Solid state lighting apparatus including separately driven LED strings and methods of operating the same

1. A solid state lighting apparatus comprising:

a first string of Light Emitting Diodes (LEDs) configured to operate in response to a rectified ac voltage having a cycle including a null time interval when the first string is off; and

a second string of LEDs, separate from the first string of LEDs, configured to emit light during a portion of the null time interval, wherein the null time interval comprises a variable contiguous combination of a phase cut dimming time interval of the cycle and an off time portion of the cycle when a level of the rectified ac voltage is insufficient to forward bias any LED included in the first string of LEDs, the variable contiguous combination being responsive to a phase cut dimmer input.

2. The apparatus of claim 1 further comprising:

a first driver circuit configured to provide a first current to the first string of LEDs that changes with a phase of the rectified ac voltage; and

a second driver circuit configured to provide a second current to the second string of LEDs that is substantially constant as the phase of the rectified ac voltage changes.

3. The apparatus of claim 2 wherein the second driver circuit is further configured to provide the second current outside the null time interval.

4. The apparatus of claim 1 further comprising:

a first driver circuit configured to bias the first string of LEDs so that all of the LEDs in the first string of LEDs remain off during the null time interval and configured to forward bias the LEDs in the first string of LEDs according to a sequence outside the null time interval.

5. The apparatus of claim 4 further comprising:

a second driver circuit configured to bias the second string of LEDs so that all of the LEDs in the second string of LEDs remain on during all of the null time interval.

6. The apparatus of claim 4 further comprising:

a second driver circuit configured to bias the second string of LEDs so that at least one of the LEDs in the second string of LEDs is forward biased during all of the null time interval.

7. The apparatus of claim 4 further comprising:

a second driver circuit includes a DC/DC converter circuit configured to bias the second string of LEDs to emit a substantially constant level of light over the cycle of the rectified ac voltage.

8. The apparatus of claim 7 wherein the DC/DC converter circuit comprises a boost circuit coupled to the rectified ac voltage and configured to provide a substantially constant dc current to the second string of LEDs over the cycle of the rectified ac voltage.

9. The apparatus of claim 7 wherein the DC/DC converter circuit comprises a switched mode power supply circuit coupled to the rectified ac voltage and configured to provide a constant dc current to the second string of LEDs over the cycle of the rectified ac voltage.

10. The apparatus of claim 7 wherein the DC/DC converter circuit includes a switch configured to control power delivery to the second string of LEDs by modifying a duty cycle of a pulse width modulation signal provided to the switch, to reduce the constant dc current.

11. The apparatus of claim 1 wherein the first string of LEDs and the second string of LEDs include identical colors of LEDs and the first string of LEDs is configured to emit about a first percent of a total lumen output of the apparatus and the second string of LEDs is configured to emit about a second percent of the total lumen output, wherein the first and second percents are configured to maintain the total lumen output on a black body radiator curve at a respective color temperature.

12. The apparatus of claim 1 wherein the first string of LEDs includes blue-shifted-yellow LEDs configured to output about 80 percent of a total lumen output of the apparatus and the second string of LEDs includes red LEDs configured to output about 20 percent of the total lumen output.

13. The apparatus of claim 1 wherein the second string of LEDs is configured to emit light outside the null time interval when the first string of LEDs is on.

14. A method of operating a solid state lighting apparatus, the method comprising:

driving a first string of Light Emitting Diodes (LEDs) in response to a rectified ac voltage having a cycle including a null time interval when the first string is off; and

driving a second string of LEDs, separate from the first string of LEDs, to emit light during at least a portion of the null time interval, wherein the null time interval comprises a variable contiguous combination of a phase cut dimming time interval of the cycle and an off time portion of the cycle when a level of the rectified ac voltage is insufficient to forward bias any LED included in the first string of LEDs, the variable contiguous combination being responsive to a phase cut dimmer input.

15. The method of claim 14 further comprising:

providing a first current to the first string of LEDs that changes with a phase of the rectified ac voltage; and

providing a second current to the second string of LEDs that is substantially constant as the phase of the rectified ac voltage changes.

16. The method of claim 15 wherein the second driver circuit is further configured to provide the second current outside the null time interval.

17. The method of claim 14 wherein driving the first string of LEDs comprises biasing the first string of LEDs so that all of the LEDs in the first string of LEDs remain off during the null time interval and to forward bias the LEDs in the first string of LEDs according to a sequence outside the null time interval.

18. The method of claim 17 wherein driving the second string of LEDs comprises

biasing the second string of LEDs so that all of the LEDs in the second string of LEDs remain on during all of the null time interval.

19. The method of claim 17 wherein driving the second string of LEDs comprises biasing the second string of LEDs so that at least one of the LEDs in the second string of LEDs is forward biased during all of the null time interval.

20. The method of claim 17 wherein driving the second string of LEDs comprises biasing the second string of LEDs to emit a substantially constant level of the light over the cycle of the rectified ac voltage.

21. The method of claim 20 wherein driving the second string of LEDs comprises providing a substantially constant dc current to the second string of LEDs over the cycle of the rectified ac voltage.

22. The method of claim 21 further comprising:

modifying driving the second string of LEDs to less than a level of the substantially constant dc current as a level of dimming is reduced.

23. The method of claim 22 wherein modifying driving of the second string of LEDs comprises modifying a current provided to the second string of LEDs based on an RMS voltage determined over the cycle responsive to phase cut dimming of the apparatus.

24. The method of claim 22 wherein modifying driving of the second string of LEDs comprises modifying a current provided to the second string of LEDs based on the phase cut dimming time interval during the cycle.

25. The method of claim 14 wherein the first string of LEDs and the second string of LEDs include identical colors of LEDs,

wherein driving the first string of LEDs comprises driving the first string of LEDs to emit about a first percent of a total lumen output of the apparatus; and

wherein driving the second string of LEDs comprises driving the second string of LEDs to emit about a second percent of the total lumen output, wherein the first and second percents are configured to maintain the total lumen output on a black body radiator curve at a respective color temperature.

26. The method of claim 14 wherein the first string of LEDs includes blue-shifted-yellow LEDs configured to output about 80 percent of a total lumen output of the apparatus and the second string of LEDs includes red LEDs configured to output about 20 percent of the total lumen output.