Analysis and design of a HID lamp ballast with sinusoidal waveform superposed with 3rd harmonic

This paper deals with the supply of HID lamps at high frequency avoiding acoustic resonances in the arc discharge. With the proposed ballast the lamp is supplied with a sinusoidal waveform superposed with third harmonic, thus approximating to the square waveform but reducing the harmonic content. The proposed topology is based on two half bridges operating at frequencies equal to the fundamental and third harmonics, plus a simple LLC resonant tank. In this way, the two harmonics, fundamental and third, are superposed in the lamp current. The complete analysis of this new power topology is also performed in the paper, obtaining both output voltage and switch currents. A design procedure for a 35 W metal halide lamp is also presented. The experimental results obtained from the prototype probe that lamp operation is stable at frequencies where acoustic resonances were previously observed supplying with pure sinusoidal waveforms.