Small Question on SMPS Tranformer Isolation Material

[homebrew]

Hi,

can anybody tell me what the yellow sticky tape inside typical SMPS transformers actually is?

It looks like electrical tape - which it is certainly not. Kapton-Tape it isn't either.

Btw., when winding a transformer myself, is Kapton a viable (and safe) option to use?

[Richard Head]

It's polyester tape. Normally 50um thick but to check clean the glue off and measure with a micrometer.
Not nearly as temperature hardy as polyimide, but easier to use as it stretches nicely.

[T3sl4co1l]

Mylar/polyester, with acrylic adhesive.  3M Scotch something or other is pretty standard.

Kapton/polyimide is fine too, but rather expensive and higher temperature than you need. (Ironically, polyimide with acrylic adhesive isn't much higher temperature than polyester.  The silicone adhesive stuff is good for plenty, though.)

Tim

[Richard Head]

Tim
I've dismantled seriously overheated HF transformers that use polyimide tape and the copper wire is almost black due to overheating but the polyimide tape can still be unwound and is virtually untouched. When polyester tape is used it turns into a melted gooey mess. Obviously, no normal person would design a transformer to operate anywhere near these limits but it's good to know what gives when you push the limits. 

[T3sl4co1l]

Yeah, normal magnet wire enamel melts and solders at the temperatures where polyimide just begins to sweat.  To use it to its fullest advantage, you must use high temperature (usually, polyimide as well!) enamel, and bobbin, and either keep the core a whole lot cooler (ferrites run out of steam much above 100C!), or use a higher temperature material (NiZn ferrite or powdered iron)!

Tim

[homebrew]

Thank you very much for the infos!

I have Kapton laying around. And for a one-off thing, I don't care the costs that much.