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Old 26th October 2005, 05:10 AM   #41
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Quote:
Originally posted by jackinnj
you can observe the phenomena Jan describes with a decent 'scope using the AC input -- there can be many microvolts of RF ripple which, because of their comparatively short wave-length -- propogate themselves all over the place. it's not just the diode, it's the transformer leakage inductance, interwinding capacitance, etc.

with one very inexpensive, beastly torroid bought off EBay I experienced zero, zip, nada, niente RF ripple on the DC power rails using MUR860's. with another high quality EI transformer from a cannibalized HP meter you could detect the ripple with an RFI sniffer on the HP 3585a spectrum analyzer. it pays to measure.

by the way, Analog Devices has reprinted their amusing tale of ADC problems in the teeth of RFI.
Hi to all

In my opinion jackinnj tell us the most interesting!

So far as I know it`s in the case of a main-rectifier not important how quick the diode switches.

BUT: every diode has a capacity (schottky has a bigger on!) as also a smaller over the secondary windings.

These capacitys get in resonance with the also always present leakage-inductance as soon the current through the diode goes to zero.

As both are small the frequency is high.

Toroids have in contrast to EI`s smaller leakage-inductances so in the resonance-circuit is stored less energy and so the hf soon "disappears".

If you put some damping-R in the resonance-circuit (in form of a snubber over the diode) the hf disappears even quicker.

Without leakage inductances there will be no hf-noise!


Jan

You done already some simulations may be You are so kind to do some more.

For example You can switch 2mH as leakage-inductance before the diode and over the "ideal-diode-model" some 100pF.

You should use small timesteps so it takes a time!

Please observe what happend at the node
leakage-inductances/diode.

You can also try a "real model diode".
What happend with a snubber?

Regards
Heinz!
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Old 26th October 2005, 08:24 AM   #42
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Heinz,

I have earlier done some simulations more or less of the kind you ask for. I haven't saved any data, though, so I can only recapitulate from memory. Originally I was just curious to see if the phenomenon could be produced in a Spice simulation at all. It turned out to be no problem to show turn-off oscillations for diodes. However, not surprisingly there must be inductance in the circuit. I also compared a few of the diode models that come with LTSpice, and while the ordinary (don't know if they were fast recovery, though) diodes could quite a lot of oscillation, the Schottky diode didn't show any oscillation (or so little that it was negligible in comparison with the others. I also did some snubber experiments, but I never tried to calculate optimum values la Hagermann, so I just experimented a bit to see that the snubbers affected the oscillations.

I think you are missing the point about Schottky diodes. It is true that the diode capacitance takes part in the LC tank that causes oscillation. However, there must be an excitation signal to get an oscillation. With ordinary diodes, it is the reverese current at turn off that excites the LC tank. With Schottkys, there is no reverse current, so there is practically no excitation signal. At least that is how I have understood it.
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Old 27th October 2005, 12:45 AM   #43
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snip/Crister
However, there must be an excitation signal to get an oscillation. With ordinary diodes, it is the reverese current at turn off that excites the LC tank. With Schottkys, there is no reverse current, so there is practically no excitation signal. At least that is how I have understood it.

Hi Crister,
thank You for reply.

The "excitation signal" is coming from a sudden revers-VOLTAGE
over the diode when in case of a big leakage-inductance (Ls) the diode-current goes zero.
This voltage-jump charges the capacity in the diode. This charged capacitor then starts to "play" with Ls : hf is there!

But from what came this revers-voltage jump?

If a Ls is present the current through the diode lags after the mainvoltage. When current goes to zero the voltage on the anode goes down to the mainvoltage.
Do of this current-lagging the mainvoltage is now already much smaller so the anode steps down:
this is the excitation signal!

For better clarity I have done some simulations.
I use only the ideal-diode or a On-Schottky from LtSpice.

I think You will agree that in both no revers-current will appear .

Because big attachments are forbidden I must sent some posts.
Here you can see the circuit.
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File Type: jpg test-circuit.jpg (57.7 KB, 1442 views)
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Old 27th October 2005, 12:54 AM   #44
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This is the simulation with a small Ls (200uH) and a perfect diode
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File Type: jpg diode ideal-1pf-ls 200uh.jpg (66.1 KB, 1349 views)
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Old 27th October 2005, 12:59 AM   #45
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Again perfect diode but a bigger Ls (2mH). looks ok.
Red is current blue is the voltage at anode green is the dc on kathode.
Current is lagging, so anode must jump down!
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Old 27th October 2005, 01:04 AM   #46
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Perfect diode but with 100pF and small Ls (200uH). Again no problem!
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Old 27th October 2005, 01:11 AM   #47
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As above but now with Ls 2000uH
(a do a mistake: I write in screen 200uH)......... Hf is there!
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Old 27th October 2005, 01:17 AM   #48
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Now a Schottky MBRS 340 (ON 3A-60V) with Ls 2mH!
Looks not nice!!
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Old 27th October 2005, 01:20 AM   #49
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Here also the MBRS 340 but with small Ls (200uH).
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Old 27th October 2005, 01:24 AM   #50
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Again MBRS 340 with big Ls (2mH), but with a small (not optimal!)
snubber (100pF / 1k): much better!

Regards

Heinz!
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