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Old 27th October 2005, 01:23 PM   #61
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Quote:
Originally posted by Christer
Eva,

I agree one should be careful interpreting the simulation results. When I did simulations on this I was mainly interested in the qualitative behaviour, that is, whether such a phenomenon could be provoked at all in a simulation, since it was claimed to exist in reality.
Chris:

You've seen this before, problem is that I can't find the reference in my notebooks to the actual transformer I used, although I am pretty sure that the diode was an MUR860:

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Old 27th October 2005, 03:30 PM   #62
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I think that these pictures contribute to prove that ringing in 50Hz power supplies is not related to diode reverse recovery but just to transformer leakage inductance resonating with diode capacitance, and all diodes have capacitances, particularly schottky ones (altough they tend to be more constant than in conventional diodes).

Note that diode capacitance varies a lot with instantaneous reverse-bias voltage and this may constitute by itself a good high frequency ringing triggering signal.

Concerning snubbers, knowing the ringing frequency F when no snubber is connected, there is a basic rule of thumb that states that the relationship between C and R should be F = 1 / (R*C). So damping ringing in a somewhat optimum way is just a matter of finding the biggest R and smallest C capable of producing reasonable ringing suppresion while satisfiying that formula. At least, it works quite well for SMPS.

Also, one of the things that you learn about ringing when working with SMPS is that the wrong values of snubber R and C may boost ringing sometimes instead of just damping it, so choosing component values randomly is not a good idea at all. (Why audio people is so hesitant to just take an oscilloscope and look at what is actually happening inside their circuits??)
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Old 27th October 2005, 03:34 PM   #63
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Quote:
Originally posted by Eva
(Why audio people is so hesitant to just take an oscilloscope and look at what is actually happening inside their circuits??)
... becuase rather many hasn't got the equipment and/or aren't very interested. It's sufficient if it "sounds good".
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Old 27th October 2005, 03:41 PM   #64
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Or because people who claim there are RF problems from reverse recovery also say it takes RF equipemnt to measure it and it cannot be seen on a scope. Whether they are right, I don't know, but at least this makes it a bit pointless to even try. I did once, failing to see anything on my scope, although some people seem to have succeeded. If I remeber correctly, Till once posted some scope pictures that showed a very different ringing pattern than Jacks, so he succeded to capture something.
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Old 27th October 2005, 05:31 PM   #65
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These captures were taken from the power supply of an actual audio amplifier while playing loud music. The transformer is made with a big EI core that I scrapped from an old B/W TV set from 1960s and whose secondaries I rewound to match my needs. There are two diode bridges, one for each rail, and they are conventional KBPC2506-like (actually old FAGOR FB2506 from 1980s), those big ones with square shape and fast-on connectors. There are two 10.000uF 63V filter capacitors connected in paralell for each rail.

Red trace shows transformer secondary current measured with a 0.022ohm non-inductive resistor soldered directly to the diode bridge terminals, the trace actually shows the voltage drop across the resistor, so each 0.022V are 1 A. Blue trace shows voltage waveform at the diode bridge terminals. The time base and volts/div scale of each capture are shown in its header.

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I think that the pictures are rather self explaining, they show a lot of facts abut 50Hz power supplies and about diode behaviour, like:

- The very high peak to average ratio of the current waveform.
- How the diode current rises and falls gradually.
- How the diode behaves when it's "asked" to stop conducting while driven from a highly inductive source (the downslope after the glitch).
- Some 200Khz ringing, altough the currents involved are negligible.

These details are hardly seen on any simulator. Enjoy it.
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Old 27th October 2005, 05:54 PM   #66
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Eva:

Don't want you to do all the work again -- but did you look at the data when the amp was just idling, or just above the point where the diodes are conducting --

the other thing to try is examining the traces with a low power device (i.e. preamp) where R(Load) is some tens of milliamps.

Jack
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Old 27th October 2005, 06:29 PM   #67
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The amplifier was playing loud music, I have already mentioned it. Furthermore, each capture was done exactly during the period of increased current consumption that follows each bass transient. All captures were done almost at the same peak pulse current level.

I also observed the behaviour of the system as volume was increased starting from idle. The magnitude of the overshoot just when te diode turns off appears to be quite proportional to the peak pulse current. However, the number of times that it rings appears to be constant with load, and the current fall slope when the diode is "asked" to stop conducting is also constant, it does not change with load, it appears to be related only to mains waveform voltage slope, and the leakage inductance of the transformer (plus mains distribution network leakage inductance itself!!!).

I have made more captures. The two first ones show what happens when the amplifier is muted (bias removed). The three last ones show what happens when the amplifier is idle (biased but with no signal applied).

It's obious that things get better as load current is reduced. Concerning the noise that appears on the current trace, it's picked up RF from ambient, it doesn't change at all when the amplifier is phisically disconnected from mains, and it disappears if I short the probe. In this age of movile phones and wireless ADSL internet access it's quite easy to pick remarkable amounts of RF with a small loop antenna.

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Old 27th October 2005, 07:50 PM   #68
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Quote:
Originally posted by Eva


Click the image to open in full size.


- The very high peak to average ratio of the current waveform.
- How the diode current rises and falls gradually.
- How the diode behaves when it's "asked" to stop conducting while driven from a highly inductive source (the downslope after the glitch).
- Some 200Khz ringing, altough the currents involved are negligible.

These details are hardly seen on any simulator. Enjoy it.
This pic is quite close what i saw on scope when measuring that second transformer, oscillations just attenuated bit slower in my tests.

BTW. I got almost identical looking waveforms on spice as they were in my real-word measurements, just add 100k resistor over diode so that it is not ringing forever like in Heinz sims.
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Old 27th October 2005, 09:41 PM   #69
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It is interesting to note, though, that Hagerman, who seems to be considered the authority on snubber calculations (at least on this forum) uses Spice simulations to demonstrate the effect of snubbers and his simulations show turn-off ringing of the type that powerbeckers simulations showed (and that I recall from my own simulations too).
www.hagtech.com/pdf/snubber.pdf
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Old 27th October 2005, 10:48 PM   #70
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The following captures were taken in the same cconditions as the ones from my first post, at more or less the same peak pulse current level and exactly the same circuit.

Some of you may be wondering what happens if a 100nF 100V capacitor is paralelled to the AC terminals of the diode bridge... Well, thiese two captures show the result :

Click the image to open in full size.
Click the image to open in full size.

As it can be seen, the high frequency current transient is almost supressed, but the 200Khz ringing is still here, the capacitor may actually boost the ringing. Well, but what happens if a 47 ohm resistor is connected in series with the 100nF capacitor in order to make a 200Khz snubber according to the F=1/(R*C) formula? These captures show again the result:

Click the image to open in full size.
Click the image to open in full size.

Now, the 200Khz ringing is perfectly suppressed (the snubber formula works!!!) but the high frequency transient suffers much less attenuation. Let's think a bit, aren't there two independent resonance phenomena at two independent frequencies? Of course there are. So let's add a 22nF capacitor directly across the AC terminals of the diode bridge while keeping also the 47 ohm and 100nF snubber. These captures show what happens:


Click the image to open in full size.
Click the image to open in full size.

Oh!! Wonderful!! It damps everything!! (Now try to model this tale of diode stored charge, non-linear capacitances and transformer winding distributed L and C in PSpice )

I have also looked at what happens at low currents: The high frequency glitch becomes unmeasurable, while the 200Khz ringing associated with the 100nF capacitor without resistor is still present. I find these experiments quite funny, because it's a very easy way to prove that most common practices associated to audio circuits are pointless.
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