Please help clarify this doc on Earthing and EMC of Offline SMPS

[treez]

Hi
Please 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”…

   https://www.edn.com/Pdf/ViewPdf?contentItemId=4169156

Some of the statements of this doc appear to be begging for clarification…As follows below....
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Pg 2 top

Without a metal enclosure, whether Earthed or not, there is little chance that an
off-line switching power supply can ever comply with the radiated emission limits.

…can this be right?, I have seen many plastic enclosed offline flybacks which pass radiated regulations.

Pg 2

But the metal enclosure (of a power supply) is naturally eyed as an excellent fortuitous heatsink by engineers. So power semiconductors are often going to be mounted on it (with insulation). However by doing this, we also create leakage paths (resistive/capacitive) from the internal subsystems/circuitry to the metal chassis. Though these leakage currents are small enough not to constitute a safety hazard, they can present a major EMI problem. If these tiny leakage currents are not 'drained out', the enclosure will charge up to some unpredictable/indeterminate voltage……..So we need to connect the enclosure to Earth

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 2

Generally, if the equipment is designed not to have any Earth connection (i.e. a two wire AC cord), there will usually be no metal enclosure present either. Keeping the issue of radiation limits aside for now, the good news here is that no significant common mode (CM) noise can be created, simply because CM noise needs an Earth connection by its very definition. Therefore a CM filter is superfluous and can be omitted.

…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.

Pg2

One of the simplest ways of suppressing any noise is to provide decoupling between the nodes involved. For CM noise this means connecting high frequency ceramic capacitors between the L and E wires and also between the N and E wires, possibly at several points along the filter. But each of these CM line filter capacitors also unintentionally pass some of the AC line current into the chassis (besides the CM noise). To reduce the chances of a fatal electrical shock, safety laws restrict the total amount of current that can be injected into the Earth/enclosure. This in turn means that we have to limit the total CM filter capacitance.

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 3
The following concerns the use of balanced (symmetrical) filters in offline SMPS EMI filters…

In general we try to maintain balanced impedances because any imbalance basically causes some of the CM noise to get converted to DM noise. When this happens the resulting EMI spectrum may be rather confusing to analyze and fix,

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 4

As per safety standards, we cannot for example, just wind the two windings of an off-line CM choke carelessly overlapping on each other. We need to maintain a certain specified separation. Nor can we just use any bare toroid to wind them on. We need an approved coating, or a suitable designed bobbin.

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 4

A bare ferrite can be a very good electrical conductor, especially if it is the more commonly used manganese-zinc ferrite, as opposed to nickel-zinc formulations. This can be confirmed by simply pressing the tips of an ohmmeter at two points on the surface of a typical ferrite core lying around in the lab.

Suirely this is not right?, its very difficult to get any kind of resistance reading by probing ferrite surface, even with Mn-Zn ferrite.

…The following thread confirms that Mn-Zn ferrite is essentially not conductive…

https://www.edaboard.com/showthread.php?383678-Common-mode-noise-reduction-by-connecting-ferrite-SMPS-core-to-ground

Pg5

Line to Line capacitors are called 'X-capacitors'. X-caps when used before the input bridge (in offline applications) must be safety approved, but after the bridge it's basically a 'don't care' situation.

I 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?

The use of non X2 rated capacitors upstream of the bridge was previously considered here…

 https://www.edaboard.com/showthread.php?367959-Not-using-X2-rated-capacitors-in-AC-filter-for-offlien-SMPS

Pg 6

We should be conscious however that large input (X2 rated) capacitances can cause undesirably high inrush surge currents at power-up. This may also cause eventual failure of the X-cap, especially if it is the very first component after the AC input inlet. Film caps may self-heal from such an event each time, but after every event, the capacitance gets degraded just a little. Finally, after many such events, there is a cumulative effect, and we would be left with a capacitor that is barely one. Therefore, despite EMI concerns, we should rather place X-caps after any input surge protection element (e.g. NTC thermistor or wirewound resistor), and perhaps even after a front-end choke.

