June 2009

EMC filters

New applications thanks to lower leakage current

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Increasing numbers of electronic appliances and their interaction mean that suitable EMC filters are more important today than ever before. They are needed not only to observe the EMC equipment limits (radiated interference) but also to ensure reliable operation of the equipment even under harsh conditions. This also means protection from interference coming from other equipment and from the power line (perturbations). Reliable EMC protection contributes significantly to assuring machine availability, thus also providing a useful sales argument for manufacturers of systems, machines and installations.

The 2-line filters of the new SIFI® series from EPCOS are now used successfully in the most diverse applications. Thanks to innovative materials, the dimensions of the new SIFI series have been reduced still further compared to their predecessor types while retaining the same current capability. In addition, constructional improvements have also led to cost reductions.


All filters of the three SIFI families are now also available with a reduced leakage current of only 0.002 mA, making them suited for applications in medical technology.

EPCOS currently offers three new SIFI families: SIFI-F (B84111F), SIFI-G (B84112G) and SIFI-H (B84113H). They were developed as standard modular filters for single phase systems (2-lines) with various attenuation characteristics. Fig. 1 is designed as a selection guide so that EMC filters with correctly dimensioned properties and thus the most cost-effective solution can be found in a few steps.

The new SIFI families differ mainly in their attenuation properties and dimensions. SIFI-F (B84111F) has the smallest dimensions and covers the range of normal requirements on interference suppression. Even a limited available space is usually sufficient for SIFI-F, as the package of the 10-A version including terminals and attachment clips requires a footprint of only 60 x 60 mm2.

In the case of higher requirements on attenuation properties, SIFI-G (B84112G) is recommended. Especially at frequencies below 1 MHz, it offers an improved asymmetrical insertion loss compared with SIFI-F. Fig. 2 shows the asymmetrical insertion loss (common mode) of the 3-A versions of SIFI-F (B84111FB30), SIFI-G (B84112GB30) and SIFI-H (B84113HB30) as a function of the frequency.


Asymmetrical insertion loss for 3-A versions of SIFI-F, SIFI-G and SIFI-H as a function of the frequency.

If the insertion loss of SIFI-G is not sufficient, then SIFI-H (B84113H) should be used. This is a two-stage filter with the highest insertion loss. Starting from as low as about 0.1 MHz up to about 50 MHz it reliably reduces the conducted symmetrical and asymmetrical interference voltages. Depending on the interference source, this allows high requirements such as the limits of class C1 to EN 61800−3 (2004) for conducted disturbance voltages to be observed even for strong sources of interference.

When the suitable SIFI family (F, G or H) has been selected on the basis of the attenuation requirements, the leakage current must be considered. All new SIFI filters are available in both standard and medical versions. The standard version has, depending on the type, a leakage current in the range from 0.5 to 3.5 mA. In the version for medical technology, the leakage current was limited to a maximum of 0.002 mA, as strict limits apply in this sector.

When selecting the rated current, filters are available from 3 A to 36 A. The rated voltage is 250 V DC/AC for all types. Optimization of the components used allows the maximum ambient temperature to be increased to 100 °C, corresponding to climate category 25/100/21 to IEC 60068-1. This leads to a reduced and thus improved current derating at higher temperatures. The complete SIFI range has naturally been approved to UL, cUL and ENEC. This facilitates the approval of the end product for the North American and European markets. The connections used for SIFI-G and H are tab connectors up to 16 A and threaded stubs starting from 20 A. SIFI-F has tab connectors up to 20 A and threaded stubs from 25 A.

Broad range of applications

The medical version is used wherever the leakage current must be kept low. This can be the case for X-ray equipment, computed tomographs, ultrasonic and other diagnostic equipment.

Effective and reliable EMC is particularly important in medical equipment, as patients come into close contact with medical diagnostic equipment. Accordingly, leakage currents must be kept low and equipment malfunctions must be excluded, especially in life-support systems. But the SIFI medical version may also be used in other applications where the leakage current must be limited – e.g. where a ground fault circuit interrupter is used.


Distrurbance voltage against frequency without EMC filter (above) and with EMC filter B84112GG125 (below). The blue curves show the measured average peak values, the red curves show the results of the quasi-peak measurement.

The standard versions of the SIFI are used in almost all areas of industrial electronics, both in AC and in DC applications. Thanks to their high performance and compact dimensions, SIFIs are incorporated into welding equipment, measuring equipment, machine control systems as well as fitness equipment and the telecommunications equipment. They have also proven themselves many times over in power supplies for small machines, switching cabinets and fan installations.

They are also used increasingly in solar inverters. Fig. 3 shows an example of an disturbance voltage measurement of a solar inverter. In the first case – without EMC filters – some of the values are significantly above the limits stipulated for class A (industrial environment). In the second case, a SIFI-G B84112GG125 (25A) with enhanced attenuation was used. It allows the noise voltages to be reduced to below the class-A limits to DIN EN 55011 (2007).

All SIFIs are now available from stock in small quantities. The ordering codes start with B84111F* (SIFI-F), B84112G* (SIFI-G) and B84113H* (SIFI-H). The predecessor SIFI product families, SIFI-A, SIFI-B, SIFI-C, SIFI-D and SIFI-E, are also still being manufactured. However, new SIFI versions should be preferred for new designs. In general, SIFI-A can be replaced by SIFI-F, SIFI-B by SIFI-G and SIFI-C by SIFI-H.


When mounting the filter, the package should as far as possible be connected with ground across a large area (surface without lacquer) of the other modules. This is particularly important for interference frequencies >1 MHz. At such high frequencies, a ground connection via a cable strand must absolutely be avoided (see diagram).

A cable of 10 cm length has an impedance of about 140 nH. This already results in an impedance of 17 Ω with an interference frequency of 20 MHz, for instance. This impedance is too high for a ground connection, so that practically no filter effect is achieved in the higher frequency range irrespective of the filter used. All filter concepts will fail in this case, whether they have one or two stages. Only a low-impedance wide-area ground connection will help in this case.

A frequently occurring fault source in practice is a lack of separation of interference-emitting and filtered lines. This can result in a coupling of interference and considerably reduce the filter effect. So care must always be taken to ensure spatial separation of unfiltered and filtered lines. Where this is not possible, grounded metal parts or cable channels should decouple the lines from each other. Another solution would be a right-angled crossing or twisting of the lines. This can reduce the magnetic coupling. When shielded lines are used, the shielding must be connected to the reference potential along a large area on both sides.



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