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Class D Switching Power Amplifiers and Power D/A conversion

Shielding air core inductors (T-Amp)
Shielding air core inductors (T-Amp)
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Old 16th October 2007, 06:50 PM   #1
xyrion is offline xyrion  Croatia
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Default Shielding air core inductors (T-Amp)


I wanted to know if the inductors should be shielded individually or can I shield them all together as a group?

I got 4 air cores from Yeo, installed them on my T-Amp and then wrapped (grounded)copper braid from old coax around all 4. Would wrapping each inductor separately be an improvement over this?
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Old 17th October 2007, 08:12 AM   #2
Eva is offline Eva  Spain
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What you have done is reducing inductance by 50% to 90%... Any shorted turn style shielding placed nead an air core inductor will reduce inductance a lot...

Least leaky inductors are ferrites with internal air gap, followed by MPP/iron powder toroids.

Air cores for class D are a big no-no unless you place them in big individual shielding boxes whose walls are away from the windings.
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Old 17th October 2007, 12:48 PM   #3
xyrion is offline xyrion  Croatia
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I think I wasn't clear enough on this. Copper braid is insulated and so are the air cores, there is no electrical contact between the two. If that's what you meant with shorted turn style shielding (sorry, I've never heard the expression). If you were only referring to the distance between a shield and a inductor wire, do you think that a ~3 mm gap be good enough?

I've read people have great results with air core inductors on Tripath amps even if they are theoretically not a good match, and that shielding should take care of their biggest problem. That's why I'm using them.
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Old 19th October 2007, 04:26 AM   #4
vectorplane is offline vectorplane  United States
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Originally posted by xyrion
...shorted turn style shielding (sorry, I've never heard the expression).
If you've never heard the expression, you really haven't read enough

And 3mm is too close.

You need to better understand magnetic fields and electromagnetics.

Any metal that completely encircles a coil, is going to act as the secondary of a transformer, of which your coil is the primary.
And because the metal goes all the way around, it's a shorted one-turn secondary.

I'm not familiar with what they do in Class D amps, but the only inductors that can safely be shielded without shorting are CORED inductors, where the core is a completely closed magnetic path.
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Old 19th October 2007, 08:40 PM   #5
xyrion is offline xyrion  Croatia
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Thanks for a quick lesson, vectorplane. I have a lot to learn in this field.
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Old 19th October 2007, 09:19 PM   #6
Pano is offline Pano  United States
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Shielding air core inductors (T-Amp)
Shorted turns....

I've been working with shielding air core inductors for over a year now and have not found a grounded copper shield to cause any trouble.

Yeah, it should, but in my work with air core inductors it has not. I see little or no change in the RF fundamental post filter. But I also don't see any change in the 0.75-1MHz fundamental inducted into adjacent components, so maybe the shielding ain't that great. I "think" the harmonics are attenuated, but at present do not have a way to measure them.

Copper isn't going to be the greatest RF shield, so there is work to be done there. Mu-Metal, perhaps?

Still, the best bet is to use good inductors that don't pollute. Toroids or shielded bobbins come to mind.
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Old 20th October 2007, 06:58 AM   #7
vt4c is offline vt4c
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For the record, the Charlize 2 comes with shielded air-core inductors.
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