Earthing a metal SMPS enclosure and the secondary ? - Page 1
Forum section: electronics / general

author: Faringdon
author_rank: Super Contributor
timestamp: March 08, 2022, 10:20:51 am
content: Supposing you have an isolated offline 20W SMPS on a PCB in an equipment. The equipment has metal walls in its enclosure. -As such, im sure you would agree that this metal case must be earthed.Also, on the offline SMPS PCB there is also some analog circuitry with microcontroller etc which is supplied by the offline SMPS.Would you agree that the secondary of the SMPS must be connected to earth to reduce noise problems generally? Also, do you agree that there should be only one earth path from the mains earth, then to the secondary GND, then this earth being wired from there (secondary GND) to the chassis, where earth connects to the chassis in only one single place?....Would you agree with that?Also, do you agree that this means that the best way round this would be to connect the incoming mains lead earth to the offline SMPS PCB…..then to run a thickly insulated, thick earth wire from the earth point at the SMPS mains input, to the secondary ground…then to run another earth wire from secondary ground to the metal enclosure (chassis)?Can you also categorically agree with us that in no circumastance should secondary ground be left floating? (ie, it should be earthed as described?)

author: dmills
author_rank: Super Contributor
timestamp: March 08, 2022, 12:06:12 pm
content: Nope!Mains earth must go directly to chassis and there are even rules about how you connect it to chassis (At least for UL).From there you can take a wire to the supply secondary, but the primary safety earth must go to the chassis first.Seriously Treez you have been doing mains connected switchers for HOW long, read the bloody standards for mains safety, they are sort of basic to what you do.

author: penfold
author_rank: Frequent Contributor
timestamp: March 08, 2022, 02:25:04 pm
content: Quote from: Faringdon on March 08, 2022, 10:20:51 amIf that metal can in any way be reached by human hands, then it must be earthed. If it was behind many layers of insulation, maybe not. If it is a commercial product, there is no room for opinion, there is a standard that dictates the minimum requirements for safety.Quote from: Faringdon on March 08, 2022, 10:20:51 amNo. It is entirely possible to design an SMPS without earth than one with. It is also possible to create horrific noise problems with either earthed or non-earthed secondary. That's why power electronics designers exist, they can work out which configuration would be best for the application.Quote from: Faringdon on March 08, 2022, 10:20:51 amLike a star-point, sure so long as you meet the safety requirements.Quote from: Faringdon on March 08, 2022, 10:20:51 amNo, that sounds horrendous.Quote from: Faringdon on March 08, 2022, 10:20:51 amStrongly disagree. Safety and noise requirements can be met without an earthed secondary.Earth isn't magic, it doesn't just absorb noise. There are bad PSUs with either configuration out there, don't let a few cheap bad experiences of isolated secondaries taint your opinion of it.

author: Faringdon
author_rank: Super Contributor
timestamp: March 09, 2022, 09:15:04 am
content: Thanks.Though , Do you agree if the secondary of an offline SMPS is earthed, then common mode noise will generally be worse, because radiation can go into earth from the secondary side and thats what common mode noise is....ie, energy coupling away to earth rather than returning in either live or neutral?.....Basically, by earthing the secondary GND, you have earthed one side of the CM antenna - so yes CM radiation will be worse ....?

author: Gyro
author_rank: Super Contributor
timestamp: March 09, 2022, 11:42:52 am
content: To save everyone's' time, I have linked some of your previous threads relating the same subject. Maybe you should revisit the answers. Do you agree treez?https://www.eevblog.com/forum/renewable-energy/earthing-the-chassis-has-nothing-to-do-with-emc-improvement/https://www.eevblog.com/forum/projects/pcb-layout-video-is-bringing-up-questions/https://www.eevblog.com/forum/projects/grounding-earthing-and-shielding-in-an-electronics-system/https://www.eevblog.com/forum/renewable-energy/earthing-the-secondary-of-an-offline-isolated-smps/https://www.eevblog.com/forum/renewable-energy/isolated-secondaries-of-smps-need-1meg-discharge-resistors/https://www.eevblog.com/forum/renewable-energy/how-to-earth-a-pcb-in-a-metal-enclosure/https://www.eevblog.com/forum/renewable-energy/offline-smpss-isolated-secondary-must-be-earthed/

author: Faringdon
author_rank: Super Contributor
timestamp: March 10, 2022, 08:30:35 am
content: Thanks very much...And i think all those posts show just what a grey area this is.....i mean....those posts also contain this...https://www.eetimes.com/safety-concerns-for-practical-emi-line-filters/#...in the above, (a Dr Murthy backs the document), and it very clearly says that earthing in an SMPS is definitely needed to reduce EMC problems.

