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    What should you pay attention to when designing common mode inductors?
    Date:2016-04-05 08:42


    As we all know, typical EMI filters include common mode inductors, differential mode inductors, and X, Y capacitors.
            The Y capacitor and the common mode inductor attenuate the common mode noise. In high frequency noise, the inductor exhibits high impedance characteristics and reflects and absorbs noise. However, the capacitor has a low impedance and changes the noise direction of the main line.
           The common mode inductor has the same number of turns and produces two magnetic fluxes of equal magnitude and opposite directions. These two magnetic fluxes cancel each other out. Therefore, the magnetic core is placed in an unbiased state. The differential mode inductor has only one winding and requires the core to provide a completely unsaturated linear current. This is quite different from the common mode inductance. To prevent magnetic saturation, the differential mode inductor must use a low effective magnetic permeability core (air gap ferrite or iron powder core).
           However, a common mode inductor can use a higher permeability core and a higher inductance can be obtained with a relatively small core. So what should we consider when designing?
           The technicians of Shenzhen Suqian Electronics have compiled the most basic design considerations, as follows:
            1. The basic parameters required for common mode inductor design are: input current, impedance and frequency, and core selection.
            2. The input current determines the wire diameter required for the winding. When calculating the wire diameter, the current density is usually taken to be 400 A/cm3. However, this value must vary with the temperature rise of the inductor. Typically, the windings operate with a single wire, which reduces high frequency noise and skin effect loss.
            3. The impedance of the common mode inductor is generally specified as the minimum value at the given frequency condition. The linear impedance in series provides the generally required noise attenuation. Unfortunately, linear impedance is known to a relatively small number of people, so designers often test common-mode inductors with a 50W linear impedance stabilizing network and become a standard method for testing common-mode inductor performance. But the results obtained are usually quite different from the actual ones. In fact, the common mode inductor will produce a -6dB attenuation per octave (the angular frequency is -3dB for the common mode inductance). This angular frequency is usually very low, so that the inductive reactance can provide impedance. Therefore, the inductance can be expressed by the following formula: Ls = Xx / 2πf (1).
           Inductors are known to all, but it is worth mentioning that the design must pay attention to the core, the core material and the number of turns required:
           1. First, the first step of the design is the selection of the magnetic core model. If there is a specified inductor space, we will select the appropriate core model according to this space. If there is no regulation, the magnetic core model is usually selected at will.
            2. The second step is to calculate the maximum number of turns the core can take. The common mode inductor has two windings, generally a single layer, and each winding is distributed on each side of the core, and the two windings must be separated by a certain distance. Double-layer and stacked windings are also used occasionally, but this practice increases the distributed capacitance of the windings and reduces the high frequency performance of the inductor. Since the wire diameter of the copper wire is determined by the magnitude of the linear current, the inner circumference length can be calculated from the radius of the inner circle of the core minus the radius of the copper wire. Therefore, the maximum number of turns can be calculated by the wire diameter of the copper wire plus the insulation and the circumference occupied by each winding.
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    Sqzhengsong@sohu.com. Suqian Zhengsong Electronics Co., Ltd. All rights reserved

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