Research On Multi-Mode Smooth Switching Strategy Of Isolated Buck-Boost Converter

The DC distributed power system with the intermediate bus structure requires the intermediate bus converter to work efficiently and stably in a wide input voltage range,and has the function of electrical isolation.The four-switch buck-boost（FSBB）converter,which has been widely studied and applied,has the following advantages:it can boost or decrease the input voltage;the input voltage and the output voltage have the same polarity;and the voltage stress of the switch is low.However,application of the FSBB converter is limited due to the lack of electrical isolation.At the same time,under the multi-mode operation,the FSBB converter faces the problem of mode switching dead zone.The mode switching dead zone results in a large ripple in the output voltage,and even leads to system instability.Thus,based on the FSBB converter,this thesis studies the multi-mode smooth switching strategy of the isolated buck-boost converter to achieve good mode switching performance and dynamic response.Firstly,this thesis replaces the buck cell or the boost cell of the FSBB converter with isolated cells,and obtains a family of isolated buck-boost converters.Buck full-bridge（Buck-FB）converter is selected as the research object of the thesis.The working principle of the Buck-FB converter is analyzed in detail and the relationship between input voltage and output voltage is obtained.Aiming at the problem of mode switching dead zone under high-efficiency multi-mode control,a duty cycle D₂-D₁coordinate system is established,and a three-mode modulation strategy with hysteresis is designed.The AC small-signal model of the Buck-FB converter working in CCM is established to obtain the control-output transfer functions of different operating modes.Then,the working principle of the traditional linear control strategy is analyzed in detail on the basis of the three-mode modulation strategy with hysteresis.Considering the influence of the right-half plane zero of the boost mode,the PI controller is designed according to the boost mode by using the closed-loop control theory of linear systems.The simulation results of mode switching and load transient show that the traditional linear control strategy occurs unstable oscillation when the operation mode is switched between the buck-boost mode and the boost mode.In order to obtain better mode switching performance,it is necessary to use smaller K_por K_i.This also means poor dynamic response and tedious parameter tuning.In addition,it is difficult to ensure the stability of the system by using the linear loop to control the nonlinear system in the boost mode.It is difficult to find uniform PI parameters that can satisfy different working modes.Aiming at these problems,a nonlinear control strategy with threshold comparators is proposed.It uses threshold comparators to convert the analog signal of error voltage into a set of level signals based on the proposed three-mode modulation strategy with hysteresis.The digital controller thus increases or decreases the duty cycle to achieve closed-loop regulation.It avoids the tedious modeling and parameter optimization process while improving the anti-interference ability.The simulation results show that the proposed nonlinear control strategy can easily and effectively achieve good mode switching performance and dynamic response.Finally,an experimental prototype with 16～40V input,12V output,400W power and quarter brick size was developed.The correctness of the analysis of the working principle of the converter is verified by the open-loop experiment.The effectiveness of the control strategy is verified by closed-loop experiments.The experimental results are in good agreement with theoretical analysis and simulation results.The integrated planar magnetic technology is used for the magnetic components in the prototype.The power density of the prototype is greater than 380W/inch³,and the peak efficiency is up to 95.6%.