Working with telephone company power supplies for
about 44 years has shown me that Will Matney's, (et al),
comments are dead on.
For me, power supplies were in two usual flavors:
High Voltage/Low Current and Low Voltage/High Current.
For the most part, the most frequent failures have been
with semiconductor junctions in switching type supplies.
After that, semiconductors in Class-A (ferromagnetic)
rectifiers exhibited the shortened MTBF.
What was found was that Power Line Disturbances
on the order of 1 X 10 e-12 seconds, and sometimes
with potentials of thousands of volts were trashing
the junctions.
Heavy equipment on the same line as the rectifiers, such as
elevator motors, HVAC equipment, assorted compressors,
all using open-frame contactors were inducing spikes in
the a.c. mains supply and these spikes just went right
through the filtration steps and wore down the diodes.
Years ago, when semiconductors were less impervious
to PLD's, (over-engineered rectifier banks, etc.),
the frequency of power supply failures was much lower.
As equipment evolved to faster microprocessor times
and switching supply circuits additional protection
needed to be added to the power plants in the form of
MOV's, choke-reactors and gas-discharge devices.
In one circumstance the general manager just didn't
believe that PLD's were the cause of his frequent
failures and we were authorized to rent a Power Line Analyzer
with a strip-chart recorder. Every morning at 0200,
when the building heating saystem cut back in there was
a huge spike of around 3,000 volts for only 12 microseconds.
It took a ferromagnetic conditioner with a buck and boost
winding and capacitor to filter that spike, but it worked.
Hal Mandel
W4HBM
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