High-efficiency adaptive DC/AC converter

• US 6,396,722 B2
• Filed: 05/07/2001
• Issued: 05/28/2002
• Est. Priority Date: 07/22/1999
• Status: Expired due to Term

• Current Assignee: O2 Micro International Limited (O2 Micro Incorporated)
• Original Assignee: O2 Micro Incorporated
• Inventors: Lin, Yung-Lin
• Primary Examiner(s): Patel, Rajnikant B.
• Application Number:
  US09/850,222
• Publication Number:
  US 20010036096A1
• Time in Patent Office: 386 Days
• Field of Search: 363/98,132,17,89,56,37
• 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 inverter circuit, comprising:

an input voltage source;

a plurality of switches arranged in a full bridge configuration wherein opposite corners of said full bridge configuration being selectively coupled to said voltage source;

a transformer having a primary side and a secondary side, said primary side being selectively coupled to said voltage source in an alternating fashion through said full bridge switch configuration;

a load coupled to said secondary side of said transformer;

a pulse generator circuit generating a first pulse signal for driving one of the corner switches of said full bridge configuration;

a feedback control loop circuit receiving a feedback signal indicative of power being supplied to the load and adapted to generate a second pulse signal for controlling the conduction state of a switch on the opposite corner of said switch being controlled by said first pulse signal, said second pulse signal having a first state which overlaps said first pulse signal to deliver an amount of power to said load determined by said feedback signal, and a second state which overlaps the first signal with a predetermined minimum overlap to deliver a predetermined minimum power to the load.

2. An inverter circuit as claimed in claim 1, said feedback control loop circuit comprises an error amplifier 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;

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

3. An inverter circuit as claimed in claim 2, further comprising a ramp signal generator circuit receiving said first pulse signal and generating a ramp signal approximately 180 degrees out phase from said first pulse signal, said feedback control loop circuit further comprises a comparator comparing said ramp signal to said DC output signal of said switch and generating said second pulse signal.

4. An inverter circuit as claimed in claim 3, wherein the intersection of said DC output signal and said ramp signal determines the overlap between said first and second pulse signal.

5. An inverter circuit as claimed in claim 2, further comprising protection circuitry comprising a current sense comparator, said current sense comparator comparing said feedback signal to a second reference signal;

wherein if the compared value of said feedback signal and said second reference signal is within a predetermined range said current sense comparator generates a first control signal to control the conduction state of said switch corresponding to said first conduction state; and

wherein if the compared value of said feedback signal is outside a predetermined range said current sense comparator generates a second control signal to control the conduction state of said switch corresponding to said second conduction state.

6. An inverter circuit as claimed in claim 5, further comprising a sense resistor coupled between said load and said secondary side of said transformer, said sense resistor generating said feedback signal.

7. An inverter circuit as claimed in claim 5, said protection circuitry further comprising an overvoltage protection circuit comprising a voltage comparator comparing a predetermined voltage reference to a voltage signal indicative of load voltage conditions and generating a voltage control signal.

8. An inverter circuit as claimed in claim 7, further comprising a pulse generator generating said first pulse signal having a 50% duty cycle, pulse generator receiving said voltage control signal and/or said control signal generated by said current sense comparator, said pulse generator shutting off if said voltage control signal and/or said control signal generated by said current sense comparator is indicative of an open lamp or short circuit condition at the load during operation.

9. An inverter as claimed in claim 2, wherein said reference signal represents desired load conditions.

10. An inverter as claimed in claim 5, wherein said second reference signal represents the minimum or maximum current permitted by said switches and/or said load.

11. An inverter as claimed in claim 7, wherein said third reference signal represents the maximum voltage at said secondary side of said transformer.

12. An inverter circuit comprising:

an input voltage source;

a full bridge circuit comprising a first pair of switches arranged at opposite corners of a full bridge circuit, and a second pair of switches arranged at other opposite corners of said full bridge circuit, said first and second pair of switches being selectively coupled to said input voltage source such that said first pair of switches defines a first conduction path and said second pair of switches defines a second conduction path;

a transformer having a primary side and a secondary side, said primary side selectively coupled to said input voltage source in an alternating fashion through said first and second conduction paths;

a load coupled to said secondary side of said transformer;

a pulse generator generating a first pulse signal for controlling the conduction state of one of said first pair of switches;

a feedback control loop circuit receiving a feedback signal indicative of power being supplied to said load and adapted to generate a second pulse signal for controlling the conduction state of a different one of said first pair of switches, said second pulse signal having a phase overlap with said first pulse signal to deliver power to said load only if said feedback signal is above a predetermined threshold.

13. An inverter as claimed in claim 12, wherein the amount of overlap between said first and second pulse signals determines the amount of power delivered to said load.

14. An inverter as claimed in claim 12, wherein if said feedback signal is below said predetermined threshold, said feedback loop generating said second pulse signal to have a minimum phase overlap with said first pulse signal to reduce power delivered to said load to a predetermined minimum amount.

15. An inverter as claimed in claim 12, further comprising a first driver circuit receiving said first pulse signal and generating a first complimentary pulse signal 180 degrees out of phase from said first pulse signal, said first complimentary pulse signal controlling the conduction state of one of said second pair of said switches;

and a second driver receiving said second pulse signal and generating a second complimentary pulse signal 180 degrees out of phase from said second pulse signal, said second complimentary pulse signal controlling the conduction state of the other of said second pair of said switches.

16. An inverter as claimed in claim 12, wherein said first pulse signal comprises a PWM signal having approximately 50% duty cycle.

17. An inverter as claimed in claim 12, said second pulse signal having a first state which overlaps said first pulse signal to deliver an amount of power to said load determined by said feedback signal, and a second state which overlaps the first signal with a predetermined minimum overlap to deliver a predetermined minimum power to the load.

18. A DC to AC inverter circuit comprising:

a plurality of switches arranged in a full bridge configuration wherein switches on opposite corners of said full bridge being controlled in an alternating fashion;

a pulse generator circuit generating a first pulse signal for driving one of the corner switches of said full bridge configuration; and

a feedback control loop circuit receiving a feedback signal indicative of power being supplied by said switches to a load, said feedback loop adapted to generate a second pulse signal for controlling the conduction state of a switch on the opposite corner of said switch being controlled by said first pulse signal, said second pulse signal having a first state which overlaps said first pulse signal to deliver an amount of power to said load determined by said feedback signal, and a second state which overlaps the first signal with a predetermined mininum overlap to deliver a predetermined minimum power to the load.

19. An inverter circuit comprising:

a full bridge circuit comprising a first pair of switches arranged at opposite corners of a full bridge circuit, and a second pair of switches arranged at other opposite corners of said full bridge circuit, said first and second pair of switches being selectively coupled to said input voltage source such that said first pair of switches defines a first conduction path and said second pair of switches defines a second conduction path;

a pulse generator generating a first pulse signal for controlling the conduction state of one of said first pair of switches;

a feedback control loop circuit receiving a feedback signal indicative of power being supplied to a load coupled to said switches, and adapted to generate a second pulse signal for controlling the conduction state of a different one of said first pair of switches, said second pulse signal having a phase overlap with said first pulse signal to deliver power to said load only if said feedback signal is above a predetermined threshold.