…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?


Pg7

Safety regulations for Nordic regions (and Switzerland) may require each Y-cap shown in Figure 1 to be actually two Y2 capacitors in series (or a single Y1 capacitor). Historically, this has been necessitated by the fact that Earthing is poor in those geographical regions. In fact, it used to be pointed out that even the central meeting room of the Norwegian safety agency NEMKO (literally Norwegian Electric Material Control) did not have any Earth point in the wall outlets. Therefore, practically speaking, a lack of Earth is not considered a fault condition in many parts of the world, but is just a normal condition (this actually also includes about 1/3rd of homes in the US!). Therefore, very often, whether the equipment is supposed to be Earthed or not, it is expected to have reinforced insulation anyway. The Earthing if present, is just for helping out with EMI.

“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 9

Engineers are often mystified to find that making the DM choke out of (low permeability) powdered iron or lossy ferrite helps too, when all else has failed, despite all the talk about DM noise being essentially a low-frequency emission. The reason seems to be as follows. The CM noise in a power supply is actually a nonsymmetric mode at its point of creation, though ultimately, by cross coupling it does tend to spread into both the lines equally. We can show that nonsymmetric noise can be considered to consist of part CM and part DM components. Therefore in practice, we do get a fair amount of high frequency DM noise too from nonsymmetric CM noise. That is where high bandwidth/low permeability/lossy materials help in DM noise suppression.

….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 10

CM noise suppression is usually said to require a very 'good' connection to Earth.

..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.

 

[mtwieg]

Pg 2 top

…can this be right?, I have seen many plastic enclosed offline flybacks which pass radiated regulations.

Yeah their statement is overly broad.

Pg 2

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?

Yes, when there's no protective earth connection, a plastic housing is safer than a metal one.

Pg 2

…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.

Again, their statement is overly broad, CM EMI can be created due to parasitic capacitance through the air. Usually a well designed layout alleviates this. But a CM choke usually doesn't help much, unless its impedance is incredibly high.

Pg2

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 .?

No, you should look up limits on earth leakage current/touch current during a single fault condition. Those are usually what limit the size of Y capacitors.

Pg 3
The following concerns the use of balanced (symmetrical) filters in offline SMPS EMI filters…

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.

The total inductance is going to be the same, though. Splitting it into two smaller inductors/capacitors shouldn't be significantly larger or more costly.

Pg 4

Suirely this is not right?, its very difficult to get any kind of resistance reading by probing ferrite surface, even with Mn-Zn ferrite.

Yeah not sure what's up with this.

Pg7

“The Earthing if present, is just for helping out with EMI”……
…surely this is wide of the mark?.....it doesn’t matter whether or not the building is actually connected to 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?

I think they mean that in some regions a protective earth connection is not relied on for protection (more stringent insulation and creepage/clearance is used). In such a case, adding a protective earth wouldn't actually improve safety significantly. But it would possibly help with EMC matters.

Not sure if that's all true, but I think that's their meaning.

Pg 9

….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?

Adding a DM choke is certainly helpful, but it won't replace the CM choke.

Pg 10

..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.

I question whether those CM chokes actually mitigate the CM EMI, then.

 

[treez]

Yes, when there's no protective earth connection, a plastic housing is safer than a metal one.

Thanks, i see what you mean...of course, even if there is no actual "Planet" earth connection, connecting the "earth" will allow the RCD to trip. Would you agree?
I cant undertsand why the article never even mentions RCDs? (the doc is called “Safety concerns for practical EMI line filters”)
RCDs shut off the electricity to a house when too much current flows in the earth...this is surely the single most safety conscious method around?...and it doesnt need a "planet" earth connection.
(which is just as well, because 1/3rd of USA homes dont have an "planet" earth connection...as the doc says.)

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I question whether those CM chokes actually mitigate the CM EMI, then.

