Research On Cascaded DC/DC Converter Based On Flying-Inductor Module

The cascaded DC/DC converter is one of the most important types in power electronic devices,and it is composed of several modules in series and parallel.It is also widely used in DC power generation systems.The cascaded converter with high-step ratio is applied in fuel cell power generation and photovoltaic power generation.The photovoltaic power generation efficiency can be improved by transforming the structure of distributed PV generation system to cascaded structure.The flying-inductor has functions of buck and boost.It can also achieve bidirectional power flow.Therefore the flying-inductor can be used as a fundamental module for cascaded converter.The research concentrates on cascaded DC/DC converter based on flying-inductor and the main content is as follows:(1)The topological structure,working principle,parameter design and control strategy of the flying-inductor are analyzed.The AC small signal model of flying-inductor is established.The transferring function of the converter under the double closed-loop control strategy is deduced.And the influence of the PI parameters of the voltage loop and the current loop is studied by drawing the picture of zero-pole distribution of the transferring function.The correctness of the relevant theoretical analysis and the feasibility of the design are verified by simulations and experiments,which provides the basis for the subsequent cascaded system research.(3)The structure of the cascaded DC/DC converter based on flying-inductor module is studied.There are three different kinds of converters that are composed of input-parallel-output series connection.The output side of the converter not only includes the output voltage of all modules but also the input voltage of the converter,which increases the high-step ratio of the converter.The carrier phase shift is used to reduce the output voltage ripple of cascaded converter.The amplitude and frequency of carrier are same,but there is 360°/n phase difference among them,in which n represents the number of modules in the system.(4)A renewed type of distributed MPPT is proposed,which realizes independent MPPT by parallelling connection of flying-inductor module with photovoltaic cell board in series branch.Besides,the output voltage of flying-inductor undertakes differential voltage in series connection of flying-inductor output capacitor in photovoltaic branch,which guarantees the stability of the output voltage branch.The method of single peak MPPT is adopted for the tracking of maximum power of photovoltaic panels,which is easy to control.(5)Regarding the above theoretical analysis,the effectiveness and correctness of the theoretical analysis and control strategy are verified by simulation model in Matlab/Simulink and hardware-in-the-loop simulation platform based on dSPACE1103.