I'm estimating actual peak current limit & air gap height based on the actual transformer core parameter in the datasheet. This flyback transformer is designed based on 24V output with 50W rated power. Below is my procedure:

  1. Calculate primary inductance based on minimum frequency 55kHz at minimum input voltage (85V). Based on that, I will get below value

Calculation 1

As you can see from above image, the primary inductance, Lp = 280uH. I fixed it as 300uH with 0.307 turns ratio. Based on that, I'll get peak current 2.76A with maximum duty cycle ~45%

  1. Then I calculate the area of product, Ap based on above value using below formula. Based on that, I will get Ap = 5303mm^4

Calculation 2

  1. After that, I search PQ core material that have nearest value to my Ap = 5303mm^4. From the datasheet, I choose PQ 26/25 due to the total area is 6000m^4 which is almost near to my Ap value.
  2. From PQ 26/25 datasheet (Part No.: PC95PQ26/25Z-12 ), I checked NI limit & air gap specification. From my previous calculation (Lp = 300uH, Nsp = 0.307), I will get Al = 443nH/turns^2. From the curve, when Al = 443nH/turns^2, NI = 90A.turns. See below



  1. Based on above, I can conclude that the total peak current for my actual transformer core will be 3.5A (based on NI = 90A.turns) which is slightly higher compared to initial calculation & air gap is 0.2mm. Correct?
  2. From the curve, if primary turn increase, AL value will decrease, NI value will increase, therefore total peak current can be increase. Correct?

Please comment if found any missing/wrong step.


  • \$\begingroup\$ AL value will decrease - for a given core set, AL remains static. \$\endgroup\$ – Andy aka Feb 26 '20 at 11:49
  • \$\begingroup\$ Really? Do you refer to "maximum" AL that fixed in each core? I believe that maximum AL should be static \$\endgroup\$ – Lutz Fi Feb 26 '20 at 11:57
  • \$\begingroup\$ AL is fixed by the core irrespective of number of turns. It's the reciprocal of reluctance and that makes it purely a core parameter. See this. \$\endgroup\$ – Andy aka Feb 26 '20 at 12:03
  • \$\begingroup\$ @Andyaka Another reluctant answer ? :-) . Sorry too bad a pun to resist. \$\endgroup\$ – Russell McMahon Feb 26 '20 at 12:19
  • \$\begingroup\$ @RussellMcMahon I am reluctant to agree with you henry. \$\endgroup\$ – Andy aka Feb 26 '20 at 12:20

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