Re: Shottky Diodes in SSTC ---> Not A Necessity

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

Original poster: "Terry Fritz" <teslalist-at-qwest-dot-net>

Hi,

Recently the FET drivers on one of my big high voltage supplies literally 
burst into flames.  Non of the FETS were damaged since they had extra big 
transorbs across the gates.  Only $7 worth of drive chips and some PC board 
repair were needed as opposed to all blown FETs and probably a blown HV 
converter/multiplier (fatal since it is potted).

Protecting gates with transorbs is cheap and easy and can save not only the 
FETs, but a lot of repair work, or the unit itself.

Normally these types of protection serve no use.  They are there for 
abnormal times - which are measured in microseconds...  Tesla coils are 
really vulnerable since the secondary can unexpectedly arc to things like 
the AC power cord, control circuits etc.  Also, if something does fail, 
transorbs and such can limit the damage to only one or two devices instead 
of all the FETs and most of the control circuits behind it.  In MY OLTC 
where I run say 500 amps through a little TO-247 device, the transorbs not 
only protect the gate circuits from direct hits but also protect the drive 
system in case a FET nukes.

Of course, on one's Tesla coil, party hardy ;-))  But don't go whining if 
one day you find all the FETs and drives blown ;-)))


Cheers,

         Terry


At 03:42 PM 3/15/2003 -0500, you wrote:
> > You're not going to believe this, but I can *almost* say that
> > back-to-back zeners from gate to source are unecessary too. This is
> > pushing it, I know, but I can't ignore my own findings. I have gotten
> > 16" arcs without any zeners in a IRFP460 full bridge. The gate
> > signals were still very clean with no Vspikes past + and - 20V. I
> > did, however, use a very powerful driver I designed that has an
> > output impedance of less than 100 milliohms.
>
>Well, I completely disagree with this statement especially when using high
>priced FETs such as the IXFN24N100s which I use for my SSTCs which cost over
>$50.00 each.
>Protection devices such as these (transient voltage suppressors) are
>especially required in SSTCs where there is high noise, kickbacks, etc... On
>my SSTCs i get transient
>voltage spikes over 100Volts on my FET gates with no protection devices.
>
>Remember, just because you don't see an immediate need for them, that they
>are unecessary.  Its the scenario you can't think of that can really ruin
>your day.  I'd rather
>blow out some zener diodes and my FET drivers any day of the week rather
>than blowing my IXFN24N100s ! ! ! !
>
>The Captain
>
>
>
>
>
> > Anyhow...SSTC's are cool!
> >
> > Take care,
> >
> > Justin Hays
> > KC5PNP
> > Email: justin-at-hvguy-dot-com
> > Website: www.hvguy-dot-com
> >
> >
> >
> >
> >