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Class D Switching Power Amplifiers and Power D/A conversion

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Old 16th January 2005, 10:19 AM   #11
classd4sure is offline classd4sure  Canada
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Those are things to look out for, nice to have them mentioned, but I'm not convinced it's a problem. Every single topology has areas that need careful attention.. "If designed right" applies to everything.

Sonically speaking, it is far superior.

That's why phase oscillation is nice, it is easy to take post filter feedback.

The Fs can vary but it isn't that big a variation, nothing to be concerned with. Clipping can be problematic, but you can easily clamp the input to avoid that.

Fs variation is alot worse for hysteresis modulators, but once again..they can, and should be stabilized just by modulating the hysteresis band.

Why clamp the output, if the load is removed you still have post filter feedback.

When I speak of self oscillating you can be assured I have post filter feedback firmly in mind, and not pre filter or even mixed. We have to learn from what works and improve on that instead of always trying to reinvent the wheel.

Anyway, properly designed, there's still no downside, it has every advantage over other methods.

I did mention the Zappulse though, it has pre filter feedback, load dependant frequency response... it isn't a design I'd base my own on.
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Old 17th January 2005, 09:02 PM   #12
genome is offline genome
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Just found a great educational website of Class D amp.

It looks like a final project report of college students, which is good since the level is closer to beginers' such as me. Also, parts used were quite availible and I believe everyone can dig some replacement from his collections.
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Old 18th January 2005, 05:39 AM   #13
analogspiceman is offline analogspiceman  United States
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Last September there was a thread all about the workings of various self oscillating class d schemes:
Non clocked (e.g. free running, self oscillating) class d

One little known design fact about the phase shift type of d-amp is that the phase shift network must provide 180 degrees of shift at amplifier quiescence and only 90 degrees of shift at when the amplifier output is railed. The usual feedback phase shift scheme is to augment the a dominant pole (90 degrees shift - perhaps from the output inductor) with two identical high frequency real poles at the intended oscillation frequency. These are in the form of RC "noise" filters (45 degrees shift each) placed in the feedback path (one is often right at the comparator input). When the amplifier is nearly railed these RC section must only supply 10 or 20 degrees of additional shift over the constant 90 degrees from the dominant pole, thus, the operating frequency must drop under the nominal RC frequency by a factor of about three.

The obvious way to limit this drop in operating frequency is to increase the rate of phase change of the feedback phase shift network. This can be done by increasing the Q of the phase shift elements (if gain peaking can be tolerated) or by adding in additional low Q, low pass elements (i.e. more RC sections).
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