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Flyback Transformer Design

Exp: 
 input 90~264Vac
output 19V 4.74A (90W)
選擇鐵心大小  POT-3319 Ae:1.474cm^2
工作頻率    f:70KHZ
工作週期 0.4 at 90Vdc
       

PWMMOS=12A/600V
               

SBD=20A/100V
        Maximum repetitive reverse voltage (VRRM
               
Forward Current Rating (IAVG)

              The diode manufacturers provide de-rating curves so for example at a lead temperature of 100 °C a 3 A PN diode is actually a 1.5 A diode!

               As a general rule of thumb select the current rating such that IAVE = 3 x Iout irrespective if the diode is a PN or a Schottky type.

      

or

               

Reverse recovery time (trr)


      To prevent large reverse currents from flowing through the diode when the MOSFET in turns on, applying a reverse voltage, the diode should have a reverse recovery time less than switching time.

               
               
      This means a recovery time of 14 us or better.
      Longer duration than this causes large primary side current spikes at the turn on event and high diode dissipation.

               

        An ideal current waveform, the spike now seen is purely due to parasitic capacitance, largely the capacitance of the primary winding.
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