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Class D Switching Power Amplifiers and Power D/A conversion

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Old 7th September 2009, 04:37 PM   #21
Pafi is offline Pafi  Hungary
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definitely EMI as well
Yes, but how could have generate variable difference freq? f-f=0! And why only at 150-200-250-300-350 kHz? There must be an 50 kHz (and harmonics) signal source!

Leigh! Was the SMPS really totally disabled?
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Old 7th September 2009, 08:06 PM   #22
81bas is offline 81bas  Germany
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Hi!

Do you use 78XX or 79XX in you schematics (7805, for example)? They can oscillate ignoring any bypass capacitors...
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Old 8th September 2009, 08:57 AM   #23
nitrate is offline nitrate  United Kingdom
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I do have a 7805 reg, and to make matters worse this reg follows an lm switching reg that just happenZ to switch at around 50Khz. Can it be that obvious??

I doubt it as i have run the amp without the switching reg and there was no difference, the noise was still there. The 7805 reg was still in place tho. There are also two DC-DC converters on board. They seem clean and are well decoupled. Maybe i'm being a bit picky here. The noise is very low in the background and i'm pretty sure it will be below the noise floor when i go closed loop. Maybe i'm at the limit of this technology. If this is a design error i'd like to fix the problem properly rarther than simply covering it up with feedback

Even though i have not done a schematic yet i have my proto PCB layout that i can attach so you have an idea how things are wired and you can work out the schematic from that. I'll post an actual schematic this weekend. The layout shows ground tracks but on the physical board the entire bottem layer is one large copper area as in the photos above.
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Old 8th September 2009, 09:37 AM   #24
Pafi is offline Pafi  Hungary
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the only other thing that is switching in the room is the fet bridge
Quote:
There are also two DC-DC converters on board.


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They seem clean and are well decoupled.
Do you really think they are isolated from the carrier or PWM signal for at least 80 dB ?!? How could you possibly know? Are you able to see 10 micrometer untriggered signal fluctuations on your scope???
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Old 8th September 2009, 09:45 AM   #25
nitrate is offline nitrate  United Kingdom
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I can see most things on my scope. Not sure what you mean by 10 micrometer??

But if the two DC-DC converters were causing the intermodulation then there would be more than one tone as there are two converters and the tones would interfere with each other too. It would cause a real mess. This may sound stupid but does anyone think 50Hz mains could be a culprit here??


Leigh
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Old 8th September 2009, 10:05 AM   #26
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there would be more than one tone
1: not neccessarily perceptable both.
2: I do hear two tones!

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and the tones would interfere with each other too
Not really. (They do, but you don't have any chance to notice it.)
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Old 8th September 2009, 04:04 PM   #27
81bas is offline 81bas  Germany
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The diodes in RD networks after comparator on the layout are directed in wrong way... In this direction this will work for charging of capacitor (before optocoupler), and discharging will be done via resistor... Is it in your real prototype so too?

And where are your bypass cpacitors after DC-DC converters?

Last edited by 81bas; 8th September 2009 at 04:11 PM.
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Old 8th September 2009, 09:35 PM   #28
Pafi is offline Pafi  Hungary
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Leigh!

From RB0505 datasheet:
"Ripple and Noise (20MHz limited) Single output types 100mVp-p max.
Dual output types ±75mVp-p max."

Do you call it well decoupled?
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Old 9th September 2009, 08:14 AM   #29
nitrate is offline nitrate  United Kingdom
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The DC-DC converters are well decoupled with tantalum caps directly on the base of the MAX626 driver ic, and also the IL711 magnetic coupler. I have used them before like this with no problems. However i must say there is too much of a coincidence that the running freq of these converters is approx 50KHz. Common sense forces me to think these converters could be the source of the noise. I'll whip them out this weekend and try some fancy bootstraping to see if the problem vanishes. Just because i have used them successfully in other amps doesnt mean that they are not interfering on this occasion. I really cant think of any other noise source, Well spotted Pafi.

Regarding the diode directions, they are correct. Yes, this is a real working prototype. The diodes and resistors set my dead time.

I'll let you know if bootstraping solves the noise issues, won't be till weekend tho... time is a premium this end LOL

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Old 9th September 2009, 01:45 PM   #30
81bas is offline 81bas  Germany
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Originally Posted by nitrate View Post
Regarding the diode directions, they are correct.
Yes, agree... This is because of inverting MAX626...


Also, simply interesting, do you see any ringing on this prototype?
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