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This paper proposes a novel single-stage high-power-factor electronic ballast for metal halide lamps. By integrating two buck-boost typed power-factor-correction converters, a full-bridge inverter and a bidirectional buck converter, a single-stage electronic ballast with symmetrical circuit topology is derived. The circuit component count can be effectively reduced and circuit energy-conversion efficiency is improved. With a tactful control scheme, the proposed electronic ballast can output a low-frequency square-wave voltage to drive metal halide lamps. The problematic phenomena of acoustic resonance can be eliminated. A high power factor at the input line is assured by operating the buck-boost converters in discontinuous conduction mode. The operation modes, design equations and design steps for the circuit parameters are proposed. A prototype circuit designed for a 70-W metal halide lamp was built and tested to verify the analytical predictions. Satisfactory performances are obtained from the experimental results.