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[LTSpice] Beginner - help with capacitor multiplier?
[LTSpice] Beginner - help with capacitor multiplier?
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Old 3rd November 2016, 10:09 AM   #1
bugbear is offline bugbear  United Kingdom
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Default [LTSpice] Beginner - help with capacitor multiplier?

Background;

My actual purpose is to make an external PSU for the HiFiBerry DAC+ Pro.

Since this point is upstream from a rather good onboard regulator (ADP150), I think it makes sense to have a good, clean PSU, with excellent filtering, but no regulation; the ADP150 can deal with the low frequency ripple. We need 5V, 100mA.

I have very little electronic design or build background.

So (in my first foray into LTSpice) I tried to simulate a basic reservoir/filter cap followed by a capacitor multiplier.

This worked beautifully under AC analysis, dropping 1V of ripple to 10uV at 1KHz, and 1uV by 10KHz, where it stays. Lovely.

But this was with a "npn" in LTSpice. As soon as I try a modelled part from the library, the ripple goes no lower than 2uV.

Can anyone tell me what property of a "real" transistor is causing this?

Clearly, there's a progression from "simulated ideal components" to "simulated real component" to actual build, but I'd like to clear the second hurdle before I try the third.

(circuit attached)

BugBear
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File Type: asc psu_mult_filt.asc (2.4 KB, 39 views)
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Old 3rd November 2016, 10:25 AM   #2
AndrewT is offline AndrewT  Scotland
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possibly capacitance.
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Old 3rd November 2016, 11:38 AM   #3
Mooly is offline Mooly  United Kingdom
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[LTSpice] Beginner - help with capacitor multiplier?
A couple of things here. Firstly your ripple of a sine wave superimposed on DC bears no resemblance to real ripple that has a reservoir cap quickly charging and then slowly discharging between each cycle.

We can fix that by simply changing the voltage source to be a pulse of suitable rise and fall times although its now up to us to add the amount of ripple... guesswork.

We could also use a bridge rectifier and an AC source. This is much more accurate. Also, for simple simulations like this try setting the run time to be much longer. Here I have it set to 10 seconds and to start saving data after 9.92 seconds. That lets things settle down, just as they would in a real circuit.

The ripple is now around 5 mv pk/pk which seems reasonable. Have a play with these two versions and look at the settings for the voltage source in the first example.
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File Type: asc psu_mult_filt rev 1.asc (2.3 KB, 37 views)
File Type: asc psu_mult_filt rev 2.asc (3.0 KB, 32 views)
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Old 3rd November 2016, 12:34 PM   #4
bugbear is offline bugbear  United Kingdom
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Quote:
Originally Posted by Mooly View Post
A couple of things here. Firstly your ripple of a sine wave superimposed on DC bears no resemblance to real ripple that has a reservoir cap quickly charging and then slowly discharging between each cycle.

We can fix that by simply changing the voltage source to be a pulse of suitable rise and fall times although its now up to us to add the amount of ripple... guesswork.
Very neat; thank you.

Quote:
We could also use a bridge rectifier and an AC source. This is much more accurate. Also, for simple simulations like this try setting the run time to be much longer. Here I have it set to 10 seconds and to start saving data after 9.92 seconds. That lets things settle down, just as they would in a real circuit.
That all seems most reasonable.
Quote:

The ripple is now around 5 mv pk/pk which seems reasonable. Have a play with these two versions and look at the settings for the voltage source in the first example.
How did you measure that ripple? I can see a nice flattish trace, but can't quantify it.

But, more importantly, on the AC Analysis, my 1V "trash" source doesn't appear to escape the diodes.

Even vRect has AC at 1e-27V !!

In my original model, the AC source called "ripple" serves the dual purpose of representing actual PSU ripple from a bridge rect, but also representing (in a sort of FFT sense) the trash and noise a real transformer/diode system spews out.

BugBear
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Old 3rd November 2016, 12:42 PM   #5
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by bugbear View Post

Can anyone tell me what property of a "real" transistor is causing this?
The default NPN has the parameter Vaf=∞, which in practice means no Early effect.
Since it is the dominant source of imperfection in a cap mult application, this only leaves very minor degradations, hence the good (but impossible) performance
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Old 3rd November 2016, 12:42 PM   #6
Mooly is offline Mooly  United Kingdom
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[LTSpice] Beginner - help with capacitor multiplier?
I measured the ripple on the first revised sim by just probing the output such that you get to see the full value displayed.

I just want to try something on the other one.
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File Type: png Ripple.png (126.2 KB, 200 views)
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Old 3rd November 2016, 12:44 PM   #7
Mooly is offline Mooly  United Kingdom
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[LTSpice] Beginner - help with capacitor multiplier?
^ Thanks Elvee. I didn't know that was what caused the default model to vary so much.
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Old 3rd November 2016, 12:45 PM   #8
bugbear is offline bugbear  United Kingdom
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Quote:
Originally Posted by Elvee View Post
The default NPN has the parameter Vaf=∞, which in practice means no Early effect.
Since it is the dominant source of imperfection in a cap mult application, this only leaves very minor degradations, hence the good (but impossible) performance
I understand very little of that, but I'm sure google will help me.

Thank you.

EDIT; https://en.wikipedia.org/wiki/Early_effect

That's way above my pay grade.

BugBear

Last edited by bugbear; 3rd November 2016 at 12:52 PM.
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Old 3rd November 2016, 01:01 PM   #9
Mooly is offline Mooly  United Kingdom
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[LTSpice] Beginner - help with capacitor multiplier?
Quote:
Originally Posted by Mooly View Post
I just want to try something on the other one.
Didn't work

This does though You could rig up the AC voltage source + bridge to determine the 'true ripple' of your circuit and then feed those values into the simulated ripple source. That seems to overcome the problem of the simulation constantly settling to a final value. Even running for 100 secs still leaves the output voltage with a definite falling value making it hard to estimate the ripple.

Also try this.

Now I can't do a screen shot but here is what you do.

1/ Run the sim and probe the incoming ripple and the final ripple. As you mentioned, the final ripple is just a straight line, no resolution is possible.

2/ Place your cursor over the straight final ripple somewhere over at the left hand side and holding a left click drag a box over the whole trace.

3/ Keep doing that and you will go from this to this.
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File Type: png R3.PNG (54.4 KB, 192 views)
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Old 3rd November 2016, 01:25 PM   #10
AndrewT is offline AndrewT  Scotland
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Alternatively click on the margin where the vertical scale is placed.
That opens a dialogue box to allow you to manually set the scale limits.
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