Research On Optimal Design And Synchronous Rectifier Of LLC Resonant Converter

LLC resonant converter can achieve zero-voltage switching (ZVS) for primary side switches and zero-current switching for secondary side rectifiers, so it features simple structure and high efficiency. And it is widely adopted in front-end DC/DC converters in recent years. Nowadays, with the tendency of high power density and high efficiency in power supplies, the LLC resonant converter is even more attractive and applied to more and more applications. At the same time, new problems about optimal design and synchronous rectifier also appear.High frequency transformer is the key component in the LLC resonant converter. This paper mainly focuses on the optimal design of the planar transformer for the LLC resonant converter. Different from conventional power transformer, the transformer in LLC resonant converter is a combination of inductor and transformer. The primary side current and secondary side current is not in phase, which affects the power loss of the windings. In this paper, the effects of winding structure and air gap on winding loss are analyzed respectively. Based on the loss analysis, some optimization method to design a LLC planar transformer are presented and discussed. Finite element analysis (FEA) simulation is used to aid the theoretical analysis. A prototype is built to verify the analysis and FEA simulation.The theoretical analysis for the capacitive switch in load transient and startup of LLC resonant converter is firstly introduced in this paper. With the simulation, the principle of working point variation in load transient is illustrated. And the discriminance for capacitive switch in load transient is also proposed. Further more, the requirement of ensuring inductive switch during startup is also proposed. The analysis and calculation are verified by simulation and experiment.Synchronous rectification is also a big challenge for the LLC resonant converter. This paper also proposes a novel current driving scheme for synchronous rectifier (SR) in the LLC resonant converter with a voltage doubler rectifier to further improve the performance of the LLC resonant converter. In the proposed current driving scheme, a simple current transformer (CT) with only one secondary winding is used to drive two SRs in the voltage doubler rectifier. Furthermore, the reflected current from the CT will feed to the output. Therefore, the proposed current driving scheme features simple structure, low power loss and good performance. The working principle and design consideration is given. And a prototype is built to verify the theoretical analysis.