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Old 2nd August 2010, 11:31 PM   #1
klausix is offline klausix  Germany
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Default Class-D Amp LC-Filter: Coil heating

Hi All!
I am currently designing a Class D Amp based on TIs TAS5504 and TAS5152. I use the TAS5152 in 2x BTL configuration with 2x 10uH and 1x 0.47uF for each BTL. PVdd is 36V

So far it's working, but most of the coils I tested for the output filter heat up for 20-30C above Tamb in idle (!) mode.
And those Coils are listed as "Coils for Class-D Amps". So I guess the resonant frequency of the core shouldnt be a question.
But to check it: (1) what should be the resonance frequency of the core when switching is done at 384kHz?

Interesting is that the coils do not heat up that strong when I place them in the TI eval-Board. So (2) my layout could be a reason? (I have a very compact board)
Or could there be some jitter on the PWM caused by a decoupling failure? (3).
I measure betweeen 0.5-1.0V Vss ripple at 384kHz. This seems to be a normal value to me (which was also confirmed by a Spice simulation).

Any help, hints or notes would be highly appreciated!

Thx and best regards,
Klaus!


P.S.: I foud the "heating in inductor thread", but this ended in nowhere
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Old 3rd August 2010, 07:11 AM   #2
Hitec is offline Hitec  Finland
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The core losses take place also at idle because of the 50% PWM signal. I guess the temperature rise you are experiencing is normal (just for empirical reasons). I had a problem with a coil that heated up and after a while a really annoying noise prevented listening to the amplifier. I repaired it by installing a small computer fan

But anyway one should check the core losses with given frequency & voltage. If I remember right, those are the parameters that affect the core losses. But check them somewhere!

I have also had that kind of ripple in my amplifiers.
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Old 3rd August 2010, 10:39 AM   #3
Pafi is offline Pafi  Hungary
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Quote:
(1) what should be the resonance frequency of the core when switching is done at 384kHz?
I don't know what kind of resonance do you refer to, but anyhow, this is irrelevant.

Quote:
(2) my layout could be a reason?
Maybe. If there is no air-flow allowed, then it will be warmer obviously. And if a coil has a strong stray magnetic field, then it can heat up the metal parts next to it.

Quote:
Or could there be some jitter on the PWM caused by a decoupling failure? (3)
Basically it can't cause more heat.
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Old 3rd August 2010, 11:58 AM   #4
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When I first stated designing class-D amps, I tried various types of output filter choke core. Most of the dust-iron cores sold for winding output inductors in buck regulators have far too high hysteresis losses when used at the 300kHz or so in a class-d amp. For the last couple of designs I have done for the company I work for, I have used gapped RM cores in Ferroxcube 3F3 material. (Epcos N87 ferrite also works well). The white ferrite toroids with the precision-cut gap from Ferroxcube also work well, but are difficult to buy in small quantities.
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Old 3rd August 2010, 12:07 PM   #5
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well, it depend on your output power,20-30is normal with 20-40% power ,and the material of your coil is critical to the D amp ,i recommend you the T106-2 coil is perfect for the performance
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Old 3rd August 2010, 12:44 PM   #6
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Quote:
Originally Posted by Ouroboros View Post
The white ferrite toroids with the precision-cut gap from Ferroxcube also work well, but are difficult to buy in small quantities.
Tell me about it; I've tried many times.

Group buy? Who wants to share 1000 pieces?
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Old 3rd August 2010, 01:37 PM   #7
authlxl is offline authlxl  China
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Core type error,because you can not select it on the base of color,the core material is very important.

I have developed this amp years ago ,use tas5504 +tas5261.works well.

If friends like ,pls connect us for the right core .
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Old 3rd August 2010, 01:48 PM   #8
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The white "Polo Mint" gapped toroids from Feroxcube are in 3C20 material. the characteristics of this ferrite grade are at:
http://www.ferroxcube.com/prod/assets/3c20.pdf
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Old 5th August 2010, 08:40 PM   #9
klausix is offline klausix  Germany
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[QUOTE=Pafi;2261526]I don't know what kind of resonance do you refer to, but anyhow, this is irrelevant.
Well, I just know from speaker-cabinets that you should use them only/ideally at frequencies above double then the resonant frequency. So i expected that you should use coils in a similar way. With voltage/current frequencies below the resonant f.

But if it doesnt matter, it's fine!
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Old 5th August 2010, 08:46 PM   #10
klausix is offline klausix  Germany
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Quote:
Originally Posted by Hitec View Post
The core losses take place also at idle because of the 50% PWM signal. I guess the temperature rise you are experiencing is normal (just for empirical reasons). I had a problem with a coil that heated up and after a while a really annoying noise prevented listening to the amplifier. I repaired it by installing a small computer fan

But anyway one should check the core losses with given frequency & voltage. If I remember right, those are the parameters that affect the core losses. But check them somewhere!

I have also had that kind of ripple in my amplifiers.
I see - I have a basic misunderstanding here: I though: no current through the coil, so no heat! But the heat is generated by the toggling "re-magnetisation" through the switching voltage within the core?!
(or am i again wrong...?)
(but the energy creating the heat in the core must come from somewhere. So there must be some current flow. isnt it? hmmm.....)
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