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Self-Oscillating Analog-to-PWM Converter for Class-D
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SiliconWizard:

--- Quote from: Evan.Cornell on January 02, 2020, 05:00:10 pm ---I did run FFT directly on PWM output and additionally on output with AES17 20kHz filter (someone reverse engineered AudioPrecision AES17 lpf box component values). Filtered output had the desired reduction above 20kHz, but nothing thru the audio band, as expected.

Where do you think dedicated triangle wave + comparator modulator lies in terms of linearity, compared to self-oscillating, and then your own PDM modulator?

--- End quote ---

A (very) good triangle wave generator-based design would probably be better, especially for generating PWM signals (both PDM modulators discussed here are not really PWM, not fixed-frequency), at lower modulation frequencies. But implementing a good triangle generator is not trivial if you want to go all discrete.

Both the design you posted and mine generate relatively high-freq signals. Yours at around 1MHz, mine a decade higher... this would cause issues as I said to drive power output stages. Driving power MOSFETs in the 10MHz range requires care to avoid unacceptable losses.

In practical implementations, I still think a sigma-delta modulator would yield the best results and keep part number low. Mine is pretty simple, but you could get better results implementing one with higher order on a CPLD or FPGA for instance. But the thing to consider is the modulation frequency - sigma-delta modulators would  tend to work best at higher frequencies, which makes power stages a bit trickier/more expensive to design.
pwlps:
FYI here is another self-oscillating audio-to-PWM converter using an asynchronous sigma-delta modulator : https://www.eevblog.com/forum/projects/simple-lifi-transmitting-audio-signal-with-a-led/msg2282771/#msg2282771

This was for a different application but I would be interested to know how these different approaches (PDM vs true PWM via clocked or asynchronous sigma-delta) compare in class-D applications.
Yansi:
Thing is, PWM based class D amplifiers suck big time and I wouldn't even bother trying. *

PDM on the other hand, is a different matter.

And do not forget to include a global feedback, including (preferably) even the output reconstruction filter. Having a good open-loop linear modulator is great, but useless in the end.

EDIT: * Linear voltage-to-PWM modulator by itself is useless, as the output linearity will suffer severely by adding a power stage, for example by adding a dead time and non-equal channel paths or Ton/Toff times of transistors.
pwlps:

--- Quote from: Yansi on January 02, 2020, 07:23:45 pm ---Thing is, PWM based class D amplifiers suck big time and I wouldn't even bother trying. *

PDM on the other hand, is a different matter.

And do not forget to include a global feedback, including (preferably) even the output reconstruction filter. Having a good open-loop linear modulator is great, but useless in the end.

EDIT: * Linear voltage-to-PWM modulator by itself is useless, as the output linearity will suffer severely by adding a power stage, for example by adding a dead time and non-equal channel paths or Ton/Toff times of transistors.

--- End quote ---

Yes but a linear voltage-to-PWM modulator does not introduce a quantization noise like in PDM so in principle much lower clock frequencies can be used ?  Actually I also tried a PDM version of the transmitter in my li-fi project with clocked 1-bit sigma-delta and I had to increase the frequency by a factor of 10 to get rid of the noise.  I would say PDM can only be better if a clever noise shaping is done digitally.

Edit:
I just found this article: https://www.eetimes.com/class-d-audio-amplifiers-what-why-and-how-part-5/#
Yansi:
Does not work like that I think.  Quantization noise will still be there in form of a jitter (timing issues especially the power stage. You can't make infinitely short pulse, neither any longer than the PWM period is set.

PWM makes for an inefficient modulation, usually requiring more transitions for describing the same signal. Also, signal tracking will be worse than with a PDM.

Most class D amps are constructed as somewhere in between fixed frequency PWM and fixed clock gated PDM. 

The simplest and most scalable form of modulator is with the second order integration (see for example IRS2092). This type of modulator is very reliable and gives very good results.  Straight PWM won't touch it.

//EDIT: Regarding your article you've found:

--- Quote ---"An alternative to PWM is pulse-density modulation (PDM), in which the number of pulses in a given time window is proportional to the average value of the input audio signal. Individual pulse widths cannot be arbitrary as in PWM, but are instead “quantized” to multiples of the modulator clock period. 1 bit sigma-delta modulation is a form of PDM."
--- End quote ---

PDM does not need to have any clock and does not need to be quantized to a modulator clock.  Again, see the second order integration loop, like the IRS2092 has.  The frequency changes with the signal, as required.

It is similar to comparing PWM (SVPWM) modulation in a motor drive to a DTC, with the latter one being superior in the fast signal tracking ability.
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