If you look at 3:41 of this video…
https://www.youtube.com/watch?v=t5eGfaqjV3g

..or 00:29 of this one
https://www.youtube.com/watch?v=W70vmxUg-sE

..you will see the black offline power supply of this device (on the floor in the first one) . This power supply has no earth connection (just live and neutral)…but the power supply has two common mode chokes at its AC input…one a torroid of diameter 18mm, the other a torroid of diam = 9mm.
They must be of use, surely?

 

[mtwieg]

Thanks, i see what you mean...of course, even if there is no actual "Planet" earth connection, connecting the "earth" will allow the RCD to trip. Would you agree?
I cant undertsand why the article never even mentions RCDs? (the doc is called “Safety concerns for practical EMI line filters”)
RCDs shut off the electricity to a house when too much current flows in the earth...this is surely the single most safety conscious method around?...and it doesnt need a "planet" earth connection.
(which is just as well, because 1/3rd of USA homes dont have an "planet" earth connection...as the doc says.)

Because relying on someone else installing a RCD in order for your PSU to be safe is not good practice. If instead you integrated a RCD in your own PSU, then that would be nice. But that's certainly not a practical solution in most cases.

..you will see the black offline power supply of this device (on the floor in the first one) . This power supply has no earth connection (just live and neutral)…but the power supply has two common mode chokes at its AC input…one a torroid of diameter 18mm, the other a torroid of diam = 9mm.
They must be of use, surely?

I think you might have linked the wrong videos... no shot of any electronics.

In any case, no, simply seeing a component on a product doesn't prove it needs to be there.

 

[treez]

I think you might have linked the wrong videos... no shot of any electronics.

...its the product, the "smoothskin bare"..its offline power supply has no earth connection but does feature significant common mode chokes.
It was designed and built by an enormously successful (enormous) SMPS company...one of the world's biggest and best. The product has a great record in service.
The product was made for the Braun corporation...one of the finest cosmetic product co's in the world.

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Another point is the PI Expert Design software by Power.com…if you do a offline flyback design in it, (with no earth connection, only Live and neutral) , it always puts a large common mode choke at the input…and has done for at least the last decade.

 

[mtwieg]

...its the product, the "smoothskin bare"..its offline power supply has no earth connection but does feature significant common mode chokes.
It was designed and built by an enormously successful (enormous) SMPS company...one of the world's biggest and best. The product has a great record in service.
The product was made for the Braun corporation...one of the finest cosmetic product co's in the world.

Right, but the videos don't show the choke you describe. What is the point?

Another point is the PI Expert Design software by Power.com…if you do a offline flyback design in it, (with no earth connection, only Live and neutral) , it always puts a large common mode choke at the input…and has done for at least the last decade.

So...?

Without some defined capacitance/impedance to earth, common mode currents can't exist. Any design which leaves out that info isn't credible.

 

[treez]

Without some defined capacitance/impedance to earth, common mode currents can't exist. Any design which leaves out that info isn't credible.

Thanks, the capacitance to earth is the stray capacitance from the product , to the surrounding planet earth...thius is why we believe the CM choke is needed, even though theres no actual earth wire connection to the product.

 

[c_mitra]

If these tiny leakage currents are not 'drained out', the enclosure will charge up to some unpredictable/indeterminate voltage...

First, I did not read the original. Second, you have asked several questions all at a time.

This is the way as I see the statement:

Earth provides a reference point for potential measurement. Actual potential is not really meaningful, only the potential difference (that is the reason multimeters have two leads to measure potentials). Hence, in all electrical circuits, earth simply means a common reference point for measurement of any potential in the circuit under test.

At the earth's surface, the electric field is approx 100V /m, that means if you are standing in the open, your head is having a potential of about 150V! Of course this is not damaging!

The problem is that if the equipment is not connected to the (mother) earth, you must measure the voltage from the common circuit reference ground (assume you are in a plane) but that potential will be indeterminate with respect to your own body.

Consider a power supply running with a cell (battery) and the power supply produces 1000V across the terminals. Do this gedanken: you touch one terminal (or the other, does it matter?) of the power supply; do you get a shock?

