The primary-side output voltage of a Fly-Buck converter must be no more than one half of the minimum input voltage. For a minimum VIN of 18 V, the primary output voltage (VOUT) must be no higher than 9 V. To generate an isolated output voltage of VOUT(ISO) = 12 V, a transformer turns ratio of 1 : 1.5 (N1 : N2) is selected. Using this turns ratio, calculate the required primary output voltage VOUT using Equation 22.
The 0.7 V subtracted from VOUT(ISO) represents the forward voltage drop of the secondary rectifier diode. Fine tuning the primary side VOUT1 may be required to account for voltage errors due to the leakage inductance of the transformer and the resistance of the transformer windings and the low-side MOSFET of the LM5160.