Research On A Five-level DC-DC Charging Module

With the promotion of electric vehicles in the field of urban public transportation and the development of power battery fast charging performance,the demand for high-power chargers with rated power of 350 k W and more is gradually increasing.In this thesis,aiming at the application of high power charging for electric vehicles,the overall efficiency of charger/charging station is improved by increasing the voltage level.On the premise of ensuring low voltage stress of power devices,the multi-level DC/DC converter which is appropriate for high DC bus voltage level is studied.LLC resonant soft switching technology combined with the corresponding control strategy is used to realize wide range regulation of output voltage.The following aspects are mainly studied:Firstly,the research status and characteristics of multilevel converters are compared and analyzed,a five-level topology with better comprehensive performance is selected as the research object.Then,this thesis analyzes its working principle and different working modes.Considering the low modeling accuracy of the traditional Fundamental Harmonic Analysis method when it is far away from the resonant frequency,the established mathematical model is modified by combining the time domain analysis method,the related working characteristics of the converter are analyzed.In the meanwhile,the advantages and disadvantages of phase-shifting control strategy and frequency conversion control strategy are compared and analyzed,and a hybrid control strategy is adopted to achieve higher working efficiency in a wide voltage range.Secondly,the influence of resonant parameters on the performance of the converter is analyzed.According to certain design steps,the design of resonant parameters is completed.On this basis,the rationality of parameter design and the feasibility of closed-loop control strategy are verified by MATLAB simulation.Then,the specific hardware circuit design is completed,including the selection of main circuit parameters,the design of inductors,transformers and other magnetic components,as well as the specific design of drive circuit and sampling circuit.By establishing the loss model of the converter,the loss distribution of each part is given.On the basis of hardware circuit platform,the software control system is analyzed and designed based on STM32F103,and the specific implementation scheme of the proposed control strategy is given.Finally,this thesis completes the relevant debugging of the experimental prototype,the key waveforms are tested and analyzed.And the performance of the closed-loop system under different disturbances is tested,which verifies the feasibility of the control strategy of the prototype.