High-efficiency adaptive DC/AC converter

• US 7,515,446 B2
• Filed: 11/17/2005
• Issued: 04/07/2009
• Est. Priority Date: 04/24/2002
• 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:
  US11/282,873
• Publication Number:
  US 20060077700A1
• Time in Patent Office: 1,237 Days
• Field of Search: 363 96- 98, 363/31, 363 40- 41, 363/95, 363/25, 363/89, 363/17, 363/132, 363/34, 363/37, 323282-285, 323/224, 315/160, 315/127, 315/128, 315/117, 315/225, 315/307, 315/244, 315/291
• US Class Current: 363/98
• CPC Class Codes: H02M 1/0032 Control circuits allowing low power mode operation, e.g. in standby mode
• CPC Class Codes: H02M 7/4815 Resonant converters H02M7/4811 and H02M7/4826 take precedence
• CPC Class Codes: H02M 7/53871 with automatic control of output voltage or current
• 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: 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 inverter circuit, comprising:

an input DC voltage source;

a plurality of switches for selectively passing said DC voltage source;

a transformer having a primary side and a secondary side, said primary side being selectively coupled to said DC voltage source and receiving DC voltage from said DC voltage source in an alternating fashion through said switches;

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

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

a protection circuit coupled to said timer circuit for shuting down said plurality of switches when said first voltage signal exceeds said predetermined threshold for said predetermined duration; and

a feedback control loop circuit coupled to said plurality of switches for receiving a feedback signal indicative of power being supplied to said cold cathode fluorescent lamp load from a component receiving current from said secondary side and for controlling a conduction state of at least one of said switches so that said switches have a plurality of operating modes comprising a first mode in which said switches deliver an amount of power to said cold cathode fluorescent lamp load determined by said feedback signal, and a second mode in which said switches deliver a predetermined minimum power to said cold cathode fluorescent lamp load for a predetermined time duration.

2. A DC to AC inverter circuit as claimed in claim 1, wherein said feedback control loop circuit comprises:

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

a flow-through switch coupled to said error amplifier and having a first conduction state wherein the output of said flow-through switch comprises a DC signal indicative of said first mode of said switches, and a second conduction state wherein the output of said flow-through switch comprises a DC signal indicative of said second mode of said switches.

3. A DC to AC inverter controller circuit comprising:

a plurality of drivers for controlling a plurality of switches in an alternating fashion allowing a transformer to receive DC voltage in an alternating fashion;

a voltage input for electrically coupling to a capacitor divider and to a load and for receiving a first voltage signal representing a voltage across said load;

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

a protection circuit coupled to said timer circuit for shutting down said plurality of switches when said first voltage signal exceeds said predetermined threshold for said predetermined duration; and

a feedback control circuit for receiving a feedback signal indicative of power being supplied to said load from a component receiving current from a secondary side of said transformer and for controlling a conduction state of at least one of said switches to deliver power to said load not exceeding a predetermined amount during a predetermined start time duration.

13. A DC to AC inverter controller circuit as claimed in claim 3 wherein said load comprises a cold cathode fluorescent lamp.

14. A DC to AC inverter controller circuit as claimed in claim 13 wherein said predetermined duration is sufficient for ignition of said cold cathode fluorescent lamp when properly operating.

15. A DC to AC inverter controller circuit as claimed in claim 13 wherein said predetermined start time duration is sufficient for ignition of said cold cathode fluorescent lamp when properly operating.

16. A DC to AC inverter controller circuit as claimed in claim 13 wherein said predetermined threshold represents a value of said voltage across said load greater than a minimum striking voltage of said cold cathode fluorescent lamp and less than a rated voltage of said transformer.

17. A DC to AC inverter controller circuit as claimed in claim 13 wherein said transformer has a primary winding and a secondary winding.

18. A DC to AC inverter controller circuit as claimed in claim 17 wherein said predetermined duration is sufficient for ignition of said cold cathode fluorescent lamp when properly operating.

19. A DC to AC inverter controller circuit as claimed in claim 17 wherein said predetermined start time duration is sufficient for ignition of said cold cathode fluorescent lamp when properly operating.

20. A DC to AC inverter controller circuit as claimed in claim 17 wherein said predetermined threshold represents a value of said voltage across said load greater than a minimum striking voltage of said cold fluorescent lamp and less than a rated voltage of said transformer.

4. A liquid crystal display comprising:

a plurality of switches being controlled in an alternating fashion for allowing a transformer to receive DC voltage in an alternating fashion;

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

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

a protection circuit coupled to said timer circuit and said plurality of switches for shutting down said plurality of switches when said first voltage signal exceeds predetermined threshold for said predetermined duration; and

a feedback control loop circuit coupled to said plurality of switches for receiving a feedback signal indicative of power being supplied to said load from a component receiving current from a secondary side of said transformer and for controlling a conduction state of at least one of said switches, said feedback control loop circuit comprises an operation state in which said switches deliver an amount of power to said load determined by said feedback signal; and

an ignition state in which said switches deliver power to said load not exceeding a predetermined amount during a predetermined start time duration.

5. A liquid crystal display as claimed in claim 4 wherein said load comprises a cold cathode fluorescent lamp.

6. A liquid crystal display as claimed in claim 5 wherein said predetermined duration is sufficient for ignition of said cold cathode fluorescent lamp when properly operating.

7. A liquid crystal display as claimed in claim 5 wherein said predetermined start time duration is sufficient for ignition of said cold cathode fluorescent lamp when properly operating.

8. A liquid crystal display as claimed in claim 5 wherein said predetermined threshold represents a value of said voltage across said load greater than a minimum striking voltage of said cold cathode fluorescent lamp and less than a rated voltage of said transformer.

9. A liquid crystal display as claimed in claim 5 wherein said transformer has a primary winding and a secondary winding.

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

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

12. A liquid crystal display as claimed in claim 9 wherein said predetermined threshold represents a value of said voltage across said load greater than a minimum striking voltage of said cold cathode fluorescent lamp and less than a rated voltage of said transformer.