Distorsions in class D

[Bruno Putzeys]

Hysteresis amps are a bit more amenable to intuitive analysis and therefore may be a more practical starting point for beginners. I began that way

One important thing: Get the positive (=hysteresis) feedback from the power stage as well, not from the comparator output. Otherwise you'll find the switching frequency changing with power supply voltage.

A good hysteresis value is 1/1000.

Attached is a drawing which I put up earlier somewhere (it could have been this thread), showing a hysteresis amp with mixed feedback. The "comparator" symbol includes the switching stage.

[ChocoHolic]

Hi Bruno,

...great to get your support!

To pick also the positive feedback from the power stage was exactly, what I tried first in an older set up, with a discrete differential and some crosswise full symmetric feedback of the full bridge . But simulation surprised me with some difficulties in proper oscillation, sometimes. It was caused by the neg feedback, which acted so earlier, due to improper delays of the two halfbridges.
Unfortunately the halfbridge which should give the pos feedback acted later than the other one.....
By picking the positive feedback from the comparator oscillation became perfectly reliable, because this shifted the small time delay between positive and negative feedback to the desired direction (comparison level definitely has jumped before the neg feedback can change anything..).
Anyway, I will try your proposal here again. As in this circuit positive and negative feedback are derived by the same halfbridge, I think it should work fine.

Frequency modulation with changing rail voltage:
Is frequency modulation a major concern?
I took care about the neg feedback to be picked from the power stage, because the gain of the power stage is directly proportional to the rail volatge.
But which drawbacks will I face due to some variation in the switching frequency?

Feedback level about 1/1000:
So according your experience my ramping level of several hundrets of millivolts at the input stage is high above the optimum?
You would prefer ramping levels around +/50mV, correct?
Due to possible issues with saturation in the comparator input stage?

Bye
Markus

[classd4sure]

Hello,

I think the problem with frequency modulation lies simply in limiting it to a desired value, so that it doesn't start at say 100khz and wind up at 2Mhz. It's an extrem example but gives you the idea.

To avoid that you have to have the hysteresis set points ride the reference signal, and the control signal as well. That stabalizes the switching frequency a fair amount. It's not easy though, after many many failed attempts, I'm a little afraid of the hysteresis modulator these days. Phase modulation is easier for me to grasp the concept of, there's alot more material out there to be found on phase shift oscillators/circuit examples etc than there are for hysteresis modulators.

I would like to learn more about them as well. Bruno actually emailed me a full circuit schematic of his SODA amplifier over a year ago, and I learnt alot from it, but I couldn't figure out the hysteresis aspect of it, never tried duplicating it either, or match it to what I thought I wanted to achieve (mueta). After having read the mueta patent, it's a little more complicated than I'd first assumed, to say the least.

I sure would like to try getting one going though (hys. modulator, not mueta), call it a grudge at this point.

Regards,
Chris

[phase_accurate]

but I couldn't figure out the hysteresis aspect of it,

It is basically the same circuit one uses when building a rectangular oscillator with an op-amp, improved by an overall feedback loop.

Regards

Charles

[Bruno Putzeys]

Hang on... are you saying you were using one half bridge for the hysteresis and the other one for the feedback? The two half bridges will never operate identically, so you need to use both for the negative feedback (even if you decide to use comparator feedback for hyst.).

Attached is how to make a simple full-bridge self-oscillating amp. This circuit is used in this product (and others in that range).
http://www.bwspeakers.com/index.cfm/...el%20ASW%20CDM

The disadvantage of allowing the switching frequency to change with power supply is not enormous, unless you intend the circuit to operate over a wide supply range. It's just nice that you can have one modulator setting that you don't have to change when making new amps based on the same circuit.

The frequency swing with signal of hysteresis amps is already quite large. Increasing hysteresis will make this situation worse (capacitor charging can no longer be simplified as a linear ramp). If the gain of your amp is small, larger hystereses are not such a big problem, but if you want to have a gain of 30, well...

[ChocoHolic]

....as expected, in my current schematic there are no big issues if I pick
the pos feedback from the power stage. For this, I disconnected R9 from "HB" and wired it to the center tap of the left halfbridge. I adjusted the value more or less according the increased voltage swing to 39k, may be 33k would match best....

I worked fine, but the distorsions doubled....???
May be the starting conditions of the outputfilter does not match anymore to the new slightly higher switching frequency???

Harmonics analysis with simulation is a mess as soon as you reach distorsion levels below 1%.... (just IMHO)...

Bye
Markus

[ChocoHolic]

Yepp, I adjusted the frequency back to it's former value (more or less).
Now my former adjustment of the starting conditions for the capacitors and inductors are matching well again.

THD is even slightly better, than in the first posting. Now: 0.02%


Dear Bruno,
...saw your last posting right now.
Yes, in one of my simulations, I picked pos feedback and neg feedback from different halfbridges..., ... not so good ... I agree...

In my current simulation I am picking pos and neg feedback from the same halfbridge. The second halfbridge is running simply inverted with additional control.
My next step would be to use a differential amp for an additional outer feedback/gain-system. The feedback would be picked from behind the filter, from both bridge tails. I will post the schematic and the results, after finishing.
But I think the THD is already quite good and simulation results seem to be dominated from not perfectly matching start conditions of the capacitors and inductors. In real life the additonal outer loop may be more important.... as the real life full bridge will be not as nice as my simplified model...

By the way: Do you have a special trick for perfect start conditions?
I cannot let run the simulation for thousands of signal periods just in order to get correct steady state values....

Bye
Markus

[ChocoHolic]

...good schematic hint in your attached small gif-image....
Thanks!
Will help me with my older approach...

[Bruno Putzeys]

Originally posted by ChocoHolic
In my current simulation I am picking pos and neg feedback from the same halfbridge. The second halfbridge is running simply inverted with additional control.

You mean the second half bridge is running off the same comparator but the feedback loop doesn't see it? That is just as bad as the previous case.
Try wobbling the power supply to see what I mean. PSRR=0

Originally posted by ChocoHolic
By the way: Do you have a special trick for perfect start conditions?

In the rare instances that I perform time-domain simulations, I do so with an "idealised" power stage. I don't ever simulate complete amps.
I'm a bit bemused by the local hobby in this thread of trying to optimise a class D amp on a spice simulation.

[ChocoHolic]

Hi Bruno!
Glad to hear your opinion on optimizing everything with simulation....
I am 200% with you in this regard!

PSSR=0?
Doesn't the second halfbridge bring a system inherent high PSSR?
As you switch the rail modulation in positive AND negative direction to the load, the low frequency modulation in the load current should be very low... I thought... ???!!!
....hm, I think you might be right.... uhps!
If the feedback is from one end only, then the rail modulation will be given non compensated to the input......


Anyway, I am already simulating your cross symetric proposal.
Apart from some convergence issues (as usual) it was working fine from the very beginnig. I split the ramping cap and put the center tap to ground in order to overcome this....
...before this the differential showed heavy common mode jumping, because the power stages do not always switch perfectly at the same time....

Attached file shows the full symmetric arrangement.


Bye
Markus