Why does the flyback circuit need an air gap?

The high frequency transformer of flyback switching power supply is in the first quadrant of hysteresis loop. During the on period of the switch tube, the primary coil of the transformer only stores energy, and during the off period, the energy stored in the primary coil is transferred to the secondary, so it is both a transformer and an energy storage inductor. Opening a certain air gap in its central column can reduce the residual magnetic field, improve the DC magnetic field strength of the magnetic core, make it bear a lot of ampere turns, prevent the magnetic core from saturation, and achieve the required inductance by adjusting the air gap (AL-Value=L 1/N 1? )。