Re: SSTC probs

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Darren Freeman by way of Terry Fritz <twftesla-at-qwest-dot-net>" <free0076-at-flinders.edu.au>

At 04:04 AM 23/11/2001, you wrote:
>Original poster: "by way of Terry Fritz <twftesla-at-qwest-dot-net>" <Fucian-at-aol-dot-com>
>
>Hi guys, this is for the SSTC experts.Ive been using a simple 555 timer and
>IRF740 MOSFET to run a true tesla coil secondary.I get pretty good output for
>the fact its way out of tune.My input power is a 4.5amp variac.When I try to
>go abouve maybe 70 volts, the MOSFET makes a small little pop noise and
>output goes to zip...WHY?Ive killed maybe 10 now!Whats happening?Ive got a
>150amp 1200volt hyperfast diode on the output to the coil.Anode towards the
>MOSFET and cathode to the coil.I also have another identical diode going
>across the source and drain.They both get a little warm.The MOSFET is on a
>fan cooled heatsink.It gets really hot.


I know what it's like to cook lots of MOSFETS..

I'm not sure what you mean by the diode between the coil and the MOSFET, 
I'd need a circuit diagram. With only one MOSFET it sounds a little weird, 
are you running it in the flyback configuration? If so, I'd suggest two 
MOSFETS in a half bridge with a high current capacitor to block DC, 
otherwise you will dissipate a fair amount of heat in your diodes due to 
flyback pulses, to say the least.

If the heatsink is getting really hot, and the above isn't the main 
problem, then I'd say your gate drive is too weak. You need a low impedance 
source with the right ringing suppression. All of this needs a CRO or you 
won't have any idea how you're going.

Good luck, I've had some fun with my SSTC, but it's shelved because I took 
out two 900V MOSFETS.. I've gone through too many of them to try again =)

>Matt

Have fun,
Darren Freeman