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Class 2 SMPS - Removing the Y capacitor - Good design solution wanted !
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pedrocelli:
I'm new in this blog and doing DIY projects related to vintage computers for many years. Often, I've stumbled over the the electric shock effect, when working with class 2 SMPS together with other earth grounded instruments or computers. It's about the Y capacitator in almost every SMPS connecting secundary and primary side. Especially in class 2 (not earthed) PSU, I can't understand, why the existing design tradeoff resulting in a typical "touch current" and high "touch voltage" is so commonly accepted. When connecting these PSU or its connected devives to other circuits, it may cause problems or even damages. How can it be, that there is no good solution for class 2 PSU ?

I understand from the authors at Electrical Engineering Stack Exchange that it's purpose is to suppress the high frequency common mode signal comming from the flyback transformer. It is necessary to put capacitors between the input and output side of the power supply with a capacitance substantially higher than the capacitance in the flyback transformer. This effectively shorts out the high frequency and prevents it escaping from the device.
(see https://electronics.stackexchange.com/questions/216959/what-does-the-y-capacitor-in-a-smps-do )

Dr. Jochen Jirmann is decribing the high frequency part in very detail in his interesting article I've translated into english.(attached PDF file)
However why do we have to live with this tradeoff ? I would rather like to remove the Y capacitator and add some components to the secundary side for high frequency supression and having a totally isolated secondary output line, no humm no electric shock.
Probaly some PSU designers had some good ideas in the past, I would rellay like to know. Or any good designs found by reverse engineering of well performing class 2 PSU ?

T3sl4co1l:
The only alternative is a well shielded transformer.  That would take heroic effort to solve, so, you're better off just clipping a ground lead on there and using a three prong cord like everything else.

Why do they put up with 'touch current'?  Because it meets the safe limit, end of story.  Is it annoying?  Is it still potentially unsafe?  Sure.  There's arbitrariness, compromise, in most standards.

If you can find a medical grade power supply to replace it, those use shielding and minimal size Y caps to meet more stringent standards.

Tim
Ian.M:
The problem is that HF coupling from the chopper transistor end of the primary to the secondary.  Without the Y capacitor between the 0V side of the primary DC bus and output 0V, and without an inter-winding screen, you'd get far more leakage current at the chopper frequency than is permitted at the line frequency as it is typically at least three orders of magnitude greater than the line frequency.

If the PSU has a three wire input including Ground, the output 0V can either be directly grounded or the HF current returned to ground via a capacitor then the Y capacitor can be fitted between the 0V side of the primary DC bus and Ground to complete the return path for the HF inter-winding leakage current.

The only other options are a foil inter-winding screen returned either to Ground (with a Y cap to the 0V side of the primary DC bus) or to the 0V side of the primary DC bus, or a split bobbin design with much greater primary-secondary spacing, but both push up the transformer cost so are typically only seen in medical devices and other applications where a low capacitive leakage floating supply is required.
pedrocelli:
Thank you for the suggestions and explanations. What I had in mind was a kind of "add-on" to a Class 2 PSU on the secondary side when the Y-cap was removed. Just enough to block RF transmission to the output lines.
Connecting the Y-cap to an incoming ground line ... does this really eliminate the secondary RF component or will the ground line be just a good antenna?
T3sl4co1l:
Well, that's the point of the Y cap, it is already "enough" to deal with EMI.  Hopefully.

Connecting it to ground instead of secondary, guarantees full EMI on the secondary. That's a non-starter.

Tim
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