When reverse engineering a common mode choke from a line filter I found a wrap of very thin metallic foil inside the plastic housing of the toroid core. A little research on magnetic materials indicated to me, that the core material has to be some kind of permalloy because the L/N² value is exceptionally high, about 40uH/N². And then I asked myself the following question: wouldn't it make sense to build the core from solid permalloy instead of the thin foil? After all the only benefit of the multilayer construction I see is the reduction of eddy current losses. Why would I want to reduce them in a common mode choke instead of exploiting them?