Patent ID: 8378579

Claim:
A ballast circuit capable of reducing a filament heating voltage below a desired maximum voltage level when heating a lamp filament of a gas-discharge lamp, comprising: an inverter having at least one inverter switch device for converting a DC voltage into an AC voltage to power the gas discharge lamp; an inverter controller operable to control a switching frequency of the inverter switch device, the inverter controller having a feedback terminal for receiving an overvoltage control signal and being responsive to adjust the switch frequency of the inverter switch device to reduce a signal level of the overvoltage control signal; a filament heating component coupled to the inverter to receive the filament heating voltage, the filament heating component comprising a filament heating winding; and a control loop operably associated with the filament heating component and the feedback terminal of the inverter controller, the control loop including a voltage regulator that is operable to cause the transmission of the overvoltage control signal when a voltage level of the filament heating voltage is at or above the desired maximum voltage level, a secondary winding magnetically coupled to the filament heating winding to receive a feedback control signal associated with the filament heating voltage, and a high pass filter coupled between the voltage regulator and the secondary winding, the high pass filter having a corner frequency selected such that the feedback control signal is blocked during steady state operation.