The Auxiliary transformer has the output voltage and current that are closely related to the input voltage and current. some conditions the time of power flows into the transformer from the supply, while no power flows out. Later, power flows out of the transformer into the load while no power flows inside. Leakage inductance can be quite large, leakage inductance is the part of the inductance of a winding that is not linked to other winding. If you also get the problem where one winding is better linked to the primary than another winding. there is any leakage inductance, if you get a big spike in primary side of the transformer, auxiliary voltage rises too high, causing the regulator to shutdown or fail. Actually this transformer is used in FLYBACK CONVERTER.
