# What is the difference between magnetic and primary inductance?

Electrical Engineering Asked on December 2, 2020

I try to calculate ramp compensation resistor (Rcomp) for flyback SMPS converter design. For this, I examined slope compensation section at page 15 in NCP1252 datasheet by OnSemi.

But, there is a parameter which I don’t understand. Calculations are below as screenshot:

Mixed point !!!
Ns/Np = 0,085
Lout = 27µH
Lmag = 13mH (?) WHY?

As I know, L2 = L1 x (N2/N1)^2
So, Lout = Lmag x (0,085)^2 = 27µH –> Lmag must be 3,7mH
But, Lmag = 13mH in NCP1252 datasheet

Are magnetic and primary inductance different each other?

Are magnetic and primary inductance different each other?

No, they are not different unless you want to take into account leakage inductance (2 to 5%). I think you are not understanding the type of circuit they refer to: -

Page 16 and 17 are referring to a 2 switch−Forward Power supply specification and they use an extra inductor in the output circuit seen as L1 in the above schematic. That will be the 27 uH they refer to: -

Correct answer by Andy aka on December 2, 2020