Forming main inductor for high-power boost converter

[Bootstrapper]

Hi,

I'm about to build the big brother of that nice little boost converter shown in Elektor magazine. Full output power is approx 250W (25V @ 10 A). So the main inductor needs to pass roughly 25 amps at full load (from a 12-V supply) and if a small margin is added, the total current would be 30 amps. Fine, next thing is to find a suitable core.

I happen to have several large yellow-white toroidal cores salvaged from a telecom power supply. After some intense googling these were identified as T157-26 (2P75) cores. The inductance required is 33 uH and I came up with 18 turns. But there is one problem: I wound quickly 15 turns of wire around the core to see what kind of inductance it forms, but I can measure only 11 uH. As far as I know, the type of wire shoudn't affect this much to inductance. This lead me wondering whether the core is permanently saturated and hence unusable.

Have I done/calculated something wrong? I used the following links as a reference:

www.micrometals.com/images/tcolorcode.jpg

http://www.ferroxcube.com/prod/assets/2p.pdf

Thanks for inductive advice!

p.s. These cores were used in inductors that passed maximum of 75 amps DC current, so if they are not saturated, they can easily withstand my requirements. I calculated the worst-case flux density to be 700 mT (40 amps DC!) - well below the saturarion limit.

p.p.s. I also have several small toroid inductors with only 5 turns, but inductance as high as 95 uH. These are form the same power supply as the cores mentioned earlier. The "wire" used in these comprises hundreds of ultra-thin wires wound in parallel, the diameter of this thick bundle of wires is roughly 4.5 mm! I might use one of these as an alternative.

[cerrem]

If you could post schematic it would help...
Measuring inductance is not always straight forward....
The excitation current and or voltage used durring measuring is critical...so is the frequency used...
What frequency is the SMPS at ???
You would want to proper small gauge of wire to avoid skin effect at your switching frequency...then parallel the appropriate number of windings to meet the proper Circular Mills per Ampere needed for your temperature rise requirement..
The spec sheet provided the core material characteristics....I would need to see the physical data of the core size, ect...
This way you can then calculate the Al , which is the mHenries per 1000T as a start....Then you can figure the turns needed for 33uH...
Have you figured the Energy of the core???

Chris