Research On Bidirectional Series Resonant Converter With Frequency Tracking Control

With the construction of smart grid and the renovation of traditional power grid,the solid state transformer plays an important role in smart grid,which can not only provide simple and stable grid-connection for a variety of AC and DC power supplies,but also improve the power quality of the grid.In many solid state transformer topologies,the three-stage topology can realize bidirectional energy flow,simple control,complete function and modular cascade with a good symmetry,so it has been applied more widely.In this paper,the DC/DC isolation stage of the three-stage topology is studied,and the bidirectional series resonant converter is selected as the subject investigated.First of all,the paper analyses the working principle of bidirectional series resonant converter when it works forward and reverse under different operating frequency.Based on this,the voltage gain,impedance characteristics,no-load and short circuit situation of the converter are studied by the fundamental wave analysis.Besides,the conditions of realizing the power switching devices’ soft switching are discussed.Because the influence of the converter’s parasitic parameters under high frequency operation cannot be ignored,different parasitic parameters of the converter are taken into consideration,and the methods to reduce those influences are put forward.When the operating frequency of the resonant converter is equal to the resonant frequency,the combination of the resonant inductor and capacitor is equivalent to the short circuit.Under this condition,the topology of the converter is completely symmetrical and the efficiency is highest.Therefore,the frequency tracking control is necessary.This paper analyzes five methods of frequency tracking control as the phase-locked-loop(PLL),zero current time detection,current harmonic detection,virtual current sensor,and maximum output voltage tracking control.And the advantages and disadvantages of each method are compared.Then the maximum output voltage tracking method is selected and verified by simulation and experiment.Finally,according to the technical parameters,the power circuit of the converter is designed in detail,and a prototype of 1kW series resonant converter has been developed.The rationality of the practical applications is verified by PSIM simulation and experiments on the prototype.