Why some power supplies have Secondary side referencing to Ground??

[chitransh92]

Hello,
I have been looking at some power supply (AC to DC) circuit designs and there is at least one thing that has made me think "Why?"
Quite comfortable to understand the input circuit which includes Protection, Filter, switching circuit but when it comes to say 24V DC output I see there are a couple capacitor connected between (+24V and Earth) and (Return and Earth).
This is selective and not seen in all the designs.

I have below question to ask on this:
1. Are these capacitor for EMI protection and work as Common mode filters or for Surge Events??
2. Why these are not in all the designs??
3. If this is beneficial for the product how do we select the capacitor values in terms of Farads and Voltage??

Thank you.

[robertferanec]

This would be in EMC area ... and .... for some time I have been trying to find someone who could answer these questions. I hope in future I will create a video about it.

[rajat_jain_785]

In a Flyback AC-DC Converter, there is parasitic capacitive coupling between primary and secondary sides of the transformer which may be different for positive and negative terminals of respective primary and secondary sides of transformer. Although most of the common mode current on the primary side is filtered through the input common mode filter, the common mode energy that leaks to the secondary side through these unequal parasitic capacitors also needs to be removed. Keeping in mind that even 10s of uA of common mode current can cause your converter to fail EMI test, people generally will put capacitors on the secondary side between positive and negative terminals to the Chassis either just to be on the safe side or after their device has failed the EMI test.
There are excellent material on this by Biricha Digital: https://www.youtube.com/watch?v=JQkNqY0I02Y and by Robert Bolanos: https://www.youtube.com/watch?v=kuS-XrQU1QI

[chitransh92]

Well explained rajat_jain_785 .
Thank you for clearing the doubts...

[anovickis]

Most of the DCDC chip vendors cover this in some detail,
there is typically a capacitor placed between secondary and primary sometimes this is known as a "Y" capacitor - which provides a local return path of those currents - and they will take the shortest path

I don't want to post a particular manufacturers data - but google "flyback common mode noise y capacitor"
The need for this becomes greater when there is more capacitive coupling between primary and secondary - and also your circuits power, and noise tolerance on the output