Research On Reactive Power Control And Efficiency Optimization In Light Load Of Inverter Based On Boundary Current Mode

With the gradual depletion of petroleum,coal and other traditional energy sources,clean and pollution-free renewable new energy sources such as solar energy have developed rapidly,and photovoltaic power generation has also occupied an increasing share of the global electricity market.Compared with the traditional photovoltaic grid-connected power generation system,microinverters have gradually become a research hotspot in recent years because of the advantages of modularization,easy installation and maintenance,and high reliability.This paper mainly studies the control scheme and corresponding improvement methods of the two-stage microinverters' back-stage inverters,aiming at further improving the efficiency and output current quality.In this paper,the working principle of a full-bridge inverter based on Boundary Current Mode(BCM)control scheme is analyzed in detail.Soft-switching is realized without adding additional circuits.The working characteristics of the inverter under the traditional unipolar and bipolar modulation strategies are studied.The unipolar modulation strategy has high efficiency,but current distortion exists.The bipolar modulation strategy has high output current quality,but its efficiency is hard to raise.To overcome the shortcomings of existing control schemes,a hybrid modulation strategy based on BCM is proposed.By switching between two modulation strategies in one switching cycle,the switching frequency can be adjusted,while retaining the high efficiency advantage of unipolar modulation strategy,the current distortion problem can be solved.The operation principle and characteristics of the inverter under the modulation strategy are analyzed in detail,and the control parameters are optimized.The realization of reactive power output of the inverter under the improved control scheme is further discussed.The feasibility of the proposed control scheme is verified by simulation.Because of the low efficiency of light-load caused by the high switching frequency in Boundary Current Mode,this paper choose Discontinuous Current Mode(DCM)to carry out research.The operation principle and working characteristics of the inverter under DCM are analyzed in detail.Based on the traditional scheme of fixed turn-off time,this paper choose to change the turn-off time according to different loads to further the efficiency under light load.Besides,aiming at the problem that the effect of inductance current control is not good under the traditional intermittent current mode control mode,a digital-analog combination method is proposed,which improves the effect of inductance current control and optimizes the THD of output current.The effectiveness of the improved method is verified by simulation.The experimental platform of 300 W two-stage microinverter is designed and built.The inverter in BCM and DCM are experimentally verified.The experimental results show that the inverter in both current modes can realize soft-switching with high efficiency.The highest efficiency of BCM can reach 98.5%,and both can output high quality voltage and current.The results are consistent with the theoretical analysis and simulation waveforms,which verify the correctness and superiority of the proposed control scheme.It is suitable for small power inverters such as microinverter.