Measurement and Diagnosis of Common Mode Inductors - An Overview of Common Mode Chokes

Filter design assumes that the two parts, common mode and differential mode, are independent of each other. However, the two components are not truly independent because the common mode choke can provide considerable differential mode inductance. This differential mode inductance can be simulated by a discrete differential mode inductor.

　　
In order to take advantage of the differential mode inductance, common mode and differential mode should not be done at the same time but in a certain order during the design of the filter. First, the common mode noise should be measured and filtered out. By using Differential Mode Rejection Network (DMRN), the differential mode component can be eliminated, so the common mode noise can be measured directly. If the common mode filter is designed so that the differential mode noise does not exceed the allowable range at the same time, then the mixed common mode and differential mode noise should be measured. Since the common mode component is known to be below the noise tolerance, it is only the differential mode component that is exceeded and can be attenuated by the differential mode leakage inductance of the common mode filter. For low-power power systems, the differential-mode inductance of the common mode choke is sufficient to address the differential-mode radiation because the source impedance of the differential-mode radiation is small, so only a very small amount of inductance is effective.

　　
Although a small amount of differential mode inductance is very useful, too much differential mode inductance can magnetically saturate the choke. Magnetic saturation can be avoided by making a simple calculation based on equation (2).