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.
Dave
Dave
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.
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.
Ipanema said:
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.
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
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
Dave,
Thanks for the explanation. The problematic I mean is how would they affect my audio system e.g. burn tweeter, bad sound, interfere with my TV etc. I would like to know whether the amount of EMI present in the filterless class D chip offered by TI or Maxim will cause any problems mentioned above.
As for as I know, the output LC filter only filter off EMI at the output connected to speaker. With or without LC filter, the EMI issue still exist on the amp board. Hence, good PCB layout and shelding requirement are required for both filter or filterless design.
Regards.
Thanks for the explanation. The problematic I mean is how would they affect my audio system e.g. burn tweeter, bad sound, interfere with my TV etc. I would like to know whether the amount of EMI present in the filterless class D chip offered by TI or Maxim will cause any problems mentioned above.
As for as I know, the output LC filter only filter off EMI at the output connected to speaker. With or without LC filter, the EMI issue still exist on the amp board. Hence, good PCB layout and shelding requirement are required for both filter or filterless design.
Regards.
The speaker will act like a filter which is part of the premise why filterless class D works. But, the inductance is not the same as a true inductor. The output inductors can run quite warm. This heat will now have to be dissipated from the voice coil so make sure you derate the driver. Also since the inductance is less, not as much high frequency will be attenuated so it will start to become harsh which is why I don't use filterless above 5W -10W or so. Unfortunaltey even though the speaker will attenuate some high frequency, it will only attenute differential mode high frequency so it will not reduce EMI like a ferrite bead (plus it is also at the end of the speaker wire in stead of getting rid of it at the source). Speaker drivers are also not low impedance at high frequencies.
You will see output frequency response graphs for class D ICs based on filter values for 4, 8 or whatever load impedance. This is theoretical and not real world. As pointed out a speaker really is an inductor. So as frequency rises so does the impedance. You must do an impedance plot vs frequency of the speaker cabinet to see what the impedance of the speaker is where the LC filer starts to roll off. This should be the entire network. If there are crossovers, zobels, and multiple drivers in the speaker cab, they must ALL be connected while doing the impedance plot. The point is to see the true impedance of what the amp sees. You may find an 8ohm cabinet may really be 12-20 ohms at 20kHz. This will effect the filter response. If Z is higher than what the filter is designed for, you could get an unmatched load which may cause the lowpass LC filter to have a high Q. You could actually get a rise in output power at that freq which may cause damage or sound harsh. If real Z is lower than what the filter is designed for you will be attenuating some high frequencies. It's a shame IC manufacturers don't explain that better.
Dave
You will see output frequency response graphs for class D ICs based on filter values for 4, 8 or whatever load impedance. This is theoretical and not real world. As pointed out a speaker really is an inductor. So as frequency rises so does the impedance. You must do an impedance plot vs frequency of the speaker cabinet to see what the impedance of the speaker is where the LC filer starts to roll off. This should be the entire network. If there are crossovers, zobels, and multiple drivers in the speaker cab, they must ALL be connected while doing the impedance plot. The point is to see the true impedance of what the amp sees. You may find an 8ohm cabinet may really be 12-20 ohms at 20kHz. This will effect the filter response. If Z is higher than what the filter is designed for, you could get an unmatched load which may cause the lowpass LC filter to have a high Q. You could actually get a rise in output power at that freq which may cause damage or sound harsh. If real Z is lower than what the filter is designed for you will be attenuating some high frequencies. It's a shame IC manufacturers don't explain that better.
Dave
Shame on the enclosure manufacturers to not compensate their loudspeakers for a flat impedance curve, adding compensation network both for inductance and motional impedance of their drivers.
Shame on the reviews to not put accent about the importance of a near flat impedance curve when their review speaker enclosures.
Drivers are reactive in nature so a traditional coned speaker will have a, voice coil that exhibits resistance, inductive reactance and a little capacitive reactance. So it will exhibit different impedance at different frequencies. That is why an 8 ohm driver measures 5-6 ohms on a multimeter, and the impedance increases with increasing frequency. As with most things, the laws of Physics rule.
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