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pietroboni , 10-09-2023, 03:27 AM

Hi,
I have a PCB inside a plastic case supplied in AC 230V. With the AC input cable also comes the Protective Earth connector (PE).
The AC voltage is then filtered and supplies an AC/DC module, PCB mounted, that gives 12V DC to the whole board.
It is mandatory for me that the Vout- of the AC/DC module (for me it is the GND node) is connected with low impedance (<5 ohm) to PE from DC to at least 10kHz; so a fully isolated solution is not feasible.

Now my question is: what is the best GND-PE connection strategy, EMC-wise?
Here are some random thoughts:

• I have often been told that you should have a single GND-PE connection, for example through 0R resistor, but I don't fully understand why this should be better than a "strong" connection with GND power planes polygons
• I have found out that the position of the GND-PE connection on the PCB matters a lot: connection far from the AC/DC resulted in better radiated emission, but once I added Y capacitors (L-PE and N-PE) in the AC filter, I was forced to move the GND-PE connection close to the filter itself because otherwise the conducted emissions went crazy
• I did not try any kind of "low-pass" GND-PE connection (inductor, ferrite bead), but I was wondering what effect if could have on EMC emissions

I understand that this is a chaotic matter, but I would love to hear your experience and thought about this topic.
Thanks

qdrives , 10-09-2023, 02:34 PM

A Gnd polygon IS a single connection. It is difficult to "disconnect" it though.

PE is the reference for EMC measurements. So you want to avoid having high frequency current going over it.
And yes, the Y caps are best places close to the rest of the filter as it is part of the filter.

The filter should be placed close to the connector. I personally would also have the Gnd-PE connection there.

Adding components to the PE connection is a component that can fail. Perhaps not the best choice.

Out of curiosity - which standard or regulation requires this?