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Class D Switching Power Amplifiers and Power D/A conversion

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Old 16th October 2011, 06:37 PM   #11
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Well, without feedback you will participate on dead time distortion in full extend anyway. For educational purpose OK.

The triangle of the modulator has a direct influence as well, but slightly less then one would expect from shape. Especially at low modulation levels the impact is small, but increasing with larger music signals.
When using a simple RC, then my rule of thumb for Hifi is to keep the magnitude of the triangle less than 8% of the rectangular magnitude. In your picture you show something like 30%. Might be still acceptable for educational purpose.
Feel free to simulate your amp with an ideal power comparator in order to get a feeling for the modulator distortion (simulation of a real switching stage with dead time distortion is also possible but by far more time consuming).

Bread board, protoboard PCB:
Well, everything is possible. Especially when using a proto board with copper back plane.
The key is the understanding about which loops and inductances have to be kept small. Proper PCBs usually are slightly less beasty, while unfortunate wiring on bread boards easily happen to be extremely unfortunate.
But also a PCB does not guarantee good results, when routing ignores physics.
Especially HF ringing in the power stage may completely spoil everything.
Understanding about... takes a lot of reading (app notes + forum) and experimenting.
As a hint:
Pick your schematic and add the parasitic capacitances of the power MOS switches and then add a inductor for every wire (rule of thumb 0.5-1nH per mm of wire length).
Now your schematic is looking pretty ugly - almost as ugly as reality.
Possible resonances everywhere.
Most critical loop: Pos rail - upper MosFet - lower MosFet - neg rail - caps from neg rail to pos rail - to upper MosFet.
Keep this loop as small as possible and add a RC-snubber directly from drain to source at each MosFet.
Second critical loops: Gate drive loops.
Reality will strike back worse and worse the more power you intend to implement.
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Old 16th October 2011, 10:09 PM   #12
Derfpa is offline Derfpa  Sweden
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You're absolutely true, there would be nasty uncontrollable distortion with deadtime without feedback. This thing with feedback doesn't seem too bad actually.

I'm thinking more and more about building with feedback and skip the PWM completely... We'll see when my goodie package arrive and ill get started to play for real. :P

I actually had weird RL-resonants when i played a few evenings ago with the PWM. Not until now i realized that the probably were from the bread board itself!
But as it seems, everything counts, and its just to eliminate as many possible flaws as possible. As one guy i know said: "Why even bother using a bread board? Just make it right, etch it and solder it with SMD's" Guess he have a point! Read that that's what Bruno Putzeys did too and probably still is doing.
RC-snubber is actually a new term for me, but a little bit of googling made it all clear and understandable. But i don't understand gate drive loops, what is that really?
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Old 17th October 2011, 08:47 AM   #13
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[QUOTE=Derfpa;2748008]
I'm thinking more and more about building with feedback and skip the PWM completely... QUOTE]

No, even if you build a self-oscillating amplifier, the PWM modulator is still there. The difference is that the 'clock' input to the modulator is a signal derived from the output stage switching waveform. (Either before or after the output filter).
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Old 17th October 2011, 07:23 PM   #14
Derfpa is offline Derfpa  Sweden
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I meant a PWM-module that creates a PWM-signal from a triangle wave. But you're completely correct, I still get a PWM-signal!
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Old 17th October 2011, 07:33 PM   #15
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For the loops (and more) you may have a look here:
http://www.irf.com/technical-info/appnotes/an-1135.pdf

My first critical loop you may find in figure 13 on page 11.
My second critical loop you may find in figure 8 on page 9.

Please note:
I am not generally promoting self oscillating topologies nor clocked.
Self oscillating and clocked - both have their specific advantages and downsides. Currently I am working on a clocked one again....
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Old 17th October 2011, 08:50 PM   #16
Derfpa is offline Derfpa  Sweden
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Okay, i get your point. Interesting pdf! Although thinking about that is far in the future... Really sexy mosfets btw :$ :P

I found http://eprints.nottingham.ac.uk/501/1/final.pdf a while ago.... I really should put my brain together and understand most part of it, because I think it contains a lot of good stuff!
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Old 18th October 2011, 07:01 PM   #17
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COXnCANDY !
Are you really sure that this lecture is helpful for pain relieve during building your first class D amp???
Aside this doubt, it seems to be a helpful analysis of feedback distortion mechanism. ...and I am loving this sentence:

"The next steps in the calculation are algebraically cumbersome and shed little further light on the problem, so the full details are not presented here."
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Old 22nd October 2011, 03:25 PM   #18
Derfpa is offline Derfpa  Sweden
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Well, I'm not into understanding that completely (yet), but it feels a bit stupid to apply feed back and cross my fingers that things will turn out alright without understanding whats happening. :P But maybe that's the way to go, and just keep as a rule of thumb to filter the signal after the mosfets, where the signal goes to the speakers and also to negative feedback.

I had some thoughts about using S/H-circuits and a integrator to compare the output to the input... but all that is to complicated for me, but i sense there's some potential maybe Maybe possible to solve with an atmega... but that's much further into the future :P

Just had my exam-week (phew!) and also received my package of various component (yay!), so hopefully ill get time soon to start playing a bit more
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Old 23rd October 2011, 01:04 PM   #19
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For sure it will not be sufficient just to apply feedback and hope things will be fine.
But a math model for the distortion mechanism of feedback in class D amps with triangle modulator -
That is pretty advanced, even if you stick to the simple set up of COXnCANDY, where they do the maths for a pre filter feedback.

Such topic may become important after you solved more basic points like:
- Handling the power stage, including real life layout and real life components.
- How much feedback to apply, without getting supersized carrier residuals that completely spoil the operation?
(Hint: You would not need the COXnCANDY paper for this, but they have a nice picture, which helps for this on page 8. Besides the discussion about distortion, you have to ensure that dv/dt of h(t) never exceeds dv/dt of v(t), also at higher modulation levels. Well in reality the propagation delay will in fact allow to slightly violate this rule, but nothing to win there.. )
- Fundamental control theory to design a stable loop, especially when heading for post filter feedback.

You would be the first, who suffers from feedback distortion mechanism before solving the three topics above.
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Old 23rd October 2011, 01:08 PM   #20
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BTW:
Keeping my fingers crossed that everything went right with your exam.
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