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Old 18th August 2008, 11:02 PM   #1
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Default Schottky & Co.

(The thread name sounds like an old Kipling novel... HI hi...) Ok: I'm just learning how components works, yes I'm studying and I'm afraid that the 'diode moment' is arrived... Well, I understand what they are doing in the dark belly of my amp, but I would like to replace them with some better Schottky or something fast recovery or soft recovery or not recovery at all
These are the original parts:
BAW62
IN60
IN4002.
I bought some Schottky 11DQ10 to replace IN4002. Which I could choose to replace the other two?
Thank you so much for your comprehension
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Old 18th August 2008, 11:18 PM   #2
sam9 is offline sam9  United States
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What is the function of the diode(s) you want to replace. If they are used to bias the base of a transistor (~0.7V), then a Schottky won't work because the drop across the junction is different.
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Old 19th August 2008, 07:45 AM   #3
Mooly is offline Mooly  United Kingdom
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Soft recovery diodes are often used in switch mode PSU's that run at many KHz and also TV line output stages etc. An ordinary diode just can't turn on and off quickly enough here.
In a linear PSU (mains transformer 50/60 Hz) some swear by shottkys and soft recovery diodes, but you need to be careful.
The BAW62 is I think only rated at half an amp or so max peak current, and this is a true high speed device. The IN4002 is a standard 1 amp rectifier as used in PSU's. Shottky devices are based on an entirely different principle of operation and have drawbacks. Then there are the "Glass passivated" types.
A good mod is just to add 0.01 mfd caps directly across the 4 diodes in a bridge. If you swap diodes you need to consider the P.I.V. and both the max repetitive and non repetitive currents that the device will see.
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Old 19th August 2008, 11:59 AM   #4
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Thank you for your explanation.

Now I've found this advice:

"upgrade BAW62 (both preamp and poweramp) to fast diodes with slow recovery characteristic (Onsemi UG4D)".

Could it works?
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Old 19th August 2008, 12:47 PM   #5
Mooly is offline Mooly  United Kingdom
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You mean soft recovery . This is a Glass Passivated device as I mentioned earlier. You would see these in high frequency PSU's etc used as efficiency diodes.
You really need to know what the diodes in the amp do before you can think about replacing them and that really means studying the circuit. Not what you wanted to hear is it
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Old 19th August 2008, 01:01 PM   #6
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Ok. Thank you so much. I'll definitely do this way.
Cheers
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Old 23rd August 2008, 09:43 PM   #7
Gubbe is offline Gubbe  Sweden
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Hi Mooly!
You seem to know a lot about rectifier diodes, I have been looking the most low noise diodes I can find, but I think that the parameters are contradicting, som say fast are good, but then there are some that say that to fast cause ringing. Can you recommend some good diodes for rectification? I need 1.2A @ 20V and 50mA @ 500V.
I have looked into the SONIC range and they seem good, but a little to expensive, have you any experience whit these or other diodes designed for low EMI?
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Old 24th August 2008, 06:53 AM   #8
Mooly is offline Mooly  United Kingdom
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Hi,
Well, no easy answer in that many will disagree with what I say
High speed, soft recovery diodes have a very specific use. If you want to rectify the AC from a mains transformer just think what that voltage looks like. It's a low frequency sine wave.
A high speed diode is used for example to rectify the voltage from the winding on a switch mode PSU or a line output transformer. This may be anything from a squarewave ( with very fast rise/fall times ) to something little more than a short duration high voltage "pulse" at anything up to 100 khz or so. An ordinary diode would literally burn up as the switching losses would be so high.
There are no such problems with a 50/60 sine wave.
So on that point there is no advantage in using anything other than ordinary diodes.
Now think of a half wave rectifier ( a bridge is just the same ) and a single smoothing cap. The cap is charged to the peak value of the incoming AC ( less diode drop ). Now put a load across the cap. The diode "tops up" the cap every half cycle, but it only comes into conduction when the rising part of the AC equals the DC voltage on the cap. At this point, some diodes can generate interference -- just for a few 10's of micro seconds -- a lot of which depends on the characteristics of the transformer, inductance of wiring etc.
It's easy to avoid this completely, just add a 10 nf cap across each diode and possibly a snubber network over the secondaries. Would using high speed diodes be better ? On their own, maybe, but adding the caps just totally swamps the effect and in my opinion is the best option.
So what to recommend. Nothing exotic for sure 1.2 A at 20 Volt -- but whats the surge current, what's the repetitve peak current the diode will see, mostly this depends on the value of the caps it is working into. For the 500 volt supply I probably would use something like a BY399 etc.
Just remember when it comes to PSU's layout and grounding are the most important factors.

Edit, actually it's a pretty good all rounder http://www.datasheetcatalog.org/data.../118792_DS.pdf
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