High-efficiency adaptive DC/AC converter

• US 20050030776A1
• Filed: 09/07/2004
• Published: 02/10/2005
• Est. Priority Date: 07/22/1999
• Status: Active Grant

• Current Assignee: O2 Micro International Limited (O2 Micro Incorporated)
• Original Assignee: O2 Micro International Limited (O2 Micro Incorporated)
• Inventors: Lin, Yung-Lin
• Granted Patent: US 7,417,382 B2
• Time in Patent Office: Days
• US Class Current: 363/98
• CPC Class Codes: H02M 1/0048 Circuits or arrangements for reducing losses using snubbers H02M1/34
• CPC Class Codes: H02M 7/4815 Resonant converters H02M7/4811 and H02M7/4826 take precedence
• CPC Class Codes: H02M 7/523 with LC-resonance circuit in the main circuit
• CPC Class Codes: H02M 7/5387 in a bridge configuration
• CPC Class Codes: H05B 41/2828 using control circuits for the switching elements H05B41/2827 takes precedence
• CPC Class Codes: H05B 41/2855 against abnormal lamp operating conditions
• CPC Class Codes: H05B 41/3927 by pulse width modulation H05B41/3922 takes precedence
• CPC Class Codes: Y02B 20/00 Energy efficient lighting technologies, e.g. halogen lamps or gas discharge lamps
• CPC Class Codes: Y02B 70/10 Technologies improving the efficiency by using switched-mode power supplies [SMPS], i.e. efficient power electronics conversion e.g. power factor correction or reduction of losses in power supplies or efficient standby modes

Abstract

A CCFL power converter circuit is provided using a high-efficiency zero-voltage-switching technique that eliminates switching losses associated with the power MOSFETs. An optimal sweeping-frequency technique is used in the CCFL ignition by accounting for the parasitic capacitance in the resonant tank circuit. Additionally, the circuit is self-learning and is adapted to determine the optimum operating frequency for the circuit with a given load. An over-voltage protection circuit can also be provided to ensure that the circuit components are protected in the case of open-lamp condition.

Claims

1. A DC to AC cold cathode fluorescent lamp inverter circuit comprising:

a plurality of switches for selective coupling to a voltage line;

a transformer coupled to said switches for increasing voltage;

a feedback signal line receiving a feedback signal indicative of a current through a cold cathode fluorescent lamp load; and

a feedback control circuit coupled to said feedback signal line and said switches for adjusting the power delivered to said load only if said current is above a predetermined threshold.

2. A circuit as claimed in claim 1, wherein said feedback control circuit reduces power to said load to a first power level when said current is below said predetermined threshold.

3. A circuit as claimed in claim 1, wherein said feedback control circuit reduces power to said load to zero when said current is below said predetermined threshold.

4. A circuit as claimed in claim 1, wherein said predetermined threshold indicates an open lamp condition at said load.

5. A DC to AC cold cathode fluorescent lamp inverter circuit comprising:

a plurality of switches for selective coupling to a voltage line;

a transformer coupled to said switches for increasing voltage;

a feedback signal line receiving a feedback signal indicative of a voltage across a cold cathode fluorescent lamp load; and

a feedback control circuit coupled to said feedback signal line and said switches for adjusting the power delivered to said load only if said voltage across said cold cathode fluorescent lamp load is below a predetermined threshold.

6. A circuit as claimed in claim 5, wherein said feedback control circuit reduces power to said load to a first power level when said voltage across said cold cathode fluorescent lamp load is above said predetermined threshold.

7. A circuit as claimed in claim 5, wherein said feedback control circuit reduces power to said load to zero when said voltage across said cold cathode fluorescent lamp load is above said predetermined threshold.

