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 to the rectifier circuit;
a diode coupled to 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 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, resulting in an average input current being proportional to the amplitude of the rectified input voltage while the pulse current is substantially constant when 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 , wherein the circuit implements a floating buck-boost or a floating buck topology.
4. The circuit of claim 1 , where the diode is a Schotky diode.
5. The circuit of claim 1 , further comprising:
a capacitor coupled in parallel with the output load.
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. The circuit of claim 1 , where the output load includes one or more light emitting diodes.
12. A method of driving an output load, comprising:
generating a first signal for commanding a switch to allow an inductor coupled to the output load to transfer energy provided by an input voltage source to an output load, the first signal having a first frequency with a duty cycle in proportion to an amplitude of the input voltage source;
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 to provide a substantially constant current pulse to the output load.
13. The method of claim 12 , wherein the output load includes one or more light emitting diodes.
14. The method of claim 12 , where the first signal is a Pulse Width Modulation (PWM) signal.
15. The method of claim 14 , further comprising:
using the switch and PWM signal to regulate peak or average current flowing in the output load.