High Frequency Electronic Ballast for Metal Halide Lamps Based on a PLL Controlled Class E Resonant Inverter

In present paper, the operation of a class E resonant inverter used to supply a low-wattage metal halide lamp at 1 MHz is analyzed. A single LCC series-loaded resonant tank is used to provide the ignition voltage and also to supply the lamp at steady state. A low-cost PLL based control circuit is used to provide a constant phase shift between the output voltage and the MOSFET control pulses. This strategy allows obtaining a nearly constant output power against strong changes in the lamp equivalent resistance. The design of the resonant tank can be made to assure ZVS operation during the warm-up phase and also to avoid high voltage across the switch. The proposed circuit also provides a fast lamp warm-up providing full light output a few seconds after ignition. At the end of the paper, the theoretical results are compared with the experimental measurements made using a laboratory prototype