Load control device for high-efficiency loads
A two-wire load control device (such as, a dimmer switch) for controlling the amount of power delivered from an AC power source to an electrical load (such as, a high-efficiency lighting load) includes a thyristor coupled between the source and the load, a gate coupling circuit coupled between a first main load terminal and the gate of the thyristor, and a control circuit coupled to a control input of the gate coupling circuit. The control circuit generates a drive voltage for causing the gate coupling circuit to conduct a gate current to thus render the thyristor conductive at a firing time during a half cycle of the AC power source, and to allow the gate coupling circuit to conduct the gate current at any time from the firing time through approximately the remainder of the half cycle, where the gate coupling circuit conducts approximately no net average current to render and maintain the thyristor conductive.
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 between the first and second main terminals;
a gate coupling circuit electrically coupled to the first main terminal of the thyristor and arranged to conduct current through the gate terminal of the thyristor;
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; and
a control circuit configured to control the gate coupling circuit and the controllable switching 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, the control circuit being configured to control the gate coupling circuit and the controllable switching circuit to allow the gate terminal of the thyristor to conduct current again at a first time between the firing time and a second time that occurs after the firing time and before the end of the present half-cycle;
wherein the control circuit is configured to render the controllable switching circuit non-conductive between approximately the second time and the end of the present half-cycle to prevent current from being thereafter conducted through the gate terminal of the thyristor during the present half-cycle.
2. The load control device of claim 1 , 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 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.
3. The load control device of claim 2 , wherein the control circuit is configured to control the gate coupling circuit to allow current to be conducted through the electrical load after the controllable switching circuit is rendered non-conductive during the present half-cycle.
4. The load control device of claim 3 , wherein the gate coupling circuit comprises two MOS-gated transistors electrically coupled in anti-series connection between the first main terminal of the thyristor and the controllable switching circuit.
5. The load control device of claim 4 , wherein the control circuit is configured to generate two drive signals and to apply the drive signals to respective ones of the MOS-gated transistors to independently control the MOS-gated transistors.
6. The load control device of claim 5 , wherein the control circuit is configured to render both of the MOS-gated transistors conductive at the firing time, the control circuit further 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.
7. The load control device of claim 6 , wherein the configuration of the gate coupling circuit is such that the first one of the MOS-gated transistors blocks current at the beginning of the next half-cycle, and the second one of the MOS-gated transistors conducts current the until the end of the present half-cycle.
8. The load control device of claim 6 , wherein the gate coupling circuit comprises two control inputs for receiving the respective drive signals 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 terminal of the respective MOS-gated transistor when the MOS-gated transistors are rendered conductive at the firing time.
9. The load control device of claim 4 , wherein the control circuit is configured to generate a single drive signal for controlling the gate coupling circuit, the gate coupling circuit comprising a control input for receiving the drive signal from the control circuit.
10. The load control device of claim 1 , wherein the control circuit is 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.
11. The load control device of claim 10 , wherein the control circuit is further configured to:
control the magnitude of the 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 the firing time during the present half-cycle of the AC power source;
maintain the drive signal at the first magnitude from the firing time until the second 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 second time; and
control the magnitude of the drive signal to the second magnitude to render the gate coupling circuit non-conductive between approximately the second time and the end of the present half-cycle.
12. The load control device of claim 11 , further comprising:
a power supply configured to generate a supply voltage for powering the control circuit.
13. The load control device of claim 12 , 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.
14. The load control device of claim 12 , 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.
15. The load control device of claim 11 , wherein the control circuit is configured to maintain the drive signal at the first magnitude from the firing time until the second time to provide constant gate drive to the thyristor between the firing time and the second time.
16. The load control device of claim 1 , further comprising:
a wireless signal receiver configured to receive a wireless signal;
wherein the control circuit is configured to determine a desired intensity for the electrical load in response to the wireless signal.
17. The load control device of claim 16 , wherein the wireless signal receiver comprises a radio-frequency receiver configured to receive a radio-frequency signal.
18. The load control device of claim 1 , further comprising:
an intensity adjustment actuator configured to receive a user input;
wherein the control circuit is configured to determine a desired intensity for the electrical load in response to actuations of the intensity adjustment actuator.
19. The load control device of claim 1 , wherein the gate coupling circuit is arranged to conduct current through the electrical load when the thyristor becomes non-conductive after the second time.
20. The load control device of claim 1 , further comprising:
a resistor coupled between the gate coupling circuit and the second main terminal of the thyristor, whereby a path is provided for current to flow through the electrical load after the thyristor becomes non-conductive during the present half-cycle.