Modeling Of Electromagnetic Interference And Design Of The Shielding Winding In Flyback Transformer

Flyback converter is an isolated conversion circuit based on Buck-Boost converter.It can achieve the electrical isolation between the input and output of the circuit,and have the advantage of simple topology,high efficiency and wide range of voltage rise or drop,so it is widely used in various low-power occasions.However,the development trend of miniaturization and high frequency of power electronic equipment makes the electromagnetic compatibility(EMC)design of flyback converter more difficult.The high operating frequency makes more serious electromagnetic interference(EMI),the miniaturization leads to more complicated electromagnetic environment of the equipment,and the volume of the EMI filter needs to be reduced.In order to realize the EMC design of the flyback converter,this paper analyzes the influence of distributed capacitance between the transformer windings on EMI,discusses the capacitive modeling method of the transformer and the effect of the shield structure on EMI,and proposes a design method of transformer shielding winding based on the cancellation principle of common mode(CM)noise.It first analyzes the EMI generation mechanism and interference source characteristics in the flyback converter,and reveals the important role of the transformer in the EMC design.Then it is clarified that the distributed capacitance between transformer windings is the main factor of conducted EMI,and the model of distributed capacitance under two mechanisms are compared.The EMI suppression measures are introduced from three aspects,and the influences of transformer winding structure and shielding method on the suppression effect of CM noise are further discussed.Finally,a design method of the number of transformer shield winding turns is proposed to suppress the CM noise in flyback converter.Based on the design method,the composition and calculation method of CM current in a small power flyback converter with different shielded winding turns are discussed,the relationship between the CM current and the number of transformer shielding winding turns is explored,and the optimal solution of the number of turns of the shielded winding is obtained by solving the relationship equation.Then the CM noise of the flyback converter at 150 k Hz～30MHz is measured,and the design method of the number of transformer shielding winding turns is verified in low frequency by comparing the CM noise in converters with different number of transformer shielding winding turns.