LAUSR

To achieve ultra-deep dimming ratio in LED driver application, the resonant process of LLC resonant converter considering the critical parasitic components has been analyzed in detail with time-domain solution in this paper. The effects of three parasitic components, including the equivalent parasitic capacitor Cp in parallel with the transformer, the equivalent output capacitor Ca of the bridge switches and the parasitic capacitor CL of resonant inductor, on the resonant current during the dead-time and switching frequency under light-load condition have been taken account. Brief equivalent circuits during the resonant process of LLC resonant converter and corresponding mathematical functions based on time-domain analysis are presented. After that, accurate frequency curve of the converter and required dead-time in variation with output current have been calculated out. Theoretical analysis show that the LLC resonant converter can achieve ultra-low dimming ratio without entering the burst mode with appropriate external capacitor in parallel with the output of the bridge and zero voltage switching (ZVS) of switches by self-adjusting the dead-time. Finally, an experimental prototype has been built up to verify the theoretical analysis and 0.5% LED dimming ratio has been achieved. The theoretical analysis results meet the experimental results very well, which demonstrates that the time-domain solution presented in this paper can achieved very high accuracy.