Patent ID: 8847512

Claim:
An electronic ballast comprising: an inverter circuit comprising a power supply having positive and negative output terminals, a controller, and a pair of switching elements coupled between the positive and negative terminals of the power supply, the switching elements responsive to control signals from the controller to oscillate at an operating frequency and to generate an output voltage at first and second inverter output terminals between the switching elements; a main inverter tank coupled to the first inverter output terminal; a filament heating circuit further comprising a first capacitor coupled to the second inverter output terminal, a primary winding of a filament heating transformer coupled on a first end in series with the first capacitor, a second capacitor coupled to the second end of the primary winding, and a clamping circuit coupled to the second capacitor and effective during a preheat mode of operation to clamp an amplitude of the voltage across the primary winding of the filament heating transformer to an amplitude of the input voltage from the power supply.