Optimization of universal ballasts through magnetic regulators

Nowadays there are few commercial types of ballasts able to control fluorescent lamps with different power ratings. One of the main issues is how to get optimum operation for each lamp in this condition. This paper tries to demonstrate how to accomplish this objective using a magnetic regulator, i.e. using a variable inductor. This variable inductor is controlled by a dc current delivered by a forward converter directly supplied from the ballast dc bus voltage. This control current allows changing the resonant tank, adapting it to the working parameters imposed by each lamp, ensuring near resonance working conditions. Design criteria and practical verifications are included to confirm this new technique. Theoretical predictions are verified with the experimental results for three TLD Philips fluorescent lamps, from 18 W to 58 W.