selfmade AF-transformer

Hello guys,

I was looking for an AF-transformer with a ratio of 1:10. It must have a center tap on the primary winding. The ones I found so far do have the desired ratio, but no center tap. Using two of them in series will result in either frying the transformer or the controlling MosFet transistor, most likely both of them. The reason is clear. Switching the same polarity to the same terminal of the transformer will saturate the iron core and what will be left is a little bit of resistive load. (4 - 8 Ohm)

So I decided to make my own transformer. I don't know how to calculate an AF-transformer.

So any hint towards the calculation formulas will be appreciated. Googling did not bring about anything usable.

Thanks

Boncuk