Hybrid control system in an efficient LED driver

This paper presents a unique hybrid control system arising from an efficient multi-string LED driver with pulse-width-modulated (PWM) dimming scheme. The LED driver is based on a single inductor multiple output boost converter. It has two levels of switching, a high frequency switching whose duty cycle is the control input of the boost converter, and a low frequency switching whose duty cycle adjusts the dimming ratio. Due to the time-sharing scheme for multi-channels, feedback control for one channel is applied during only a portion of the dimming period. The switching and the control scheme yield several operation modes which are described by continuous-time systems. As a result of PWM dimming, one of the states periodically jumps to a fixed value and the stability needs to be examined as that of a discrete-time system. The control objective is to ensure stability and to minimize the deviation of LED current from the rated value by choosing circuit and feedback control parameters. Through detailed state-space analysis in continuous-time and discrete-time, this paper examines conditions for stability, evaluates the LED current deviation and utilizes the relationships for the design of circuit and control parameters. Computational results, simulation and experimental waveforms validate the analytical results and effectiveness of the control scheme.