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In this paper, a new dimmable electronic ballast for metal-halide (MH) lamps based on a three-stage structure is proposed. The presented dimmable electronic ballast can dim smoothly without acoustic resonance from 50% to 100% brightness. A novel current-control method with constant-power capability is also presented to solve the power-drifting problem resulting from the V- I characteristic variations between lamps or caused by the aging of lamp. The dimming capability allows the ballast to adjust its working condition according to the environments or the dimming command; this further improves the energy efficiency. Finally, a 150-W MH lamp ballast prototype is built to verify the feasibility of the proposed methods.