Electronics > RF, Microwave, Ham Radio

[Solved]Type 43 core resonance (Hartley oscillator frequency jump during tuning)

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JohnG:
Saturation is important if you have a core whose properties can be degraded upon saturation, which includes a large subset of NiZn cores, the main topic of this thread. If you are using MnZn cores, then generally it is not a big deal, though it can be. I have seen a number of designs (mostly buck converters) where total losses went through the roof because the MnZn core was pushed too hard, the inductance dropped at the flux peaks, and ripple and RMS current went up a lot. The additional core losses did not limit this to any significant degree.

I did not say these were good designs, BTW, but they were existing commercial designs. You might be surprised (or perhaps not) at the number of existing shipping designs where inductor current has never been measured.

Thanks for the link. It's the most comprehensive list of core manufacturers I have ever seen.

John

T3sl4co1l:
Reminds me, once upon a time we (PPoE) were tasked with a custom induction heater, lower frequency rating than usual (a kHz or so).  So, did the design work, needs a transformer of such-and-such, called around a few shops, eventually got a reasonable quote (a toroid about a foot across, wound on fine, I think 3 thou, GOSS), got the parts in and starting testing protos.  One transformer, after being hit with saturation (either because it was direct drive from an H-bridge, or through a coupling cap but without damping so the startup transient heavily magnetized it), never ran quiet again, always with a noticeable buzz (which was especially noticeable at this frequency!).

I suppose the same thing was probably in play, something like, residual magnetism, probably in a nonuniform manner (are there usually sites pinned so hard they only shift up at saturation?), or because of inhomogeneity (could be a lumpy formulation I suppose -- patches in the steel, or pockets in ferrite?), or because of geometry (of note, the start/end of the core spiral won't be magnetized equally with the rest; not sure how well glued down they were, I never saw the interior of those things), and so the magnetostriction was permanently increased despite the presumable degaussing effect of running at AC for millions of cycles.

Which, I think, is a non sequitur, the AC exposure part?  It seems tempting to think about continuous wobbling as tending to drive a system to equilibrium, like shaking a pile of sand, or straightening out a piece of wire: even if you don't deform it much on each step, you're still deforming it back towards that base condition.  But magnetic materials aren't in equilibrium like that, there may be a lower energy state with spontaneous magnetization (indeed there is, hence domains even at zero net magnetization) than at zero.  And so, magnetization can persist more or less forever, until the coercive force is finally exceeded.  (Of course, we use an oscillating decaying field in the hope that, at the initial peaks, all important sites are being pushed around, and so as time goes on, they're pushed around a little less, and a little less, averaging zero as the amplitude falls below the coercive threshold.)

Hm, if that's the case [high-coercive-force defects], I suppose degaussing -- well, it should still do the job, but it may take much more external field than you would otherwise expect; and maybe it's not feasible at all at that point to degauss it (could such a defect exist, which, once it takes a set in one direction, takes significantly more force to reverse it..??), but maybe it's just more practical to anneal above Tc at that point, assuming you can (heat ferrite to ~soldering temperature, good to go; obviously, a bit harder to do that for steel!).

Hadn't thought of it that way before, but that seems reasonable... domain pinning could be a population thing, where most take low force (giving the Barkhausen effect in aggregate), but very few have extraordinarily high coercivity, and then in combination with NiZn's higher magnetostriction, you get the observed effect.  I would imagine the distribution is a power law of some sort.  Maybe the population statistics vary with material as well, and it happens to be worse for NiZn?

I wonder how you would quantify that... Well, if it's a magnetization thing, and if you set up to measure Barkhausen noise, I suppose that would be a possible route.  Basically count the steps and see at what points the largest steps fall at; then reverse it and hope to count the same again.  Well, there's not really any reason for domains to be consistent from cycle to cycle, I suppose, is there... it's not a perfectly fixed array of discrete dipoles, huh.  But also maybe you can still identify, oh maybe even with local field probes around the core, such localized sites, that only change at extremely high field strength?

The ultimate implication being, if you knew exactly where those sites were, you could, like, literally drill them out of a core, ending up with a quieter / lower remenance part as a result.  Like defects in semiconductors causing yield reduction instead of absolute failure, it's a statistical process, not a homogeneous one.  Which, I mean, it's no accident, ferrites are semiconductors too, just ferromagnetic ones.

Tim

JohnG:
Those are great questions for a materials scientist >:D.

More seriously, magnetic materials have been poorly modeled for a century now. Maybe things are starting to improve, but I am not so sure that a full physics model, if ever developed, will be all that useful to an engineer. This is especially true for bulk materials. There are umpteen variations on the Steinmetz equation, and they are still all a fancy curve fit in the end.

On top of that, materials science is an expensive endeavor, and ferrite cores are cheap. I would guess that the margins are too low to justify serious material development, at least for most of the companies that make cores. No wonder progress is so slow.

Capacitors seem to make slightly faster progress, but not by much.

John

WPXS472:
AT my previous employer, we had a serious problem caused by magnetostrictive resonance in a ferrite core. Seems a tech was using a magnetic screwdriver too close to the core. This was a major CATV manufacturer, and they got in a panic thinking that they were going to have to replace a bunch of product. It took a couple of days to identify the issue. The core was made by a major foreign brand. I found that heating beyond the curie point cured the problem. My suggestion that they change vendors to an American core that didn't exhibit this property fell on deaf ears. Since the problem had only actually been identified with 2 units, they decided to put a label on them saying not to use magnetic tools, and let the sleeping dog lie.

szoftveres:
Annealing/de-magnetizing in real time (just for the sake of science):
https://youtu.be/Z_t8PBLibXA
Interesting to see how the mu disappears (S1,1 shoots up at the end of the video) with temperature.

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