# smps inductance

Discussion in 'General Electronics Chat' started by m1ch43l, Dec 30, 2014.

1. ### m1ch43l Thread Starter Active Member

Aug 16, 2012
62
1
Hi all,
I have an experiment im working on and i need to know the typical inductance of a computer smps primary coil [the biggest transformer in the device]. Im driving it with 50Khz to 70Khz squarewave signal.

Ive tried the age-old inductance formula for cylindrical inductors for a coil i wound on the smps core with 19SWG wire(1.006mm), 3cm long 1cm diameter and one layer thick. I ask this because on spice, it shows outrageous voltages and current at the low-side mosfet switch [i know that this usually happens within a small timespan].
For the record, i'll use this just as a pointer for my project to see if i'm going nuts with inductance calculations.
Three questions...
1. Anyone have an idea or measured value?
2. Does one use the value 720 or 150 for iron core for the relative permeability of the core?
3. Does one have to match the inductances of the primary coil and the secondary coils in any transformer?

Last edited: Dec 30, 2014
2. ### m1ch43l Thread Starter Active Member

Aug 16, 2012
62
1
i calculated the inductance of one smps with roughly 33 turns of primary coil. It had two coils but i went for the innermost coil and got 6mH while using relative permeability value of 720, radius of 0.003Metres and a height of 0.014M.
Can anyone confirm this?

3. ### Alec_t AAC Fanatic!

Sep 17, 2013
7,023
1,453
Do you know the core is iron? I think most are ferrite.
Without knowing the core material, cross-section, and whether or not there is an air gap it is impossible to calculate the theoretical winding inductance.

4. ### m1ch43l Thread Starter Active Member

Aug 16, 2012
62
1
Its a ferite core, no air gap and its open coil space from the centre of the centre–leg is 1.5cm radius by 1.4cm high. Other details are un the above posts.