Flyback Converter Magnetics

We have also placed a dot on one end of each of the windings. All dotted ends of a transformer are considered to be mutually equivalent. All non-dotted ends are also obviously mutually equivalent. That means that when the voltage on a given dotted end goes high (to whatever value), so does the voltage on the dotted ends of all other windings. 

That happens because all windings share the same magnetic core, despite the fact that they are not physically (galvanically) connected to each other. Similarly, all the dotted ends also go low at the same time. Clearly, the dots are only an indication of relative polarity. Therefore, in any given schematic, we can always swap the dotted and non-dotted ends of the transformer, without changing the schematic in the slightest way. 

Let us calculate what Vinr is. This voltage translation across the isolation boundary follows from the induced voltage equation applied to each winding 

Note that both windings enclose the same magnetic core, so the flux t is the same for both, and so is the rate of change of flux d(j)/dt for each winding. Therefore 

Note During the on-time, the primary side is the one determining the voltages across all the windings. And 

---

The flyback inductor actually behaves both as an inductor and a transformer. It stores magnetic energy as any inductor would, but it also provides mains isolation (mandated for safety reasons), just like any transformer would. In the forward converter, the energy storage function is fulfilled by the choke, whereas its transformer provides the necessary mains isolation. 

---