Design And Research On DC-DC Converter For Electric Vehicles

Increasingly serious air pollution and increasing environmental pressure have accelerated the pace of energy structural reform,and reducing the use of fossil energy has become an effective way to solve environmental problems.With the gradual introduction of electric vehicles into public life,the bidirectional DC-DC converter,as the core of its power system,can complete the high-power charging and discharging of the supercapacitor and ensure the stability of the bus voltage through bidirectional power conversion,which enables electric vehicles to switch power quickly and stably during startup,acceleration,deceleration and braking.It can be seen that the bidirectional DC-DC converter has a great influence on the battery life and power performance of the electric vehicle.Therefore,the output power,dynamic characteristics and response speed should be mainly considered when designing the bidirectional DC-DC converter.In view of the above problems,the development history and research status at home and abroad of electric vehicles and bidirectional DC-DC converters for automobiles are firstly analyzed in the thesis,and then the development trends are discussed.The structural characteristics and composition principle of electric vehicle power system are studied,and the application advantages of multi-energy source hybrid energy storage system are analyzed based on storage battery and super capacitor,as well as the application requirements of bidirectional DC-DC converter in hybrid energy storage system.Secondly,the topology,working principle,performance advantages and disadvantages of the basic bidirectional Buck/Boost converter are studied.And based on this a bidirectional DC-DC converter for electric vehicle is designed.In order to meet the high power demand of electric vehicles,reduce the electrical stress of the device,improve the power density and optimize the efficiency,a 30 kW bidirectional DC-DC converter power circuit is designed by using interleaved parallel Buck/Boost topology.Aiming at the problem of phase-to-phase current shift when Buck/Boost converters are running in parallel,the method of external characteristic droop,automatic current sharing and master-slave setting are studied,and the current sharing control circuit is designed by using the automatic current sharing chip through performance analysis.In order to drive the IGBT tube reliably,the totem pole circuit is designed to increase the driving current,which can realize the safe driving.Meanwhile,the circuit parameters are calculated and the device selection is completed.Then,equivalent models of circuit components averaging method based on non-ideal active devices in different modes of the converter are established,including the circuit equivalent under large signal analysis and dynamic small signal disturbance,and various transfer functions are solved.Accroding to the control targets and objects in different modes of bidirectional DC-DC converters,closed-loop control networks based on different controllers are designed.By comparing its output characteristics,the system adopts digital fuzzy PI controller and further proposes a control mode with operation mode switching logic to realize good and stable power switching of the bi-directional DC-DC converter in different modes.Finally,the simulation model of bidirectional DC-DC converter is built by Simulink software platform.The simulation results show that the Buck/Boost converter running in parallel has good performance under the staggered conduction control,with small output ripple and large transmission power.Moreover,the converter based on fuzzy PI control has good dynamic stability and short response time.In order to verify the correctness of the proposed method,an experimental test platform is built to test the load characteristics,output waveform and transmission efficiency under different modes.The test results are basically consistent with the simulation results,indicating that the circuit design of the bidirectional DC-DC converter is feasible and has practical application value.