Patent ID: 7843139

Claim:
An electronic ballast for driving a gas discharge lamp having a plurality of lamp filaments, the ballast comprising: an output circuit operable to receive a high-frequency AC voltage and comprising an inductor; a plurality of filament windings magnetically coupled to the inductor, each of the plurality of filament windings connectable to at least one of the plurality of filaments of the lamp and operable to supply an AC filament voltage to one of the plurality of filaments; a control winding magnetically coupled to the inductor; a controllably conductive device having a control input and first and second terminals coupled such that the controllably conductive device is operable to control a magnitude of a voltage across the control winding; and a control circuit coupled to the control input of the controllably conductive device to selectively render the controllably conductive device to be conductive and non-conductive, so as to control the magnitude of the voltage across the control winding; wherein the control circuit is operable to control the controllably conductive device to adjust the magnitude of the voltage across the control winding to be equal to approximately zero volts, such that each of the plurality of AC filament voltages has a first magnitude, the control circuit further operable to control the controllably conductive device to adjust the magnitude of the voltage across the control winding to be greater than zero volts, such that each of the plurality of AC filament voltages has a second magnitude greater than the first magnitude, so as to heat the filaments of the lamp.