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Addressing EMI of common mode chokes

The presentation by Sam Ben-Yaakov “Common mode chokes: There is more than meets the eye” explores the complexity of common mode chokes (CMCs) in mitigating electromagnetic interference (EMI) in power systems. The speaker begins by explaining the need for filters to reduce ripple current from switch-mode units to meet EMI standards. Two types of conducted EMI are introduced: differential mode, where current flows between lines, and common mode, where currents flow through parasitic capacitances to the ground. To address these, common mode filters, including coupled inductors or chokes, are employed. Unlike separate inductors, chokes efficiently handle DC currents by balancing MMFs (magnetomotive forces) and isolating common mode currents. Differential mode filtering can also be achieved through leakage inductance, but achieving sufficient attenuation often requires additional filters, which can introduce challenges due to high DC currents and core saturation.

The speaker demonstrates the limitations of standard approaches to modeling and simulating common mode chokes, such as using LTspice. Real-world behavior diverges due to complexities like parasitic resonances, varying permeability of ferrite cores, and frequency-dependent losses. These factors affect impedance, attenuation, and resistance. For instance, at high frequencies, permeability drops, causing inductance to taper off, while losses (represented by imaginary permeability) increase. Commercial common mode chokes are highlighted as complex devices, with performance influenced by construction details, such as winding placement and core material. The presentation concludes by emphasizing the nuanced nature of CMCs and their attenuation behavior, particularly at higher frequencies, where performance can degrade due to parasitics and core limitations.