Application Note AN-22 TOPSwitch transformer turns problem

4 turns secondary is a calculated value. It is calculated based on the inductance and the peak flux density BP which is required to below 4200Gs.

First, calculate the duty cycle and turn ratios based on the reflected output voltage Vor. Then calculate the average current and the peak current of the primary side current based on the duty cycle and the current ripple ratio Krp. And then calculate the transformer based on the duty cycle and the current ripple too. The secondary turn numbers can be calculated, since both transformer effective area Ae, maximum current limit of the part, peak flux density and the transformer inductance are all known.