Single Switch Off-Line Ballast With PFC for Low Metal Halide Lamps

This paper proposes a novel complete electronic ballast for Low Wattage Metal Halide lamps. The lamp driver is based on the integration of a flyback PFC input stage and a buck-boost inverter stage. The resulting circuit supplies the lamps properly, operating from the mains facility. The main feature of the proposed ballast is that through an adequate integration of the stages, the ballast operates with only one controlled switch. This switch is shared by the active PFC input stage, by the output high-frequency inverter stage as well as by the igniter. The control of the ballast is carried out by adjusting the maximum current through the switch and by fixing the duty ratio. Thus, the resulting output characteristic is suitable for a proper operation of these discharge lamps, including ignition, warm-up stage and steady- state operation. The obtained high frequency square waveforms in the lamp avoid the apparition of acoustic resonances. A complete prototype has been built and tested, and experimental results are shown.