Current-equalization techniques for electronic ballasts

This paper presents active current-equalization (CE) techniques for equalizing the lamp currents of multi-lamp electronic ballasts. Multi-lamp products, such as the backlighting for liquid crystal display (LCD) panels, have to be capable of equalizing luminance. Due to the deviations of lamps and ballasts, the currents flowing through the lamps are not equal, which causes poor-quality backlight sources for LCD panels. Additionally, since the size of thin-film transistor LCD (TFT-LCD) monitors is becoming larger, a multi-lamp drive system must provide higher levels of voltage and current and more power for longer length lamps and more lamps. Since the number of lamps that can be driven by one inverter is limited, distributed multi-ballast drive systems are becoming popular. In order to achieve current equalization for distributed multi-ballast drive systems, the active CE technique is proposed to equalize every output current of the ballast. In this paper, the three-ballast drive system, for distributed single-lamp inverters, is implemented with the active CE technique. In addition, the CE ability in dimming range is demonstrated by experiments in order to verify the performance of the proposed active CE circuit scheme.