Control Techniques for a Current-Mode-Controlled Merged-Energy-Buffer-based Two-Stage Electrolytic-Free Offline LED Driver

This paper presents improved control techniques for a current-mode-controlled offline LED driver that uses a merged energy buffer for twice-line-frequency energy buffering. The LED driver has two power conversion stages: a four-switch buck-boost PFC stage and an LLC resonant dc-dc stage. To achieve a high power density, the PFC stage uses a small inductance, making its current control loop design challenging. A systematic methodology for the design of this control loop that ensures adequate bandwidth, phase margin and low-frequency gain is presented. The small capacitors and the switching action of the merged energy buffer cause wide variations and abrupt jumps in the input voltage of the dc-dc stage. To maintain a tightly regulated LED current, a feedforward-enhanced feedback controller is designed that leverages a computationally efficient converter model. These control strategies are experimentally validated through a 150-W electrolytic-free offline LED driver prototype, which achieves a high power density of 50 W/in
³
.