Solid state lighting circuit and control method
A lighting circuit makes use of a switch mode power converter to drive a solid state lighting arrangement having a lighting unit and a lighting control switch. A pulse width modulation control signal is applied to the lighting control switch for dimming control with an activation pulse during which the lighting unit is activated. The power converter delivers a triangular current waveform and the activation pulse is delayed until a predetermined point within the triangular current waveform. This ensures that the PWM pulse during which the lighting arrangement is turned on has timing which is synchronized with the timing of the switch mode power converter. The prevents beat frequencies so that visible flicker is avoided.
1. A lighting circuit, comprising:
a switch mode power converter having a main control switch;
a solid state lighting arrangement comprising a lighting unit and a lighting control switch, at the output of the switch mode power converter;
a pulse width modulation controller for providing a pulse width modulation control signal to the lighting control switch for controlling dimming of the solid state lighting arrangement, the pulse width modulation control signal having a predetermined frequency and period and comprising pulses during which the lighting unit is activated; and
a power converter controller for controlling the main control switch independently of the pulse width modulation control signal,
a sync circuit for generating a synchronization signal (SYNC) based on a voltage over the main control switch,
wherein the switch mode power converter is adapted to deliver a triangular current waveform which cycles between upper and lower threshold currents,
and wherein the pulse width modulation controller is adapted to delay the start of each of the pulses until a predetermined point within the triangular current waveform, wherein the synchronization signal is used to identify the predetermined point within the triangular current waveform.
2. The lighting circuit as claimed in claim 1 , wherein the each of the pulses are at the start of the pulse width modulation control signal, and the pulse width modulation controller is adapted to delay the start of the pulse until said predetermined point within the triangular current waveform.
3. The lighting circuit as claimed in claim 1 , wherein the pulse width modulation controller is adapted to delay the start of the pulse until the triangular current waveform reaches a next upper or lower threshold current.
4. The lighting circuit as claimed in claim 3 , wherein the pulse width modulation controller is adapted to delay the start of the pulse until the triangular current waveform reaches a next upper threshold current.
5. The lighting circuit as claimed in claim 1 , wherein the power converter controller is adapted to switch the main control switch at the start of the pulse width modulation control signal.
6. The lighting circuit as claimed in claim 1 , wherein the power converter controller is arranged to operate at a frequency of at least 100 kHz and the predetermined frequency of the pulse width modulation controller is at most 25 kHz.
7. The lighting circuit as claimed in claim 1 , wherein the solid state lighting arrangement comprising a plurality of lighting units in series, each with a respective lighting control switch mounted in parallel on each lighting unit.
8. The lighting circuit as claimed in claim 7 , wherein each lighting unit has a different output color or color temperature.
9. The lighting circuit as claimed in claim 1 , wherein the switch mode power converter comprises a buck converter.
10. A method of controlling a lighting circuit, comprising:
operating a switch mode power converter having a main control switch to deliver power to a solid state lighting arrangement which comprises a lighting unit and a parallel lighting control switch, wherein the switch mode power converter delivers a triangular current waveform which cycles between upper and lower threshold currents;
generating a synchronization signal (SYNC) based on a voltage over the main control switch;
operating a pulse width modulation controller for providing a pulse width modulation control signal to the lighting control switch for controlling dimming of the solid state lighting arrangement, the pulse width modulation control signal having a predetermined frequency and period and comprising pulses during which the lighting unit is activated; and
wherein the switch mode power converter is controlled independently of the pulse width modulation control signal and wherein the method further comprises:
delaying the start of each of the pulses until a predetermined point within the triangular current waveform, wherein the synchronization signal is used to identify the predetermined point within the triangular current waveform.
11. The method as claimed in claim 10 , wherein the pulse is nominally at the start of the pulse width modulation control signal, and the method comprises delaying the start of the pulse until said predetermined point within the triangular current waveform.
12. The method as claimed in claim 10 , comprising delaying the start of the pulse until the triangular current waveform reaches a next upper or lower threshold current.
13. The method as claimed in claim 12 , comprising delaying the start of the pulse until the triangular current waveform reaches a next upper threshold current.
14. The method as claimed in claim 10 comprising switching the main control switch of the switch mode power converter at the start of the pulse width modulation control signal.
15. The method as claimed in claim 10 , comprising operating the power converter controller at a frequency of at least 100 kHz and operating the pulse width modulation controller at the predetermined frequency of at most 25 kHz.