New HF square-waveform ballast for low wattage metal halide lamps free of acoustic resonances

High intensity discharge (HID) lamps are, among all artificial light sources, the most suitable when it comes of getting both high light power levels and good color rendering. In low wattage metal halide (MH) lamps, acoustic resonances (AR) become a very important drawback for the use of high frequency (HF) electronic ballasts. In this paper, a new HF ballast for MH lamps is presented. This ballast provides a HF square-waveform to the lamp, avoiding the appearance of AR. It operates from European AC mains (230 _VRMS, 50 Hz), and it has been designed to meet the European standard IEC-61000-3-2. Also, a complete analysis of the circuit operation is done. Nevertheless, in HID lamps, the HF equivalent resistance of the lamp varies highly in starting and warm-up stages, as well as in the ageing process. This variation was reviewed in the paper, in order to fix the design properly. This way, the proposed ballast deals with load variations, ensuring nominal average power regardless of lamp equivalent load. Built prototypes and experimental results carried out are also shown in this digest.