Design of output inductor for class D amplifier

[zilog]

I want to have a discussion regarding the design of output filter inductors for class D power amplifiers. I already know the basics - how to design inductors with regard to cross sectional area, Bmax and Imax, but I want to dig deeper - what materials are the best to use, and why?

I want to leave the powder iron core (amidon toroidal -2 material) I am using now and instead use the more space efficient ferrite cores available (RM-cores). I have understood that the B-H curve can be vastly linearized by introducing a large air gap, but I do now know what ferrite parameters that are important to gain an output inductor that introduces as little distortion as possible (is as low permability as possible after gapping desired? Does 1/mu correspond to linearity?). Is the area of the hysteresis loop important with regard to distortion, or does it only indicate power loss? Is the permeability versus frequency relationship important? I guess it is, and therefore I suspect 3F3 material to be good, correct?

Finally, can I hope to outperform an amidon T106-2 inductor using say an RM-10 core with ferrite material?

Edit: what is a reasonable limit on Bmax for hifi-use? Is there any formula to calculate the distortion introduced from a certain inductor?

[SpittinLLama]

Could be an interesting thread. Unfortunately I don't have anything to contribute but will be following it to learn. I hope others can post some good info. Right now it is black magic to me (which just means I don't understand enough about it all).

-SL

[BWRX]

Originally posted by zilog
I want to have a discussion regarding the design of output filter inductors for class D power amplifiers. I already know the basics - how to design inductors with regard to cross sectional area, Bmax and Imax, but I want to dig deeper - what materials are the best to use, and why?

I have understood that the B-H curve can be vastly linearized by introducing a large air gap, but I do now know what ferrite parameters that are important to gain an output inductor that introduces as little distortion as possible

This is a great topic that I know some members here are capable of contributing to. There is also some good info out there that just takes some time to uncover.

Here's a good read to start out with: http://focus.ti.com/lit/ml/slup124/s...0saturation%22

As far as inductors with as little distortion as possible, you need to find a good compromise between the inductor parameters that will be largely dictated by the switching frequency and power output of the amplifier the inductor will be used in.

Never allow a core to saturate and you will probably ok in terms of linearity. As you know, an air core inductor will not saturate, but the field is not nearly as well contained as it is with other core materials.

[Lars Clausen]

This can be a very interesting thread! I can think of at least EVA who is an expert on this, and hopefully will contribute.

The choke is always a crucial contributor of performance in a ClassD amplifier, and there are so many parameters in the design of the choke, that it can almost be considered a science of it's own right. Therefore, i think many commercial producers will protect their choke design as part of their IP.

[soongsc]

I remember another thread talking about choke design for Tripath based amps.

UcD180ad Vs 41Hz Audio AMP5 (Tripath TA2022)

Started here
UcD180ad Vs 41Hz Audio AMP5 (Tripath TA2022)

[Pano]

I'll repeat - this could be a very interesting thread!

I have worked with about 10 different inductors on the little T-Amps. Small bobbin types, shielded bobbins, aircore both home made and factory, and toroids of various types.

They do sound different, sometimes a lot, sometimes a little. Mostly I find that if the core is big enough and doesn't saturate, the inductors sound much the same.
But there are other concerns.

In the RF domain they are certainly different, different core materials attenuate the RF more or less. I have some measurements comparing the RF output of two different inductors. Will try to dig them up and graph them - then post here. Choosing a core material that is designed to work at and above the amp's switching frequency is key to reducing the RF harmonics. High permeability allows few turns, lower DCR, but will saturate earlier. Aircore inductors radiate a lot of RF. The radiation needs to be addressed or it will find its way back into the audio circuits. Sounds bad.

I have been trying for months to get some good IM distortion measurements done on various inductor types. That would tell us a lot.

A good start to a good thread. I hope that this discussion can remain both technical and subjective. We are most interested in the subjective change to the music, but want to know the technical how and why the changes are happening.

Thanks to Zilog for starting this.

[zilog]

It would also be nice to adress the question of getting hold of RM ferrite cores for DIYers, any good source for both magnetics and semiconductor stuff? When I have designed out all my issues, I would like a place to order the needed componentes, preferrably in Sweden or at least in Europe.

[phase_accurate]

RM Cores are not that difficult to get at all, depending on what material you'd like ti use.

Regards

Charles

[phase_accurate]

Is the permeability versus frequency relationship important? I guess it is, and therefore I suspect 3F3 material to be good, correct?

N87 by EPCOS seems to be quite similar to 3F3:

http://www.epcos.com/web/generator/W...F_SIFERRIT.pdf

It can also be had as gapped halves of EFD cores:

http://www.epcos.com/inf/80/db/fer_01/05140515.pdf

If you use a core that is already gapped from the beginning you will end up with less stray-field outside the coil compared to the insertion of insulation material.
You will still have to take care of the strayfield inside however. Take some measures to avoid this area with your windings.

Regards

Chasrles

[Ouroboros]

In the Epcos range the ferrites that are of interest (in terms of increasing performance are:

N27
N67 (similar to Ferroxcube 3F3)
N87

You can get EFD and ETD cores in all these materials. These cores have the gap in the centre leg, so you don't get much of a stray external field.

N27 and N67 are easier to obtain than N87, which is a more recent material.

Thinking of output inductors: has anyone cut a slot in a ferrite toroid with a diamond faced tile-cutting tool? I think that a tile-cutting disc is about the only thing that would produce a neat cut in ferrite. The problem is that the slot would be a bit wider than required.