Re: SSTC, Class DE High Power Half Wave Inverter Design

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

Original poster: "by way of Terry Fritz <twftesla-at-qwest-dot-net>" <Kchdlh-at-aol-dot-com>

In a message dated 01/28/2002 9:52:05 PM Pacific Standard Time,
tesla-at-pupman-dot-com writes:


>
> Subj:SSTC, Class DE High Power Half Wave Inverter Design 
> Date:01/28/2002 9:52:05 PM Pacific Standard Time
> From:<mailto:tesla-at-pupman-dot-com>tesla-at-pupman-dot-com
> To:<mailto:tesla-at-pupman-dot-com>tesla-at-pupman-dot-com
> Sent from the Internet 
>
>
>
> Original poster: "David Sharpe by way of Terry Fritz <twftesla-at-qwest-dot-net>"
> <sccr4us-at-erols-dot-com>
>
> All
>
> While researching the web for DE inverter designs this article popped
> out of google (www.google-dot-com)
>
> http://www.chipcenter-dot-com/analog/images/tn/tn038.pdf
>
> This inverter is scaleable from 50Khz to better then 6Mhz, and has
> very high efficiencies, and supposedly is relatively resistant to load
> impedance swings.  Circuits and full design info supplied.  Looks
> good I've already downloaded it for my SSTC file.  Good stuff...
>
> Regards
> Dave Sharpe, TCBOR
> Chesterfield, VA. USA
>
>



That was a good find!  It's a PhD thesis, of course.  The candidate did an
excellent job--very thorough design and analysis work, in my opinion.  Although
his circuit design incorporates the use of a resonant load, I should think that
it should quite well drive a non-resonant Tesla coil primary.  In such a case,
of course, operation would not be class DE but rather, class D; but at 100 KHz
or so, not a problem.  And in such a case, one could excite his driving
circuits with the secondary's return-signal, as I do, thus making the system
self-tuned.

A more fundamental limitation, for Tesla-coil work, of a half-bridge circuit
such as this design incorporates, is that the signal voltage that can be
applied to the primary is limited to what the half-bridge MOSFETs can
withstand: perhaps 400V for 500V MOSFETs.  That limitation is what I've tried
to overcome with my "current-loop" design.  In that design, I incorporate,
presently, 5 MOSFET pairs and 5, 160V power sources in a "daisy chain" primary
configuration.  That applies some 800V--less MOSFET drops, of course--of signal
to my 4-turn, 12"-dia. primary.  But that is still very much less, it must be
said, than the punch that can be delivered by a spark gap since that can
deliver maybe 10x that voltage.

The following has always seemed to me to be important:  1.  It's amperes x
turns (I x T) that generates the primary flux which is what excites the
secondary.  2.  One can best increase I x T by increasing I in preference to
T.  This is because, while I varies directly with applied voltage V, the
primary's impedance varies as the square of T and thus I will vary inversely as
the square of T.  And because of that, to maintain the same I while increasing
T, V must increase as the square of T, whereas in maintaining the same T while
increasing I, V must increase only directly with I.  3.  Thus, make V high
while keeping T low.  Does that make sense?  Perhaps that's already a "given"
in the coiler community.

Ken Herrick