RE: SSTC questions / body diode stuff

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Original poster: "Justin Hays by way of Terry Fritz <teslalist-at-qwest-dot-net>" <pyrotrons2000-at-yahoo-dot-com>

Hi Everyone,

Jan, I see your point, but will still have to kindly disagree with
you.

Only after the freewheeling current spike has been dissipated, it is
safe to turn on the other MOSFET (in half-bridge) or pair of MOSFET's
(full bridge). If the MOSFET's are still turned on when the diodes
are still conducting...you get an "ON" DIODE on top, and an "ON"
MOSFET on bottom! This equals shoot-through current, similar to
having the wrong MOSFET's turned on at the same time. This is why I
think dead time could be added to allow slow diodes (body diodes) to
fully turn off before the next switching transistion happens. Indeed,
Trr goes up significantly with increased current, but the current
spike itself is so short, I think this effect can be dismissed.

Also:

Contrary to popular belief, oscillator-based SSTC's do NOT switch in
a ZVS fashion, even if the system is in perfect tune. The signal from
the control IC (TL494 in many designs) is not locked onto the
secondary coil output. The two signals drift in and out of phase in
respect with each other...the end result is that MOSFET's switch all
over the place in respect to the sinusoidal output of the resonator.
And since this current/voltage relationship is directly coupled to
the primary, you see what happens. Only feedback or PLL coils switch
in a ZVS fashion. And only PLL coils can switch at *perfect* zero
crossing. Why? Big MOSFET gates cause an RC time delay with feedback
coils. This has the tendency of pushing things out of phase, and more
towards non-ZVS. The bigger the MOSFET gate capacitance, the more
non-ZVS in a feedback (antenna) coil.

If the two signals (HV output and oscillator signal) are
locked...then your logic electronics is probably responding to
conducted/radiated noise from the output??

Comments/flames welcome...

Justin Hays, www.hvguy-dot-com

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