Powering high-efficiency lighting devices from a triac-based dimmer

1. A circuit for powering one or more lighting devices from an output of a thyristor-switched dimmer circuit coupled to an AC voltage source, the circuit comprising:

a switching power converter having an input for coupling to the output of the thyristor-switched dimmer circuit, a magnetic storage element for storing energy transferred from the input, and at least one output for supplying energy to the one or more lighting devices from the magnetic storage element; and

a control circuit for operating the switching power converter during an active portion of half cycles of a voltage of the AC voltage source in which the switching power converter transfers energy from the output of the thyristor-switched dimmer circuit, wherein the control circuit determines durations of the active portion of the half cycles such that sufficient energy is transferred from the input to operate the one or more lighting devices until a beginning of an active portion of a subsequent half-cycle, wherein subsequent to the active portion of the half-cycles, a high impedance level is presented to the output of the thyristor-switched dimmer circuit during an idle period that extends until the subsequent half-cycle, wherein the half cycles extend between adjacent zero-crossings of the voltage of the AC voltage source.

2. The circuit of claim 1 , wherein the switching power converter comprises:

a first power converter stage having an input coupled to the output of the thyristor-switched dimmer circuit;

a link capacitor coupled to an output of the first power converter stage; and

a second power converter stage having an input coupled to the capacitor for transferring energy from the link capacitor to the one or more lighting devices.

3. The circuit of claim 2 , wherein first power converter stage is coupled to the control circuit so that the control circuit controls the first power converter stage to transfer energy during the active portions of the half-cycles to charge the link capacitor in excess of the energy transferred from the second power converter stage to the one or more lighting devices during the corresponding half-cycles.

4. The circuit of claim 1 , further comprising a load circuit for dissipating energy provided from the output of the thyristor-switched dimmer circuit during the active portions of the half-cycles, wherein the control circuit controls the load circuit to provide a minimum predetermined impedance level at the output of the thyristor-switched dimmer circuit.

5. The circuit of claim 1 , further comprising a zero-crossing predicting circuit for predicting a zero-cross time of the voltage of the AC voltage source, wherein the control circuit has an input for receiving an output of the zero-crossing predicting circuit for indicating a beginning of the half-cycles of the voltage of the AC power source.

6. The circuit of claim 5 , wherein the zero-cross predicting circuit is a circuit that computes the zero-cross time of the voltage of the AC power source from a waveshape of the voltage at the output of the thyristor-switched dimmer circuit during the active portions of the half-cycles.

7. The circuit of claim 5 , wherein the zero-cross predicting circuit and the timer form part of a phase-lock loop that that calculates the zero-cross time of the voltage of the AC power source from phase comparisons of multiple periods of the AC power source with a phase of the timer.

8. A method of powering one or more lighting devices from an output of a thyristor-switched dimmer circuit, comprising:

supplying energy to the one or more lighting devices by converting energy drawn from the output of the thyristor-switched dimmer circuit;

determining a duration of active portion of half-cycles of a voltage of an AC voltage source that supplies an input of the thyristor-switched dimmer circuit for which the switching power converter transfers energy from the output of the thyristor-switched dimmer circuit, wherein the duration is determined such that sufficient energy is transferred from the input to operate the one or more lighting devices until a beginning of an active portion of a subsequent half-cycle; and

controlling an impedance presented to the output of the thyristor-switched dimmer circuit such that a high impedance level is presented to the output of the output of the thyristor-switched dimmer circuit during an idle period immediately subsequent to the active time period, wherein the half cycles extend between adjacent zero-crossings of the voltage of the AC voltage source.

9. The method of claim 8 , wherein the converting comprises:

storing the energy drawn from the output of the thyristor-switched dimmer circuit in a link capacitor in a first converting stage; and

transferring energy from the link capacitor to the one or more lighting devices by a second converting stage.

10. The method of claim 9 , wherein the first converting stage transfers energy during the active period to charge the link capacitor in excess of the energy transferred by the second converting stage to the one or more lighting devices during at least a portion of a power line cycle of the AC power source.

11. The method of claim 8 , further comprising dissipating energy provided from the output of the thyristor-switched dimmer circuit during the active portions of the half-cycles through a loading circuit, wherein the controlling controls the loading circuit to apply a predetermined minimum impedance level at the output of the thyristor-switched dimmer circuit during the active portions of the half-cycles.

12. The method of claim 8 , further comprising predicting zero-cross times of the voltage of an AC power source coupled to an input of the thyristor-switched dimmer circuit and wherein the determining commences the active portions in conformity with the predicted zero-cross times.

13. The method of claim 12 , wherein the predicting computes the voltage of the AC power source from a waveshape of the voltage at the output of the thyristor-switched dimmer circuit during the active portions of the half-cycles.

14. The method of claim 12 , wherein the predicting is performed by a phase-lock loop that that calculates the zero-cross time of the voltage of the AC power source from phase comparisons of multiple periods of the AC power source with a phase of the timer.

15. An integrated circuit for operating a circuit that powers one or more lighting devices from an output of a thyristor-switched dimmer circuit, comprising:

a power converter control circuit having an input for coupling to the output of the thyristor-switched dimmer circuit and an output for controlling a switching power converter for supplying the one or more lighting devices from the output of the thyristor-switched dimmer circuit; and

a timing control circuit for operating the switching power converter during an active portion of half cycles of a voltage of the AC voltage source in which the switching power converter transfers energy from the output of the thyristor-switched dimmer circuit, wherein the control circuit determines durations of the active portion of the half cycles such that sufficient energy is transferred from the input to operate the one or more lighting devices until a beginning of an active portion of a subsequent half-cycle, wherein subsequent to the active portion of the half-cycles, a high impedance level is presented to the output of the thyristor-switched dimmer circuit during an idle period that extends until the subsequent half-cycle, wherein the half cycles extend between adjacent zero-crossings of the voltage of the AC voltage source.

16. The integrated circuit of claim 15 , wherein the power converter control circuit controls a two cascaded switching power converter stages coupled by a link capacitor that stores energy converter by a first one of the power converter stages from the output of the thyristor-switched dimmer circuit and wherein a second one of the power converter stages is controlled by the power converter control circuit to transfer energy from the link capacitor to the one or more lighting devices.

17. The integrated circuit of claim 16 , wherein the power converter control circuit controls the first power converter stage to transfer energy during the active portions of the half-cycles to charge the link capacitor in excess of the energy transferred from the second power converter stage to the one or more lighting devices during the corresponding half-cycles.

18. The integrated circuit of claim 15 , further comprising a load circuit for dissipating energy provided from the output of the thyristor-switched dimmer circuit during the active portions of the half-cycles, wherein the control circuit controls the load circuit to provide a minimum predetermined impedance level at the output of the thyristor-switched dimmer circuit.

19. The integrated circuit of claim 15 , further comprising a zero-crossing predicting circuit for predicting a zero-cross time of the voltage of the AC voltage source, wherein the timing control circuit has an input for receiving an output of the zero-crossing predicting circuit for indicating a beginning of the half-cycles of the voltage of the AC power source.

20. The integrated circuit of claim 19 , wherein the zero-cross predicting circuit is a circuit that computes the zero-cross time of the voltage of the AC power source from a waveshape of the voltage at the output of the thyristor-switched dimmer circuit during the active portions of the half-cycles.

21. The integrated circuit of claim 19 , wherein the zero-cross predicting circuit and the timer form part of a phase-lock loop that that calculates the zero-cross time of the voltage of the AC power source from phase comparisons of multiple periods of the AC power source with a phase of the timer.