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

A comment on MOSFETs:  You want to watch those internal diodes; they're
slow in turning off.  It took me a long time to become reminded of that
from someone's website posting a while back.  At t.c. frequencies you
need to turn on & off within, at most, a microsecond & usually those
diodes won't cut it.  I use a Schottky diode in series on the drain side,
to increase the drop, and then a fast diode across the Schottky + MOSFET.

On Tue, 16 Nov 1999 20:52:29 -0700 Tesla List <tesla-at-pupman-dot-com> writes:
>Original Poster: Eddie Burwell <eburwell-at-HIWAAY-dot-net> 
>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 
>>flyback.  Does anybody recommend a specific amount of turns?  I was
>>thinking of doing 8-10.  Is the number of turns on the winding 
>>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. 
>input capacitance. It will require a fairly strong gate drive circuit 
>charge and discharge the gate quickly. In order to minimize the on 
>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 
>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 
>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 
>proportional to the number of turns and the area and inversly 
>to the voltage across the the winding. If the core saturates the 
>will begin to draw more current(possibly much more). So if you up the 
>voltage or decrease the turns on the primary may need to cut back the 
>with to prevent saturation.
> Oh yeah, some form of current limiting on the power supply might be a 
>idea if you intend to draw arcs of the secondary.
>Hope this is of some use,
>Eddie Burwell

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