DC/AC cold cathode fluorescent lamp inverter

• US 7,881,084 B2
• Filed: 06/10/2008
• Issued: 02/01/2011
• 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:
  US12/136,597
• Publication Number:
  US 20080246413A1
• Time in Patent Office: 966 Days
• Field of Search: 363 16- 20, 363/25, 363/31, 363 39- 41, 363/91, 363 95- 98, 363/131, 363/132, 315127-128, 315/160, 315/194, 315/209, 315/219, 315/225, 315/224, 315/247, 315/86, 315/277, 315/291, 315/307, 315/309, 307/45, 307/46, 307/553, 307/359
• 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 DC/AC cold cathode fluorescent lamp (CCFL) inverter circuit includes a transformer with a primary winding and a secondary winding for providing increased voltage to a CCFL, a first and second MOSFET switches for selectively allowing direct current of a first polarity and a second polarity to flow through the transformer respectively. The primary and secondary windings of the transformer are electrically coupled to ground. A capacitor divider is electrically coupled to the CCFL for providing a first voltage signal representing a voltage across the CCFL. A first feedback signal line receives the first voltage signal. A timer circuit is coupled to the first feedback signal line for providing a time-out sequence of a predetermined duration when the first voltage signal exceeds a predetermined threshold. A protection circuit shuts down the first switch and the second switch when the first voltage signal exceeds the predetermined threshold after the predetermined duration.

Claims

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

a step-up transformer with a primary winding and a secondary winding for providing increased voltage to a cold cathode fluorescent lamp, wherein said primary winding and said secondary winding are electrically coupled to ground;

a first MOSFET switch coupled to said step-up transformer for selectively allowing direct current of a first polarity to flow through said step-up transformer;

a second MOSFET switch coupled to said step-up transformer for selectively allowing direct current of a second polarity to flow through said step-up transformer;

a capacitor divider electrically coupled to said cold cathode fluorescent lamp for providing a first voltage signal representing a voltage across said cold cathode fluorescent lamp;

a first feedback signal line coupled to said capacitor divider for receiving said first voltage signal from said capacitor divider representing said voltage across said cold cathode fluorescent lamp;

a timer circuit coupled to said first feedback signal line for providing a time-out sequence of a predetermined duration when said first voltage signal exceeds a predetermined threshold; and

a protection circuit coupled to said timer circuit, said first switch and said second switch for shutting down said first switch and said second switch when said first voltage signal exceeds said predetermined threshold after said predetermined duration.

2. A DC to AC cold cathode fluorescent lamp inverter circuit as claimed in claim 1, wherein said predetermined duration is sufficient for ignition of said cold cathode fluorescent lamp when properly operating.

3. A DC to AC cold cathode fluorescent lamp inverter circuit as claimed in claim 1, wherein said predetermined threshold represents a value of said voltage across said cold cathode fluorescent lamp greater than a minimum striking voltage of said cold cathode fluorescent lamp and less than a rated voltage of said step-up transformer.

4. A DC to AC cold cathode fluorescent lamp inverter circuit as claimed in claim 1, further comprising:

a sense resistor electrically coupled to said cold cathode fluorescent lamp and electrically coupled to ground for providing a second voltage signal representing current through said cold cathode fluorescent lamp;

a second feedback signal line coupled to said sense resistor for receiving said second voltage signal from said sense resistor representing said current through said cold cathode fluorescent lamp; and

a feedback control circuit coupled to said second feedback signal line for adjusting power to said cold cathode fluorescent lamp to a power level such that said second voltage signal approaches a reference value representing desired load conditions of said cold cathode fluorescent lamp.

5. A DC to AC cold cathode fluorescent lamp inverter circuit as claimed in claim 4, wherein said reference value has a minimum representing minimum current through said cold cathode fluorescent lamp and a maximum representing maximum current through said cold cathode fluorescent lamp.

6. A DC to AC cold cathode fluorescent lamp inverter circuit as claimed in claim 1, further comprising:

an ignition circuit coupled to said first switch and said second switch for providing lower amount of power to said cold cathode fluorescent lamp prior to ignition of said cold cathode fluorescent lamp.

7. A DC to AC cold cathode fluorescent lamp inverter circuit as claimed in claim 1, further comprising:

a third MOSFET switch coupled to said first MOSFET switch and said step-up transformer for providing a first electrical path through said primary winding of said step-up transformer to ground when said third MOSFET switch and said first MOSFET switch are simultaneously on;

a fourth MOSFET switch coupled to said MOSFET second switch and said step-up transformer for providing a second electrical path through said primary winding of said step-up transformer to ground when said fourth MOSFET switch and said second MOSFET switch are simultaneously on;

a sense resistor electrically coupled to said cold cathode fluorescent lamp and electrically coupled to ground for providing a second voltage signal representing current through said cold cathode fluorescent lamp;

a second feedback signal line coupled to said sense resistor for receiving said second voltage signal from said sense resistor representing said current through said cold cathode fluorescent lamp; and

a feedback control circuit coupled to said second feedback signal line, said first switch and said third switch for adjusting time when said third switch and said first switch are simultaneously on such that said second voltage signal approaches a reference value representing desired load conditions of said cold cathode fluorescent lamp.

