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Old 20th April 2009, 06:11 AM   #1
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Default Hybrid Output Inductor

I might use an apparently novel output inductor type instead of the type 2 red iron powder toroid that is often preferable.

I am considering winding a straight, single layer coil with a diameter equal in size to a small toroid that I have plenty of to spare. Then I plan to use a dremel cutting wheel to slice the toroid in half. I would then bend the coil in half and cap each double end with a half toroid to direct the emitted magnetic field between adjacent coil ends. I would hope this can provide the high saturation level of air core without emitting a big external magnetic field.

Since this approach might have a high Q, it might not be ideal for a class D amplifier. Yet I am still interested in it, maybe at least as an option for SMPS somehow.
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Old 20th April 2009, 09:42 AM   #2
Pafi is offline Pafi  Hungary
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Default Re: Hybrid Output Inductor

Do you mean this way? (attached) This has a very low Al value, you needed a very large number of turns, and this leeds to high loss, and yet the stray field wont be low really. You'd better cut the toroids into quarters (or smaller parts), join them back together with 0.3...0.8 mm gaps, and wind it normally. It's good to leave the areas over the gaps free of wires, but not neccessary.

High Q is not a problem for class D audio output filter, but a requirement.
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Old 20th April 2009, 10:18 AM   #3
Pafi is offline Pafi  Hungary
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Default Re: Re: Hybrid Output Inductor

This is a really high Q inductor style, with a moderate stray field, and quite easy to do. Of course one should use more wire for optimum result, and maybe little wider gaps, and a good ferrite material (N87, etc...).
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Old 20th April 2009, 12:48 PM   #4
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Default Re: Re: Re: Hybrid Output Inductor

Quote:
Originally posted by Pafi
Do you mean this way? (attached) This has a very low Al value, you needed a very large number of turns, and this leeds to high loss, and yet the stray field wont be low really. You'd better cut the toroids into quarters (or smaller parts), join them back together with 0.3...0.8 mm gaps, and wind it normally. It's good to leave the areas over the gaps free of wires, but not neccessary.

High Q is not a problem for class D audio output filter, but a requirement.
In a gapped toroid, the magnetic field bulges externally at each gap. I've noticed that in images of a straight coil, the field remains inside the coil except at the ends.

I wonder why iron powder is preferred to gapped ferrite besides magnetic field containment. Could that be the only reason? I think some damping property is given by the core material.
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Old 20th April 2009, 07:34 PM   #5
Pafi is offline Pafi  Hungary
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Electrone!

Quote:
In a gapped toroid, the magnetic field bulges externally at each gap.
Yes, but the smaller the gap, the lower the bulge. This because I use 4 smaller gaps instead of 1 big. Additionally: magnetic forces of the opposite gaps cancels (more-or-less) each other when you are relatively far from the core. This has much less leakage then an air-core has, and also it has significantly lower loss (at moderate freq).

Quote:
I've noticed that in images of a straight coil, the field remains inside the coil except at the ends.
OK, but with this arrangement you get a very low Al also. Why is it any better then an air-core toroid (wich is also unusable at this freq because of too low Al)?

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I wonder why iron powder is preferred to gapped ferrite besides magnetic field containment.
Because no manufacturer makes ferrite toroid slices.
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Old 21st April 2009, 01:06 AM   #6
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Quote:
Originally posted by Pafi
Electrone!



Yes, but the smaller the gap, the lower the bulge. This because I use 4 smaller gaps instead of 1 big. Additionally: magnetic forces of the opposite gaps cancels (more-or-less) each other when you are relatively far from the core. This has much less leakage then an air-core has, and also it has significantly lower loss (at moderate freq).


I've made air core class D output inductors as in the diagram, but without the ferrite ends, and with the adjacent coils touching, for my experimental projects. I've thought a similar thing about those stray fields canceling at a distance.

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OK, but with this arrangement you get a very low Al also. Why is it any better then an air-core toroid (wich is also unusable at this freq because of too low Al)?
I've tried air core toroids, but they are harder to wind as with other ones. I think i am going to try an output choke that is just 10uH. It is easy to wind on a straight former like a stick pen.



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Because no manufacturer makes ferrite toroid slices.
Ahh!
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Old 21st April 2009, 01:52 AM   #7
tinitus is offline tinitus  Europe
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Default Re: Re: Re: Re: Hybrid Output Inductor

Quote:
Originally posted by Electrone


I wonder why iron powder is preferred to gapped ferrite besides magnetic field containment. Could that be the only reason? I think some damping property is given by the core material.

The iron powder crossover toroids from danish DCA and maybe also Jantzen are supposed to be of a special mix that holds air and works like airgap
But those are usually very big

Im not sure whether other makes work the same way, but I dont think so
The smaller common ones in question here I reckon are the same used fore small noise chokes fore mains filter
I dont think they are special

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Old 21st April 2009, 10:43 AM   #8
Pafi is offline Pafi  Hungary
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Quote:
I've made air core class D output inductors as in the diagram, but without the ferrite ends, and with the adjacent coils touching, for my experimental projects.
Had it high Q? Had it low DCR? Was it small?

Quote:
I've thought a similar thing about those stray fields canceling at a distance.
And you were right, but your initial field strength were huge, and cancellation is not perfect (generally). A small gap has much smaller leakage.

Quote:
It is easy to wind on a straight former like a stick pen.
OK, but what will hold it during usage?

tinitus!

Quote:
The iron powder crossover toroids from danish DCA and maybe also Jantzen are supposed to be of a special mix that holds air and works like airgap
Every iron powder core contains "distributed gaps", but they have one ore more of the following drawback:

- expensive (or very expensive)
- high loss
- too small Al
- reduced effective area (at 0.3...0.5 Tesla the permeability starts to fall)
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Old 21st April 2009, 11:01 AM   #9
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Default Re: Re: Re: Re: Re: Hybrid Output Inductor

Quote:
Originally posted by tinitus



The iron powder crossover toroids from danish DCA and maybe also Jantzen are supposed to be of a special mix that holds air and works like airgap
But those are usually very big

Im not sure whether other makes work the same way, but I dont think so
The smaller common ones in question here I reckon are the same used fore small noise chokes fore mains filter
I dont think they are special

Indeed, there are a lot of different compositions of iron core types, but when I tested several different ones, only the red type 2 from Amidon seemed to work acceptably. I just tried the offerings of the one company.
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Old 21st April 2009, 11:14 AM   #10
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Quote:
Originally posted by Pafi
[B]

Had it high Q? Had it low DCR? Was it small?
Air cores generally have high Q, I'd say. It was composed of about 90 inches of 20 gauge wire. It wasn't real small though, being composed of a coil 2cm in diameter and 8cm long that was then folded back at half its length.



Quote:
And you were right, but your initial field strength were huge, and cancellation is not perfect (generally). A small gap has much smaller leakage.
I'm glad that we get some things right to lessen the misery in life.

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OK, but what will hold it during usage?
Epoxy might work.
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