[Amps] HV switch-mode power supply

[Larry]

The switch more power supply topic seems to pop up more frequently these 
days. It has always been of interest to me although I have not pursued 
any designs lately.  Some 40+ years ago, I built switch mode supplies to 
replace the dynamotors and vibrators in my AM mobile rigs. Of course 
back then we just called them "transistor supplies". They were always 
free running multivibrators and ran at frequencies between 100 Hz and 2 
kHz due to speed limitations of the germanium transistors and slow 
rectifier diodes. I wound a few toroids myself but usually scrounged 
them from commercial radios, etc.

Since I have a current need for a few HV supplies (rebuilding the 
VHF/UHF station with 3-4 separate amps + supplies), I keep thinking 
about building some "modern" switchers.

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? I would like to be able to start/stop the supply with the 
PTT switch for reasons which I won't go into here. I don't mind a large 
inrush current if I can make the design handle it.

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 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.

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.

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.

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.

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.

Discussion appreciated.

73, Larry

Larry - W7IUV
DN07dg
http://w7iuv.com