Digital synchronous rectification controller for llc resonant converters

This paper introduces a robust, hardware efficient, mixed-signal control loop that governs the switching actions of the secondary-side synchronous rectifier (SR) switches of LLC resonant converters. The new SR control method minimizes switching and conduction losses of the SR switches through online optimization of their on-off timing using an auto-tuning process that takes into account the effect of parasitic elements, mainly leakage inductances. In this controller, the information from body diode conduction detection circuits across the SR switches and switching frequency available from digital controller are utilized by a digitally implemented auto-tuning algorithm to determine optimal switching times to achieve zero current switching. In comparison with the existing SR controllers the introduced solution has more precise driving scheme. Moreover, the controller requires simple hardware implementation. Experimental results obtained with a 350 W, 400 V to 12 V, isolated LLC experimental resonant converter, verify the operation of the introduced SR driving scheme and show efficiency improvement over the whole operating range resulting in up to 9% reduction of converter power losses during light load operation.