Simple protection circuit and adaptive frequency sweeping method for CCFL inverter

• US 8,063,570 B2
• Filed: 07/01/2008
• Issued: 11/22/2011
• Est. Priority Date: 11/29/2007
• Status: Expired due to Fees

• Current Assignee: Monolithic Power Systems Incorporated
• Original Assignee: Monolithic Power Systems Incorporated
• Inventors: Zhang, Junming, Yang, Eric, Ren, Yuancheng, Yao, Kaiwei
• Primary Examiner(s): Owens; Douglas W
• Assistant Examiner(s): A; Minh D
• Application Number:
  US12/166,140
• Publication Number:
  US 20090140655A1
• Time in Patent Office: 1,239 Days
• Field of Search: 315/224, 315/225, 315/306, 315/308, 315/312, 315/209.R
• US Class Current: 315/224
• CPC Class Codes: H05B 41/2855 against abnormal lamp operating conditions

Abstract

A CCFL inverter circuit integrates a feedback circuit and protection circuit together. For both in-phase and out-of-phase applications, sensed lamp voltages can be used for open lamp and short lamp detection and sensed currents can be used for open lamp detection. The driving circuit adjusts the open lamp frequency by using a duty cycle control signal so that the driving circuit can always achieve the desired lamp voltage gain.

Claims

1. A method for detecting an open lamp condition in a discharge lamp system, comprising:

providing a DC bias voltage to a detector circuit that is coupled to a discharge lamp, said detector circuit having

a sensing resistor coupled to said discharge lamp; and

a diode coupled to said sensing resistor and to said discharge lamp;

deriving a current signal from said detector circuit having said sensing resistor and said diode, said current signal corresponding to a current of said discharge lamp and said DC bias voltage; and

if said current signal satisfies an open lamp condition, triggering an open lamp protection process.

2. The method in claim 1, further comprising:

coupling said current signal to a protection trigger circuit on an integrated circuit level to trigger the open lamp protection process.

3. The method in claim 1, wherein said current signal is related to a maximum sensing current of said discharge lamp.

4. The method in claim 3, wherein said detector circuit is an OR gate detector circuit.

5. The method in claim 3, wherein the open lamp protection process is triggered when the lowest value of said current signal in one switching cycle is higher than a predetermined threshold voltage.

6. The method in claim 5, wherein said detector circuit is an AND gate detector circuit.

7. The method in claim 5, wherein the open lamp protection process is triggered when the highest value of said current signal in one switching cycle is lower than a threshold voltage.

8. A method for detecting an open lamp or a shorted lamp condition in a discharge lamp system, comprising:

providing a DC bias to a detector circuit that is coupled to a discharge lamp, said detector circuit having

a sensing resistor coupled to said discharge lamp; and

a diode coupled to said sensing resistor and to said discharge lamp;

deriving a current signal from said detector circuit, said current signal corresponding to a current of said discharge lamp and said DC bias voltage;

if said current signal satisfies an open lamp condition, triggering an open lamp protection process; and

if said current signal satisfies a shorted lamp condition, triggering a shorted lamp protection process.

9. The method in claim 8, further comprising:

coupling said current signal to a protection trigger circuit on an integrated circuit level to trigger the open lamp protection process or the shorted lamp protection process.

10. The method in claim 8, wherein said current signal is related to a maximum sensing current of said discharge lamp.

11. The method in claim 10, wherein said detector circuit is an OR gate detector circuit.

12. The method in claim 10, wherein the open lamp protection process is triggered when the highest value of said current signal in one switching cycle is higher than a first predetermined threshold, and the shorted lamp protection process is triggered when the lowest value of said current signal in one switching cycle is higher than a second predetermined threshold.

13. The method in claim 8, wherein said current signal is related to a minimum sensing current of said discharge lamp.

14. The method in claim 13, wherein said detector circuit is an AND gate detector circuit.

15. The method in claim 13, wherein the open lamp protection process is triggered when the lowest value of said current signal in one switching cycle is lower than a first predetermined threshold, and the shorted lamp protection process is triggered when the highest value of said current signal in one switching cycle is lower than a second predetermined threshold.

16. A circuit for detecting an open lamp condition and triggering an open lamp process in a discharge lamp system, comprising:

a direct current (DC) bias source coupled to at least one discharge lamp, said DC bias source being configured to apply a DC voltage to said at least one discharge lamp;

a detector circuit coupled to said at least one discharge lamp and to said DC bias source;

said detector circuit having

a sensing resistor coupled to said at least one discharge lamp; and

a diode coupled to said sensing resistor and to said at least one discharge lamp, said sensing resistor and said diode being configured to output a current signal corresponding to a current of said at least one discharge lamp and said DC bias voltage applied to said at least one discharge lamp; and

a protection triggering circuit for receiving said current signal from said detector circuit and triggering the open lamp protection process if said at least one discharge lamp is open.

17. The circuit in claim 16, wherein said circuit is used in connection with a plurality of discharge lamps and said detector circuit is an OR gate detector circuit comprising:

a plurality of sensing resistors coupled to said plurality of discharge lamps wherein one sensing resistor corresponds to one discharge lamp; and

a plurality of diodes coupled to said plurality of discharge lamps wherein one diode corresponds to one discharge lamp.

18. The circuit in claim 16, wherein said circuit is used in connection with a plurality of discharge lamps and said detector circuit is an AND gate detector circuit comprising:

a plurality of sensing resistors coupled to said plurality of discharge lamps wherein one sensing resistor corresponds to one discharge lamp;

a plurality of diodes being coupled to said plurality of discharge lamps wherein one diode corresponds to one discharge lamp; and

wherein said DC bias source is coupled to said plurality of diodes through a pull-up resistor.

19. The circuit in claim 16, wherein said circuit is used in connection with a plurality of discharge lamps and said detector circuit comprising:

a first plurality of sensing resistors coupled to a first plurality of discharge lamps wherein one of said first plurality of sensing resistors corresponds to one of said first plurality of discharge lamps, voltage of said first plurality of sensing resistors being in phase;

a first plurality of diodes coupled to said first plurality of discharge lamps wherein one of said first plurality of diodes corresponds to one of said first plurality of discharge lamps;

a first additional diode coupled to said first plurality of diodes;

a second plurality of sensing resistors coupled to a second plurality of discharge lamps wherein one of said second plurality of sensing resistors corresponds to one of said second plurality of discharge lamps voltage of said second plurality of sensing resistors being about 180 degree out-of-phase with said first plurality of sensing capacitors;

a second plurality of diodes coupled to said second plurality of discharge lamps wherein one of said second plurality of diodes corresponds to one of said second plurality of discharge lamps;

a second additional diode coupled to said second plurality of diodes; and

wherein said DC bias source, is coupled to said first plurality of diodes and said second plurality of diodes.

20. A method for adjusting a switching frequency in a discharge lamp system, comprising:

applying a direct current (DC) bias voltage to a discharge lamp;

deriving a current signal from a detecting circuit coupled to said discharge lamp, said detector circuit having

a sensing resistor coupled to said discharge lamp; and

a diode coupled to said sensing resistor and to said discharge lamp;

wherein said current signal corresponds to a current of said discharge lamp and to said applied DC bias voltage;

comparing said current signal to a predetermined reference and generating an error signal; and

adjusting said switching frequency according to said error signal.

21. The method in claim 20, wherein said current signal is related to a maximum sensing current of said discharge lamp.

22. The method in claim 20, wherein said current signal is related to a minimum sensing current of said discharge lamp.