Whats wrong with large filter caps for Gainclone?

[Pedja]

Christer, I actually made these graphs having the transformer (.subckt) in circuit but values used shouldn’t relate to any real transformer. As about the transformers, generally their R is pretty low, L is pretty high but AFAIK you always have some pitfall. As about the diodes, I tried some others in this circuit and yes they do interfere into the look of these graphs. Gen Semi’s models of SB560 Schottky which I in fact use make these graphs looking very nervous (like the left half of the two later graphs). Hence the simulations I posted here should be understood more like the orientation of what happens, no more than that. My impression is that a lot will depend on the actual implementation and used parts. I like SPICE to model the filters, to determine operating points, to estimate the stability and even to estimate the distortion (yes) and some other things as well, but this is the area where I don’t believe entirely that the right thing can be easily made. Finally, my original intention wasn’t to get the insight about the whole circuit but about one partial phenomenon – influence of the capacitance value on the noise generation. If I had success trying to isolate that phenomenon then I done something.

So Jeremy, whatever I posted here, only the measurement will be the right thing. Can’t wait to hear your results.

Pedja

(circuit used for sims below)

[Christer]

Pedja,

My point was not to complain about you not taking the diode
noise into account (and I don't think you understood it that
way either). I just meant to point out that there are several
mechanisms here that contribute to the noise spectrum. One
is the shape of the charge current curve, which you investigated,
and the other is noise from diode shut-off, which depends on
the charge currents, of course. Since the latter is conisdered a
real issue, and not only a theoretical exercise, by many, it should
not be entirely left out of the "equation" when discussing the
capacitor size.

[millwood]

Originally posted by kropf
The next question is what happens when we have a 1kHz or 20kHz signal superimposed on the 120 Hz charging pulses.
Jeremy

that's actually fairly easy to estimate using data we have so far. the voltage drop wouldn't be as much, and there wouldn't be much impact on the charging current.

I did that with a transistor driven by a signal source as load, and in pedja's model the current sources can be modulated.

[millwood]

i put in a small cap cross the diode (0.01u - 0.1u) and it made no material difference to the current's FFT. Now coming to thinking about it, even with a large cap (10,000uf), energy over 5Khz on the diode current is almost zero. so I suppose the RF emission from rectifiers isn't a big deal.

[Pedja]

Originally posted by kropf
The next question is what happens when we have a 1kHz or 20kHz signal superimposed on the 120 Hz charging pulses.

First, again the graph without the superimposed signal (as the first graph in the post # 82) but, for the convenience, here showing somewhat higher bandwidth. 1000uF cap, 0.1 Ohm ESR, 100mA





AC voltage source 10kHz/10mV put in series after the rectifier




With higher drawn current, as the noise level starts to become higher anyway the differences between these two graphs at the HF start to become smaller but, of course, base freqs (10k and first harmonics) of the injected signal remain.

However, note that this is more serious problem than it could seem at the first sight. The simplest answer is: such signal will make problems. But the source of such signal shouldn’t be a voltage source in series after the rectifier, in reality it is rather a current source in parallel with the cap (a second device plugged to the same supply). Anyway, a lot will depend on the cap’s AC properties. And diodes are definitely important here.

Of course, the source of the external noise can be also at the transformer’s primary side, right at the secondary, between the secondary and the rectifier… Seems to me like a huge topic.

Pedja

[JOE DIRT®]

try the test out with a random signal (pink noise)

[janneman]

Originally posted by apassgear


Well said Jan,

From my point of view, i agree with every word, and not taking in consideration pedja's modeling or testing myself, just as an understanding of this efects on my humble mind.

Still, it is nice if ones mental gymnastic is (sort of) confirmed by measurements. But I was wrong on one account. I said that increased load increases the rippel (true) and thus increases hf noise (not true). This is borne out by Pedja's curves (the one were he says: "this cap likes 350mA better than 100mA". I think what I missed was that if you increase load, and increase rippel, the diodes open up earlier in the cycle. The current pulse is flattened and the net effect seems to be LESS hf noise.

Jan Didden

[millwood]

Originally posted by janneman
The current pulse is flattened and the net effect seems to be LESS hf noise.

Jan Didden

that is very correct. In a way, the high peak current on the diode is due to the RC constant mismatch on both sides of the diode: RC constant is higher on the load side of the diode than on the source side of the diode. So if the two match exactly, you will not see any ripple current on the cap.

In this particular case, the output voltage (as applied on the load) will be exactly the same as the input voltage (on the other side of the diode).

[Pedja]

Originally posted by millwood
i put in a small cap cross the diode (0.01u - 0.1u) and it made no material difference to the current's FFT. Now coming to thinking about it, even with a large cap (10,000uf), energy over 5Khz on the diode current is almost zero. so I suppose the RF emission from rectifiers isn't a big deal.

Just a cap across the diode shouldn’t be as effective as you would want. And check the real RF, make a high resolution analysis and see what happens in MHz and GHz range.

Originally posted by millwood
… in pedja's model the current sources can be modulated.

Originally posted by Pedja
… in reality it is rather a current source in parallel with the cap (a second device plugged to the same supply).

Sorry, I overlooked this notice of yours, yes, these two options boil down to the same.

Originally posted by JOE DIRT®
try the test out with a random signal (pink noise)

This can show a general ability of the supply to shunt the noise but this would definitely demand some further (better) modeling of the caps, diodes, other parasitic properties and I don’t have a time to do this now (maybe soon)… Anyway, do you know the model statement for the pink noise?

Pedja

[JoeBob]

With all this talk of capacitor sizes, it makes me wonder if anyone has tried a PI filter, I haven't heard any talk about it.

Adding an L would smooth things out, but are there any obvious downsides I don't know about?