ESD problem on SMPS

Try to figure out if the input or output is the cause. I would try wrapping the L&N input wires around a ferrite bead, then testing again. The same with the output, perhaps just a bead on the +5V. Have you grounded the Common?

My thinking is the fast transients for the 6kv (or8kv) ESD test is being conducted into the supply. You can get the EMC lab reports from Aimtech, they should show how the device was setup to be tested. After all this has all the approvals even 610000-4-2.

Cheers

What happene if you put a ferrite bead in that free earth wire? Near the power suplly of course?

This looks like the ESD event capacitively couples over from the otherwise not connected PE wire into the line and neutral wires in your power cord. The same might happen with the metal enclosure, provided there is enough capacitance between it and the switcher module plus power cable.

Without knowing the innards it's anyone's guess what happens inside. A possible scenario: The ESD spike goes into the direction where the switcher in there "thinks" there was a dip in the input voltage and then goes through an undervoltage lockout scenario, followed by a re-start. Other than TVS'es and/or filtering on the outside there probably isn't much for you to do about it, other than finding a better switcher module.

You could try a common mode choke at the input, right at the switcher module.

--
Regards, Joerg 

http://www.analogconsultants.com/

cause the supply to drop its 5V output for as much as 100ms. It always re covers, but that is bad for microcontroller circuits. I have simplified th e circuit as much as possible to demonstrate it is the power supply and not the other circuitry per se:

terminals. The earth ground wire is left disconnected, as there is no ear th connection on the power supply. The other end of the cord is plugged in to a properly wired outlet.

limit resistor connected across the output to monitor the presence of power supply output. By the way, the low tech monitoring is on purpose to avoid introducing any other factors into the test.

ps out. If I use a completely separate earth ground wire, and place the en d of it near the power supply and hit it with ESD, no problem. However, th is thinking doesn't translate when I try the device the power supply is use d in. The device is in a metal case connected to earth ground, and this da mn power supply likes to drop out when the case is hit with ESD. Doesn't m atter if the case ground is provided with a separate wire or not. I've tri ed connecting the earth ground to the power supply output common, but that does no good neither (BTW Aimtec suggested that. They weren't much help ot herwise. The PS is rated such that it's allowed to malfunction, but must r ecover). A new power supply and different cord was tested, along with othe r outlets. And this isn't a cheap PS either; right around $30 in singles.

at solving this.

lem is it would need to be about 30,000 uF.

No surprise there. A clip-on ferrite is for EMI, it does next to nothing against ESD. You could try a CM choke but only if it has several millihenries. Has to sit right next to the switcher module.

--
Regards, Joerg 

http://www.analogconsultants.com/

Or even a Cx EMI line cap may help. A clip on bead wouldn?t do much, but 2 or 3 turns thru a bead may do more.

Cheers

Or even a Cx EMI line cap may help. A clip on bead wouldn?t do much, but 2 or 3 turns thru a bead may do more.

Cheers

Or, maybe it thinks the output voltage has gone too high and falsely comes out of regulation for a moment ?

Does the time it takes to recover change with output load resistance ? That might give a clue.

Have you tried small common mode caps to ground from all input and output lines ? Maybe tie them to a foil or copper wrapping to encase the power supply ? And then try adding a cap from that common connection to ground ?

boB

On a sunny day (Tue, 05 Feb 2013 18:09:50 -0800) it happened Joerg wrote in :

Bit strange idea, but would a fat transzorb from that ground wire to metal of the module work, maybe in combination with the ferrite filter?

No, because AFAIK there is no metal. And the OP wrote, quote "I've tried connecting the earth ground to the power supply output common, but that does no good neither (BTW Aimtec suggested that". This is not much of a surprise because the hiccups usually happen on the primary side. Either the IC goes through an UVLO sequence in the unlikely event that there is an IC in there at all, or the more likely transistor oscillator trips over itself.

--
Regards, Joerg 

http://www.analogconsultants.com/

On a sunny day (Wed, 06 Feb 2013 07:52:25 -0800) it happened Joerg wrote in :

OK, but somehow the pulses come in via the ground wire it seems. There can even be crosstalk from the ground wire to say for example output. Logic then dictates these pulses should be filtered, so say shorted against some system ground that is safe (interference proof). If that is not possible then changing the circuit would be needed, not an option here perhaps.

So whats your idea?

Which is not connected to anything at the switcher, meaning it can only be coupled into line&neutral.

As I said, I'd first try a common mode choke. TVS across the input may be another option but before I'd go that far I'd look for a better manufacturer.

--
Regards, Joerg 

http://www.analogconsultants.com/

On a sunny day (Wed, 06 Feb 2013 09:31:23 -0800) it happened Joerg wrote in :

Yes, right, faster, cheaper better.... Perhaps:-)

Ok, maybe a solution. I tried ferrite beads on the input, but no luck. Ho wever I found a very similar supply made by CUI; same footprint, but only 1

2V out, not 5V+12V. However this isn't a big a problem to solve. It's che aper too, and seems to be stable under the given conditions.

I like their LNK-series deivce but not too fond of the TOP-series. Anyhow, those TOP256 have a line surge shutdown protection. So if there is an ESD surge that protection feature could be tripped and then it goes through the re-start sequence. There is also an output OVP protection and the effect is very similar.

--
Regards, Joerg 

http://www.analogconsultants.com/

maybe there's a current sense resistor in the ground line?

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another test. It's a TOP256.

The topswitch feedback pin needs local decoupling with a ceramic part, not usually included in the application literature, to prevent UVLO events that are associated with transients on the switch drain - whether self-generated or otherwise.

If this local decoupling is not present, that's a long-term fix, probably outside your development budget or time schedule.

RL