James, I looked at your schematic.

The threshold for the mosfet you are using is quite low, but with any real curre
nt (such as the 3-4 Amp inrush current of your lamp when it is cold), the mosfet
 doesn't really turn on all the way till you have about 7 volts or so from gate
to source.  If you turn it on all the way and turn it on quickly (it can turn on
 in 60 nanoseconds, it should not get hot, since its on resistance is only .15 o
hms!  This is true even if you do it at 15 thousand times per second, as long as
 you turn it on quickly, and turn it off quickly.

The voltage translation circuit to take the 5 volt swing from the PIC and make i
t into a fast 10 volt swing with high current drive capability uses a number of
parts, so there are now mosfet driver circuits to do that for you.  However you
would do better by getting some "logic level" mosfets that turn completely on at
 5 volts, such as the International Rectifier IRL series.  The IRLIZ24N is a 55
volt device, on resistance is 0.06 Ohms!, and rated for 14 Amps continuous.  The
re are lots of others in that series that should work fine in your circuit.
The IRL540N is my favorite, 100 Volts, .044 Ohms On, 30 Amps continuous, and Dig
ikey sells them at $1.41 each when you buy 10.

The PDF file for the  3055E mosfet you are using can be found at:
http://www.st.com/stonline/books/pdf/docs/3036.pdf
   Regards,
      Ron Fial

--------------------------------------------
At 01:15 PM 6/25/98 +1000, you wrote:
>Thanks Ron, a most impressive answer.  I understood what you said and
>will test on the bench tonight.
>
>For your interest, here is the schematic.  The prototype is using a
>16F84, the final will use a 12C509.
>
>--
>James Cameron                              (james.cameron@digital.com)
>Digital Equipment Corporation (Australia) Pty. Ltd. A.C.N. 000 446 800
>Attachment Converted: "c:\program files\eudora\attach\schematic-ax4.gif"
>
At 11:35 AM 6/25/98 +1000, you wrote:
>G'day,
>
>Amateur wants to learn.
>
>My MOSFETs are getting hot and I'm wondering why.  Is my PWM frequency
>too high?  Am I simply pushing them too hard?
>  ...snip....
>James Cameron                              (james.cameron@digital.com)
>Digital Equipment Corporation (Australia) Pty. Ltd. A.C.N. 000 446 800
>

Regards,
  Ron Fial