On Tue, 15 Jun 99 10:34:15 MDT "dhaupt" <dhaupt@mail.bewellnet.com>
writes:
>Hi Marv,
>
>Nice notes from you recently on the amps reflectors about SMPS design.
> I agree with you - the most difficult part of an SMPS design would
>seem to be the transformer. I've considered the concept of not merely
>doing a series/parallel transformner combination, but even perhaps
>parallelling outputs after rectification. It might seem that the
>rectifiers will isolate the transformers from each other so you don't
>get high circulating currents in case of transformer imbalance.
Hi Dave,
How much power are you looking to run? A single transformer should
suffice for up to 3KW. If you need more, I think seriesing either
transformers (run from synchronized sources) or entire supplies (more
fault tolerant) is the way to go.
>You mention Litz wire techniques. How do you determine when Litz wire
>techniques are required, and how do you determine their benefits? The
>one time I've worked with Litz wire, was when I had to design a loop
>antenna for, of all things, 2.257 KILOHERTZ! The radiation resistance
>of this loop antenna was milliohms, so the skin effect of a single
>conductor was huge in comparison. The difference in efficiency (as
>measured by driving the antenna with a known current, then measuring
>the magnetic field a few feet away - had to face the fact that at that
>frequency, there is no far field detectable!), was about 40dB better
>using Litz wire.
I had a table somewhere that escapes me at the moment... but, if I
recall correctly #26 was a good choice for negligible loss at 100KHz.
>I have never done a ground-up SMPS. I have finished the control loop
>on several that other engineers gave up on. Control loop design isn't
>all that hard, really, as I see it. One of the designs I finished was
>a 6kVA switcher driving a radar tube. 6kV output at 1amp average,
>drawn in 10 amp peaks at 10% duty cycle. That was nearly 15 years
>ago. So it seems that a 1.5kVA switcher for ham amps shouldn't be out
>of the question.
Yep, quite doable. and no where near as tricky as 15 years ago!
>How would you start the transformer design? And who would be good
>suppliers of core material? Any thoughts on references to magnetics
>design texts?
It is really rather simple. All you need is core that can support
the power at the chosen frequency of operation and provide enough winding
area for the necessary coils.
Magnetics, Inc. (www.mag-inc.com), in Pennsylvania, has a a few large
toroids that are applicable and they also have some very complete design
notes. There may be a problem with availability. If they don't happen
to have a part in stock you wouldn't want to pay for the minimum run!
The key formula is: Np = E * 10^8 / ( K * F * Ac * B)
where Np = Primary turns
E = Primary voltage
K = a constant equal to 4 for square wave operation
F = frequency
Ac = core cross section in square mm
B = flux density in Gauss
So at 375 volts at 100 KHz and 1000 Gauss operating flux using a core
with a 2 square mm cross section the answer is 46.875 turns... 47 or 48
is close enough for ham work.
The secondary works just the same as in a low frequency
transformer... secondary voltage =primary voltage * turns ratio. So, if
you wanted 3000 volts out, from 375 volts in, an 8-1 turns ratio with the
48 turn primary gives a 384 turn secondary. Add a turn or two (8 volts
per turn) to the secondary to compensate for copper losses if you really
want to get fancy.
Now, the not fun part... that primary is best made up of about 25 (or
so) strands of #26... 3 strands of #26 will suffice for the secondary.
It is best to wind both coils equally spaced around the core. ... and
don't forget the mylar tape to keep the HV windings away from each other
and the primary!
A fine point. In order to reduce interwinding capacitance a method to
wind the secondary would be to wind a number of individual coils and
series them up externally.
If you also required, perhaps,a 350 volt screen source an auxiliary
secondary with 45 turns (of #26 or slightly smaller... but, you already
have the big roll of #26!) would be just about right.
Time to wind?
Magnetics (as well as many other suppliers) have cores suitable for
the boost inductor in the PFC section.
73,
Marv WC6W
*
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