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:
- Calculate primary inductance based on minimum frequency 55kHz at minimum input voltage (85V). Based on that, I will get below value
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%
- Then I calculate the area of product, Ap based on above value using below formula. Based on that, I will get Ap = 5303mm^4
- 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.
- 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
- 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?
- 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.