Electronic dimming methods for solid state electronic ballasts

Abstract

A circuit and method for controlling a power supply are disclosed, with specific application to dimming of a resonant inverter solid-state fluorescent lamp ballast. The control circuit produces a pulse train signal with pulses of variable duty cycle. The signal is integrated to provide a DC signal which the solid-state ballast uses to control the level of dimming of the lamp. When the duty cycle of the pulses is increased, the level of the DC signal increases, so that the ballast dims the lamp. Conversely, reduction of the pulse duty cycle increases the brightness of the lamp. The control circuit incorporates delay circuitry which suppresses the pulse output at powerup, so that the lamp starts at full intensity. Thereafter, the delay circuitry adjusts the pulse signal so that the lamp intensity is adjusted smoothly to the desired level. A reset circuit resets the delay circuitry in case of a momentary power failure so that the lamp will restart at full intensity and then smoothly dim to the desired intensity, rather than starting at a low intensity. A brightness control circuit allows the user to set the desired light intensity, and an adjustable pulse control circuit allows limitation of the maximum amount of dimming. Overcurrent circuitry disables pulse output if excessive current is drawn from the circuit.