Re: Sheppard-Talyor circuit up on my website/ low Z construction technique

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

Original poster: "Jan Wagner by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jwagner-at-cc.hut.fi>

Hi Rob,

> Now I bet that the FETs will explode with violence :) instead of just
> burning out.  Gee, then I don't have to probe them with an ohmmeter. 

Yup, they just might. This exploding thing must be genetical or something,
it seems all mosfets have that behaviour preprogrammed. :-)

Anyway, there's a small but transistor-deadly mistake in your schematic!

You should have a ultrafast recover diode from the node between D24 and
R36 pointing towards ground, that is, clamping positive voltages to
ground. And that diode is missing, at least in your schematic.

(initially hard to believe maybe, but that's the right way round for the
diode - the Sheppard Taylor is a bit odd as the second stage is fed from
the storage cap with a negative voltage, not positive)

Without the diode, positive going voltage appearing on the transformer
primary (secondary side fed by the TC) is not clamped to any rail. Via D24
and D25 it may make the drain of M17 see an overvoltage, causing it to
fail. I don't know why your simulation worked ok, though... maybe zener
effect of the mosfet drain-source?

The diode works a bit like the flyback diode in flyback converters.

Hope this helps! And saves your expensive mosfets from further failures...

cheers,
 - Jan

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