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.

You are carefully winding the wire onto the form, correct.  You have to
wind the wire without cris-crossing it.  IOW, the turns should be side
by side and not crossing the previous turn, otherwise it's a choke and
not a coil.

michael brown

"In the land of the blind, he who has one eye is king"

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