In old magazine (Radio – Ð Ð°Ð´Ð¸Ð¾) I found small article about calculating PWM power transformers. The formulas are for toroid core transformers, but I think, they are usable in standard E core transformers too.
1. Calculate the power of transformer: P
n , W. The power of transformer is sum of all output powers multiplied by 1.3.
2. The core max power: P=S
max/150 , W. f=Hz, S
c=area of core section in cm
o=area of the “hole” in cm
max= max induction in Teslas, from core datasheet.
3. Number of turns in primary winding: w
c , in turns.
4. Max current in primary winding: I
1 , A. (Î· = ~0.8)
5. Diameter of wire: d
1max , mm. diameter is equal 0.6 multiplied by square root of max current.
6. Secondary (ant other) winding: w
1 , in turns.