N_PS is a ratio of primary winding turns to secondary winding turns and although each winding must have a whole number of turns, the ratio of the two is not required to be a whole number. The choice of N_PS influences the design tradeoffs on the voltage ratings between primary and secondary switches, and the balance between the magnetic core and winding loss of the transformer, which are explained in more detail as follows:

1. Maximum N_PS (N_PS(MAX)) is limited by the maximum derated drain-to-source voltage of Q_L (V_{DS_QL(MAX)}). In the expression below, ∆V_CLAMP is the total voltage deviation above V_BULK when the primary TVS or RCD clamp absorbs the leakage inductance energy of the trasnformer when Q_L turns off. V_O is the output voltage, and V_F is the forward voltage drop of the secondary rectifier.
   Equation 21.
2. Minimum N_PS (N_PS(MIN)) is limited by the maximum derated drain-to-source voltage of the secondary rectifier (V_{DS_SR(MAX)}). In the expression for N_PS(MIN), ∆V_SPIKE should account for any additional voltage spike higher than V_BULK(MAX)/N_PS that occurs when Q_SR is active and turns-off at non-zero current in AAM and ABM modes.
   Equation 22.
3. The winding loss distribution between the primary and secondary side of the transformer is the final consideration. As N_PS increases, primary RMS current reduces, while secondary RMS current increases. Conversely, as N_PS decreases, primary RMS current increases, while secondary RMS current reduces.