Electronic ballast for HPS lamps with dimming control by variation of the switching frequency. Soft start-up method for HPS and fluorescent lamps

This paper presents the design of a resonant inverter that unifies the two basic functions of an electronic ballast: the generation of an initial overvoltage and current control after start-up. The chosen topology is a half-bridge parallel resonant inverter (PRC); its behaviour in the frequency domain is reviewed when driving a high-pressure sodium (HPS) lamp. The dimming control is performed by shifting the switching frequency of the resonant inverter within the range above resonance. The ignition of the lamp is divided into two periods: start-up time and warm-up time. A soft-start method minimizes the initial overvoltage applied to the lamp, and reduces the current stress on the circuit components. This method is also studied for fluorescent tubes. The experimental results are shown for the commercial HPS lamp OSRAM NAV70E and compared to the results achieved with the fluorescent tube SYLVANIA F-40.