A term I have used in the past to denote two windings tightly wound together is that they are 'bifilar wound'. This does, of course, suggest that the two windings are similar in wire gauge and turns - which may not be the case where there is a large difference in primary to secondary volts and, to minimise interwinding voltage breakdown, the windings need to be separated. Would appreciate a video on how you measure leakage inductance.

This video links up nicely with the Miller Plateau video. (Flyback leakage inductance/switch-off/+ve/snubber)

I have very large ferrite core it’s a 20kw transformer I use 24 gauge copper wire 6 together for a smps welder am building everything work but the transformer make a lot of noise when the overloaded senor reduces the duty I don’t have any issues when the arc welding but when it sticks the senor activate and the noise is there i gat wire space of 1 cm per turn and use glue to stop wire from moving when the high current is welder mother moves is a toroidal core but still the noise I rewind it for no space and wire all wound tightly the with the right phase but still noise the current sensor controller is the issues I use CT senor and a small ferrite ring to remove the frequency from getting into the ic the same thing happens the frequency is 50khz is there anything that can be down to stop the noise when current sensor activated

Does it change based on the transformer type (IE, C-core, R-core)?