I'm actually amazed that I was remotely correct about how to shield the fringe flux radiation. Though I am aware of the existence of a Faraday's cage, and came the foil assumption based on that. However, I did think distancing the air gap from the litz wire would actually work.
Yeah
I'm not clear if you meant to wrap the gap, all the way around, with a solid (shorted) turn of foil -- if so, that's a big difference,
but just that subtle change (leaving it overlapping but insulated) makes it workable. (I mean, besides the problem that air gap fringing is just too vicious to shield.)
Placed between windings, this is also how an electrostatic shield is made.
Shields also enforce field uniformity, so they're useful in precision (balanced / bridge) transformers and RF.
There's one thing I'm not clear on. Being there will be a slight increase in the coil area where the spacer exists, wouldn't that increase the AE at the spacer, affecting the inductance value?
Coil area doesn't matter: very little flux flows in the air beneath the coil. That's the point of the core.
That said...
The fact that the fringing extends out from the core, means, yes, the effective area is increased at that point!
So you're again not entirely wrong!
The effect is that, as you increase gap, the Ae of the gap increases as well (basically by adding gap length to all sides; if you assume it remains a square, that makes the new area,
Ae' = Ae + 4 * (l_g * H + l_g^2)
Where H is the height of the core. (Or similarly for a round core, changing H for D (diameter), and 4 for pi, so that the diameter is increased the same way.)
That means you need slightly more gap than calculated (from a linear method), because the gap has lower reluctance (more area) than you were expecting. And that also means this is only important when the gap is relatively large compared to H, say 5% or more.
Even further into the weeds, we can contemplate the core geometry itself; the fringing flux affects the core as well, so that flux is concentrated on the edges of the core faces (recall how metal filings hang off the
edges of a bar magnet!), which makes the corners saturate earlier. The core face could perhaps be rounded off, which would make the fringing volume even bigger (a downside), but would have the effect of sharpening saturation (i.e., the whole core saturates at once, more nearly).
But that's more of a digression, than anything you'd do; it's much cheaper to simply get the next larger core size, than to try and wring every last mT out of a core.
I just want to be sure I'm understanding this correctly. Are you telling me that If I use Litz I don't have to use a spacer ?
I was always planning on using Litz Wire for this project. Being I'll be operating the inductor at 100kHz, I figured it was a prerequisite, due to skin effect.
Yes, though it depends on the Litz of course. Regular Litz, for carrying currents in cables, let's say, won't work as well once it's inside a transformer: because of proximity effect. Proximity effect is exaggerated when you have multiple layers carrying current in the same direction, so it's hardest on multilayer inductors. Transformers (including coupled inductors like flyback transformers) can be wound with alternating primary and secondary layers, to minimize proximity effect. (This is one advantage of forward converters: the primary and secondary currents oppose, so that a construction with alternating layers doesn't incur any extra proximity losses. Flyback converters don't carry primary and secondary currents simultaneously, so the losses aren't reduced by current cancellation: but the reduction of leakage inductance is more critical for this, so the alternate construction is still important.)
Further, because the fringe field is intense, it might be that you need Litz made with several gauges finer strands, to avoid excessive loss near the fringe.
For sure, don't make a marginal design where the Litz is
just barely good enough in the rest of the winding -- then the part deeper inside (subject to proximity effect or fringe eddy currents) will overheat.
But in general: absolutely, without a doubt, Litz is better, whether it's placed near the gap or not. There may still be some advantage to avoiding the fringe area, but likely you'll be fine to put it there: don't worry.
You definitely have to worry if your strands are rather coarse. Why coarse strands? You'll find sometimes it's easier/cheaper to build a transformer with several individual strands, laid down in parallel, or twisted together. Maybe you don't have Litz that size, or don't want to splurge for it. An inductor designed for low current ripple will even be fine to make out of solid wire. But if these materials are wound on a cut and gapped core, they may burn up near the gap, due to induced eddy currents (instead of load current). This would be a situation where you'd add a spacer in the middle.
There are also good reasons to add a different kind of spacer, like this,
Which has more to do with reducing the capacitance between sections. The insulation voltage also goes up. These are commonly seen in high voltage transformers (including CCFL drivers).
And, apparently, whatever this transformer was doing. It was removed from a CRT monitor, so a couple of those windings are mains voltage input (~350V, probably switched at ~140kHz -- flyback), a couple would've been ~100V output (for deflection and CRT driver), and a couple for logic supply (3.3 to 12V, say).
The single split bobbin design is also very common in mains transformers, where the huge isolation voltage is a plus, and the high leakage inductance is not-a-bug-it's-a-feature (crappier regulation, but reduced harmonics, fault current, and in the smaller sizes (< 20VA or so), tolerance of continuous short circuits).
Tim
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