Analog versus Digital

W hen considering alternative approaches to the design of dimmable electronic ballasts for fluorescent lamps, the lamp power regulation method employed is the critical factor. It determines the linearity (i.e., the linear response of the light output with the dimming control input), the smoothness of dimming and, most importantly, the stability of the light output. Both digital and analog approaches are possible. The method most often used in dimming ballasts involves controlling the lamp power by modulating the frequency of the switched dc bus. This is done through a series inductor and a parallel resonant output capacitor network. Increasing the resonant circuit’s frequency reduces the lamp current and vice versa (Fig. 1). Pulse-width modulation of the half-bridge voltage also is used in some dimming designs to widen the dimming range. However, this is not usually necessary.