Scott Dattalo wrote:


>Again, in my experience with switching power supplies of this type,
>you want to have very hard MOSFET on/off transitions. If you don't,
>you will have a less-than-efficient design. This extra power loss
>winds up mostly in the MOSFET.

I'm not trying to knock your apparent knowledge in this area, but
I assumed you were implying that the PIC's port pin was inadequate
to drive a MOSFET with a large capacitance. As for size, I picked an
IRLL014 device out of the IRF Hexfet data book with the
following characteristics:

     Vdss  = 60V
     Rdson = 0.2ohm
     Id    = 2.7A
     Ciss  = 400pF

     Package = SOT223 (small indeed)

     Under ambient temperatures this device will dissipate 2W
     into a 1" sq. area of the PCB.


IRF has a table of turn-on and turn-off times for standard CMOS
and TTL gate drives for logic level MOSFETS in thier APP note AN971.
I picked a worst case approximation for the above device
compared to a device with similar gate and miller
capacitance. I rounded up both times to the nearest 100nS.

     ton  = 200nS
     toff = 100nS

I assumed a boost converter topology for simplicity and convenience:

     Vin  = 10V
     Vout = 16V
     Fsw  = 20KHZ
     L    = 100uH

Energy of the inductor storage cycle:

     Ipk = 2A
     ton = 20uS
     Rdson = 0.2Ohm

            /20uS
           |                 2
     Els = |     (Ipk/ton * t) Rdson dt  = 5.33uJ
           |
           /0

Energy of the turn-on was approximated with a linear transition from
10V to 0V with a load current on the inductor as an initial condition
which is approximated as a constant for the turn on switch time. This
should give a VERY converative estimate.

     Iload = 0.8A
     Von   = 10V
     tson  = 200nS


             /200nS
            |
     Eson = |     ( Von - (Von/tson)*t )(Iload) dt = 0.8uJ
            |
            /0

Energy of the turn-off was approximated with a linear transition
of Voltage from 0 to 16V and current from Ipk (2A) to 0A during
the turn-off.

     Von   = 16V
     Ipk   = 2A
     tsoff = 100nS

              /100nS
             |
     Esoff = |   (Von/tsoff * t)(Ipk - (Ipk/tsoff)*t) dt = 0.53uJ
             |
             /0

Total Enery per cycle times the frequency gives the power:


     Pdiss = (5.33uJ + 0.8uJ + 0.53uJ) * 20KHZ = 133mW


No heat sink required. No special gate drive.


                                         Mike