At 04:14 AM 08/03/2012, Forrest Christian wrote:
>I have an application where I'm considering just using the voltage drop
>produced by Rds(on) across a FQB22P10 fet in the place of a current
>sense resistor.   This is one of those applications where the exact
>current draw is not really needed, but instead it is more being used as
>a 'load present' detection.   The load is actually around 0.5A - which
>based on the just under 0.1 Ohm Rds(on) resistance, I'm right where I
>need to be for my 20V/V current sense amplifier....
>
>Whenever I consider something like this, I worry that I've missed some
>critical piece of 'semiconductor physics' which would make this not work.
>
>Is there a reason I shouldn't do this, or why I wouldn't get at least
>somewhat accurate results?
>
>-forrest

I don't think so. Of course it's fairly sensitive to junction temperature
(maybe 2:1 over a moderate Tj range), and the minimum Rds(on) is not
guaranteed at all, but it should work just fine if you are only
looking for an "order of magnitude" type result. I think it's a reasonable
guess that Fairchild will not soon use an unduly large die area and thus ge=
t
a very low Rds(on) compared to the guaranteed/typical values. Strange
things can happen in the future though- and it's possible that 10 years
from now 0.01 ohm Rds(on) MOSFETs could be inexpensive enough to be
marked as FQB22P10 parts and would still meet all the published=20
specifications.
You may wish to take that small chance. 5*.5

And, as Russ points out, your sense amplifier has to withstand the=20
'off' voltage
without being destroyed (or causing objectionable current flow into the
load).

Personally, I think I'd think the few cents for a 0.1 ohm resistor would
likely be a more economical overall and more generally satisfactory
alternative. You would still have to worry about protecting the amplifier
from over-voltage if a large transient or fault current could occur.

BTW, detecting a relatively large 'on' voltage of a MOSFET is frequently
used as a method of detecting a short circuit, so there are examples of
this method being used in motor controls and such like.


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