i2 is the steady current that flows through RL once the supply voltage V is established across the primary. However as the transistor Q1 is turned on/off intermittently we now consider a situation when Q1 is turned on after an off state. It should be noted that during off state we assume the current through the primary coil to be zero. So the magnetic field which had been feeding energy into the secondary coil by its decay has reduced in magnitude during the time toff. The current in the secondary coil determines the magnetic field strength completely as there is no other current(here primary) to sustain the magnetic field. When Q1 is turned on, due to absence of resistance in primary the sustainence of magnetic field is rapidly taken over by the primary current. Very rapidly an equivalent current required to sustain the existing magnetic field in the core is setup in the primary and the current in the secondary also ceases at the same pace, essentially energy is conserved. Additionally due to increase in the current in the primary a steady current or voltage is established in secondary during ton(equation A4, Figure 2)). Since the currents in primary and secondary are in opposite directions the magnetic fields produced by the two coils in the core are in opposite directions. These respective currents are also established very rapidly without any hinderance. To start with we