Hi Doug:
> done, the entire safety approval reduced to a simple
> cfm rating fan for a chip both on the secondary
> side of the power supply.
The issue for me is: What is the safety requirement
that requires cfm (I presume a minimum cfm)?
Reading between the lines...
The fan cools the chip.
The chip runs warm/hot and requires forced-air
cooling.
The chip heats the printed wiring board on which
it is mounted.
The temperature of the printed wiring board is
a function of the power dissipated by the chip
and by the cooling of the chip. Without the
cooling, the PWB temperature would exceed the
limits specified in the standard.
So, I presume the safety requirement is that of
temperature of the PWB. Without the fan, the PWB
temperature would rise above the limit value specified
in the standard.
For the purposes of safety, nobody cares whether the
fan is effective at cooling the chip, or even if the
chip gets so hot as to self-destruct. We are only
concerned with the temperature of the PWB.
Working with these data, I see a number of ways out
of this predicament.
1. Control the fan by manufacturer's name and model
number. The cfm is not necessary. We simply
know by test that the cooling provided by this
specific fan is sufficient to keep the PWB from
exceeding the allowable limit.
(Since the fan is a secondary circuit motor, you
will have to comply with those requirements, but
they are a separate issue to that of the PWB
temperature.)
2. Control the fan by electrical ratings and physical
size. The electrical ratings (power) are
proportional to cfm.
3. Exempt the secondary PWB from the temperature
requirements. Since the PWB is a secondary
circuit (I presume SELV), then there is no shock
hazard in the event of failure of the PWB insulation.
So, the PWB provides no safety function in terms of
electric shock, and therefore the temperature
requirements need not apply.
However, failure of the fan is an abnormal/fault
condition for determining fire hazard. So, the
unit must be tested for 7 hours without the fan
running to determine no fire.
> So, I'm wondering some of the following:
>
> 1. Have any you ever run into something
> like this before?
>
> 2. If you have, what did you do about it?
I have not answered your questions. But, I often
invoke these kinds of reasonings so that we don't
get stuck with traditional or single solutions that
otherwise would be onerous or impossible.
Best regards,
Rich
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