Neil,

TI's knowledge base server is presently down so I can't give you a link
at the moment however, when they come back up, look at the Unitrode
Power Supply Design Seminar Archives.  I'd recommend slup062.pdf,
slup073.pdf (both on transformer design), slup105.pdf (on coupled
inductor design - which is what you are doing) and slup123.pdf through
slup129.pdf a good basic magnetics handbook.

Best regards,

Dave

PicDude wrote:
>
> So in my experiments with switcher chips, I've found a
> common point of failure.... the transformer.  And I guess
> I need to learn more about designing/building a transformer
> for switching-freq applications.  Here's why I say this...
>
> Over the past couple weeks, I've built a number of switcher
> circuits... 5 buck circuits (using LM2678 and MC33167 chips),
> 1 boost circuit (LM2588), and 3 flyback circuits (LM2588,
> and MC33167).  Using off-the-shelf inductors, the buck and
> boost converters work.  Replacing those inductors with my
> own home-wound inductors, they still work very well.
>
> BUT.... for any of the circuits that require a transformer,
> they don't work.  I've even followed guidelines for adding
> additional outputs to a couple of the buck designs by tapping
> off the inductor to make a transformer, but the primary
> outputs still work while the tapped-off secondary outputs
> do not.
>
> No, not very scientific w/o an oscope, but it seems obvious
> enough that it's my transformers.
>
> SPECIFIC EXAMPLE ... LM2678 buck ckt (9Vin, 5Vout) works
> well with my own inductor -- 21 turns of 18AWG on a T130-3
> toroid.  Followed NS's doc for adding another output f/12V
> (48 turns of 18awg) but that failed, leaving me with ~3-4V
> out.  The 5V output still functions fine though.
>
> Anyone have good info/links on how to design/wind transformers?
> I've searched the local bookstores, but no dice.
>
> Cheers,
> -Neil.
>
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