New control strategy in square-wave inverters for low wattage metal halide lamps supply to avoid acoustic resonances

High intensity of discharge (HID) lamps is the most suitable lamps when it comes to get both high light power levels and good colour rendering. In HID lamps, a lot of theoretical advantages are obtained if they are supplied with high frequencies electronic ballasts: higher efficiency, optimisation of system cost, weight and size, control and remote operation capability, etc. Nevertheless, at these frequencies, acoustic resonances appear in this kind of lamps. In low wattage metal halide lamps, the risk of appearing acoustic resonances is the highest among all HID lamps. To obtain a lack of acoustic resonances in HID lamps, lamp power frequency harmonics must have low amplitude, lower than a certain threshold value, related to activation energy. A supply system can be designed in order to guarantee, at nominal operation, this low power harmonics levels. Nevertheless, in HID lamps, the equivalent resistance of the lamp varies highly in starting and warm-up stages, as well as in the ageing process. So, the system must be designed to adequate the supplying conditions to this load variations. In this paper a new control strategy for high frequency (HF) square-wave inverters is presented, in order to keep this lamp power harmonics in low levels, regardless of the lamp equivalent load. The proposed ballast is presented, as well as the built prototypes and experimental results carried out.