When designing the filter, it is assumed that the two parts of the common mode and the differential mode are independent of each other. However, these two parts are not truly independent, because common mode chokes can provide considerable differential mode inductance. This part of the differential mode inductor can be modeled by a discrete differential mode inductor.
In order to utilize the differential mode inductance, the common mode and the differential mode should not be performed simultaneously in the filter design process, but should be done in a certain order. First, common mode noise should be measured and filtered out. With the Differential Mode Rejection Network, the differential mode components can be eliminated, so the common mode noise can be directly measured. If the common mode filter is designed to make the differential mode noise not exceed the allowable range, then the mixed noise of the common mode and the differential mode should be measured. Since the common mode component is known to be below the noise margin, the excess is only the differential mode component, which can be attenuated by the differential mode leakage inductance of the common mode filter. For low power supply systems, the differential mode inductance of the common mode choke is sufficient to solve the differential mode radiation problem because the source impedance of the differential mode radiation is small, so only a very small amount of inductance is effective.
Although a small number of differential mode inductors are very useful, too large differential mode inductance can magnetically saturate the choke. A simple calculation can be made according to formula (2) to avoid the occurrence of magnetic saturation.
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