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There are a variety of terms used by manufacturers to describe their filters; they include “noise filters”, “EMI filters” and “EMI/EMC” filters. To best answer this question “are they the same thing?” we need to review the differences between EMI and EMC.
From a legislative viewpoint, EMI (ElectroMagnetic Interference) is the measurement and restriction, to defined limits, of unwanted conducted or radiated electrical noise from a product. Under this term, a power supply and/or the equipment it is powering is considered the “source”.
EMC (ElectroMagnetic Compatibility) immunity on the other hand is the ability to withstand, again to defined limits, a variety of external electromagnetic signals. Under this term, a power supply and/or the equipment it is powering is considered the “victim”.
Regulation of both the “source’s” emissions and the ability of the “victim” to function under those emissions (and other external influences), guarantees the end product a defined level of performance.
Regulation of conducted and radiated EMI in Europe is commonly defined by two standards. EN 55011 covers industrial, scientific and medical appliances and EN 55022 information technology and telecommunications equipment. The less stringent Class A level is used for industrial and Class B for medical and household applications.
See Fig 1
Regulation of EMC immunity in Europe is defined by EN 61000, which is a very broad set of standards. For the purpose of this article, only the two sections applicable to a typical EMI/EMC filter used with a power supply will be discussed.
EN 61000-4-4: Electrical fast transient/burst immunity test – here bursts of electrical noise are injected on the input lines to simulate inductive switching, relays, etc.
EN 61000-4-5: Surge immunity test – here single pulses are injected on the input lines to simulate lightning strikes and high energy switching.
It should be noted that both of the above sections include different test voltages (ranging from 500V to 4,000V peak) and different performance criteria. With criteria A the product will continue to operate as normal during the test, B states a recovery after the test, C requires user intervention to restart the product and D is a loss of function that is not recoverable.
Certainly any filter with suitable attenuation characteristics will reduce EMI and can to some extent improve EMC immunity due to their inductive and capacitive content.
Filters like TDK-Lambda’s RSAL and RSMN series contain amorphous cores that attenuate the bursts and pulses described in the EMC immunity EN 61000-4-4 & -5 sections and allow the product to be compliant. (see FAQ “Selecting a line filter to reduce input transients”)
Are EMI and EMC filters the same thing? Yes, although actual product EMC immunity performance will depend on the filter characteristics, the test voltage level and the desired performance criteria.
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