[Elecraft] KPA500

[Tom W8JI]

> Isn't the I-E stack of a conventional power transformer 
> just a rectangular
> binocular core made out of steel?

Almost. It is impossible to grain orient the entire core, 
and there are core areas out near the corners that have very 
little flux but add weight and cost. Rounder cores are 
nicer.

The performance difference depends on the particular core, 
how the core is configured or stacked, how conductors are 
arranged, and where and how we are measuring the flux.

In an E-I lamination the normal stacking procedure is to 
alternate single or groups of laminations. E from one 
winding end, then E from the other end. The result of this 
is much less air gap because I's are sandwiched between E's 
at both ends. The result is very little air gap in the flux 
path. The more the manufacturer interleaves the E's and I's, 
the less flux leakage there is.

Chokes on the other hand are generally made with the E's all 
from one side, and a little paper spacing all the I's from 
the E's. This makes an intentional air gap. The gap reduces 
tendency to saturate, and can actually increase inductance 
when dc current is present. It also causes very high levels 
of flux leakage. Boy Scouts within miles of a typical filter 
choke can no longer rely on their compasses.

Toroids are not perfect. Over half of the wire in the 
winding is outside the window. Outside the hole, that wire 
adds a lot of resistance compared to the inductance! This is 
where the dual C core transformers come in. With a thick 
core, most of the copper is inside the window. Pound for 
pound and physical volume for physical volume, they have the 
best ESR performance. They are really nothing more than two 
toroids side by side with a common winding sharing both 
cores.