A novel low-cost electronic ballast to supply metal halide lamps

This paper presents a novel single-stage electronic ballast with high power factor. The ballast circuit is based on the integration of a buck converter to provide the power factor correction and a flyback converter to control the lamp power and to supply the lamp with a low frequency square-waveform current. Both converters work in discontinuous conduction mode, which simplifies the control. In spite of being an integrated topology, the circuit does not present additional stress of current in the main switch, which handles only the flyback or buck current, depending on the operation mode. In order to supply the lamp with a low frequency square-wave current, to avoid acoustic resonances, the flyback has two secondary windings that operate complementary at low frequency. Results of acoustic resonances excited by the high-frequency ripple in the square waveform applied to the lamp are presented, in order to design the circuit. The design procedure of the converters is also detailed. Experimental results from a 35 W metal halide lamp are presented, where the proposed ballast reached a power factor of 0.956, a total harmonic distortion of 30%, complying with IEC 61000-3-2, and an efficiency of 90%.