[ARC5] Switching DC-DC Power Supplies.

[Ben Hall]

Hey gang,

I was hoping that my Chinese DC/DC converters from the e-place would 
have arrived in today's USPS mail, but alas...'twas not to be.  Maybe 
tomorrow, but I'm not hopeful.  It left Flushing, NY, on the 9th and 
hasn't been tracked again.  :(

However, the guy in the Big Brown Truck (BBT) dropped off a Mouser order 
that contains the parts I need to build a DC/DC switching supply as 
shown in the March 2015 Elektor magazine article entitled "350-V Step-Up 
Converter, High voltage from a 30 V bench power supply."

(Elektor is a pretty good magazine.  It is by a company in the 
Netherlands and seems to be more technical than QST's of late.  I 
recommend it highly.)

Unfortunately the article is in the member's only area, but that's okay. 
While the author (Martin Ossmann) wanted to turn his 0-30 VDC bench 
supply into to a 0-350 VDC supply, his solution does not have regulation 
on the output.  That's not a criticism, it wasn't something he 
needed/wanted.

On the other hand, I suspect most people here are looking for a 
regulated supply and not something to convert a bench variable into an 
HV variable supply.

Martin's supply is impressive in that it is rated for 50W output and, as 
you've already seen, you can buy the magnetics and the formers from 
Mouser.  350 VDC at 50W is about 140 mA.  Fully isolated.  Nice!  All 
from a rather small ETD29 core.

In the Mouser package was also a few ETD49 cores that are roughly twice 
the size.  I suspect the design will scale with the larger core.  We'll see.

In my past failed experiments with homebrew DC/DC converters, I've 
always run into issues with the magnetics.  (plus some exciting 
exploding MOSFETS!)  Martin seems to have a good grasp on that and does 
get into saturation in the article which I suspect has been one of my 
issues in the past.  (I was also not going off of any proven design, but 
was winging it with what I had in the junk box salvaged from old 
computer power supplies.)

My plan is to build one of his supplies and see how it does.  I don't 
care about regulation for now, I want to see if I can get it to the 
point where the output is sufficiently filtered for radio use.

If that's a success, my next step is to add regulation to the output. 
His converter is built around the Texas Instruments TL494 switching 
controller, which has comparators built-in to allow for output voltage 
regulation.  They are not used in Martin's design, but not difficult to 
add I suspect.

The problem I'm not real sure how to get around at the moment is that I 
think using the TL494's comparators may require elimination of the 
output isolation.

I need to look at some ATX computer power supply schematics.  IIRC, they 
commonly use the TL494 controller, are isolated output, and have at 
least one output regulated.  That, however, is relying on some old 
memories and is likely wrong.

On the other hand, using the circuit as-is as the high voltage supply to 
some sort of regulator circuit (MOSFET zener, maybe?) would work and 
retain isolation.

We shall see.  I've got the PC board laid out, etched, and partially 
drilled.  Winding the ETD29 core will be my first task, and probably the 
most difficult.  I will keep everyone informed.

thanks much and 73,
ben, kd5byb