A Compact Electrolytic-Free Two-Stage Universal Input Offline LED Driver With Volume-Optimized SSC Energy Buffer

This paper presents a single-phase, two-stage electrolytic-free offline LED driver that utilizes a hybrid film-ceramic stacked switched capacitor (SSC) energy buffer in place of limited-life electrolytic capacitors for twice-line-frequency energy buffering. The proposed LED driver comprises a front-end power factor correction (PFC) stage, followed by an isolated dc-dc conversion stage, with an SSC energy buffer connected across the intermediate dc bus. For the design of SSC energy buffer, an improved capacitance ratio optimization methodology is presented that incorporates the energy densities of commercially available capacitors and minimizes the passive volume of the buffer. Compared to a single capacitor, the optimized SSC energy buffer, comprising three switches, reduces the passive volume of the energy buffer by greater than 40%. A simple and robust control scheme is also proposed to interface the SSC energy buffer with the PFC converter. A prototype 300-W LED driver based on the proposed architecture, designed for universal input voltage range (90-305 V
_rms
) and 12-V output voltage is built and tested. The SSC energy buffer achieves a peak efficiency of 98.5% and maintains an efficiency of above 96% across a wide output power range, thus minimally impacting the overall system efficiency. Finally, the impact of the intermediate dc-bus voltage ripple on overall LED driver efficiency is investigated. It is found that there is negligible change in the efficiency when the intermediate dc-bus voltage ripple ratio is varied between 3% and 7.5%.