Loosely coupled transformer windings

Steve, the way you show it is generally the way it is done, except that the center leg of the core is often a separate chunk of core with an air gap at each end, that is slid between the primary and secondary sides of the outer core. Almost any microwave oven that does not have inverter drive will have one of these in a kilowatt size. Many high pressure sodium and mercury lamp ballasts are made this way, also. The difference for your case, I think is that the core will be ferrite instead of laminated silicon steel.

You might be able to get by with a toroid core with a the primary (and feedback) on one half, the secondary on the other half, and a ferrite bar insert across the diameter between them. But it would probably be more reproducible with UU cores.

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Excellent, thanks John. I thought I had the concepts right, but just wanted to be sure. I've wound my share of simple inductors in the past, but nothing like this. I like your lateral thinking - a bar across a toroid. In this application, the impedance of the secondary isn't too critical, but I'll have a play around now that I know I'm on the right track and increase it as much as possible. A question - what do you mean by UU cores? (Two "U" cores?)

... Steve

There are lots of other possibilities, also. Keep in mind that the top and bottom bar in your drawing are essentially equi-potential (from a magnetic field standpoint) nodes, so the ordering of the parallel branches is not very important. The flux shunt could be on either side of either the primary or secondary branches. It doesn't need to be between them. Think E cores with center or one leg ground off to make a gap. There are E core shapes made that have all 3 legs the same cross sectional area for this purpose. And, of course, you could add an external inductor in series with a tightly coupled secondary and get a very similar effect.

Right. I don't have keyboard characters for a pair of Us facing each other to form a loop. ;-)

My drawing probably wasn't too clear, but there was an air gap in the middle leg. (It looks like a washer.) I figure that I can wind the 3 windings, then increase the air gap to reduce the coupling to the secondary until, under load, it drops from 1000V of "turns-ratio" voltage to about 100V-200V of "loaded" voltage, while still keeping close coupling between primary and feedback windings, (or two halves of a centre-tapped primary for push-pull driving).

I don't quite follow what you mean here.

Yep, I've also been thinking about that. It would make starting easier, too, especially if I adopted a method of shorting across the second side of the heater filaments to heat them, then open that circuit to dump the series inductor's energy across the tube for ignition, similar to mains operated fluoros.

Incidentally, in the end I'll also add heater windings, which should have reasonably close coupling, but one thing at a time, I reckon.

... Steve

View in "Symbol" font:- ÌÉ

... Steve

Question for the OP Why would you NEED a loosely coupled winding? Why can't you use a tightly coupled winding? Mark

Or a capacitor - as used in CCFL inverters.

I understand, but increasing the gap between the core halves will also increase the magnetization current of the primary because of the lowering if its inductance and also slightly lower the coupling factor to the tightly coupled secondary.

Most E cores have a center leg with twice the cross sectional area of the legs on each side, with the assumption that all the coils will be placed on the center leg, and the flux will split into two equal fractions and half will pass through each of the outside legs. But I have seen a few E cores that have a constant cross sectional area in all parts. These allow for some creative winding designs with various windings on different legs.

Sounds like a plan.

Question for the OP Why would you NEED a loosely coupled winding? Why can't you use a tightly coupled winding? Mark

I thought the purpose was clear. I want the secondary to have loose coupling so that the output voltage is high under no load, then drops considerably under load once the tube ignites.

... Steve