Load control device for high-efficiency loads

1. A method of controlling power delivered from an AC power source to a lighting load to control an intensity of the lighting load, the method comprising:

conducting current through first and second main terminals of a thyristor to energize the lighting load, wherein the thyristor is electrically coupled between the AC power source and the lighting load;

determining to operate in one of a first gate drive mode and a second gate drive mode;

controlling a gate current path to conduct a pulse of current through a gate terminal of the thyristor to render the thyristor conductive at a firing time during a half-cycle of the AC power source;

when operating in the first gate drive mode, rendering the gate current path non-conductive at an end of a pulse time period after the firing time during the half-cycle of the AC power source; and

when operating in the second gate drive mode, maintaining the gate current path conductive after the end of the pulse time period to allow at least one other pulse of current to be conducted through the gate terminal of the thyristor after the pulse time period during the half-cycle of the AC power source.

2. The method of claim 1 , further comprising:

when turning on the lighting load, operating in the first gate drive mode over a first plurality of half-cycles of the AC power source, and operating in the second gate drive mode over a second plurality of half-cycles of the AC power source after the first plurality of half-cycles.

3. The method of claim 2 , wherein operating in the first gate drive mode over the first plurality of half-cycles of the AC power source further comprises:

controlling the pulse time period to a minimum pulse time period during a first one of the first plurality of half cycles; and

increasing the pulse time period during a subsequent one of the first plurality of half cycles.

4. The method of claim 3 , wherein operating in the first gate drive mode over the first plurality of half-cycles of the AC power source further comprises:

periodically increasing the pulse time period over the first plurality of half-cycles of the AC power source.

5. The method of claim 4 , wherein periodically increasing the pulse time period over the first plurality of half-cycles of the AC power source further comprises:

periodically increasing the pulse time period over the first plurality of half-cycles of the AC power source until the pulse time period is equal to a maximum pulse time period.

6. The method of claim 5 , wherein operating in the second gate drive mode over the second plurality of half-cycles of the AC power source further comprises:

maintaining the gate current path conductive for the maximum pulse time period in each of the second plurality of half-cycles;

wherein the maximum pulse time period is dependent upon a target intensity to which the control circuit is configured to control the intensity of the lighting load.

7. The method of claim 4 , wherein periodically increasing the pulse time period over the first plurality of half-cycles of the AC power source further comprises:

periodically increasing the pulse time period over a number of half-cycles of the AC power source, where the number of half cycles is dependent upon a target intensity to which the control circuit is configured to control the intensity of the lighting load.

8. The method of claim 4 , wherein periodically increasing the pulse time period over the first plurality of half-cycles of the AC power source further comprises:

increasing the pulse time period in each of the first plurality of half-cycles.

9. The method of claim 1 , wherein controlling the gate current path to conduct the pulse of current through the gate terminal of the thyristor to render the thyristor conductive at the firing time during the half-cycle of the AC power source further comprises:

controlling a gate coupling circuit of the gate current path to conduct the pulse of current through the gate terminal of the thyristor, the gate coupling circuit connected to the gate terminal of the thyristor.

10. The method of claim 9 , wherein controlling the gate current path to conduct the pulse of current through the gate terminal of the thyristor to render the thyristor conductive at the firing time during the half-cycle of the AC power source further comprises:

rendering a controllable switching circuit of the gate current path conductive prior to controlling the gate coupling circuit, the controllable switching circuit electrically coupled in series between the gate coupling circuit and the gate terminal of the thyristor to conduct current through the gate terminal of the thyristor.

11. The method of claim 10 , further comprising:

when operating in the second gate drive mode, rendering the controllable switching circuit non-conductive before an end of the half-cycle of the AC power source to prevent further pulses of current from being conducted through the gate terminal of the thyristor.

12. The method of claim 11 , wherein the thyristor is capable of commutating off after the controllable switching circuit is rendered non-conductive, the method further comprising:

maintaining the controllable switching circuit non-conductive until at least the beginning of a subsequent half-cycle of the AC power source.

13. The method of claim 1 , further comprising:

when operating in the second gate drive mode, rendering the gate current path non-conductive before an end of the half-cycle of the AC power source to prevent further pulses of current from being conducted through the gate terminal of the thyristor.

14. The method of claim 13 , wherein the thyristor is capable of commutating off after the gate current path is rendered non-conductive, the method further comprising:

conducting current through an alternate current path and the lighting load after rendering the gate current path non-conductive until the end of the half-cycle.

15. The method of claim 13 , wherein the thyristor is capable of commutating off after the gate current path is rendered non-conductive, the method further comprising:

rendering a controllable switching circuit conductive after rendering the gate current path non-conductive, the controllable switching circuit electrically coupled in parallel with the first and second main terminals of the thyristor; and

maintaining the controllable switching circuit conductive until the end of the half-cycle to conduct current through the lighting load after the thyristor commutates off.

16. The method of claim 1 , further comprising:

receiving a first zero-cross signal that provides an indication of a zero-crossing of the AC power source;

wherein controlling the gate current path to conduct the pulse of current through the gate terminal of the thyristor to render the thyristor conductive at the firing time during the half-cycle of the AC power source further comprises controlling the gate current path to conduct the pulse of current through the gate terminal of the thyristor to render the thyristor conductive at the firing time during the half-cycle of the AC power source based on the first zero-cross signal.

17. The method of claim 16 , further comprising:

sampling the first zero-cross signal during a zero-cross window; and

determining if the first zero-cross signal indicated the zero-crossing of the AC power source during the zero-cross window.

18. The method of claim 17 , wherein determining to operate in one of the first gate drive mode and the second gate drive mode further comprises:

operating in the second gate drive mode while turning on the lighting load;

determining that the first zero-cross signal did not indicate the zero-crossing of the AC power source during the zero-cross window while turning on the lighting load using the second gate drive mode; and

subsequently operating in the first gate drive mode while turning on the lighting load in response to determining that the first zero-cross signal did not indicate the zero-crossing of the AC power source during the zero-cross window while turning on the lighting load using the second gate drive mode.

19. The method of claim 1 , wherein determining to operate in one of the first gate drive mode and the second gate drive mode further comprises:

operating in the first gate drive mode while turning on the lighting load; and

operating in the second gate drive mode when in a steady state condition.

20. The method of claim 1 , wherein determining to operate in one of the first gate drive mode and the second gate drive mode further comprises:

while turning on the lighting load, operating in the second gate drive mode;

detecting a fault condition while turning on the lighting load using the second gate drive mode;

subsequently, while turning on the lighting load, operating in the first gate drive mode over a first plurality of half-cycles of the AC power source while turning on the lighting load, and operating in the second gate drive mode over a second plurality of half-cycles of the AC power source after the first plurality of half-cycles.