| Under the background of vigorously pushing forward the reform of energy structure and advocating the conversion of new and old kinetic energy,new energy distributed generation,electric vehicles and other fields have developed rapidly.Among them,the bidirectional DCDC converter plays an irreplaceable role in the energy conversion between the DC bus and the energy storage system.Therefore,it is of great significance to carry out an in-depth study on the bidirectional DC-DC converter to improve its performance and transmission efficiency.In this thesis,firstly,the topology of bidirectional DC-DC converter is investigated and classified,and CLLLC bidirectional resonant converter is selected as the research topology.The current research work of CLLLC bidirectional resonant converter is summarized from four aspects: analysis method,control strategy,soft start and commutation control,efficiency improvement technology.Secondly,the working principle and characteristics of CLLLC bidirectional resonant converter in different working range are analyzed.First harmonic approximation(FHA)is used to model CLLLC bidirectional resonant converter,and its characteristics are studied comprehensively.In addition to the basic voltage gain characteristics,the monotonicity of voltage gain,impedance characteristics and efficiency transmission characteristics are also obtained.At the same time,the influence of resonance network parameters on the characteristics of the converter is analyzed in detail.Based on the results of the characteristic analysis,two extremely important design points are obtained: the minimum switching frequency and the maximum switching frequency.Taking the voltage gain and zero voltage switching(ZVS)of two points as the limiting conditions and optimizing the transmission efficiency as the goal,a design method for resonant network parameter that realizes soft switching and high efficiency transmission within the full load range is proposed.This method is a sequential design,without iteration and simulation trial.According to the design example,the design of resonance network parameters and the selection of components are completed.In addition,in order to solve the problems of zero current switching(ZCS)loss,excessive switching frequency,narrow voltage gain range and low efficiency in the overresonance interval of the CLLLC bidirectional resonant converter,the control strategy of variable-frequency and phase-shift is proposed.The control strategy can not only make full use of the advantages of wide voltage gain range and high efficiency of variable-frequency control,but also solve the shortcomings of variable-frequency in over resonance range.The rationality of resonance point as switching point of variable-frequency control and phaseshift control is analyzed in detail,and the realization method of variable-frequency and phaseshift control is proposed.At the same time,it is concluded that the parameters of resonance network can be further optimized by using variable-frequency and phase-shift control.Finally,an experimental prototype of CLLLC bidirectional resonant converter is made.The experiment verifies the voltage gain and soft switching characteristics of the converter under different load and input voltage when using variable-frequency control and phase-shift control.The transmission efficiency is compared,which proves the correctness of the proposed design method of resonant network parameter and the feasibility of the control strategy of variable-frequency and phase-shift. |
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