LM5026: the Vds stress of the MosFET on the Freewheel end is abnormally large when the 48Vin is no-loaded, but this abnormal stress will disappear when the output is increased to 1A.

Hi,

It looks to me with some load like 1A, when turn off Q204, its current still in freewheeling from its source to its drain. So Q204 is turned off while the L201 current still freewheeling through Q204, i.e., Q204 turned off with ZVS so after Q204 off, and when L201 current freewheeling to zero L201 current won't reverse flow through Q204.

When no load, when Q204 turns off, it is with L201 current reversely flowing and flowing from Q204 drain to source so when Q204 turns off, that di/dt and associated inductance including parasitic and L201 will cause a voltage spike V = L201 x di/dt. It looks this is why. You can test to find L201 current reversely flow when Q204 turns off at no load.

If you want to avoid L201 current reverse flowing at no load, you would need to increase L201 inductance value, or increase fsw, or set up some minimum load.