8. A circuit as claimed in claim 5, wherein said feedback control circuit reduces power to said load to zero when said voltage across said cold cathode fluorescent lamp load exceeds said predetermined threshold for a predetermined period of time

9. A circuit as claimed in claim 5, wherein said predetermined threshold indicates an open lamp condition at said load.

10. A liquid crystal display unit comprising:

a liquid crystal display panel;

a cold cathode fluorescent lamp for illuminating said liquid crystal display panel;

a plurality of switches for selective coupling to a voltage line;

a transformer coupled to said switches for increasing voltage;

a feedback signal line receiving a feedback signal indicative of a current through said cold cathode fluorescent lamp; and

a feedback control circuit coupled to said feedback signal line and said switches for adjusting the power delivered to said cold cathode fluorescent lamp only if said current is above a predetermined threshold.

11. A display unit as claimed in claim 10, wherein said feedback control circuit reduces power to said load to a first power level when said current is below said predetermined threshold.

12. A display unit as claimed in claim 10, wherein said feedback control circuit reduces power to said load to zero when said current is below said predetermined threshold.

13. A display unit as claimed in claim 10, wherein said predetermined threshold indicates an open lamp condition at said load.

14. A liquid crystal display unit comprising:

a liquid crystal display panel;

a cold cathode fluorescent lamp for illuminating said liquid crystal display panel;

a plurality of switches for selective coupling to a voltage line;

a transformer coupled to said switches for increasing voltage;

a feedback signal line receiving a feedback signal indicative of a voltage across said cold cathode fluorescent lamp; and

a feedback control circuit coupled to said feedback signal line and said switches for adjusting the power delivered to said cold cathode fluorescent lamp only if said voltage across said cold cathode fluorescent lamp is below a predetermined threshold.

15. A display unit as claimed in claim 14, wherein said feedback control circuit reduces power to said load to a first power level when said voltage across said cold cathode fluorescent lamp load is above said predetermined threshold.

16. A display unit as claimed in claim 14, wherein said feedback control circuit reduces power to said load to zero when said voltage across said cold cathode fluorescent lamp load is above said predetermined threshold.

17. A display unit as claimed in claim 14, wherein said feedback control circuit reduces power to said load to zero when said voltage across said cold cathode fluorescent lamp load exceeds said predetermined threshold for a predetermined period of time

18. A display unit as claimed in claim 14, wherein said predetermined threshold indicates an open lamp condition at said load.

19. A method for controlling a DC to AC cold cathode fluorescent lamp inverter circuit comprising:

providing a first pulse signal to a first transistor;

selectively coupling said first transistor to a voltage;

providing a second pulse signal to a second transistor;

selectively coupling said second transistor to said voltage;

receiving a feedback signal indicative of a current through a cold cathode fluorescent lamp load; and

adjusting the power delivered to said load only if said current is above a predetermined threshold.

20. A method as claimed in claim 19 further comprising:

reducing power to said load to a first power level when said current is below said predetermined threshold.

21. A method as claimed in claim 19 further comprising:

reducing power to said load to zero when said current is below said predetermined threshold.

22. A method as claimed in claim 19 further comprising:

detecting an open lamp condition at said load; and

reducing power delivered to said load after such open lamp condition.

23. A method for controlling a DC to AC cold cathode fluorescent lamp inverter circuit comprising:

providing a first pulse signal to a first transistor;

selectively coupling said first transistor to a voltage;

providing a second pulse signal to a second transistor;

selectively coupling said second transistor to said voltage;

receiving a feedback signal indicative of a voltage across a cold cathode fluorescent lamp load; and

adjusting the power delivered to said load only if said voltage across said cold cathode fluorescent lamp load is below a predetermined threshold.

24. A method as claimed in claim 23 further comprising:

reducing power to said load to a first power level when said voltage across said cold cathode fluorescent lamp load is above said predetermined threshold.

25. A method as claimed in claim 23 further comprising:

reducing power to said load to zero when said voltage across said cold cathode fluorescent lamp load is above said predetermined threshold.

26. A method as claimed in claim 23 further comprising:

detecting an open lamp condition at said load; and

reducing power delivered to said load after such open lamp condition.