First you say :

Primary:  18.18uh...
Secondary:  0.96uh...

then you say :

Primary:  18.5uh...
Secondary: 0.96 ratio...

Why *ratio* in the second case ?

From the first cse I get a ratio of about 0.05 (or 20
depending on which way you look at it).

Jan-Erik Soderholm

-----Original Message-----
From: PicDude [mailto:picdude@NARWANI.ORG]
Sent: den 5 mars 2003 19:30
To: PICLIST@MITVMA.MIT.EDU
Subject: [EE]: Transformer Q...


In my ongoing saga to get this DC-DC converter working, I'm
questioning my transformer design, and would appreciate any
help on this from you gurus in this area, as I've not done
many of these in the past.  The design spec (from National's
SMS tool) calls for...

Primary:  18.18uh, 33.45 mOhms.
Secondary:  0.96uh, 58.46 mOhms.

I've calculated this as followings (using a T-130-3 (gray)
toroidal core)...
Primary:  23 turns of 20-awg, yielding 18.5uh, ~33 mOhms.
Secondary:  22 turns of 20-awg, yielding 0.96 ratio, ~32 mOhms.

The secondary resistance is much lower than required, but is
that a problem?  NS's websim tool seems in indicate that it
is not.

I've wound the coils non-overlapping this time, btw.  See..
http://www.narwani.org/neil/stuff/transformer-03.jpg

For coil phase/orientation, I'm winding with the same direction
(start over the core, going down thru the middle, and moving
counter-clockwise as I wind the turns), with the start of each
winding marked as the "dot".  See the following pic, in which
my "dots" are on A1 and B1.
http://www.narwani.org/neil/stuff/transformer-phase.jpg

Can anyone spot a problem here?

Thanks,
-Neil.

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--
http://www.piclist.com hint: The PICList is archived three different
ways.  See http://www.piclist.com/#archives for details.