8.2.2.2.1 Primary-to-Secondary Turns Ratio (N_PS)

N_PS influences the design tradeoffs on the voltage rating between primary and secondary switches, and the balance between the magnetic core and winding loss of the transformer, which are explained in 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 voltage above the reflected output voltage. It can be either the ripple voltage of C_CLAMP in AAM mode, or the voltage over-charge of C_CLAMP by the leakage energy as Q_H is disabled in LPM mode. V_O is the output voltage, and V_F is the forward voltage drop of the secondary rectifier.
Equation 19.
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_H is active and turns-off at non-zero current in AAM mode.
Equation 20.
3. Since the high-frequency transformer is usually a core-loss limited design instead of a saturation-limited design, the minimum duty cycle (D_MIN) at V_BULK(MAX) is more important. Lower D_MIN increases core loss at V_BULK(MAX), so this constraint creates another limitation on N_PS(MIN).
Equation 21.
4. 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.