Modeling And Suppression Of Electromagnetic Interference In Flvback Converter

Flyback converters are widely used for implementation of low-power and multiple-output isolated power supplies.However,as the frequency of the switching mode power supply(SMPS)tends to be higher and the size of SPMS tends to be smaller,switching speed becomes higher and the layout of printed circuit board(PCB)becomes more compact.As a result,the electromagnetic interference(EMI),which is caused by the switching of power semiconductor device,becomes more serious.When electromagnetic compatibility(EMC)is not realized in SMPS,there may be two problems.The electronic load,which is sensitive to the electromagnetic noise,will work abnormally;besides,other power equipment,which is connected to the grid,will be interfered by the excess EMI.In order to realize EMC of SMPS,this paper reveals why the common mode(CM)noise can be cancelled and how the electric field affects the devices.It analyzes the effect of transformer structure on CM noise and the effect of shielding layer structure on near field interference.And based on the above theory,it proposes the method of cancelling the CM noise in flyback converter and the method of suppressing near field interference.It first analyzes the CM interference paths in flyback converter,sets up its equivalent circuit model and introduces the measurement methods of EMI and CM noise.Then it analyzes the time domain waveform of CM current.According to the waveform of voltage across MOSFET on the primary side and the waveform of voltage across the diode on the secondary side,it explains how the CM noise is cancelled.In the prototype of SMPS used in this paper,the CM currents produced by the primary side and the secondary side could be cancelled,which is illustrated by the measurement results.Based on the CM coupling mechanism and the electric field distribution of flyback transformer,it derives the formulas of CM currents in flyback transformers,and introduces a coefficient Ck to estimate the suppression performance of CM currents in flyback transformer.Then how to estimate the suppression performance of CM currents through a test is explained.And why this method is effective is also explained.Besides,the relation between the test results and the CM noise is illustrated.What's more,the effects of winding structure and shielding layer structure on the test results are studied.Finally a method to design shielding layer structure for suppressing CM noise is proposed,which is validated to be effective.Take a near field interference problem as an example,it determines that the PCB trace is the victim and the EMI source is MOSFET.The near field interference mechanism is analyzed after the determination.Then the effects of thickness,area and position of the shielding layer on near field interference are analyzed.At last,an improved shielding method is proposed,which is validated to be effective to suppress near field interference by measurement results.Through the method,the EMI of SMPS meets the requirement of EMC standard.