A ballast having a rapid inverter frequency control circuit that is used
for programmed operation during lamps starting, during normal operation
of all lamps and during no lamp(s) operation. A method and circuit
controls frequency shift via a combination of different feedback
circuits. A control signal for the inverter controller is synthesized as
a sum of phase shifted inverter output voltage feedback signal and lamp
current sense signal indicating status of each lamp connected to the
ballast output.