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Class D Switching Power Amplifiers and Power D/A conversion

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Old 12th March 2005, 06:37 AM   #11
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Quote:
Originally posted by IVX
Hi,
In my experiments UcD show decent clipping behavior.
Why we need the effective mosfet dv/dt control circuit?
Need? That depends. Are the mosfets failing mysteriously? Does the output stage produce excessive EMI?

C5/R14 and C6/R18 limit mosfet turn-on dv/dt by stealing just enough drive current. This has little or no influence on clipping behavior.

Regards -- analog(spiceman)
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Old 12th March 2005, 07:32 AM   #12
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Here is a simulation of the LeapFrog controlled UcD power circuit for a 10kHz square wave at two levels into a 6 ohm load. Current in the upper trace compares actual inductor current to its sensed feedback signal.
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Old 12th March 2005, 07:49 AM   #13
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Here is a simulation of the LeapFrog controlled UcD power circuit for a 10kHz square wave into an open circuit, a 6 ohm load, and a short circuit. Inductor current is shown in the upper trace.
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File Type: gif lf10khz_open_6r_short.gif (15.6 KB, 926 views)
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Old 12th March 2005, 08:02 AM   #14
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Hi,

thank you very much for your effort. I am eagerly awaiting continuation. Would you care to show and comment results (not immediately but sometime along your analysis) to what I think is the most acid test for classD: square wave output of unloaded amplifier. I think that LF must excell in that region too.

Best regards,

Jaka Racman
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Old 12th March 2005, 08:09 AM   #15
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Quote:
Originally posted by analogspiceman
Here is a simulation of the LeapFrog controlled UcD power circuit for a 10kHz square wave into an open circuit, a 6 ohm load, and a short circuit. Inductor current is shown in the upper trace.
For comparison, here is a simulation of the unmodified UcD power circuit for a 10kHz square wave into an open circuit, a 6 ohm load, and a short circuit. Inductor current is shown in the upper trace.

Please note that this result is only based on my guess at the UcD180 schematic as shown earlier in this thread.
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Old 12th March 2005, 08:24 AM   #16
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Hi,

you simulate faster than I type.

Thanks,

Jaka Racman
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Old 12th March 2005, 08:45 AM   #17
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Quote:
Originally posted by Jaka Racman
Hi, thank you very much for your effort. I am eagerly awaiting continuation. Would you care to show and comment results (not immediately but sometime along your analysis) to what I think is the most acid test for classD: square wave output of unloaded amplifier. I think that LF must excel in that region too.

Best regards,
Jaka Racman
Okay, Jaka, just for you, in order to maximize the cleanliness of the no load response, I've turned down the voltage loop gain a notch or so (I also cut back on the limit level for current a tad, too).

Quote:
Originally posted by Jaka Racman
Hi,
you simulate faster than I type.

Thanks,
Jaka Racman
Not really. I'm only using an old 266MHz pentium II (I like it because the kids don't want to use it).

Regards -- analog(spiceman)
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Old 12th March 2005, 11:37 PM   #18
IVX is offline IVX  Russian Federation
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i just now try simulating too, and i've noted worse THD(.2% vs .02% for UcD at the same comparator based) at the low levels .5-1W(fall I-feedback resolution?). However, for mid power up to around clipping THD almost constant (.1%). Clipping itself is very smooth. THD for 1khz vs 10khz almost equal too (AD826 model used).
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Old 13th March 2005, 12:37 AM   #19
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Quote:
Originally posted by IVX
i just now try simulating too, and i've noted worse THD(.2% vs .02% for UcD at the same comparator based) at the low levels .5-1W (fall I-feedback resolution?). However, for mid power up to around clipping THD almost constant (.1%). Clipping itself is very smooth. THD for 1khz vs 10khz almost equal too (AD826 model used).
Hi Ivan,
Thanks for the report. What simulator are you using? (I use LTspice - the best, IMO, and it's free, too!)

I was planning to zip together my simulation files and post them at some point. I have detailed, componnent level models (I've posted gifs of the schematics for those) and idealized models that run faster and are probably better for comparing the basic concepts rather than the strengths of the particular implimentations. The idealized models can also be set up to do ac analyses (closed loop frequency response, loop gain, output impedance) in addition to the usual switching transient type.

Leapfrog depends on sensed current, the quality of which affects the performance (although the outer voltage loop corrects for current sensing anomolies that are within the reach of the voltage loop's lessor loopgain). The cheap and easy current sensing scheme I've shown seems to have lower distortion with higher dc bias. Note that I haven't investigated its sensitivity to component variations or power supply noise, so whether or not it is the best choice remains to be seen.

Also, note that the voltage loop gain of leapfrog can be increased to where it matches UcD (but then the large signal transient response rings just as much). When these gains are the same, THD may be the same (haven't checked yet). Much to do.

Regards -- analogspiceman
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Old 13th March 2005, 08:24 AM   #20
subwo1 is offline subwo1  United States
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Hi analogspiceman, thanks for explaining the leapfrog method well. For example, I was unaware of its usefulness in certain applications. That current sensing circuit is indeed interesting.

I think LTspice is excellent too. It has cool features like the ability to mirror circuit elements ranging from a single component to an entire circuit. Plus, I find its intuitive user interface very helpful.
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