Re: More SSTC Questions

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

Original poster: "Malcolm Watts by way of Terry Fritz <twftesla-at-qwest-dot-net>" <m.j.watts-at-massey.ac.nz>

Hi Matt,
          Some speculation on my part and I don't have time to cover 
old ground (posting a url relevant to the current query would be a 
big help ;) but have you disabled the parasitic drain-source diode in 
the MOSFETs? Are the MOSFETS getting hot prior to dying? I take it 
drain source voltages are OK (little or no deadtime). What is the 
mode of failure (guess = drain-source short circuit)? The 555 may 
simply be taking a hit via the gate circuit when the MOSFET dies. The 
gate drive transformer is a fairly universal way of feeding 
controlled signals to the gates and coupling the gates tightly 
together (sort of an exclusive OR function). Sorry to hear about your 
troubles.

Regards,
Malcolm

 
On 26 Dec 2001, at 21:12, Tesla list wrote:

> Original poster: "by way of Terry Fritz <twftesla-at-qwest-dot-net>"
<Fucian-at-aol-dot-com>
> 
> Hi all, I am wondering if a getdrive xfmr is all that neccesary?Can i use a 
> half bridge w/o the stupid transformer?Or is it really that hard to make?
> 
> I am still killing mosfets and now 555s!I have tried putting an MOV across 
> the source and drain and still no help.I have tried all the diodes and still 
> no help!?Whats wrong?
> 
> I know a half-bridge is better but I REALLY want to make the 555 and 1
MOSFET 
> ciruit work.Success would be full variac voltage(120v) without the parts 
> frying.Now WHY is the MOSFET dying with the diodes in place also?Wont a 
> ultrafast diode stop CEMF?I also have tried the schotty in series with the 
> MOSEFT source and then another ultrafast diode going drain to 
> ground.(Correct?)
> 
> I have measured the gate voltage at about 15volts.
> 
> What could kill the MOSFET now?CEMF should not be an issue.The internal
diode 
> heating should not either.Ugggggggg!!!!!!!
> 
> Thanks,
> Matt
> 
> 
> 
>