Models for RF transformers with extreme ratios?

A bandwidth of around an octave implies _some_ resonance.

You'll have a huge juggling job between leakage inductance and primary inductance.

The more loss you can stand the better chance you'll have of making it work. I'd feel a strong sense of accomplishment if I got 50% of my input power coming out of my secondary.

I recall reading in some ARRL publication or another (the one on transmission line transformers, I think) that to achieve extreme ratios you can often do better using two stages -- in your case perhaps three?

Disclaimer: never done it, I wasn't there, it's not my fault, etc.

On extreme ratios you just can't avoid it.

Yes, and you can never have half turns like at some restaurants.

Unfortunately that won't work in this case. It's usually only ok if you have an active stage inbetween.

Maybe you could come visit at the hospital if it blows up in my face ;-)

Please give some more information about why you can never have half turns. I remember overheating a transformer that used a half turn. Never tried half turns again. But what is the reason the half turn gets hot? Mike

With a toroid core, it should be obvious why you can have only integral numbers of turns. In an E-I core, or a pot core with openings on both sides, you can have a wire exit a different place than it entered. The loop then closes around one of the outside "legs" of the core. Note that this is equivalent to a full number of turns around the center post, and one turn around the outer post, with the two connected in series. IF the magnetics are balanced, the field in the outer leg will be half the field in the center leg. But this happens only if there is no current in the turn around the outer leg. Note that the "half turn" is not strongly coupled to the rest of the turns, and as a result adds a lot of leakage inductance. I don't see why the "half" turn itself should get hot, but if it diverts the magnetic field into the other leg in such a way that it significantly increases the core loss in that leg, it could lead to excess power loss in the transformer.

Cheers, Tom

Which got me to thinking: you can keep the magnetics in the two outer legs balanced (that is, the rate of change of flux per unit time) if you put a turn around each and put those two turns in parallel. However, each will see half the flux that's in the center leg, so will contribute half a turn's voltage...this could be an interesting way to get a high step-up ratio with fewer secondary turns: only 50 turns instead of 100, to get a 1:100. That could be an advantage in keeping the parasitic capacitance on the secondary at bay, though the effective capacitance is generally a very weak function of the actual number of turns--and for modest permeability cores, significantly lowers the pri:sec coupling as compared with having the windings co- axial.

Here is a patent on the subject:

there is a pretty picture of exactly that in Keith Billings SMPS book.

in some cases the leakage can be exploited.

I have a humungous iron powder core with 8.5 turns on it. its an inductor, so I dont care about the leakage.

Cheers Terry