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

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

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.

Rob.

-- 
-----------------------------
The Numeric Python EM Project

www.pythonemproject-dot-com