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We investigated ballast for electrodeless lamps operating at 13.56 MHz. Higher-efficiency ballast circuits are required bacause of problems at high frequency. Our investigation focused on class-D circuits. We applied numerical model analysis and conducted experiments to determine the conditions that maximize efficiency. These conditions occur when the output current is simultaneous zero and the voltage across the switching device is virtual zero. We found that the virtual zero of the voltage is the crossing point between the zero line and the extrapolation line of the voltage slope.