Flyback transformers come in two flavors: CCM (continuous conduction mode) and DCM (discontinuous conduction mode). The DCM version is the one we will develop here.
The idea of the DCM flyback is that the SEC (secondary) current drops back to zero before the next ON time for the PRI (primary). Generally, a DCM flyback is designed at the point between CCM and DCM.
When the switch is ON, the PRI inductance is storing energy due to a PRI current. The moment the switch is turned OFF, the energy stored is then:
This energy is then transferred to the SEC side where the load attached depletes that energy. The DCM is depleted entirely of its SEC energy before the next ON time the PRI sees. The number of times per second this occurs is the power delivered and is this energy multiplied by the frequency.