Power filter measurement diagnostics
Oct 30, 2018

The design of the line filter can usually be considered from both 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 that its inductance is extremely high and the volume is small. An important issue to consider when flowing a loop is its leakage inductance, which is the differential mode inductance. Usually, the way to calculate the leakage inductance is to assume 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. The impact of this error may not be negligible when designing chokes with optimal performance.

The importance of leakage

How is the leakage inductance formed? The toroidal coil that is tightly wound and wound around one full turn, even if there is no magnetic core, all the 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 safety reasons, the coils on the core are 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.

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