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Old 10-19-2013, 06:20 AM   #1
Fabie
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Default technical question: schottky diodes in 5v and 3.3v versus schottky diodes in 12v line

I noted that all ATX PSUS from all manufacturer uses fast schottky diodes for the 5v and 3.3v , but on the 12v they use 1 or some inferior diodes

for example for the 5v and 3.3 volts, generally they use schottky stps3045 or mbr2045ct, 30A40V, 30A30V etc ( very low forward voltage) generally 0.45 or 0.55

but for the 12v volts line they use schottky diodes which technically are inferior( high forward voltage) generally 0.85 or 0.90
examples they use MBR20100CT ,S30c100c etc


this is normal in good power supplies and OEM
so

why they use inferior diodes for the 12v lines?

can a fast schottky 30A45V be used in the 12 v line?

Last edited by Fabie; 10-19-2013 at 06:23 AM.
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Old 10-19-2013, 04:19 PM   #2
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You can usually replace the fast recovery rectifers on the 12V rail with schottky rectifers, but make sure they rated for at least 60V.
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Old 10-21-2013, 02:28 PM   #3
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Quote:
Originally Posted by c.hegge View Post
You can usually replace the fast recovery rectifers on the 12V rail with schottky rectifers, but make sure they rated for at least 60V.
ok but is there a specific explanation for this? which issue can happen
ie for example a schottky 2045 or 3045 supports 45v if the line is only 12v, why them can't be inserted?
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Old 10-21-2013, 07:15 PM   #4
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The VRRM is the Peak Repetitive Reverse Voltage, and is different to the forward voltage. Many cheaper designs produce a lot of reverse voltage spikes, which it is the rectifier's job to filter out. Some power supplies get away with 40V diodes on the 12V, but most do not. If you go too low on that voltage, the diode will fail, usually within a few minutes of being powered on.
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Old 10-22-2013, 01:13 PM   #5
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Fast recovery diodes end up used on the +12V rail because the voltage drop is less of an efficiency hit at that voltage, and FRDs are cheaper than Schottkies per A of current rating.

Replacing them with a drop-in Schottky can cause issues if the feedback circuit can't compensate for the higher voltage coming out of the rectifier.
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Old 10-22-2013, 03:27 PM   #6
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^
True, hence why I said usually, and not always. I've done it several times before, and it has worked, but you will occasionally find one where the feedback loop doesn't like it. Often, though, the 12V voltage will be a little higher, so I wouldn't advise it if you plan to use the PSU with a Socket A machine
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Old 10-23-2013, 07:43 AM   #7
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thanks masters for the answers, Im understanding now

I just tried an HBR3045 on an old oem PSU which originally had a F12cxxx (12A)rectifier
it worked, the PSU started normally, voltage climbed from 12.1v to almost 12.4v
I left the PSU running for a few hours with only 1 fan and nothing happened
but I think different history is inserting the PSU in a computer stressing the 12v line
so

yet do not have the courage to insert this PSU on one of my computers,
what could happen?
if the PSU is destroyed I do not care
but is this dangerous for the motherboard or cpu?
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Old 10-23-2013, 12:16 PM   #8
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Yes. Using an underspec's rectifier could cause it to short under load, which could damage the motherboard and/or other components.
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Old 10-23-2013, 12:21 PM   #9
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Seeing as how you replaced a 12A rectifier with a 30A one, overloading it is unlikely to be a problem. There could be control loop issues under a dynamic load, though. High-power, multi-rail SMPS design is part of the dark arts (or at least differential calculus).
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Old 10-23-2013, 02:52 PM   #10
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I was thinking more along the lines of the VRRM being too low. See http://www.badcaps.net/forum/showthr...?t=8171&page=4 Goodpsusearch used a 30A 40V rectifier on the 12V rail, and it shorted shortly after the PSU was powered on. My concern was that a similar thing might happen here.
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