Research On CLLLC Resonant Bidirectional DC/DC Converter

In recent years,as the global energy crisis is becoming very serious,more and more new energy power generation systems are used in social production and daily life.In order to solve the problem of unstable power generation of photovoltaic,wind and other new energy systems,it is necessary to use the energy storage system to maintain the stability of power quality by bidirectional DC/DC converter.Meanwhile,bidirectional DC/DC converter has wide range of applications in electric vehicles,DC distribution network and aerospace power supply system.How to improve the efficiency and power density of bidirectional DC/DC converter,realize wide range voltage regulation and fast and smooth switching of forward and reverse energy transmission has become the research hotspot of bidirectional DC/DC converter in recent years.There are many topologies of bidirectional DC/DC converters,among which resonant DC/DC converters have better soft switching characteristics.In this thesis,CLLLC resonant converters are studied in depth with the application background of energy storage system in new energy power generation system.Firstly,the advantages and disadvantages of various bidirectional DC/DC converter topologies and control strategies are compared and analyzed.The bidirectional CLLLC resonant converter is selected as the research object,and a novel synchronous frequency conversion control strategy is proposed.By synchronously changing the switching frequency of the primary and secondary side switches,the forward and reverse energy flow of the converter is smoothly switched.Meanwhile,the converter is guaranteed to have a relatively wide range of voltage gain regulation,and all switches have excellent soft switching characteristics.The working principle of the converter in different states is analyzed,a more accurate fundamental equivalent circuit model and a simplified timedomain analysis method are proposed.The voltage gain formulas under different states of the converter are deduced respectively.The soft switching conditions of CLLLC converters controlled by synchronous frequency conversion under different states and the transmission characteristics of smooth switching of forward and reverse energy flow directions are analyzed in detail.Secondly,a energy storage system of new energy generation is taken as an example,from the perspectives of voltage gain,frequency conversion range,soft switching conditions and so on.The resonant parameter design scheme of the synchronous inverter CLLLC converter is given.According to the voltage gain characteristics of the converter and the power transmission characteristics of biditection smooth switching,the closedloop and energy commutation control scheme of the converter is presented.The selection of power components such as switching device and resonant capacitor of the converter and the design of magnetic components such as transformer and inductor are completed..Finally,PSIM simulation software is used to verify the working principle and soft switching characteristics of the converter.A laboratory prototype with input voltage of100 V,output voltage of 400 V and power of 1kW has been built.The experimental results show that: compared with the traditional single-side frequency conversion control method,the CLLLC resonant converter controlled by synchronous frequency conversion can achieve wider voltage gain adjustment range.All switches have good soft switching characteristics,which can achieve higher efficiency.Moreover,the power transmission direction of the converter can be changed quickly and smoothly only through the frequency conversion.The experimental results verify the correctness of the theoretical analysis in this thesis.