Common mode Choke inductor’s Measurement Diagnosis 1

author: MagTop
common mode choke,filter inductors
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1. Overview  power line common mode choke
The design of power filter can be considered from common mode and differential mode. The most important part of the common mode filter is the common mode choke. Compared with the differential mode choke, a significant advantage of the common mode choke is its high inductance value and small volume. An important problem to be considered when designing the commonmode choke is its leakage inductance, that is, the differential mode inductance. Usually, the leakage inductance is calculated by assuming that it   is 1% of the common mode inductance, in fact, the leakage inductance is between 0.5% and 4% of the common mode inductance. When designing chokes with optimal performance, the effect of this error may not be ignored.

2. The importance of leakage
How does leakage develop? A tightly wound, fully wound loop, even without a magnetic core, has all its magnetic flux concentrated in the "core" of the coil. However, if the loop is not fully wound, or not tightly wound, then the magnetic flux will leak out of the core. This effect is proportional to the relative distance between turns and the permeability of the coil core.
Common-mode chokes have two windings that are designed to cause the current they flow through to travel in opposite directions along the coil core so that the magnetic field is zero. If, for the sake of safety, the coil on the core is not double-wound, there will be a considerable gap between the twowindings, which will naturally cause flux "leakage", that is to say, the magnetic field is not really 0 at each point of concern. The leakage inductance of common mode choke is differential mode inductance. In fact, the magnetic flux associated with the differential mode must leave
the core at some point. In other words, the magnetic flux forms a closed loop outside the core, not just inside the annular core.
If the core has a differential mode inductance, then the differential mode current will make the magnetic flux in the core deviate from zero. If the deviation is too large, the core will have magnetic saturation phenomenon, making the common mode inductance basically the same as the inductance without a magnetic core.

3. Overview of common mode chokes
In the design of filter, it is assumed that the two parts of common mode and differential mode are independent of each other. However, the two partsare not truly independent, as a common mode choke can provide considerable differential mode inductance. This part of differential mode inductance can be simulated by discrete differential mode inductance.
In order to make use of differential mode inductors, common mode and differential mode should not be carried out simultaneously in the design process of filters, but should be done in a certain sequence. First, common mode noise should be measured and filtered out. By means of Differential Mode Rejection Network, the differential mode components can be removed so that the common mode noise can be measured directly. If the common-mode filter is designed to simultaneously keep the differential mode noise within the allowable range, then the mixed noise of the common-mode and the differential mode should be measured. Because the common mode components are known to be below the noise tolerance, the only components that exceed the limit are the differential mode components, which can be attenuated by the differential mode leakage of the common mode filter. For low power supply systems, the differential mode inductance of a common mode choke is sufficient to solve the problem of differential mode radiation, because the source impedance of differential mode radiation is small, so onlya very small amount of inductance is effective. Although a small number of differential mode inductors is very useful, too large a differential mode inductor can cause magnetic saturation of the choke.