High-efficiency adaptive DC/AC converter

• US 7,515,445 B2
• Filed: 04/24/2002
• Issued: 04/07/2009
• 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:
  US10/132,016
• Publication Number:
  US 20020180380A1
• Time in Patent Office: 2,540 Days
• Field of Search: 363 96- 98, 363/31, 363 39- 41, 363/25, 363/20, 363/95, 363/132, 363/131, 363/19, 363/17, 363/91, 315/160, 315127-128, 315/307, 315/309, 315/311, 315/225, 315/247, 315/86, 315/175, 315/277, 315/291, 315/224, 315/194, 315/209, 315/219, 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 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. An inverter controller circuit, comprising:

a pulse generator circuit for generating a pulse signal for controlling a first switch and a second switch in an alternating fashion so as to allow a primary winding of a transformer to selectively couple to a DC voltage source;

a capacitor divider electrically coupled to a load and for providing a first feedback signal representing a voltage across said load; and

a protection circuit coupled to said pulse generator circuit and to said capacitor divider and for shutting down said first switch and said second switch when said first feedback signal exceeds a predetermined threshold after a first predetermined duration.

2. An inverter controller circuit as claimed in claim 1, wherein said load comprises a cold cathode fluorescent lamp powered by said transformer.

3. An inverter controller circuit as claimed in claim 2, wherein said first predetermined duration is sufficient for ignition of said cold cathode fluorescent lamp.

4. An inverter controller circuit as claimed in claim 2, 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 transformer.

5. An inverter circuit controller circuit as claimed in claim 2, further comprising:

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

6. An inverter circuit controller circuit as claimed in claim 1, further comprising:

a first feedback signal line coupled to said capacitor divider and for receiving said first feedback signal from said capacitor divider representing said voltage across said load; and

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

7. An inverter circuit controller circuit as claimed in claim 6, further comprising:

a sense resistor electrically coupled to said load and electrically coupled to ground and for providing a second feedback signal representing current through said load;

a second feedback signal line coupled to said sense resistor and for receiving said second feedback signal from said sense resistor representing said current through said load; and

a feedback control circuit coupled to said second feedback signal line and for adjusting power to said load to a power level such that said second feedback signal approaches a range of reference values representing desired load conditions of said load.

8. An inverter circuit controller circuit as claimed in claim 7, wherein said range of reference values has a minimum representing minimum current through said load and a maximum representing maximum current through said load.

9. An inverter circuit controller circuit as claimed in claim 7, wherein said feedback control circuit comprises:

an error amplifier for comparing said second feedback signal to a predetermined reference signal and for generating an error signal indicative of a difference between said second feedback signal and said predetermined reference signal; and

a flow-through switch coupled to said error amplifier and having a first conduction state in which said transformer delivers an amount of power to said load according to said second feedback signal and also having a second conduction state in which said transformer delivers a predetermined minimum power to said load for a second predetermined time duration.

10. An inverter circuit controller circuit as claimed in claim 1, wherein said transformer comprises said primary winding and a secondary winding for providing an increased voltage to said load, and wherein said primary winding and said secondary winding are electrically coupled to ground.

11. An inverter circuit controller circuit as claimed in claim 10, further comprising:

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

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

a sense resistor electrically coupled to said load and electrically coupled to ground and for providing a second feedback signal representing current through said load;

a feedback signal line coupled to said sense resistor and for receiving said second feedback signal from said sense resistor; and

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

12. A liquid crystal display unit comprising:

a liquid crystal display panel;

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

a transformer with a primary winding and a secondary winding coupled to said cold cathode fluorescent lamp and for providing increased voltage to said cold cathode fluorescent lamp;

a pulse generator circuit for generating a pulse signal for controlling a first switch and a second switch in an alternating fashion so as to allow said primary winding of said transformer to selectively couple to a DC voltage source;

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

a protection circuit coupled to said pulse generator circuit and for shutting down said first switch and said second switch when said first feedback signal exceeds a predetermined threshold after a first predetermined duration.

13. A liquid crystal display unit as claimed in claim 12, wherein said first predetermined duration is sufficient for ignition of said cold cathode fluorescent lamp.

14. A liquid crystal display unit as claimed in claim 12, 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 transformer.

15. A liquid crystal display unit as claimed in claim 12, further comprising:

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

16. A liquid crystal display unit as claimed in claim 12, further comprising:

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

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

17. A liquid crystal display unit as claimed in claim 16, further comprising:

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

a second feedback signal line coupled to said sense resistor and for receiving said second feedback 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 and for adjusting power to said cold cathode fluorescent lamp to a power level such that said second feedback signal approaches a range of reference values representing desired load conditions of said cold cathode fluorescent lamp.

18. A liquid crystal display unit as claimed in claim 17, wherein said range of reference values 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 liquid crystal display unit as claimed in claim 17, wherein said feedback control circuit comprises:

an error amplifier for comparing said second feedback signal to a predetermined reference signal and for generating an error signal indicative of a difference between said second feedback signal and said predetermined reference signal; and

a flow-through switch coupled to said error amplifier and having a first conduction state in which said transformer delivers an amount of power to said cold cathode fluorescent lamp according to said second feedback signal and also having a second conduction state in which said transformer delivers a predetermined minimum power to said cold cathode fluorescent lamp for a second predetermined time duration.

20. A liquid crystal display unit as claimed in claim 12, wherein said primary winding and said secondary winding are electrically coupled to ground.

21. A liquid crystal display unit as claimed in claim 12, further comprising:

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

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

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

a feedback signal line coupled to said sense resistor and for receiving said second feedback signal from said sense resistor; and

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

22. A method for providing power to a cold cathode florescent lamp, said method comprising:

generating a pulse signal for controlling a first switch and a second switch in an alternating fashion;

selectively providing a DC voltage to a primary winding of a transformer according to said pulse signal;

receiving a first feedback signal representing a voltage across said cold cathode florescent lamp from a capacitor divider electrically coupled to said cold cathode florescent lamp; and

shutting down power to said cold cathode florescent lamp when said first feedback signal exceeds a predetermined threshold after a predetermined duration.

23. A method as claimed in claim 22, wherein said predetermined duration is sufficient for ignition of said cold cathode fluorescent lamp and 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 transformer.

24. A method as claimed in claim 22, further comprising:

providing a time-out sequence of said predetermined duration when said first feedback signal exceeds said predetermined threshold.

25. A method as claimed in claim 22, further comprising:

providing a second feedback 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 feedback 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 feedback signal approaches a range of reference values representing desired load conditions of said cold cathode fluorescent lamp.

26. A method as claimed in claim 25, wherein said range of reference values has a minimum representing minimum current through said cold cathode fluorescent lamp and a maximum representing maximum current through said cold cathode fluorescent lamp.

27. A method as claimed in claim 22, further comprising:

reducing power to said cold cathode fluorescent lamp prior to ignition of said cold cathode fluorescent lamp.

28. A method as claimed in claim 22, further comprising:

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

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

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 feedback 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 feedback signal from said sense resistor representing said current through said cold cathode fluorescent lamp; and

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