How are Class D output filters designed

[enzowho 10+ year member]

How do you design a output filter for a class D audio amplifier? It seems that the variation in the load inductance (say roughly .5 mH - 8 mH) would cause major problems for the design. Do you isolate the output filer from the load somehow or perhaps make the output inductance >> maximum load inductance?

Perhaps I am looking at this the wrong way. The LC combination is used to create a low pass filter and remove your higher order switching harmonics. Once these are removed, you are left with your fundamental harmonic and a smoothed waveform. If the load's inductor value changes very drastically this will shift the corner freq and possibly allow unwanted harmonics to be seen at the load. Generally speaking, is there something I missed?

Anyways, thanks in advance for any help you can provide.

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[yacob.naif 10+ year member]

I don't think too many people take the conditions that a varying inductance is going to create with regards to the output filters into consideration when buying an amp, installing an amp, or quite frankly, even listening to an amp.

I know, ignorance is bliss, meh.

Could be time for a trip to the library?

 

[Raven 10+ year member]

I would like to think that such things aren't even considered when making an amplifier. Rather, the amp is made to create it's smoothest response over a non-reactive load like a resistor. It's the woofer that handles everything else.

 

[enzowho 10+ year member]

I think I finally got it. So the speaker can be modeled as an inductor and resistor in series, where the inductor has an impedance of jwL (as freq goes up so does impedance). This means that the high frequency harmonics will not go through the speaker, but instead through the parallel cap (which as a low impedance at these high freq). Therefore, the output filter can be designed for any speaker inductance without adversely affecting the corner freq (obviously, this does not pertain to resistance of the speaker).

If I am wrong please point it out, but this seems to make sense to me.

 

[Raven 10+ year member]

Are you talking about inductor-less passive low pass crossovers?

 

[yacob.naif 10+ year member]

I wrote it out below, this should explain it to you.

Both the light and the dark. The then and the now. The always will be and the never was.