Patent ID: 7772783

Claim:
A dimmable electronic ballast for an electrodeless discharge lamp, comprising: an inverter circuit that, in response to a drive signal having a variable drive frequency, converts the DC power from a DC power supply circuit into the high-frequency power having the operation frequency corresponding to the drive frequency; a resonance circuit that receives the high-frequency power and then produces high-frequency resonant power based on a resonance characteristic, said resonant power being variable output corresponding to the operation frequency; an induction coil that receives the high-frequency resonant power and then generates a high-frequency electromagnetic field to apply the electromagnetic field to the electrodeless discharge lamp; and a start circuit having a variable time constant, said start circuit sweeping the drive frequency through a time constant for start or restart so that the voltage applied across the coil by the high-frequency resonant power is raised from voltage lower than start voltage and restart voltage for starting and restarting the lamp to voltage higher than the start voltage and the restart voltage; wherein the time constant for start during a start period for starting the lamp at the beginning of dimming is larger than the time constant for restart during a restart period for restarting in the dimming period.