Efficiency Improvement of Single-Stage AC-DC LLC Converter Using a Line Cycle Synchronous Rectifier (SR) Driving Strategy

Synchronous rectification that is being widely used in high-power Inductor-Inductor-Capacitor (LLC) DC-DC converters to improve efficiency can be challenging in single-stage AC-DC LLC converters with high output voltage levels (i.e., >100V) where synchronous driving ICs cannot be used. In this paper, a simple line cycle SR driving strategy with direct MCU control is proposed for single-stage AC-DC LLC converters. The principles of operation and methodology behind the proposed line cycle SR driving strategy are discussed. Simulation and experimental results validated the performance of the proposed SR driving strategy for a 250 V to 400 V output voltage range AC-DC LLC converter. A full-load efficiency of 98.1 % was achieved for the 250 V output voltage condition and a full-load efficiency of 97.3 % was achieved for the 400 V output voltage condition. Compared with a fixed ON time method, around 0.5 %, and 0.8 % efficiency improvements were observed in 250 V and 400 V output voltage conditions, respectively. In addition, it is observed that the proposed simple SR driving method obtains the same efficiency levels as more complex adaptive SR driving approaches.