04-02-2013, 10:18 AM
EMC filters
1EMC filters.pdf (Size: 883.99 KB / Downloads: 18)
EMC basics
Legal background
Electromagnetic compatibility (EMC) has become an essential property of electronic equipment. In view of the importance of this topic, the European legislator issued the EMC Directive as early as 1996 (89/336/EEC): it has since been incorporated at national level by the EU member states in the form of various EMC laws and regulations.
The EU’s new EMC Directive (2004/108/EC of December 15, 2004) contains several significant innovations compared to the version in force since 1996. It will become binding on all equipment put on the market after the elapse of the transitional period in July 2009. The most important changes include:
Regulations for fixed installations
Abolition of the “competent body”
Conformity assessment may also be made without harmonized standards
New definitions of terms (”equipment”, “apparatus”, “ fixed installation”)
New requirements on mandatory information, traceability
Improved market surveillance
The definition of “apparatus“ has now become clearer, so that its scope of validity now covers only apparatus that the end user can use directly. Basic components such as capacitors, inductors and filters are definitively excluded.
The “essential requirements” must be observed by all apparatus offered on the market within the EU. This ensures that all apparatus operate without interferences in its electromagnetic environment without affecting other equipment to an impermissible extent.
Directives and CE marking
Manufacturers must declare that their apparatus conform to the protection objectives of the EMC Directive by attaching the CE conformity mark to all apparatus and packaging. This implies that they assume responsibility vis-à-vis the legislators for observing the relevant emission limits and interference immunity requirements.
The interference immunity requirements in particular are becoming increasingly important for the operators of apparatus, installations and systems, as their correct functioning can be ensured only if sufficient EMC measures are taken. However, the need for constant functionality also implies high availability of installations and systems and thus represents a significant performance figure for the cost-effective operation of the equipment.
It should be noted that the CE conformity mark not only asserts electromechanical compatibility but also confirms the observance of all the EU Directives applying to the product concerned. The most important general directives apart from the EMC Directive include the Low-Voltage Directive and the Machinery Directive.
EMC standards
Dedicated product standards or product family standards are available for many kinds of equipment (see Section 1.9). All equipment not covered by these EMC standards are assessed on the basis of the generic standards. Special rules apply to larger and more complex installations which are assembled on site and are not freely available commercially (see Chapter “Application notes”).
Characteristics of interferences
In order to be able to choose the correct EMC measures, we need to know the characteristics of the interferences, how they are propagated and the coupling mechanisms. In principle, the interferences can also be classified according to their propagation mode (Figure 6). At low frequencies, it can be assumed that the interferences only spreads along conductive structures, at high frequencies virtually only by means of electromagnetic radiation. In the MHz frequency range, the term coupling is generally used to describe the mechanism.
Analogously, conducted interferences at frequencies of up to several hundred kHz is mainly differential-mode (symmetrical), at higher frequencies, it is common-mode (asymmetrical). This is because the coupling factor and the effects of parasitic capacitance and inductance between the conductors increase with frequency.
EMC measurement methods
As previously mentioned, an interference source causes both conducted and radiated electromagnetic interferences.
Propagation along lines can be detected by measuring the interference current and the interference voltage (Figure 7).
The effect of interference fields on their immediate vicinity is assessed by measuring the magnetic and electric fields. This kind of propagation is also frequently termed electric or magnetic coupling (near field).
In higher frequency ranges, characterized by the fact that equipment dimensions are in the order of magnitude of the wavelength under consideration, the interference energy is mainly radiated directly (far field). Conducted and radiated propagation must also be taken into consideration when testing the susceptibility of disturbed equipment.