Small-signal modeling of self-oscillating amplifiers

[sovadk]

Hi

Back in August i wrote the attached rapport on small-signal modelling of self-oscillating amplifiers. By computing the equivalent gain of the hysteretic comparator or just the comparator (for an UCD solution), one can find the transfer function of the system or even more interesting the noise transfer function of the system (Shows how good the system is to suppress errors like distortion). The reason why self-oscillating amplifiers outperform clocked amplifiers is because their equivalent comparator gain is the largest. This results in more gain to suppress errors, or in other words a noise transfer function with a higher bandwidth.

There is also an example of how to make a UCD with a higher order
feedback loop that gives 20dB of extra gain to suppress errors.

http://www.student.dtu.dk/~s042302/d...scillating.pdf

I've also written a second report on self-oscillating amplifiers (it was the first of the two reports I wrote). I deals with the following topics:
Design of passive phase oscillating (UCD) and hysteretic oscillating amplifiers.
Design of additional active poles to increase the loop bandwidth and thereby suppress distortion.
How to get a constant oscillating frequency for both phase and hysteretic self-oscillating amplifiers.
Some layout considerations.
Schematic for a dicrete comparator + gate driver with under 100ns of propagation delay.

http://www.student.dtu.dk/~s042302/d...amp_master.pdf

[TOINO]

[IVX]

sovadk, thank you so much for really interesting papers!

[BWRX]

Originally posted by IVX
sovadk, thank you so much for really interesting papers!

Agreed. Thanks for posting them here for us to check out. I think I have to break out my engineering books to get reacquainted with the math!

[IVX]

Wonderful, that by using one of such approaches, i've got indeed .00075% at 15W with THD/Freq flat (measured in 20-20khz BW, however amp does sound very close to the usual UcD400, so sound aren't automatically better if more zeros after point in the THD specs), or .0003% (maybe less even, sorry -no APsys2) if 2-3Khz THD rising, also same approach used in the 400VDC H-bridge for .0017% at 50-100Hz>600W@230VAC.

[sovadk]

Thanks TOINO, IVX and BWRX

Originally posted by IVX
Wonderful, that by using one of such approaches, i've got indeed .00075%....

The feedback approach or what are you reffering to?

In the "Small-signal modelling of self-oscillating
switch-mode amplifiers" report I cite a paper by Lars Risboe. I've attached it here if some of you are interested. He explains very well how to model the small-signal properties of a comparator.

[sovadk]

Part 2.

[analogspiceman]

A proper review of your papers would require more time than I've been able to find so far (but I do plan to make a proper response eventually). -- a.s.

[Eva]

Sovadk thank you very much for publishing your papers here, they are among the most interesting stuff that I have ever found in this forum.

[sovadk]

I appreciate your interest.

I've found out that self-oscillating converters (audio amplifiers or power supplies) also goes under the name sliding mode regulators. I've found several papers about sliding mode control, that among others describe self-oscillating converters with other power stages than a buck half- or full-bridge. This is very interesting since
it suddenly turns out that there's much more information out there, than I thought previously.
Searching google or even better IEEE.explorer with "Sliding mode control for switched mode power supplies" or something similar, really gives a lot of interesting hits. http://www.hait.ac.il/jse/B/vol0236B/jse15.pdf is one example.

Does any of you know something about sliding mode control?