High Speed Diodes

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Fred Dieckmann

diyAudio Retiree
Joined 2002
#1
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.

International Rectifier - Page Not Found

The following are also very useful:

IIS 8.5 Detailed Error - 404.0 - Not Found
IIS 8.5 Detailed Error - 404.0 - Not Found
Vishay - Products

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
 
#4
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/appnote_number/849/ln/en

Also an-15 from:
http://www.powerint.com/PDFFiles/
 
#5
The oldfashioned Way

sam9 said:
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?"
Click to expand...


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".:idea:
 
#7
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.
 
#8
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.
 
#10
The capacitors across the diodes...

Hi Sy

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.
Click to expand...

Me to...is one of my prefered tests...nothyng best that "see" whats is going on with realy music signals..
I wonder if anybody is noticing a trend here
Click to expand...

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!!
 
#11
waveforms or spectra which show some kind of significant difference?

Even though the 60Hz driving waveform is slow, when the diode switches on you will have higher speed transient. To make matters worse the peaks of the voltage waveform are clipped and contain high frequency harmonics and RFI components that can couple to the bridges trough parasitic winding capacitance in the transformer.

The article http://www.gensemi.com/appnotespdf/quik108.pdf
refers to an article by Rick Miller with pictures and data in the “Audio Amateur” of Jan ‘94. The spectra were measured with a spectrum analyzer with an RF amplifier and high pass filter between the DC supply the analyzer. very many high end products are designed with high speed soft recovery diodes now since this been know about for quite some time.

Shown is the part of the supply in a McCormack preamp.
 

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#13
McCormack strategy...

MackCormack strategy...for the two schools of thougt been happy.(fast versus regular diodes)...they use boths!!

D3,4,5,6---Fast

D7 ....to....D14 ---regular 1N4007...

So the best of both wolds!😉

We can called it...new hibrid rectification!!!:nod:
 
#15
McCormack strategy

There may be some conventional diode for the Digital volume control interface. I have seen high speed diodes in several McCormack products. Steve was one of the first to do this in commercial products. i think the high speed diodes were from Harris.

Picture of 100 volt Shottkys and paralelled Panasonic FCs in Creek integrated.
 

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#16
Diodes issues...

Yes i see a point in favour of high speed diodes as they have less parasitic capacitance bettwen anode and cathod...and is that quality (low capacity) that is lost when you add a capacitor across the diode...

The low capacity in a diode helps to isolate fom the noisy secundary transformer...and from the noise in the mains( via internal capacity of the transformer)...from the delicat audio circuits!

If you add a capacitor acrss the capacitors...you shunt this noise to the circuit and lost the diodes(high speed) advantage!!
 
#20
Schottky rectifiers turn on fast; if the power supply capacitor has
drained significantly during the half-cycle, then the turn-on surge
will 'ring' the secondary winding. This ringing may be at a lower frequency than the RFI generated internally by the rectifier.
This may be a more significant problem with a power amplifier
under heavy load than a preamp running at a constant and
very modest power level.

I have found that a snubber across the secondary winding can
greatly reduce this ringing; 0.47 uF and 100 ohms is a good starting point for experimenting. It is necessary to tune both
value for best results, and small transformers benefit the most.

A shunt capacitor across the rectifier helps to isolate the RFI
generated by the rectifier itself, but a snubber is better and
the actual values are different from the snubber used on a
transformer secondary.

I've had trouble measuring the RFI from rectifiers, even with a
100 Mhz oscilloscope. A cheap transistor AM band radio might
make a good detector. I haven't tried this.

Of course there should be a good RFI filter on the power line.
 
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