author: penfold
author_rank: Frequent Contributor
timestamp: March 10, 2022, 09:15:19 am
content: Quote from: Faringdon on March 10, 2022, 08:30:35 amQuoting from point 9 in that reference:Quote from: EETimesThere is always an exception or a "but" if you attempt to discuss topics like that in a general manner. There are specific solutions to specific problems in specific devices, the converse of that being for any specific solution there will be a specific device with a specific problem where that same solution has been demonstrated as detrimental.Conducted emissions are just slightly less complex than radiated, relying slightly less on specific geometries so general rules are slightly more forthcoming - there'll still be specific exceptions though.If you're just looking for a simple, single-pass design procedure to produce a good PSU, I highly doubt you'll be able to come up with one. If you just go around reading EETimes and EDN articles then it is more important to look at what properties make their discussion unique from any other and what of those properties apply to your specific problem.

author: Faringdon
author_rank: Super Contributor
timestamp: March 13, 2022, 06:44:21 pm
content: Here's a very simple diagram of what's going on regarding common mode emissions. The actual source of the emissions is represented by a thevanin source (Vth and Zth) bridging the isolation barrier in the SMPS. Between the primary and mains (or the LISN, in a test setup) is a line filter with impedances Z1 (the series chokes) and Z2 (Y capacitors). On the secondary side, Z3 represents the impedance from the secondary side to earth.It's clear that if Z3 is an open circuit, both Ith and Icm are zero. Shorting the secondary to earth is equivalent to making Z3=0.As such, the following diagram offers proof that common mode emisisons will be worse when the secondary is connected to earth......When this is agreed with, we come to the next question, which is, OK, earthing the secondary is lousy for Common mode EMC (but sometiems you have to do it, or have to allow it) ...so how exactly do we do it in order to minimise that badness?

author: dmills
author_rank: Super Contributor
timestamp: March 14, 2022, 02:11:23 pm
content: Class Y cap between primary and secondary side to provide a return path and a decent line filter to discourage that which remains?I thought this was completely standard practise?Not that I do much with mains switchers, rather just buy them in.

author: Faringdon
author_rank: Super Contributor
timestamp: March 15, 2022, 05:57:06 pm
content: QuoteThnaks yes i agree......and those steps would indeed be taken...but you have one more EMC card to play....ie. how to do the earth wiring to the secondary ground....The situation here is that you have a secondary which is connected to earth with an earth wire.It is postulated, that if this is ever done, then you should have the earth wire that comes from the mains input cable, being run across the isolation barrier to the secondary side.Is this a good postulate?Comms wires etc result in earth connections to the secondary ground anyway.....but you can still run the earth cable as described above...to improve EMC....agreed?..if you do then you surely reduce the size of earth loop antennae?

author: dmills
author_rank: Super Contributor
timestamp: March 15, 2022, 06:59:18 pm
content: The safety earthing conductor goes directly to chassis on a dedicated stud near the mains filter on most of my designs, this satisfies UL as well as minimising the loop inside the electrically noisy chassis and hence minimising the potential for a failure due to that loop picking up a stray clock and allowing the mains cable to re radiate it.Since the secondary earth is purely functional I generally use the chassis at whatever point is convenient. My input RFI filter is probably good for several tens of dB where it matters, and good enough is good enough.Quite often I will rely on the class Y and input filter can to deal with the power supply conducted bollocks and have my actual interesting doings tied down to the chassis all along the back edge.I mean you are INSIDE a metallic enclosure that is many skin depths thick up where it counts, and the IO holes all have filters and finger stock, right? What happens inside the faraday cage stays inside the faraday cage to the extent you protect the IO cables to make them part of the cage.

author: Cerebus
author_rank: Super Contributor
timestamp: March 16, 2022, 01:33:52 am
content: Quote from: Faringdon on March 10, 2022, 08:30:35 amQuote from: Shank’s LawWhich is another way of saying that just because you can find one person with "Dr." in front of their name to say something it doesn't make it necessarily correct or a good idea.

