Study On LLC Three-port DC/DC Converter With PWM+ Variable Inductor Control

With the increasingly great consumption of fossil energy,the environmental problems become more and more serious,which leads to more attention on the renewable energy.And the photovoltaic(PV)power generation has become a hot research field due to the advantages of clean and high efficiency.For distributed photovoltaic power generation systems,in order to realize the power flow among the units(photovoltaic,energy storage and load),the integrated three-port DC / DC converters(TPC)have been widely used,which features high power density,high efficiency,and simple control.Thus,in this paper,the researches on the TPCs are carried out.Firstly,the TPC topologies are deeply analyzed.According to the power transmission and control requirement of the three-port conversion system,five topology structures are derived.By the comparative analysis of them,the advantages and disadvantages are presented.Then,based on these topology structures,the non-isolated,partially isolated and fully isolated TPC topologies are derived by means of component sharing,etc.,and compared.Based on the comparison results of these topology structures and the three isolation types,a guidance for the selection of TPC topologies in different applications are given.Then,based on the selection method of the TPC topology,the interleaved Buck+Full-bridge LLC TPC is selected for the rooftop distributed PV generation system.This TPC has the advantages of high power density and high safety,and it is feasible to realize the voltage matching of the transformer.What’s more,the zero voltage switching(ZVS)of the switches and the zero current switching(ZCS)can be realized in a wide range.Thus,it is worthy of research.In this paper,the operating principle of this TPC is analyzed firstly,and six resonant states and the typical resonant mode are mathematically calculated based on the time domain analysis.Then,the boundary between the resonant modes in the ZVS/ZCS region and their adjacent modes is derived.In addition,the dc gain characteristics of this TPC is calculated.Based on the qualitative and quantitative analysis of the TPC,a pulse width modulation(PWM)plus variable inductor multi-loop control strategy is proposed.The MPPT of PV or the constant current charging or constant voltage charging of the battery is realized by regulating the duty cycle of switches,and the constant load voltage is realized by the variable inductor.Compared with PWM plus pulse frequency modulation control strategy,the proposed control strategy can reduce the difficulty of magnetic component design and drive circuit design,and avoid the problems such as Electronic-magnetic-interference(EMI)to some extent.Thus,based on this control strategy and the time domain analysis of the TPC,the parameters especially the resonant unit are designed with the objective of gain requirements,the ZVS/ZCS realization of all the semiconductor devices,and the optimization of the RMS value of the resonant inductor current.Finally,the simulation model of the TPC including the main circuit,control circuit,MPPT algorithm module and variable inductor module,and a 500 W experimental prototype is built.And the steady-state waveform of the TPC,the dynamic switching process under different operating conditions are simulated and tested by experiments,which verifies the accuracy of the mathematical analysis results,the effectiveness of the proposed control strategy and the rationality of the parameter design results.