How to Earth a PCB in a metal enclosure? - Page 1
Forum section: electronics / general

author: Faringdon
author_rank: Super Contributor
timestamp: July 11, 2021, 04:17:27 pm
content: Hi,There is a PCB in a metal enclosure. It comprises circuitry which is supplied by an isolating offline SMPS, which is situated on the same PCB. The secondary GND is to be connected to Earth. The chassis is also to be connected to Earth.How do you achieve this, and cause the least common mode emissions/radiated emissions?Do you use setup A or B? (attached)(Note that there is an Earth ground loop which includes the Y capacitor that straddles the transformer isolation barrier…this loop has far smaller area in setup A.)

author: pardo-bsso
author_rank: Frequent Contributor
timestamp: July 11, 2021, 05:25:45 pm
content: Unless there is a compelling reason I usually go with one connection only at the mains connector and everything tied there.Also , the garden section at Woolworths or similar stores might have something that might be handy too.

author: Just_another_Dave
author_rank: Regular Contributor
timestamp: July 11, 2021, 05:48:34 pm
content: In my experience, the ground of the secondary side of most circuits is left floating unless some requirement for your particular application forces you to connect it to earth (someone with more experience in EMC might know in which cases it should be earthed). By doing that, discharges to the user are avoided if he touches any component not connected to ground. However, it is also possible to earth it using setup BOn the other hand, setup A (secondary and primary grounds connected) should be avoided as that would eliminate the galvanic isolation provided by the transformer

author: Faringdon
author_rank: Super Contributor
timestamp: July 11, 2021, 06:21:14 pm
content: QuoteThanks, i apologise for my poor drawing......that link in "A" doesnt connect pri and sec gnds...it connects earth to secondary GND.QuoteThanks,So by this do you mean you prefer “A”?Or do you literally mean that you would take a wire from secondary ground right back to the earth place at the mains input connector , where earth connects to chassis?As you know, a lot of comms lines connect to devices which use Earth as ground, and so you often need to earth your secondary because of this. As you know, connecting the secondary of an offline SMPS to earth always means poorer EMC performance, though it just needs to be tolerated for various reasons.As you know, when the secondary ground of an offline SMPS is connected to earth ground, then we have earthed one side of the “common mode antenna”….which makes radiated emissions far worse.As you know, its common to earth the secondary of an offline SMPS, otherwise if left floating it may float up to some random potential…often mains peak.But which of "A" and "B" in the top post gives worse EMC performance?

author: Just_another_Dave
author_rank: Regular Contributor
timestamp: July 11, 2021, 07:37:17 pm
content: Quote from: Faringdon on July 11, 2021, 06:21:14 pmIn that case, both can be used, but connecting it directly to the mains seems a betters solution as the current path will be shorter and therefore its impedance will be lower. Additionally, in setup b if a high current circulates through the earth, the chasis could be charged to a significantly high connection. Therefore, for high power devices I’d use setup A (connected through the pcb or through an external wire) or leave the secondary side of the converter unconnected to any ground (an optocoupler or similar component can sometimes be used to solve the risk of ending up using the earth of the communication wire if something fails if the output of the SMPS)The only case I know in which setup B is preferable is when a lot of heat needs to be dissipated from that circuit, as the ground connection to the chasis can be used to reduce the thermal impedance of the device