Method and apparatus for detecting and correcting improper dimmer operation

1. A method of detecting and correcting improper operation of a lighting system including a solid state lighting load, the method comprising:

determining first and second values of a phase angle of a dimmer using a phase angle detection circuit, the dimmer being connected to a power converter driving the solid state lighting load, the first and second values corresponding to consecutive half cycles of an input mains voltage signal;

determining a difference between the first and second values; and

implementing a selected corrective action when the difference is greater than a difference threshold, indicating asymmetric waveforms of the input mains voltage signal.

2. The method of claim 1 , wherein the step of implementing the selected first corrective action comprises:

determining whether a corrective action is already active; and

implementing a highest priority corrective action as the selected corrective action when it is determined that no corrective action is already active.

3. The method of claim 2 , wherein the step of implementing the selected corrective action further comprises:

determining whether at least one other corrective action is available when it is determined that a corrective action is already active.

4. The method of claim 3 , wherein the step of implementing the selected corrective action further comprises:

implementing a next highest priority corrective action as the selected corrective action when it is determined that at least one other corrective action is available.

5. The method of claim 3 , further comprising:

shutting down the power converter when it is determined that at least one other corrective action is not available.

6. The method of claim 5 , further comprising:

determining third and fourth values of the phase angle of the dimmer, the third and fourth values corresponding to consecutive half cycles of the input mains voltage signal;

determining a difference between the third and fourth values; and

activating the power converter when it is determined that the difference between the third and fourth values is less than the difference threshold, indicating symmetric waveforms of the input mains voltage signal.

7. The method of claim 1 , wherein the step of determining the first and second values of the phase angle comprises:

sampling digital pulses corresponding to the waveforms of the input mains voltage signal; and

determining lengths of the sampled digital pulses, the lengths corresponding to a level of dimming of the dimmer.

8. The method of claim 1 , wherein the corrective action comprises switching in a resistive bleeder circuit in parallel with the solid state lighting load.

9. The method of claim 1 , wherein determining the difference between the first and second values comprises:

storing the first value as a previous half cycle level;

storing the second value as a current half cycle level; and

subtracting the stored current half cycle level and the previous half cycle level.

10. The method of claim 1 , wherein implementing the selected corrective action when the difference is greater than a difference threshold eliminates flicker of light output by the solid state lighting load.

11. A system for controlling power delivered to a solid state lighting load, the system comprising:

a dimmer connected to voltage mains and configured to adjustably dim light output by the solid state lighting load;

a power converter configured to drive the solid state light load in response to a rectified input voltage signal originating from the voltage mains; and

a phase angle detection circuit configured to detect a phase angle of the dimmer having consecutive half cycles of the input voltage signal, to determine a difference between respective values of the consecutive half cycles, and to implement a corrective action when the difference is greater than a difference threshold, indicating asymmetric waveforms of the input voltage signal.

12. The system of claim 11 , wherein the power converter operates in an open loop or feed-forward fashion.

13. The system of claim 11 , wherein the phase angle detection circuit detects the phase angle by sampling digital pulses corresponding to waveforms of the input voltage signal and measuring the consecutive half cycles based on lengths of the sampled digital pulses.

14. The system of claim 13 , wherein the phase angle detection circuit determines the difference between the respective values of the consecutive half cycles by subtracting the lengths of the sampled digital pulses corresponding to the consecutive half cycles, respectively.

15. The system of claim 11 , wherein the phase angle detection circuit comprises:

a processor having a digital input;

a first diode connected between the digital input and a voltage source;

a second diode connected between the digital input and ground;

a first capacitor connected between the digital input and a detection node;

a second capacitor connected between the detection node and ground; and

a resistance connected between the detection node and a rectified voltage node, which receives the rectified input voltage,

wherein the processor is configured to sample the digital pulses corresponding to waveforms of the input voltage signal at the digital input and to measure the respective values of the consecutive half cycles based on the lengths of the sampled digital pulses.

16. The system of claim 11 , wherein the phase angle detection circuit is further configured to select the corrective action having a highest priority.

17. The system of claim 16 , wherein the phase angle detection circuit is further configured to shut down the power converter when the selected corrective action is implemented, but the difference between the respective values of the consecutive half cycles continues to be greater than the difference threshold.

18. A method implemented by a phase angle detection circuit for eliminating flicker from light output by a light emitting diode (LED) light source driven by a power converter in response to a phase chopping dimmer, the method comprising:

detecting a dimmer phase angle by measuring half cycles of an input voltage signal;

comparing consecutive half cycles to determine a half cycle difference;

comparing the half cycle difference with a predetermined difference threshold, wherein the half cycle difference being less than the difference threshold indicates that waveforms of the input voltage signal are symmetric, and wherein the half cycle difference being greater than the difference threshold indicates that the waveforms of the input voltage signal are asymmetric; and

implementing a corrective action when the half cycle difference is greater than the difference threshold.

19. The method of claim 18 , further comprising:

comparing the half cycle difference with the predetermined difference threshold after implementing the corrective action; and

implementing another corrective action when the half cycle difference is greater than the difference threshold and another corrective action is available for implementation.

20. The method of claim 19 , further comprising:

shutting down the power converter when the half cycle difference is greater than the difference threshold and another corrective action is not available for implementation.