[SOLVED] What does this capacitor do in SMPS?

Further to this thread https://www.edaboard.com/showthread.php?t=231900, can you point me to the "verbose discussions at edaboard about the topic." I am interested in the comment "if there is optocoupler current or voltage feedback it should be in place!"

the capacitor is partly to make a low impedance path for "bad boy" high frequency noise to return back to the primary of the power supply.
But also the Y cap has a noted leakage current level....in other words, it basically has a resistor in parallel with it (effectively.). This is very important as it stops the secondary from getting induced up to the highest peak voltage that exists in your system (usually mains peak).
At a big global TV company, we used to augment this effective resistor with an actual 1 megohm resistor in parallel with the y cap.....so as to discharge the secondary.
When you went for coffee break people would sneek to your bench , and snip off the 1 meg resistor, and when you came back and started testing, you would touch the components on the secondary as before in order to see how hot they were getting, and would get a nasty capacitive electric shock due to the removal of the 1 meg resistor....then everyone would start laughing. This is a TRUE story.

Trees first sentence is essentially the proper explanation. This "bad boy" noise current flows thru parasitic capacitances across isolation barriers, notably the optoisolator and transformer.

When I worked at a radar place, we had to measure the minute leakage current of the y cap and record it...if it was too low, it was a fail.
So I don't think its just about common mode noise problem as in my fist sentence above.

The Radar application may have had some specific requirements, which would have made it mandatory a certain leakage current value.

For the majority of applications though, is shunting away high frequency common mode current, as you had previously explained.

Treez and schmitt trigger, thanks for your replies.
Treez, thanks also for your anecdotes. Coincidentally my question was prompted by getting a tingle from the DC output of a plug pack. My DMM showed 140VAC, but only < 1V with the capacitor removed.

I don't know about any other country, but here in Oz our mains is multiple earthed neutral. The cap, being connected to the negative output of the bridge rectifier, couples > 300v negative half wave pulses to the secondary with respect to ground. Of course the impedance is high so that any load (even the DMM or me) reduces this voltage markedly. Seems that the effectiveness of that capacitor as a bypass depends on the secondary negative being grounded as it is in PC power supplies here via the 3 pin mains plug. So I cannot see any reason to have this cap in a plug pack without a mains earth pin. What do you think?

I think its still needed.
it staves off common mode problems.
Powerint.com app notes on offline psu's explain the y cap well

I may be that you don't care much about emc regulations, buf if radiated and conducted interferences matter somehow, you won't hardly get off without an Y capacitor.

It's true it involves a certain earth leakage current, up to several 100 µA for usual capacitor values. That's not safety critical, but can be well perceived when touching the SMPS secondary.

if you pick up 140vac on an isolated secondary, then that could be just mains hum induced into the scope cable.
I presume you were just putting the scope tip on the isolated secondary 0v rail?

Hi treez, No I am measuring it between the floating secondary and ground which is why I can feel it. I am quite wrong; as has been said, the capacitor does a lot for EMI. But it also has this undesirable side effect of coupling AC to an ungrounded secondary. This explains it and offers solutions: **broken link removed**
Thanks for the discussion.