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Old 21st November 2012, 06:07 PM   #1
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Default SystemD_2kW, any interest for an open design?

SystemD_MD is by far to complex for an open DIY design.
Also I have the feeling that the class D community is heading for higher power.

Since two weeks I am thinking about a DIY friendly design, that could deliver about 2kW into 4R in bridged mode. I am considering to develop this as an open design for DIY class D enthusiasts.

Rough outline of SystemD_2kW:
- Useable as a halfbridge with 1kW/2R
- Useable in bridged mode with 2kW/4R
- High performance and well balanced properties, but of course not as blameless as SystemD_MD
- Simplified implementation of the triangle shaping method known from SystemD_MD
- Instead of mostly discrete, heading for a reasonably simple design
- Selfoscillating, because that's the way to keep component count attractive for DIY
- Hysteresis oscillator, not UcD
- Restrict the really critical parts to MosFets, level shifter and layout

Why a hysteresis oscillator, while the entire classD world is on the UcD trip (or on the IRAUD trip with delta sigma...)?
Compared to UcD the hysteresis oscillator needs just two resistors more and offers the chance to adjust the frequency shift by a dynamic hysteresis control.
At this point I have to highlight, that I am not the only one who has thoughts into this direction. I will post my circuit for the dynamic hysteresis control soon and would love to see postings with different circuits for a dynamic hysteresis control of other enthusiasts, in case you are willing to share it with the DIY community.
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Old 21st November 2012, 06:45 PM   #2
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man, if nothing else, I would love to see this forum get some proper amp (layout), I ready to see, how this will gor, and respect for your work here, really
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Old 21st November 2012, 08:01 PM   #3
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There are others who can do such designs as well.
Also in the 25W-1200W there are people actively working and for sure there are already valuable contributions including layouts.
I still have not proven that I can design a 2kW class D amp.
The 1.2kW of the bridged MD is the strongest class D amp I have designed so far. Up to now, I just think I could also do 2kW with good quality.

Attached my dynamic hysteresis control.
Please don't be shocked by the LT1711, it is just an avaiable comparator in LT Spice. For the real design I am intending LM160/LM360.
Also please do not wonder about the series connection of a 33V zener and 6.2V zener. LTSpice simply did not offer a 39V zener....

Functionality of the circuit:
At low output voltage Q7 and Q8 are completely OFF, polarity of Ube is reversely biased. No influence on the hysteresis rectangle formed by Rmk1 and R2.
Q9 and Q10 are running at low biasing (theoretically for fundamental circuit no bias needed). When the difference between output voltage and rails becomes smaller than the zener value of 33V+6.2V zeners, then Q10 or Q9 will be driven with increasing currents and pull up or down Q7 or Q8, which in consequence will more and more clamp the hysteresis rectangle. The smaller the magnitude of the rectangle, the higher the operating frequency. It is easy to control the frequency shift this way.
Still a good portion of frequency reduction at high levels is desired, because this is reducing the switching stress of the MosFets. The control circuit shall just avoid really heavy voltage ripple on the speaker signal as it usually happens in ultra simple selfoscillating amps.
In fact this principle is suffering from the parasitic phase shift oscillator mechanism, which will not allow unlimited high frequencies. Transfer function and propagation delay will limit the max. possible switching frequency, even if the hysteresis would be set to zero.
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File Type: jpg DynamicHysteresisControl-800x800.jpg (53.3 KB, 4542 views)
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Old 21st November 2012, 08:39 PM   #4
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Attached you can see the influence on the intended amp, when running to the clipping limit with a 2kHz sine wave.
Basically also without dynamic control the amp is behaving in an acceptable way, but with dynamic control the ripple remains really nice even during last switching events.

First picture without dynamic control.
Second picture with dynamic control.

Edit: The blue trace is showing the hysteresis rectangle, which has a constant magnitude without dynamic control, but with control its magnitude decreases at high output levels.
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File Type: jpg withouthysteresiscontrol-800x800.jpg (59.9 KB, 4324 views)
File Type: jpg withhysteresiscontrol-800x800.jpg (57.0 KB, 4102 views)

Last edited by ChocoHolic; 21st November 2012 at 08:42 PM. Reason: Edit: The blue trace is showing the hysteresis rectangle...
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Old 22nd November 2012, 07:36 PM   #5
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The difference is more obvious, when watching signals which run close to the rail - like large rectangles.
At 150Vpp output voltage the version without dynamic control shows approx 10Vpp switching ripple, while the version with dynamic control is showing approx 4Vpp.
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File Type: jpg Rect_wo_dynamiccontrol.jpg (88.1 KB, 3913 views)
File Type: jpg Rect_with_dynamiccontrol.jpg (80.9 KB, 3739 views)
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Old 23rd November 2012, 06:31 PM   #6
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Slightly more than 300 views within two days.
Only one reply from a DIYer.

This and the monologue of my last thread is telling me that there is just little interest in class D DIY.
Also it tells me that there are around 20 knowledgeable class D freaks watching the thread, but not willing to share anything, because of the 280 CopyCats that lurk around the thread in silent spy mode .
In history there have always been these technically ignorant guys who only want to suck&sell the design, but it is sad they became powerful enough to kill the DIY-spirit in the class D section.

Not sure whether I will completely stop the thread
- or continue, but restrict the design details to DIYers who I know more or less.
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Old 24th November 2012, 08:10 AM   #7
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C'mon Choco,

Don't loose the hope.

Here is one of my early [rookie] DIY attempts that came very well.
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Old 24th November 2012, 08:13 AM   #8
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And one day i made this

One shouldn't be pessimistic. I know copy cats are everywhere and are hard bent to making big monies.

And its very sad and serious issue that people who don't know anything technically , ripe the fruits from the hardwork done by others.
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Last edited by Workhorse; 24th November 2012 at 08:28 AM.
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Old 24th November 2012, 08:17 AM   #9
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The schematic which you have posted has analog discrete front end and a chip comparator afterwards, i think it would be much better if we can make a similar discrete comparator also.
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Old 24th November 2012, 09:17 AM   #10
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Markus, the problem with your thread is it is too high technical level.
Adjusting hysteresis window to get tighter switching frequencies range is scary. I've tried modulating hysteresis window (for PLL-ing phases in multiphase) and I'm still scared too. This was strainghtforward dynamic adjustment of comparator supplies.
It is not unusual that jazz piano virtuoso gets less audience then a boysband if you know what I mean.
From my side, I think I can contribute to this open design in certain parts of modulator circuitry, switching part and layout if you'd wish.


Last edited by darkfenriz; 24th November 2012 at 09:20 AM.
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