| The dual power supply system is a new type of automobile energy supply system,which can improve the reliability and practicability of the automobile distribution system,and solve the problem that the original single power supply system can not access the high-power power equipment.As the core part of the dual power system,the bi-directional DC-DC converter needs to control the energy flow in the system according to the running state of the generator and the SOC state of the battery.However,the conventional bi-directional DC-DC converter has low power density,it can only step up or decrease voltage in a single direction,which can not adapt to the situation where the range of input and output voltage changes widely.Therefore,the paper takes Four Switches Buck-Boost Converter as the basic topological structure of bi-directional DC-DC converter,and studies its working principle and control method deeply,and makes corresponding experiments and analysis.In order to reduce the input/output voltage/current ripple and improve the power density of the converter,a bi-directional DC-DC converter based on two-phase interleaved parallel technology is studied.The working modes and waveforms of the converter in Buck mode,Boost mode and Buck-Boost mode are analyzed.The inductance current rippleΔiL1,ΔiL2 and charging current rippleΔicharge1of the converter in different working modes are calculated.According to the specific parameters of the converter,the energy storage inductor,input and output filter capacitor are designed,and the type of switching device is selected.In order to improve the dynamic performance of bi-directional DC-DC converter and meet the demand of precise control of voltage and current under constant current and constant voltage charging mode,the control strategies of the converter under different working modes are studied.The AC small signal model and control block diagram of bi-directional DC-DC converter are analyzed and established,and from the point of view of the optimal robust performance and dynamic response performance of the system,the closed-loop controller of voltage and current in boost and buck modes is designed.The simulation and experimental results show that the controller of the inner current loop and the outer voltage loop has better robust performance and dynamic response speed,whether the system works in constant current charging mode or constant voltage charging mode.In order to avoid the voltage and current spikes of bi-directional DC-DC converter during mode switching and affect the reliable operation of the system,the mode switching method between different working modes of the converter is studied.This paper analyzes and designs the operation process of the software control system of bi-directional DC-DC converter,expounds the basis for selecting the charging direction of the converter and the judgment conditions for the switching of constant current and constant voltage charging mode,and studies the control strategy for the smooth switching of Boost and Buck modes of bi-directional DC-DC converter.According to the results of the simulation and experimental,the control strategy realizes the judgment between different charging modes,and solves the problem of discontinuous output voltage gain when the Boost and Buck modes are switched.In order to verify the rationality of the scheme,a 1k W bi-directional DC-DC converter battery charging and discharging experimental platform is built.The hardware circuit of the bi-directional DC-DC converter is designed,and the efficiency test under the three working modes and the charging and discharging curve of the battery and lithium battery are carried out.The result of experimental indicate that the minimum efficiency of bi-directional DC-DC converter is 93.6%at full load,which can achieve constant current and constant voltage stage charging control strategy and meet the actual design requirements.The bi-directional DC-DC converter studied in this paper realizes the controllable bidirectional flow of energy between the original vehicle battery and the lithium battery,which lays the foundation stone for the coordinated operation of the vehicle dual eletrical supply system. |
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