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Old 26th October 2009, 02:29 AM   #1
rickco is offline rickco  France
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Default Class D output low pass filter EMI suppressor

Hi All,

I am designing an 30Khz low pass filter for the output of a 160W class D amplifier (STA328) for a telecom module where EMI are important to check.

Here is the schematic, recommanded in STA328 datasheet, on which I have added ElectroMagnetic Issue (EMI) suppressor (according to friend opinion) :

Click the image to open in full size.

L5 and L6 are Coilcraft SER2918H-153KL
Coilcraft SER2900 Surface Mount Power Inductors
http://www.coilcraft.com/pdfs/ser2900.pdf

L10 and L11 are EMI Suppressor Wurth Electronik 742792906
http://www.we-online.com/katalog/med.../742792906.pdf


1- According to the characteristic of L5 and L6, do you think L10 and L11 are usefull ?
In fact, in this document, we can see that L5 and L6 are 2,60mOhm ESR on low frequency and 2kOhm at 10Mhz, whereas L10 and L11 are 10kOhm at 20mHz.
Do you think L5 and L6 play the role of EMI suppressor PLUS 2nd order audio filter (with C37), and that L10/11 are useless? Or do you think I should keep L10 and L11 if I want to minimize efficiently EMI.


2- STA328 can output 6,5 A per channel on 36V. It is referenced as a 160W audio amplifier with 4Ohm output. How can I calculate the max power dissipation that goes in R31 and R32 (RC snubber network) and in R33 and R34 ?

Thank you for your help
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Old 26th October 2009, 10:00 AM   #2
Eva is offline Eva  Spain
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If the PCB layout is done properly and the LC resonance on the power stage (due to parasitic L of chip package and PCB, and MOSFET d-s C) are damped the amount of EMI on the output is going to be very low. I usually achieve an output waveform which just contains a tiny glitch on zero crossing and no ringing. The glitch may be further reduced with ferrite beads and 1n caps to gnd.

In a bad layout these resonances usually appear somewhere between 20 and 100Mhz.

In which frequency range EMI is required to be low?

Remember that the most harmful EMI is common-mode and usually comes from having input, supply and output wiring on different corners of the PCB and referenced to grounds that bounce (show some AC potential) on every swiching event.
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Old 26th October 2009, 10:21 AM   #3
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Hmmm... this seems to be a professional work related project?!
If you get paid... (is there in diy.audio.com such as a free lunch?)
Just hit the private message and we will make consultation contract to get your device through EN standards.
If it's not pro, many will help you through th swamp.
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Old 28th October 2009, 03:14 PM   #4
rickco is offline rickco  France
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This project will be used as support for research in animals communication.
I dont know if someone will win money one day, but I think the answers to this question can interest me AND a lot of this forum readers.

EMI should not disturb the telecom part of the projet using 433Mhz or 915Mhz or 2,4Ghz, so EMI must be low in all ISM band.

Thank you Eva for your advices but in all case, do you think L10 and L11 are useless according to L5 and L6, that have in the same way, an ESR (equivalent serial resistance) that grow exponentially above 5Mhz ?

Last edited by rickco; 28th October 2009 at 03:20 PM.
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Old 28th October 2009, 06:15 PM   #5
Eva is offline Eva  Spain
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5.6uH and 1nF resonate at 2Mhz ( Fr=1/(2*pi*sqr(L*C)) ) This is not good for post-filter feedback or for EMI around 2Mhz. On the other hand ferrite beads are lossy, they appear as series resistance too above 1Mhz, not just inductance, so they don't resonate with low value filter capacitors. On the other hand, a class D amplifier is unlikely to produce anything above 100Mhz so no problem with 433Mhz stuff and above.
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