Googling "transformer core cross section" turns up more hits than you can shake a stick at... is there something else you need?

Dan - Owner
http://www.Hi-TecDesigns.com

Given you have known supply conditions the dominant factors will be the maximum peak flux density your core material can handle without saturating, and the core aperture you need to be able to fit conductors. Suppliers of small cores usually provide some simplified design equations.

Time to open some USB power supplies "chargers" (120-240 VAC in, 5 volts DC / often 1000 or 2000+ mA out) and see how they do it.

For transformer design two different methodologies can be selected from. The Kg approach which refers to core geometry or the Ap approach which refers to area product. Excellent examples of how to do the design can be found in books by Colonel McLyman (i.e. Transformer and Inductor Design Book). The problem is that not every transformer core manufacturer has the proper published data to immediately follow one approach or another completely through to the end. Also, not all transformer core manufacturers publish the same kind of data for their cores, so knowledge of how to calculate some of the needed information from the published data is needed. Some transformer core companies will have some data in English units, and other data in the MKS system. If you are designing a transformer that will require eventual UL or CE approvals, careful attention to insulation, spacing, materials, etc will be required for the actual construction.

Earlier editions of McLyman's books can be found as pdf on the internet (he worked for JPL and designed power supplies for space probes, so early editions were in the public domain). Also, you can go to www.coremaster.com and look at their application notes (written by McLyman). Coremaster makes a more exotic nanocrystalline core, so depending upon your end application their offerings may not be what you need, but his approach is there in full detail and you can use the app notes to design using cores from other companies. Also, look at http://coefs.uncc.edu/mnoras/files/2013/03/Transfo... (something I stumbled upon while googling to write this reply).

There are lots of strange numbers and equations involved that to the unfamiliar may look like voodo magic. But once you do it a few times you come to realize that Steimetz equations and coefficients are a means of putting the manufacturers charts of material characteristics into an equation, and those numbers to a strange power for thermal are really just figuring temperature rise of a transformer from a straight thermal design standpoint as if the transformer was just a rectangular box.