Research And Application Of Conducted EMI Model For Magnetic Components In Power Adapter

Electromagnetic interference(EMI)noise will become one of the major challenges for power designers.Magnetic components not only affect the efficiency,volume,and weight of power supplies but also have a key impact on EMI.In this thesis,the magnetic components including transformer and EMI filter in flyback converter are taken as the research platform.The high-frequency EMI model and the EMI mechanism of the magnetic components are analyzed.The schemes of suppressing EMI noise and the evaluation method of EMI characteristics from the perspective of magnetic components are proposed.The volume and weight of the EMI filter mainly depend on the volume and weight of magnetic components and the integration scheme of common mode(CM)and differential mode(DM)inductors is a significant approach to reduce the volume and weight.Based on the concept of dual toroid magnetic core combination,a double 8-shaped winding wiring scheme used to achieve the integration of CM and DM filter inductors is proposed.Compared with the traditional CM filter inductor,the proposed integrated inductor can increase DM inductance and lower DM magnetic field leakage without sacrificing CM inductance.The CM and DM impedance,the CM and DM insertion loss(IL),and the CM and DM noise spectrums are tested to verify the effectiveness of the proposed CM and DM inductor integrated scheme.The generation and conduction mechanism of CM noise in flyback converter is investigated.The transformer is regarded as a key impedance of the CM noise conduction path,which has a significant impact on CM noise level.A practical and simple method for evaluating the CM noise suppression characteristics of the transformer which only requires a signal generator and oscilloscope is proposed,and this kind of evaluation method can be widely applied in the quality control in production lines.Meanwhile,the influence of the potential difference between the static points of the primary and secondary sides on the dynamic capacitance is analyzed,which can be used to choose the proper resistance value in the proposed practical and simple evaluation method.Y-capacitor is an effective method to suppress CM noise,but its value is limited by power frequency leakage current safety regulation,and even cannot be used in some applications.The balance winding scheme of the transformer is proposed to suppress CM noise.The balance winding is inserted between the adjacent primary and secondary windings of the transformer,and the potential balance can be achieved between the adjacent primary and secondary winding layers.Meanwhile,the influence of the turn spacing of the balance winding on the balance of the CM noise electric charges is further considered by 3D FEM simulation tool HFSS.The experiment results verified that the proposed balance winding scheme can suppress the CM noise in the transformer coupling path and achieve no Y-capacitor design in the flyback converter.In addition to the CM noise flowing through the transformer coupling path,there is also the CM noise flowing through the other parasitic capacitance between the voltage hot point and the ground.An evaluation method is proposed to measure the parasitic capacitance between the voltage hot point and the ground by two-port instruments.Combining with HFSS simulation,the CM noise flowing through the transformer coupling path can be accurately designed and the two parts of the CM noise can be canceled with each other.Thus,the total EMI noise measured at LISN can be further reduced.The influence of switch position on the potential distribution of the primary and secondary windings of the transformer and the phase of CM noise sources between primary and secondary sides is analyzed.It is pointed out that forming the CM noise cancellation mechanism in the transformer coupling path should satisfy the antiphase CM noise sources in primary and secondary sides or the same polarity potential in primary and secondary windings.For flyback converter with secondary synchronous rectification,this thesis also proposes the scheme of placing the primary MOSFET on the high side,then the CM noise cancellation mechanism can be formed.Meanwhile,the proposed circuit topology can also achieve sharing of the PCB trace between the drain of the primary MOSFET and the positive terminal pin of the bus capacitor,which can help to improve the heat dissipation of the primary MOSFET.