It is a common failure in many switching power supplies to expect that
the power line voltage will snap on quickly, and snap off quickly when
the supply is activated, or deactivated.
If the mains supply hangs around in certain brown out voltage ranges, it
can fool the start up control circuitry into exceeding the ratings on
the bootstrap circuitry, or the inrush limiting circuitry, and toast parts.
Better supplies have timers that make sure that the inrush protection,
or boot strap power only last for the amount of time expected, and
shut down if those values are exceeded.
The usual UPS can be complicit in these failures because they just pass
the mains power through to the load, and don't switch over to inverter power
until a programmed amount of brown out has occurred... if at all.
Basically they pass the crappy brown out power on to the load.
-Chuck Harris
Andrea Baldoni wrote:
Dear fellow experimenters, hello.
Yesterday, in the zone near my lab, the weather forecast was of a big storm.
I was here doing some work when the sun got covered by clouds and started
a strong wind. The neon lights went away for a fraction of a second, not
enough for the PC to reset, then, something like ten seconds later, the lights
went out, I heared a multiple clicking sound from the electric switch box, then
the UPS alarm.
I thought it was a blackout, but no, the UPS was in overload alarm, there
was smell of something burned and all chain of three automatic switches
powering it tripped.
It turned out that the power supply of my server, protected by the line
interactive UPS burned out. I opened it, and the chain of resistors and
diodes giving the startup power to the control IC UC3843 arched and the IC
itself exploded (more details on the circuit later on request, I still didn't
reverse engineer it).
To this point, nothing exceptional: an overvoltage came from the line and,
even if nothing else of the many devices in the lab (fortunately) had been
affected, this power supply died. I didn't saw any lightning however and no
thunder.
The strange thing came this morning, when a customer, having the office
19Km far from my lab in a zone populated enough to have between us some small
towns and industrial areas, called because his server was off. Guess? Exactly
the same power supply, exactly the same components arched, and the control IC
exploded in the same way. Also in their office, no other equipment was
affected, everything was running and the UPS didn't went into overload alarm
(or maybe it did and recovered by itself, I don't know in effect).
Surely we are on the same grid, even if the transformers are obviously
different, but despite I have other customers in the zone (none with the same
power supply model), I didn't get any other call (while usually I do, when a
lightning strike), and, no one reported to me to have had problems with other
equipment.
Now I just had the idea to check the logs of the servers to see if they powered
off at the same time. They did not, the one of my customer powered off
17 hours earlier, when the storm was still far away (it's unlikely the sudden
power off erased 17 hours of logs).
Could be just a coincidence?
Best regards,
Andrea Baldoni
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