Determining Bobbin Fit Window (BFW) Transformer Parameter

Posted by: Gononda on 05/29/2012

In PI Expert v8.6.0.2, I am making use of a E25/13/7 (EF25) core, which is included in the Default Component Library of PI Expert (see attached image for a screenshot of the transformer parameters as shown in the Component Library).

According to this image:
http://www.powerint.com/sites/default/files/BFW.png

a good approximation for the Bobbin Fit Window (BFW) parameter is:

Bobbin Fit Window (BFT) = AW (Bobbin Winding Area) / BW (Effective Winding Width of Bobbin)

= 61.0 mm^2 / 15.30 mm = 4 mm (for the E25/13/7 EF25 core)

So why was BFT = 7.26 mm used in the library for this core?

Here is the datasheet of the core (B66317G0000X127) I'm going to use:
http://www.epcos.com/inf/80/db/fer_07/e_25_13_7.pdf

Also: How do you determine LT (length of a single turn of wire) from the datasheet? Shouldn't I use I-N (average length of turn) as shown on the datasheet?

Comments

Hi.

You are absolutely right, 7.26mm is a bug. Thanks for spotting it.

I measured on a EF25 we have in stock and got 4.3mm. I think your estimate is a good one since you only have the datasheet. If you want a better measurement you'd need to have the actual bobbin and core.

This will be fixed in the next patch. Again, thank you very much for spotting this bug!

Thanks for your quick response. Can you perhaps also shed some light on my last question about how to determine LT (see last part of my first post)?

From your example datasheet, you would need to do the following assuming you are using the bobbin B66208B.

the center leg is 9.6mm (on the outside), and it is a square so 9.6 * 4 would be your LT. The reason why we don't use the mean length is because the software will already take into account the layers, for the added length of wire on subsequent layers.