A novel electrode power profiler for dimmable ballasts using DC link voltage and switching frequency controls

A novel electrode power profiler for dimmable ballasts using a series-resonant parallel-loaded inverter is presented. Dimming of fluorescent lamps and control of the electrode power are achieved by simultaneously adjusting the DC link voltage and the switching frequency of the inverter. The current gradient along the electrode, which is due to the distributed lamp current property on the electrode, is deliberated in formulating the power and voltage models of the electrode. As the electrode heating relies on two near-orthogonal current components including the lamp current and the resonant tank circuit current, a simple feedback control is developed to derive the electrode power. Reduction of the electrode power (due to the decrease in the lamp current) during dimming is compensated by increasing the switching frequency of the inverter, in order to increase the resonant tank circuit current. Experimental results of a T8 36 W prototype are verified with theoretical predictions.