Research On Bi-directional DC-DC Converter And Its Control Strategy

With the rapid advance of renewable energy,the bi-directional DC-DC converters have been widely used in more and more applications like lithium-ion energy storage systems.The bi-directional DC-DC converters with wide input voltage range that are used to lithium-ion battery charging applications have been researched.The two stage bi-directional DC-DC have been adopted,which includes two parts,LLC resonant converter and Buck/Boost converter.As a former stage,the LLC resonant converter is a symmetrical type.The equivalent circuit of FHA is analyzed,and the working principle,as well as gain characteristics,are introduced.The implementation scheme of the soft switching scheme of Buck/Boost converter have been analyzed,so that the working mode of the inductor current zero crossing is established.On this basis of this,the opening condition of the ZVS switch tube is deduced.The two stage converter combines the advantages of Buck/Boost and full-bridge LLC resonant converter.In the guarantee of the bidirectional flow of energy,the converter will not only meet the requirement of wide voltage ratio range,but also keep the soft switching character of the LLC resonant converter.The parameters design of the LLC+Buck/Boost bi-directional DC-DC converter are studied,which mainly include the selection of high frequency transformer ratio,resonant element,filter capacitor,filter inductance,vibration capacitor and power device.The control strategy of the bidirectional DC-DC converter is analyzed.The open-loop constant frequency control is adopted in the LLC converter,which can realize the voltage matching and isolation function.Reasonable design of the double closed-loop control is adopted in the Buck/Boost converter,which is responsible for reducing the voltage,regulating the output voltage and current,as well as charging-discharging of the battery.In the environment of MATLAB/Simulink simulation,the simulation model of the two stage LLC+Buck/Boost converter is built.The simulation work is carried out under different input voltage.Based on the analysis of simulation results,correctness and feasibility of the two stage bidirectional DC-DC converter have been verified.