author: Faringdon
author_rank: Super Contributor
timestamp: March 16, 2022, 10:25:02 am
content: QuoteThanks, yes i agree, there is little choice here, it has to be done.....And from that point where the earth goes straight off to the chassis, i would also take the earth from there , over to the secondary ground....ie, the point where the earth breaks out of the mains cable to go to chassis....from there...kind of bring it back to run close to L and N....and then after the mains input connector to the SMPS.......run it across the smps isol barrier and connect it to the secondary....if this is not done, and the secondary gets grounded via comms wire earths, then i believe its big trouble for EMC.....i hear what you are saying, you connect secondary earth to chassis near the secondary....i actually think you can reduce the earth loop area if you run the said earth wire to secondary from the mains cable.I am sure all would agree that the way wiring is done is critical for EMC....and as you know, its no different for earth wiring.If the secondary of an SMPS is to be connected to earth......then you must do so....but if you do it......then also have the earth connection from the incoming mains cable that is spoken of above.....so why is it that we dont see this earth wire being used inside SMPS's?.....is it because its inconvenient and people just elect to use more common mode filtration instead?......i'd say it'd be better to use less CM filtration and just have the earth wire as described......the earth wire as described will kind of short out any other earth wires that are in there.....which is good as it stops those earth loops from impacting so heavily on EMC.

author: penfold
author_rank: Frequent Contributor
timestamp: March 16, 2022, 03:00:52 pm
content: Quote from: Faringdon on March 16, 2022, 10:25:02 amDo some measurements and report back then? Any calculations to back it up? In what regard would it be better?What would the effect on transients be if earth is on the secondary side before first?

author: dmills
author_rank: Super Contributor
timestamp: March 16, 2022, 04:09:47 pm
content: Also note that that earth wire will have WAY more inductance then the large flat panel of the chassis, I would be surprised if it reliably added anything.In production, 'WIRE' is a 4 letter word, they should be avoided as far as reasonably possible.Besides, who cares, it is current circulating inside the box, as long as it don't get out (Which is to say as long as you provide a return path for the input CM current that does not leave the box, which the line filter does), it is not a big concern IMHO.As I say, good enough to meet whatever standard you are targeting is good enough, nearly nobody chooses a product because is is 6dB better on conducted emissions then a competitor (As a HF radio geek, I really wish they would)....

author: Faringdon
author_rank: Super Contributor
timestamp: March 16, 2022, 08:09:20 pm
content: QuoteThanks, and it sounds like you do agree with the postulation about the earth wire....just that in a metal case it could well pass EMC anyway...though i hasten that you would probably very much agree with the postulation in the case of plastic enclosures?And also, cheap metal enclosures have indents and gaps which arent perfectly sealed, so even a metal enclosure can emit those radiated emissions as you know.QuoteYes, i agree, and radiated emissions tests is also a 4-letter-word in some cases........so i would compromise that a PCB placement for the connection of the postulated earth wire should always be added....and if necessary to pass EMC, then it can be fitted, ..but hopefully not needed.QuoteThanks, i would need access to an EMC chammber and all the kit, which i dont have unfortunately....but youre right, that would be the way to prove itAnother point about the postulated earth wire is that it should obviously be passaged away from the switching nodes, for obvious reasons. (ie not right over the switching fets.

author: dmills
author_rank: Super Contributor
timestamp: March 16, 2022, 10:08:11 pm
content: Conducted emissions tests stop at 30MHz IIRC, so for most things slot radiators are of minimal concern (Because they are electrically short and so inefficient), if it comes to that the case itself is usually electrically small.What you want to avoid is the earth wire acting as one half of a dipole with some other signal cable playing the other half.Conducted emissions from the power supplies are a rather 'meh' sort of problem, easy to fix and trivial to do precompliance on (Cheap LISN and not very fast SA will get you in the ballpark, you can probably be setup for well under £3k on ebay).It is the radiated harmonics of things like HDMI clocks and similar things that cause the real heartburn because that shit radiates and slots will do it for that.

author: penfold
author_rank: Frequent Contributor
timestamp: March 16, 2022, 11:04:27 pm
content: Quote from: Faringdon on March 16, 2022, 08:09:20 pmCalculations and simulations work outside of an EMC chamber, don't they? Even a hypothetical sketched design or some physical constraints would help

author: Faringdon
author_rank: Super Contributor
timestamp: March 17, 2022, 08:32:02 am
content: QuoteThanks, that is a very good point.The opposite of this "earth wire postulation", as you know, is to have no earth wire from sec to input of the smps, and just have the earthing coming in on whatever comms cables are connected to the secondary..........Its the fact that this is so obviously spelling out EMC disaster, that we know that we must then move to enhance the situation by using the "postulated eath wire".

author: penfold
author_rank: Frequent Contributor
timestamp: March 17, 2022, 09:57:36 am
content: Quote from: Faringdon on March 17, 2022, 08:32:02 amMust we really? Is that genuinely the only option you can think of? Is there really nothing obvious?