Research Of Photovoltaic Grid-connected Flyback Micro-Inverter Based On DCM Variable Frequency Control

With the current global energy crisis and environmental crisis becoming more and more prominent,it is urgent to accelerate the transformation of primary energy consumption structure based on clean energy.Photovoltaic grid-connected power generation is one of the most potential alternative technologies for clean energy,and its development has been widely concerned by countries all over the world.Due to the shortcomings of traditional centralized photovoltaic power generation,such as large area,low tracking accuracy of the maximum power point and obvious hot spot effect,distributed photovoltaic power generation technology based on rooftop photovoltaics has developed rapidly.As a bridge between distributed photovoltaic power generation and power grid,micro-inverter has become a hot research topic in the field of distributed photovoltaic power generation because of its modular production and convenient expansion.This paper analyzed and compared the common grid-connected micro-inverter topology.Through comparison and analysis,the single-stage flyback grid-connected micro-inverter with simple topology and convenient control are finally determined as the research object.Not only the research status and key technologies of flyback grid-connected micro-inverters are summarized and analyzed,but also the control method of flyback grid-connected micro-inverters is studied in this paper.Firstly,the operation principle of single-ended flyback grid-connected micro-inverters and their operation characteristics in Continuous Conduction Mode(CCM),Discontinuous Conduction Mode(DCM)and Boundary Conduction Mode(BCM)are analyzed.The relevant formulas in three modes are also deduced.In order to solve the switching loss problem of the flyback photovoltaic grid-connected micro-inverter working at discontinuous conduction mode when the sinusoidal wave is near zero or the instantaneous output power is low due to the constant frequency or dual frequency control,a novel DCM variable frequency control strategy is proposed in this paper.With this control strategy,the switching frequency of the inverter changes with the instantaneous output power variation.Compared with the traditional constant frequency and dual frequency control strategies,the DCM variable frequency control strategy can ensure that the flyback main switching frequency of the inverter is lower when the instantaneous output power is low,thus reducing the switching loss of the inverter to improve the overall efficiency of the inverter.Secondly,according to the DCM variable frequency control strategy proposed in this paper,the realization method is designed concretely,and the characteristics of flyback grid-connected micro-inverters controlled by DCM variable frequency are analyzed.Through simulation analysis,the control strategy of DCM variable frequency is further optimized,and the flyback grid-connected micro-inverters controlled by DCM variable frequency with the lowest frequency limit are proposed to reduce the total harmonics distortion of the inverterst ensures the quality of grid-connected current.Finally,the circuit parameters of flyback grid-connected micro-inverters are analyzed and designed in detail,especially the design of flyback transformers.A 125 W single-phase photovoltaic flyback grid-connected micro-inverter experimental prototype based on TMS320F2812 is developed.The corresponding hardware circuit design,software program design,experimental debugging are finished.The experimental results verify the effectiveness of the proposed DCM variable frequency control strategy.