Coupled Inductors--how coupled is coupled?

The "coupled-inductor" label comes about because it is first and foremost an inductor, with a carefully designed gap, to store energy in the winding. The second transformer winding goes along for the ride.

By contrast "transformer" windings don't generally store energy, but transmit it directly to a secondary. Also, the manufacturers of coupled inductors use that name, not transformer, so if you searched with your preferred term, you wouldn't find these parts. You can treat these parts as transformer if you wish, but in many of our applications most of the analysis will concentrate on the magnetizing inductance.

How are you choosing your preferred inductance value? Based on Newark's special sales price?

Well, for junkbox-type protos that's not totally irrational, but in this ca se it's mostly the tradeoff between loss in the sync buck chip and in the c oil resistance, with cost as the penalty function.

The Bourns SRF1260-101 has about 350 mOhms in each winding, which makes it reasonable to drive two in parallel with an LM3103 switcher. With loosey-g oosey coupling like that (k=0.985), I did have to dork the primary-side s etpoint as a function of supply voltage, but it looks like it should work.

I don't care too much about efficiency in this application--it's only a cou ple of watts overall--but installed cost matters a lot.

Fun.

Cheers

Phil Hobbs

I should add, these are 1:1 coupled inductors; yep, larger ratio are called flyback transformers. ;-)

Here I'm using one as, well, an autotransformer or something.

I'm now doing the board layout, and I've got too used to fast stuff, packing things as close together as possible. Now I'm dominated by HV clearance rules, so I have to keep parts far from each other.

Freaky.