Offline SMPS Earthing & EMC article begs for clarification - Page 1
Forum section: electronics / beginners

author: ocset
author_rank: Super Contributor
timestamp: May 06, 2019, 11:50:16 am
content: HiPlease could anyone elaborate on the meanings of various things in this (attached) document on Earthing & EMI Filters?..The document is called “Safety concerns for practical EMI line filters”…Code:https://www.edn.com/Pdf/ViewPdf?contentItemId=4169156Some of the statements of this doc appear to be begging for clarification…As follows below....* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * ** * * * * * * * * ** *Pg 2 topQuote…can this be right?, I have seen many plastic enclosed offline flybacks which pass radiated regulations.Pg 2Quote…..So an unearthed metal enclosure around a power supply can “charge up to some unpredictable/indeterminate voltage”…..The thing is, in many parts of the world, the earth connection to the households etc does not exist, due to the terrain…this is prevalent in many parts of Switzerland, and even in a third of USA homes…..so for them, metal cased SMPS’s are going to be highly dangerous?Pg 2Quote…Surely this is not correct?, offline SMPS’s which have no earth connection, usually always have a common mode choke at their input. This is because "planet" earth is all around everything, and noise inside the SMPS will couple to it, even through the air.Pg2Quote…Surely if the chassis is solidly connected to earth ground, then its impossible to get an electric shock by touching it?, no matter how much Y capacitance connects to it? Surely the reason to restrict Y capacitor size is so that the current from the Y capacitor doesn’t trip the RCD’s .?Pg 3The following concerns the use of balanced (symmetrical) filters in offline SMPS EMI filters…Quote…..Now I agree with this, but debate whether the problem of CM noise getting converted to DM noise is really a problem?, because one simply needs to filter it out with a DM filter.Another point is that a balanced filter potentially needs more inductors (one in line and one in neutral), and so is inconvenient and more costly.Pg 4Quote…..I would have thought as long as the CM choke was wound with wire coated in acceptable level of insulation, then the torroid would not have to have an “approved coating”? Also, how would an approvals agency test whether a torroid coated in an “approved coating” had been used?......there is no safety test involving connection of any voltage higher than mains peak to the live and neutral connections…and torroids wound with ordinary enamelled copper wire would pass this….even with non coated torroids.Pg 4Quote…..Surely this is not right?, its very difficult to get any kind of resistance reading by probing ferrite surface, even with Mn-Zn ferrite.Pg5QuoteI agree that this is how the standards appear to tell it…..but why should an interposing bridge rectifier give creedance to the use of non X2 rated capacitors?Are we saying that if a big mains transient happens in an offline SMPS, then a non X2 rated capacitor is going to be protected from it just because its fitted downstream of the bridge rectifier?...Also, are the safety agencies going to conscientiously take the schematic and bill of materials and check that all DM mode capacitors upstream of the bridge rectifier are X2 rated?Pg 6Quote…Have to agree with this one, but it fails to state the real problem concerning cumulative damage to metal-film X2 capacitors due to repeated exposure to high inrush currents….and that is that the X2 capacitor can become highly conductive, and heat up and eventually explode in service…I don’t know why the article failed to mention this?Pg7Quote….“The Earthing if present, is just for helping out with EMI”………surely this is wide of the mark?.....surely it doesn’t matter whether or not the building is actually connected to "planet" earth ground…but surely an earth connection is useful (even if it doesn’t ultimately connect to “planet earth”) because it can be used to trip an RCD and save someone’s life?Pg 9Quote….this suggests that its always adviseable to add some small value inductors (using nickel-zinc or powdered iron core) to the line and neutral inputs of offline SMPS…to handle the 10-30MHz noise….because they will give a joint CM and DM filtration…….and this is prefereable to using a small CM choke…because a small CM choke won’t offer enough filtration to the high frequency DM components which always accompany high frequency CM components. Would you agree?Pg 10Quote..Surely this is not always the case?…many offline SMPS’s do not have an earth connection, and still get round it by using common mode chokes.

author: ejeffrey
author_rank: Super Contributor
timestamp: May 06, 2019, 05:37:53 pm
content: Quote from: treez on May 06, 2019, 11:50:16 amThat is to restrict the current if the earth connection is not present or broken. This can happen due to a broken connection, intentional use of a "cheater plug", or attachment to an ungrounded outlet where permitted. This is all part of the "no single point of failure". If the ground conductor is carrying significant current from the EMI filter, then a single fault (broken ground conductor) results in a dangerous configuration -- high voltage present on the case with significant current capacity behind it.This is a much bigger problem in (parts of) Europe than the US both due to the higher voltage -- the half-rail voltage is 115 V instead of 60V, and because of the shucko style plug allowing grounded plugs to attach to ungrounded outlets. They usually have RCDs, but those RCDs are usually sized too big to prevent dangerous currents from flowing. I think 30 mA RCDs are common, but 5 mA is easily enough to be dangerous.QuoteI would read that differently. The safety ground can't be assumed to be present for safety purposes because it is often not. Therefore devices must be designed to be safe (including against "single" failures) with or without an earth connection. Agree that connection to planet earth is irrelevant -- it is the missing ground wire to the outlets that is the problem.

