Patent ID: 8659233

Claim:
A ballast for operating one or more fluorescent lamps, the ballast comprising: a DC power circuit operative to receive an AC input and to produce a DC output; an inverter operatively coupled to the DC power circuit to convert the DC output to produce an inverter output to power at least one fluorescent lamp, the inverter including a frequency control circuit operative to control a frequency of the inverter output; and a preheating circuit operatively coupled with the inverter to modify at least one impedance in the frequency control circuit to control the frequency of the inverter output to be in a first range during a preheating period following application of power to the DC power circuit to preheat at least one cathode of the lamp using power from the inverter output and to control the frequency of the inverter output to be in a different second range following ignition of the lamp; where the frequency control circuit of the inverter includes a frequency control capacitor with a first terminal connected to control terminals of a first and a second inverter switching device, and a second terminal, and where the preheating circuit is operative to modify a capacitance value of the frequency control circuit to control the frequency of the inverter output.