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Fitting a design to specific transformer parameters

Posted by: luukmeijwaard on

Hello,

I'm tasked with designing a 260-410V in, 12V/0.833A (10W) out flyback converter using the EP-3 INN3692C for a graduation assignment, and i'm trying to evaluate my design in PI Expert online. This is my first design in PI expert so all of this could be the result of my lack of experience. As a starting point i'm trying to figure out whether this would be possible with EP-20 ferrite (EP20-3C81-A250) since i have it readily available. I have made a custom core component with all the correct specifications, which i'm using in my design. SInce the INN3692C is unavailable in PI expert i'm using the INN3678C instead.

One of the design specifications is to ensure that the converter works in the Constant Conduction Mode until the output power dips below 3.75W. For this reason i am trying to design for a specific value of primary Inductance, which PI Expert doesn't seem to allow since there's is no way for primary inductance to be defined by the user. The cores i'm supposed to use have a specific air gap size of 460um, which i'm not able to define myself either.

The transformer specifications are:
Primary inductance : 7.388mH
Primary/secondary ratio : 7:1
Primary windings: 175
Secondary windings: 25

I have tried approximating my design by changing the parameters that are available, by changing the reflected output voltage i've been able to at least get the number of primary and secondary windings right but the primary inductance calculated by the software is way off; 110uH instead of 7.388mH. The primary inductance value seems unaffected by the amount of turns, which strikes me as odd since it's dependant on Al and the amount of turns. Is there any way i could specify the Inductance and air gap myself? If not, what could be the reason for this difference in inductance value?

Personally i find it frustrating that there's no user input for these parameters, but perhaps i'm tackling the design the wrong way.

Any thoughts are welcome!

Kind regards,

Luuk

link to the design : https://piexpertonline.power.com/piexpert/design/select?share=570721880d7d74733c18f6844cf3832a14f23f0465ebeb0abc78af16d3382069

Comments

Submitted by PI-Yoda on 05/16/2020

Hi Luuk,

PIExpert could be used to recover design parameters with known transformer and spec.
But not for ANY transformer!
The reverse engineering – fit a design around transformer is possible when the transformer parameters were similar to the one delivered as a result of a normal design process.

The problem with your specific transformer is that there is no set of design parameters and controller available leading to such a transformer, for that particular spec. Or even close.

DER-745, DER 752, and some other examples could be used as a starting point and to modify them for another core, but the inductance is impossible to match.

Best Regards,