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Old 24th July 2012, 10:20 PM   #281
DF96 is offline DF96  England
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Quote:
Originally Posted by MagicBox
looking at the sensitivity of the last two scope images they are both 200mV/Div
True, I hadn't spotted that. Another fine theory comes crashing down!
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Old 25th July 2012, 03:36 AM   #282
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Quote:
Originally Posted by MagicBox View Post
Just using electrolytes is not sufficient, they are too slow to supply the initial current, but a fraction after T=0 they do start to show their significance. (avoiding the L of the supply wire). No, these electrolyte buffers always need an MKT film cap of about 100nF and if you want to go for the best parallel it with a ceramic of around 100pF.

You people must not forgot that it is the audio signal that is pulling this 'instant' current through the output devices. No; it is the closed-loop HF regulation frequency that must be supplied instant current through local caps.
Yes. Totally agree! Howver, paralleling electrolytics with film is often risky, due to the possibility of forming resonant LC tanks. Each case will need to be looked at for resonance issues. That having been said, I would often like to try to add up to about 1 uF of good film, there, right at the load.

A small cap is also needed there for HF bypassing, to short-circuit high frequencies to ground, because there is a "hidden" HF feedback path through the power rail, for most transistor amp circuits.

Also, I hope everyone realizes that paralleling the electrolytics with small film caps, here, is meant to be only right at the active load device. Doing the same thing at the reservoir caps is a bad idea and can't do anything good, anyway, if there is more than about an inch or two of rail conductor before the load.
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Old 25th July 2012, 03:50 AM   #283
fas42 is offline fas42  Australia
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Quote:
Originally Posted by gootee View Post
Yes. Totally agree! Howver, paralleling electrolytics with film is often risky, due to the possibility of forming resonant LC tanks. Each case will need to be looked at for resonance issues. That having been said, I would often like to try to add up to about 1 uF of good film, there, right at the load.
Electro's with film is not a problem, but film in parallel with film can be. Electro's ESR is too high to create issues, but the high Q of films will cause peaks if the values are wrongly picked.

One rule of thumb is that film values very close to each, adjacent in the standard range are fine; and also if spaced by about 50 times or more. The latter is the crucial factor to keep in mind.

So:

2.2nF || 2.7nF || 3.3nF - good
2.2nF || 120nF || 1.2uF - also good

but ...

2.2nF || 10nF || 180nF - bad!!!

Frank
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Old 25th July 2012, 04:22 AM   #284
gootee is offline gootee  United States
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Quote:
Originally Posted by MagicBox
I don't know if it would be neglible when talking the size of currents in a decent amp. The L could be just high enough to 'choke' the T=0 current and prohibit initial demand. I was about to put a simulation together of a full rectified supply with buffercaps and ESR simulation, along with the chopper circuit to view the transient response. I wanted to model the supply wire in there too.
The conductor self-inductance, for ANY length of conductor, WILL always prevent supplying a current at the correct time. The only question is how bad the error will be.

Quote:
Originally Posted by tsiros View Post
i doubt a 200nH inductance would have any significance

current of what size are we talking about here?

What?! 200 nH of inductance is extremely significant and should probably be the main point of much of this whole thread! That's the whole reason that the decoupling caps are so important (and the reservoir caps less so)!

Of course, if you don't mind blurred edges and a poor soundstage image, and spiky signal voltages on your rails, then don't worry about the rail inductance.

But if the goal is not high-fidelity audio reproduction, here, then why even bother discussing this at all?

Thinking only about the total reservoir capacitance will never get us there.

Inductance causes delays in current delivery, ruining the all-important transient response.

Inductance also causes time-varying currents to induce voltages across it, polluting our power and ground rails.

Even a very-small-amplitude current, if varying quickly-enough, will induce a very large voltage, across a small inductance. (The amplitude of the induced voltage does not depend on the amplitude of the time-varying current, at all, only on its rate-of-change vs time and the inductance.)

Our power supply is (or should be) all about accurately providing large, high-precision, time-varying currents, while trying to keep the voltage rails at a constant voltage. Conductor self-inductance is one of our main enemies!

The power rail conductor's inductance means that the voltage at the PSU output is NOT the same as the voltage anywhere else on the power rail, and the voltage at the ground of the PSU caps is not the same as the voltage anywhere else on the ground rails. (Those are "bad things".)

For the fastest transient signal and HF loop currents that we must be able to provide, exactly when and as demanded, anything more than about 10 to 15 nH (and often much less) means that we FAIL.
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Old 25th July 2012, 04:30 AM   #285
gootee is offline gootee  United States
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Quote:
Originally Posted by fas42 View Post
Electro's with film is not a problem, but film in parallel with film can be. Electro's ESR is too high to create issues, but the high Q of films will cause peaks if the values are wrongly picked.

One rule of thumb is that film values very close to each, adjacent in the standard range are fine; and also if spaced by about 50 times or more. The latter is the crucial factor to keep in mind.

So:

2.2nF || 2.7nF || 3.3nF - good
2.2nF || 120nF || 1.2uF - also good

but ...

2.2nF || 10nF || 180nF - bad!!!

