US Patent 7239094 Electronic ballast with adaptive lamp preheat and ignition

An electronic ballast includes a microcontroller with software to provide an adaptive lamp preheat and ignition operation. The microcontroller commands a test frequency from the inverter and detects the frequency response of the resonant output circuit by measuring the voltage across the resonant capacitor. The measured voltages are compared to one or more reference voltages as the frequency is varied to select the optimal inverter frequency. An algorithm or look-up table is used to set the inverter frequencies for the lamp preheat and ignition phases.