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Old 16th March 2003, 02:49 PM   #1
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Default High Speed Diodes

Since there has been a lot of discussion about the merit of high speed diodes, I thought that the subject deseved it's own thread. The following application notes gives an excellent overview of the design parameters for high speed soft recovery diodes and how they can introduce RF noise into an audio circuit. Don't get freaked out by the semiconductor physics but pay attention to the definition of terms such as reverse recovery time and recovery softness. Appendix 1 is very useful for understanding how to compare parameters of diodes from different manufactures.

http://www.irf.com/technical-info/wh.../murdiodes.pdf

The following are also very useful:

http://www.ixys.com/t03232ka.pdf
http://www.ixys.com/t052599a.pdf
http://www.gensemi.com/appnotespdf/quik108.pdf

Grey would have a fatal stroke over me posting references this technical, but he does not seem to be around lately. You don't need to understand all of the information in articles to get some very useful imformation out of them. Required reading for power supply perfectionist in my opinion.

Have fun,
F.R.E.D.*

* fast recovery epitaxial diode
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Old 16th March 2003, 03:24 PM   #2
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Default Also -- another link

Linear Technology discusses EMI/RFI from fast acting diodes in Application Note 70 -- note at www.linear.com you can search the apnotes numerically by going into "Publications"
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Old 16th March 2003, 03:35 PM   #3
sam9 is offline sam9  United States
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To the extent I can follow these paoers, I keep hearing a voice in my head ask: "But doesn't an old fashoined bridge rectifier with RC snubbers do about as well?"
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Old 16th March 2003, 03:59 PM   #4
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Default EMI/RFI from fast acting diodes

Thanks jackinnj. I figured the other kids would come out and play
on this thread and I don't think read this one.


http://www.linear.com/pdf/an70.pdf

"doesn't an old fashoined bridge rectifier with RC snubbers do about as well?"

I don't think so. There is no reason you can't use snubbers with the FREDs also which is kind of implied in the references. Another good one that discusses snubbers is:

http://www.maxim-ic.com/appnotes.cfm...mber/849/ln/en

Also an-15 from:
http://www.powerint.com/PDFFiles/
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Old 16th March 2003, 04:10 PM   #5
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Default The oldfashioned Way

Quote:
Originally posted by sam9
To the extent I can follow these paoers, I keep hearing a voice in my head ask: "But doesn't an old fashoined bridge rectifier with RC snubbers do about as well?"

Hi sam9
I replaced all 1N4002 powersupplydiodes in my Philips CD-650 by 11DQ10 Schottky's and I removed all snubber caps across the diodes. This includes the diodes for the +32V for the display.
It made a dramatic difference to the sound. Voices became much less screamy. All music became more "musical".
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Old 16th March 2003, 04:17 PM   #6
Wombat is offline Wombat  Germany
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alfsch did some tests here and found the BA159 to perform surprisingly good.

replacement diodes for 1N4007

They are easy to get here and are cheap.
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Old 16th March 2003, 04:26 PM   #7
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Default Schottky's

Schottky diodes are very fast and can create lots of RFI as well.
Snubbers without resistors tend to move the ringing to another frequency and can create an even more troublesome undamped ringing. This was not meant as a snub to your post Elso, but as a snubber post to you.*

For some infor on snubber design:
http://www.hagtech.com/pdf/snubber.pdf

I wonder if anybody is noticing a trend here?

Later,
Fred

* I believe that might qualify as a pun but I guess I will be forgiven....... eventually.
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Old 16th March 2003, 04:44 PM   #8
SY is offline SY  United States
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When building an amp, I look at what's on the DC rails with a scope and a coupling cap while signals (music or test, depending) are running through it. I can't ever recall seeing artifacts attributable to the use of slow diodes (like 4007) actually appearing on the PS rails. For 50/60 Hz power supplies, I wonder if it's worthwhile to go high-speed. Has anyone got DC rail waveforms or spectra which show some kind of significant difference? If there really is something there, I'd certainly upgrade the 4007oids I usually use.
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Old 16th March 2003, 04:49 PM   #9
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If snubbed, What values of R's and C's do you suggest?

Chris
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Old 16th March 2003, 04:54 PM   #10
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Default The capacitors across the diodes...

Hi Sy

Quote:
When building an amp, I look at what's on the DC rails with a scope and a coupling cap while signals (music or test, depending) are running through it. I can't ever recall seeing artifacts attributable to the use of slow diodes (like 4007) actually appearing on the PS rails.
Me to...is one of my prefered tests...nothyng best that "see" whats is going on with realy music signals..
Quote:
I wonder if anybody is noticing a trend here
When one experience is the oposite of whats been discussed ...!!!

For the record...my experience is that the capacitor across the rectifier diodes allways degrad perfomance...sorry!!
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