Apparatus and method for controlling the filament voltage in an electronic dimming ballast

A circuit for an electronic ballast for controlling a plurality of AC filament voltages provided to a plurality of filaments of a gas discharge lamp, the circuit comprising:

• a plurality of filament windings magnetically coupled to an inductor of an output circuit of the ballast, the plurality of filament windings each connectable to one of the plurality of filaments of the lamp and each operable to provide one of the plurality of AC filament voltages to one of the plurality of filaments;
• a control winding adapted to be 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 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.

A method for controlling a plurality of AC filament voltages provided to a plurality of filaments of a gas discharge lamp in an electronic ballast comprising an output circuit including an inductor, the method comprising the steps of:

• magnetically coupling a plurality of filament windings to the inductor,
• connecting each of the filaments of the lamp to one of the plurality of filament winding;
• providing each of the plurality of filaments with one of the plurality of AC filament voltages;
• magnetically coupling a control winding to the inductor;
• adjusting a magnitude of a 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;
• and
• adjusting 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.