UCC28780: UCC28780+GaN+UCC24612 SR MOS damage

Part Number:UCC28780

I use the UCC28780+GaN in my 65W A+C project, (has two ports: type A+type C).

 I open the port A output when the C port has 20V output, then the SR MOS damage.

My RM8 transformer turns ratio is 6.5, SR MOS Vds is 150V,

Do you have any suggestions to avoid this damage? thx.