Electronic ballast with high power factor
This invention provides an electronic ballast for a fluorescent lamp, including a rectifier for converting an alternating current input voltage into a direct current output voltage, and a circuit including a combined power factor correction (PFC) stage and an inverter, wherein the PFC stage and the inverter share a switch. Also provided is a controller for an electronic ballast, including a duty ratio controller that controls a duty ratio of a switch of the ballast, and means for adjusting the duty ratio according to a nonlinear function, so that an arc is sustained across the lamp. The controller and the ballast allow dimming of the fluorescent lamp while maintaining a high power factor.
1. An electronic ballast circuit for a fluorescent lamp, comprising:
a rectifier for converting an alternating current input voltage into a direct current output voltage; and
a circuit including a combined power factor correction (PFC) stage and an inverter;
wherein the PFC stage and the inverter share a switch;
wherein the electronic ballast circuit operates with only one switch.
2. The electronic ballast circuit of claim 1 , wherein the PFC stage includes a single ended primary inductor converter (SEPIC).
3. The electronic ballast circuit of claim 1 , wherein the inverter is a resonant inverter.
4. The electronic ballast circuit of claim 1 , wherein the shared switch is the only switch.
5. The electronic ballast circuit of claim 1 , further comprising an EMI filter.
6. The electronic ballast circuit of claim 1 , wherein the fluorescent lamp is a compact fluorescent lamp (CFL).
7. The electronic ballast circuit of claim 1 , wherein the inverter is a current fed inverter.
8. The electronic ballast circuit of claim 7 , wherein the current fed inverter includes a C-L-L resonant circuit.
9. A fluorescent lamp comprising the electronic ballast circuit of claim 1 .
10. The fluorescent lamp of claim 9 , wherein the fluorescent lamp is a CFL.
11. The electronic ballast circuit of claim 1 , including means for operating the shared switch at a selected duty ratio.
12. The electronic ballast circuit of claim 11 , wherein the duty ratio is variable.
13. The electronic ballast circuit of claim 12 , wherein the duty ratio is variable in accordance with a line voltage applied to the electronic ballast circuit.
14. A method for operating a fluorescent lamp, comprising:
converting an alternating current input voltage into a direct current output voltage;
connecting the direct current output voltage to a circuit including a combined power factor correction (PFC) stage and an inverter; and
connecting an output of the circuit to the fluorescent lamp;
wherein the PFC stage and the inverter share a switch;
wherein operating the fluorescent lamp uses only one switch.
15. The method of claim 14 , including using a SEPIC for the PFC stage.
16. The method of claim 14 , including using a resonant inverter for the inverter.
17. The method of claim 14 , wherein the shared switch is the only switch.
18. The method of claim 14 , including filtering the input voltage with an EMI filter.
19. The method of claim 14 , wherein the fluorescent lamp is a CFL.
20. The method of claim 14 , including using a current fed inverter for the inverter.
21. The method of claim 20 , wherein the current fed inverter includes a C-L-L resonant circuit.
22. The method of claim 14 , including operating the shared switch at a selected duty ratio.
23. The method of claim 22 , including varying the duty ratio.
24. The method of claim 23 , including varying the duty ratio in accordance with the input voltage.
25. The method of claim 22 , including adjusting the duty ratio according to a nonlinear function, so that an arc is sustained across the lamp during changes in supply voltage.