Self-oscillating high frequency converter with power factor correction
A self-oscillating converter includes a power transistor coupled to a primary winding for controlling current flow in the primary winding, and a turn-on circuit configured to turn on the power transistor for maintaining oscillation in the self-oscillating converter. The self-oscillating converter also includes a turn-off circuit configured to turn off the power transistor to maintain an on-time of the power transistor at a pre-set value for power factor correction, and modulate the on-time of the power transistor to regulate the output current in the load device.
1. A self-oscillating converter with power factor correction and output current regulation, the self-oscillating converter comprising:
a primary winding for coupling to an input voltage and providing a regulated output voltage to a load device;
a power transistor coupled to the primary winding for controlling current flow in the primary winding;
a first auxiliary winding coupled to the primary winding for providing a turn-on signal to the power transistor for maintaining oscillation in the self-oscillating converter;
a second auxiliary winding coupled to the primary winding for providing a turn-off signal to the power transistor to maintain an on-time of the power transistor at a pre-set value for power factor correction; and
a sensing circuit for sensing an output current in the load device and providing a modulating signal to modulate the on-time of the power transistor to regulate the output current in the load device.
2. The self-oscillating converter of claim 1 , further comprising a turn-off circuit coupled between the second auxiliary winding and the power transistor and configured to provide the turn-off signal, wherein the turn-off circuit comprises a ramp circuit coupled to the second auxiliary winding for providing the turn-off signal to the power transistor to maintain the on-time of the power transistor at the pre-set value.
3. The self-oscillating converter of claim 2 , wherein the ramp circuit in the turn-off circuit comprises a first resistor and a charging capacitor coupled in series to the second auxiliary winding to provide a first charging current to maintain the on-time of the power transistor at the pre-set value.
4. The self-oscillating converter of claim 3 , wherein the modulating signal from the sensing circuit activates a second charging current to the ramp circuit to modulate the on-time of the power transistor to regulate the output current in the load device.
5. The self-oscillating converter of claim 1 , further comprising a turn-on circuit coupled between the first auxiliary winding and the power transistor and configured to provide the turn-on signal, wherein the turn-on circuit comprises a first branch and a second branch connected in series, the first branch including a first resistor and a capacitor connected in series, and the second branch including a diode and a second resistor connected in series.
6. The self-oscillating converter of claim 1 , wherein the self-oscillating converter is a non-isolated Buck converter.
7. The self-oscillating converter of claim 1 , wherein the self-oscillating converter is a non-isolated Buck-Boost converter.
8. The self-oscillating converter of claim 1 , wherein the self-oscillating converter is an isolated switch mode power converter.
9. The self-oscillating converter of claim 1 , wherein the sensing circuit comprises an error amplifier characterized by a bandwidth substantially lower than a frequency of an AC input voltage.
10. The self-oscillating converter of claim 9 , wherein the AC input voltage is a 60 Hz signal, and the bandwidth of the error amplifier is approximately 2 to 3 Hz.
11. The self-oscillating converter of claim 1 , wherein the load device comprises a light-emitting diode (LED) device.
12. The self-oscillating converter of claim 1 , further comprising an over-current protection circuit, wherein the over-current protection circuit is coupled between a gate node of the power transistor and a ground node.
13. The self-oscillating converter of claim 1 , wherein the power transistor comprises a JFET (Junction Field Effect Transistor).
14. The self-oscillating converter of claim 1 , wherein the power transistor comprises a gallium nitride (GaN) transistor.
15. A method for driving a load device with a self-oscillating converter, comprising:
coupling a primary winding of the self-oscillating converter to receive an AC input voltage and providing an output voltage to the load device;
controlling current flow in the primary winding using a power transistor coupled to the primary winding;
providing a turn-on signal to the power transistor for maintaining oscillation in the self-oscillating converter, using a first auxiliary winding coupled to the primary winding;
providing a turn-off signal to the power transistor to maintain an on-time of the power transistor at a pre-set value for power factor correction, using a second auxiliary winding coupled to the primary winding; and
sensing a current in the load device and providing a modulating signal to modulate the on-time of the power transistor to regulate an output current in the load device, using a sensing circuit.
16. The method of claim 15 , further comprising providing the turn-off signal using a turn-off circuit coupled between the second auxiliary winding and the power transistor, wherein the turn-off circuit comprises a ramp circuit coupled to the second auxiliary winding for providing the turn-off signal to the power transistor to maintain the on-time of the power transistor at the pre-set value.
17. The method of claim 16 , further comprising forming the ramp circuit in the turn-off circuit with a first resistor and a charging capacitor coupled in series to the second auxiliary winding to provide a first charging current to maintain the on-time of the power transistor at the pre-set value.
18. The method of claim 17 , further comprising using the modulating signal from the sensing circuit to activate a second charging current to the ramp circuit to modulate the on-time of the power transistor to regulate the output current in the load device.
19. The method of claim 15 , further comprising providing the turn-on signal using a turn-on circuit coupled between the first auxiliary winding and the power transistor and configured to provide the turn-on signal, wherein the turn-on circuit comprises a first branch and a second branch connected in series, the first branch including a first resistor and a capacitor connected in series, and the second branch including a diode and a second resistor connected in series.
20. The method of claim 15 , wherein the sensing circuit comprises an error amplifier characterized by a bandwidth substantially lower than a frequency of the AC input voltage.