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PostPosted: Thu Jul 19, 2007 2:29 pm 
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http://www.irf.com/technical-info/refde ... udamp4.pdf


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PostPosted: Thu Jul 19, 2007 2:35 pm 
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the best inductor, according to IR is Panasonic ETQA21ZA (ETQA17B) series


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PostPosted: Thu Jul 19, 2007 7:05 pm 
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dimitri wrote:
the best inductor, according to IR is Panasonic ETQA21ZA (ETQA17B) series

Maybe it has to do with inter winding capacitance...

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PostPosted: Fri Jul 20, 2007 1:20 am 
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since the output filter is not compensated for, using "soft" magnetic devices especially helps
having a smooth saturation (stay away from it anyway) reduces distortion

of course you get less inductance per size, when you go for soft magnetics. but you guys don't believe in free lunches anymore, do you? ;)

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PostPosted: Fri Jul 20, 2007 10:10 am 
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Hi,

That's a valide finding Dimitri, however I think that placing their particular selection in the spotlight as being "the best" takes their point out of context. The only important take-away from that is simply that the inductor matters.

Sadly, they didn't dare scratch the surface as to how or why, but it's a fairly involved topic worthy of a few application notes by itself.

The component selected I find interesting only because suitable off the shelf coils are rare, and this would OK at least for certain values and at lower power.

The best text I've seen on the topic to date follows, by JohnW:

Quote:
In fact with the larger Amidon Type 2 red cores I achieve about 0.0015% @ 120W 8 Ohms, Open Loop (384 KHz). The primary distortion components being 3rd harmonic & odd order components.

The primary cause of the odd order distortions is the rounding effect of the B-H characteristics. Ideal magnetic materials have a Square-Loop characteristic with very high permeability and insignificant stored energy until finally driven into saturation. This is called a “Sharp saturation” characteristic.

A rounded or “Soft saturation” characteristic exhibits a gradual reduction of incremental permeability until finally the core is completely saturated. Magnetic “Hard spots” and inside corners will also cause rounding of the B-H characteristics. In some inductor filter applications, this rounding effect is akin to a “Swinging Choke”, and is actually desirable – but not for Class D output filters!

Rounding effects in metal-alloy cores are normally quite low. However, in composite metal powered cores, non-magnetic “Gaps” exist between the discrete magnetic particles. It's these distributed non-magnetic regions that cause the significant rounding of the B-H characteristics – these regions also are responsible for storing energy within the core. Similar non-magnetic areas occur among the sintered particles in ferrite cores.

1. The distributed reluctance of these tiny “Gaps” causes the flux and the flue change to be discrete across the entire core, rather then as a discrete flux change boundary moving from inside to outside for a ungapped idealized metal alloy core.

2. At lower flux densities, the flux tends to concentrate via the easiest paths (i.e. paths with the lowest reluctance), where the magnetic particles are in close proximity too each other. As the flux density increases, these “easier routes” are the first to saturate. These portions of magnetic particles saturate first becoming non-magnetic, resulting in their flux paths becoming “less easy”. Incremental flux increase shifts to adjacent paths where the magnetic material has yet to saturate, but where the gap is somewhat wider. This process continues, effectively widening the incremental distributed gap as the flux density increases. Thus, the incremental permeability (and therefore inductance) is progressively reduced, and is observed by the rounding of the B-H characteristics.

The solutions to reducing the effect are: -

1. Manufacture the cores with a material that exhibits Square-loop characteristics

2. Reduce the irregular non-linear gap area by manufacturing the cores with highly polish magnetic material thereby reducing their uneven surface area and - higher density material (i.e. reduced particle size).

3. Adding a discrete air gap, the rounding effect of the B-H characteristics disappears –dominated by the linear high reluctance of the Air gap, thus linearizing the inductance characteristics. If an Air gap is used, then the copper windings must be kept well away from this gap to reduce the “Fringe” effects.

Cheers,

John


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Quote:
For the record, I would like to thank Jaka Racman for his extremely useful leads while I researched the cause of the odd order distortion introduced by magnetic cores.

I’ve not built a high power Class D amplifier above 150W 8 Ohms or so, so I cannot comment on the performance of the Amidon cores above this power level.

I use the T106 core, and can just about wind 18uH on a single layer using 1.5mm Dia. Wire (for 18uH I counted about 36 turns 1.5mm, on the T106 type 2 core).

It’s not just a simple matter of material type, particle size & shape is critical, finer particles (denser material) result in lower distortion – which is a real concern for QC control. To achieve finer / smoother particles the magnetic powered is “rumbled” in a large drum for many days. The longer the rumbling process, the finer / smoother the particles.

I understand that Micro-Metals also have a similar core to the Amidon type 2 – however I have no experience of there variant.

Bigger cores and thicker wire can only be better - so I don't see higher powers being a problem if you scale up the core size.

A interesting effect I’ve observed with the Type 2 core material is that the distortion level remains the same for any power from about 1W to 120W, below 1W there’s a sharp step drop in distortion to the level of the HBridge output.

John


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Quote:
I have not measured a step change in inductance at lower power levels with the Amidon cores, so I’m at a loss to explain the distortion “Step Effect”, and can only speculate that it’s some “weird” hysteresis effect – but I would be grateful for a better explanation!

Air-cores not surprisingly, do not exhibit this effect.

Attached is some info I grabbed from an ST app note AN1013, which mentions a “Step” in the THD vs. Pout plots. However the app note suggests that this mechanism is akin to Class AB bias in the output stage, which is a valid explanation only when THD of the HBridge output is “High” - however my inductor Test Jig output stage has a typical THD at the HBridge node Pre inductor of 0.000025% 100W (highest harmonic below -120dB).

I’ve found that even with high order correction Post inductor, the absolute quality of the inductor is still very audible. The Amidon type 2 have the effect of “sharpening” detail very slightly, however with most normal Class D circuits this effect will be buried under many other problems.

In listening test, the Amidon cores performed the best out of Magnetic cores I’ve tested.

Sorry to have digressed this thread,

John


That at least scratches the surface. No free lunch means doing your homework because there are no easy answers to such a complex topic.

Cheers,
Chris


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PostPosted: Fri Jul 20, 2007 11:48 am 
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http://www.ferroxcube.com/appl/info/cla ... lifier.pdf :good:


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PostPosted: Fri Jul 20, 2007 1:31 pm 
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ok, thanks


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PostPosted: Fri Jul 20, 2007 2:20 pm 
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Not a bad link at all Ivan, more marketing than substance though, but gapped ferrite is good. Then, there's X types of ferrite .... :doh:

Knock yourselves out:
http://www.mag-inc.com/library.asp

Dimitri, are you thinking of building that amp?? :cool:


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PostPosted: Fri Jul 20, 2007 4:20 pm 
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>>Dimitri, are you thinking of building that amp??

no, I don't like it


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PostPosted: Fri Jul 20, 2007 4:22 pm 
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dimitri wrote:
>>Dimitri, are you thinking of building that amp??

no, I don't like it


Neither do I.


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PostPosted: Sat Aug 11, 2007 3:11 pm 
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small-signal measurements of some inductors: 2.75V rms @ 8 Ohm
inductor (1) 2nd -81dB 3rd -97dB


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PostPosted: Sat Aug 11, 2007 3:13 pm 
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inductor (2) 2nd -81dB 3rd -88dB


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PostPosted: Sat Aug 11, 2007 3:17 pm 
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inductor (3) 2nd -80dB 3rd -91dB 4th -95dB 6th -99dB


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