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Interesting failure of buck converter

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mikeselectricstuff:
I have a prototype board using a ST ST1S14PHR buck converter, converting 48V to 13.5V at 1.25A max output current.
I'm seeing an odd failure mode - after it's been running hot at high loads for a few 10s of secs, instead of the ST1S14 going into thermal shutdown, the diode fails to a near-short.
At lower loads ( ~1A) the thermal protection works OK, the chip cycling on and off.
I've confirmed that it is the effect of heat ( from the chip - the diode produces minimal heat itself), as putting a soldering iron on its ground pin induces the same failure quite quickly.

The diode is an STPS5L60U, rated 60V 5A. (Datasheet recommends STPS3L60, the 3A version)

I know Shottkys get quite leaky at higher temps, so my current theory is the 60v rating isn't enough when it gets hot. I've ordered some 100v diodes to try as I can't find any to hand.


 

Fraser:
Failure of the diode appears to be a challenge to other designers as well.

See here....

https://electronics.stackexchange.com/questions/86791/cause-of-diode-failure-in-step-down-switching-mode-power-supply

May be nothing to do with your failure mode but some interesting comments. High voltage ringing looked to be one possibility and with high currents and any inductance high frequency, high voltage ringing could potentially occur ? An RC snubber does look like a good idea if this is suspected.

Another suggestion mirrors yours. That of thermal runaway due to external heat causing increased leakage.

Hope you solve the mystery.

Fraser

Fraser:
The last comment on that referenced thread looks pretty convincing......
 To quote it here .......

“It will be over-voltage spikes that are causing the Schottky shorts (many years experience in SMPSU). If you look at the waveform with a higher bandwidth scope you will seen them. 100 MHz not really good enough, suggest 500 minimum to see all. Use snubber as suggested or higher voltage rating.

I realise this is very late, but may help others, who find the thread as I did.”

End quote.

Hope this helps

Fraser

T3sl4co1l:
Shouldn't be much overshoot on turn-on though, at least not unless the bypassing is terrible?

That diode is rated for avalanche, so it's very suspicious that it would fail due to overvoltage without also heating up.  It's probably not rated for much avalanche, so maybe it fails pretty suddenly when subjected to a large enough surge?

The other way diodes can fail suddenly, is large peak currents.

And dV/dt, but I don't really know what that does for diodes (and MOSFETs).  It's hard to even generate that much rate, though.  (Easier with GaN nowadays.)

Tim

thinkfat:
I would expect the diode to generate some heat. At the given conversion ratio, there should be a significant ripple current which the diode has to bear.

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