Use of saturable inductor to improve the dimming characteristics of frequency-controlled dimmable electronic ballasts

This paper presents an investigation into the use of saturable inductor to improve the dimming characteristics of frequency-controlled dimmable electronic ballasts with a half-bridge series-resonant parallel-loaded inverter. The concept is based on the fact that the effective resonant frequency increases as the inductor current increases, if the unsaturable inductor in the resonant tank circuit is replaced with a saturable one. Susceptibility of the lamp power to the switching frequency variation at low luminous level can be reduced. This can lessen the problem of lamp flickers at dimmed level and result in a wider dimming range and improved controllability. The method gives a simple solution of improving the dimming characteristics without changing the circuit structure. A fundamental frequency model that includes the characteristics of the nonlinear lamp resistance and filament resistance is applied in the analysis. A PSIM simulation model for the saturable inductor is developed. Theoretical predictions are verified with the experimental results of a 36-W T8 prototype.