Allocating transformer pins

Posted by: ralphs99 on 05/19/2009

Hi to the Forum,

I'm a first time user of PI expert. So far I'm very impressed with the attention to detail in the software. Excellent work!

I'm trying to create a low power design with a single primary winding, a stacked secondary and 2 separate auxilliary windings that will be on the primary side of the insulation. I was able to fudge the design by pretending that my auxilliary windings are on the secondary side. So far so good.

Now when it comes to transformer selection I'm unable to find a bobbin with more primary pins than seconday pins. I need at least 6x pri, and 3x secondary pins. Plus maybe a pin for a shield. It seems to me that either side of the bobbin can be used as a primary as long as sleeving is used to maintain the safety requirements. I'm looking at an EEL-19 core at the moment. RM8 might be a possiblity but that one doesn't seem to be supported for my design.

However I am unable to force my preferred pin selection, as PI expert complains than I may not place a secondary winding termination on a primary pin.

So I'm stuck with trying to specify the transformer the way I want. I'd like to get a sample transformer made by PI, and so need to specify it correctly. Any suggestions on how to proceed?

Cheers, Ralph

回應

Yes I understand. PI Expert will not allow you to pretend that pins are available on the primary side... Its the built in checking for safety that comes back to haunt you in this case.
Solution - Add a dummy EEL-19 core (same specs as EEL19 core) and a dummy bobbin to the database. Name this new transformer core as EEL19-Custom. (Watch the demo video named " Adding a Custom Transformer into the Component Library" for details - http://www.powerint.com/en/design-support/pi-expert-design-software)
Make sure that bobbin has (atleast) 7 pimary side and (at least) 4 secondary side pins. Next make sure that optimization uses this particular transformer EEL19-custom (watch the following video "Designing with a Fixed PI Device and/or a Fixed Transformer Core" for details from the following site - http://www.powerint.com/en/design-support/pi-expert-design-software

That should solve your problem.

Great, thanks for the reply. I'll give your suggestions a try.

I also have another question about the airgap that's identified for the core by PI expert. I've noticed that when I choose a larger core than really required for the power level, the gap length increases. Once it gets beyond a certain length, the numbers changes from blue to red. Is this a warning of some sort?

Cheers, Ralph

Yes it is warning you that it is violating the upper limit for gap length. As your gap length increases it also increases the fringing flux. There are losses (and thermal hotspots) associated with fringing flux so beyond a certain calculated limit the software flags a warning (by turning red) indicating that gap length is too large.

Thanks to your advice, I've finished my design using a fudged EEL19 transformer with 2 primary pins and 10 secondary pins. All good!

However, the 'transformer construction' tab has only one of my primary windings under the principal insulation layer. If I now submit this design to PI for a custom transformer, I won't get what I'm after. What is the best way to specify the design for a custom transformer? Should I manually write up the winding instructions? Can PI's winding dept accept this? Is there a way to trick the application into giving me the winding instructions I want?

Cheers, Ralph