an ac - ac ballast system for a discharge lamp , which includes a pfc converter which incorporates an isolation transformer , and a dc - ac inverter provided on the secondary side of the isolation transformer . because the ac line input is isolated from the lamp load by the transformer in the pfc converter , the switching frequency of the pfc converter can be advantageously significantly higher than the lamp current frequency , to thereby enable a significant reduction in the size and weight of the ballast system , without an increase in the emission of emi radiation from the lamp . in this regard , the ballast system of the present invention can be thought of as a two - frequency ballast system having an isolated pfc converter .