High frequency transformer function
In a word, isolation and transformer. And the ac signal in the transformer is the same. Because a magnetic field is used to transmit energy (and signals), the ground on the side of the primary and the side of the secondary need not be equipotential, and the difference in the ratio of the primary to the secondary also affects the output voltage. However, according to the question put forward by the subject, it seems that the subject does not quite understand the various dc/dc converters. Yes, you are right, PWM signal control transistor provides the required duty cycle, if the requirements are not high, this signal can be used directly output. The inductance/capacitance/resistance network at the back is used to stabilize the voltage and the current. Some of the switching power supplies that use transformers make use of the inductors in the transformer belt, "integrating" the separate inductors. The actual analysis can be completely separate inductor and high frequency transformer to calculate.
The small smps high frequency transformer has ee transfomer, ef transformer , ec transformer, etd transformer, pq transformer etc.