What is EMI and EMC ?

May 15, 2023

What is EMI and EMC ?


EMC: Electro Magnetic Compatibility, electromagnetic compatibility, is the interaction of electrical and electronic equipment with its electromagnetic environment and other equipment. All electronic devices have the potential to emit electromagnetic fields. With the pervasiveness of electronic devices in everyday life — televisions, washing machines, electronic ignition lights, traffic lights, cell phones, ATM machines, anti-theft tags, to name a few — the potential for devices to interfere with each other is high.

This means that every piece of equipment that emits electromagnetic waves or interference must limit it to a certain level, and each individual piece of equipment must have sufficient immunity to electromagnetic interference in the environment in which it is intended to operate.

If we still feel abstract, let’s give a few examples to illustrate:

Have we ever seen such a phenomenon in our life?

If the two people in the picture are equipment, then the electromagnetic compatibility of the two is unqualified. On the one hand, it shows that the interference generated by the component 1 that takes off the socks obviously affects the component 2 and makes it unable to work normally. On the other hand, it shows the anti-interference ability of the component 2. Not enough to be affected by bobbin 1…

EMC includes the following three meanings:

EMC (electromagnetic compatibility) = EMI (electromagnetic interference) + EMS (electromagnetic immunity) + electromagnetic environment

1. EMI: Electro Magnetic Interference, electromagnetic interference, that is, equipment or systems in a certain environment should not generate electromagnetic energy exceeding the requirements of the corresponding standards during normal operation. EMI is a product of “fastness”. The operating frequency of the product IC will become higher and higher, and the EMI problem will become more and more serious; however, the test standard has not been relaxed, and there is only the possibility of tightening it;

2. EMS: Electro Magnetic Susceptibility, electromagnetic immunity, that is, the equipment or system in a certain environment, in normal operation, the equipment or system can withstand the electromagnetic energy interference within the range specified by the corresponding standard.

3. Electromagnetic environment: the working environment of the system or equipment.

Here we use an old picture as a simple example of what an EMI looks like. On the left, you’ll see a photo taken from an old TV. Since it wasn’t designed for EMI, older TVs are very susceptible to EMI and the failures it causes in its environment. The figure on the right shows the result of this interference.

Electromagnetic compatibility is divided into EMI (electromagnetic interference) and EMS (electromagnetic susceptibility). Electromagnetic interference propagates through the power line, the frequency is below 30MHz, and mainly interferes with the audio frequency band. Since the switching power supply for computers has a metal shell for shielding, it is mainly conduction interference. The size of conducted interference is an important criterion to measure the quality of computer power supply. It includes two meanings: one is to prevent the electromagnetic interference on the grid from entering the grid through the electromagnetic interference generated by the power supply itself, and affect the normal operation of the host system; the other is to prevent the host itself from generating Electromagnetic interference from the system enters the power grid and affects other electrical appliances. We may have such experience in our daily work that when the computer is turned on, the nearby electrical appliances such as TV and stereo cannot be used normally, which is the effect of conduction interference. And electromagnetic radiation will bring harm to human health, so good.


Regarding electromagnetic compatibility (EMC), the internationally accepted standards are EU 89/336/EEC Directive (ie, EMC Directive), US Federal Code CFR 47/FCC Rules, etc. Only the EMC letter after the CCC certification mark in my country indicates electromagnetic compatibility certification. The power supply should meet civil standards. The switching power supply is a kind of power supply that rectifies the power frequency AC into DC, then turns it into high frequency AC through the switch, and then rectifies it into a stable DC power supply, so that there is noise and switching waveform generated by the rectification waveform distortion of the power frequency power supply. If the noise leaks out on the input side, it will appear as radiation interference, and if it leaks out on the output side, it will appear as ripple.


Electromagnetic Interference (EMI), referred to as EMI, has two types of conducted interference and radiated interference. Radiated interference means that the interference signal generated by electronic equipment transmits the interference signal to another electrical network or electronic equipment through space coupling; conducted interference mainly refers to the interference signal generated by electronic equipment through conductive media or public power lines.


Electromagnetic compatibility (Electromagnetic compatibility), referred to as EMC, refers to the operation of the system or equipment in its electromagnetic environment in compliance with the requirements and does not cause large electromagnetic interference to any equipment in the environment. EMC includes two requirements: on the one hand, it means that the appliance has a certain degree of immunity to electromagnetic interference in the environment, that is, electromagnetic susceptibility; Electromagnetic interference cannot exceed certain limits.


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