Turns of the auxiliary winding (N_A) is an integer value usually chosen to provide a nominal V_VDD that satisfies all devices powered from V_VDD, such as a gate driver, UCC28782, etc. N_A is determined by the following design considerations:

1. V_VDD must be lower than the maximum rating voltage of VDD pin (V_VDD(MAX)) at maximum output voltage and rectifier forward drop (V_O(MAX) + V_F). V_VDD(MAX) is also limited by the lowest voltage rating of any other devices connected to the VDD pin. Use the lower result of the two following options, where applicable.
   1. For designs with a fixed output voltage or a narrow output range, the maximum Auxiliary winding turns (N_A(MAX)) is given by the following equation.
      Equation 36.
   2. For designs with wide-output voltage range (such as with USB-PD or PPS or similar) where the boost circuit is likely to be used, leakage inductance may peak-charge the BIN capacitance. The internal boost switch has a maximum rating of 30 V, so a 24-V Zener diode is often used as a clamping device to avoid overstress on BSW. This clamping voltage sets a lower limit on N_A(MAX) and is given by the following equation.
      Equation 37.
2. The nominal V_VDD should consider the impact on the stand-by power. Higher V_VDD results in a static-loss increase with the total bias current of the devices connected to the VDD pin.
3. V_VDD should be higher than the 13-V threshold voltage of survival mode (which is the sum of V_VDD(OFF) and V_VDD(PCT)) at the minimum sustained output voltage (V_O(MIN)). ΔV here represents the voltage difference between the nominal V_VDD and the survival-mode threshold. A minimum of 3 V is a recommended design margin for ΔV.
   Equation 38.

N_A(MIN) must also accommodate the highest V_VDD(OFF) threshold of other devices powered by VDD, if any. Select an integer value for N_A between the lowest N_A(MAX) and the highest N_A(MIN) with consideration of #2. For best performance, design the DC resistance of the auxiliary winding to be < 0.1 Ω.