors blocks current at a beginning of a next half-cycle, and the second one of the MOS-gated transistors conducts current until the end of the present half-cycle.

19. The load control device of claim 15 , wherein the thyristor is able to commutate off after the control circuit renders the controllable switching circuit non-conductive, the control circuit being configured to control the gate coupling circuit to prevent the thyristor from becoming conductive until the control circuit controls the MOS-gated transistors to render the thyristor conductive during a next half-cycle.

20. The load control device of claim 14 , wherein the gate coupling circuit comprises two control inputs for receiving the respective drive voltages from the control circuit, the gate coupling circuit being configured to conduct through each of the control inputs an amount of current appropriate to charge an input capacitance of a gate of the respective MOS-gated transistor when the MOS-gated transistors are rendered conductive at the firing time.