For you, the potential at either of the jacks is indeterminate. But this is a crude analysis because the charges produce a field and the field can be measured (really?) and is associated with a potential. (absolute measurement of potential at a point or the electric field is tricky). That is where the EMI comes because changing electric fields are very very real and measurable.

Sometime back you posted the test report for a power supply: the isolation was tested with 600VDC (and not a 600V pulse or 600V 50Hz AC)- this is a kind of, say, very good joke.

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A bare ferrite can be a very good electrical conductor...

Ferrites are usually non-conducting but there are specs (see, e.g., http://www.mmgca.com/catalogue/MMG-FKAF.pdf) that claim considerable conductivity (nothing to boast about but, still).

Many non-metallic high temp superconductors share similar crystal structure (spinel; common structures for ferrites).

I do not consider bare ferrites as very good electrical conductor (most are excellent insulators)

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RCDs shut off the electricity to a house when too much current flows in the earth...

Most work by detecting any imbalance current between the phase and return line. It really does not care how and where the "missing" charges are going but the only place (logically speaking) they can go is to earth (because the generators are earthed at one end). The third pin is a safety point and not to be used for electrical connection (deliberately).

 

[mtwieg]

Thanks, the capacitance to earth is the stray capacitance from the product , to the surrounding planet earth...thius is why we believe the CM choke is needed, even though theres no actual earth wire connection to the product.

Right, but the amount of capacitance must be known, at least approximately. If it's just 10pF, then that's probably low enough that a choke isn't necessary.

 

[treez]

Right, but the amount of capacitance must be known, at least approximately. If it's just 10pF, then that's probably low enough that a choke isn't necessary.

Thanks, i think the point is that on the face of it, if there is no earth connection wire to the product, then it appears that there cannot be any use for a common mode choke…(unless the attachable load say has a big earthed plate on it or something)
The thing is, the stray capacitance to planet earth is often not going to be known, it depends where the product is going to be placed when its sold…..so therefore, it seems that designers always use significant common mode chokes.
For example, take LED TVs…….they often don’t have an earth wire connection, but page 2 of this app note talks about the need to use common mode choke’s in TV’s…(second paragraph up from the bottom)
https://www.ti.com/general/docs/lit/getliterature.tsp?baseLiteratureNumber=slua878&fileType=pdf
Then there is aeroplane power supplies…no planet earth up there……but they often use common mode chokes….admittedly the aeroplane chassis forms the “planet earth”, but it just goes to show…..

 

[Easy peasy]

Enough ( i.e. more and more ) CM choke will cure a power supply of emissions problems - even with no earth reference - if you take the CM impedance up high enough ( at the right frequencies ) then you simply cannot get too many uV when you try to measure it ... hence the proliferation of large hi-mu CM ferrites on power and signal leads etc ... they do actually work ...

 

[treez]

Thanks EP, I see your point…
Below is shown the two AC input common mode chokes that feature in a 70W offline flyback SMPS (240VAC) for a sold consumer product in Europe. The bigger one is 10mF, and the smaller one is 1mF. There is no earth connection in this power supply….yet it still has common mode chokes
This product was made by one of the biggest and best SMPS companies in China….the product itself has great reliability. I believe common mode chokes are definitely needed, whether or not there is an earth connection to the product….(due to stray coupling to the “earth” in whatever the surroundings are.)
I think the situation is , that in actual fact, much of our electrical theory goes by what is called the “low frequency approximation”……in other words, we kind of consider electrical current to be electron flows confined into wires and circuits……in actual fact, that model only works at low frequencies…………its an inaccurate model, and at higher frequencies (eg microwave frequencies) its inaccuracies are shown up…..the actual truth, is that all electricity is actually a 3D mesh of electromagnetic waves……just like in Maxwells Laws……this is part of the reasoning why common mode chokes are always needed…whether or not there is an earth connection.

