Phase shift modulation-based control of amplitude of AC voltage output produced by double-ended DC-AC converter circuitry for powering high voltage load such as cold cathode fluorescent lamp

Abstract

A double-ended, dc-ac converter supplies ac power to a load, such as a cold cathode fluorescent lamp used to back-light a liquid crystal display. first and second converter stages generate respective first and second sinusoidal voltages having the same frequency and amplitude, but having a controlled phase difference therebetween. By employing a voltage controlled delay circuit to control the phase difference between the first and second sinusoidal voltages, the converter is able to vary the amplitude of the composite voltage differential produced across the opposite ends of the load. The converter may be either voltage fed or current fed.

Description

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims the benefit of previously filed, co-pending U.S. patent application Ser. No. 60/589,172, filed Jul. 19, 2004, by R. Lyle et al, entitled: “Phase Shift Modulation for Double Ended, Push Pull Inverter,” assigned to the assignee of the present application and the disclosure of which is incorporated herein.

FIELD OF THE INVENTION

The present invention relates in general to power supply systems and subsystems thereof, and is particularly directed to a method and apparatus for controlling the amplitude of an AC voltage supplied to a high voltage device, such as a cold cathode fluorescent lamp of the type employed for back-lighting a liquid crystal display.

BACKGROUND OF THE INVENTION

There are a variety of electrical system applications which require one or more sources of high voltage AC power. As a non-limiting example, a liquid crystal display (LCD), such as that employed in desktop and laptop computers, or in larger display applications such as large scale television screens, requires an associated set of cold cathode fluorescent lamps (CCFLs) mounted directly behind it for back-lighting purposes. In these and other applications, ignition and continuous operation of the CCFLs require the application of a high AC voltage that can range on the order of several hundred to several thousand volts. Supplying such high voltages to these devices has been customarily accomplished using one of several methodologies.

A first approach involves the use a single-ended drive system, wherein a high voltage AC voltage generation and control system is transformer-coupled to one/near end of the lamp, while the other/far end of the lamp is connected to ground. This technique is undesirable, as it involves the generation of a very high peak AC voltage in the high voltage transformer circuitry feeding the driven end of the lamp.

Another approach involves the use a double-ended drive system, wherein a high voltage AC voltage generation and control system is transformer-coupled to one/near end of the lamp, while connection from the voltage generation and control system to the other/far end of the lamp is effected through high voltage wires. These wires can be relatively long (e.g., four feet or more), and are more expensive than low voltage wires; in addition, they lose substantial energy through capacitive coupling to ground.

Another approach is to place a high voltage transformer and associated voltage switching devices, such as MOSFETs or bipolar transistors, near the far end of the lamp; these devices are connected to and controlled by a local controller at the near end of the lamp. This approach has disadvantages similar to the first, in that the gate (or base) drive wires are required to carry high peak currents and must change states at high switching speeds for efficient operation. The long wires required are not readily suited for these switching speeds, due their inherent inductance; in addition they lose energy because of their substantial resistance.

SUMMARY OF THE INVENTION

In accordance with the present invention, disadvantages, such as those described above, of conventional high voltage AC power supply system architectures, including systems for supplying AC power to CCFLs used to back-light an LCD panel, are effectively obviated FIG. 10 diagrammatically illustrates an example of a system incorporating a DC to AC controller and driver according to one embodiment of the current invention.1006 in a system such as but not limited to that of system 1000 of FIG. 10systems such as that in FIG. 10 having display 1004 and having the double-ended, push-pull DC-AC converter architecture of the present invention, which is operative to drive opposite ends of a load, such as a CCFL 1006, with a first and second sinusoidal voltages having the same frequency and amplitude, but having a controlled phase difference therebetween. By controlling the phase difference between the first and second sinusoidal voltages, the present invention is able to vary the amplitude of the composite voltage differential produced across the opposite ends of the load.

While we have shown and described several embodiments in accordance with the present invention, it is to be understood that the same is not limited thereto but is susceptible to numerous changes and modifications as known to a person skilled in the art. We therefore do not wish to be limited to the details shown and described herein, but intend to cover all such changes and modifications as are obvious to one of ordinary skill in the art.

