Most EMI filters must use a common mode inductor. Since the common mode inductor has a high impedance over a wide frequency range, high frequency noise generated by the high frequency switching power supply can be suppressed. The common mode inductor is why the core is in front of the ring. Here, what are the factors that need to be considered when designing?
The basic parameters required to design a common mode inductor are input current, impedance, and frequency. The input current determines the size of the winding conductor. When calculating the wire diameter, the calculated value of 400 amps per square centimeter is generally used, but other calculated values may be used depending on the acceptable temperature rise of the inductor. Single-strand wire is used in almost all cases because it is not only the cheapest, but its copper loss due to high-frequency skin effects helps to reduce noise.
The impedance of an inductor is typically taken as the minimum at a given frequency. This impedance is in series with the line impedance to achieve the desired noise attenuation. Unfortunately, line impedance is mostly unknown, so designers often use a 50Ω line impedance stabilization network (LISN) test filter. This has become the standard method for testing filter performance, but the result may be an -6 dB increase in attenuation per octave when the actual true-order filter exceeds the corner frequency. The corner frequency is generally low enough to make the inductive reactance a major part of the impedance, so the inductance can be calculated using the following equation: Ls = Xs / 2πf.
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 design is often to choose the core size. If the design has dimensional requirements, then as long as the core can be guaranteed to meet these requirements after winding, you should choose the largest size core that can be satisfied. If there is no size limit, you can choose the core size at will.
This is followed by the calculation of the maximum number of turns required to be wound on the core. Usually they are single layers, one is wound at one end of the core and isolated from each other. Double-layer and stacked windings are sometimes used, but these two windings increase the distributed capacitance of the windings, which reduces the high frequency performance of the inductor.
Since the wire diameter has been determined by the line current, the inner circumference can be calculated from 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 diameter of the wire plus the thickness of the insulation. After calculating the maximum number of turns, the next step is to select the material and determine the inductance. The choice of materials is based on many factors such as operating temperature, frequency range and cost. But first verify the selected core size, other factors can be considered after sale. So choose the material with the right permeability and then calculate the inductance.
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