As Steve says, you need to look at the transformer as an inductor. As such, you need to know the specs of your powdered iron core, number of turns of wire etc to calculate the inductance.
Firstly, you need to calculate the amount of energy you need out
This determines the amount of energy you need to store in the inductor plus some more to cover losses.
Once you know the amount of energy to be stored, then calc your inductance required by the volts you're applying and the nominal pulse width.
Once you know the required inductance then you can calculate the number of turns for your given powdered iron core.

The inductance determines the pwm frequency.

Re-arrange to suit the major requirements.

A powdered iron core will saturate and can only handle a certain flux density which determines the amount of power you can transfer.