Claims

1. An apparatus for supplying ac power to a high voltage load comprising first and second push-pull dc-ac converter stages which are operative to drive opposite ends of said load with first and second sinusoidal voltages having the same frequency and amplitude, but having a controlled phase difference therebetween, which is effective to vary the amplitude of the composite ac voltage differential produced across the opposite ends of said load.

2. The apparatus according to claim 1, wherein a respective converter stage contains a pair of pulse generators which generate phase-complementary pulse signals of the same amplitude and frequency, and having a 50% duty cycle, said phase-complementary pulse signals being used to control ON/OFF conduction of a pair of controlled switching devices, current flow paths through which are coupled between a reference voltage terminal and opposite ends of a voltage-fed center-tapped primary coil of a step-up transformer, said step-up transformer having a secondary coil thereof coupled to a resonant filter circuit that is operative to convert a generally rectangular wave output produced across the secondary winding of the step-up transformer into a generally sinusoidal waveform.

3. The apparatus according to claim 2, wherein the phase of the sinusoidal waveform produced by the resonant filter circuit of one of said converter stages is controllably shifted by a prescribed amount relative to the phase of the sinusoidal waveform produced by the resonant filter circuit of another converter stage, so as to modify the amplitude of the composite ac voltage differential produced between said opposite ends of said load.

4. The apparatus according to claim 3, further comprising a voltage-controlled delay circuit which is operative to impart a controlled amount of delay to pulse trains produced by pulse generators of said one of said converter stages relative to the pulse trains produced by pulse generators of said another of said converter stages, said controlled amount of delay between the two pulse trains controlling the amplitude of the composite ac voltage differential produced across the opposite ends of the load.

5. The apparatus according to claim 1, wherein a respective converter stage contains a pair of pulse generators which generate phase-complementary pulse signals of the same amplitude and frequency, and having a 50% duty cycle, said phase-complementary pulse signals being used to control ON/OFF conduction of a pair of controlled switching devices, current flow paths through which are coupled between a reference voltage terminal and opposite ends of a current-fed, center-tapped primary coil of a step-up transformer, said primary coil being coupled with a capacitor, so as to form a resonant tank circuit therewith, said step-up transformer having a secondary coil that is operative to produce a generally sinusoidal waveform.

6. The apparatus according to claim 5, wherein the phase of the sinusoidal waveform produced by the secondary coil of the step-up transformer of one of said converter stages is controllably shifted by a prescribed amount relative to the phase of the sinusoidal waveform produced by secondary coil of the step-up transformer of another of said converter stages, and thereby modify the amplitude of the composite ac voltage differential produced between said opposite ends of said load.

7. The apparatus according to claim 6, further comprising a voltage-controlled delay circuit which is operative to impart a controlled amount of delay to pulse trains produced by pulse generators of said one of said converter stages relative to the pulse trains produced by pulse generators of said another of said converter stages, said controlled amount of delay between the two pulse trains controlling the amplitude of the composite ac voltage differential produced across the opposite ends of the load.

8. The apparatus according to claim 1, wherein said load comprises a cold cathode fluorescent lamp.

9. The apparatus according to claim 1, wherein said first and second push-pull dc-ac converter stages include respective first and second resonant filter circuits which are operative to convert first and second generally rectangular wave voltages produced by said first and second push-pull dc-ac converter stages to said first and second sinusoidal voltages.

10. A method for supplying ac power to a high voltage load comprising the steps of:

(a) providing first and second push-pull dc-ac converter stages which are operative to produce first and second sinusoidal voltages having the same frequency and amplitude, but having a controllable phase difference therebetween;

(b) driving opposite ends of said load with said first and second sinusoidal voltages; and

(c) controlling the phase difference between said first and second sinusoidal voltages, so as to modify the voltage differential between said first and second sinusoidal voltages applied to said opposite ends of said load;

wherein controlling the phase difference comprises applying a delay to pulse trains produced by pulse generators of one of the push-pull dc-ac converter stages relative to pulse trains produced by pulse generators of the other push-pull dc-ac converter stage.

