Power supply EMP protection

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I have a synchronous isolated forward converter which produces 8V@3A.
The converter will be connected to about a hundred meters of wire.
What would be a reasonable EMP protection in this case? The EMP may
be caused by a nearby lightning (no direct hit possible, "just" the
induced transients). To protect the signal lines I use a 75V GDT
+ a 50mA TBU + a TVS + a series resistor, just in case. But there
are no TBUs for currents above 500mA. Should I try to emulate one
with discretes? What are the high reliability approaches in
such a case? If that matters, the output filter is composed of
2 poly tantalums (150uF/30V) and 2 ceramics (22uF/35V).

The converter board is designed with quick replacemnt in mind
and there will always be a spare one, but it would be good to
forget about such failures in the first place.

    Best regards, Piotr

Re: Power supply EMP protection
My starting point is to connect a hefty TVS in parallel with the output
caps, add a series 100uH (give or take) choke to the V+ line and connect  
a GDT between its other end and V-. The GDT side will be the PSU output.  
A kind of a PI filter. Does it make sense?

PSU -*-L-*-hostile_world
      |   |
     TVS  GDT
      |   |
-----*---*----

    Best regards, Piotr

    



Re: Power supply EMP protection
On 13/06/18 20:44, Piotr Wyderski wrote:
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Don't forget to put something on the ground wire. At least:

PSU -*-L-*-hostile_world
      |   |
     TVS  GDT
      |   |
-----*-L-*----

Pere

Re: Power supply EMP protection
On Wed, 13 Jun 2018 20:16:04 +0200, Piotr Wyderski wrote:

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This is a really hard problem.  If the powered unit at the end of the  
100m wire can be fully floating, it gets easier.  Although if somebody  
were to have their hand on it when the EMP occurred they'd get a big jolt.

But, if both ends need to be tied to a local ground, then it is really  
tough.  The 100m wire forms part of a loop, the earth completes the loop.
Depending on how close the lightning bolt is, it couples magnetically to  
the loop, inducing a current.

For signal wires, you can put some resistance in series, which makes the  
GDT and other protectors work a lot better.  Obviously, for multiple amps  
at really low voltage, you can't have much series resistance at all.

Is there any way you can put the converter at the load end of the wire?
The input end of the converter might be a lot more robust against  
transients.

I've had plenty of stuff I thought was quite robust damaged by NEARBY  
lightning, and I'm talking about hits that were several hundred meters  
away!  The longer the wires, the more current you develop.

Jon

Re: Power supply EMP protection
Jon Elson wrote:

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There will only be this multiwire cable (probably ethernet or something  
comparable), no earth connection. So I am trying to protect against the
surges induced in this cable only. Would using a shielded cable help  
noticeably? I know what a Faraday cage is, but I don't know how would  
the FTP-like cat5e aluminum foil shield behave during such a massive dI/dt.

    Best regards, Piotr

Re: Power supply EMP protection
On Wed, 13 Jun 2018 20:59:02 +0200, Piotr Wyderski wrote:


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There's no way you can power anything requiring 8 V 3 A through an  
ethernet cable. especially 100m of it.  POE specs something like 250 mA  
at 48 V,  which will deliver 12 W max.  MAYBE, maybe you could deliver 12  
W to a load 100 m away with the POE scheme, where the power flows over  
two pairs of wires.  But, 8V/3A is just not going to work with those tiny  
wires.

I don't think the shield will do anything at ALL for the EMP case.  The  
magnetic field just goes right through the shield (and induces the same  
voltage on it, too.)

Jon

Re: Power supply EMP protection
On Thu, 14 Jun 2018 13:56:06 -0500, Jon Elson wrote:


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Prepare to be pleasantly surprised:

https://en.wikipedia.org/wiki/Power_over_Ethernet

802.3at-2009[6] (PoE+ or PoE plus) up to 25.5 W (Type 2)
802.3bt[11]  55 W (Type 3) and up to 90-100 W (Type 4)

Re: Power supply EMP protection
On Mon, 18 Jun 2018 03:17:42 -0000 (UTC), Przemek Klosowski

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While the CAT standards specify how well each individual pairs should
be twisted, but do these specify how individual _pairs_ are twisted
together ?

