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Class D Switching Power Amplifiers and Power D/A conversion

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Old 14th January 2013, 05:37 PM   #221
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My english is not so good, what i want to say - with ucd like feedback i have cracking sound at clipping, but with the pre-clipper band aid everything is ok, 2r-open load; i have tryiet your modulator schematic and at clip with 4r load everything is about the same, switching frequency drop, producing the same cracking sound; then i disconected the load (with 100hz sinewave for testing still clipping) and BooM...
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Old 14th January 2013, 05:42 PM   #222
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Discrete class d can have some strange behaviours.
You can sometimes get bus pumping causing the power supply to rise above the capacitors rated voltage. Inputting low frequency sine waves is the worst culprit.

Sometimes the clipping can cause Vb to discharge and you lose the top transistor gate drive or even worse the gate drive goers into linear mode and blows up the transistor.
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Old 14th January 2013, 10:09 PM   #223
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Hi Ionut,
...as Nigel says there are many possible reasons to struggle with...
In your case you have tried two very different modulators, but found the same issue. This points to an issue in the circuit parts which were the same in both trials.
If your level shifter and/or power stage is in trouble with clipping, then generally loop structures with integrating portions in the loop gain will be more critical. Depending on the issue in the level shifter and/or switching stage this might even lead to defects - fitting to your experiment.
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Old 15th January 2013, 06:00 PM   #224
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...on request... here the LTspice file for the functional principle including dynamic hysteresis.
Attached Files
File Type: asc SystemD_2kW_4DIYaudio.asc (14.3 KB, 126 views)
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Old 15th January 2013, 06:39 PM   #225
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Quote:
Originally Posted by ChocoHolic View Post
Today I set up a small jig for testing the intended multi outline before going on with the layout.
Also this is helpful for comparing the comparators.
The jig is build with identical routing as intended for the amp.
Attached some pictures showing the different positions of LM306 or MAX913 or LM361.
All comparators are working correctly with the intended routing.
The electrical testing was done with input signals of +/-10mV and a frequency of 2MHz.
LM306 is the slowest and shows slightly more self disturbance when driven with rectangles, but for a so called 'scrappy' comparator it is really good.

Besides measurements with rectangles, I also measured with +/-10mV triangles.
And there we can see, why I am stating that the fastest high end comparators are not the best choice for classD. When driven with small and slowly creeping signals these ultrafast high gain devices start to show serious jitter.
MAX913 performs great with rectangular input signal of reasonable size, but for input signals with low dv/dt it generates by far more jitter than a LM306 or LM361.
I am curious if we will see impact from this effect on the noise figures of the amp.

Overall the LM361 seems to perform very well.
My thanks to Kanwar that you pushed me to add it, despite its unpleasant size.

Edit:
All measurements were done with 10:1 probes.
Upper trace is the input voltage of the comparator.
Lower trace is the output voltage of the comparator.
Quite interesting.
Could it be layout and GND (no plane), which makes the MAX913 perform so badly? ... would guess that this matters more and more, the faster the device is.
Thanks for this investigation.
/Baldin
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Old 15th January 2013, 09:35 PM   #226
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The top layer of this bread board is a GND plane, or at least a GND grid.
A completely closed plane might be better -yes.
In any case within a certain environment with some noise the picky comparators are likely to show more jitter than the less picky ones.
Please note that the MAX913 does not perform poor. The input signal is very small and with low dv/dt and for sure contains some noise in itself.
Of course this test situation magnifies the differences on purpose.
In the amp the comparator will operate with signals that show dv/dt of approx one order of magnitude higher. This is the good side.
The bad side is that the switching power stage will generate a more noisy environment than the test set up...
We will see if the found differences turn visible in the amp.
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Old 16th January 2013, 04:53 PM   #227
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Good points.
I have been using MAX913 for some time, and have found it to be way easier to work with than both LT1016 and LT1711. It does require good layout and good decoupling, but it is fast and uses relatively little power.
What you call jitter her couldn't it be fast and precise reaction on changes in the input signal. And if the signal contains noise, you'll see smalle differences in switching times and therefore jitter. If the noise is higher in frequency than the signal, I guess you can place a LP filter to reduce the noise and thereby reduce the jitter.
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Old 27th January 2013, 03:29 PM   #228
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Default V1.0 of PCB

One piece of V1.0 ordered.

V1.0 is always the most thrilling version.
Waiting until PCB arrives ==> some weeks.
Building ==> some more weeks.
Debugging ==> don't ask.

Attached two PDF files which show top side (GND plane) and bottom side in order to give some food to the thread.
Attached Files
File Type: pdf PCB_V1_Front.pdf (206.9 KB, 228 views)
File Type: pdf PCB_V1_Back.pdf (131.1 KB, 182 views)
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Old 27th January 2013, 04:36 PM   #229
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Looks nice

What are the MBR7030 used for?
Why not more SMD components? Would probably make it easier to have a less broken GND layer.

Year always a long wait for PCB ... have just ordered PCBs myself .... but in the oposite direction 5x5cm boards hopefully capable of between 100 and 150W ... let's see
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Last edited by Baldin; 27th January 2013 at 04:45 PM.
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Old 27th January 2013, 05:01 PM   #230
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Both MBR7030 avoid that the body diodes will be flooded.
The question about body diodes is one of the oldest in classD and always a hot discussion up to which power level at which switching frequencies it is making sense to use them.
Simulation showed that in this project the IRFP4668 would struggle hard with its body diodes.
Vienna Tom and Workhorse pointed out that already top brands had tried to go this path, but had issues with reliability.
So I stepped into manual calculation and noticed that the simulation was misleading in a forgiving way.
Altogether a clear indication to spend the additional components.

I have chosen a low amount of SMD components, because many DIYers complain about SMD. So this design is a trade off.
Intended for high performance, high power - still DIYable for experienced enthusiasts who like the design.
Of course not a beginner project, because of it's complexity and involved high voltages.
In case you want to build it, please wait until the design is ready and has reached a sufficient level of maturity.
If you read this thread from the beginning you will find the evolution and many design considerations.
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