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The paper proposes a robust resonant ignition technique for low frequency square wave (LFSW) ballasts without requiring additional igniter components or increase in component size or weight. The LC filter of the LFSW ballast is used at start-up in a resonant mode for lamp ignition. In order to achieve the high voltage gain required for lamp ignition, significantly higher current and volt-seconds are imposed on the filter inductor during resonant ignition than in steady-state LFSW operation. The proposed solution uses a Kool Mu soft saturation inductor core material to prevent core saturation during ignition without requiring an increase in core size or weight from the design for steady-state LFSW operation. A digital phase controller is used during the ignition sequence to gradually sweep the phase lag of the inductor current from 90deg down towards resonance. This results in a robust ignition sequence while maintaining zero voltage switching despite variations of inductance value in the presence of soft saturation core dynamics. Experimental results are presented to demonstrate reliable ignition of 150 W metal halide (MH) and 150 W high pressure sodium (HPS) lamps using the proposed techniques.