Duty factor probing of a triac-based dimmer
A power supply circuit for operating high-efficiency lighting devices from a thyristor-controlled dimmer determines the dimming value, i.e., the dimmer duty factor by periodically probing the dimmer output. A minimum conductance is applied across the output of the dimmer during probing intervals that begin at the turn-on time of the dimmer and last until enough information has been gathered to correctly predict a next zero crossing of the AC line voltage that supplies the input of the dimmer. The dimming value is determined from the time interval between the predicted zero-crossing and a next turn-on time of the dimmer. The probing can be performed at intervals of an odd number of half-cycles of the AC line frequency so that a DC offset is not introduced within internal timing circuits of the dimmer. The AC line frequency can also be determined from a time interval between the predicted zero crossings.
1. A method of determining a dimming control value of a thyristor-switched dimmer, the method comprising:
predicting zero-crossings of a voltage of an AC power source that supplies power to the thyristor-switched dimmer;
probing the output of the thyristor-switched dimmer by periodically or intermittently applying a minimum conductance across the output of the thyristor-switched dimmer from a turn-on time of the thyristor-switched dimmer circuit at least until the predicting has gathered sufficient information to predict the next zero-cross time of the voltage of the AC power source;
detecting a leading edge of a waveform of an output voltage of the thyristor-switched dimmer circuit subsequent to the next zero-cross time; and
determining the dimming control value of the thyristor-switched dimmer from a time interval extending from the next zero-cross time of the voltage of the AC power source to the detected leading edge.
2. The method of claim 1 , wherein the probing applies the minimum conductance until the next zero-cross time.
3. The method of claim 1 , wherein the probing is performed at intervals corresponding to an odd number of half-cycles of the voltage of the AC power source.
4. The method of claim 1 , wherein the probing applies the minimum conductance to the output of the thyristor-switched dimmer circuit by controlling a power converter that transfers power from the AC power source to one or more lighting devices such that the power converter control circuit transfers energy continuously during the durations of the probing cycles.
5. The method of claim 1 , wherein the probing applies the minimum conductance to the output of the thyristor-switched dimmer circuit by controlling a controllable load impedance applied to the output of the thyristor-switched dimmer circuit.
6. 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 zero-crossing predicting circuit for predicting a next zero-cross time of a voltage of an AC power source that supplies power to the thyristor-switched dimmer circuit;
a probing control circuit that intermittently or periodically applies a minimum level of conductance to the output of the thyristor-switched dimmer circuit from a turn-on time of the thyristor-switched dimmer circuit at least until the zero-crossing predicting circuit has gathered sufficient information to predict the next zero-cross time of the voltage of the AC power source;
a detector for detecting a leading edge of a waveform of an output voltage of the thyristor-switched dimmer circuit subsequent to the next zero-cross time; and
a control circuit that determines the dimming control value of the thyristor-switched dimmer from a time interval extending from the next zero-cross time of the voltage of the AC power source to the detected leading edge.
7. The integrated circuit of claim 6 , wherein the probing control circuit applies the minimum level of conductance until the next zero-cross time.
8. The integrated circuit of claim 6 , wherein the probing control circuit applies the minimum level of conductance at intervals corresponding to an odd number of half-cycles of the voltage of the AC power source.
9. The integrated circuit of claim 6 , wherein the probing control circuit applies the minimum level of conductance level to the output of the thyristor-switched dimmer circuit by controlling a power converter control circuit that powers the one or more lighting devices such that the power converter control circuit transfers energy continuously during the durations of the probing cycles.
10. The integrated circuit of claim 6 , wherein the probing control circuit applies the minimum level of conductance to the output of the thyristor-switched dimmer circuit by controlling a controllable load impedance applied to the output of the thyristor-switched dimmer circuit.
11. A method of powering one or more lighting devices from an output of a thyristor-switched dimmer circuit, comprising:
converting power from the output of the thyristor-switched dimmer circuit to supply the one or more lighting devices;
predicting a next zero-cross time of a voltage of an AC power source that supplies power to the thyristor-switched dimmer circuit;
probing the output of the thyristor-switched dimmer by periodically or intermittently applying a minimum conductance across the output of the thyristor-switched dimmer during durations of probing cycles, wherein a rate of initiating the probing cycles is less than a half-cycle rate of an AC power source that supplies the thyristor-switched dimmer circuit, whereby some half-cycles of the AC power source are not probed, wherein the durations of the probing cycles extend from a turn-on time of the thyristor-switched dimmer circuit at least until the zero-crossing predicting circuit has gathered sufficient information to predict the next zero-cross time of the voltage of the AC power source.
12. The method of claim 11 , further comprising:
detecting a leading edge of a waveform of an output voltage of the thyristor-switched dimmer circuit subsequent to the next zero-cross time; and
determining a dimming control value of the thyristor-switched dimmer from a time interval extending from the next zero-cross time of the voltage of the AC power source to the detected leading edge.
13. The method of claim 11 , wherein the probing is performed at intervals corresponding to an odd number of half-cycles of the voltage of the AC power source.
14. The method of claim 11 , wherein the probing applies the minimum conductance to the output of the thyristor-switched dimmer circuit by controlling the converting to transfer energy continuously during the durations of the probing cycles.
15. The method of claim 11 , wherein the probing applies the minimum conductance to the output of the thyristor-switched dimmer circuit by controlling a controllable load impedance applied to the output of the thyristor-switched dimmer circuit.
16. 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 for controlling a power converter for supplying the one or more lighting devices from the output;
a zero-crossing predicting circuit for predicting a next zero-cross time of a voltage of an AC power source that supplies power to the thyristor-switched dimmer circuit;
a probing control circuit that intermittently or periodically applies a minimum conductance to the output of the thyristor-switched dimmer circuit during durations of probing cycles, wherein a rate of initiating the probing cycles is less than a half-cycle rate of an AC power source that supplies the thyristor-switched dimmer circuit, whereby some half-cycles of the AC power source are not probed, wherein the durations of the probing cycles extend from a turn-on time of the thyristor-switched dimmer circuit at least until the zero-crossing predicting circuit has gathered sufficient information to predict the next zero-cross time of the voltage of the AC power source.
17. The integrated circuit of claim 16 , further comprising:
a detector for detecting a leading edge of a waveform of an output voltage of the thyristor-switched dimmer circuit subsequent to the next zero-cross time; and
a control circuit that determines the dimming control value of the thyristor-switched dimmer from a time interval extending from the next zero-cross time of the voltage of the AC power source to the detected leading edge.
18. The integrated circuit of claim 16 , wherein the probing control circuit applies the minimum conductance at intervals corresponding to an odd number of half-cycles of the voltage of the AC power source.
19. The integrated circuit of claim 16 , wherein the probing control circuit applies the minimum conductance to the output of the thyristor-switched dimmer circuit by controlling the power converter control circuit such that the power converter control circuit transfers energy continuously during the durations of the probing cycles.
20. The integrated circuit of claim 16 , wherein the probing control circuit applies the minimum conductance to the output of the thyristor-switched dimmer circuit by controlling a controllable load impedance applied to the output of the thyristor-switched dimmer circuit.