A Feed-forward Scheme For An Electrolytic Capacitor-less AC/DC LED Driver To Reduce Output Current Ripple

Light Emitting Diode (LED) as a new light source with high efficiency, long lifetime, small size,rich color and dynamic control etc, is widely used in all kinds of lighting field. The key toguaranteeing the LED quality and performance is a high reliability, high efficiency, high power factor,low cost and long lifetime LED driver. This thesis is dedicated to the design of a control scheme for aproposed flicker-free electrolytic capacitor-less ac/dc LED driver to reduce the output current ripple.The flicker-free electrolytic capacitor-less ac/dc LED driver comprises an electrolytic capacitor-lessflyback PFC converter and a buck/boost bidirectional converter. The buck/boost bidirectionalconverter serves to absorb the ac current component in the output current of the PFC converter.Consequently, the driving current for LED is a pure dc current, and no flicker occurs. To removeelectrolytic capacitor of the bidirectional converter, the capacitor is intentionally designed to have alarge voltage ripple to reduce the capacitance. The large voltage ripple will cause that the trackingability of inner current loop of the bidirectional converter with dual closed-loop control is reduced.Then, the steady-state error of the bidirectional converter input current tracking and the ripple ofdriving current for LED increase. Therefore, a feed-forward control scheme for the flicker-freeelectrolytic capacitor-less ac/dc LED driver is proposed. The modulation wave signal producing theduty cycle of the bidirectional converter is provided by feed-forward path, so the bidirectionalconverter closed-loop control effect is reduced. As a result, the input current steady-state error ofbidirectional converter decreases and the driving current ripple is reduced. According to the principleanalysis, the feed-forward control scheme circuit is designed and simulation results are presented toverify the effectiveness of this method.Based on the theoretical analysis above, a33.6W prototype has been built and tested in the lab.Experiment results are presented to verify the effectiveness of the feed-forward control scheme inthe flicker-free electrolytic capacitor-less ac/dc LED driver for reducing the LED driving currentripple and eliminating flicker.