So far I have calculated the primary turns using the formula:

Npri = V*10^8/(4.44*F*B*Ae), giving me 20 turns for a frqeuency of 300kHz, B of 75mT, and an Ae of 0.991cm^2.

However when attempting to use the attached equation to determine my secondary turns with Faraday's Law, my secondary turns are calculated as 169 when rounded up, for a Vo1 of 753 to account for diode drops, a period of 3.33uS, 150mT flux swing (2xoperating flux) and 0.991x10^-4 (the area again but this time in metres? I was going off the equation which seems to use metres)

This gives a turns arrangement of [20 20 169 169 169 169] for my two primary and four secondaries respectively. This seems excessive, since this is a turns ratio of 169/20 = 8.45, which given my primary voltage of 200V should generate 1690V... but I originally only needed the 750V across each sec winding.

Where am I going wrong? Does this mean that my primary turns will have to increase due to the large number of secondary windings to get my correct voltage ratio, since these values seem to be the minimum required values. If I calculate the primary turns according to the voltage ratio and the secondary turns required I would need 2 x 45 turns on the primary.

Does this suggest the core or the material I am using is not suitable?

I am new to transformer design so as much detail as possible is appreciated. Thanks!