Air core toroidal inductor for class D amp output? - diyAudio
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Old 5th July 2012, 07:25 AM   #1
banana is offline banana  Hong Kong
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Default Air core toroidal inductor for class D amp output?

Anyone have experience with them? I mean toroidal inductors wounded on non-ferrous material like wood, plastic or simply air.

Some quick google I came across with these:
Click the image to open in full size.
http://engineering.dartmouth.edu/ind.../pesc2007b.pdf
http://thayer.dartmouth.edu/inductor.../apec2008a.pdf

Unfortunately their inductance are 100 times smaller than what we need for class D amplifier output filter. But the construction idea look promising.

Any idea?
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Old 5th July 2012, 04:33 PM   #2
Eva is offline Eva  Spain
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With an air core, even if inductor is toroidal, the number of turns required becomes quite high resulting in problems with high winding capacitance and wire resistance.

Low permeability iron powder RF materials like micrometals -2 (u=10) allow for good compromises between linearity, turn count and capacitance.

It's high permeability cores (like 26 material with u=75) what result in poor linearity, but it's not a good idea to go down to the u=1 permeability of the air either.
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Old 5th July 2012, 05:03 PM   #3
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Quote:
Originally Posted by Eva View Post
It's high permeability cores (like 26 material with u=75) what result in poor linearity,.
Really? I didn't know that. What makes that so?

thanks

gene
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Old 5th July 2012, 06:12 PM   #4
banana is offline banana  Hong Kong
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Originally Posted by Eva View Post
With an air core, even if inductor is toroidal, the number of turns required becomes quite high resulting in problems with high winding capacitance and wire resistance.
For example a 25mm height, 50mm outter diameter toroid, with 88 turns of AWG20 wire, yields 0.23 ohm. Is it very bad?

Ness Engineering Tech Data - Toroid Formulas
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Old 5th July 2012, 08:32 PM   #5
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Originally Posted by gearheadgene View Post
Really? I didn't know that. What makes that so?

thanks

gene
Besides different shape of the magnetization curves, with u=75 you will typically also end up in a design with pretty low number of turns (nice for Rdc), but you will typically end up with a design that drives the core to higher flux densities (not nice...).
With lower permeability you can find a better trade off between Rdc and flux density.
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Old 6th July 2012, 01:02 AM   #6
Eva is offline Eva  Spain
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Given the same core size and turn counts, a material producing 10 times more inductance than air is always going to exhibit better linearity and less losses than one producing 75 times more inductance than air. The extra inductance always comes at a cost. At high current (magnetizing force) the 75u material losses extra permeability and ends up being 10u too, while the 10u material just stays at 10u all the way.

See this figure:
Click the image to open in full size.

And this is the trick with iron powder toroids

For SMPS, chokes with inductance smoothly falling as current increases are useful, they prevent discontinuous mode and reduce ripple and stress at low current, but for class D they aren't useful.
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Last edited by Eva; 6th July 2012 at 01:12 AM.
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Old 6th July 2012, 10:55 PM   #7
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Hi Eva,
I agree on the choice, but do not feel comfortable with some simplifications.
To compare in a fair way you must not compare same number of turns.
From application view you will need a certain inductance.
Consequently with a material of u=10 the number of turns will be higher by factor sqrt(7.5) compared to the design with u=75.
Unfortunately the same applies for the magnetizing force.
Fortunately the material with u=10 allows not just factor sqrt(7.5) but factor 20 higher magnetizing forces without much dropping and consequently is the more fortunate choice for class D.
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Old 6th July 2012, 11:56 PM   #8
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Air cored inductors work great; I applied them in NPX (former Philips) TDA8920 class d amps.
DC resistance is to pay attention to, but capacitance is a non-item because of the very low source impedance and the "few" number of windings.
Inductance is linear as is to be expected from air cored inductors.
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Old 7th July 2012, 03:33 AM   #9
gootee is offline gootee  United States
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What value of inductance are we talking about?

Madisound or Parts-Express might have what you need. They have lots of air-core inductors for speaker crossovers, too (along with vary-large-value film caps).

But for a small-valued one, you could so-easily wind your own. Here is a good air-core inductor calculator:

Pronine Electronics Design - Multilayer Air Core Inductor Calculator

For lower resistance, just use larger-gauge magnet wire. I did a 2.8 mH crossover coil with something like 130 feet of 10-gauge magnet wire and it was around 15 milliohms or less, if I recall correctly. (Yes, it was very large and heavy, and the wire alone was about $40 per coil, but would probably be much more costly now.)
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Old 7th July 2012, 12:42 PM   #10
banana is offline banana  Hong Kong
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Originally Posted by gootee View Post
For lower resistance, just use larger-gauge magnet wire. I did a 2.8 mH crossover coil with something like 130 feet of 10-gauge magnet wire and it was around 15 milliohms or less, if I recall correctly. (Yes, it was very large and heavy, and the wire alone was about $40 per coil, but would probably be much more costly now.)
Let's say inductance value of 30uH, that is 0.03mH.

In case of toroidal structure, the inner diameter limited the size of wire to be used if we keep it single layer. 10 gauge is seriously thick! How would you managed to pull it tight??
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