UCC28070: Current sense transformer selection

Sam,

Transformer design is beyond the scope of this forum. Besides, without knowing the whole system requirement, it is impossible for us to provide recommendations.

Best, 

Hello Sam, 

The UCC28070A datasheet has significant information and guidance on how to select the current transformer (CT) in Section 7.2.2.7 on page 37 (Section 8.2.2.7 page 32 of UCC28070 datasheet).  The key parameter is to have a extra Volt-second margin in the core in order to accommodate several cycles where complete reset of the magnetizing current of each switching cycle may not have been accomplished within the available off-time.  

This condition is most likely to happen around the AC-line zero-crossings where the MOSFET on-time is maximum (at DMAX) and off-time is minimum within the switching cycle.  Perhaps the worst event where such a condition might happen is during the recovery time from a sudden load step, where the peak inductor current may become maximum at the same time as on-time goes to DMAX until the output voltage has recovered back to regulation.  
You don't want the CT to saturate because the UCC28070 will lose control of the inductor current. 

I recommend using the -070A datasheet because it has more updated information about timing control and inductor design.  (The UCC28070 datasheet is not yet updated.) 

Regards,
Ulrich