[Amps] Peter Dahl transformers

[Manfred Mornhinweg]

Bill,

> Manfred, in spite of the problems for the homebrewer, I'm still 
> interested in switching power supplies for high power amps.

It's true that doing the whole development work to build just one unit
(and possibly publish it) isn't very efficient. But ham radio isn't
about providing the most cost-effective solution! It's about learning
radio, electronics, and having fun doing so. In this way, for me it has
always been very attractive to develop some piece of equipment for ham
radio use, just if I was developing something for commercial production
(which I have very rarely done, buy the way, my job of many years having
been in a research organization, and not in a commercial company).

So I applaud any ham who develops a good design, rather than to just
copy what others have been doing for decades.

> Do you have any schematics or sources of design information for:

> 1. A 50 VDC 50 amp supply and 2. A 3000 VDC 1 amp supply?

Usually you won't ever find a perfect, ready, complete, bug-free design
for anything you want to build - specially if you want it to meet YOUR
specifications! But the datasheets for power supply controller ICs
contain many referential designs that are useful as starting points.
Often only small changes are needed to adapt them to your particular needs.

For power supplies of this power class, I would use a full bridge of 4
MOSFETs. The 50V supply is relatively easy and simple. If you find
ferrite cores large enough, you could use just a single transformer.
Otherwise, like if you are bound to what Amidon sells, you will need to
combine several cores.
The control could be simple pulse width modulation, like implemented in
the old and well known LM3524, TL494 or KA7500 chips, or you could use
fixed 50% duty cycle with phase shift modulation, as implemented in the
  UC3875. I actually would prefer the latter approach, but it might be 
necessary to either use a bleeder resistor to impose a minimum load, or 
else simply switch off the supply during RX. During TX the amplifier's 
idle current is enough minimum load.

The entire primary side can be the same for both the 50V and 3000V
versions. But the high voltage version would probably be easiest and
most reliable if you split it up into about 8 separate, rather small
transformers delivering a lower voltage each, rectify and filter each
output separately, and then place them all in series. The primaries 
would all go in parallel. This allows you to use inexpensive, 
off-the-shelf diodes and capacitors, and you don't need any 
voltage-equalizing resistors. The transformers must be all insulated for 
the FULL high voltage, though. An easy way to do that is winding on a 
bobbin that has no side walls, leaving a few millimeters on each side 
without wire but with interlayer insulating sheets (mylar or the like),
then fill the ends with an insulating material, which could be epoxy
glue or silicone caulk. In a production setting this would be done with
vacuum potting.

> I'm looking for designs where the heavy lifting (engineering) has
 > already been done.

The problem is that apparently everybody is doing that! Someone HAS to 
make that first detailed design, and publish it instead of sitting on 
it!  I don't have any complete, fully developed designs on hand for 
either of those specs. Sorry!

The main problem is that people developing circuits for an employer 
usually have to sign over the rights to that employer, who usually would 
never publish them. And too few people do such designs just for their 
own use, and then put them in the public domain.

I have found that I can only very rarely use a published design just as 
it is. Usually I can't find all the same parts, or the specs are not the 
exact ones I need, and so on. Instead of modifying existing designs, I 
usually prefer to start from scratch and roll my own, but of course I do 
look at all designs I can find, to learn how others have solved specific 
problems, and copy the good ideas, if I find any!  Then I publish what I 
cobble together.

The data sheets and application notes by the semiconductor manufacturers 
are typically the best freely available sources of information and 
reference designs. I suppose that there must also be engineering books 
with good designs, but the non-ham books I have seen so far are for the 
most part either very poor, or purely theoretical, or very expensive, 
and usually at least two of these! And the ham books rarely contain much 
modern technology at all, and good ones are becoming ever scarcer.

If you are serious about building either of those power supplies, I 
suggest that you start by collecting all the data sheets and application 
notes about suitable controller chips, and read and fully understand 
them. That should give you the main information you need to do a 
detailed design yourself. You will probably find some practical circuit 
examples that are roughly in the correct power class - you can largely 
use one of them, and just adapt the secondary side to your voltages. And 
if you have specific questions about the details that are not 
sufficiently covered in those papers, I can try my best to help you. 
Just don't ask me to teach a complete course on switching power supplies 
by e-mail, which is time-consuming! The basics are well covered in many 
freely available papers.

Manfred

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