Comprehensive Analysis of Synchronous Rectifying Signal Delay of High-Frequency LLC Resonant DC/DC Converter

LLC resonant dc/dc converters are widely applied in various scenarios, such as electronic device power supplies, vehicle chargers, etc. To improve the efficiency of the LLC converter, synchronous rectifiers (SR) are employed. However, due to various reasons, such as the hardware propagation delay, the SR gate signal can lag behind the secondary-side current particularly when the switching frequency of the LLC converter approaches the mega-Hertz level. This delay has a critical impact on the operation mode and the output characteristic of the LLC converter. Starting with the SR delay mechanism, this article comprehensively analyzes the operation of the LLC converter and derives the time-domain model under SR signal delay. The output gain and accurate power loss model of MOSFETs under different working modes are built to study the impact of SR signal delay on the LLC converter. The relationship between the SR signal delay and the LLC converter characteristics is quantified and summarized. Two new working stages of the LLC converter are revealed. Simulation and experimental studies are conducted to validate the theoretical analysis results. This article can provide the theoretical basis for optimizing the design of a high-frequency LLC converter.