Research On Electromagnetic Characteristics Of The Magnetic Components Of EMI Filter

The EMC problems of the switch mode power supply become increasingly serious because of its trendency of higher frequency,smaller volume and higher efficiency.The common mode choke of EMI(Electromagnetic Interference)filter is a key device which is used to restrain EMI noise and is an important element affecting the EMC behaviors,however,it is easy to be interferenced by the ouside magnetic field and the ability of inhibiting EMI noise would be decreased.Common mode choke is the mainly research object of this paper and its leakage inductance calculation,near-field coupling characteristic,new integrated structure and magnetic core saturation characteristic are deeply analyzed,which provides new ideas and methods to restrain EMI noise and improve EMI filter performance.In the aspect of the calculation of the common mode choke leakage inductance,the error of the empirical formula in existing papers which only applys to common mode choke with less turns is verifed to be large through experiment.By the simulation analysis of the relationship between the inner dimeter,outside diameter,height and winding angle and the leakage inductance of common mode choke,its mathematical mode of the leakage inductance is deduced in this paper,and the coefficients of each variable are fitted out by software.Finally,the mathematical model of the leakage inductance is validated not to be limited by turns and layers through testing samples,and the error of the leakage inductance is controled within 10 percent so the accuracy of leakage inductance calculation of the common mode choke is substantially improved.In the aspect of the common mode choke near-field coupling characteristics,the symmetry and asymmetry planes of the common mode choke,capacitor and transformer are analyzed by qutoing the judgement methods of the symmetry and asymmetry plane,then the optimal layout is obtained by analyzing the elimination principle and condition of the two coupling elements based on the Curie law of symmetry.The feasibility and the convenience of the theory method is validated by comparing the insertion loss and the EMI noise in different PCB layouts between the common mode choke and capacitance.In the aspect of the new integrated structure of the common mode choke,a new stucture consists of two common mode chokes of which directions are same or opposite is proposed,in which the two common mode chokes have the same parameters with traditional ones except half of the height.By simulating the leakage magnetic flux of the traditional common mode choke and the new integrated stucture common mode chokes and analyzing the equivalent measuring circuit of the common and differential mode noise voltage of the new integrated common mode choke,it is concluded that the common mode choke which is formed by two stacked common mode chokes which are in series and the winding directions of them are opposite has the advantage of the bigger leakage inductance and not being easily affected by the outside interference magnetic field.The experiment validates the correction of former analysis.In the aspect of magnetic core saturation characteristic of the common mode choke,the effect of dc bias and temperature on the magnetic core saturation is analyzed by making a simulation analysis.Measures of improving the magnetic core saturation are proposed through changing the inner shape to an oval shape and adding non-magnetic materials to inner ring of the magnetic core,both the effect of temperature and dc bias on the magnetic core saturation and the effectiveness of two improvement measures are verified by the experiment,what is more,the improvement of the common mode choke carrying with inner-oval magetic core is obvious.