The more I dig in magnetism formulae the more vague things get. So I decided to make a few experiments. First I took a transformer and short-circuited its secondary winding. Result was drastic drop in primary windings inductance. There lies the answer. You need to know your output load (resistance) and connect equivalent resistor to transformers secondary winding. Then measure primarys inductance. Knowing your switching frequency and voltage assuming less than 80% duty cycle, finding the period (t) should not be very confusing. Then use this formula: (I = U*t/L) and you'll get your amperage which the transistors or fets must tolerate. It was supposed to be obvious because I recently measured effect of resistance on capacitance. It works vice versa. The more resistance the less capacity, at least from oscillation perspective. This makes contactless heart rate monitor possible.
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Measuring the primary inductance with the secondary shorted is a standard test for transformers to determine their leakage inductance. The lower, the better. For an ideal transformer, it would be zero.
In switch mode power supplies, the output of a transformer is intended to be connected, through a rectifier, to an inductor (note 1). It is that inductance, and a capacitor, that forms the integrator that smooths the pulses from the transformer into DC. The control loop of the SMPS looks at the output voltage and adjust the duty cycle to keep the voltage where it's supposed to be. Use of a resistor instead of an inductor to limit the current would be an unnecessary waste of energy.
Most power supplies that are used in desktop computers fall into two groups. Half bridge (these will have a 115/230 Volt switch) or full bridge (these generally have PFC also)
In the half bridge supplies, one end of the primary sits at ~170 Volts DC and the other end gets alternately pulled up to +340 Volts or down to 0 Volts.
In full bridge supplies, both ends of the primary are switched. First the left side to 0 Volts and the right to ~ +390 Volts. Then both are off, Then they change polarity. then both off again. Repeat. (note 2)
Note 1. This assumes that we are talking about forward topology based SMPS using true transformers, and not flyback type SMPS that use coupled inductors.
Note 2. Some systems, such as phase shift switching, use different timings.