Posted on 27 December 2019

Basics of EMC and EMI


Originally electronics was synonymous with telecommunications (radio, television, telephony). During operation these electronic devices often produced frequencies and harmonics which interfered with the correct operation of other equipment. These undesirable effects were known as RFI (RFI = radio frequency interference = high-frequency disturbance). Since the last few decades the number of electrical and electronic devices has become so large that it is no longer only a problem of high-frequency suppression (radio-frequency suppression) but a much broader definition needs to be used. That is why we now talk about EMC (electromagnetic compatibility). With EMC we mean that electrical setups can function normally in the electromagnetic environment and that in addition they do not cause unacceptable effects on equipment in the vicinity. The electromagnetic transmission between the "source" of the disturbance and the "receiver" can occur via radiation (RE = radiated emission and RS = radiated susceptibility) or via conduction (CE = conducted emission and CS = conducted susceptibility). Undesired radiation is counteracted via screening of electrical equipment and cables. The undesired conduction via mains-, control- or signal lines is counteracted by the use of EMI–filters (EMI = electromagnetic interference). These EMI-filters are most frequently LC-filters which are designed as low pass filters. Some manufacturers provide filter modules which only need to be connected between the mains and the equipment.

With EMC we also make a distinction with respect to the frequency spectrum of the interference:

A. Source of disturbance

  1. Discreet frequency spectrum (high-frequency generator, μ-processor systems, radio and TV receivers, SMPS,...)
  2. Continuous frequency spectrum (household appliances, power semiconductors, discharge lamps,...)

B. Receiver (of disturbances)

  1. Small band sensitivity (radio and TV receivers, modems,...)
  2. Wide band sensitivity (control systems, analogue and digital systems, process computers,...)

In figure 1 the above information is graphically represented.

Electromagnetic interaction and location of the EMI-filters

Figure 1. Electromagnetic interaction and location of the EMI-filters

EMI - filter

We distinguish two types of disturbance according to the current direction of the disturbance. The first type is the differential mode noise that occurs in the signal line and in the ground line in opposite directions as shown in figure 2. To suppress this disturbance we use capacitor Cx in parallel between the two lines (figure 3). Additionally a small coil ( = choke; e.g. 1μH) can be placed in the signal line but this does produce an undesired voltage drop. In addition this choke needs to be suitable for the nominal consumer current, a reason to avoid using this choke if possible.

Differential mode disturbance


Suppression of differential mode disturbance

Figure 2. Differential mode disturbance


Figure 3. Suppression of differential mode disturbance

A second sort of disturbance is the common mode noise that operates in all lines in the same direction. In the case of an AC-supply line the disturbance operates in both wires in the same direction (figure 4) and in the case of a signal cable the disturbance is in all cores of the cable in the same direction.

Common mode disturbance

Figure 4. Common mode disturbance

To counteract the common mode noise we combine two methods. In the first solution (figure 5) we connect an inductor in both lines.

Common mode choke

Figure 5. Common mode choke

Both coils are wound on a ferrite core in such a way that they are in series for the influence of ICM, but in opposite directions for the differential mode current ID. The self induction has no effect on the current ID.

Equivalent circuit for common mode choke

Figure 6. Configuration and equivalent circuit of a common mode choke

Figure 6b shows the equivalent circuit of such a common mode choke. This common mode choke produces no distortion of the signal, which is important if the signal lines are carrying video signals.

A second method involves a metal enclosure which includes a bridging capacitor (line bypass capacitor) Cy as shown in figure 7. The disturbance flows via the signal lines and via Cy back to the casing and via the stray capacitance to the source of the disturbance.

Application of a metal enclosure

Figure 7. Application of a metal enclosure

Figure 8 shows an example of an EMI-filter commonly used in the supply cable on the AC-input side of equipment (for example a computer).

EMI-filter at the supply-side of equipment

Figure 8. EMI-filter at the supply-side of equipment




  1. In the case of a single phase mains the differential mode current consists of the 50Hz-current together with the disturbance. The common mode disturbances normally have higher frequencies (10kHz to 10MHz). They flow via parasitic capacitances back to the source of noise.
  2. A Cy is placed between every line and the metal enclosure. This "line bypass capacitor" is usually only a few nF, and this is to limit the leakage current VωCy.
  3. Practical values for the common mode choke are for example 500μH up to 2mH.
  4. Cx (across-the-line capacitor) suppresses the differential mode disturbance. A practical value is for example 0.015μF.
  5. In the case of a frequency converter, depending on whether it is a single phase or three-phase supply an appropriate filter is included in the input.


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- who has written 7 posts on PowerGuru - Power Electronics Information Portal.

Professor Dr. Jean Pollefliet is the author of several best-selling textbooks in Flanders and the Netherlands

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