Frank
Frank,

It has already been shown to be a problem, many times by many people. I don't feel like debating it again. See the thread on "Paralleling Film Caps with Electrolytics". Also, note that electrolytics' ESRs get very tiny at high frequencies, and they actually turn inductive instead of capacitive at high frequencies when viewed on a network analyzer, and HF is where the resonance problems occur.

The film-cap value-spacing thing looks interesting, though.

Cheers,

Tom

Last edited by gootee; 25th July 2012 at 04:36 AM.
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Old 25th July 2012, 04:51 AM   #286
gootee is offline gootee  United States
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Just did a quick LTSpice simulation of a 40V linear power supply with a 14000 uF reservoir cap and an 8 Ohm load, with no ESR and with 0.1 Ohm ESR. The attached ripple voltage plot shows the effect. It's due to ESR.
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File Type: jpg ripple_w_wo_esr.jpg (164.9 KB, 364 views)

Last edited by gootee; 25th July 2012 at 04:54 AM.
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Old 25th July 2012, 04:55 AM   #287
MiiB is online now MiiB  Denmark
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This with the inductance was why I designed my PCB with an El-cap+small decoupling cap right at each output device...

I have some 100V 220uF copper infused aluminium/polypropylene capacitors that I use in my speakers, could be that I should try a few of those as the final bank, they have for their size extremely low ESR as they are designed with copper-metalized end-caps and welded terminals.
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Old 25th July 2012, 04:58 AM   #288
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Quote:
Originally Posted by gootee View Post
Just did a quick LTSpice simulation of a 40V linear power supply with a 14000 uF reservoir cap and an 8 Ohm load, with no ESR and with 0.1 Ohm ESR. The attached ripple voltage plot shows the effect. It's due to ESR.
Nice work !
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Old 25th July 2012, 05:16 AM   #289
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Quote:
Originally Posted by gootee View Post
It has already been shown to be a problem, many times by many people. I don't feel like debating it again. See the thread on "Paralleling Film Caps with Electrolytics". Also, note that electrolytics' ESRs get very tiny at high frequencies, and they actually turn inductive instead of capacitive at high frequencies when viewed on a network analyzer, and HF is where the resonance problems occur.
Bit of a thread to digest, so I'll wind up my energy to chomp my way through it later on. At a quick glance, I can see that people get into trouble by ignoring the lead and connections' inductances; this will kill any benefit of bypassing, possibly make it worse, unless it's done precisely where it's needed. Rule of thumb, the smaller the cap the closer it has to be where the HF noise is the actual problem.

In general, ESR is relatively constant over the range that the cap deals with, it's the impedance that starts capacitive, momentarily becomes resistive, and then remains forever inductive, with rising frequency.

Frank
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Old 25th July 2012, 05:47 AM   #290
gootee is offline gootee  United States
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Quote:
Originally Posted by MagicBox View Post
I don't know if it would be neglible when talking the size of currents in a decent amp. The L could be just high enough to 'choke' the T=0 current and prohibit initial demand. I was about to put a simulation together of a full rectified supply with buffercaps and ESR simulation, along with the chopper circuit to view the transient response. I wanted to model the supply wire in there too.
If you're using LTSpice you could save some time and download some of the ones I've already done.

Make sure that you also model the inductance in the caps, and both the ESR and ESL in the supply line, and in all of the connections if you use parallel caps. For traces or wires, most people use estimates of 1 mOhm and 25 nH per inch (1 nH per millimeter) of conductor.

If you parameterize both of those values in an LTSpice inductor, as equations based on a length parameter, it makes it easy to change the length, and then you can also do mutliple runs that automatically step through ranges of lengths, etc.

For the chopper, there are several ways to do it. One simple way is to just use a current source with pulse parameters, pulling through the output resistor. A voltage source actually works, too, since it "holds off" the power supply voltage. Or you can use an actual device, as you suggested. It's more realistic but not quite as easy to interpret the results.

I have a downloadable LTSpice model of a linear PSU that doesn't include many of the conductor parasitics, but does include a "real" transformer model (not just coupled inductors), at Spice Component and Circuit Modeling and Simulation . (There is also one there with no transformer model, which might run faster.)

You can just chop out the regulators and the soft-start circuits and probably one of the rails entirely, and add the parasitics needed.

There is another model , of just parallel caps and rails, downloadable from the post at paralleling film caps with electrolytic caps .

NOTE that I accidentally used 15 nH per inch for the ESL of the conductors, which needs to be changed to 25 nH per inch, everywhere.

One big problem is that the ESR of electrolytic capacitors changes dramatically with frequency, which is difficult to model with LTSpice. There are equations on my psu model's scheamtic for changing the ESR for different frequencies, but it's a pain, and there's still no way to accurately model ESR for signals with wide spectral content, like fast edges. However, Cornell Dubilier does have a Java applet that automatically creates downloadable spice models for (their) electrolytic capacitors, that include the frequency and temperature dependence of the ESR! It also just plots these characteristics for you, if you're not into spice. The link is at

paralleling film caps with electrolytic caps

but also read through the next page or so.

That whole thread is really good but THIS is an interesting post, and is also relevant to our reservoir caps:

paralleling film caps with electrolytic caps

Cheers,

Tom
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