Overview:
The traditional way to demagnetize a transformer on a forward converter is to use a second winding that is wound in two wires with the primary winding so that when the power switch is turned off (usually a power FET), the magnetizing current continues to flow. Over. Such a circuit typically limits or clamps the drain-source voltage of a field effect transistor (FET) to twice the DC supply rail voltage. This technique of using a recovery winding can also be successfully used in a flyback structure to solve the problem of leakage inductance.
Note that in any flyback converter, the flyback transformer (multi-winding inductor) is far from perfect, and its (primary to secondary) leakage inductance can reach 5% of the primary magnetizing inductance. The leakage inductance (LLK) is equivalent to the power FET (drain connection). More complicated is that the parasitic output capacitance of the FET and LLK constitute a series resonant circuit. When the FET is turned off, very high overvoltages and ringing occur. The higher the Q value of the circuit, the higher the ringing voltage. This situation can cause huge EM interference and reduce the reliability of the FET due to the increase in the FET drain voltage. 
Figure 1(a) shows a modified demo board (Spirit Viper17L) with a recovery winding added to the flyback converter. There are important considerations in this circuit: Resistors RS1 and RS2 are sense resistors that monitor the current; the range of the measured current is directly obtained from these resistors. The transformer ratio is consistent with the original transformer ratio. The recovery winding N has only a close magnetic coupling with the primary winding N, making the two windings double-stranded. The two wires are wound side by side on the core or the bobbin at the same time to form a double-strand winding. This approach maximizes coupling and achieves a tight match between parasitic capacitance and inductance. The coupling between the primary winding and the other windings is not as important.
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