Advanced LISN-Based Testing Method Reveals Common Mode Choke Saturation Behavior

Common mode chokes play a critical role in reducing electromagnetic interference (EMI) in power electronic systems. However, accurately measuring the saturation behavior of their magnetic cores remains a technical challenge, especially when the core is partially or completely affected by bias current.
The LISN (Line Impedance Stabilization Network) principle offers a practical solution by analyzing how common mode filter attenuation changes when the inductance decreases due to 60Hz line current.

The test method requires an oscilloscope and a Differential Mode Suppression Network (DMRN). During testing, the oscilloscope monitors the line voltage waveform, while the DMRN measures emission changes through the LISN output. When the common mode choke operates within its linear magnetic region, emission variations remain limited, typically within 6–10dB.

However, when the magnetic core approaches saturation, the filter attenuation decreases significantly. As the input surge current increases, EMI emissions rise accordingly. Under severe saturation conditions, the filter may lose most of its suppression capability, with emission levels exceeding 40dB.
The LISN-based evaluation method provides valuable insight into the relationship between magnetic saturation and EMI filter degradation. It helps engineers optimize core selection, winding design, and filter performance for demanding power applications.