Research On Bidirectional Symmetric CLLLC Resonant Energy Storage Converter

In order to improve energy utilization efficiency and improve power quality,energy storage technology has been further developed.In the energy storage converter,DC/DC is an important link of energy transfer,so it is very important to improve its working efficiency.By referring to the literature,this paper summarizes and analyzes the characteristics of various bidirectional DC/DC converters,and determines the bidirectional symmetric CLLLC resonant converter as the research target of this paper.Based on the traditional analysis and control methods,the parasitic parameters and efficiency optimization problems are further studied.Firstly,the topology and working principle of the bidirectional symmetric CLLLC converter are analyzed,and a mathematical model is established by using the traditional fundamental analysis method to study the voltage gain characteristics of the converter.The influence of the parasitic junction capacitance of the output switch on the working waveform of the resonant converter is also analyzed,and the voltage gain formulas containing the parasitic junction capacitance of the secondary switch,the distributed capacitance of the transformer and the leakage inductance are deduced respectively.Combined with the specific analysis of parasitic parameters,it is used to guide the parameter correction of voltage gain range deviation in the actual circuit debugging process.Secondly,the optimization conditions of the parameter design of the bidirectional symmetric CLLLC resonant converter are described,and the factors such as efficiency,soft switching,frequency modulation ability and no-load characteristics are fully considered.The process parameter design scheme is given by taking the 2.5k W fullbridge resonant converter as an example.The design and selection of main circuit components such as converter switch tube,high frequency transformer,resonant inductor and resonant capacitor are completed,and corresponding loss model is established.Through the simulation to verify the frequency modulation ability,soft switch implementation and other characteristics,as well as the influence of various parasitic parameters on the working characteristics,the forward and reverse switching simulation verification is completed.Thirdly,the synchronous rectifier control strategy and soft start control strategy based on software control are introduced,and the phase-shifting control strategy of bidirectional symmetric CLLLC resonant converter is analyzed in detail for the full bridge circuit.Combined with the variable frequency control and phase shifting control,the variable frequency phase-shifting piecewise-mixing control based on the expansion of voltage gain range is proposed.The phase-shifting control is used to replace the overresonant region in the original frequency conversion control,and the turn-off loss of the switch tube in the corresponding working region is reduced preliminarily.The effectiveness of the control strategy is verified by simulation.The traditional frequency conversion control inevitably works in the over-resonant region in the two-way operation.It has disadvantages such as large turn-off current of the switch tube on the input side,high switching frequency and large reverse recovery current of the diode on the output side,which results in low overall efficiency in the two-way operation.In order to optimize the bi-directional efficiency of the converter,an improved synchro-hybrid control strategy is proposed,which works forward in the under-resonant region of the frequency conversion control and backward in the quasi-underresonant region of the synchro-hybrid control.The forward and reverse directions have the same zero-current turn-off characteristics.Through simulation calculation of switch loss and comparison with traditional frequency conversion control and DAB loss,it is verified that the improved synchronous hybrid control has higher efficiency in both positive and negative operation.Finally,the resonant converter studied in this paper is verified by experiments.The hardware circuit construction and software design of the 2.5KW full-bridge resonant converter and 500 W half-bridge resonant converter are completed,and the dynamic control strategies mentioned above,such as frequency conversion control,phase shift control and soft start,are verified by experiments.Experimental results show that the improved variable frequency phase-shifting synchronous hybrid control strategy based on efficiency optimization proposed in this paper has the same zero-current turn-off characteristics at the low-voltage side switches when working in the positive and negative directions.