Design approach for the cold cathode fluorescent lamps backlight inverters with lamp current balancing chokes

Parallel resonant ballast inverters are often used to drive fluorescent light application including backlight for the cold cathode fluorescent lamps (CCFL) for liquid crystal display (LCD) units. Besides the traditional requirements of high efficiency, low EMI noise and dimming capability, there are two important requirements. Those are the array light level evenness and the quality of the lamp current waveform. The former is critical especially for very large screen display where CCFL are still attractive. And the latter affects the lifespan of the lamps if the harmonic content of the lamp current is high. Both of these performances are affected by the parasitic elements of both the lamp and the balancing chokes. The issue has never been tackled from an overall point of view. In this paper, theoretical model will be developed and design procedure proposed to determine the critical design parameters. Experiments based on a laboratory 200W prototype were conducted to verify the performance of the proposed circuit model. Although CCFL lamp was used for illustration and experimentation in this paper, the results of this paper are applicable to other lamp that use resonant circuit and inductor balancing chokes in the ballast circuit.