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What is EMC

2021/04/21 > Back

EMC stands for Electromagnetic Compatibility. This term conveys the meaning of the need to sustain both of the capabilities as following:
1. Not causing electromagnetic interference to other devices.
2. The ability to maintain the inherent performance even subjected to electromagnetic interference from other devices.

1. Not causing electromagnetic interference to other devices:
    Means that without this proviso devices could give EMI to other devices. EMI stands for Electromagnetic Interference. Since emitting electromagnetic waves may link to interference, the term “Emission” often used within expression.

2. The ability to maintain the inherent performance even subjected to electromagnetic interference from other devices:
    The term is EMS (Electromagnetic Susceptibility). Used in pair with EMS is “Immunity”, which requires a device has the immunity to external EMI in case causing malfunction.

It tells the reason why we say EMC contains the EMI and EMS from above explanation. From the standpoint of EMC, the requirement of 1 is not to produce or minimize EMI from itself, the requirement of 2 is a tolerance that do not occurs malfunction when subjected to EMI from external devices.

The goal of EMC is to manage the electromagnetic environment that made up of three elements: space, time, and spectrum. To the space in the home, there may have computers, stereos, refrigerators, air conditioners, mobile phones and other electrical products working at the same time. As spectrum, ITU has planned an available radio spectrum between 10kHz and 400GHz, entering the sound frequency at a lower level and light waves at a higher level, so no radio industry can get out of the range of the spectrum described above. As in the same space, at the same time plus a variety of spectrum, EMI electromagnetic interference will occur.

The types of EMI are conducted emission and radiated emission. Conducted emission is that propagated through wires or PCB wiring; Radiated emission is a type of noise that emitted (radiated) through the air. According to the type of conduction, the further classification of conducted emission has two types: "differential mode noise" and "common mode noise".

For differential mode noise, the noise voltage occurs across power supply lines, it is called “differential mode” because it is a mode in which a noise current flows on the same path as the power supply current. For common mode noise, the noise voltage doesn’t occur across power supply lines, Instead across power supply lines and GND, the noise comes from the leaks through the capacitor and returns to the power line through the earth. It is called "common mode" because it is a mode that the noise currents flow in the same direction on the power supply positive (+) and negative (-) sides.

In consideration of the space and cost, for many engineers it was the first attempt to prevent electromagnetic interference by installing power line filters. Line filters is the kind of electronic filter that is placed between electronic equipment and a line external to it, to attenuate EMI (also known as RFI (radio-frequency interference)) between the line and the equipment, it suppress unwanted interferences from correct signals in the bands. By reducing emission to a certain extent of the EMC standards, prevent equipment occurs system performance degradation or malfunction.

Data Source: Techweb