Re: VTTC controller - First Light

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

Original poster: "K. C. Herrick by way of Terry Fritz <twftesla-at-qwest-dot-net>" <kchdlh-at-juno-dot-com>

Shad (& all)-

Comments interspersed--

On Wed, 13 Nov 2002 07:44:14 -0700 "Tesla list" <tesla-at-pupman-dot-com>
writes:
 > Original poster: "Sundog by way of Terry Fritz <twftesla-at-qwest-dot-net>"
 > <sundog-at-timeship-dot-net>
 >
 > Hi Ken,   Comments interspersed
 >
 > At 09:21 AM 11/12/2002, you wrote:
 > >Original poster: "K. C. Herrick by way of Terry Fritz
 > ><twftesla-at-qwest-dot-net>" <kchdlh-at-juno-dot-com>
 > >

[snipped]

 > >3.  Sounds as if your FETs are turning >off< quite rapidly since
 > the
 > >excursion to -300 takes only ~200 ns.  They don't turn on fast
 > enough.  I
 > >suppose you have checked the gate waveforms themselves?  You could
 > use
 > >N-channel FETs, of course, by merely 1:1-transformer-coupling
 > their
 > >drives.
 >
 >
 >
 > The fets are turning *on* quite rapidly.  The drop from -300v to 0v
 > is
 > taking place in about 1.6uS. Once the FET is off, it slowly climbs
 > back up
 > to -300v.
 >    A harder turn off is necessary to a point.  Where a FET creates a
 > lot of
 > heat while switching, a tube only heats up as it's conducting.
 > Also, the
 > voltage on the grid of the tube that was just turned off is already
 > back
 > down to -170v or so when the opposite tube switches on, which is a
 > high
 > enough voltage on the grid to prevent tube conduction.   I haven't
 > run
 > across any N channel fets with a high negative voltage rating, so
 > far
 > everything I've found is all P channel.
 >
 >
 > >4.  If it's the case instead that it's the return to -300 that's
 > slow,
 > >then they're, of course, turning >on< slowly.  In any case in a
 > push-pull
 > >situation, I'd think slow-fast, or fast-slow, would be good since
 > that
 > >would afford a dead-zone between 1 tube turning on & the other off
 > & vice
 > >versa.  But during that dead zone, the primary is going to ring
 > something
 > >fierce--if untuned & unless clamped.
 >
 >
 >
 > I think you are seeing the fets backwards.  The -300v supply runs
 > through a
 > 3kohm resistor to limit current, and continues on to ground.  The
 > FET is
 > between the resistor's output and ground.  When the fet is off,
 > there's
 > -300v on the grid, when the fet is on, it's shorting that line
 > directly to
 > ground, and the grid voltage drops to zero.  Given the ramp down to
 > -300v,
 > there should be minimal dead-time on the tubes between switching,
 > and the
 > scope didn't show much feedback into the primary.

I'm confoozed.  Do I really have to take pencil & paper & draw the
circuit or can I continue just to think about it...?  When the FET goes
on, it shorts the -300 at the grid (delivered via the resistor) to
ground.  That happens in ~1.6 us and that's not fast in my book.  That
should happen in n.g.t. ~200 ns.  When the FET goes off, then the grid
voltage returns to -300 in 200-300 ns, per your 11/11 posting, which is
quite respectable considering that you're pulling it there via a sizeable
resistance.  Thus, it's the turning-on that seems to be the problem,
perhaps because they're P-channel MOSFETs (I've never used them).
 >
 >
 > >5.  I should think you could use a totem-pole configuration of 500
 > V
 > >FETs, driving the grid & connected between 0 and -300 V, to avoid
 > the use
 > >of the series resistor.

 >
 > I'm no EE, can you please elaborate on that a little?  I thought a
 > 500v FET
 > would smoke if put between a -300v line and ground.  I'm also under
 > the
 > assumption that fets are mainly current-carrying devices, as I could
 > get
 > very little voltage swing across it without a load to deliver it
 > into.

If a single N-channel FET, its source would have to be at the -300
potential, along with its gate circuit, of course, so driving it would be
a bother unless you were to use a small 1:1 transformer.  But in that
configuration, a 500 V N-channel should work fine.  IRF 840?...something
like that.  As to the totem-pole configuration, that's just two
transistors "stacked" one above the other.  For N-FETs and starting from
the "top", connected to ground potential, you'd have a series circuit of
top drain/top source/bottom drain/bottom source with the last-named
terminal connected directly to -300.  You'd need two small 1:1
transformers, or one 1:1:1 transformer, to drive the gates.  You'd also
need some circuitry in the gate circuits to keep both FETs from
conducting at the same time during the switching-times; that's a no-no!
The grid would be driven from the top source/bottom drain connection.
 >
 > >6.  You mention using 2 MOTs in series:  I hope they're insulated
 >> for it.

 > So far, I've had no problems.  The first mot's core is grounded, and
 > the HV
 > output is connected to the core of the second MOT.  So far, there's
 > been no
 > trouble [knock on wood], and my buddy has run MOTs in this way for a
 > long
 > time without failure.  The second MOT is mounted on standoffs.

That's fine, but the worrisome part is the core:primary insulation.  I
wouldn't do it but then I'm a bit leery of less-than-100 KHz-or-so H.V.

[snipped]

Ken Herrick