11. The method according to claim 10, wherein a respective one of said first and second push-pull dc-ac converter stages contains a pair of pulse generators which generate phase-complementary pulse signals of the same amplitude and frequency, and having a 50% duty cycle, said phase-complementary pulse signals controlling ON/OFF conduction of a pair of controlled switching devices, current flow paths through which are coupled between a reference voltage terminal and opposite ends of a voltage-fed center-tapped primary coil of a step-up transformer, said step-up transformer having a secondary coil thereof coupled to a resonant filter circuit that is operative to convert a generally rectangular wave output produced across the secondary winding of the step-up transformer into a generally sinusoidal waveform for application to a respective end of said load.

12. The method according to claim 11, wherein step (c) comprises controllably shifting the phase of the sinusoidal waveform produced by the resonant filter circuit of one of said converter stages by a prescribed amount relative to the phase of the sinusoidal waveform produced by the resonant filter circuit of another converter stage, so as to modify the amplitude of the composite ac voltage differential produced between said opposite ends of said load.

13. The method according to claim 12, wherein step (c) comprises imparting a controlled amount of delay to pulse trains produced by pulse generators of said one of said converter stages relative to the pulse trains produced by pulse generators of said another of said converter stages, said controlled amount of delay between the two pulse trains being effective to control the amplitude of the composite ac voltage differential produced across the opposite ends of the load.

14. The method according to claim 10, wherein a respective converter stage contains a pair of pulse generators which generate phase-complementary pulse signals of the same amplitude and frequency, and having a 50% duty cycle, said phase-complementary pulse signals being used to control ON/OFF conduction of a pair of controlled switching devices, current flow paths through which are coupled between a reference voltage terminal and opposite ends of a current-fed, center-tapped primary coil of a step-up transformer, said primary coil being coupled with a capacitor, so as to form a resonant tank circuit therewith, said step-up transformer having a secondary coil that is operative to produce a generally sinusoidal waveform for application to a respective end of said load.

15. The method according to claim 14, wherein step (c) comprises controllably shifting the phase of the sinusoidal waveform produced by the secondary coil of the step-up transformer of one of said converter stages by a prescribed amount relative to the phase of the sinusoidal waveform produced by secondary coil of the step-up transformer of another of said converter stages, thereby modifying the amplitude of the composite ac voltage differential produced between said opposite ends of said load.

16. The method according to claim 15, wherein step (c) further comprises imparting a controlled amount of delay to pulse trains produced by pulse generators of said one of said converter stages relative to the pulse trains produced by pulse generators of said another of said converter stages, said controlled amount of delay between the two pulse trains controlling the amplitude of the composite ac voltage differential produced across the opposite ends of the load.

17. The method according to claim 10, wherein said load comprises a cold cathode fluorescent lamp.

18. The method according to claim 10, wherein said first and second push-pull dc-ac converter stages include respective first and second resonant filter circuits which are operative to convert first and second generally rectangular wave voltages produced by said first and second push-pull dc-ac converter stages to said first and second sinusoidal voltages.

19. An apparatus for supplying ac power to a high voltage load comprising: first means for driving a first end of said load with a first sinuosoidal ac voltage derived from a dc input voltage; second means for driving a second end of said load with a second sinuosoidal ac voltage derived from a dc input voltage, said second sinuosoidal ac voltage having the same frequency and amplitude as said first sinusoidal ac voltage; and third means for controlling the phase difference between said first and second sinusoidal ac voltages, so as to vary the amplitude of the composite ac voltage differential produced across said first and second ends of said load.

20. The apparatus according to claim 19, wherein each of said first and second means comprises a pair of pulse generators which generate phase-complementary pulse signals of the same amplitude and frequency, and having a 50% duty cycle, said phase-complementary pulse signals being used to control ON/OFF conduction of a pair of controlled switching devices, current flow paths through which are coupled between a reference voltage terminal and opposite ends of a voltage-fed center-tapped primary coil of a step-up transformer, said step-up transformer having a secondary coil thereof coupled to a resonant filter circuit that is operative to convert a generally rectangular wave output produced across the secondary winding of the step-up transformer into a generally sinusoidal ac waveform.

21. The apparatus according to claim 20, wherein said third means is operative to controllably shift the phase of the sinusoidal waveform produced by the resonant filter circuit of one of said first and second means by a prescribed amount relative to the phase of the sinusoidal waveform produced by the resonant filter circuit of the other of said first and second means, so as to modify the amplitude of the composite ac voltage differential produced between said first and second ends of said load.