8. A liquid crystal display unit comprising:

a liquid crystal display panel;

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

a step-up transformer with a primary winding and a secondary winding coupled to said cold cathode fluorescent lamp for providing increased voltage to said cold cathode fluorescent lamp, wherein said primary winding and said secondary winding are electrically coupled to ground;

a first MOSFET switch coupled to said step-up transformer for selectively allowing direct current of a first polarity to flow through said step-up transformer;

a second MOSFET switch coupled to said step-up transformer for selectively allowing direct current of a second polarity to flow through said step-up transformer;

a capacitor divider electrically coupled to said cold cathode fluorescent lamp for providing a first voltage signal representing a voltage across said cold cathode fluorescent lamp;

a first feedback signal line coupled to said capacitor divider for receiving said first voltage signal from said capacitor divider representing said voltage across said cold cathode fluorescent lamp;

a timer circuit coupled to said first feedback signal line for providing a time-out sequence of a predetermined duration when said first voltage signal exceeds a predetermined threshold; and

a protection circuit coupled to said timer circuit, said first switch and said second switch for shutting down said first switch and said second switch when said first voltage signal exceeds said predetermined threshold after said predetermined duration.

9. A liquid crystal display unit as claimed in claim 8, wherein said predetermined duration is sufficient for ignition of said cold cathode fluorescent lamp when properly operating.

10. A liquid crystal display unit as claimed in claim 8, wherein said predetermined threshold represents a value of said voltage across said cold cathode fluorescent lamp greater than a minimum striking voltage of said cold cathode fluorescent lamp and less than a rated voltage of said step-up transformer.

11. A liquid crystal display unit as claimed in claim 8, further comprising:

a sense resistor electrically coupled to said cold cathode fluorescent lamp and electrically coupled to ground for providing a second voltage signal representing current through said cold cathode fluorescent lamp;

a second feedback signal line coupled to said sense resistor for receiving said second voltage signal from said sense resistor representing said current through said cold cathode fluorescent lamp;

a feedback control circuit coupled to said second feedback signal line for adjusting power to said cold cathode fluorescent lamp to a power level such that said second voltage signal approaches a reference value representing desired load conditions of said cold cathode fluorescent lamp.

12. A liquid crystal display unit as claimed in claim 11, wherein said reference value has a minimum representing minimum current through said cold cathode fluorescent lamp and a maximum representing maximum current through said cold cathode fluorescent lamp.

13. A liquid crystal display unit as claimed in claim 8, further comprising:

an ignition circuit coupled to said first switch and said second switch for providing lower amount of power to said cold cathode fluorescent lamp prior to ignition of said cold cathode fluorescent lamp.

14. A liquid crystal display unit as claimed in claim 8, further comprising:

a third MOSFET switch coupled to said first MOSFET switch and said step-up transformer for providing a first electrical path through said primary winding of said step-up transformer to ground when said third MOSFET switch and said first MOSFET switch are simultaneously on;

a fourth MOSFET switch coupled to said second switch and said step-up transformer for providing a second electrical path through said primary winding of said step-up transformer to ground when said fourth MOSFET switch and said second MOSFET switch are simultaneously on;

a sense resistor electrically coupled to said cold cathode fluorescent lamp and electrically coupled to ground for providing a second voltage signal representing current through said cold cathode fluorescent lamp;

a second feedback signal line coupled to said sense resistor for receiving said second voltage signal from said sense resistor representing said current through said cold cathode fluorescent lamp; and

a feedback control circuit coupled to said second feedback signal line, said first switch and said third switch for adjusting time when said third switch and said first switch are simultaneously on such that said second voltage signal approaches a reference value representing desired load conditions of said cold cathode fluorescent lamp.

15. A method for providing power to a cold cathode fluorescent lamp of a liquid crystal display unit, said method comprising:

providing direct current of a first polarity through a primary winding of a transformer;

providing direct current of a second polarity through said primary winding of said transformer;

providing increased voltage from said transformer to said cold cathode fluorescent lamp;

providing a first feedback signal representing a voltage across said cold cathode fluorescent lamp from a capacitor divider electrically coupled to said cold cathode fluorescent lamp;

receiving said first feedback signal representing said voltage across said cold cathode fluorescent lamp;

providing a time-out sequence of a predetermined duration when said first voltage signal exceeds a predetermined threshold; and

shutting down power to said cold cathode fluorescent lamp when said first voltage signal exceeds said predetermined threshold after said predetermined duration.

16. A method as claimed in claim 15, wherein said predetermined duration is sufficient for ignition of said cold cathode fluorescent lamp when properly operating and said predetermined threshold represents a value of said voltage across said cold cathode fluorescent lamp greater than a minimum striking voltage of said cold cathode fluorescent lamp and less than a rated voltage of said transformer.

17. A method as claimed in claim 15, further comprising:

providing a second voltage signal from a sense resistor electrically coupled to said cold cathode fluorescent lamp and electrically coupled to ground representing current through said cold cathode fluorescent lamp;

receiving said second voltage signal from said sense resistor representing said current through said cold cathode fluorescent lamp; and

adjusting power to said cold cathode fluorescent lamp to a power level such that said second voltage signal approaches a reference value representing desired load conditions of said cold cathode fluorescent lamp.

18. A method as claimed in claim 17, wherein said reference value has a minimum representing minimum current through said cold cathode fluorescent lamp and a maximum representing maximum current through said cold cathode fluorescent lamp.

19. A method as claimed in claim 15, further comprising:

providing lower amount of power to said cold cathode fluorescent lamp prior to ignition of said cold cathode fluorescent lamp.

20. A method as claimed in claim 15, further comprising:

providing a first electrical path through said primary winding of said transformer to ground when a first plurality of switches are simultaneously on;

providing a second electrical path through said primary winding of said transformer to ground when a second plurality of switches are simultaneously on;

providing a second voltage signal from a sense resistor electrically coupled to said cold cathode fluorescent lamp and electrically coupled to ground representing current through said cold cathode fluorescent lamp;

receiving said second voltage signal from said sense resistor representing said current through said cold cathode fluorescent lamp; and

adjusting time when said first plurality of switches are simultaneously on and said second plurality of switches are simultaneously on such that said second voltage signal approaches a reference value representing desired load conditions of said cold cathode fluorescent lamp.