Here is the flyback that I m studying. It is a flyback transformer. enter image description here

I am confused about the different turns ratio. If I suppose the input voltage minimum and so the duty cycle maximum (i.e D = 0.5), according to the relation which gives the output voltage in function of the input voltage, we should have :

$$\frac{Vin_{minimum}}{Vout+V_{forward}}\ = N$$

Nevertheless according to this formula I do not get the different turn ratio of the transformer.

For example by using my formula, the turn ratio between the 24V output voltage and the input voltage should be :

$$\frac{290}{25}\ = 11.6$$

Nevertheless, the turn ratio of the transformer is :

$$\frac{52}{4+3+3+6}\ = 3.25$$

It seems to be very different from my formula ... I do not understand :( Besides the duty cycle is regulated from the 24V. Any turn ratio corresponds to what I find with the formula ... I obviously miss something ...

Here is what I think the bottom coil section should be superior or equal to the one which follow it. Look the figure at the top of the pictures enter image description here

Thank you very much

  • 3
    \$\begingroup\$ You do need to ask a question Jess and you do need to show the numbers you calculated. Your formula is correct but you can't have fractional turns, you must always round down N (increase secondary turns) and live with the consequences. \$\endgroup\$
    – Andy aka
    Commented Apr 7, 2020 at 14:34
  • \$\begingroup\$ Thank you Andy for your comment. I edited the post for being more understable :D \$\endgroup\$
    – Jess
    Commented Apr 7, 2020 at 14:46

1 Answer 1


The first error is in assuming the 24 volts is produced by all 16 secondary turns - look at the first tap point at the top of the 6 turn winding - this is the ground reference for those voltages. In other words the 24 volts is produced from 10 turns and not 16: -

enter image description here

I would say that the next error is in assuming that the minimum supply for the primary is 290 volts. This may not be your error but someone else's. Given the turns ratio of 52:10 to make the 24 volt output, I would say that the minimum DC supply on the primary is more likely to be 125 volts DC or around 87 V RMS after accounting for the bridge rectifier. It's likely that the minimum supply is 87 volts AC because that just about makes the transformer suitable for 85 to 264 volts AC i.e. the universal supply range of 85 - 264 V RMS.

  • \$\begingroup\$ You re right ! I don't know where was my mind ... I was so in despair. For the next part, you may be right ! I ;) It clearly makes sense !! \$\endgroup\$
    – Jess
    Commented Apr 7, 2020 at 15:04
  • 1
    \$\begingroup\$ Glad to help and lift the misery LOL. \$\endgroup\$
    – Andy aka
    Commented Apr 7, 2020 at 15:10
  • \$\begingroup\$ "I m back ! " Question : Do you agree that the coil section at the bottom of the transformer shoud be higher or equal to the coil section which follow it, as the coil at the bottom provides the current of all the secondary supplies. (-15V; 7V5; 15V; 24V) \$\endgroup\$
    – Jess
    Commented Apr 10, 2020 at 9:43
  • \$\begingroup\$ I don't understand. What do you mean by the coil section which follow it? \$\endgroup\$
    – Andy aka
    Commented Apr 10, 2020 at 9:45
  • 1
    \$\begingroup\$ Yes, it makes sense to have the lowest 3 turn coil have a larger copper wire (if needed). It doesn't apply to the -15 volt coil because any current circulates back to 0 volts and that coil doesn't contribute to any of the voltages on the positive side. \$\endgroup\$
    – Andy aka
    Commented Apr 10, 2020 at 10:40

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