Research On Bi-directional DC-DC Converter Of Auxiliary Power System For Fuel Cell Vehicles

With the rapid development of fuel cell vehicles,people pay more and more attention to the power electronics technology applied to fuel cell vehicles.Due to the weak output characteristics of the fuel cell,the voltage drastically decreases when vehicles start and accelerate which results in a lower mobility of the automobile.As a new type of energy storage device,the super capacitor has the characteristics of large power,fast charging and discharging speed.What is more,it can be recycled many times.As an auxiliary power supply,the bidirectional DC-DC converter is connected with the fuel cell,which not only makes up for the shortcomings of the weak output characteristics of the fuel cell,but also realizes the recovery of the braking energy and improves the energy utilization efficiency.This paper focuses on the development of bi-directional DC-DC converters for auxiliary power system of fuel cell vehicles,and analyzes their topological structure,mathematical model and control strategy respectively.Finally,the hardware and software of the control system are designed.First of all,the paper analyzes the selection of the bi-directional DC-DC converter topology,and then studies the topology of the converter from two aspects.At the same time,relevant design requirements are put forward to the research object of the paper.By comparing the advantages and disadvantages of the isolated and non-isolated topology structures,the non-isolated topology structure is selected according to the requirements of this paper.Then the voltage stress and current stress of the four common non-isolated bidirectional DC-DC converters are analyzed.The bi-directional half-bridge Buck-Boost topology is used as the main circuit topology of this thesis.Secondly,the working principle of the selected bi-directional half-bridge Buck-Boost topology is introduced.The state space averaging method is used to mathematically model the Boost mode and the Buck mode respectively,and their respective small signal models are established.Because PI control is widely used to compensate for the rapidity and stability of the system in the past,this paper combines the voltage and current input and output characteristics of bidirectional DC-DC converters based on PI control,and adopts different methods for different operating modes.The control method adopts voltage and current double closed-loop control in Buck mode to realize the constant voltage charging of the super capacitor.At the same time,the inner current loop limits the size of the current,prevents the current from being too large,and damages the switching elements;in the Boost mode,Using the method of compound control,it is possible to dynamically select its output characteristics and to achieve automatic switching of output.Through Simulink simulation of the different operating modes of the converter,it is found that the stability and dynamic response capabilities of the converter are improved,and the effectiveness and feasibility of the design are verified.Finally,in order to realize the control strategy of the bidirectional DC-DC converter,the DSP is taken as the control core,the control system of the control system is designed withhardware and software,the design of the main functional circuit is introduced,and the implementation is explained in the software design introduction.The flow of energy flows in both directions.