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Common mode inductance leakage inductance introduction
Sep 18, 2018



Common mode inductance leakage inductance introduction


How is the leakage inductance of the common mode inductor (filter) formed? A toroidal coil that is tightly wound and wound around a full circumference, even without a magnetic core, all of its magnetic flux is concentrated in the "core" of the coil. However, if the toroidal coil is not wound for a full circumference, or if the winding is not tight, the magnetic flux will leak out of the core. This effect is proportional to the relative distance between the turns and the magnetic permeability of the spiral core. The common mode choke has two windings that are designed such that the current they flow through is opposite in direction as the coil core conducts, thereby making the magnetic field zero. If, for the sake of safety, the coil on the core is not wound in a double wire, there is a considerable gap between the two windings, which naturally causes the magnetic flux to "leak", which means that the magnetic field is at various points of interest. It is not really 0. The leakage inductance of the common mode choke is a differential mode inductance. In fact, the flux associated with the differential mode must leave the core at some point. In other words, the flux forms a closed loop outside the core, not just within the toroidal core.


If the core has a differential mode inductance, then the differential mode current will cause the magnetic flux in the core to deviate from the zero point. If the deviation is too large, the core will be magnetically saturated, so that the common mode inductance is basically the same as the inductance without the magnetic core. same. As a result, the intensity of the common mode radiation is as if there were no chokes in the circuit. The magnetic flux deviation caused by the differential mode current in the common mode toroidal coil can be obtained by:


Where is the amount of flux change in the core, Ldm is the measured differential mode inductance, is the differential mode peak current, and n is the number of turns of the common mode coil.


Since the B total can be controlled to be less than B saturation, thereby preventing the magnetic saturation phenomenon of the core, there are the following rules:

In the formula, it is the differential mode peak current, Bmax is the maximum deviation of the magnetic flux, n is the number of turns of the coil, and A is the cross-sectional area of the toroidal coil. Ldm is the differential mode inductance of the coil.


The differential mode inductance of the common mode choke can be measured as follows: short the ends of one leg, and then measure the inductance between the other legs, the value of which is the differential mode inductance of the common mode choke.


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