The design of Transformer is by a simple graphical method. All the major parameter （e.g. Turn-Ratio, Primary minimum Turn, Primary Inductance）
Determine Turns-Ratio – The used of Turn-Ratio could affect the CC/CV turning point and maximum switching voltage stressed across the FET/BJT. （take a 5V output design as example, and reference to graph-1, it shown the high/low limit is between 12.2-15.8 and the calculated reflected voltage is 70-90V）
Determine minimum Primary Turns – The minimum number of turn affected the operating flux density and must be keep below the maximum limit （0.32T）. This graph show a minimum turn to be used for a specific core size. （e.g. EPC17）. The Vin resistor could also affect the operating duty （or VT） and a higher value would result a higher output power. The value used must recalculated if Vin resistor value has changed from it standard value. （e.g. 4.63M）
Determine Primary Inductance – This parameter affect the maximum power point for a design to be deliver. For a Adapter, it limit the over power limit & for Charger, it affect the CV/CC turning point. The Vin resistor could affect the operating duty （or VT） and a higher value would result a higher output power. The value used must re-adjusted if Vin resistor value has changed from it standard value. （e.g. 4.63M）
