Electronic – Common ground on a switch mode power supply

groundingpower supply

I've got a switch mode power supply and I have a question on the grounding of the PCB. Documentation on the PSU can be found here:

https://docs-emea.rs-online.com/webdocs/1621/0900766b81621ef9.pdf

On the back of the pcb you can see 2 of the 4 screws holes with a star. I assume that these are meant to be screwed into a chassis where these screw holes will connect to chassis ground.

enter image description here

A schematic representation can be seen here :

enter image description here

M1 (the one on the AC side of the board is called "M1 – Safety Ground". The other one (on the DC side, not named but labelled M2) is not electrically connected to M1.

According to the documentation it states :

Grounding Required

M1 is safety ground.
For better EMC performance, please secure an electrical connection between M1,M2 and chassis grounding.

The chassis where I will be hosting this power supply has the green/yellow wires (ground) hooked up to the chassis.

enter image description here

So the questions :

  • Do I connect 2 wires from M1 and M2 to this green/yellow wire ? I don't have any corresponding screw holes so I was thinking about soldering a wire on M1 and on M2 and hooking them up to the green/yellow wire you see in the picture above.
  • Perhaps a stupid question, but if it's always the end-goal to have these 2 grounds connected (will be the case if you screw it in), then why aren't these electrically connected in the first place ?

The PSU would be attached to the chassis like this (using a single screw as the others don't correspond with the ones in the chassis) :

enter image description here

Best Answer

If you don't care for EMC, you can solder the green/yellow earth wire to both the holes. Because an electrical connection may not serve as mechanical connection, you must secure them mechanically, so they don't come loose.

Why they didn't connect M1 and M2 on the PCB?
Likely for space: a chassis trace on the primary side of the PCB (having live parts) requires clearance, double isolation, which is relative much space.
And maybe for meeting the EMC requirements