Re: MMCs, LTR coils, 45kv diodes and MOSFET flyback drivers

At 08:07 AM 11/14/99 -0700, you wrote:
>Original Poster: Bryan St <warp27-at-juno-dot-com> 

>	The IRF 640 18amp 200v MOSFET is only $1.41 in MOUSER.  Would two in
>parallel be good for high power flyback driving?  The MOSFET will be
>driven by a multivibrator with PWM, so I have to wind one winding on the
>flyback.  Does anybody recommend a specific amount of turns?  I was
>thinking of doing 8-10.  Is the number of turns on the winding something
>I should spend time tinkering with?  

The IRF 640 sounds like a good choice to me. If your brewing this setup from
scratch I can recommend a few suggestions. Each IRF 640 has approx. 1200pF
input capacitance. It will require a fairly strong gate drive circuit to
charge and discharge the gate quickly. In order to minimize the on state
resistance you will want to drive the gate to 10-15v. If the gate drive is
not strong enough the mosfets will not turn on all the way and will probably
get hot quick. Even with good gate drive there will still be some power loss
in the mosfets due to the inductive kickback from the primary. This kickback
will be absorbed by the IRF 640's internal souce to drain zener.

  The thing you will want to watch for with the number of turns is
saturating the core.  The time required to saturate the core is basicaly
proportional to the number of turns and the area and inversly proportional
to the voltage across the the winding. If the core saturates the circuit
will begin to draw more current(possibly much more). So if you up the input
voltage or decrease the turns on the primary may need to cut back the pulse
with to prevent saturation.

 Oh yeah, some form of current limiting on the power supply might be a good
idea if you intend to draw arcs of the secondary.

Hope this is of some use,
Eddie Burwell