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Any electromagnetic or radio frequency energy that unintentionally disturbs the functionality of an electrical or electronic equipment is termed as Electromagnetic Interference (EMI). This interfering energy can be produced by a device or equipment itself or by other devices within close proximity. But if the equipment can function without loss in quality or reliability under influence of such an EMI, then the equipment is said to have Electromagnetic Compatibility (EMC) or to be under EMI control.
Some common examples of Electromagnetic Interference (EMI) are
Electronic design engineers have long been puzzled by various concerns pertaining to EMC. This easily stems from myriad of challenges they face in debugging and resolving EMI/EMC effects (noise problems, signaling issues, current paths, emissions & immunity) on circuits and equipments.
An impressive gamut of technologies such as Smart Grid, Broadband over Power Line, Photovoltaics, Global Earth Observation System of Systems (GEOSS), Nanotechnology, Ultra High-frequency Communication Networks, Intelligent Transportation System and Advanced Health Care shall soon make their presence more visible. Such advances in modern electronics will indeed add to the challenge of managing EMC. In the light of this design engineers will have to develop deeper understanding into various products to be designed in the future and their use. As a case in point, an established and expansive infrastructure like power lines could be used alternately for internet broadband thereby drastically reducing the cost of laying new cables provided the high EMI emissions can be made compatible to the connecting electronic equipment.
An important aspect emanating from the above argument is the fact that even as EMC compliance is getting much stricter, it is no longer just a regulatory requirement but also strongly demanded by industries and consumers alike.
Several international organisations like IEC, TC77, CISPR, CEN, FCC, SAE and IEEE EMC Society work to promote international co-operation and disseminate enhanced awareness on standardization of the EMI and EMC norms. This is essential to ensure that equipments from different parts of the world are capable to work smoothly with each other. With a view to furthering this initiative, various standards are being put together and distributed among engineers the world over.
Today, various product certifications of different countries call for certain norms to be followed. Examples of such certifications are CE Marking, FCC etc.
Before developing a product, one should first identify EMC standards that the product should comply with. This depends on various factors like the category of the product, field of application and the certification required before deployment. These standards are called project standards. Normally these address the basic standards like IEC or CISPR (known as testing standards) and also specify the requirements for various tests to be applied to the product.
For example, if a modem is required to be CE marked, then one should follow BS EN 55022 standard for emission testing, measurements and limits and BS EN 55024 standard for immunity testing, measurements and limits. In doing so, adequate attention should be given to ensure only latest version of the standard is followed, e.g. BS EN 55022:2010 & BS EN 55024:2010, where ‘:2010’ stands for the last publication year. These standards can be referred to as the project standards for a modem. They also specify various tests to be carried out as per IEC 61000-4 & CISPR 11 and CISPR 22, which are called testing standards.
It is for the design house to see that due diligence goes in maintaining adherence to all applicable project standards.
The project standards help designers identify least design requirements for a product. For example, project standard BS EN 55024:2010 specifies that a modem must withstand a surge of 1KV, 10/700 tr/th μs at the mains input port without failure. This becomes the least design requirement for surge protection on the mains input port of the modem. Listing out such minimum design requirements at the beginning of the design cycle is a part of good practice of EMC consideration. Adopting this method during the initial phase of component selection, circuit design, PCB layout and product design can facilitate essential EMC performance.
Two different approaches can be followed for EMC development & testing of electronic circuits.
This should be followed when designers have acquired adequate experience and possess practical know-how in various issues related to EMC. Using this method, the complete electronic circuit can be designed at once keeping in mind the least design requirements as discussed above. The product is then tested for EMC. If it passes at first effort, the design can be frozen. This being a fast turn approach, it helps minimize time and costs, provided the results are positive. On the flip side, if the product fails to comply with any EMC test, then the whole product has to be redesigned. Hence this approach should be followed only when designers have demonstrable experience and are confident of meeting stipulated EMC guidelines for the least design requirements. Importantly, this method mostly suits comparatively small and not so complex products only
This modular design approach is more popular among design engineers. Here, the circuit is divided into various modules according to the testing requirements. Each module is developed, tested and finalized individually before integrating them to form the complete product. Since individual modules would have already cleared all required EMI/EMC tests, the product generally passes such tests at first pass. Also each certified module can be used in several products without the need to redesign or recertify.
Although at first glance this approach seems tedious and time consuming, it has a higher probability of passing the tests. In the long run, it reduces the development costs significantly, as the library of the finalized modules grows.
For all the approvals / markings like FCC, IEC & CE, third party testing and verification for EMC compliance is mandatory, i.e., the tests should not be done either by the manufacturer or by the approval or marking authority. Although manufacturers may establish their own test laboratory for developmental testing, a third party testing is still required before the final approval or certification can be granted. Many test laboratories offer EMI/EMC related testing at a reasonable cost. Also some laboratories offer developmental test shifts in which the parameters for the tests are maintained but any test failure will not reflect on the certification process. These developmental test shifts are also available at lower costs. So even if a product fails a certain test, engineers still can make necessary modifications and redo the test for verification. In the development test shifts, sequence of tests need not be maintained, making it less taxing on product engineers.
For EMC, the foremost concerns are: Does the Device Under Test (DUT) cause other devices or systems to fail, and does the DUT itself fail due to lack of immunity to external electromagnetic energy?
A DUT can be tested under Class A or Class B as defined below
Class A – DUT passed the tests as per industrial requirements.
Class B – DUT passed the tests as per commercial requirements.
The DUT is listed under any one of the below mentioned criteria after the final testing is done.
Criteria A – The DUT withstood disturbance without any functionality loss.
Criteria B - The DUT lost functionality during the test, but recovered automatically (without operator interventions) after the test.
Criteria C – The DUT lost functionality during the test, and recovered after operator interventions.
Criteria D – The DUT could not withstand disturbances and suffered a permanent failure.
Following are tests carried out on a DUT during EMC compliance procedures -
(1) Electrostatic Discharge (ESD) test according to IEC-61000-4-2
(2) Radio Frequency Immunity test according to IEC-61000-4-3
(3) Electric Fast Transient (EFT) test according to IEC-61000-4-4
(4) Surge test according to IEC-61000-4-5
(5) Conducted Immunity test according to IEC-61000-4-6
(6) Power Frequency Magnetic Field test according to IEC-61000-4-8
(7) Voltage Dip test according to IEC-61000-4-11
(1) Conducted emission test according to CISPR 22
(2) Radiated emission test according to CISPR 22
Ensuring products or equipments are designed to work suitably in its intended electromagnetic environment while being able to survive high levels of immunity to maintain reliability and quality is therefore now becoming inevitable. This heightened sense of awareness towards EMC/EMI compliance is due to the fact that EMC compliant design methodology will evidently ease customers’ development costs (for product development), reduce verification schedule and boost product competitiveness.