# Flyback Transformer Primary Inductance Calculation

Can we prove the inductance value of this datasheet using calculation based on it test condition (10kHz,100mV)?

Previously I can get almost similar result using LCR meter however I want to know how the value is derived without measurement. Is it possible? Source Design Note – DN05086/D

Thanks.

• You can use this formula to find the primary inductance value : Npri^2/( R core+R gap) R is the reluctance. – Delphesk Feb 25 at 9:34
• however, if you don't know the core material used and the air gap i am not sure you will be able to calculate it. – Delphesk Feb 25 at 9:39
• Typically it is measured as it can be difficult to calculate directly. An air gap is introduced to set the inductance. The bigger the gap the lower the for a theoretical formula see this answer to a related question. This formula tends to underestimate the inductance however because it does not allow for 'fringing' effects. – Warren Hill Feb 25 at 9:41

I want to know how the value is derived without measurement

The inductance of a flyback transformer is based on: -

• Number of turns (actually it's number of turns squared that's proportional to inductance)
• Core permeability (more permeability means more inductance)
• Core dimensions (a shorter mean length and bigger cross sectional area increase inductance)
• Core gapping (if used, L will reduce with more gap)
• Operating frequency (if above a certain point L reduces due to permeability changes and, above usually a higher frequency, self resonances can make inductance measurements pointless)

And you need to assess if the peak current is causing saturation because that reduces effective inductance.

• Hi @Andy aka, thank you very much for your answer. Ya, I totally agree with this. – Lutz Fi Feb 26 at 11:22

What is your purpose here? Incoming goods inspection of actual parts? Or something theoretical?

If it's theoretical, then Andy's answer is good provided you have all the measurements including turns count, mechanical drawings, and the datasheet for the correct grade of ferrite.

If you have a tray of actual parts in front of you, all the calculations in the world won't help if the assembler picked the ferrite core from the wrong bin, or used the wrong brand of cigarette paper to gap the transformer. Then there is no substitute for measurement.

If an LCR meter isn't available, there are other approaches : calculate the capacitor value that will resonate it at 10kHz, and measure the resonant peak frequency, for example.

• Hi @Brian Drummond, this is only for my reference. Basically I understand that we need to use step mentioned by Andy. – Lutz Fi Feb 26 at 11:25