Earthing the secondary of an offline , isolated SMPS - Page 1
Forum section: electronics / general

author: Faringdon
author_rank: Super Contributor
timestamp: July 29, 2021, 06:41:06 pm
content: If you earth the isolated secondary of an offline smps, can you confirm that an earth connection to the secondary, must come from the earth wire in the mains cable that supplies the smps?Also, that the smps must feature y caps from live and neutral to earth, as well as a y cap across the smps isolation barrier?Also, that the "earth loop" that runs through the y caps across the transformer (across the isolation barrier) and also through the earth connection that runs across the isolation barrier of the smps, (to secondary ground) must be as narrow in area as possible?

author: f4eru
author_rank: Super Contributor
timestamp: July 29, 2021, 09:22:40 pm
content: Like this?

author: Ian.M
author_rank: Super Contributor
timestamp: July 29, 2021, 11:18:09 pm
content: Luvverly!!! 'Proper job' that!Around a decade ago I saw a photo of aSuicide Shower'grounded' to the earth in a small plant pot (containing a plant) in a cup holder mounted to the shower wall! Unfortunately I've never found it again . . .

author: Faringdon
author_rank: Super Contributor
timestamp: July 30, 2021, 07:32:59 pm
content: Thanks, good pics, the thing about earthing the isolated secondary of an offline SMPS, is that no book in the world tells how it should be done.The earth loop must be reduced in area.Why is it that just because its an "earth" loop, every book on earth literally says its free from EMI measures, and only concerned with safety?

author: TimFox
author_rank: Super Contributor
timestamp: July 30, 2021, 08:17:48 pm
content: During the second Gulf War, the American military used some shoddy contractors to build barracks, etc. in Iraq. There were an unconscionable number of personnel electrocuted in the showers and swimming pools due to bad electrical installation, as well as fires. Seehttps://www.govinfo.gov/content/pkg/CHRG-110hhrg48065/html/CHRG-110hhrg48065.htmfor the official government investigation.

author: Gyro
author_rank: Super Contributor
timestamp: July 30, 2021, 08:20:15 pm
content: Quote from: Faringdon on July 30, 2021, 07:32:59 pmBecause the length and impedance of the earth lead going back to the supply has far too high an impedance art RF to make any difference to EMI of course.EMC mittigation measures must be very local to the noise generating device (Y caps, filters, etc).The purpose of the earth lead is to provide a defined reference, to avoid Y cap leakage from elevating the secondary, or for safety in the case of Class I.

author: Faringdon
author_rank: Super Contributor
timestamp: July 31, 2021, 09:29:14 am
content: Thanks, When you earth the secondary of an offline SMPS, then you unfortunately unavoidably create an earth loop.Your task then is to reduce this loop...since part of this loop is the dreaded isolation barrier that the noisy transformer spans across.The attached diagram is what i mean....Do you agree with this?As you know, mains earth has two equal purposes..1...safety2....EMC mitigationas discussed by the following article...https://www.edn.com/safety-concerns-for-practical-emi-line-filters/

author: Gyro
author_rank: Super Contributor
timestamp: July 31, 2021, 09:58:23 am
content: No, mains earth connection has nothing to to do with EMC mitigation. Having a short filter loop path within the smps certainly does (including creating a local RF 'earth' node within The PSU (the mains filter).Whether the SMPS / case is connected to earth by some wandering length of mains flex and the house static wiring or not is entirely irrelevant to EMC mitigation.So no, I most certainly don't agree with your false statement.You seem very pleased with your mains filter article from some contributor to EDN, which you seem to think gives total proof to your misguided ideas. If only you spent as much time applying your education to your job as you do searching the internet for the odd snippet that might give some credence to your unshakable ideas, and trolling forums.

author: tggzzz
author_rank: Super Contributor
timestamp: July 31, 2021, 11:52:32 am
content: Quote from: Faringdon on July 31, 2021, 09:29:14 amDoes that article explain why protective mains earths were mandatory long before EMI/EMC was even a concern?

