What is it, how it works and how they choose most appropriate EMC filter?

These are some of the questions whose answer is the key to be in the best position to comply with EMC legislation.

As an introduction, EMC filter is usually a passive element  , with one or several stages   to attenuates RF noise in both differential mode and common mode in a given frequency spectrum called “rejection band”

Usually between 10kHz (or 150kHz) and 30MHz.

The effectiveness of an EMI filter is given by the insertion loss (both common and differential mode):

I.L. = 20 log V1/V2

 Where, V1 is the voltage without the filter installed and V2 is the voltage with the filter installed.

Since the insertion loss of a filter with the frequency are given for a specific impedance (50 ohms), these curves (common and differential mode) should only be used to give us an idea of their ability to attenuate radio frequency noise to select it, depending on the final standard or regulation to be met by the device (FCC, VDE, EN, etc.).

If we make a simple classification of the filters from the its attenuation point of view we can establish the following:

20-50dB: Standard 40-70dB: Medium 60-80dB: High 70-90dB: Excellent

The correct selection of a filter type, in the beginning is given by the type of noise source, and also by the complex impedance seen mainly from the load (from the noise source).

And here it is, where the concept of the filter as "Miss-matching network”, take effect, so that the more miss-matched is in front of the “Line” and in front of the “Load” the more effective is:  The subject is to reject as much energy as possible (out of the band-pass: 50/60Hz AC or DC in case of DC filters).

Therefore, the same filters (same values and topology) provide different attenuation values depending on the real impedance (which varies both in magnitude and phase) of the load.

This is the reason why PREMO has filters with suitable designs for different applications (different noise sources), inverters, converters DC / DC variable speed drives, etc.:

http://www.grupopremo.com/in/file/462

So after an initial approach to the most suitable filter, it is definitely mandatory to make an EMC (conducted emission) test to determine their suitability.

In addition to the insertion loss, another important parameter when selecting a filter is concerning the safety: the leakage current.

The leakage current is mainly determined by the capacitance between phase (or neutral) and protection wire (or chassis), ie the "Y" capacitor.

In general, the maximum allowable leakage current can range from 0.1 mA for electro-medical equipment to 5mA or more for industrial equipment. For electro-medical equipment in contact with the patient the maximum allowable leakage current is 5uA (this can only be achieved with no capacitors in "Y").

In the PREMO EMC filters catalog following references FF,FG/FGR, FI, FT, FTA, FTC, FTCV, FTN and FTO  can be found as the most relevant for applications in electro-medicine and those families for finished industrial environment LL (low leakage) that are specially designed to keep the leakage current below a certain value.

To avoid an important stress that cause a significant reduction of their reliability, ie the MTBF, it is important to note the rated current of the filter in the working environment od the device, ie we select the current range of the filter taking into account the maximum operating temperature, or rather the temperature at which it is anticipated that it will work for 80% of his life:

The typical curve of current derating (with temperature) of an EMI filter is:

EQ:

where,

e.g.:

10A filter, defined at 40 ºC and a maximum operating temperature of 85 °C, if we use it at 65 °C, the rated current must be reduced to:

I = 10 * (85-65)/85-40)^0,5 = 6,6A

A properly installed EMC filter on the border between the device and the network acts as a bi-directional filter and thus serves both to the EME (electromagnetic emission) and the EMS (electromagnetic susceptibility). Both phenomena must be treated to operate in a given electromagnetic environment and comply with European legislation and other international standards.

Therefore, the mounting and wiring of the filter is critical to its operation, especially in the upper frequency band (15MHz-30MHz) of the conducted emission spectrum (10kHz-30MHz) where crosstalk coupling becomes relevant.

Example of a bad cable routing in the mounting of an EMC filter

Both the connection to chassis and the position of the input-output cable are two aspects that must be taken into account if we want to improve the filter response at high frequencies (15MHz-30MHz) significantly.

Conducted Emission in a shielded          Conducted Emission in field,

room, using a LISN (line Impedance        using a voltage probe 

Stabilization Network)

Antonio Rojas

Marketing Manager

Grupo Premo