Research On Four-Switch Buck-Boost Converter For Fuel Cell System

As the key equipment connecting different voltage levels and different modules in the fuel cell system,the performance of the DC/DC converter directly affects the safe and stable operation of the fuel cell system.As a DC/DC converter with simple structure,common input and output and wide voltage range,four-switch Buck-Boost(FSBB)converter is very suitable for the fuel cell system with limited power density and weight.On the one hand,the fuel cell system has higher requirements for the input and output current ripple of the converter;on the other hand,most of the existing control strategies have problems such as complex control or unsmooth mode transition,which cannot get a better performance of the FSBB converter.Therefore,aiming at the above problems,the working principle of the FSBB converter in the fuel cell system,the smooth mode transition strategy under the graphical method,the small signal model and the four-phase interleaved topology has been studied in this paper.The research content is mainly divided into the following three parts:1.Firstly,the topology and working principle of FSBB converter are introduced.Based on the four working modes of FSBB converter,the common control strategies of the converter are summarized,theoretically calculated and compared.2.Secondly,to make the transition between Buck mode and Boost mode smoother,a graphical method is used to analyze the relationship of the duty cycle between Buck bridge and Boost bridge and its changing trajectory during the working mode transition of the FSBB converter.The reason for the blind zone of the converter under the traditional two-mode control strategy is also obtained.Furthermore,to eliminate the blind zone of voltage gain and minimize the duty cycle change of the switching devices,a three-mode smooth mode transition strategy is proposed and compared with other mode transition methods.And the superiority of the proposed control strategy is verified in the end.3.Finally,the small signal model of the converter under the proposed three-mode control strategy is obtained by the state space averaging method.What’s more,the controller of the converter under voltage and current closed loop control is designed.Furthermore,the overall design scheme of the FSBB converter prototype with operating frequency 100 kHz and maximum power 3300 W is proposed.Also,the hardware and software design of the main circuit,control circuit and auxiliary source circuit are introduced in detail.Under the conditions of wide input and output voltage,four-phase interleaved and mode transition,the feasibility of the proposed smooth mode transition strategy is verified in the end.