Russell McMahon wrote:

> Nice FETs!
> IRF1404 is rated at 162A (junction) and 75A package.
> P/Fet = I^2R = 225 x .004 = 900 mW/FET
> A vertically mounted TO220 package will about handle that without any
> assistance at all

> You don't say what the application and switching speed is, and this can
> greatly influence decisions here.
> You would have to ensure that the FET was fully enhanced and that
> dissipation due to switching transitions were well managed.


I'm glad you added that last bit! Even in a well
designed h-bridge the FETs can spend a typical 5%
of the total timeslice in switching transitions...

Even as a rough calc with 12v supply and the 30A
mentioned, you can expect about 6v and 15A average
(90W!) for 5% of the time, which is 4.5W, many
times the on dissipation. If the supply is 24v or
more this figure can get out of hand very quickly.
(before some people start arguing average vs RMS
power etc etc it's not worth it as the exact shape
of the slewing curve or ramp will vary with load
as well as the 5% figure).

With high power drivers I was actually taught to do
it the old fashioned way, which beats any calcs <grin>
what you do is mount the transistors or FETs on the
sink you will use, run them up with a DC supply at
measured volts and amps (measured dissipation) and
using a thermometer measure the 'C/W of the installation
itself. Then when the driver is actually running you
can simply measure the sink temperature and KNOW what
your switching losses are.

Like Russell I would go with *some* heatsinking,
probably a vertical alloy bracket as a minimum.
And forget heat calcs, run the thing and measure. ;o)
-Roman

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