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Old 3rd March 2011, 11:50 AM   #171
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It is highly unlikely a 1uF cap will cause oscillation, when the 100uF cap (already in place) does not.
Like I said, the supply is stable now. Correcting the compensation through the feedback loop did it.
All of this, and other design specifics, were presented in detail earlier in the thread.
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Old 5th March 2011, 12:11 PM   #172
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Film caps have MUCH lower ESR than electrolytics. Generally for electrolytics, the more F the lower ESR. The higher V the higher ESR. The larger size the lower ESR.

Having a very low ESR capacitor at the output has the effect of disconnecting the feedback loop because the output voltage no longer responds directly to a change in load or drive.

A 100uF lytic will generally have ESR>.1R. This is usually not lerge enough to goof up any regular. A film cap on the other hand has say ESR<.001R. This greatly increases the Q of any reactive network including this cap.

It doesn't sound as if you have actually tried testing the regulator with film caps. What puzzles me then is why you show a .3R resistor in series with C12. I could reason this is ESR, but I don't see it on any other cap, for instance the 100uF cap where it might be more important. If this was a film cap, then I can understand giving it ESR if it helps make the regulator stable. But if this cap needs ESR, then I don't suppose the regulator is stable into film caps.

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Old 5th March 2011, 01:05 PM   #173
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Quote:
Originally Posted by keantoken View Post
Having a very low ESR capacitor at the output has the effect of disconnecting the feedback loop because the output voltage no longer responds directly to a change in load or drive.


It doesn't sound as if you have actually tried testing the regulator with film caps. What puzzles me then is why you show a .3R resistor in series with C12. I could reason this is ESR, but I don't see it on any other cap, for instance the 100uF cap where it might be more important. If this was a film cap, then I can understand giving it ESR if it helps make the regulator stable. But if this cap needs ESR, then I don't suppose the regulator is stable into film caps.
Kean,
Your first quoted statement above is incorrect.
It's plain on the schematic and the photos of the PCB that there is a film cap in place on the output - the 10nF (this, along with the 1uf in series with 300m ohm (revised to 4.7 ohms)) keep the output impedance low when the loop gain is falling off.
Would it bring you any satisfaction if I were to put a 1uF film cap across the output and photograph the results?
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Old 5th March 2011, 01:22 PM   #174
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Quote:
Originally Posted by MJL21193 View Post
Would it bring you any satisfaction if I were to put a 1uF film cap across the output and photograph the results?
Here it is:
1uF cap across the output. Supply at 15VDC drawing ~1.3A:

_0247.JPG

Overall view, showing scope output:

_0250.JPG

Close up of scope:

_0251.JPG

No signs of instability. The regulator is still working extremely well to maintain voltage and kill ripple (as clearly seen in the scope shot).

I hope this answers some of your questions.
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Old 5th March 2011, 01:33 PM   #175
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My satisfaction is no prerequisite, I was only pointing out something you may want to consider and test. 10nF is not always large enough to cause oscillation. A test that would satisfy me is connecting a 1uF film cap between the terminals and testing with a square wave.

If my first statement is incorrect, I would like to know why.

I understand now that the 4.7R+1uF network is a zobel network. However if your intent is to keep output impedance reasonable at HF, hasn't the 100uF cap accomplished this? It should have lower impedance at HF than the 4.7R+1uF network. Is it possible that the increase from 300mR to 4.7R helped performance because it increased the total ESR at output?

EDIT: You posted before I did! Looks fine to me...

- keantoken

Last edited by keantoken; 5th March 2011 at 01:36 PM.
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Old 5th March 2011, 02:02 PM   #176
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Quote:
Originally Posted by keantoken View Post
A test that would satisfy me is connecting a 1uF film cap between the terminals and testing with a square wave.
You mean the transient response test? That takes time to set up and I have other things to do at the moment. One of those is testing a new amp that is being powered by this supply. Pumping 10khz squarewaves through the amp near clipping has no effect whatsoever on the supply stability - that should be a reasonable test.

In all fairness and in the name of good scientific procedure, I disclose that the above scope shot was with the probe at 10x (not intentionally).
I redid the same load test, and here is the scope with the probe at 1x:

_0254.JPG

Still real clean with no discernible ripple or oscillation.
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