Electronics > Projects, Designs, and Technical Stuff
Transformer design: how to size a core?
Buk:
--- Quote from: T3sl4co1l on October 16, 2021, 08:35:07 am ---Yeah I mean it fits, you can pull off a winding factor of 50% -- but it's going to look like, well, that; and all that crowding increases proximity effect, while the turn length greatly increases as well. So the efficiency falls; the price of compactness.
--- End quote ---
You might find this: Designing Low-Power Flyback Inductors Using Tiny Toroids (Part 1): A Boost Converter Application
and this: Designing Low-Power Flyback Inductors Using Tiny Toroids (Part 2): Calculating Losses
interesting; or not.
T3sl4co1l:
What a strange article; lambda is used for flux, the application is not introduced so there's no motivation or direction behind the subject, and no mention is made of a suitable controller -- the most particular it gets is naming a 2N7000, a TO-92 device of all things. And he goes directly to #26 powdered iron, literally the worst choice available, without explanation. This feels so... vintage? Yet the copyright is proudly 2019.
I think I get the idea of what's being done there -- it's hard to review because it's all over the place, the core selection is unexplained, then the core loss flux density formula is given, then the power loss is given -- it's like it's completely in reverse logical order.
So in the end he's got 8uH Lp, rated for 210mA. Sounds very much like one of these:
https://www.coilcraft.com/en-us/products/power/coupled-inductors/1-n-shielded-coupled/lpr/lpr4012/lpr4012-103d/
which is a smaller outline at a tiny fraction of total height. They also have core loss calculations on site; though I'm not sure if the coupled inductors are supported?
Inputting similar numbers, Micrometals recommends very far from #26; T37-6 and T30-2 are typical examples, giving excellent low losses (43, 35mW), though mind that these are single-winding inductor figures, and the copper losses would be about 40% higher in a flyback transformer.
To his credit, the core loss calculations seem consistent with what Micrometals' calculator says. It would seem the inexplicable core choice, or lack of motivation, is the only real problem; the calculated results are sound. :-+
Tim
Buk:
--- Quote from: T3sl4co1l on October 16, 2021, 08:49:58 pm ---What a strange article; lambda is used for flux, the application is not introduced so there's no motivation or direction behind the subject, and no mention is made of a suitable controller -- the most particular it gets is naming a 2N7000, a TO-92 device of all things. And he goes directly to #26 powdered iron, literally the worst choice available, without explanation. This feels so... vintage? Yet the copyright is proudly 2019.
--- End quote ---
I don't see any reference to "2N7000" or "TO-92" in either of the articles I linked? (Sorry one of the links was broken, I'll correct it shortly.)
--- Quote ---I think I get the idea of what's being done there -- it's hard to review because it's all over the place, the core selection is unexplained, then the core loss flux density formula is given, then the power loss is given -- it's like it's completely in reverse logical order.
So in the end he's got 8uH Lp, rated for 210mA. Sounds very much like one of these:
https://www.coilcraft.com/en-us/products/power/coupled-inductors/1-n-shielded-coupled/lpr/lpr4012/lpr4012-103d/
which is a smaller outline at a tiny fraction of total height. They also have core loss calculations on site; though I'm not sure if the coupled inductors are supported?
Inputting similar numbers, Micrometals recommends very far from #26; T37-6 and T30-2 are typical examples, giving excellent low losses (43, 35mW), though mind that these are single-winding inductor figures, and the copper losses would be about 40% higher in a flyback transformer.
--- End quote ---
I can't argue with your logic; but 3 observations:
* I suspect the choice of core for the example was deliberately obscure by way of demonstrating that it could be done, not necessarily that is was a good solution to anything in particular
In support of that suggestion, I offer another article of his which might clarify some of the motivation:
Single-Bundle Windings Make It Easier To Build Custom Magnetics In-House
* I haven't seen the references to TO-92 packaged transistors you saw; but when it comes to electronics, I like vintage. Components big enough for me to see with my tired old eyes; recognisable component values; no need to for an STM, nor the dexterity of an eye surgeon, to fit (and remove) from boards. You can keep your smt ... :P
* I think the guy likes to explore the obscure and forgotten topologies; and to prototype. (Which makes him useful to me for this.)
(BTW: I got really excited about this, before I realised I'd need a stack of 20 in parallel :palm: )
--- Quote ---To his credit, the core loss calculations seem consistent with what Micrometals' calculator says. It would seem the inexplicable core choice, or lack of motivation, is the only real problem; the calculated results are sound. :-+
--- End quote ---
That's good to hear.
T3sl4co1l:
The paragraph on page 3, "For a small converter, a small MOSFET switch such as a 2N7000 is used. ..."
It's interesting, subtle; merely change a single digit and it seems current enough (2N7002 is the SOT-23 version, still very much in use for jellybean purposes, alongside BSS138, BSS84 and others). Just as well, it could mean the "original", generic part, and there's not actually anything to read into. But so too, the choice of #26 (and yeah, a possible motivation is to "make do" with it). We can only speculate, short of e-mailing the guy, heh.
I saw the reference to, but didn't look at, the "single bundle" article. I don't have a problem with that, at least from what I gather from its use in the first document. Indeed, I've done that plenty of times myself, and actually I'd recommend you do it as well, if you can. The biggest benefit is: getting excellent transmission line transformer type coupling between windings. Especially so, if you pair primary and secondary wires (as pairs or quads), and then bunch up however many it is you need to wire up in series and parallel, for primary and secondary. The biggest downside is: you have to painstakingly ohm out every single goddamned wire, to make sure you're getting them all properly in series and parallel..! (Even with color coding, you'll only have so many colors to choose from, but for your case, dozens of strands to cover, so...) But yeah, fine for one-offs.
BTW, if you're curious about off-the-shelf possibilities, shop around for inverter transformers. Nothing wrong with using a, say, 240V to 5V, flyback transformer in reverse! Or maybe a forward type would be more suitable (do you really know what inductance you need, as yet?), or multi-secondary so you can wire them for a CT'd primary, etc. They'll likely be bulkier than what you may be looking for, on account of the squarer shapes (mostly ferrite E-core styles), bobbins with rows of pins, and reinforced insulation, and they tend to be expensive (in part just because you're buying tiny quantities through a distributor, prices are kind of irrelevant -- but anyway, the alternative is your labor, potentially a big savings?).
CCFL transformers are also interesting, but probably much too high a ratio for what you want, and they only come in so many sizes. I guess if you wanted to rewind one you could. They tend to have a low profile and high aspect ratio, so might still fit inside your cell-sized enclosure, and I think the power level is about right, give or take size selection that is.
Tim
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