PI Xls Designer V9.0.5.1 : User Interface Problems Transformer Parameters ...
Dear All,
I try to compute my wide input 90..350VDC 24Vout / 7Watt design. However I'm getting
read remarks on the transformers Airgap section.
Q : What is the right way to specify interleaved (sandwich) primary windings ?
Q : Where can I specify the core material and the air-gap I can actually buy ?
For example if I would like to use a RM8 core from EPCOS : B65811J0400A0
Material is standard N87, the air gap is 0.18 mm etc etc. Where can I specify all
the core parameters in order to use them in the spreadsheet ?
For me it doesn't make to much sense if I select an air-gap I can't order without
doing custom cutting the cores surface. A custom air-gap order increases the
BOM cost a lot.
I'm new to this Xls designer. Therefore it may be a stupid question and if so I
appologize.
br
Markus
Hi Markus.
First of all rest assured your questions are not stupid at all.
1 - To change to interleaved primary windings click on design tree view (on the top left, just below the new file icon). If the design tree view taskbar is visible, then click on the "Primary and Bias" option under the "Winding construction" section. There you will have a check box for this option and others (like Z winding, or tape between primary layers).
2 - We use a default core material on PIXls (PIExpert allows you to specify a core, but unfortunately LNK-HP is not available on PIExpert at this point). So to specify you just need to fill out lines 54 to 57 on the spreadsheet which will basically overwrite our default transformer selection.
Unfortunately you can not specify air gap, but you can use goal seek (in the "tools" tab). This goal seek allows the software to change one variable in order to reach your goal. In your case though this won't be a very good solution since you are very far away from your goal.
I took the liberty to try and optimize the design for 0.18 gap. Notice that I corrected the transformer parameters according to this: http://www.epcos.com/inf/80/db/fer_07/rm_8.pdf
I also changed the Vmin line to 90VDC according to your spec. Notice that to meet the gap I had to increase Kp. I also had to change the IC because I wasn't able to make the minimum secondary conduction time (line 38) with the original one you were using. By the way there is a small bug where we mention 1.11us in the minimum, but in fact we recommend 1.4us. This will be corrected in the next patch.
In the end I achieved your desired gap, but keep in mind that although this will decrease your transformer cost, it may increase other points in your board. A more discontinuous board may have a bit more EMI (due to higher peak currents). You will also have more secondary ripple current, which may affect your output cap selection.
Hopefully this will help you move forward. By the way, there are a few videos in the download section of PIExpert.