Research Of Efficiency Improvement And High Frequency Technology For Bidirectional CLLC Resonant Converter

With the continuous development of renewable energy power generation,as the main carrier of its access to power grid,DC microgrid has become the inevitable development direction of power grid technology in the future.In DC microgrid architecture,high-power bi-directional DC/DC converter plays a key role in charging energy storage medium and regulating DC bus power,which is crucial to the efficient and reliable operation of the whole DC microgrid.Bidirectional CLLC resonant converter has become one of the preferred topologies for bidirectional DC/DC converter due to its excellent soft switching performance and high conversion efficiency.However,in the practical design and application of bidirectional CLLC converter,there are the following difficulties:(1)there are many resonant elements with high degrees of freedom,and in order to take into account bidirectional operation,it is difficult to design resonance parameters with better performance;(2)Due to the bidirectional operation of the converter,the traditional rectification scheme which uses low-cost and highly reliable synchronous rectifier driver chip is relatively complicated;(3)The converter using Si MOSFET is restricted by the device itself in the aspect of frequency increase,which hinders the further improvement of power density.Therefore,the parameter design,synchronous rectification strategy and high-frequency technology of bidirectional CLLC converter are deeply studied and discussed in this paper.In this paper,the working mode of bidirectional CLLC converter under pulse frequency modulation is firstly introduced,the equivalent analysis of the converter is carried out by using fundamental wave analysis method,and the voltage gain expression is deduced.On this basis,the influence of each parameter in the gain expression on the gain curve is studied in detail,and the constraint conditions of parameter selection are summarized based on the principle of safe,correct and efficient operation of the converter,and a complete set of parameter design flow is given.Then by combining with the requirements of the project,the CLLC converter with switching frequency of 100 k Hz is studied and designed.In order to realize high efficiency in forward operation mode(high voltage to low voltage),a digital synchronous rectification control strategy based on drain-source voltage detection of rectifier is designed.Its control principle and algorithm are described in detail.The theoretical prototype of 1k W bi-directional CLLC converter built in the laboratory verifies the correctness and effectiveness of the parameter design and synchronous rectification strategy.The maximum efficiency of the prototype is 97.3% in forward operation and 95.7% in reverse operation.Further,in order to reduce the volume of magnetic components and improve the power density of the converter,based on the above digital synchronous rectification strategy,the high frequency implementation technology based on GaN HEMT with high FOM and planar transformer technology is studied,and the switching frequency level of the converter is increased to 500 k Hz.Based on the detailed design of the drive circuit of GaN HEMT and transformer winding structure,the paper finally developed an experimental prototype of a bidirectional CLLC converter with 1k W output power and 500 k Hz switching frequency level.The volume of its magnetic components is significantly smaller than before.The experimental results show that the hardware design of the converter is reasonable,the digital control synchronous rectification strategy is still suitable at high frequency,and the maximum efficiency of the prototype is 95.3% and 91.6% respectively in forward and reverse operation.