# Calculating the number of turns for my inductor

so i've calculated that I need an inductance of 0.150mH for my inductor. However, I'm trying to figure out the number of turns I need for it. # I've got an answer of 24 turns but this doesn't feel right to me. Could someone double check my calculations?

below is the datasheet for the toroid I used as well as the graph(from the datasheet) I used.DATASHEET FOR TOROID ***TOROID datasheet (using 3C90)

If anyone is interested what I was following for my calculations.

Many thanks!

• Are you trying to build a voltage controlled variable inductor? You linked the material spec for 3C90 but you haven't linked the toroid data sheet - you need to do that. Apr 18 '20 at 12:48
• It's for my boost converter. This is the data sheet for the toroid, the on im using is the 3C90 farnell.com/datasheets/650988.pdf Apr 18 '20 at 12:56

I've got an answer of 24 turns but this doesn't feel right to me. Could someone double check my calculations?

The data sheet for the toroid tells us that $$\A_L\$$ is this: -

And the relationship between $$\A_L\$$ and inductance (L) is this: -

$$L = N^2\cdot A_L$$

So, if you have 24 turns and $$\A_L\$$ is 1170 $$\nH/turn^2\$$, inductance is 673.92 uH.

Don't derive this yourself because the data sheet has the numbers directly "inside" $$\A_L\$$.

If you need 150 uH then you'll need about 11 turns (141.6 uH)

If you are operating closer to saturation (as you appear to be saying) then $$\A_L\$$ reduces proportionally with permeability. So, with a H-field of 60 At/m, the relative permeability has dropped from 2300 to 1000 hence, $$\A_L\$$ drops by the same amount from 1170 to 507.

Now if 24 turns are used, I calculate an inductance of 293 uH (still higher than your calculations). I estimate 17 turns would be needed to give 146.5 uH.

However, the bigger question for me is why you are using this toroid to make a boost converter. At a 60 Ah/m H-field and an effective core length of 30.1 mm (see above), the MMF (magneto motive force) becomes 60 x 0.0301 = 1.806 ampere-turns AND, given that you have possible 17 turns, the maximum current you can push through the toroid (for reasonable efficient operation) is 106 mA and this is a pretty low power boost converter in my humble opinion.

• Thanks for the explanation, I see where I went wrong :) I’m attempting a MPPT for a solar base system. And I need to be careful with the voltage and current as the microcontroller have a limiting voltage rating Apr 18 '20 at 13:24