piclist 1995\12\01\185115a >
Thread: PIC MOSFET control
picon face BY : mlk@spam_OUTASU.EDU

Hi all,

       I have a puzzler which is related to PICs only because I use a PIC to
control the circuit.  If you want to save your mail reading bandwith
for PIC only questions skip to the next message now.....

       I am using a PIC to switch a full-wave
rectified signal to a load through a MOSFET.  The actual circuit is a PIC
16C57 output driving a CMOS 4050 buffer which drives the gate of an IRF520
n-channel MOSFET.  The switched signal is a full-wave rectified 24vac
(rms) so that means roughly 12volts rms (upper half of the waveform).
Here is the circuit:

                                  ^         _      _      _
                                  |        / \    / \    / \
                               12vrms     /   \  /   \  /   \
                                  24vac (rms), full-wave rectified
                                   load (4 ohm, pure resistance)
       PIC-------->MHC14050------->IRF520 gate         } rds(on)=0.27ohm
                                  ground (common to PIC and rms waveform)

I use the PIC to sense the zero of the rectified signal and turn on the
MOSFET at low voltage (approx. 1 volt).  The MOSFET turns off at the
appropriate point to provide the desired power to the load.  This all
works like a champ.

       My problem is that the MOSFET is getting very hot.  It is rated
at 8 amps and 40 watts with rds on of 0.27 ohm (at 5 amps).  Thus far I have
only used about 30 percent duty cycle to the load.  By my calculations that
should be just over 2 watts in the MOSFET even at 100% duty cycle.  But
that little puppy is cooking.

       I hesitate to display my ignorance but am I just missing
something basic about the use of MOSFETs?  I did not expect to be near
the limit for power usage at all.  If any of you see an obvious solution
please let me know.

       It is probably most appropriate to respond directly to me at
.....mlk.....spamRemoveMEasu.edu.  This is not just PIC related so let's not clog the list
bandwidth with discussion of MOSFET physics.

Thanks all for your patience,

Martin Kirk
Arizona State University
(602) 582-5718

See also: www.piclist.com/techref/microchip/devices.htm?key=pic
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