There are two primary functions that an input EMI filter can perform:
1. Minimise conducted emissions to avoid interfering with neighbouring equipment
2. Attenuate incoming electrical noise that could damage the system
Regarding conducted emissions, although most power supplies meet internationally recognised regulations for EMI, noise is additive and if there are multiple power supplies with noise generating loads such as high speed processor boards, it can result in the end equipment exceeding allowable limits.
If the noise is only slightly out of specification, then a (lower cost) single stage filter may suffice. If the noise is considerably out of specification then a higher performance two stage filter will be required.
To find out more, please visit the TDK-Lambda AC-DC Power Supplies FAQs page
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