I had a bit of rattle about this here before but I hope you folks ain't fed up with me yet.:) So after some trial and error i've built a working smps for my amplifier , it's pretty high power ,when testing dummy loads i got up to 2KW , since I don't need more for my project I never went further as I had no need. It's not regulated but it dropped some 15 volts at max dummy load so works for me. The problem is this , after trying out some leftover ferrite cores simply to understand that there's no luck with them I bought a new EPCOS made core.It's the ETD59 , the material is N87 with permeability of 5300nH. the size of the core is 59.8 x 31.2 x 22.1mm Based on other smps using the same materials I made 12 turns in the primary (maybe 11 can't remember) And the secondary even though doesn't play a role in this problem is some 2x7 turns.the primary is wound with some thick litz wire salvaged from some old smps.Haven't exactly measured but I would say some 3 mm2. the transformer frequency is +- 50 Khz, driven by a pair of half bridge IGBT'S. The problem is this , under load the core heats up , I kind of predicted this so I made my heatsink such that it also touches the core , in other words I have an aluminum box with large surface areas and everything is thermocoupled to the box. But still I think the core gets too hot even with lighter loads , something probably isn't right.The windings themselves stay cool , only slightly warm but then it could also be the effect from the heat given off by the core. When I was younger i made a few mains transformers with hand and I once got my turns calculations for the primary wrong and ended up winding fewer than necessary turns , but back then the windings got warm not the core , since there wasn't enough inductance and the wires went through some resistive/ohmic heating on each mains cycle. So I wonder what could be the problem , my own guess is too few or too many turns in the primary, rather too few but I'm not sure.