Hello guys, I’m planning to build a flyback SMPS (12V, 3A). I’ve collected all the necessary parts and components for this project from the local market. For this project, I’m using an EE/25/13/7 ferrite core. Below are the dimensions of the ferrite core that I measured.



To build this SMPS, I’m following the book “Switching Power Supply Design” by Abraham I. Pressman, Keith Billings, and Taylor Morey. But I’m facing a major problem here. In the book, the formula given to calculate the number of turns on the primary side of the flyback transformer includes a term “A[sub]lg[/sub],” which represents the air gap.
Here is the formula,


However, the EE/25/13/7 ferrite core that I bought does not seem to have any air gap, just like shown in the picture. My question is: if there is no air gap, how do I calculate the number of turns for the primary side? In this condition, the transformer core will saturate.

If there actually is an air gap, how can I determine its value?

 

My impression is that in a transformer, an air gap is used to pprevent core magnetic saturation. Core saturation may not be an issue in your design. The number of primary turns depends on the amount of power to be transformed versus the saturation intensity, since that limits the transformed power. Primary current beyond what produces saturation serves no useful purpose. So a bit of math is required, in addition to knowing the saturation intensity level of your core material. Your application may not require an air gap.