Single-Switch Offline Ballast With PFC for Low-Wattage Metal Halide Lamps

This paper proposes a novel complete electronic ballast for low-wattage metal halide (LWMH) lamps. The lamp driver is based on the integration of a flyback power factor correction (PFC) input stage and a buck-boost inverter stage. The resulting circuit supplies the lamps with a high-frequency (HF) square current waveform, operating from the mains facility. The main feature of the proposed ballast is that through an adequate integration of the stages, the circuit operates with only one controlled switch. This switch is shared by the active PFC input stage, by the output HF 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 HF square waveforms in the lamp avoid the apparition of acoustic resonances. A complete prototype has been built and tested, and experimental results are shown.