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Class D Switching Power Amplifiers and Power D/A conversion

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Old 15th January 2008, 03:20 AM   #121
MikeHunt79 is offline MikeHunt79  United Kingdom
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Quote:
Originally posted by djQUAN
that's great....

this is what started it all for me.

I haven't done some power testing yet. the entire car setup of mine is out and I have to rebuild a new amprack. will update as to how it goes.
I only just found that site yesterday - it's awesome.

I am now currently in the process of modifying an old broken kenwood amp so I can run an AMP9 from it.

All I need to do is wind a toroid and I should be set.

I'm looking forward to seeing when your is done tho... Class D in a car is awesome, as it uses less battery power... in theory at least.
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Old 17th January 2008, 12:28 PM   #122
djQUAN is offline djQUAN  Philippines
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I have tested it in teh car but the EMI generated is massive. there is noise like static coming out of the other two amplifiers. beside it.

I'm suspecting it's due to the output inductors mounted off board. the total board height is just low enough that I might be able to still mount the inductors on board so that's the project for tonight.
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Old 17th January 2008, 01:07 PM   #123
Eva is offline Eva  Spain
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Disable or mute the class D module, put a dummy load on the SMPS and check if you have noise in the other amplifiers. Is the class D ground floating? How is it coupled to the 12V ground?

I was expecting your class D module to be quite clean as it should be expected from a finished product, but now I think that it isn't. First of all, consider adding 3mm thick aluminium oxide spacers between the class D transistors and the heatsink to reduce capacitance. Then you may have to add some common-mode filtering. Don't bother about the inductors that much...
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Old 17th January 2008, 01:19 PM   #124
djQUAN is offline djQUAN  Philippines
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the class D ground is directly connected to chassis same as the SMPS input ground connected to chassis. but there is no direct connection from SMPS input to class D gnd.

the interference is happening only at a particular volume. above and below that volume, there is no interference.

the kit came with 3mm oxide spacers and I have them installed.

where do I add common mode filtering?

I have used the module for about a year without problems and nothing major changed in the amplifier besides the output inductors mounted off board and class D gnd connected directly to chassis. before, it was done via 100nF caps to chassis.
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Old 17th January 2008, 01:28 PM   #125
Eva is offline Eva  Spain
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Why did you remove the 100nF capacitors?
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Old 17th January 2008, 01:41 PM   #126
djQUAN is offline djQUAN  Philippines
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there was no option for grounding on the board, I do it by connecting the ground terminal (supply gnd) at the PCB to chassis via a thick wire.

since I rebuilt the amp, I thought no cap is better EMI shielding than through a cap to chassis so I connected it directly.
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Old 17th January 2008, 04:13 PM   #127
djQUAN is offline djQUAN  Philippines
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ok, I changed the grounding from direct connection to via 3pcs 100nF caps and 100R resistor all in parallel

mounted the inductors right onto the board

added 100nF cap across thermistor for thermal protect (because the wire runs near the fets I thought it might cause interference to the PWM controller)

and none of them worked.

I noticed that the EMI happens at above a certain current draw I jsut thought it disappeared above a level before because it was masked by the music but actually, it occurs above a certain current draw and remains constant when current draw increased.

I realized this when I wired the subs in series when it was in parallel before. the result was that the volume had to be increased before the interference appeared.

time to take the scope to the garage. I think it has something to do with the SMPS.
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Old 17th January 2008, 06:06 PM   #128
djQUAN is offline djQUAN  Philippines
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I've confirmed it's the SMPS. the drain waveforms get erratic like the oscillator stops at moderate currents.

when I took it back onto the bench, it runs perfectly. the only difference from bench and in-car conditions is that the bench supply is not that 'stiff'. in my car, I have a truck batt under the hood, 4ga cable and two 1F caps just beside the amp.

right now, I have this thing in my workshop and used another amp for the meantime in the car.

I haven't tried removing a cap that isn't on the board layout that I added for soft start when the amp recovers from over-voltage or over temp condition. it's a 100uF cap from pin3 of TL494 to gnd so I don't know yet if it has anything to do with the problem.

I need some rest so this amp will have to wait.
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Old 21st January 2008, 12:55 PM   #129
djQUAN is offline djQUAN  Philippines
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darn it.

I fixed the amp. the problem was grounding issues and I'm getting pretty nasty spikes at the grounds.

fixed it by using jumpers and taking grounds from the main input caps for the control circuit and adding bussbars for the fets.

seems to work fine now with a solid ground.

lesson learned: I'll go back to double sided boards for my next SMPS and make sure about proper grounding of critical paths.
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Old 21st January 2008, 01:50 PM   #130
luka is offline luka  Slovenia
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Say what??? how do you get spikes from that short path from fets to ground wire? resistance of it should be pritty low....
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