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

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

Original poster: "Malcolm Watts by way of Terry Fritz <twftesla-at-qwest-dot-net>" <m.j.watts-at-massey.ac.nz>

On 2 Jul 2002, at 20:54, Tesla list wrote:

> Original poster: "rob by way of Terry Fritz <twftesla-at-qwest-dot-net>"
<rob-at-pythonemproject-dot-com>
> 
> Tesla list wrote:
> > 
> > Original poster: "Justin Hays by way of Terry Fritz <twftesla-at-qwest-dot-net>"
> <pyrotrons2000-at-yahoo-dot-com>
> > 
> > Hi Rob (and All),
> > 
> > I agree with Jan to some extent, putting some kind of clamp on the
> > transformer primary is probably a wise idea....but...
> > 
> > Only if I interpret the schematic correctly, this topology doesn't
> > ever hard-switch the primary. So I suppose that since there is no
> > fast switching of large currents (IN THIS STAGE), there is minimal
> > voltage spiking (L di/dt) due to uncoupled inductance (leakage L) in
> > the primary of the transformer. Seems to me that there is a nice
> > round pump of current fed into the primary versus a hard-switched,
> > square-wave slam of current. I also suppose that this is the essence
> > of this topology...low EMI!
> > 
> > So aside from that stage (the output stage I'll call it) the MOSFET's
> > are no doubt operated as switches, meant to pull current into the
> > 160uH inductance and "release" this current to the rest of the
> > circuitry. So the MOSFET's are switched quickly here in order to
> > minimize I^R power loss in the junction....and therefore there will
> > most likely be large voltage spikes...
> > 
> > So it is here in this stage (where there is high-speed switching of
> > significant current) that I think there should be clamp diodes. Maybe
> > even isolation of the MOSFET body diodes (with a series shottky) if
> > it comes down to it...I've heard the body diodes can create damaging
> > reverse recovery transients if driven into forward conduction too
> > hard (from free-wheeling current).
> > 
> > Well, maybe I've missed some things it but hopefully I've got the
> > main stuff right, the basic workings of the topology and all.
> > Although I AM starting to wonder if there is hard-switched ANYTHING
> > here...
> > 
> > Take care,
> > 
> > Justin Hays
> > KC5PNP
> > Email: justin-at-hvguy-dot-com
> > Website: www.hvguy-dot-com
> 
> Hi Justin,
> 
> Well, I actually achieved short streamers with my 25mH coil with this
> converter with Vdd=30V.  I feed the coil at the base using the 1:3
> transformer tap.  Then one of the FREDS burnt up.  I am finding that the
> FETs dissipate little wattage, but the FREDs get extremely hot.  I
> suppose I'm going to nead to heat sink all three of them, but I would
> like to find a way to reduce their losses. 20W x 3 is alot of wasted
> power.  Thats just my estimate based on free air case temp.
> 
> I tried the extra diode experiment, and all I got was 4 very hot diodes
> :)  No visible output from the coil.  It does certainly reduce the
> voltage on the FETs, but it kills the coil operation.  I just got in a
> new shipment of IR IRFPG50's and 1200V Hexfreds.  I was using IXYS
> FREDS, and maybe these huge TO-247 HexFREDs will work better.

Just an observation: it looks like using a sledgehammer to crack a 
nut. There must be something fundamentally wrong, either with the 
circuit or the way it's being implemented. No?

Regards,
Malcolm