22. The apparatus according to claim 21, further comprising a voltage-controlled delay circuit which is operative to impart a controlled amount of delay to pulse trains produced by pulse generators of said one of said first and second means relative to the pulse trains produced by pulse generators of said other of said first and second means, said controlled amount of delay between the two pulse trains controlling the amplitude of the composite ac voltage differential produced across the first and second ends of the load.

23. The apparatus according to claim 19, wherein each of said first and second means comprises a pair of pulse generators which generate phase-complementary pulse signals of the same amplitude and frequency, and having a 50% duty cycle, said phase-complementary pulse signals being used to control ON/OFF conduction of a pair of controlled switching devices, current flow paths through which are coupled between a reference voltage terminal and opposite ends of a current-fed, center-tapped primary coil of a step-up transformer, said primary coil being coupled with a capacitor, so as to form a resonant tank circuit therewith, said step-up transformer having a secondary coil that is operative to produce a generally sinusoidal ac waveform.

24. The apparatus according to claim 23, wherein said third means comprises means for controllably shifting the phase of the sinusoidal waveform produced by the secondary coil of the step-up transformer of one of said first and second means by a prescribed amount relative to the phase of the sinusoidal waveform produced by secondary coil of the step-up transformer of the other of said first and second means, and thereby modify the amplitude of the composite ac voltage differential produced between said first and second ends of said load.

25. The apparatus according to claim 24, wherein said third means comprise a voltage-controlled delay circuit which is operative to impart a controlled amount of delay to pulse trains produced by pulse generators of said one of said first and second means relative to the pulse trains produced by pulse generators of the other of said first and second means, said controlled amount of delay between the two pulse trains controlling the amplitude of the composite ac voltage differential produced across said first and second ends of the load.

26. The apparatus according to claim 19, wherein said load comprises a cold cathode fluorescent lamp.

27. The apparatus according to claim 19, wherein said first means is operative to produce a first generally rectangular wave voltage and includes a first resonant filter circuit which is operative to convert said first generally rectangular wave voltage to said first sinusoidal ac voltage, and said second means is operative to produce a second generally rectangular wave voltage and includes a second resonant filter circuit which is operative to convert said second generally rectangular wave voltage to said second sinusoidal ac voltage.

28. A method for supplying ac power to a high voltage load comprised in the steps of:

(a) driving opposite ends of said high voltage load with first and second sinusoidal voltages having the same frequency and amplitude, but a controllable phase difference therebetween; and

(b) controlling the phase difference between said first and second sinusoidal voltages, so as to modify the peak voltage differential between said first and second sinusoidal voltages applied to said opposite ends of said load;

wherein controlling the phase difference comprises applying a delay to pulse trains produced by pulse generators of one dc-ac converter stage coupled to one end of the high voltage load relative to pulse trains produced by pulse generators of another dc-ac converter stage coupled to an opposite end of the high voltage load.

29. The method according to claim 28, wherein step (a) comprises providing first and second dc-ac converter stages which are operative to produce first and second generally rectangular way wave voltages, and which include respective first and second resonant filter circuits that are operative to convert said first and second generally rectangular wave voltages to said first and second sinusoidal voltages.

30. An apparatus for supplying ac power to a high voltage load comprising first and second converter stages which are operative to drive opposite ends of said load with first and second sinusoidal voltages having the same frequency and amplitude, but having a controlled phase difference therebetween, which is effective to vary the amplitude of the composite ac voltage differential produced across the opposite ends of said load.

31. The apparatus according to claim 30, wherein each of the first and second converter stages contains a pair of pulse generators which generate phase-complementary pulse signals of the same amplitude and frequency, and having a 50% duty cycle, said signals controlling a pair of switching devices, the current flow paths between said switches through which is coupled a center-tapped primary coil of a step-up transformer, said transformer having a secondary coil coupled to a resonant filter circuit that is operative to convert a generally rectangular wave output into a generally sinusoidal waveform.

32. The apparatus according to claim 31, wherein the phase of the generally sinusoidal waveform produced by one converter stage is shifted by a prescribed amount relative to the phase of the sinusoidal waveform produced by the other converter stage, so as to modify the amplitude of the voltage differential across said load.

