Inductor Design-Core selection

[Fazlay Rabbi]

I am creating a 21 to 12 volt , current rating 10A Buck converter . After calculating The value of inductor is 2.05mH . Now which type core i should use to make the inductor ?
And what is the difference between coated & uncoated toriod core ??

 

[Warpspeed]

I suggest you use the free inductor design software available from the Micrometals web site.
**broken link removed**

 

[treez]

Torroids are sometimes coated so that the enamel doesn’t get scraped off the enamelled wire when winding it round the torroid.
The coating makes it not able to run so hot, as heat is trapped in.
The coating also acts like a cushion for the winding to sink in to , stop the turns sliding around on the torroid, so you can more easily ensure good spacing of turns round the torroid perimeter.

Many of the powdered iron type torroids are giving you a non linear inductor.

Some of the powdered iron torroids cant really be used at very high switching frequencies.
If you want to use ferrite core, you could always put two or three in series to make up the inductance.

 

[FlapJack]

Coated cores have two purposes. Stop the wire from being nicked and increasing the voltage rating of the toroid. There are a few different coatings that have different voltage ratings i think in the 500 to 600 volt range.

 

[treez]

As well as micrometals, mentioned by Warpspeed, there is mag-inc.com, who do a good line in torroids...."magnetics incorporated"

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Ferrite core inductors
If it’s a ferrite core, then the inductance is just L = AL * N^2 * 1E-9

Calculating the B value at current I is also just B = (I * L) / (N * A)

Where
AL = “AL” value quoted in datasheet, its related to the ‘allowance’, the reciprocal of reluctance

A = Area
B = Flux density
L = Inductance
N = number of turns.

Non linear inductor
However, if it’s a non linear powder core inductor, then you will need to use the kind of calculation as attached here.

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The problem for non linear inductors is that at any value of H , you cannot calculate B by simply going H = B/(uo.ur)

This is because for non linear inductors, the B,H curve is not straight. So for non linear inductors, you just accept theat the value “uo.ur” merely gives the gradient of the B,H curve at a certain H value.

You have to make out the B,H curve yourself, and you do this by a kind of “numerical integration”, but really it just involves repeated application of the “y=mx+c” law.

So what you do is you approximate the B,H curve by assuming that it is made up of a series of connected straight lines. You know that your first line starts at the point B=0,H=0. You then make out a series of equally spaced H values, and calculate the value of the gradient “uo.ur” at each H value. So the gradient of the first line, which emerges up from the point B=0,H=0, you make this equal to the gradient at your first non zero H point…it is an approximation, but if you make the H values close enough, then it is sufficient.

It’s a highly time consuming method, but necessary

..................................
so anyway, if you cannot find a big enough ferrite core, then use say an amogreentech core, such as in the datasheet incided with the zip file....if each is not big enough, then dual two torroids together.

 

[goldsmith]

I am creating a 21 to 12 volt , current rating 10A Buck converter . After calculating The value of inductor is 2.05mH . Now which type core i should use to make the inductor ?
And what is the difference between coated & uncoated toriod core ??

Hi
Have a look into this topic , ( it's 1st page until the latest page ! ) i'll bet you'll enjoy it ! :wink:

https://www.edaboard.com/threads/229615/

Best Wishes
Goldsmith