After all, power in PoE is connected between the center points (via CT
transformers) of two pairs.  Any unbalance of the two pairs will
increase the interference connected to the DC power. A separate well
twisted pair for power might be a better solution compared to not so
good twisting of the (four) twisted pairs around each other.
  


Re: Power supply EMP protection
On Mon, 18 Jun 2018 08:23:18 +0300, upsidedown wrote:

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 The twist rate will have *no* effect whatsoever. What's more likely to  
cause such DC phantom powering to interfere with the differential signals  
are intermittent connections in the various connector assemblies (as well  
as any permanent joints in the cabling - badly made IDCs, not forgetting  
intermittent breaks in the single core wires making up a CATn cable due  
to damage or manufacturing defects).

 Since such DC wetting currents tend to break down the initial thin oxide  
insulating film at such poor contact sites which can block the millivolt  
to hundreds of millivolt ac signals, this issue of 'induced noise' by  
such phantom powering is not regarded as a 'problem'.

 Indeed, it is often regarded as an 'asset' since it provides an early  
warning, along with easy traceability of the location of faults that  
would otherwise produce difficult to interpret and track down symptoms of  
signal corruption (bad connections will be a problem in any case,  
regardless of whether or not phantom DC powering is involved).

--  
Johnny B Good

Re: Power supply EMP protection
On Mon, 18 Jun 2018 12:04:21 GMT, Johnny B Good

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Taking this to the extremes, think about there are two separated well
twisted pairs, so full-duplex Ethernet works fine, but these pairs run
in parallel without any twist.  

Much of lightning EMP power is at 1 MHz (wavelength 300 m) or below,
so quite a lot will be induced into 100 m nontwisted wires.

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These are known phenomenons close to kW to MW transmitting antennas
working at LF/MF/HF/VHF.

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Lightning EMP can cause similar field strengths.


Re: Power supply EMP protection
On Sunday, June 17, 2018 at 10:21:32 PM UTC-7, snipped-for-privacy@downunder.com wrote:
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Hun?  There's no common mode connection of  POE with the signal,
and no differential-mode either, because the transformers cannot
put out any representation of the cable-side  center-tap voltages
into the data-carrying computer-side windings.

If there were some capacitive ooupling, the 10 MHz to 125 MHz signalling
of data is pretty far removed from any power-supply frequency, and
the 'common'mode' rejection of the receiver makes even that hard
to make an interference out of.

Re: Power supply EMP protection
wrote:

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I am not claiming that any DC/LF voltages are going through the
transformer. However, if you intend to use DC voltages present in a
PoE cable, you must make connections to the transformer primary center
taps to extract the DC power.  

If there are some differential voltages between the pairs, it will
show up as voltages at the DC/DC converter input. The DC/DC converter
must be strong enough or some protection against DC/DC converter input
will be needed.
  
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Re: Power supply EMP protection
On Wednesday, June 13, 2018 at 11:59:09 AM UTC-7, Piotr Wyderski wrote:
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Well, that makes it easier.   Ethernet is transformer-connected, the
transformers will saturate and all that gets through in a big surge is
some capacitive common-mode.    A scattering of spark gaps or neon
lamps will take most of the sting out of a nearby ligntning strike.  

There's folk who put sparkplugs across the AC lines.   Replacing plugs is
easier than rewiring the house (don't use resistor plugs, though)..

Re: Power supply EMP protection
Sorry for sending the post too early.

Jon Elson wrote:

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That's the purpose of the TBUs: provide a really quick series dynamic  
resistance. +200 fixed ohms in front of the digital circuit, to be sure.

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Hence the initial attemp with some inductance.

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There will be many devices connected to the wire, like grapes,
each of them will have their own protector and a converter.
Sometimes an LDO, sometimes a switcher, depending on the load.
None of them is supposed to consume more than 300mA and that is well
within the existing TBU capabilities. But IMHO it doesn't solve the
problem of protecting the main PSU, no matter what voltage it produces.

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Exactly, so I am trying to design robustness into the project from the
very beginning, not to random hack the network of modules afterwards.

