RE: Schottky diodes in SSTCs

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

Original poster: "Steve Conner" <steve.conner-at-optosci-dot-com> 

 >Just been looking at a few SSTC pages and am wondering whether it is still
 >generally recommended to use schottky diodes in SSTC bridges

I used no Schottky diodes _or_ ultrafast bypass diodes in my "Dirty Weekend
SSTC" that I cobbled together last weekend. Just four IRFP460s.

My belief is that they don't actually do anything when the SSTC is well in
tune. And if you don't use the Schottky diodes, then I don't believe the
bypass diodes do anything when it's out of tune either :-o IMO, with no
schottkies, the current ends up flowing in the body-drain diodes, since a
slow diode almost always has a lower volt-drop than an ultrafast one of the
same voltage/current rating.

So far my findings are consistent with my theories... the DWSSTC has
operated happily off 240V halfwave rectified supply and not blown any FETs.
I'm now using a PLL driver circuit with a 4046 chip that self-adjusts to
stay in tune.

http://www.scopeboy-dot-com/tesla/dwsstc/arcstill.jpg

But as they say, YMMV. Using Schottky diodes and fast bypass diodes should
make your bridge totally bombproof, even against the worst-case scenario
where you drive the resonator slightly _below_ resonance. In this case a
heavy current flows with leading power factor, causing forced recovery of
the diodes. Ultrafast diodes can withstand this but the body-drain diodes
suffer and die. See

http://www.richieburnett.co.uk/

for more details

Steve C.