The inductance value is a fundamental parameter for a flyback transformer that directly determines whether it operates in CCM or DCM mode, how large the peak current will be, and whether the output remains stable.

The core of a plug–in transformer is large, providing ample space for winding, making it easy to achieve an inductance value within the specified tolerance of ±5%。But the patch core is too small—so, with the same number of turns, the inductance may be lower, and winding more turns isn’t possible.

The leakage inductance does not participate in energy transfer, but it generates a voltage spike at the moment when the MOS transistor turns off。The peak voltage is too high → MOS overvoltage → the machine explodes.

The rectangle structure of the plug-in transformer is loose, and the leakage is generally controllable. The patch scheme has small magnetic cores and compact circles, and leakage may be higher or lower (depending on circle method), but cannot be assumed to be the same as the plug.

Pre–substitution verification: What is the leakage inductance ratio for the patch configuration, and do the RCD parameters need to be adjusted simultaneously？

This is a solder–pad substitution that is most easily overlooked—and also the most dangerous parameter.

The plugin’s core is large, Bmax operates steadily at a working point of 0.25 T, and there is ample headroom。However, the patch magnetic core area is small, and the Bmax will be higher at the same power.The flyback topology uses unidirectional flux and has no positive–negative symmetric space； at 100°C, PC40’s Bsat drops to just 0.35–0.39 T.

Once Bmax exceeds 0.3 T, the risk of core saturation during start-up and load jumps rises sharply.