T25*15*13 Toroidal Wirewound Inductors

T25*15*13 Toroidal Wirewound Inductors

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 common mode 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.

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 two windings, 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.