Low Frequency Architecture for Multi-Lamp CCFL Systems with Capacitive Ignition Monm Doshi (I), Jianjian Bian ( I ) , Regan Zane ( I ) and Francisco J. Azcondo [ I ) Colorado Power Electronic Center (CoPEC) Department of Electrical and Computer Engineering University o f Colorado at Boulder Boulder, CO 80309-0425 regan.zane@colorado. edu
(’) University of Cantabria Electronics Technology, Systems and Automation Engineering Department Ave. de 10s Castros Sin 39005 Santander, Spain
[email protected]
Absfruci - This paper presents a low frequency architecture for driving parallel cold cathode fluorescent lamps (CCFts) in large screen LCD ’I’V backlighting applications. Key to the architecture is a proposed capacitive coupling approach for ac lamp ignition. The system consists of a single high voltage converter, an ac lamp ignition circuit, current regulation devices and a single primary controller. The topology is capable of driving an arbitrary number of parallel lamps with independent accurate lamp current regulation, while maintaining high efficiency and achieving significant size, weight, and cost reduction when compared to typical high frequency ac ballast designs, Experimental results for a pair of parallel 800 V 40 cm CCFLs demonstrate simultaneous ignition and dc current regulation.
I.
INTRODUCTION
The popularity of thin, light weight, wide screen televisions has resulted in tremmdous interest and development of large screen liquid crystal display (LCD) TVs. The increase in LCD screen size has created a significant demand for longer CCFL designs and parallel architectures suitable for efficient drive of large CCFL arrays with high luminance uniformity and long life [l-31. High frequency LCC resonant inverters, based on push-pull (Royer oscillators) or bridge (full or half) topologies are commonly used as electronic ballasts for powering single and dual CCFL backlighting systems [4-81. The resonant circuits, generating sinusoidal waveforms of 25-1 00 kHz,have high losses associated with high frequency capacitive coupling, resonant circulating currents and also cause luminance uniformity degradation due to thermometer effect [9-121. As the number of lamps per system grows (some estimates greater than 40), it will not be feasible to provide an individual LCC drive per lamp due to the significant increase in size, weight, cost, complexity of enclosure design and losses [Ill. One solution is to drive multiple lamps with a single LCC ballast. However, it is generally not possible to simultaneously maintain high efficiency, proper parallel lamp ignition, and individual lamp current reguration in such designs. In this paper, we present a low frequency architecture
suitable for high efficiency drive of large CCFL arrays. The low frequency approach removes many of the challenges and drawbacks associated with high frequency drive, including capacitive coupling, thermometer effect luminance uniformity degradation, and electromagnetic interference (EMI). The architecture i s capable of driving a large parallel CCFL array with only a single high voltage converter, resulting in reduced size, weight, and cost over existing designs. Lamp ignition in the low frequency system relies on a unique capacitive coupling approach, resulting in near operating voltage lamp ignition for smooth operating waveforms and long lamp life. While capacitive coupling is based on similar principles to normal operation of electrodeless and extemal electrode fluorescent lamps (EEFLs) [13-181, displacement current is only required in our proposed architecture as a short pulse during cold lamp ignition. The architecture is based on a very low frequency drive, a capacitively coupled lamp ignition circuit and low voltage self-biased current limiting (CL) devices as shown in Fig I . Low frequency square wave electronic baIlasts, based on synchronous buck converter or
This work is co-sponsored by National Semiconductor Corporation (through CoPEC), the National Science Foundation (under Grant No. 0348772), and the Spanish Government (through project CICYT TEC 2004-02607iMIC).
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We experimentally tested our theory on lamp ignition by gradually applying dc voltage to lamps of varying length up to the rated lamp r m s voltage. As expected, no lamps achieved ignition under these conditions. We then wrapped a small copper foil around the center of the lamp, and applied a high voltage, high frequency drive to the copper foil (just above rated lamp operation) with the lamp electrodes grounded. This resulted in lamp ignition and full operation in shorter lamps (
