Hi John:
> >The Bad: some FETs fail very violently, and can actually be a fire hazard
> >and/or shock hazard in open-frame switchers;
>
> Really? There doesn't seem to be enough combustible material to cause a
> fire hazard, and an open-frame switcher always has to be in some sort of
> outer enclosure, doesn't it? So it's not clear how a shock hazard could
> arise.
In the case of switching FETs, the power dissipated
in the FET resistance can raise the encapsulent
material to ignition temperature in which case it
will burn until consumed or until the power to the
FET is cut off. (While the encapsulent material is
flame-retardant, flame-retardant materials burn as
long as thermal energy is applied to the material.)
Depending on the specific construction, the flame
>from the burning encapsulent material could ignite
nearby materials. However, the standards do not
allow non-flame-retardant material near mains
circuits and, ipso facto, near switching FETs.
The thermal energy from an overheated switching FET
may damage basic insulation -- such as the insulation
between the FET and its heat sink. Heat sinks are
either connected to one side of the mains, or are
connected to ground. If connected to ground, the
connection should meet the requirements for a
protective ground.
So, the failure of switching FETs could give rise
to both fire and shock, but should not do so if the
equipment construction is according to our various
safety standards.
Best regards,
Rich
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