RE: Help! My Coil Blew Up

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

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

 >I guess in spite of the age of the individuals involved, this SSTC crowd is
 >just another "old boys" club.

Touch?e

The reason you probably got no replies is that there is just so many things
it could be. SSTCs are complicated as heck and there are so many factors
interacting.

Theoretically, if you are driving all four FETs off a single gate drive
transformer, it should be impossible for them to cross-conduct. I think what
is more likely is that your FETs overheated due to poor gate drive. It could
be too much ringing on the gate waveform causing them to fall in and out of
conduction, or slow rise/fall times causing excessive switching losses. When
they overheat, they fail short circuit. If one FET shorts, it will take out
the other one on that side too.

Another possibility is that your drive circuit was driving them at the wrong
frequency to get zero current switching. If the frequency is slightly below
resonance, the resonator load starts to look capacitive. (See Richie
Burnett's website, http://www.richieburnett.co.uk ) A capacitive load forces
the FETs to switch high currents which again could overheat them.

I tend towards the "wrong frequency" explanation myself. The antenna
feedback circuit isn't very "scientific", the phase shift (and hence the
oscillation frequency) is set by a bunch of things that aren't well defined,
like the capacitance of the antenna clamp diodes and so on. As an EE it
gives me hives just looking at it.

When reading the above, bear in mind I've never built an ordinary "inverter"
type of SSTC, so it might be complete BS. If anyone out there is fond of the
antenna feedback circuit, flame away, but you won't change my mind :P

Steve C.