TRIAC dimmable electronic ballast with lamp power equalization

Abstract

A two-stage, two-wire TRIAC dimmable electronic ballast for fluorescent lamps is presented in this paper. It is constructed by using a flyback converter as the input power factor corrector to supply a half-bridge series-resonant parallel-loaded inverter to ballast the lamp. The flyback converter is operated in discontinuous conduction mode so that the filtered input current profile is same as the TRIAC-controlled voltage waveform. The switches in the inverter are switched at a constant frequency slightly higher than the resonant frequency of the resonant tank. Based on the constant average input current characteristics of the inverter, dimming operation is simply achieved by pulse-width-modulation control of the magnitude of the flyback converter output voltage. No synchronization network is required between the input and output stages. In addition, a novel power equalization scheme is developed so that the dc link voltage (and hence the lamp power) is in linear relationships with the firing angle of the TRIAC. The average output voltage of the dimmer controls the equalized flyback converter output voltage. Modeling, analysis, and design of the ballast will be described. A prototype has been implemented to verify the experimental measurements with the theoretical predictions.