COLD-CATHODE FLUORESCENT LAMP MULTIPLE LAMP CURRENT MATCHING CIRCUIT

Abstract

A method of matching the brightness of a plurality of lamps driven by an AC drive current may comprise the steps of: selecting a first one of said lamps having a lowest brightness; and reducing the AC drive current in a second lamp periodically so as to reduce the brightness of the second lamp to match that of the first lamp. According to another embodiment, a reference brightness may be selected, or optionally a reference AC current level, and the method may reduce the drive current periodically so as to set the lamp brightness in relation to the reference brightness or optionally the reference AC current level. A circuit for driving a lamp with controllable brightness may comprise: a drive circuit for supplying an AC drive current to the lamp; and a reducing circuit, advantageously comprised in an IC, for reducing the AC drive current in the lamp periodically to set the AC drive current in the lamp to match a reference. The reference may correspond to the brightness of another lamp driven by the AC drive circuit, or to a reference AC current level. The reducing circuit may comprise a switch in series with the lamp, or a parallel connection of a resistance and a switching device connecting the lamp to ground. The AC drive current may be reduced by a predetermined number of half-cycles, preferably by one half-cycle, of said AC drive current. The AC drive current may be reduced by supplying periodic OFF pulses to the reducing circuit, which may be spaced apart by a selected number of half-cycles of the AC drive current, for example by a frequency-dividing circuit which receives and divides the frequency of the AC drive current.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of a conventional CCFL circuit.

FIG. 2 is a graph showing uneven current flow in the lamps of FIG. 1.

FIG. 3 is a schematic block diagram of a CCFL circuit including a balance transformer current matching circuit.

FIG. 4 is a schematic block diagram of a CCFL circuit including a pulse skipping current matching control circuit according to an embodiment of the invention.

FIG. 5 is a graph showing lamp current waveforms with and without pulse skipping according to the embodiment.

FIG. 6 is a graph showing a pulse skipping waveform versus half-bridge voltage in the embodiment.

FIG. 7 is a schematic diagram of a CCFL circuit including a pulse skipping current matching control circuit according to another embodiment of the invention.

FIG. 8 is a schematic diagram of a function generator for generating control signals.

FIG. 9 is a timing diagram showing signals in the circuit of FIG. 8.

Claims

1. A method of matching the brightness of a plurality of lamps driven by an AC drive current, comprising the steps of: selecting a first one of said lamps having a lowest brightness; andreducing the AC drive current in a second lamp periodically so as to reduce the brightness of the second lamp to match that of the first lamp.

2. A method according to claim 1, wherein said reducing step comprises the step of reducing said AC drive current by opening a switch in series with said second lamp.

3. A method according to claim 2, wherein said switch is turned off for a predetermined number of half-cycles of said AC drive current.

4. A method according to claim 1, wherein said reducing step comprises the step of reducing said AC drive current by inserting a resistance in series with said second lamp.

5. A method according to claim 4, wherein one end of said second lamp is connected to ground via a parallel connection of said resistance in parallel with a switching device, said switching device being turned off in said reducing step so that said resistance is thereby connected between said second lamp and ground.

6. A method according to claim 5, wherein said switch is turned off for a predetermined number of half-cycles of said AC drive current.

7. A method according to claim 5, further comprising the step of generating and applying periodic OFF pulses to said switching device.

8. A method according to claim 7, wherein said OFF pulses are spaced apart by a number of half-cycles of said AC drive current selected for matching the brightness of the first and second lamps.

9. A method according to claim 8, further comprising the step of delaying said OFF pulses by a predetermined delay time corresponding to circuit parameters.

10. A method of controlling the brightness of a lamp driven by an AC drive current, comprising the steps of: selecting a reference brightness; andreducing the AC drive current in said lamp periodically so as to set the brightness of the lamp in relation to the reference brightness.

11. A method according to claim 10, wherein said reducing step comprises the step of reducing said AC drive current by opening a switch in series with said lamp.

12. A method according to claim 10, wherein said reducing step comprises the step of reducing said AC drive current by inserting a resistance in series with said lamp.

13. A method according to claim 12, wherein one end of said lamp is connected to ground via a parallel connection of said resistance in parallel with a switching device, said switching device being turned off in said reducing step so that said resistance is thereby connected between said lamp and ground.

14. A circuit for driving a lamp with controllable brightness comprising: a drive circuit for supplying an AC drive current to said lamp; anda reducing circuit for reducing the AC drive current in said lamp periodically so as to set the AC drive current in the lamp in relation to a reference.

15. A circuit according to claim 14, wherein said reference corresponds to the brightness of another lamp driven by said drive circuit.

16. A circuit according to claim 14, wherein said reducing circuit comprises a switch connected in series with said lamp.

17. A circuit according to claim 16, further comprising a control circuit for turning off said switch for a predetermined number of half-cycles of said AC drive current.

18. A circuit according to claim 14, wherein said reducing circuit comprises a resistance connected in series with said lamp.

19. A circuit according to claim 18, wherein one end of said lamp is connected to ground via a parallel connection of said resistance in parallel with a switching device, and further comprising a control circuit for turning off said switching device periodically so that said resistance is thereby connected between said second lamp and ground.

20. A circuit according to claim 19, wherein control circuit turns off said switch for a predetermined number of half-cycles of said AC drive current.

21. A circuit according to claim 19, wherein said control circuit generates and applies periodic OFF pulses to said switching device.

22. A circuit according to claim 21, wherein said OFF pulses are spaced apart by a number of half-cycles of said AC drive current selected for matching the brightness of the lamp to the reference brightness.

23. A circuit according to claim 22, wherein said control circuit further delays said OFF pulses by a predetermined delay time corresponding to circuit parameters of said drive circuit and said reducing circuit.

24. A circuit according to claim 14, wherein said reducing circuit is comprised in a single integrated circuit.