What is EMI "Electromagnetic Interference" in Power Supply ?

Hello Experts,

What is EMI "Electromagnetic Interference" in Power Supply ?

Thank you,
Lonelygirl_2012Asked:
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JohnBusiness Consultant (Owner)Commented:
A modern power supply is a switching power supply and a circuit that switches back and forth to change the level of AC input. This is used instead of a heavy power transformer. The switching circuit generates electrical (electronic) noise which is EMI. The designers of the power supply are required to mitigate the switching noise by both design and shielding.
JohnBusiness Consultant (Owner)Commented:
You should post some expertise instead of just a link - any one can google for this.
Benjamin Van DitmarsSr Network EngineerCommented:
John,

i have done alot of work in design of SMPS (Switch Mode Power Supplies).
the EMI are the guidelines inside the EMC Rules.

the basic rule is. i am not allowed to create interference with other devices. and when some device interfere with me. my device is not allowed to go crazy. in simple words.

it was for me an easy post. because our topic started wants to have the meaning and working around EMI. and this document will tell him everything he could know about this.

when he has a more specific question, i can give him a better answer.

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JohnBusiness Consultant (Owner)Commented:
@Lonelygirl_2012 - Thanks and I was happy to help.
Seasong ZHANGCommented:
In general, "Electromagnetic Interference" in Power Supply could be concluded into 2 types: one is caculated as THD(Total Harmonic Distortion) which relates to normal mode interference with frequecy below 9kHz and the other is measured by more sophisticated devices which relates to common mode interference with frequency from 150kHz to 300MHz.
Both of the 2 types of interference  could possibly make other devices around crash, or at least not work normally. So if one device has interference problem, we should add an EMI filter and L(CL) filter between that device and the power grid to protect other devices from it.

For more infomation, you should refer to standards:
EN 61000 3-2:2006 - Part 3-2:  Limits  for  harmonic  current emissions (equipment input current ≤ 16 A per phase)
EN 61000 3-12:2005 - Part 3-12:  Limits  for  harmonic currents produced by equipment connected to public low-voltage systems with input current > 16 A and ≤ 75 A per phase
EN 61000 3-11:2000 - Part 3-11: Limitation  of  voltage changes, voltage fluctuations and flicker in public low-voltage supply systems — Equipment with rated current ≤ 75 A and subject to conditional connection
And EN61000 6-3, 6-4, 4-2, 4-3, 4-4, 4-5, 4-6, 4-11, 6-1, 6-2 which I am not going to list full name of.
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