Patent ID: 8324829

Claim:
A high pressure discharge lamp ballast comprising: an inverter having a plurality of switching elements and an output coupled to a high pressure discharge lamp; a series resonant circuit coupled to the inverter output and to the high pressure discharge lamp, the series resonant circuit comprising a resonant capacitor and a resonant inductor, the resonant inductor further comprising an autotransformer having a primary winding, a secondary winding and a center tap, the resonant capacitor coupled to the center tap, the primary winding coupled between two of the plurality of switching elements, and the secondary winding coupled to the lamp and a control circuit coupled to the switching elements, the control circuit functional to operate in an ignition mode having one or more time periods during each of which a control operation is performed comprising sweeping a driving frequency of the switching elements through a predetermined range of frequencies and then controlling the switching elements at a first driving frequency less than the predetermined range of frequencies, and in a preheating mode wherein the switching elements are controlled at a second driving frequency less than the predetermined range of frequencies.