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In this paper, a simple and effective way of increasing the efficiency of a three-stage low-frequency dimmable high-intensity-discharge (HID) electronic ballast is proposed. To increase the efficiency at all dimming levels, a modified synchronous-buck converter operating in a new critical conduction mode is presented. The proposed modified topology of the synchronous-buck converter operates in a soft-switching mode and therefore significantly increases the efficiency of the three-stage low-frequency HID electronic ballast at all dimming levels. Analytical and experimental results for a dimmable 400-W HID lamp are presented.