Mains powered isolated SMPS....how to do the earthing/Diff. mode EMC?

[Flyback]

Hello,

For any given transformer isolated, mains powered SMPS, how do you know how to manage the Earth grounding configuration?, ……


…..and how do you know whether or not Earth should be wired to the SMPS at all?……………..
…and how do you know whether the chassis should be connected to earth ground or not?
[“chassis” being the chassis of the product in which the smps resides (ie the thing the smps powers)…….and/or the chassis of the smps if its metal]

And should the output return be………..
1….directly connected to earth ground?
2….Capacitively connected to the chassis and earth ground?

If earth ground is brought into an smps, should it always be connected to the chassis of the product in which the smps resisdes (the product which the smps powers)?

The powerint.com 20007 Datebook and design guide……………
**broken link removed**

…….is one of the worlds best references on smps, …….inside this databook, there are many different earth configurations, but its not clear when you should use which one.

Eg inside the databook…….

Page 2-130 has an AC split y cap connected to earth at the centre……and the output is connected to earth ground.
Page 2-50 has only a y cap from primary to secondary, and no earth ground at all.
Page 2-185 has no earth ground brought to the smps.
Page 3-18 has input neutral and output common connected.
Page 3-25 has Earth connected to the centre of the two y caps connecting primary and secondary…..and this point is also chassis connected.
Page 3-222 has earth connected to output common.
Page 3-292 has output return capacitively connected to chassis, but there is no earth connection to the chassis


Which earthing configuration should you use, when should you use it, and why?

 

[MikeMl]

If you are selling a product to the USA or Canada, then you do need a green-wire third-prong ground connected to the "chassis" to comply with the UL and CSA certification. If you need EMI filtering, then you need to include **broken link removed**

 

[KeepItSimpleStupid]

Well, my laptop power brick is only two wire and non-polaized. The brick is an SMPS for sure.

 

[Flyback]

If a mains product has a metal chassis, and is powered by an SMPS, then do you always earth the chassis?.

Can you confirm that "earthing" only affects radiated EMC , and differential mode EMC?

......And that radiated emissions problems always cause differential mode emissions problems, since whats radiated out, does not return down the neutral wire?

 

[ronv]

The earthy ground is usually used for safety reasons. Whenever you have line voltage present in a device (switch mode power supply) with only a metal case there is a danger that the line will be shorted to the metal case making it "hot". People get around this danger by putting things like wall worts in plastic cases so even if a wire is pinched there is no danger to you. You ususally only use 1 point to tie ac ground (the case ground) to dc ground. Once the voltage is isolated from the mains by the transformer in the switcher there is no more need for the earth ground. I think some use earth ground as a part of the EMI filter to make it work better so if it has an earth ground it should be used if you don't know what is inside.