# Suggestion on core calculation with air gaps

I was working with the SMPS transformers and found a transformer that had a small air gap already there and was used in charging system of a battery. I wanted the ferrite core for my own transformer but I found an air gap as shown:

1. I am using the formula of area of products for power capability but I am not sure if it will work or not because in this I have an air gap....shall I consider 90% of that result and marking 10% error?
2. secondly the inductance also decrease so I am thinking to do the number of turns per volt according to the equation shown. And then increment the number of turns per volt with 10% increase to compensate for the air gap..
3. Are my assumptions correct that the power capability will decrease due to more heating produces in the core??
4. in my actual power calculation if I am designing for 100W. I'll make it for 110-120W(due to air gap only) because now the current that will be consumed for magnetization will be more(This increase in power does not included in the one we just assume to make ie 10% approx to compensate for all the powers loss without air core loss).

So are all my assumptions correct or nearly correct in this case?? Can I start designing the transformer using these assumptions assuming the manufacturer of the charger Did the same to compensate for this air gap??? Kindly correct me in case of any wrong assumption.

Also there is a constant of topology in the products of area equation, and in flyback it is stated that air gaps are better. Does this topology constant as any relation with this air gaps??

• Nearly everything that you talk about is unreliable. You need to do proper calculations to work out how many more turns you need to add to get the required inductance. I never use area product calculations for switching transformer design. Commented Oct 24, 2023 at 17:55
• @Andyaka I'll do the calculation about the extra inductance but what about the power calculations?? I found this product of area method from many places. Commented Oct 24, 2023 at 18:02
• Do you want a forward transformer, or a flyback? As it has a gap, it will not do as a forward transformer, get an ungapped core. It will work well as a flyback, do the proper calculations for a flyback. Commented Oct 24, 2023 at 18:27
• @Neil_UK I want this for a flyback transformer. I was following the product of area method given in the link above for power calculation. But I guess that doesnt take air gap in consideration. How to calculate power with airgaps Commented Oct 24, 2023 at 18:34
– qrk
Commented Oct 24, 2023 at 20:04

The flyback converter basically works on pulsating DC voltage therefore there is a risk of core saturation. For this purpose there is an air gap introduced so that the core is not saturated.

This will make the inductance less and thus other factors had to be taken in consideration. The following book is a contains all the details about the switch mode flyback transformer and the calculations to be done in case of the air gap:

"Switching Power Supply Design, 3rd Ed. Book by Abraham I. Pressman, Keith Billings, and Taylor Morey

This book is widely available in used condition from eg Abe Books, Amazon and others.
The second edition is substantially different from the 3rd - as one of the original writers died..

A number of people online say that they consider the 2nd edition substantially superior to the 3rd edition. They speak very highly of the practical usefulness of the 2nd edition.
Caveat emptor.

Used 1st edition copies are available from about $US6.45 with free shipping in the US. . I just ordered a 2nd edition for$US16.45 (shipping to NZ and tax cost more again).
3rd edition copies are typically in the $100-$300 range.

• I removed the link to the book content because it didn't look like it was a legal copy. Commented Oct 25, 2023 at 15:27