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Old 31st December 2011, 03:44 PM   #1
gmarsh is offline gmarsh  Canada
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Default dual/quad class D inductors?

I'm working on a 2xADAU1592 based, class D amplifier card for a Boominator system. I don't have a lot of room available on the PCB, and I'm cramming in lots of other features to boot, so I can't easily fit 8 separate inductors on the card.

Originally I chose Coilcraft JA4575-AL dual inductors, but shipping and customs will end up being ~$50 CAD to get $10 worth of parts to my door - I'd really like to avoid that. Sagami has a really nice selection of class D inductors, which I've seen used on various Tripath cards, but I can't find a distributor that sells low quantities. Searching the Mouser/Digikey/Newark catalogs haven't turned up any other suitable candidates.

Anyone have any other suggestions for manufacturers?

My backup plan is coupled output inductors (eg, Coiltronics DRQ127) but it's not really my favorite solution for EMC reasons.
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Old 31st December 2011, 09:25 PM   #2
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I make my own inductors.
I use a t106-2 core with 2 metres of 18swg enamelled copper wire.

If you need smaller cores then look up Amidon they do a large range.
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Old 1st January 2012, 04:20 AM   #3
gmarsh is offline gmarsh  Canada
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In my experience, toroid inductors change inductance depending on the current going through them. Fine for a power inductor, but that can't be good for the tuning of the LC output filter of a class D amp unless the inductor is seriously oversized.

Based on that, I feel uneasy using them on a class D amp without post-filter feedback. Mind you I really should just simulate this and find out if it actually makes a significant effect on distortion performance...
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Old 1st January 2012, 06:16 AM   #4
Eva is offline Eva  Spain
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Do your own inductors. There are some essential ideas to learn in the process. You'll never get them right if you just pay for the inductors and don't care about the details.

The linearity of an iron powder inductor depends completely on material selection and magnetic flux (which depends on turn count and core size). Material -2 can be operated far enough from saturation to exhibit good linearity without requiring a big core. Even inductors that start to exhibit some inductance change above 80Apk for high power amps can be made with T184-2. Stacking cores can help too (like making a 10uH inductor good to >30Apk with stacked T106-2)

This graph shows the saturation characteristics of different materials:
I use to feel like the small child in The Emperor's New Clothes tale

Last edited by Eva; 1st January 2012 at 06:21 AM.
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Old 1st January 2012, 04:19 PM   #5
gmarsh is offline gmarsh  Canada
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Eva: I'll look into that. Thanks a lot!
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