26.1: Low-Frequency Square-Wave Drive for Large Screen LCD-TV Backlighting Systems

Abstract

paper presents a low-frequency square-wave drive, consisting of a single high voltage converter, an ac lamp ignition circuit, current control devices and a single backlight controller, capable of driving an arbitrary number of parallel cold cathode fluorescent lamps (CCFLs) with independent accurate lamp current regulation. Key to the architecture is a proposed capacitive coupling approach for ac lamp ignition that results in reliable, simultaneous ignition of parallel lamps with a maximum ignition voltage near the normal lamp operating voltage. A brief summary of the lamp model and behavior is presented to explain the findings during the capacitive ignition and low-frequency operation. Experimental results are presented demonstrating parallel lamp ignition and current regulation for four, 250 mm CCFLs. 1. Introduction screen LCD TV backlighting systems, generally consisting of 16 or more cold cathode fluorescent lamps (CCFLs), require a ballast capable of driving all or groups of lamps in parallel with accurate current control and regulation. The most common electronic ballasts, based on high frequency LCC resonant inverters, are capable of driving at most four lamps in parallel and require complex multi-winding transformers and specialized control circuitry. This solution suffers from several disadvantages, the major limitation being the inability to simultaneously maintain high efficiency, proper lamp ignition, and individual lamp current regulation (1, 2). We present a suitable architecture, capable of driving a large parallel CCFL array with high efficiency, accurate lamp current control and near operating voltage lamp ignition. The system block diagram, shown in Fig. 1, is based on low-frequency square-wave (LFSW) drive and removes many of the drawbacks associated with a high frequency drive, including energy loss through capacitive coupling, the thermometer effect luminance uniformity degradation and electromagnetic interference (EMI). The proposed architecture is capable of driving a large parallel CCFL array with only a single high voltage and high efficiency converter, resulting in reduced size, weight and cost over existing designs. Reduced ignition voltage (at near operating voltage) is achieved by a unique capacitive coupling approach, based on the similar principles to normal operation of electrodeless or external electrode fluorescent lamps. However, the displacement current is only required in our proposed architecture as a short high frequency pulse during cold lamp ignition, which triggers simultaneous lamp ignition of all parallel lamps. High impedance current source circuit (based on MOSFET current mirror) is connected in series with each lamp to ensure individual lamp current regulation and control. In this paper, we discuss a suitable architecture for large CCFL arrays and study new approaches for lamp ignition to enable low frequency drive.