33. The apparatus according to claim 32, further comprising a voltage-controlled delay circuit which is operative to impart a controlled amount of delay to pulse trains of one of said converter stages relative to the pulse trains of the other of said converter stages, said delay controlling the amplitude of the ac voltage differential produced across the opposite ends of the load.

34. The apparatus according to claim 30, wherein each of the first and second converter stages contains a pair of pulse generators which generate phase-complementary pulse signals of the same amplitude and frequency, and having a 50% duty cycle, said signals controlling a pair of switching devices, the current flow paths between said switches through which is coupled a center-tapped primary coil of a step-up transformer, said primary coil being coupled with a capacitor to form a resonant tank circuit, said step-up transformer having a secondary coil that is operative to produce a generally sinusoidal waveform.

35. The apparatus according to claim 34, wherein the phase of the sinusoidal waveform produced by the secondary coil of the step-up transformer of one of said converter stages is controllably shifted by a prescribed amount relative to the phase of the sinusoidal waveform produced by secondary coil of the step-up transformer of another of said converter stages, thereby modifying the amplitude of the ac voltage differential produced between said opposite ends of said load.

36. The apparatus according to claim 35, further comprising a voltage-controlled delay circuit which is operative to impart a controlled amount of delay to pulse trains of one of said converter stages relative to the pulse trains of the other of said converter stages, said delay controlling the amplitude of the ac voltage differential produced across the opposite ends of the load.

37. A method for supplying ac power to a high voltage load comprising:

producing first and second sinusoidal voltages having the same frequency and amplitude;

driving opposite ends of said load with said first and second sinusoidal voltages; and

controlling the phase difference between said first and second sinusoidal voltages, so as to modify the voltage difference between said first and second sinusoidal voltages applied to said opposite ends of said load;

wherein controlling the phase difference comprises applying a delay to pulse trains produced by pulse generators of one dc-ac converter stage coupled to one end of the high voltage load relative to pulse trains produced by pulse generators of another dc-ac converter stage coupled to an opposite end of the high voltage load.

38. The method according to claim 37 wherein the first and second sinusoidal voltages are converted in a first and second converter stage from a first and second rectangular waveform.

39. The method according to claim 38 wherein the first and second converter stage each contain a step-up transformer the secondary coil of which is coupled to a resonant filter circuit which is operative to convert the first and second rectangular waveform to the first and second sinusoidal waveform.

40. The method according to claim 38 wherein the phase difference is controlled by shifting the generally sinusoidal waveform produced by one converter stage by a prescribed amount relative to the phase of the sinusoidal waveform produced by the other converter stage, so as to modify the amplitude of the voltage differential across said load.

41. A system comprising:

a liquid crystal display;

a high-voltage cold cathode fluorescent lamp operative to provide backlighting to the liquid crystal display;

an apparatus for supplying ac power to the cold cathode fluorescent lamp, comprising first and second converter stages which are operative to drive opposite ends of the cold cathode fluorescent lamp with first and second sinusoidal voltages having a controlled phase difference therebetween, the controlled phase difference effective to vary the amplitude of the composite ac voltage differential produced across the opposite ends of the cold cathode fluorescent lamp.

42. The system according to claim 41, wherein each of the first and second converter stages contains a pair of pulse generators which generate rectangular waves of 50% duty cycle, the rectangular waves controlling a pair of switching devices between which is coupled a center-tapped step-up transformer, a resonant filter circuit coupled to the secondary coil of the center-tapped step-up transformer operative to convert the rectangular wave to a sinusoidal waveform.

43. The system according to claim 42, wherein the phase of the sinusoidal waveform produced by one of said converter stages is controllably shifted by a prescribed amount relative to the phase of the sinusoidal waveform produced by secondary coil of the step-up transformer of another of said converter stages, thereby modifying the amplitude of the ac voltage differential produced between said opposite ends of said cold cathode fluorescent lamp.

44. The system according to claim 43, wherein a voltage delay circuit is operative to impart a controlled amount of delay, the delay controlling the amount of the shift between the phase of the sinusoidal waveform produced by one converter stage relative to the sinusoidal waveform of the other converter stage.