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Old 20th August 2010, 06:30 AM   #1
liquias is offline liquias  Israel
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Default Schottky diode

Guys,
I am now modifying my CD's power supply. I want to replace the bridge rectifiers with Schottky diodes.

Is 1n5819 good enough (considering voltage and current are correct for the ps), or do i need some other kind of 'fast' diode ?

I have seen posts in which other people asked the same question but never seen any specific answer to this one.
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Old 20th August 2010, 06:51 AM   #2
Mooly is offline Mooly  United Kingdom
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You need to be very sure the PIV rating of the diode will not be exceeded, either in normal use or by transients... schottkys are low (40 volt for the 1N5819).

It's true that ordinary diodes can cause "commutation noise" as they come into and out of conduction on each cycle, but a small cap of around 0.01uf directly across each kills it dead.

There are many "fast" diodes such as the glass passivated "BYV" series,
http://www.datasheetcatalog.org/data...ps/BYV98_2.pdf
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Old 20th August 2010, 07:28 AM   #3
liquias is offline liquias  Israel
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Default What am I missing ?

I am looking at the datasheets of a few kinds of diodes.

If I understand correctly (I'm really not sure if I did...) the noise is generated when the diode is shut down and opens again in the next cycle. i.e. the main concern is getting a diode with a "Vforward" parameter as low as possible (fast recovery).
Unfortunately, this parameter is derived as a factor of "Vreverse" or and current capabilities.

In VYB98 Vforward is 2.3V @ 1A
In a regular 1n4007 Vforward is 0.9v @ 1A
In 1n5819 (1A 40V max) Vforward is 0.6 @1A (fastest recovery out of these three ?)

So, as far as the VYB98 is concerned, if I understood correctly (I'm guessing I didn't) I'm better off with 1n4007 ?

What am I missing ?
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Old 20th August 2010, 10:47 AM   #4
Mooly is offline Mooly  United Kingdom
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You are not missing anything

The trouble with the schottky is it's PIV rating, only you know by studying the circuit whether that rating will be exceeded or not and whether it's suitable. The schottky is the "quietest"

Fast diodes and soft recovery diodes... I tend to think of diodes such as the BYV series or devices such as BY399 etc as high speed/soft recovery types as these are the sort that were often used in power applications in SMPS or line stages of older TV's etc.
There's also the 1N4007GP. In practice these can work well.

An RC snubber across the secondary windings can also be very effective at stopping diode noise.

There's no right and wrong answer... it depends on the application and how you go about implementing it. There is nothing wrong with ordinary diodes correctly used.
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