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Old 21st August 2012, 03:05 AM   #741
gootee is offline gootee  United States
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Quote:
Originally Posted by fas42 View Post
Tom, could you post the current version of the test Spice model please, the base version thereof, so that I know exactly what you're using to give the results so far posted; to make sure that I'm in sync with what you're using ...

Thanks,
Frank
Done!
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Old 21st August 2012, 03:36 AM   #742
gootee is offline gootee  United States
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(In the transformer data table, the "654.3" for the 240 VA case should be "854.3".)

Now we can check to see how the simulation data compare to the approximate equations for selecting capacitance based on ripple voltage and average load current. I haven't done that yet but I will, unless someone beats me to it. It might be complicated a little bit by the fact that some of the simulated configurations are more viable than others, in terms of the ratings of the transformer versus the task at hand.

It did become obvious, while watching the simulations run, that the ripple voltage does not have to reach all the way down to the peak signal voltage, in order for the signal to get distorted by the effects of the capacitance being too small.

Last edited by gootee; 21st August 2012 at 03:52 AM.
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Old 21st August 2012, 07:38 AM   #743
AP2 is offline AP2  Italy
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Hello Gootee, interesting work on the simulation,sure is not simple. I have curiosity to see the dynamic behavior. you can drive with burst 1Khz (20ms/20ms) square wave, and show curves on rail vcc (both) and an output load?
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Old 21st August 2012, 09:59 AM   #744
DF96 is offline DF96  England
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Quote:
Originally Posted by gootee
It did become obvious, while watching the simulations run, that the ripple voltage does not have to reach all the way down to the peak signal voltage, in order for the signal to get distorted by the effects of the capacitance being too small.
You have 3 Vbe drops plus the 0.22 emitter resistor. I realise that the Vbe drops could be small but this means the BJTs are in collector saturation mode on peaks which could create problems at high frequencies.

With a zero resistance transformer you would need about 3500uF for 100W at 8ohms. Your simulation shows about twice this. I would expect transformer voltage drop to explain this. If you adjust your model to boost the secondary voltage you will find that a smaller cap is needed.
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Old 21st August 2012, 12:46 PM   #745
gootee is offline gootee  United States
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Originally Posted by AP2 View Post
Hello Gootee, interesting work on the simulation,sure is not simple. I have curiosity to see the dynamic behavior. you can drive with burst 1Khz (20ms/20ms) square wave, and show curves on rail vcc (both) and an output load?
Sure. I will try it this evening.
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Old 21st August 2012, 01:13 PM   #746
gootee is offline gootee  United States
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Originally Posted by DF96 View Post
You have 3 Vbe drops plus the 0.22 emitter resistor. I realise that the Vbe drops could be small but this means the BJTs are in collector saturation mode on peaks which could create problems at high frequencies.

With a zero resistance transformer you would need about 3500uF for 100W at 8ohms. Your simulation shows about twice this. I would expect transformer voltage drop to explain this. If you adjust your model to boost the secondary voltage you will find that a smaller cap is needed.
No. The simulation shows almost exactly what you predict; maybe even slightly less.

If you look at the first plot in the second set of plots (i.e. for 100W, 8 Ohms, 240 VA, "raw"), where there are enough VA available for 100W at 8 Ohms, you will see that. It's likely showing between 3000 and 3500 uF, since we're looking for the minimum, not the last 0.002% THD.

Maybe you only looked at the Cmin in the table, which is not a direct simulation result but rather something that I "derived" from the results. But I mentioned that I did not understand how to pick the Cmin, too well. The best way might be for me to look only at the raw plots, and never look at the zoomed versions. They make me forget the "min" part. <grin>


But yes. As you said earlier, you can trade excess voltage for capacitance. So far these runs are just to try to work toward a model that can predict the minimum needed capacitance for commonly-used power transformer and amplifier-output-spec configurations or combinations, to help Nico find out if there are any "rules of thumb" for reservoir capacitance. It is not even a real amplifier; just a piece torn from the output stage of one of Bob Cordell's models, minus the feedback etc. (Maybe that is another part of this simulation model that should be improved?)

Last edited by gootee; 21st August 2012 at 01:20 PM.
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Old 21st August 2012, 01:27 PM   #747
fas42 is offline fas42  Australia
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Quote:
Originally Posted by gootee View Post
So far these runs are just to try to work toward a model that can predict the minimum needed capacitance for commonly-used power transformer and amplifier-output-spec configurations or combinations, to help Nico find out if there are any "rules of thumb" for reservoir capacitance. It is not even a real amplifier; just a piece torn from the output stage of one of Bob Cordell's models, minus the feedback etc. (Maybe that is another part of this simulation model that should be improved?)
And nicely done too, Tom, .

This can all go quite some way: including feedback path, and the rest of the full amplifier circuit, will make life very interesting, because the complete amplifier has significantly poorer PSRR than just the output chunk. Fun times ahead ...

