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 series-resonant parallel-loaded inverter is presented. Dimming of the fluorescent lamp is primarily based on reducing the dc link voltage. Variations of the electrode power (due to the decrease in the lamp current) during dimming is compensated by changing the switching frequency of the inverter, so that the resonant tank circuit current is also varied. 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 and program the electrode power. Experimental results of a T8 36 W prototype are verified with theoretical predictions.