Choose the transformer primary-to-secondary-side turns ratio based on the allowed voltage stress for the output rectifier, or the primary MOSFET. For 19.5-V charger designs, it is valid to choose a turns ratio that allows the use of a more efficient 100-V Schottky rectifier.
For a good Schottky diode with 100-V reverse rating, VREV(rated), the rectifier forward voltage drop, VRECT, can be expected to be in the range of 0.4 V to 0.5 V at 3 A to 5 A, at practical operating temperatures in the region of 100°C. Allowing an 85% derating on the rectifier reverse voltage stress, Equation 23 indicates a required turns ratio of 5.734 for a maximum AC peak voltage of 373 V (264 VRMS).
Choose the bias winding turns ratio to set the nominal bias voltage for the device VDD pin. Use an initial VBIAS(target) of 12 V.
For a typical 0.7-V bias-diode drop, this equation calculates to 0.6366.
When the transformer size and type are chosen, the actual turns values can be calculated. Because the turns need to be rounded to integer values, the actual turns ratios achieved deviates from these targets. Check the final ratios to ensure that the secondary-side Schottky rectifier stress and the bias winding nominal level are acceptable. Adjust the specific turns counts to meet the target ratios.