    Best regards, Piotr

Re: Power supply EMP protection

Google  EMP mill spec

e.g. Mil-STD-185  is a PDF
http://www.dtic.mil/dtic/tr/fulltext/u2/771997.pdf


Piotr Wyderski wrote:
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Re: Power supply EMP protection

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Chuckle.  That's also what I was thinking.  

I think he means either EMC (Electromagnetic Compatibility), EMI/RFI
(Electromagnetic or Radio Frequency Interference).
<https://en.wikipedia.org/wiki/Electromagnetic_compatibility
<https://en.wikipedia.org/wiki/Electromagnetic_interference
Instead of a general acronym, perhaps disclosing which country and
which standards the power supply will need to comply.  
<https://en.wikipedia.org/wiki/List_of_common_EMC_test_standards


--  
Jeff Liebermann     snipped-for-privacy@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com
We've slightly trimmed the long signature. Click to see the full one.
Re: Power supply EMP protection
Jeff Liebermann wrote:

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No, I mean ElectroMagnetic Pulse protection and I thought it
was clear from the description of the problem. Purely a reliability
issue, nothing related to any form of compliance. And Mr NoName
is right, lightning and nukes have a lot in common, so the
military experience might be valuable here.

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As I said, it is all about making the damned thing bulletproof,
not about pleasing a civil serpent. The country is Poland, but
I don't consider our thunderbolts to be any different.

    Best regards, Piotr


Re: Power supply EMP protection
On 14/06/2018 07:26, Piotr Wyderski wrote:
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Lightning you get a huge current and if you are really out of luck  
vaporised phone conductors and an inconsolable temporarily deaf  
switchboard operator. A direct strike even with supposedly certified  
anti-lightning surge devices on the line between two buildings the  
current was so great that the thick copper earth had been mad hot and  
the potential from ground high enough to fry various components.

It was curious that buffered line drivers managed to survive by allowing  
more delicate components behind them to fail. It is too long ago to  
remember the details beyond that it took IBM most of the next day to  
rebuild the mainframe to partially working again and the entire of the  
next week before BT could get us a fully working phone system.

Nuke EMP you have the problem of sudden movement of electrons and  
breakdown voltages as the flash passes - countermeasures for that will  
not necessarily be at all relevant to a lightning strike. Beyond the  
observation that things inside hermetically sealed metal boxes tend to  
have a better chance of surviving.

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One key thing that you have to remember is that how well the protection  
circuit behaves depends a lot on how much current your reference ground  
can actually sink before it ends up floating high. There is a hell of a  
lot of current flowing in a lightning strike arc. The frozen fractal  
burn patterns on things that take a hit are very interesting.

The hit on our building pretty much vaporised the downward run of telco  
cables at the front of the building leaving a 6" stripe of soot on the  
wall. You can't really survive a direct hit but you can hope to survive  
nearby hits provided that they are not too close.

--  
Regards,
Martin Brown

Re: Power supply EMP protection
On 6/14/2018 4:01 AM, Martin Brown wrote:
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Yes. Average is 30kA, I've read. 500Mj and 15 coulombs, they say.
Source: <https://en.wikipedia.org/wiki/Lightning

Re: Power supply EMP protection
On Thu, 14 Jun 2018 10:01:54 +0100, Martin Brown

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The problem with lightnings is the ground bounce. Assume a grounding
resistance of only 1 ohm and only 1 kA of current spike, the device
potential bounces up with 1 kV compared to the neighborhood.

There are not much problem, if the device is only connected to the
mains. However, if the devices also has a connection to some other
networks (say telephone, CATV etc.) grounded at a different grounding
o. There will be kilovolts between the local bounced ground and remote
stable ground. This can cause a lot of damage, even if optoisolators
are used.

Using fibers solve this kind of problems.

With devices all connected to local ground will also survive, if
equipotential bonding has been used between local devices so that all
devices bounce up without big ground potential differences.

Using fibers or at least optoisolators also help against nuclear EMP.
It also helps using EMP protected rooms (lot of fingertock in the
doors, using long 2 m high and 1 m metallic walkways  working as
waveguides, cutting out much of NEMP energy below 75 MHz due to
wavequide cut-off).
  

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