What factors should be considered in the design of common mode inductors?

What factors should be considered in the design of common mode inductors?

Most EMI filters must use common mode inductors. Because the common mode inductor has high impedance in a wide frequency range, it can suppress the high frequency noise produced by high frequency switching power supply. Why is the core of common mode inductor circular? I have also introduced it before. What factors should be considered in the design?

Input current, impedance and frequency are the basic parameters needed to design a common mode inductor. The input current determines the size of the winding conductor. When calculating the wire diameter, the calculation value of 400 amps per square centimeter is generally used, but other calculation values may also be used according to the acceptable temperature rise of the inductor. In almost all cases, single strand wire is used, because it is not only the cheapest, but also its copper loss due to high frequency skin effect helps to reduce noise.

The impedance of inductors is usually the minimum value at a given frequency. This impedance is connected in series with the line impedance to achieve the desired noise attenuation. Unfortunately, most of the line impedances are unknown, so designers often use 50 Ω line impedance stabilization network (LISN) to test filters. This has become the standard method to test the performance of the filter, but its result may increase the attenuation by - 6dB in each octave when exceeding the turning frequency. The turning frequency is generally low enough to make the inductive reactance the main part of the impedance, so the inductance can be calculated as follows: LS = XS / 2 π F.

After knowing the inductance, the rest of the design work is to select the core and material, and calculate the number of turns.

The first step in the design is often to choose the size of the core. If there are size requirements for the design, as long as the core can still meet these requirements after winding, the maximum size of the core that can be met should be selected. If there is no size limit, you can choose the core size at will.

The next step is to calculate the maximum number of turns required to be wound on the core. They are usually single-layer, wound at one end of the core and isolated from each other. Sometimes double-layer and winding are also used, but these two kinds of winding will increase the distributed capacitance of the winding, thus reducing the high-frequency performance of the inductor.

Because the wire diameter is determined by the line current, the inner circumference can be calculated according to the value obtained by subtracting the wire radius from the inner radius of the core. The maximum number of turns can be calculated by dividing the length of the inner circumference of each winding by the wire diameter plus the insulation thickness. After calculating the maximum turns, the next step is to select the material and determine the inductance. Many factors, such as operating temperature, frequency range and cost, are taken into account in selecting materials. However, the selected core size shall be verified first, and other factors can be considered after sales. So choose the appropriate permeability material, and then calculate the inductance.

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