Hi Larry,
Some of the answers to your queries:
<snip>
>Some questions come to mind after reviewing the Feb 91 QEX article.
>The circuit in the article uses a huge line filter cap which makes step
>start seem necessary. What is the actual maximum 120 ripple permissible
>for a supply like this? How much smaller can you make the filter cap and
>still be OK?
<snip>
2000uF per KW is a good rule of thumb. Multiple caps in parallel are
typically the most economical realization.
A step start may be implemented with a power FET across a resistor in the
negative lead of the input cap(s).
>The core material is typo'd in the article schematic and I can't make it
>out. Any body know what the actual material was?
The cores were Magnetics 44916-TC-J.
The original article stacks 10 of them (enough core for 8-10 KW!) which
allows for the meager winding.
I did the math long ago: The cores are run at 1200 gauss which results in
circa 60 watts core loss.
>The transformer design seems weird. I've built VHF amps like that but
>see no reason to make the supply transformer that complicated. Is there
>some valid reason to do so? Seems like a large diameter toroid with
>conventional windings would do just fine and be easier to construct.
One must be careful to avoid leakage inductance with the large turns ratio
and high potentials involved.
The coaxial design is great in this respect though the approach shown was
not optimum in other ways.
>The "H-bridge" seems to be the choice these days. Is that for
>efficiency? My "classic" supplies just used a push-pull arrangement with
>a center tapped primary. Seems simpler, but surely there must be a
>reason why the larger supplies don't seem to use push-pull although all
>the smaller ones sure do.
The "H-bridge" is indicated at the multi KW power level. It best utilizes
the voltage and current capabilties of the power devices and provides some
inherent protection against voltage spikes as the ends of the bridge are
swamped by the input cap.
Of course, a high production volume, constant load design, such as a
microwave oven would make do with a single power switch! :-)
>Seems like it would be a good idea to make smaller "stackable" supplies.
>Say something like 700 VDC at 2.0 amps max, then stack them in series to
>get what you need, 1400V, 2100V, 2800V, 3500V. Like that.
Way, way more parts that way.
The same advantage may be obtained by winding multiple secondaries. These
may be individually rectified and seriesed after the output bridges to good
effect. 375 volt steps with a PFC input stage.
>I see no reason for feedback for voltage regulation. My old T-supplies
>were very stiff and the supply in the article claims 50 volts drop from
>ZSAC to full load. Arguments for or against? Simpler is always better as
>far as I'm concerned.
A supply of this wattage should really have a Power Factor Correcting (PFC)
front end. That would provide enough regulation for Amateur Radio. For an
exemplar, see the Texas Instruments UC3879.
Phase Shift IC's are available to drive the bridge that take care of the
overlap timing, and overload shutdown, with or without feedback. For an
exemplar, see the Texas Instruments UC3818.
>Is there a good reference for design of this type of switcher someplace.
>Preferably on-line. I have all sorts of design aids for tiny little
>computer type supplies but nothing here for BIG stuff.
For references on the PFC section see APT (now Micro Semi) app. notes:
APT9303 plus the follow up APT9901. Also TI (formerly Unitrode) DN-44
See Fairchild Ap Note 42026 (using the old Micro Linear ML4818 device) for
info on a phase shift full bridge. AB-8 may also be of interest.
I've posted a TENTATIVE schematic for a 3KW supply here:
http://wc6w.50webs.com/wc6wamps/index.html?fr43.html
>Discussion appreciated.
Your welcome!
73 & Good morning,
Marv WC6W
http://wc6w.50webs.com/
*
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