author: ocset
author_rank: Super Contributor
timestamp: May 06, 2019, 07:16:40 pm
content: The thing is, i am sure you would agree, that if a metal chassis'd offline SMPS has a broken earth connection, then it is very dangerous, because that metal chassis may get induced up to some arbitrarily high voltage?...then somebody could touch it and get a shock.

author: rstofer
author_rank: Super Contributor
timestamp: May 07, 2019, 12:25:08 am
content: In general, all exposed non-current-carrying metal surfaces are to be grounded to earth, the days of the 'hot chassis' radio and TV are long gone. The exception being 'double insulated' items. The device itself may have a tag stating that it can only be used on grounded circuits. If a buyer doesn't have a grounded system they shouldn't use the device. Inevitably, there will be several pages of safety information in several common languages.

author: bjbb
author_rank: Contributor
timestamp: May 07, 2019, 01:53:37 am
content: "In general, all exposed non-current-carrying metal surfaces are to be grounded to earth, the days of the 'hot chassis' radio and TV are long gone. The exception being 'double insulated' items. The device itself may have a tag stating that it can only be used on grounded circuits. If a buyer doesn't have a grounded system they shouldn't use the device. Inevitably, there will be several pages of safety information in several common languages."No, there is not such a requirement per IEC60950-1, nor IEC62368-1, where it is dead metal with proper spacing. Where the metal is connected to a primary-side circuit, it cannot be exposed to the end-use environment, so the issue is mute. IEC60355-x (appliances) does have additional requirements where a single-fault condition of Basic Insulation could cause an exposed dead metal surface to a hazardous voltage.That said there are some specific mechanical and electrical requirements for exposed conductive surface in the EU (LVD) and Japan, and elsewhere.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 07, 2019, 03:44:39 am
content: I'm admittedly a bit impressed. EDN articles are notoriously bad, but when even treez is finding them sorely lacking? That's something!Tim

author: ocset
author_rank: Super Contributor
timestamp: May 12, 2019, 05:59:57 pm
content: On the third page of the document linked at the top of the top post (last but one paragraph on the third page) it says that offline EMC filters should always be balanced because otherwise CM noise will get converted to DM noise, and this “converted noise” will be very hard to fix.However, page 2 of the following document…..https://www.ipen.br/biblioteca/cd/ieee/1999/Proceed/00309.pdf….states that generally CM noise occurs mostly above 2MHz, and DM noise mostly below 2MHz. It also states that any DM noise above 2MHz will generally get conveniently filtered out by PCB layout stray inductances and capacitances.….So this tends to kind of blow the first document’s statement out of the water……because it kind of poo-poos the idea that DM noise above 2MHz will be difficult to filter out…..i mean, the first doc states that CM noise converted to DM noise will be hard to filter out….but the second doc pretty much refutes this, suggesting that above 2MHz, DM noise will be generally easy to filter out. (suggesting that layout strays will mostly filter it out)What do you make of this?This idea of CM noise getting converted to DM noise also sounds like something that surely is not that common?

author: dmills
author_rank: Super Contributor
timestamp: May 12, 2019, 09:48:42 pm
content: EDN article being up to their usual high standard basically, nothing to see here!Most of those statements make sense UNDER PARTICULAR CONDITIONS, they just seem to have left out the bits that say when particular things apply or don't!Like all publications, take with a pinch of salt and do not use the word from on high as a substitute for knowing the theory and how to apply it.Most power supply EMC stuff comes down to fairly basic electromagnetism (Usually you don't need to go as far as Maxwell), I have never understood why this stuff is treated as a big mystery.Also, a bit of leakage to a metal case in a class II device thru a class Y cap (Metal cased class II do exist, see most modern HIFI amps), while it might read half mains on a high Z meter is NOT realistically any kind of danger, the source impedance is too high.Regards, Dan.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 13, 2019, 11:18:07 am
content: CM-DM conversion is normal. It just means unbalanced (asymmetrical) impedances.Tim

author: ocset
author_rank: Super Contributor
timestamp: May 18, 2019, 07:18:34 pm
content: QuoteThanks, and since CM noise is high frequency, the DM noise that it converts to is equally high, and possibly above the SRF of the DM filter inductors, thereby meaning a few low value (high SRF) filter inductors are likely to be needed at the inputs in "go" and "return".... Would you agree?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 18, 2019, 07:54:17 pm
content: Maybe.Note that it's not the parallel self resonant frequency (SRF), in fact that gives peak attenuation. It's somewhere above, at the first series resonant frequency.That usually happens at a high enough frequency that a ferrite bead is all that's needed (if anything at all) to at least dampen the resonance.The filter impedance is also usually low enough (i.e., the X cap, the impedance of which will be dominated by its ESL at this frequency) compared to the series resonant impedance that attenuation is still fair, even at that resonance.For the same reason (ESL), smaller X caps may be necessary, in combination with the smaller inductors (whether actual inductors, or FBs or CMC leakage). There usually isn't so much emission at such frequencies to require this treatment, and also this range is better treated with layout, transformer windup, and control of switching edges.Tim