Optimization Research Of DC-DC Converter In Solar-powered Aircraft Energy System

Since the 21 st century,the consumption and pollution problems of traditional fossil energy are becoming increasingly serious,which makes renewable energy attract worldwide attention,among them,solar energy is favored by many countries as a widely distributed clean energy source,and solar-powered aircraft that use it as a power source can complete ultra-long flight at high altitudes.With the continuous development of solarpowered aircraft,new requirements for light weight,high efficiency and high reliability are put forward for the DC-DC converter in its energy system,and the Four-switch BuckBoost(Four-switch Buck-Boost,FSBB)converter has the advantages of the same input and output voltage polarity,less passive components,and low voltage stress of the switch tube,which is suitable for wide input voltage occasions,therefore,this paper selects it as the DC-DC converter in the solar-powered aircraft energy system.Based on the above background,this paper mainly studies the control strategy,modeling and closed-loop control system design of the FSBB converter,so that the converter has high efficiency and good dynamic response in the full input voltage range,so as to achieve the purpose of optimizing the FSBB converter.Firstly,this paper studies the optimization of the control strategy of the FSBB converter with the high efficiency of the whole machine as the optimization goal.Analyzes in detail the two control strategies widely used in the converter control,namely dual-edge modulation and fixed phase-shift angel control of quadrilateral inductor current.By synthesizing the respective advantages of the two control strategies,the variable phase-shift angle control strategy is deduced,so that no matter which mode the converter operates in,the inductor can circulate with the minimum current,and the inductor current ripple can be minimized at the same time,thus effectively reducing the rms value of the inductor current.In order to further observe the realization of soft switching of the converter under the variable phase-shift angle control strategy,each operating mode in one switching cycle under different operating modes is analyzed,and according to the working time of each main mode and the corresponding inductor voltage,the gain expression between the input and output voltage of the FSBB converter under the variable phase-shift angle control strategy is deduced.On this basis,the LTspice simulation model of the FSBB converter is built for verification.Secondly,this paper studies the optimization design of the closed-loop control system of the FSBB converter with improving the system stability as the optimization goal.Considering that the Boost/Buck modes under the variable phase-shift angle control strategy contain three operating modes in one switching cycle,the small-signal model of the three-mode converter is deduced in this paper.Then,based on the derivation results,the small-signal modelings of the FSBB converter in Boost/Buck mode are carried out respectively,and the corresponding open-loop transfer functions are solved,furthermore,the Bode diagrams of the respective mode are drawn by MATLAB software,and the stabilities are analyzed according to the stability margin criterion.In order to make the system work stably,this paper designs the loop compensation based on the results of stability analysis combined with the current popular stability criterion,and use the bilinear transformation method to discretize the transfer function of the designed compensation system,so as to realize the compensation function of the original loop through DSP programming.Finally,this paper studies the optimization design of each part of the hardware circuit in the system with the light weight of the whole machine as the optimization goal.According to the circuit design indicators,the main circuit parameters,control circuit and auxiliary power supply in the system are designed in detail,so a 30～60V input,45 V output and 200 W rated power experimental prototype is built,and its specific power is about 1370W/kg,and the working waveforms of the FSBB converter under different control strategies and the realization waveforms of soft switching under the variable phase-shift angle control strategy are tested respectively,and the efficiency curves under different control strategies are drawn.The experimental results show that the peak efficiency of the variable phase-shift angle control strategy can reach 98.1% at full load,which verifies the feasibility and effectiveness of its application in the FSBB converter.