A single-stage high-power-factor electronic ballast with auto frequency searching capability provides a compact and efficient solution for ballasting metal halide lamps. The circuit configuration is originated from the integration of a buck-boost converter and a half-bridge resonant inverter. The buck-boost converter is designed to operate in discontinuous current mode (DCM) to improve the input power factor and at the same time to regulate the output lamp power. The resonant inverter operating at a high frequency is adopted to obtain a high efficiency on the power conversion circuit. The control strategy of auto frequency searching is realized by a microprocessor along with the acoustic resonance detection circuit. To avoid the acoustic resonance, an auto-frequency-searching method is used to search ¡§quite windows¡¨ on operating metal halide lamps with the high-frequency electronic ballast. Provided the acoustic resonance should happen to the lamp on operation, the electronic ballast will automatically change the operating frequency until a stable frequency is located. When the operating frequency has been changed, the duty-ratio of the buck-boost converter is adjusted to regulate the lamp power at the rated value. Experimental tests are carried out on a laboratory with 70-W metal halide lamps to verify the effectiveness of the auto-frequency- searching control.