Capacitance Reduction in Flicker-Free Integrated Offline LED Drivers

This article presents a deep analysis of the low-frequency ripple transferred from the bus voltage to the light-emitting diode (LED) current in integrated two-stage power converters, operating as offline LED drivers. This analysis presents a design methodology to select the optimum power control stage converter to the LED load and to reduce the bulk capacitance. The main focus is to analyze the ripple transfer ratio of the three main nonisolated dc-dc converters, namely buck, boost, and buck-boost. Moreover, this article presents a step-by-step case study for an existing converter and the proposed converter supplying a 26.9-W Luxeon 3014 LED load. The experimental results show that the integrated buck and boost converter requires a bus capacitance of 1200 μF, whereas in the integrated buck and buck-boost converter, the capacitance is reduced to 680 μF by applying the method proposed. This represents that the replacement of a boost converter by a buck-boost converter on the second stage allows a 50% reduction of the bus capacitance while keeping the same current ripple.