Frank
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Old 21st August 2012, 02:24 PM   #748
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Tom,

I may have started the thread but just considering the number of member/readers tells me that you are not wasting your time. I bet these guys are all itching for an the outcome so they can all upgrade there systems in order to achieve an example that is closer to perfection and you have been an absolute inspiration.

In my opinion, (and maybe a moderator can check the statistics) this has been one of the most read threads over a period of one month.

I applaud you contributing to this thread because it has been hugely informative and you all have done an excellent job.
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Kindest regards
Nico

Last edited by Nico Ras; 21st August 2012 at 02:40 PM.
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Old 21st August 2012, 02:56 PM   #749
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Originally Posted by gootee View Post
What a coincidence. I just came back to post the rest of it.

Below are the schematic used for the simulations, the main table of initial results, some info about the power supply behavior (because if that's no good then the data might be no good), plots of THD vs reservoir capacitance, and a zip file with the simulation files plus the Excel spreadsheet that has all of the data in it.

Please understand that I used THD here only as a tool (with an incomplete amplifier), only in order to more-easily find the minimum viable capacitance value, during C-value sweeps, to find the point (C value) where the PSU itself was no longer deforming the amplifier's output waveform due to the capacitance being too low. Otherwise, I would have had to try to very-closely inspect the output error plot and the signal output plot. And then there also would have been no real context for the swept capacitance values.

After all of that, I still didn't know exactly how to actually CHOOSE the minimum capacitance, from the data. So I tried a couple of ways:

First, I looked at the THD for a huge capacitance, then backed up until I found a THD that was 1.01 times the THD with the huge capacitance. The capacitance value found that way in labeled "Cmin 1%", in the table.

Second, I just "eyeballed" the plots and tried to pick a minimum value that was past the more-vertical section of the plot, without being too greedy about the THD, which was not changing much anyway, and while trying not to think about the uF per Amp that would result. That process was very difficult to be objective about, especially after expanding the low-level portion of each plot. The minimum capacitance found that way is labeled "Cmin from Plot", in the table, and might be more or less meaningless.

Note, too, that the plots posted as jpgs are not "to scale". The horizontal axes just have all of the data points, in sorted order. I also did some plots with linear scaling. They're in the Excel spradsheet file, in the zip file attachment, along with the actual raw data tables.

The transformer model was fed with about 144% higher voltage than I used when measuring the real transformer for the model, i.e. 173 VRMS instead of 120 VRMS. That way, it has an output level that's more like a 36 VCT transformer's secondary. Maybe I should note that when I originally tested it, I did so with both the primaries and secondaries paralleled, because a) that's how I was using it, and b) the modeling algorithm I had was only for single-primary/secondary transformers. I'm hoping that since it was in that high-current configuration when I measured it, and it was also designed for use with 230 V mains, that maybe the model won't be too far off. Anyway, I posted a little table of transfomer data that I took from the simulation, hoping that one of the transformer experts here will either pronounce it DOA or give a review of its good and bad points. (I will try to make whatever data is needed, for that, with the simulator. The original physical measurements are posted in this thread, in the transformer model schematic, or I can provide them again if that's too difficult to find.)

Oh, in order to be able to sweep the capacitance automatically, I used some equations that I found in a paper somewhere to calculate the ESR and EPR on the fly. ESR = {0.02/(cap_value*Vrating)} and EPR = {1/(0.01*cap_value)}. I used Vrating = 100 Volts. ESL was just set to 10nH.

Now that I have the setup built, it wouldn't be TOO terribly difficult to do it over again, if model changes are needed.

Cheers,

Tom
Thanks Tom. Using these results its possible to see what capacitance is required as well as transformer VA. Using this data we are able to see whats too little and whats too much. If you notice as the VA increases the rail sags less but definately there will be a point where there are diminishing returns. It is interesting that this validates our earlier predictions on expected capacitance and VA.

I am now looking forward to inclusion of decoupling caps in the simulations.
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Old 21st August 2012, 02:59 PM   #750
DF96 is offline DF96  England
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Quote:
Originally Posted by gootee
to help Nico find out if there are any "rules of thumb" for reservoir capacitance.
I thought these were already known, and based on ripple calculations. Your simulations seem to confirm this. A bit of algebra gives the minimum capacitance. Then other issues such as local decoupling and inductance tells you how to split that capacitance. ESR might mean you wish to go a bit higher in cap value, but there seems to be no hard evidence that massive caps bring any advantage. Huge caps and a huge transformer just mean narrower but taller charging pulses so more risk of induction into signal circuits, but this is hard to simulate unless you have full EM wave software.

The rule of thumb would appear to be: calculate the minimum then double it. Then make sure you have good low resistance and low inductance connections, and grounding in the right place.
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