Air Core inductor: Hole Size ! (waveforms shown)

[-_nando-_]

Hi Everyone !

Well, I'm evaluating an amp with TDA8920BJ, it runs at 327Khz, and sounds surprisingly good.

I built ALL the output inductors, and all inductors that had a small hole, heated a LOT.

First I built one with 18AWG wire, with a resonable good size hole, and it ran pretty hot. Then I thought that it was the wire loss, and I built another with 14AWG wire, with a smaller hole. It heated MORE

Then I built a BIG SIZE HOLE inductor using 16AWG wire, and it runs TOTALLY COLD !

So I built another, with a medium size hole, using the same wire (16), and it ran WARM...

WHAT makes the inductor run HOT when the hole is small ?

Ok, big hole air core inductor works FINE, and of course it sounds veeeery good... But it's damn big, and might radiate a NASTY EMI...

I soldered one LED on one of the hot inductors that I don't use anymore, and if I put it close to the hole of the big inductor, the led lights up... And yes, I BURNED the led !

The big hole inductor have a perfect waveform at the output, and the smaller hole inductors (those that heats) shows a deffective waveform.

I ordered this inductor from Farnell:

http://uk.farnell.com/jsp/search/pro...sp?sku=1077056

My GOOD (incredible GOOD !) working inductor is just FUNNY It puts out 1.16v of residual PWM, sounds great, and shows a perfect waveform !

Let's see what happens with the ordered inductor...

But, my DOUBT is... WHY air core inductors with small hole runs HOT

Here is my TDA8920BJ under testing:

At the left, is the small inductor that runs warm, and at the right there's the big inductor that runs totally cold.

[-_nando-_]

BIG hole inductor waveform

[-_nando-_]

Small Hole inductor waveform:

[-_nando-_]

NASTY EMI...

[poobah]

All other things being equal... like AC resistance etc... The smaller inductors run warmer because they have less surface area to conduct away heat... to the surrounding air.

[-_nando-_]

No, there's another thing happening. Look at the waveform !

[poobah]

Hmmmmmmmmmmmm???

Maybe you are getting some magnetic coupling into circuit?

Air core inductors throw flux EVERYWHERE.

Try moving the small away form the circuit or change the angle?

[-_nando-_]

Doest it decrease the efficience?

I'm ordering some JW Miller / Bourns inductor from digikey now.

[Pafi]

nando!

The smaller inductor have a higher flux density (constant magnetic energy in smaller place -> higher flux density). A bigger part of the flux penetrates into the wire and causes eddy current, than in the case of bigger choke.

Use litze wire! (...and RM style ferrite core with big air-gap.)

The defect in the waveform caused by stray capacitance.

[lumanauw]

Hi, Nando,

Could you wind without bobbin, to make it true aircore?