what is the name of this yellow tape around high frequency transformer?
It's an insulating, flame retardent polyester tape normally used within the layers of transformers to enhance the breakdown voltage capability between primary and secondary.



3M describe it as: -


  Yellow polyester film tape with an acrylic adhesive. This tape is
  suited to coil wrapping, wire harnesses, fractional horsepower motors
  any many more uses. Flame retardant to UL 510.


3M link