you need to learn about how the flux in the core rises with applied volts - otherwise you will saturate the core - and short the pri drivers
For example V / N Ae = dB/dt, where V = pri applied volts on one winding, N = number of turns on that winding, Ae = effective cross sectional area of the core, B = tesla, t = time
Such that; for a core of 15mm diameter, 180mm^2, 12V pri, a B swing of -150mT to +150mT, a max on time of 10uS ( 50kHz) the Npri must be > 2.222 Turns
thus the min pri turns are 3 + 3 for bifilar under these conditions