Accurate Analysis and Design of the Circuit Parameters of LLC DC–DC Converter With Synchronous Rectification

In high-load-current LLC dc–dc converter, synchronous rectification (SR) is usually used to reduce the secondary loss. As a lossless, low-cost, and small-volume SR control strategy, the drain–source voltage of SR switches sensing method is used widely in industrial applications. However, when the load current increases, the voltage ringing across SR switches during O stage is more and more severe and reaches zero eventually. When the voltage ringing reaches around zero in LLC converter, the voltage-sensing-method SR controller will generate the false driving signal, which makes SR switches turn on early and causes the converter works abnormally. This article presents the accurate analysis and design of the circuit parameters of LLC dc–dc converter to prevent the voltage ringing across the SR switches reaches around zero. The voltage ringing model is established, and the parameters design principle is given. By using the proposed design guideline, the voltage ringing across SR switches will always be higher than zero over the full load range, which eliminates the turn-on early issue or turn-on several times in one cycle issue. Therefore, this article can provide a design reference and guideline for the high-load-current and high-output-power LLC converter with voltage sensing SR control strategy. A 2.16 kW SR LLC dc–dc converter prototype is built according to the proposed design guideline, and the experimental results verify the correctness and effectiveness of the analysis.