Practical Considerations of Common Mode Inductors

Common mode inductors, more generally known as chokes, are used to filter unwanted electrical noise usually caused by the switching action of switch mode power supplies.  Specifically it filters out common mode noise or noise that is common to both the positive and negative outputs of a dc power supply, for example.  The main idea behind a common mode inductor is that you put two symmetrical windings on a magnetic core, typically a ferrite toroid core, and mount it a plastic header, see figure 1. The windings are connected so the current flow will be in equal but in opposite directions.  This keeps the core from saturating since it will have two equal but opposite magnetic fields and the full inductance of the core will help attenuate the switching noise common to both lines, see chokes/common mode inductors.

The most common core shape used in a common mode inductor is the toroid. It has two big advantages compared to other shapes. First, if you think of it as a winding on one half of a toroid opposing a winding on another half of a toroid you have two very symmetrical shapes which helps prevent core saturation. Secondly, a toroid has no mating surface compared to other core shapes so it will have more inductance and is usually 35% higher in inductance compared to a similar sized two piece core shape. The two windings on the toroid need to be as symmetrical as possible since leakage inductance will be created by the current flow. Leakage inductance can start to saturate the core which will cause the common mode inductance to decrease. An unequal number of turns on the two windings or an unsymmetrical winding shape can increase leakage inductance resulting in reduced inductance leading to reduced filtering of the noise. For more details see the theory of common mode inductors.

Typically common mode inductors handle currents ranging from tens of mA to about 20A. This means wire gauges from #38 to #12 are commonly used. Currents much above 20A are usually not wound on a toroid since the core can break due to winding stress associated with heavy gauge wire. Other core shapes and unique windings are usually used above 20A. Also multiple inductors in parallel could be used. High permeability ferrite materials are very susceptible to winding stress and inductances can be down to 50% of the expected inductance due to the winding stress on the core. Usually heating the wound cores slowly relaxes the stress and brings the inductance back up. I emphasize that the heating should be ramped up and down slowly since ferrite is a ceramic and can crack due to rapid temperature changes. 

One of the more surprising aspects of toroidal common mode inductors, or toroids in general, is that they are wound automatically, not by hand on toroidal winding machines.

written by Mike Horgan
Engineering Manager
at Butler Winding