Research On L-L Type Current-Fed Bidirectional DC-DC Converter

Energy storage systems are currently widely used in distributed micro-grids and new energy vehicles.Distributed micro-grids require energy storage systems to address the randomness and uncertainty of their power gen eration.New energy vehicles require batteries to provide power and,if necessary,become part of a distributed generation network to supply power to the grid.The main purpose of this dissertation is to study and design a converter with the advantages of wide input range,low input voltage,high input current and high voltage gain,which is suitable for energy storage system,based on existing control methods.The L-L type current-fed bidirectional DC-DC converter selected in this dissertation has two input inductors.Compared to the full-bridge topology,its voltage gain is higher and leakage current peak is smaller.Conventional single phase-shift control produces large loop currents at light loads condition and increases converter losses.The zero current single phase-shift control not only has large loop current problem,but also has leakage inductor current resonance.The leakage inductor current RMS is relatively large in dual phase-shift control.To solve the problems,this dissertation proposes a hybrid modulation based on the existing modulations.There are two modes of this hybrid modulation,Mode I and Mode II.When the load is high,the converter works in Mode II,and the leakage inductor current peak is controlled by the duty cycle of the high-voltage side switch.When the current drops to zero,keep one of the high-voltage side switches turned on to prevent the leakage inductor current from resonance.When the load is light,the converter works in Mode I.The peak value of the leakage inductor current is controlled by the phase shift angle of the converter.Therefore,the problem of loop current and leakage inductor current resonance is solved.In this dissertation,the working modes of L-L type current-fed bidirectional DC-DC converter with hybrid control method is analyzed.The characteristics of the boosting capability and soft switching range of the converter are compared under different control methods.The simulation model was built in Simulink,and the simulation results verified the correctness of the proposed control method.The converter parameters were designed and the experimental prototype was built.Experimental results show that the converter can achieve soft switc hing in wide input range and wide load range.The hybrid control method can effectively reduce the converter loop current and solve the leakage current resonance problem.