The transformer used in the STEVAL-ISA081V1 was developed in 2010 for use with the VIPER26L (60kHz); the 760871131 from Wuerth Elektronik is very similar.
However, the VIPER26H operates at the higher frequency of 115kHz, for which the transformer must be designed. If you don't want to have a customised transformer made, you could try the 750310849, which was designed for a switching frequency of 132kHz. Wuerth Elektronik or another transformer manufacturer will be happy to help you with your choice.
Hope that helps?
Good luck!
/Peter
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