Ballast with filament heating and ignition control

Abstract

A ballast () for powering at least one gas discharge lamp () having heatable filaments () includes an inverter (), a resonant output circuit () coupled between inverter () and lamp (), and a filament heating and ignition control circuit () coupled to inverter () and resonant output circuit () having a first resonant frequency and a second resonant frequency, wherein the first resonant frequency is substantially greater than the second resonant frequency. Filament heating and ignition control circuit () controls inverter () and resonant output circuit () during a preheat phase and during a normal operating phase. During the preheat phase, resonant output circuit () has an effective resonant capacitance corresponding to the first resonant frequency, and provides a first level of heating to the lamp filaments (). During the normal operating phase, resonant output circuit () has an effective resonant capacitance corresponding to the second resonant frequency, and provides a second level of heating to the lamp filaments () that is negligible in comparison with the first level of heating. Control circuit () monitors a voltage within resonant output circuit () in order to compensate for any variation in the parameters of resonant output circuit () and to ensure that an appropriate level of preheating is provided to lamp filaments ().