Loads of 15W SMPS blown up: help! 2

[GonzaloEE]

Hi all,

My company sells small 15W flyback SMPS for field appliances and surveillance equipment. One customer claims these PS fail while powering their magnetic doorlocks, even after adding a protection diode in parallel with the output.
Now we have dozens of failed PS for warranty repair, We're confused because I'm sure other customers are using these PS in much worse conditions with no problem, for up to 3 years in some cases.

Here's what we got:

Supply: Flyback SMPS, 15W, 220VAC in, 12V/15W out
Load: Magnetic Doorlock 12V, with 500mA avg./3A max consumption.
Protections: Customer is adding one 1N4007 diode in parallel with load (Id' suggest a schottky instead)

All boards have a TNY268GN switcher, an RCD snubber/clamp circuit for xformer primary, and a 2A diode bridge. Protection is composed of a 275V MOV and a 3A fuse, with no PFC nor EMI filter onboard. Most failed boards show the TNY and RCD snubber diode blown up.

I'm thinking of mains surges and load transients, though I'm not a power design expert. Any ideas for fixing this up will be highly appreciated.

Thanks in advance,

 

[IRstuff]

Under steady-state conditions, sure, but odd things can happen prior to power-up...

TTFN

FAQ731-376

 

[GonzaloEE]

Yes, it makes sense. I'll pay another visit to the client and check that out.

Thanks again,

 

[GonzaloEE]

PowerInt forums suggested that inductive kicks may come up in phase with the secondary reverse voltaje, thus exceeding the rated PIV of the schottky (60V) and blowing it up.
Any opinions?

 

[IRstuff]

And why would that only happen at this one customer?

TTFN

FAQ731-376

 

[GonzaloEE]

That's what we're all thinking all the time: We've sold some 8k pcs for 3 years, and this customer alone makes our 0.1% failure rate.
The customer claimed 100 times their field setup is OK, and he's visibly tired of all this. This company makes some 40% of our monthly income, so it's a quite delicate situation with his word against ours.
We're looking for a definite proof of failure, or incorrect setup. Otherwise, get hints for future design upgrades.