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nigelwright7557 12th December 2010 09:46 PM

The common class d output filter inductance overheating problem.
I have read in a few threads about overheating of the inductor in the output filter.

I have found using an inductor with a much higher current rating helps here.

I also noticed with a semi-discrete class d amplifier that the class d frequency also made a difference. I guess this is because a series LC is a short circuit at the resonant frequency and the further you are away from the resonant frequency the less short circuit current there.

At 100KHz I found my inductor got to 200 degrees C.
At 150KHz it got to 170 degrees C.
The resonant frequency of the output filter was 40KHz.
I can clearly see a drop in heat at the higher frequencies.

I need a faster op amp in my triangle wave generator to get the frequency higher for further tests.

newvirus2008 13th December 2010 05:09 AM

It is true but you seem to include only the "electric heat" and not the "magnetic heat".

Total heat = core loss (caused by ac) + copper loss (caused by ac&dc).

Core loss reduces with frequency (which permits smaller core in SMPS than tfr) and increases with ampere turns (resulting in larger cores for higher power applns).
Copper loss increases with frequency due to skin effect (increase in AC resistance with freq) and reduces with thickness of wire.

Hence, a good inductor is one in which both losses meet.
Also, higher current choke => larger core & heavier wire => lesser heat.

Aucosticraft 13th December 2010 05:43 AM

Good, I was looking for such a dedicated thread that would discuss LC filters of ClassD amp.
Can we discuss this spiting in smaller topics ? Like, 1) when Ideal Inductor and Capacitor is used with Ideal Resistive load. 2) Understanding and discussing copper losses. 3) Then the magnetics. Together this thread can be A to Z solutions and understanding on the subject.

We can use Spice modeling for each so easier to visualize.

Th3 uN1Qu3 13th December 2010 06:05 PM

I don't think you need Spice for this. The calculator that Micrometals offers on their site will tell you everything you need to know about your choke.

darkfenriz 13th December 2010 09:07 PM

The charts of power loss in mW/cm^3 versus delta B and frequency tell the truth about core losses.

bonalux 13th December 2010 09:27 PM

Recently i've found that the same core (MPP 60u toroidal) produced a lot of heat with 1,2mm wire wound lightly loose (50C) than the same core with 1mm wire wound very tight on the core (no space from wire to core) and epoxy glue on the outer surface (i've done that with the hope to limit vibration of the wire), the latter works nicely at 38-39C in the same amplifier. Obviously this needs further investigation but if someone wanna try this i would be glad to recive a feedback.


ontoaba 14th December 2010 02:22 AM

It is core heating, some spot of the core side has over density.
I have a way but many people not accepting this. Normally over density happening at near the coils and less at the center of inductor. To make it distributed well, use high permeability core at the center and lower at the edge, and make more flux space area (wider inductor). Avoids to use toroids.

Here is my noheatatall inductor picture.

ontoaba 14th December 2010 02:28 AM


Originally Posted by nigelwright7557 (Post 2396177)

I guess this is because a series LC is a short circuit at the resonant frequency and the further you are away from the resonant frequency the less short circuit current there.

That also true, set your filter cut off as low as you can. (40KHz or less)

Aucosticraft 28th December 2010 03:29 AM

Some 10yrs ago my employer had asked me to design SMPS and I had no knowledge of magnetics domain. Also the product was to go in huge quantity when commercialized. So had a big pressure. during this time I had some readouts on net and some discussion with the ferrite core dealers. but after that I never even spell ferrite. my sharing is based on that knowledge gathering.

Firstly heating of inductor does not necessarily mean its a problem. Heating could be core losses the coil is allowed to. First step would be knowing the frequency of operation, based on this core material is selected. Then you need to know the operating voltage. waveform, like Sine wave or square wave.. etc. current flowing through the inductor. based on this electrical parameters the capacity / size of core would be determined. We have to remember that when core saturates rest of the energy is converted to heat.
Although the above are only a basic lines of design understanding. we can still design a good efficient inductor using ready tools available on net. Here is one of the links.

Aucosticraft 28th December 2010 03:56 AM

another good guidelines

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