Primary side PFC driver with dimming capability
A primary side PFC driver circuit is disclosed that includes a switch control circuit for commanding a switch to allow an inductor coupled to an output load (e.g., LEDs) to transfer energy provided by an input voltage source. The switch control circuit provides two signals for commanding the switch. A first signal having a first frequency, with a duty cycle in proportion to the input voltage amplitude, commands the switch to allow the average input current to be proportional to the input voltage amplitude. A second signal having a second frequency higher than the first frequency pulses the output load with substantially constant current pulses based on a value of the first signal (e.g., while the first signal is high). The current pulses produce a substantially constant current in the output load.
1. A single stage, primary side, Power Factor Correction (PFC) driver circuit, the circuit comprising:
a rectifier circuit configured to rectify an alternating input voltage;
an inductor coupled in series with the rectifier circuit;
a diode coupled in series with the inductor and an output load;
a switch coupled to the inductor;
a switch control circuit coupled to the switch and configured for:
generating a first signal for commanding the switch to allow the inductor to transfer energy provided by a rectified input voltage to the output load, the first signal having a first frequency with a duty cycle in proportion to an amplitude of the rectified input voltage; generating a second signal for commanding the switch, the second signal having a second frequency that is higher than the first frequency; and
commanding the switch with the second signal based on the first signal, the commanding causing a substantial constant pulse current to be provided to the output load.
2. The circuit of claim 1 , where the rectifier circuit is a diode bridge.
3. The circuit of claim 1 , where the diode is a Schotky diode.
4. The circuit of claim 1 , further comprising:
a capacitor coupled in parallel with the output load.
5. The circuit of claim 1 , where the output load includes one or more light emitting diodes.
6. The circuit of claim 1 , where the switch is a power transistor.
7. The circuit of claim 6 , where the command terminal is a gate of the power transistor.
8. The circuit of claim 1 , where the switch control circuit further comprises:
a logic circuit configured for receiving a first waveform and a second waveform and providing an output based on the first waveform and the second waveform;
a latch circuit having a first input coupled to the output of the logic circuit and an output coupled to a command terminal of the switch, the latch circuit configured for setting a command voltage on the command terminal based at least in part on the output of the logic circuit; and
a comparator having a first input coupled to the switch, a second input coupled to a reference voltage and an output coupled to the latch circuit, where the comparator is configured for resetting the latch circuit based on a comparison of the first and second comparator inputs.
9. The circuit of claim 8 , further comprising:
a sense resistor coupled to the switch device and the first input of the comparator.
10. The circuit of claim 8 , where the latch is a flip-flop.
11. A single stage, primary side, Power Factor Correction (PFC) driver circuit, the circuit comprising:
a rectifier circuit configured to rectify an alternating input voltage;
a diode coupled in parallel to the rectifier circuit and an output load;
an inductor coupled in series with the output load;
a switch coupled to the inductor;
a switch control circuit coupled to the switch and configured for:
generating a first signal for commanding the switch to allow the inductor to transfer energy provided by the rectified input voltage to the output load, the first signal having a first frequency with a duty cycle in proportion to an amplitude of the rectified input voltage; generating a second signal for commanding the switch, the second signal having a second frequency that is higher than the first frequency; and
commanding the switch with the second signal based on the first signal, the commanding causing a substantial constant pulse current to be provided to the output load.
12. The circuit of claim 11 , where the rectifier circuit is a diode bridge.
13. The circuit of claim 11 , where the switch is a power transistor.
14. The circuit of claim 11 , where the diode is a Schotky diode.
15. The circuit of claim 11 , further comprising:
a capacitor coupled in parallel with the output load.
16. The circuit of claim 11 , where the output load includes one or more light emitting diodes.
17. The circuit of claim 11 , where the switch control circuit further comprises:
a logic circuit configured for receiving a first waveform and a second waveform and providing an output based on the first waveform and the second waveform;
a latch circuit having a first input coupled to the output of the logic circuit and an output coupled to a command terminal of the switch, the latch circuit configured for setting a command voltage on the command terminal based at least in part on the output of the logic circuit; and
a comparator having a first input coupled to the switch, a second input coupled to a reference voltage and an output coupled to the latch circuit, where the comparator is configured for resetting the latch circuit based on a comparison of the first and second comparator inputs.
18. The circuit of claim 17 , further comprising:
a sense resistor coupled to the switch device and the first input of the comparator.
19. The circuit of claim 17 , where the latch is a flip-flop.
20. The circuit of claim 17 , where the command terminal is a gate of the power transistor.
21. A method of driving a light emitting diode (LED), comprising:
generating a first waveform for commanding a switch to allow an inductor coupled to the LED to transfer energy provided by an input voltage to the LED, the first waveform having a first frequency with a duty cycle in proportion to an amplitude of the input voltage;
generating a second waveform for commanding the switch, the second waveform having a second frequency that is higher than the first frequency, where a ratio of the input voltage to the duty cycle of the second waveform is scaled to allow dimming of LED brightness; and
commanding the switch with the second waveform based on the first waveform to provide a substantially constant current pulse to the LED.
22. A method of driving a light emitting diode (LED), comprising:
generating a first waveform for commanding a switch to allow an inductor coupled to the LED to transfer energy provided by an input voltage source to the LED, the first waveform having a first frequency with a duty cycle in proportion to an amplitude of the input voltage source;
generating a second waveform for commanding the switch, the second waveform having a second frequency that is higher than the first frequency, where the duty cycle of the second waveform is generated by:
detecting a zero crossing point of the input voltage source;
synthesizing a voltage for generating the second waveform using the detected zero crossing point; and
commanding the switch with the second waveform based on the first waveform to provide a substantially constant current pulse to the LED.
23. The method of claim 22 , where an amplitude of the synthesized voltage can be set based on a brightness requirement for the LED.
24. A method of driving a light emitting diode (LED), comprising:
generating a first waveform for commanding a switch to allow an inductor coupled to the LED to transfer energy provided by an input voltage to the LED, the first waveform having a first frequency with a duty cycle in proportion to an amplitude of the input voltage;
generating a second waveform for commanding the switch, the second waveform having a second frequency that is higher than the first frequency; and
commanding the switch with the second waveform based on the first waveform to provide a substantially constant current pulse to the LED,
where a ramp rate for current charging the inductor is phase-locked to the input voltage, and maximum and minimum ramp rates corresponding to peaks and valley, respectively, of the input voltage, are used to phase the first and second waveforms.