EMI Filters

We use passive EMI filter since the beginning of time, why the research is still needed?

Reading every single paper about passive EMI filters you’ll find a statement in it, that designing and optimization of aforementioned filters is not a trivial task. If you’re familiar with basic circuit theory and know a bit about the properties of passive electric components (those being inductors, capacitors, resistors and, maybe, Common Mode Chokes, which are not that basic though) you were probably asking yourself what exactly makes this task so not trivial? Just throw some capacitors and inductors in there, get your 60 db/dec attenuation and be happy!

This article will open a series of posts in which I’ll go through every significant phenomena or aspects in a filter design to give the reader an explanation of why all those people that are saying : ”It’s not that easy to design a filter, even a passive one”, are making a good point. We’re going to focus on the EMI filters design for the power converters. Of course, to get to the truly interesting part, we must start with the basics. Remember the article about difficulties in writing and our little friend, the Tutorial Style? This guy will be the lead star of this post, please forgive me kindly.

Passive EMI filter represents a passive circuit consistent of one or several stages of inductors and capacitors. EMI filters are used to provide a high impedance for an unwanted signals (interference). An effectiveness of the filter is usually measured by its Insertion Loss (IL). IL is defined as a a ratio between a voltage measured across the load without the filter installed \(U_o\) and the voltage across the load with the installed filter \(U_o^{'}\).

\[ IL = 20 \log \left(\frac{U_o}{U_o^{'}} \right) \]

The are six classical filter topologies exist (Figure 1 - Figure 6) 1 : first stage filters – simply capacitor connected in parallel with the load or an series inductor, which will give 20db/dec attenuation. The combination of capacitor an inductor in CL or LC configuration, a second stage filters, ideally will give a 40 db/dec attenuation, and the third stage filters – CLC or π-filter and LCL or τ-filter, will give a 60 db/dec attenuation.

Typically EMI filters involve separate stages for filtering Differential and Common Mode interference. The Differential Mode currents are those that are equal in amplitude but opposite in the direction in the two conductors system, and Common Mode currents are equal in amplitude but directed in opposite direction (C. Paul, 2009). In order to suppress DM emission capacitors connected between lines or line and neutral are used (Cx capacitors) and line inductors. To suppress CM interference CMC and Cy capacitors are used (capacitors connected between line and ground wire).

The evaluation of the EMI filters is described in CISPR 17 where the precise measurement procedures and the setup are given. To measure the CM IL of the filter all inputs and outputs of the filter should be shorted. For a DM IL the decoupling transformer should be used. Typically filters are being tested with 50/50Ω terminating impedances. It should be noted, that characterization of the filter using standardized impedances only useful for a comparison of different filters but doesn’t show the real IL of the filter in a particular practical application.

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