New control strategy in a square-wave inverter for low wattage metal halide lamp supply to avoid acoustic resonances

High intensity discharge (HID) lamps are the most suitable when both high light power levels and good color rendering are researched. Some remarkable theoretical advantages would be obtained if these kinds of lamps were driven with high frequency (HF) electronic ballasts. Nevertheless, at these frequencies, acoustic resonances (AR) phenomenon appears. Low wattage metal halide (MH) lamps are, among all HID lamps, the most affected by this phenomenon. To obtain lack of AR in HID lamps, lamp power frequency harmonics must have a lower amplitude than a certain threshold value, related to activation energy. An electronic ballast may be designed to guarantee power harmonics levels lower than this threshold level at nominal operation. Nevertheless, the equivalent resistance of HID lamps varies highly in starting and warm-up stages, as well as in the aging process. Therefore, the system must be designed to be adequate for the necessary "supplying conditions" for these load variations. In this paper, a new control strategy for a HF square-wave inverter is presented. This ballast keeps lamp power harmonics at low levels, regardless of the lamp equivalent load. The proposed ballast, along with the control strategy, is presented, as well as the built prototypes and experimental results carried out. From these experiments, it can be concluded that the proposed method supplies low wattage MH lamps with lack of acoustic resonances.