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Offline SMPS with no Earth?

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Advanced Member level 5
Jun 13, 2021
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Welcome to the two most asked SMPS questions in companies across the world, please enlighten if poss....

"I am doing offline SMPS (non isolated), the enclosure is plastic, there is no earth wire taken to the product.
1...What do i connect my Y capacitors to?
2...Is it even worth having a common mode choke, if there are no Y capacitors?"


have you a serious reason to design a non isolated SMPS ?

If you have a transformer it is easy to isolate. What product do you intent to power ? The EMC requirements without Earth are not easy to meet, and also security for the user.

A few years ago some pc manufacturers have sold portable units with external charger without earth, a power cord terminated by a 2 pole connin forme of a 8. Now they all use the "trefle" conffiguration.
As you know, you can get away with no isolation with some products.
Even if its an isolated Flyback say, some would still ask questions 1 and 2 above.
(Though obviously if isolated, then a Y cap will go between pri and sec.)

Yes, I have worked for Thomson and Philips in TV SETS manufacturing, and we used something lige that :

But EMC requirements were difficult to reach.

And what about the security for the user ? Which activity domain ? UL or CEI norms can be of concern
--- Updated ---

I have bought this small supply last month :

And obviously it has this blue cap, but I have made no measurements, and if I remind, in tv sets where ground antenna can be touched we couldn't put more than 470pF for leakage current
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The salient point here is lile in this post here...

....Its the fact that, for best EMC performance and noise reduction in an offline SMPS in a totally plastic enclosure and no earth is best to actually put a metal foil shield at the inside of the plastic enclosure, and connect Y capacitors from the circuit ground to this metal foil !!

This should even be done if you are using a circuit in a plastic enclosure which is supplied by a battery !!

The above CHASSIS POST provides some proof of was no coincidence in that post, that the circuit would not work unless the chassis and circuit grounds were connected with Y capacitors. (Of course, shorting the chassis to cct ground also works, but thats like connecting chassis and cct ground with an infinite value Y capacitor.

The above is a basic Axiom of electronics. Would you agree?

Point 10 of the following states that no common mode filter is required when the product has no earth connection

..but this is nonsense....the earth is all around...and SMPS will couple noise to its surroundings, ie , such, a common mode choke is can also be enhanced by metal foil wrapping the SMPS, and Y cap'ing from circuit ground to the metal foil. Would you agree?

In the 80's I was working at Thomson factory in France, in charge of the familial computers, like Sinclair ZX81 or commodore 64.

We have tried to suppress graphic coating for the boxes of the vacuum tube tv sets, and for the aluminium plastic foil inside the computers, very difficult
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