how to choose Zener Diode rated voltage for Output Overvoltage shutdown protection

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Hi,I will make a power using TOP258PN, output 28V and 2A. The core type is EER28, Primary:77T Secondary:14T Bias:9T. I want use output overvoltage shutdown when output exceeds 20% Vout Rated. My problems is that how to choose Zener Diode rated voltage for Output Overvoltage shutdown protection.
Application Note of TOPSwitch-HX Family said that the bias winding voltage rises when primary winding voltage rises, but no said is the what connection.
Thinks !