Patent ID: 7839094

Claim:
A lamp ballast comprising: an inverter portion for receiving a direct current input from a DC bus and converting the direct current input into an alternating current output; a resonant portion that receives the alternating current from the inverter portion and supplies the alternating current to a plurality of lamps; and a filament transformer for providing a preheat current to cathodes of the lamps during a preheat phase, the filament transformer comprising: a primary winding connected to a common node between the inverter portion and the resonant portion, a first set of secondary windings inductively coupled to the primary winding of the filament transformer that apply the preheat current to the cathodes of the lamps, and a second set of secondary windings that drive transistors of the inverter to a frequency that is higher than a resonant frequency of the resonant portion during the preheat phase.