EMC (Electromagnetic Compatibility) filters are used to ensure that electrical and electronic equipment does not generate, or is not affected by, electromagnetic disturbance.
Most territories apply standards to the equipment that can be sold that include requirements to limit emissions of radio frequency interference (RFI) or susceptibility to incoming RFI. In Europe we have (EMC) Directive 2014/30/EU and in North America they have FCC Part 15, military applications have MIL specs to name three. EMC filters are very often needed to specifically help meet the conducted immunity and emissions.
EMC filters are produced using deceptively simple combinations of inductors, capacitors and resistors. “Deceptively” because the way that the components can be combined to achieve the necessary real world performance is a delicate balance between safety, physical size, cost and manufacturability. Because of the near infinite possible combinations of components there are as many unique performance characteristics as there are filter products available to buy. For any particular application/standard combination there will be many different filter products that work however there will be many others that do not. As higher performance is produced by use of larger, more numerous or more expensive components there will be many filter options that achieve a pass but at an unnecessarily high cost.
For this reason Roxburgh EMC offer a pre-compliance test service. Conducted emissions test equipment is portable so this can be carried out at our laboratory in Scunthorpe or onsite with the customer. The first stage of the process is to ask the customer to fill out a single page questionnaire. This asks for technical details relating to the operation and connection of the equipment to be tested. Also required are details of the applicable standard and, if appropriate, which limit lines from within that standard should be applied. This information is then reviewed by an engineer firstly to determine that we have the required equipment and secondly to produce an estimated test time – and therefore price.
The equipment will then be tested with no filtering to determine if indeed a filter is needed. If a fail is detected the most appropriately rated filter with the minimum performance will then be tried. If there is still a fail then the next performance level up is tested and so on. The aim is to find the product that just produces a pass with a small but significant margin to ensure that the pass can be replicated on other test setups which will have minor differences.
Roxburgh EMC engineers are happy to modify filters to create variants that are ideally suited to the application. They can also advise on other aspects of the design and construction of the equipment which will make a pass easier (and lower cost) to obtain.
After the test a full report can be provided. Manufacturers who are applying a self-certified CE mark can use the report as part of their technical file to demonstrate due diligence. Manufacturers who are using a notified body to provide third party safety certification can then engage the notified body safe in the knowledge that their equipment already passes.
Changing the EMC filter used should be approached with caution. The components and insertion loss curves can be compared to give a good idea of which filters will function similarly but in the real world small differences in specification can change a pass to a fail so it is best to carry out a new test when a new filter is to be used.
These, often unpredictable, real world effects coupled with the unique circumstances of each application mean that it is not possible to specify a filter which guarantees adherence to a particular standard. There is some potential comfort in using a very high performance filter but even one over specified filter could cost more in the long run over a test in the Roxburgh EMC pre-compliance lab and equally as important, may still not achieve a pass unless installed correctly.