"spamhog" wrote in message
...
Thanks Rich!

On Sep 17, 4:55 pm, "Rich Klestinez" wrote:
I would not go through the trouble of unbalancing the transformer

I didn't mean to... The idea was to generate two symmetrical and
opposed signals by putting an un-bal transformer between the VXO and a
first push-pull stage made with just two sections, charged with
rebuilding near-square waveforms.

I matched neither driver sections nor MOSFETs,but I did hear that both
can be a bit variable. Apparently the chip drivers are normally used
without equalization (to the surprise of quite a few authours). As for
MOSFETs, I saw several amp designs with a single separate bias
trimmer, or with a single special driver chip, for several devices. I
have separate trimmers, I didn't bother to check if the 1-mA voltage
was the same for both - but I will. That may have helped equalize the
switching point in terms of RF voltage. As for the different on-state
resistance, I guess it's more critical if one has devices in parallel
- in a push-pull a particuarly low resistance device won't hog the
current.

One other thing comes to mind is that with a torrid core at high current
and a high frequency, then symmetry of the winding itself is critical.

Yes, I've been extremely careful. Winding is trifilar, twisted, I
took care to have symmetrical twist pitch and winding spacing.

torrid core

Torrid indeed. Now I have a tiny .82 core and thin wire too. At
normal duty cycle all is fine, after 120s keydown one risks minor
burns - but the MOSFETs barely get warm.

Coincidentally, I just got several 10s of feet of Raychem "high
performance" wire after helping assemble a prototype 4-fuel car (3
liquid fuels + plugin recharger). 1.5sqmm, silver plated stranded
copper alloy, thin 600V radiation-crosslinked high temperature
insulator. I've been verbally told the 1/2sqmm variety can carry 30A
(I asked as I worried it would not bear 3.5). I will try it on a
larger core. I already recovered the ferrite of a 1-kW Hi Gain balun,
but I want to see detailed wire specs first. Once stripped, this wire
should do well in hi-Q coils too.

Awsome but really good wire!

If the Fet's are just barely warm it is my guess that you are driving them
well into saturation and that good. As far as the core getting that hot,
well that's not so good.
I digress. Some years back (70's) I tried to model the BH vs temperature of
some high power cores that I was using at 452 Khz. They were getting very
hot and we couldent figgure out why. Upon observing the BH curve at 452 Khz
at 101% saturation there were bumps and little knots on the curve, they were
really temperature dependant as well. Turned out (remember this was in the
1970's) this was a combination of grain size and a purely mechanical effect
(pizoelectric). We switched to a different material with a very fine grain
domains and the cores ran cool. I nearly wore out my Hemi 256 slide rule on
that one. Still have the darn thing. Nowdays core selection is much
easier for high current high frequency uses. Pick a core empirically do
your design and take advice from the wisdom of the below.

FYI. Adams magnetic products, Tom Corrigan Applications Engineer,
1-800-747-7543 or his direct line 630-359-9445. A really good guy and reps
several lines of cores. Of course there is always the Ferroxcube area
application engineers that are some of the best in the world that I have
worked with in the past year. They will take the time to help you, however,
you should be conversant in the terms of the language of magnetics or you
wont understand what they mean.

Its a challange, but the rewards are great. You will have a transmitter
that will be reliable and reproducible.

rich
in Texas

PS I have retired now (66 Yrs old). Not a ham yet. As soon as I get most of
my honey-doos done I am considering trying for a licence.