author: Faringdon
author_rank: Super Contributor
timestamp: July 31, 2021, 03:21:37 pm
content: Thanks, i'm sure you will have , as i have done, done radiated EMC testing on offline SMPS in metal enclosures, that were earthed by a ropey bit of mains flex, and they pass with flying colors....take them out of the metal enclosure, and they fail bad.I also conducted EMC tested a 150W Offline led driver once, and we had a common mode problem from 150khz to 6MHz.....(due to bad layout).The EMC house boss recomended we shove an earth connection on the PCB, and connect mains earth wire to it, then he said, we could do common mode filtration with a y capacitor to earth.(in the end we fixed the bad layout, but the point about that we could filter it better was correct.....he actually solved the problem with a shaeffner filter module with significant common mode filter on it, before we fixed the layout. That schaeffner mains filter module had to have a mains earth connection, and used it to connect its Y capacitor to )

author: Gyro
author_rank: Super Contributor
timestamp: July 31, 2021, 05:45:39 pm
content: Yes, I have brought a number of volume consumer products to market (Class II Metal Cased) as well as industrial street furniture (Junction controllers and associated items - awkward because despite being Class I in large metal enclosures, they also have large rubber sealed weatherproof access doors). In particular, the consumer stuff had to be tested in various operating modes where the case could be grounded (or not) by braided A/V cables, antenna lead etc.Of course if you take something with a filtering scheme designed to work inside a metal enclosure and operate it outside that enclosure, it will likely fail on radiated. That's rather outside the discussion though, isn't it. The product or system that you take to the test house either includes a metal enclosure, or it doesn't (in which case it has no business being there).It sounds as if you, or at least the test house, did investigate and (hopefully) understand why the led driver behaved differently with and without a mains earth. To me, it would have been a worst case scenario, a product that behaves differently in different installation situations (length and RF impedance of mains earth connection), something that requires an explanation in order to achieve consistent compliance in the field. Test house staff tend to have good intuition and knowledge from the range or products that they test.The above sounds like an excellent example of where an unexpected external influence, in this case the variable effect of a mains earth wire of some RF impedance, is a clue to the underlying problem with the design of the product, not a cure - Something that must resolved to achieve consistent performance in the field.As previously discussed, the mains earth connection is there for electrical safety, where required on Class I products. If it is influencing the EMC performance of a product then it points to an underlying problem that definitely needs to be investigated and resolved.Edit: Sorry, that could have been written more concisely.

author: David Hess
author_rank: Super Contributor
timestamp: July 31, 2021, 09:33:48 pm
content: Quote from: Faringdon on July 29, 2021, 06:41:06 pmThe secondary does not need to be grounded to the same earth ground as the power cable, however there are limits to how much voltage can be applied between the floating secondary and the power cable ground.QuoteThat is correct. The interference suppression capacitors minimize the loop area for differential and common mode return currents at high frequencies.

author: Faringdon
author_rank: Super Contributor
timestamp: August 01, 2021, 04:46:57 pm
content: QuoteThanks, and do you agree that if sec ground of an offline isolated smps is connected to earth, then the earth connection must come from the mains cable feeding the offline smps?...ie, as in #6 above.....and the reason being to keep the earth loop small.....ie that earth loop which goes through the y caps across the transformer isolation barrier.

author: David Hess
author_rank: Super Contributor
timestamp: August 01, 2021, 11:07:10 pm
content: Quote from: Faringdon on August 01, 2021, 04:46:57 pmFor purposes of keeping the loop area small it is irrelevant because that is what the capacitor is for.The power supply is galvanically isolated from input to output across the transformer and any optocouplers, but likely has an added capacitor from the input ground to output. It may also have a high value high voltage resistor to drain leakage current. There is no requirement that the output be grounded to the same circuit as the input, but there are limits to how much voltage can be applied between the input and output before reliability suffers.If the output was grounded to a point far from the input, then a large voltage spike induced between the separate grounding points cause the power supply to fail.