 

[Easy peasy]

Well, we all have our own take on RFI propagation ....

 

[treez]

..Yes indeed, I remember in Year 2 of Elec eng course , the Greek Microwave Doctor got up and prooved to us that when a capacitor charges up…..energy actually gets into the capacitor via electromagnetic waves flowing in and outside of the connecting wires…….not by electric current flowing in and confined to the wires…and of course he was right…he told us….”everything we have taught you in year one was a lie”….ie, the inaccurate “low frequency approximation”…………………it is this fact, ……that what we know as “electricity”, is actually a “cloud” of electromagnetic waves flowing around and inside the wires…certainly not confined to the wires…..and this is why common mode chokes are always needed….because that “cloud” of EM waves, can spew out and “connect” to earth in the surroundings very easily.
So…the theory of microwave engineering is actually true at all frequencies…..its just that at low frequencies we can “pretend” that we have the “low frequency approximation”……..and can have our inaccurate-but-satisfactory model that electrical energy flows via a stream of electrons in the wires.

Again...this is part of the reasoning why common mode chokes are always needed…whether or not there is a wired earth connection.

 

[c_mitra]

the Greek Microwave Doctor got up and prooved to us that when a capacitor charges up…..energy actually gets into the capacitor via electromagnetic waves...

Interesting.

It cannot be further from the truth.

But I wonder how did he proved it?

I admit that in the Quantum electrodynamics, electron is nothing but a quantum of the electromagnetic field. But surely you know that without an electrical connection, a simple RC circuit connected a potential source via a switch, capacitor charging is not possible?

Of course the antenna in the radio (or your mobile phone) is nothing but a capacitor (one terminal only, but that hardly matters) that charges and discharges via the electromagnetic radiation; the current also flows via the wires.

Sadly I have not followed your logic of common mode chokes and their relation to a earth connection.

 

[FvM]

the actual truth, is that all electricity is actually a 3D mesh of electromagnetic waves……just like in Maxwells Laws……this is part of the reasoning why common mode chokes are always needed…whether or not there is an earth connection.

I would like to relate this statement to the common mode choke pictured along with it. It has unlikely relevant common mode suppression above 10 or 20 MHz. If your device is able to radiate RFI, this kind of common mode choke will hardly hinder it.

Below 20 MHz, you can safely refer to a lumped ground capacitance model to analyze the RFI sourced by your device.

 

[treez]

But I wonder how did he proved it?

...with Maxwell's Laws.....the EM flows around and inside the connecting wires...that is how the energy gets into the capacitor....ie, its the EM waves that delivers the energy to the capacitor..

 

[Easy peasy]

So…the theory of microwave engineering is actually true at all frequencies…..its just that at low frequencies we can “pretend” that we have the “low frequency approximation”……..and can have our inaccurate-but-satisfactory model that electrical energy flows via a stream of electrons in the wires.

from DC to 10MHz you can regard the current in the wires - as current in the wires, air is much more resistive to current flow than it is to magnetic field flow - which is why we need mag shielding in a lot of places - regarding low freqs as EM waves is going a bit too far - our everyday approximations are very good up to 10MHz or so ...

ever tried to build an effective broadcast antenna at 531kHz..? if so you may see my point ..

 

[treez]

OK thanks, i think we agree that common mode noise always arises due to stray magnetic coupling to earth, or stray capacitive coupling to earth.....and even if the product has no hard wired earth connection, we still always need a common mode choke, because we never know how close the power supply will be operated to something which is earthed. (or indeed, to earth itself)
If we are stood on earth (ie not flying), then evertything around us, even ourselves, is in some way connected to earth.
If its in an aeroplane, then by "earth", i mean the chassis of the aeroplane, and those wires which are connected to that, which in the aeroplane, form the "earth" wiring.

 

[c_mitra]

ever tried to build an effective broadcast antenna at 531kHz..? if so you may see my point ..

I do see your point. I spent my younger days close to a naval installation and the maze always intrigued me ...

Antenna design and operation is a black art!