Transformer coupling coefficients

Really close to one, in my experience -- which has a lot more to do with RF than power supplies.

I often see the leakage inductance of a ferrite quoted as being equal to one turn of wire around the outside of the core. I _don't_ know if this is really some fundamental constant due to the geometry of the thing, or if it just works out that way, or if it's all BS. But if it's true you can back out from leakage inductance to coupling coefficients.

Yes. Leakage inductance is really what I'm after in the first place. It really is determined by geometry and the relative permeability of the core material. Windings that occupy exactly the same space would have perfect coupling. Leakage happens to the extent that this impossibility is not realized.

I suppose I could use FEMM before winding up some test samples, but for k > 0.99 I'm probably in the simulation error and noise.

Because leakage inductance can vary with the winding structure and turns ratio, a fixed value based on core structure alone is wishfull thinking.

RL

Understood. I'm not looking for a silver bullet or a "simple answer to a complex issue". I'd just like to know some typical values that people who use such things routinely may have seen in practice. That's also why I asked about k rather than leakage inductance. k does not depend on turns count, just on geometry and relative permeabilities.

I was surprised to find k's on the order of .998 in a couple of quick FEMM simulations. Maybe the question is moot.

In message , dated Tue, 29 Aug 2006, Don Foreman writes

I'm not sure what you mean by 'moot'. It can mean 'debatable', but I suspect you mean 'pointless'.

It's true that for normal constructions, k is so close to 1 that it can be assumed to be 1. It is more helpful to consider leakage inductance, which is easy to measure, at least if there are only two windings. You measure the inductance of one winding with the other winding short-circuited. You can also include leakage inductance as a discrete component in simulations.

It looks like an Anglo/Americano distinction. Merriam-Webster shows a second sense: deprived of practical significance - made abstract or purely academic. I've never heard it used in the original sense. Maybe the distinction on the west side of the pond is moot.

-- Joe Legris

In message , dated Wed, 30 Aug 2006, J.A. Legris writes

Yes, I've noticed this difference before, and decided to follow it up in this case, because it seemed simple enough to get a result without too much confusion. The transition in meaning is rather easy, but it doesn't seem to have made it into English dictionaries (no, I haven't looked at ALL of them!).

debatable -> purely academic -> pointless

You'll still need snubbers on the primary, though.

How about "don't amount to a hill of beans", or "hit don't make no nevermind, ol' Son!"

Right. k is generic to a geometry, leakage inductance is specific to a particular set of windings and the k that exists between them. For given geometry and k, leakage inductance will increase as self-inductance increases. Things are clearer for me if I separate the variables of geometry and numbers of turns because they are somewhat separate parameters.

Usually so, except for H-bridge or half-bridge configurations. Question is how much snubbing. Snubbing is dissipative so less is better -- as long as it's enough!

In message , dated Wed, 30 Aug 2006, Don Foreman writes

But it's reasonably nearly proportional to self-inductance, provided the windings are not very different in 'window-fill'.

See above.

Yes, and the proportionality constant is k -- which is what I'm asking about. I guess I'll have to do some experiments.

An overnight soak of an inductor and a transformer in some lacquer thinner did a very nice job of freeing up the little ferrite cores. Man, the one out of a fluorescent ballast has a 2.61mm gap!

In message , dated Wed, 30 Aug 2006, Don Foreman writes

k is very nearly 1. The leakage inductance is very much smaller than the winding inductance, not k (or 1/k !) times it. You may say it's (1-k) times, if you define k to make it true.

You guys are just killing me! Please read;

I believe that the coefficient k is an abstraction introduced to remind the simulator user that something is missing in the model.

It has no physical derivation.

RL

It is defined in fundamental physical terms: It is a ratio of flux linkages. Transformer theory was well-developed long before simulators or even computers existed. Many or most texts express k as lambda sub ij / lambda sub jj where lambda ij is flux linking winding i resulting from excitation of winding j.

k is defined as a ratio of flux linkages, not something I define. Leakage inductance will depend on turns ratio as well as k, but you're right in that it is a 1-nk sort of relationship.

I saw nothing in that appnote dealing with values of k found in common practice.

So far all I'm hearing is "close to 1", which isn't very helpful. The difference between .990 and .995 can be significant, and .999 works even better. Are these realistic values? I guess nobody here knows either, oh well! Thanks anyway and nevermind.

In message , dated Wed, 30 Aug 2006, Harry Dellamano writes

What astonishing insight does this give? To try to introduce all that in a news article to someone more or less beginning to understand transformers is simply counter-productive.