Patent ID: 7586268
Filing Date: 2009-09-08
Classification: H05B

Abstract:
1. 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 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 to be non-conductive; wherein when the controllably conductive device is non-conductive, each of the plurality of AC filament voltages has a first magnitude, and when the controllably conductive device is conductive, each of the plurality of AC filament voltages has a second magnitude, the control circuit operable to render the controllably conductive device to be non-conductive when an intensity of the lamp is below a predetermined threshold and to render the controllably conductive device to be conductive when the intensity of the lamp is above the predetermined threshold.