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Earthing the secondary of an offline SMPS?

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Advanced Member level 5
Jun 13, 2021
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In this thread, concerning whether or not Common Mode noise will be worse in an offline SMPS in which its secondary GND is Earth connected, please discuss the following...

Thread #3 of the below thread says that common mode noise will be worse in the above case because ***you have earthed one side of the CM antenna***

Do you know what ***you have earthed one side of the CM antenna*** means?

.......just to note, i would have thought earthing one side on an antenna makes it a worse antenna, and therefore reduces CM emissions, which sounds like a good thing?


I don´t see this "earthed one side of an antenna problem"

An offline SMPS is powered from mains. Mains usually is earthed.
Then follows a rectifier. Is the output of the rectifier still earthed or not? I´d say: partly.
Then capacitor, switcher. The switcher causes the HF problems, because the DC (capacitor) will be turned into a PWM.
And due to capacitive coupling this HF will be pushed to the secondary side.
This common mode voltage noise initially will be in the same magnitude as the PWM voltage at the switcher. more than 300V-pp.
Primary side switcher --> capacitive coupling in transformer --> secondary
Stray impedances (series, parallel) may reduce this a little.
But if you connect a capacitor from primary to secondary... it´s job is to reduce this voltage.
Let´s say the coupling capacitance is 100pF (ignoring other stray impedances) and the pri-sec-capacitor is 1nF, then idealy the common mode voltage noise can be reduced to 1/11.

Mind: this does not reduce the short circuit noise current.

An additional inductive common mode filter increases series impedcance, thus it reduces short circuit noise current and further reduces common mode voltage.

Mind: voltage always needs a reference. And current needs a loop.
A sketch that shows these "reference" and "loop" will help to understand.

An antenna probably isn't the best analogy.

Here's a very simple diagram of what's going on regarding common mode emissions. The actual source of the emissions is represented by a thevanin source (Vth and Zth) bridging the isolation barrier in the SMPS. Between the primary and mains (or the LISN, in a test setup) is a line filter with impedances Z1 (the series chokes) and Z2 (Y capacitors). On the secondary side, Z3 represents the impedance from the secondary side to earth.

It's clear that if Z3 is an open circuit, both Ith and Icm are zero. Shorting the secondary to earth is equivalent to making Z3=0.

Thanks Mtwieg, your diagram prooves beyond doubt that common mode emissions are worsened by connection of earth to the secondary.....
....and then theres the situation where the isolated output is connected to earth by comms cables which come to it from other products.......rather than the connection of z3 to earth being to the incoming mains cable to the product........and whether or not that worsens or improves common node emissions?

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