Evaluation of conducted EMI measurement without LISN using two-port ABCD network approach for EMI filter design under real operating condition

To demonstrate the impact of the line impedance stabilization network (LISN) on conducted electromagnetic interference (EMI) measurement, an in-circuit impedance extraction method based on inductive coupling approach is developed to extract the common mode (CM) and the differential mode (DM) noise impedances of AC mains, LISN, and switched-mode power supply (SMPS) under operating condition. By treating two inductive coupling probes and the device-under-test (DUT) for impedance extraction as three cascaded two-port ABCD networks, the CM and DM noise impedances can be extracted with ease. The extracted CM/DM noise impedance of AC mains is compared with the extracted SMPS and LISN CM/DM noise impedances, respectively. By analyzing the impedance comparisons, the impact of LISN on conducted EMI measurement for EMI filter design under real operating condition is evaluated. Finally, the evaluation is validated with the conducted EMI currents measured with and without LISN.