High-efficiency adaptive DC/AC converter

• US 6,804,129 B2
• Filed: 02/11/2004
• Issued: 10/12/2004
• Est. Priority Date: 07/22/1999
• Status: Expired due to Fees

• Current Assignee: O2 Micro International Limited (O2 Micro Incorporated)
• Original Assignee: O2 Micro International Limited (O2 Micro Incorporated)
• Inventors: Lin, Yung-Lin
• Primary Examiner(s): Patel, Rajnikant B.
• Application Number:
  US10/776,417
• Publication Number:
  US 20040160794A1
• Time in Patent Office: 244 Days
• Field of Search: 363/98, 363/25, 363/96, 363/97, 363/31, 363/40, 363/41, 363/95, 315/160, 315/127, 315/128, 315/175, 315/225, 315/307, 315/119
• 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 having a primary side and a secondary side, said primary side for selective coupling to said voltage line in an alternating fashion through said switches;

a feedback signal line receiving a feedback signal indicative of an electrical condition at 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 electrical condition is outside a predetermined range.

2. A circuit as claimed in claim 1, wherein said electrical condition is the current through said load.

3. A circuit as claimed in claim 2, wherein said predetermined range is zero amps to a first predetermined current value.

4. A circuit as claimed in claim 1, wherein said electrical condition is the power through said load.

5. A circuit as claimed in claim 4, wherein said predetermined range is zero watts to a first predetermined power value.

6. A circuit as claimed in claim 1, wherein said electrical condition is the voltage across said load.

7. A circuit as claimed in claim 6, wherein said predetermined range is zero volts to a first predetermined voltage value.

8. A circuit as claimed in claim 1, wherein said electrical condition is the impedance of said load.

9. A circuit as claimed in claim 8, wherein said predetermined range is an impedance greater than 130 kiloohm.

10. A circuit as claimed in claim 1, wherein said feedback control circuit reduces power to said load to a first power level when said electrical condition is outside a predetermined range.

11. A circuit as claimed in claim 1, wherein said feedback control circuit reduces power to said load to zero when said electrical condition is outside a predetermined range.

12. A circuit as claimed in claim 1, wherein said feedback control circuit reduces current to said load to a first current level when said electrical condition is outside a predetermined range.

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

14. A circuit as claimed in claim 1, wherein said predetermined range indicates a short circuit condition at said load.

17. A circuit as claimed in claim 1, wherein said electrical condition is the voltage across said load.

15. A DC to AC cold cathode fluorescent lamp inverter controller integrated circuit comprising:

a plurality of outputs for controlling a plurality of switches to selectively couple said switches to a voltage line;

a feedback signal input receiving a feedback signal indicative of an electrical condition at a cold cathode fluorescent lamp load; and

a feedback control circuit coupled to said feedback signal input and said outputs for adjusting the power delivered to said load only if said electrical condition is outside a predetermined range.

16. A circuit as claimed in claim 15, wherein said electrical condition is the current through said load.

18. A circuit as claimed in claim 15, wherein said electrical condition is the impedance of said load.

19. A circuit as claimed in claim 15, wherein said feedback control circuit reduces power to said load to a first power level when said electrical condition is outside a predetermined range.

20. A circuit as claimed in claim 15, wherein said feedback control circuit reduces current to said load to a first current level when said electrical condition is outside a predetermined range.

21. A circuit as claimed in claim 15, wherein said predetermined range indicates an open lamp condition at said load.

22. A circuit as claimed in claim 15, wherein said predetermined range indicates a short circuit condition at said load.

23. A liquid crystal display unit comprising:

a liquid crystal screen;

a cold cathode fluorescent lamp for illuminating said liquid crystal screen;

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

a transformer having a primary side and a secondary side, said primary side for selective coupling to said voltage line in an alternating fashion through said switches and said secondary side coupled to said cold cathode fluorescent lamp;

a feedback signal line receiving a feedback signal indicative of an electrical condition at 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 electrical condition is outside a predetermined range.

24. The liquid crystal display unit as claimed in claim 23 further comprising:

a graphics controller coupled to said liquid crystal screen for controlling images on said liquid crystal screen;

a microprocessor coupled to said graphics controller for controlling said graphics controller;

random access memory coupled to said microprocessor for storing instructions from said microprocessor; and

a hard disk drive coupled to said microprocessor for storing data from said microprocessor.

25. 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 an electrical condition at a cold cathode fluorescent lamp load; and

adjusting the power delivered to said load only if said electrical condition is outside a predetermined range.

26. A method as claimed in claim 25, wherein said electrical condition is the current through said load.

27. A method as claimed in claim 25, wherein said electrical condition is the voltage across said load.

28. A method as claimed in claim 25, wherein said feedback control circuit reduces power to said load to a first power level when said electrical condition is outside a predetermined range.

29. A method as claimed in claim 25, wherein said feedback control circuit reduces current to said load to a first current level when said electrical condition is outside a predetermined range.

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

detecting an open lamp condition at said load; and

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