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Old 2nd January 2008, 11:27 PM   #21
Dave Z is offline Dave Z  United States
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soongsc

I did a post on higher freq switching (post #531) that you may want to read (sorry not sure how to enter link but search my name you'll find it). As far as residual voltages though, I'm not sure what you mean. Residual where?

Dave
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Old 2nd January 2008, 11:32 PM   #22
Ipanema is offline Ipanema  Malaysia
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Guys,

If the filterless class D amp offered by TI or Maxim has such a bad EMI problem. How can they pass the CE or any radiation regulation in order to be sold to consumer?

Regards.
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Old 2nd January 2008, 11:59 PM   #23
soongsc is offline soongsc  Taiwan
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Quote:
Originally posted by Dave Z
soongsc

I did a post on higher freq switching (post #531) that you may want to read (sorry not sure how to enter link but search my name you'll find it). As far as residual voltages though, I'm not sure what you mean. Residual where?

Dave
At the output.
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Old 3rd January 2008, 12:00 AM   #24
Dave Z is offline Dave Z  United States
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It depends on the application. If your speaker wires are very short then there is less emissions. I have used TI and passed FCC and European emissions before. They usually have a ferrite bead for the really high freq stuff. PCB layout is VERY important for EMI. Power level is also important. The lower the power then the less amplitude of the switching waveform which means less emissions. The PWM output stage supply voltage sets the switching wave amplitude which would give you the output power. If one company can pass test at a given power level then so should another unless they claim to be operating spread spectrum or something. Check the PCB layout first. Shouldn't be the chips fault.

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Old 3rd January 2008, 12:00 AM   #25
soongsc is offline soongsc  Taiwan
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Quote:
Originally posted by Ipanema
Guys,

If the filterless class D amp offered by TI or Maxim has such a bad EMI problem. How can they pass the CE or any radiation regulation in order to be sold to consumer?

Regards.
I guess if it is shielded, then it is not a problem to pass the tests?
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Old 3rd January 2008, 12:05 AM   #26
Dave Z is offline Dave Z  United States
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soongsc

The output voltage is determined by the duty cycle, not the frequency. If you are at 50% duty cycle, then your output voltage will be 1/2 your supply voltage regardless if you are switching at 250kHz or 1MHz. Do you mean ripple current in those output caps?

Dave
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Old 3rd January 2008, 12:07 AM   #27
Dave Z is offline Dave Z  United States
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RF shielding generally doesn't work at that low of frequency. It has to be mitigated on the PCB. All of my designs have never been shielded to pass.

Dave
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Old 3rd January 2008, 12:19 AM   #28
Ipanema is offline Ipanema  Malaysia
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Dave,

May I know what is max. amplitude and the frequency spectrum of EMI in order to become problematic?

Maxim claim with SSM, they can lower the EMI amplitude of the switching frequency by spreading them to a wider frequency spectrum with lower amplitude, which would be less harmful than a frequency peak. But how well does it work? Anyone tested this chip MAX9709?

Regards.
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Old 3rd January 2008, 12:33 AM   #29
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Quote:
Originally posted by Ipanema
Guys,

If the filterless class D amp offered by TI or Maxim has such a bad EMI problem. How can they pass the CE or any radiation regulation in order to be sold to consumer?

Regards.
Two words: board layout.
Multilayer boards, ground planes, short wire runs, extensive decoupling, component location AND orientation...and a little black magic.

Just kidding, no magic involved. Just art and science.
Some gifted individuals have enough knowledge and experience to get it right on the first or second board attempt. The rest of the mere mortals, myself included in that group, take a little longer.

Also: length and routing of any wires.

IMHO, shielding is the last resort, when everything else has failed or where the cost of the required fixes exceeds that of the shield.
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Old 3rd January 2008, 01:11 AM   #30
Dave Z is offline Dave Z  United States
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Ipanema

Difficult question to answer the way you asked it. What do you mean by problematic? Do you mean to cause interference with other equipment or just to pass EMC requirements. For EMC requirements, different regions have different levels. The levels also change for different frequency bands, and there are conducted emissions as well as radiated emissions. I would suggest seeing if you could download the FCC part 15 subpart B for incidental radiators. Would love to help but regulatory requirements are a complete world of there own and way to extensive to explain here. For interfering with other equipment that depends on the immunity level of the other equipment. Look at GSM cell phone for example, the RF gets in to so many pieces of audio equipment but is still within required limits. GSM immunity is a completely different animal. Good thing class D is more immune to GSM phones than class AB due to the power stage not operating in the linear region.

As far as what I have experienced in designs, when switching between 200kHz and 300kHz most of the emissions have been up to 300MHz and then start to decline. Some of the spread specturm stuff does work, I've seen real world measurements of about 6dB improvement. However, even with the Spread Specturm you still MUST have a good PCB layout. you will fail tests just as misreably with spread spectrum as without SS if your layout is not optimized. The Maxim stuff is at a higher switching frequency as well so you must judge the audio performance of the higher frequency to see if it is worth it (also the higher frequency harmonics are allowed a higher level for acceptance). From what horror stories I've read on this site, it seems that most wirings / layouts are not nearly adequate for EMI reduction and may be a main culprit of some of the malfunctions I've read about. Although output filters are very important, it is much more important to have good PCB layout techniques. You could be radiating somewhere off the PCB that is not the output! Chances are, if a project is handwired, it will almost certainly fail. Good thing projects are just for personal use and only care if they work in thier surroundings

Dave
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