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FET coil questions



Original poster: "by way of Terry Fritz <twftesla-at-qwest-dot-net>" <cwillis-at-guilford.edu>


This is a set of questions for the SSTC folks.  

Background: I just lashed up a small coil that is driven from a 555 timer,
gate driver chip, and IRF740 FET with the body diode isolated with the
standard schottky and fast diode.  This coil draws about 100 watts and puts
out 6"+ sparks, with another 555 timer interrupting the main oscillator in
a staccato fashion to keep duty cycle low.  

Question #1:  Can I use an oscilloscope to make sure that the FET switching
transitions look good?  There does seem to be some high-frequency ringing
when I look at the signal recieved with the 'scope probe dangling near the
FET gate.  Does anyone know what a good (i. e. the FET is not in danger of
failing) signal should look like?

Question #2:  What is the best method for measuring peak primary current,
and how much peak current should the IRF740 be subject to?  I know the
specs say it will take 48 A or something on that order, but is it
permissible to run at 48 A peak on every pulse in this sort of application?
 I don't know the mechanism by which the FET would fail if the peak current
is exceeded.  Maybe it just burns up because the localized heat cannot be
dealt with, although the FET may be staying quite cool in general.

Question #3:  I'd like to isolate the gate with a pulse transformer, but
all the ones I have made so far seem to decrease coil performance (so I am
still using direct gate drive from the driver chip).  True, I have no clue
what type of ferrite I wound the test transformers on.  In the future,
should I get a bead that is recommended for the HF range? MF range? I may
have also read something about pulse transformers not being able to handle
pulse duty cycles different from 50% very well.  Is there truth to this
claim?  What would be the reason?

Thanks for your help!

Carl