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Non-isolated SMPS gives less common mode noise than isolated SMPS?


Well-Known Member
Regarding an SMPS with just a two wire feed (go and return), it is a fact that generally a non-isolated design will give a lesser common mode noise problem than an isolated design.
Why is this fact not stated anywhere, in any book or website?

Bottom of page 2 (2nd paragraph up) of this document
Says that ….

In a typical IC based non-isolated DC-DC SMPS, only two lines connect to the input port. So any current going in through one terminal has to go out through the other. In this configuration, the common mode noise VCOMM will always be zero.

This is simply not true. Even a two wire non isolated SMPS can radiate energy into its surroundings and thus the “go” current will not equal the “return” current.
Last edited:


Well-Known Member
Most Helpful Member
i think what that means is the noise currents in the two wires are equal but opposite, so they cancel.

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