Choke winding question

• posted

I was using a Ferroxcube calculator to work out some choke windings. These have to be multiple RM12, about 5mH and 2.5A DC, so I was looking at the maximum inductance before saturation for different cores.

A 400nH core means 4 series cores each with 57 turns giving 1.3uH each. or 3 x 250nH cores with 89 turns, 2000uH each or 10 x 1000nH cores with 22 turns, 500uH each.

The 400nH cores are probably best, 89 turns won't fit with reasonably thick wire and 10 cores is painful.

However - it seems that the /total/ number of turns is about constant. Is that a rule of thumb, just normal or just lucky?

Cheers

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Clive```
• posted

Did you play with the gap size?

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Thanks,
- Win```
• posted

The three cores modelled are the same ferrite material and the same size (RM12), they have different gaps giving them different Al values...

250nH = 901um 400nH = 511um 1000nH = 171um

Cheers

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Clive```
• posted

You can't just buy something from Coilcraft?

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John Larkin         Highland Technology, Inc
picosecond timing   precision measurement  ```
• posted

They don't have much in the way of high-inductance high-current inductors. Everybody wants small, high-frequency stuff these days. I want large high-voltage high-frequency high-power stuff. Well, at least it doesn't require too many turns.

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Thanks,
- Win```
• posted

Well, that wouldn't answer my question.

Anyway, high voltage, high temperature, constrained space mean not COTS.

Cheers

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Clive```
• posted

What frequency are you choking?

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Thanks,
- Win```
• posted

You've just run into the Hanna curves.

RL

• posted

10kHz to 100kHz.

But my question is simpler, I probably haven't explained it well. When I use a manufacturers PC software to see what's the most inductance I can get from an RM12 core with 2.5A DC going through it without it saturating, accepting that I will need multiple chokes in series to attain 5mH, it seems that the /total/ number of turns required is much the same. Is that physics or just luck?

According to the Ferroxcube software, a 400nH core with 57 turns giving

1.3mH saturates at 2.5A, so to get 5mH I'd need 4 of these, giving 228 total turns for 5.2mH or 22uH/T

250nH, 89T, 2.5A, 2mH, 3 needed, 267T for 6mH or 22uH/T

1000nH core, 22T, 2.5A, 500uH, 10 needed, 220T for 5mH or 23uH/T

All the same core material, 3 different gaps for 3 different Al figures.

It seems suspicious that these three configurations come up with the same total number of turns for the same inductance at the same saturation current. Is this something simple which is well known in the choke-winding fraternity, is it coincidence, or is it an inevitable consequence of inductor physics? It may well be an obvious thing which I simply didn't know.

Cheers

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Clive```
• posted

What is different about the cores that give them the different ratings? Is it size, materials, what? Are the winding dimensions the same? What is the same vs. what is different?

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Rick C```
• posted

.... Phil

• posted

Gap. The Ferroxcube RM12 core range offers a small series of cores with the same external dimensions, but with a progressively large gap in the centre leg.

The effective magnetic path length for all of them is 56.6mm, but the largest gap - 1.57mm - is equivalent to more than a metre of path length in ferrite and dominates the inductance of any coil wound on the core.

Whence the 160nH per root turn for a core which offers 5.6uH when ungapped.

The EPCOS data sheet isn't much different

but their application notes used to be a whole lot better.

is 6688 pages and 33.15 MB. It's more comprehensive than accessible.

The late great Tony Williams circulated a bunch of the more useful material to his friends some ten years ago in smaller chunks.

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Bill Sloman, Sydney```
• posted

Saturation occurs at the same N*I for a given core shape and size, independent of gap*.

*Not quite independent due to fringing, but in the small-gap limit, this works.

Physically, this is because Ae is the same, and you can only pack so much flux density Bsat into the core. Bsat (== Vs/mm^2) * Ae (== mm^2) == Vs (flux, integral V dt applied to the core).

The average (or effective) permeability that's best depends on a number of optimization factors, but it's in the 20-60 range. The lower A_L cores in that selection will have this property.

As you note, lower mu_eff is higher energy (because the energy is stored in the air gap, which is larger), but harder to wind (takes more copper for the same losses). (If we had superconductors, a very low mu_eff would be manageable, and we could do air-core inductors without worry.)

Tim

```--
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Contract Design ```
• posted

Thanks for the info. As to the cores, Clive says he is using the RM12 cores in 250, 400 and 1000 nH. But I don't see a 1000 nH core listed.

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Rick C```
• posted

** Huh ???

You out of your mind ??

For the same winding, increasing the gap increases sat current.

It's the reason for adding a damn gap !!!

.... Phil

• posted

I'm not using them, I'm using their design software to work out what I need. If there's no Ferroxcube 1000nH, there'll be an Epcos.

Incidentally, running the same program on a 5300nH core gives 100uH, 4 turns, 2.5A turns, 50 cores needed and 25uH/T which is still pretty close to my observation, ie I'd need 200 turns total across 50 cores for

5mH at 2.5A.

This may be obvious to someone 'skilled in the art', but it's new to me.

Cheers

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Clive```
• posted

That's 688 pages.

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Thanks,
- Win```
• posted

Oops. That's what I meant to type.

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Bill Sloman, Sydney```
• posted

Buh, why did I type N*I? I even said flux is constant later... (Thanks Phil)

Tim

```--
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Contract Design ```

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