Load control device for high-efficiency loads
A load control device for controlling the power delivered from an AC power source to an electrical load includes a thyristor, a gate coupling circuit for conducting a gate current through a gate of the thyristor, and a control circuit for controlling the gate coupling circuit to conduct the gate current through a first current path to render the thyristor conductive at a firing time during a half cycle. The gate coupling circuit is able to conduct the gate current through the first current path again after the firing time, but the gate current is not able to be conducted through the gate from a transition time before the end of the half-cycle until approximately the end of the half-cycle. The load current is able to be conducted through a second current path to the electrical load after the transition time until approximately the end of the half-cycle.
1. A load control device for controlling power delivered from an alternating-current (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 in parallel with the thyristor; and
a control circuit configured to generate at least one drive signal that is received by the gate coupling circuit, the control circuit configured to control a magnitude of the drive signal to a first magnitude to render the gate coupling circuit conductive and to a second magnitude to render the gate coupling circuit non-conductive;
wherein the control circuit is configured to:
control the magnitude of the at least one drive signal to the first magnitude to cause the gate coupling circuit to conduct 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;
maintain the at least one drive signal at the first magnitude from the firing time until a transition time before the end of the present half-cycle to allow the gate coupling circuit to conduct current through the gate terminal of the thyristor again between the firing time until the transition time;
control the magnitude of the at least one drive signal to the second magnitude to render the gate coupling circuit non-conductive between approximately the transition time and the end of the present half-cycle; and
render the controllable switching circuit conductive to allow the controllable switching circuit 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 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.
3. The load control device of claim 2 , further comprising:
a power supply configured to generate a supply voltage for powering the control circuit.
4. The load control device of claim 3 , wherein the control circuit is configured to control the magnitude of the at least one drive signal to approximately the magnitude of the supply voltage to render the gate coupling circuit conductive, and to approximately zero volts to render the gate coupling circuit non-conductive.
5. The load control device of claim 4 , further comprising:
a gate drive circuit configured to receive the at least one drive signal from the control circuit and render the at least one MOS-gated transistor conductive and non-conductive in response to the at least one drive signal.
6. The load control device of claim 5 , further comprising:
a boost converter configured to receive the supply voltage from the power supply and generate a boosted voltage;
wherein the gate drive circuit is configured to receive the boosted voltage for driving the at least one MOS-gated transistor.
7. The load control device of claim 3 , wherein the power supply is configured to conduct a charging current through the electrical load when the thyristor is non-conductive in order to generate the supply voltage.
8. The load control device of claim 2 , wherein the gate coupling circuit comprises two MOS-gated transistors electrically coupled in anti-series connection.
9. The load control device of claim 8 , wherein the at least one drive signal comprises respective drive signals coupled to the gates of the two MOS-gated transistors for individually rendering each of the two MOS-gated transistors conductive and non-conductive.
10. The load control device of claim 9 , wherein the control circuit is configured to render both of the MOS-gated transistors conductive at the firing time.
11. The load control device of claim 10 , wherein the control circuit is configured to render both of the MOS-gated transistors non-conductive at approximately the transition time.
12. The load control device of claim 8 , wherein gates of the two MOS-gated transistors are electrically coupled together, and the at least one drive signal comprises a single drive signal coupled to the gates of the two MOS-gated transistors for rendering the gate coupling circuit conductive and non-conductive.
13. The load control device of claim 2 , wherein the gate coupling circuit comprises a single MOS-gated transistor in a full-wave rectifier bridge.
14. The load control device of claim 13 , wherein the at least one drive signal comprises a single drive signal coupled to a gate of the single MOS-gated transistor for rendering the single MOS-gated transistor conductive and non-conductive.
15. The load control device of claim 1 , wherein the controllable switching circuit comprises at least one MOS-gated transistor.
16. The load control device of claim 15 , wherein the controllable switching circuit comprises a single MOS-gated transistor in a full-wave rectifier bridge.
17. The load control device of claim 1 , further comprising:
a resistor electrically coupled between the second main terminal of the thyristor and the gate terminal of the thyristor.
18. The load control device of claim 1 , wherein the thyristor is able to commutate off after the transition time during the present half-cycle, the control circuit being further configured to maintain the gate coupling circuit non-conductive until at least the beginning of the next half-cycle of the AC power source, thereby assuring that the thyristor remains non-conductive for the remainder of the present half-cycle after the thyristor commutates off.
19. The load control device of claim 1 , wherein the control circuit is configured to render the controllable switching circuit conductive before rendering the gate coupling circuit non-conductive at approximately the transition time.
20. The load control device of claim 1 , wherein the control circuit is configured to maintain the drive signal at the first magnitude from the firing time until the transition time to provide constant gate drive to the thyristor between the firing time and the transition time.