The UHP (ultra-high performance) lamp used in projectors is one of the high-intensity discharge lamps that exhibit negative resistance characteristics at high frequency. Therefore, an electronic ballast is used to generate sufficiently high voltage during start up and to provide stable arc current during steady state operation. This paper proposes a PWM variable frequency ballast to shorten the UHP startup period. The ballast circuit is composed of a half bridge series-resonant parallel-load inverter and a single-stage step-up igniter. A 120 W prototype was built to test and verify the characteristics of the proposed UHP lamp PWM ballast at cold start-up, hot re-strike, and steady state conditions. It was found that start-up time was reduced by nearly 50%. In addition, the power converter is designed to operate at zero-voltage-switching to reduce power losses.