Patent ID: 7560868

Claim:
A ballast for powering at least one gas discharge lamp having first and second lamp filaments, the ballast comprising: an inverter operable to provide an inverter output voltage having an operating frequency; a resonant output circuit coupled between the inverter and the lamp, the resonant output circuit being characterized by having a first resonant frequency and a second resonant frequency, wherein the first resonant frequency is substantially greater than the second resonant frequency; a filament heating and ignition control circuit coupled to the output circuit and to the inverter, wherein the control circuit is operable to control the inverter and the resonant output circuit such that: (a) during a preheat phase, the resonant output circuit: (i) has an effective resonant capacitance corresponding to the first resonant frequency; and (ii) provides a first level of heating to the first and second lamp filaments of the at least one gas discharge lamp; (b) during a normal operating phase following the preheat phase, the resonant output circuit: (i) has an effective resonant capacitance corresponding to the second resonant frequency; and (ii) provides a second level of heating to the first and second lamp filaments of the at least one gas discharge lamp, wherein the second level of heating is negligible in comparison with the first level of heating; wherein the control circuit is further operable: (a) to monitor a voltage within the resonant output circuit; (b) in response to the monitored voltage reaching a specified level, to provide the preheat phase wherein the operating frequency of the inverter is maintained at a first present value for a predetermined preheating period; and (c) upon completion of the preheat phase, to provide the operating phase wherein the operating frequency of the inverter is allowed to decrease from the first present value for purposes of igniting and operating the lamp.