A method for the assessment of EMI suppression performance of ferrite core loaded power cord

• Main Author: Hu, Bo
• Other Authors: See Kye Yak
• Format: Theses and Dissertations
• Language: English
• Published: 2015
• Subjects: DRNTU::Engineering::Electrical and electronic engineering
• Online Access: http://hdl.handle.net/10356/65116
• Institution: Nanyang Technological University

Clamp-on type ferrite cores are commonly employed to suppress common-mode (CM) noise currents on the attached cables without affecting the intended differential mode (OM) signal. Ferrite core manufacturers usually provide the insertion loss information of the ferrite core under pre-defined source and load conditions. Similarly, most research work has been focused on the impedance characteristics of a ferrite core and its loading effects on various types of the transmission lines. However, most of these measurement setups are still designed based on 50 0 test system. Unfortunately, the source and load impedances of the ferrite core loaded cable under the actual operating condition are usually unknown. As the insertion loss of a ferrite core depends very much on the actual source and load impedances, the predefined impedance measurement system does not reflect the true EMI suppression characteristic of a ferrite core under its actual operating configuration. An evaluation methodology of EMI suppression performance of a ferrite core loaded cable under its actual operating condition is proposed in this thesis. Based on a two-probe inductive coupling approach, the current probes are clamped on to the cable and the EMI attenuation of a ferrite core loaded power cord can be assessed more objectively without the actual operation interrupted. Taking into account the impedance of the ferrite core and the CM impedances of the intended noise source and noise termination, the true EMI suppression performance of the ferrite core located at different positions along the cable can be evaluated with confidence. Using a practical test case with an interfacing cable connecting between a mouse and a personal computer (PC), the proposed model has been validated experimentally with the measured CM noise current along the cable. The calculated insertion loss of the ferrite core based on the proposed model agrees well with the insertion loss obtained through measured CM current. Finally, using a typical power cord with a metal ground plane as the CM current return path, the EMI suppression performance of a ferrite core loaded cable is analyzed with a transmission line model terminated with the actual noise source and load. Such a model allows in-depth analysis of the influence of the length of the power cord, noise source impedance and noise termination impedance on the EMI suppression performance of a ferrite core loaded cable.