Spehro Pefhany wrote:
>
>
> The MOSFET I suggested looking at:
> http://www.irf.com/product-info/datasheets/data/irliz44n.pdf
> has compatible pinout to the TIP42C, and he can eliminate the heatsink.
> Just change the resistor values eg. to 100R series and 10K "base to emitter"
> (gate to source). It is a 55V/30A with 0.025 Ohm Rds(on) at 5V gate drive
> and 0.035 Ohm Rds(on) at 4V gate drive (maximum). R-theta-JA is 65'C/W
> for the fullpack device, so even at 0.035 Ohm (less than 200mV drop),
> the 175'C maximum Tj won't be exceeded at 5A until the ambient temperature
> is almost 120'C. (with no heatsink) That will be reliable. ;-)


Excellent info! And a nice FET. I don't think any darlington
is a good choice with a 5A 24v load. I'm going to be picky
with your fet calcs <grin> remember 65'C/W is for free standing
TO220 pack with no parts around it, and a room temp PCB. For
most situations, especially if in a small enclosed box I would
derate that to say 85'C/W.

Assuming 0.035 ohm Rds, at 5A is 0.875W. So maybe 75'C rise
will be seen over ambient, FET will get to about 100'C.

Anything over 55'C total temp at the transistor will cause
failures, probably within months as the solder weakens and
goes through enough heat/cool cycles to get circle fractures.
This is the stuff I fix all the time in TVs, and having the
infrared thermometer now it's a big eyeopener, a TV with tran
at 50'C runs for years, models with transistors at even 65'C
fail repeatedly after a few months. With some models i've
been clamping bigger heatsinks on, or using better (lowsat)
parts, having to re-engineer their crappy product so the
poor customer gets more than 6 months MTBF. :o)

I suggest 0.3W max from an un-heatsunk TO220 pack.

One last point, FETs made for 5v drive, you won't get 5v
from a PIC output pin. PIC pins source 4.2v typical, down
to 3.5v once you are sourcing 10mA or more. This is an
important point for PIC newbies. The cure is fairly simple,
add a pull-up resistor from 5v to the PIC output pin, gives
5v at the FET when on. About 1k should be fine.

Sorry to be picky, but newbies especially should be VERY
conservative with all their ratings. The expert engineers
can start to push the limits... (but then a year later i'll
be fixing their product!) ;o)
-Roman

--
http://www.piclist.com hint: To leave the PICList
mailto:piclist-unsubscribe-request@mitvma.mit.edu