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  #21  
Old 11-02-2007
Aristide1 Aristide1 is offline
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Not sure how switching PS handle the rectifier diodes, but generally bridge rectifier diodes make a fair amount of noise themselves, unless one uses fast recovery or Shottky diodes. These normally are found in high end audio circuitry when such noise is not desirable. Its also better to prevent it than to try to correct it later on.

Compared to generic bridge rectifiers these may seem costly, but it's still not that bad for playing around in a circuit like this.

Have a look around here
http://www.partsconnexion.com/catalo...onductors.html

Try this link for various high quality electrolytic caps.
http://www.partsconnexion.com/catalo...ctrolytic.html

There are tabs at the bottom of the page for different brands. Since you would use these strictly in a power supply I would tell you to look at the cheapest ones they have. There are no bad caps on this link anywhere.
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  #22  
Old 11-02-2007
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Originally Posted by Aristide1 View Post
Not sure how switching PS handle the rectifier diodes
Usually Shottky.
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  #23  
Old 11-03-2007
cypherpunks cypherpunks is offline
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Quote:
Originally Posted by Aristide1 View Post
Not sure how switching PS handle the rectifier diodes, but generally bridge rectifier diodes make a fair amount of noise themselves, unless one uses fast recovery or Shottky diodes.
Which is exactly why Schottky diodes are used universally in power supplies.

For those who care, a Shottky diode is basically "half a diode". Rather than a P-N semiconductor junction, it is metal to N semiconductor. This gives it half the forward voltage drop (saving power), and speeds up reverse recovery due to the lack of minority carriers (holes).

The thing is, all diodes require a bit of reverse current to turn off. When in forward conduction, the junction region is full of electrons to carry the current. If you just set the voltage to zero, the electrons eventually drift out of the region, but it's not a fast operation. To turn it off quickly, you have to reverse the current and suck the electrons out of the junction regions.

And then, very abruptly, the reverse current goes to zero and you have a bit of a noise spike. This is actually used on some test equipment; there are specially constructed "step recovery diodes" that switch off with a bang like a ruler hitting a desk. It turns out to be useful for generating microwaves.

Schottky diodes have a somewhat "soft" turn-off, which makes the bang less loud. And they turn off very quickly, before the reverse voltage can build up.

The downside is that they have high reverse leakage. In a power supply, a milli-amp of leakage (it's actually a few dozen micro-amps for most power Schottkies) is nothing. In a sensitive analog circuit, it would be a problem.

The one limitation on Schottky diodes in power applications is that reverse leakage increases with temperature. At high voltage, a bit of leakage heats the diode, which makes it leak more, which heats it more, and you get thermal runaway. Your diode decides it's tired of being a rectifier and is going to try playing the role of a fuse for a few milliseconds.

The alternative, which is apparently used in the OCZ ProXStream 1000W reviewed at Hardware Secrets is synchronous rectification. That's where you don't use any diodes, but turn a transistor on at just the right time. The timing is critical, but you can get higher efficiency that way, especially at low voltages where even a Schottky diode's 0.3V forward voltage drop is a significant fraction of the power consumed.
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  #24  
Old 07-24-2008
jooch jooch is offline
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Originally Posted by Aristide1 View Post
Mr Guru,

I think the author of the article was on the right track, but his testing may have been a tad premature.

Capacitors should be given time to settle down. His should burn them in for at least 100 hours and then test his updated units again for noise.

Your power supply reviews are very helpful.

Thanks
Aris
Aris, you're right on the money. Have a look at this discussion on DIYaudio.com, it confirms you're theory.
Here are the pictures for non DIYaudio users:
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  #25  
Old 03-18-2011
Olle P Olle P is offline
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This is an interesting article.
I do miss some more discussion about the options when picking new capacitors though.

- Solid versus regular electrolyte capacitors?
- Different capacitance? Is bigger better?
- The ESR, what absolute value is the upper limit for "low"? (I find 10mm aluminium capacitors with ESR ratings from 0.023 ohm up to 0.039 ohm.)
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  #26  
Old 03-18-2011
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Quote:
Originally Posted by Olle P View Post
This is an interesting article.
I do miss some more discussion about the options when picking new capacitors though.

- Solid versus regular electrolyte capacitors?
- Different capacitance? Is bigger better?
- The ESR, what absolute value is the upper limit for "low"? (I find 10mm aluminium capacitors with ESR ratings from 0.023 ohm up to 0.039 ohm.)
Solid caps pack more capacitance in lower volume, can endure higher temperatures and usually won't burst open when overstressed (but can still fail). They're also considerably more expensive.

Capacitance influences a number of things, and it's generally best to try to stay inside the original specification. It influences the period of LC circuits, it influences hold-up times, maximum energy storage during off phases of the rectification cycle, etc. In some cases, more capacitance is welcome, mostly when using filter caps in cables or modular interface boards.

ESR should be kept the same or lower as the original spec. It influences the maximum ripple voltage, and does so linearly, so at 0.0023 ohm and 10 amps, the ripple voltage would be 23 mV peak-peak, and with a 0.039 ohms and the same 10 amps, peak-to-peak ripple would be 390 mV. Of course, the final ripple/noise value depends on more than just the capacitor, but you should try to keep it as low as possible, by selecting a low and stable ESR value (it varies with time and temperature).
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  #27  
Old 04-28-2011
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Wtf, Antecs with Fuhjyyus? That's not good. I thought there were only OSTs in the NEO ECOs.
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  #28  
Old 04-28-2011
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And you guys really should have matched the ESRs correctly...Clearly you didn't...
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  #29  
Old 04-28-2011
Zero82z Zero82z is offline
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Originally Posted by TheLaw View Post
Wtf, Antecs with Fuhjyyus? That's not good. I thought there were only OSTs in the NEO ECOs.
Antec doesn't use Fuhjyyus. Maybe you should try looking at the dates next time you reply to a thread.
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  #30  
Old 04-29-2011
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Quote:
Originally Posted by TheLaw View Post
Wtf, Antecs with Fuhjyyus? That's not good. I thought there were only OSTs in the NEO ECOs.
Antec using Fuhjyyu's in CWT's is what got Antec in so much trouble almost a decade ago. OST's were in the old Seasonics.

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Originally Posted by TheLaw View Post
And you guys really should have matched the ESRs correctly...Clearly you didn't...
Quote:
Originally Posted by Zero82z View Post
Antec doesn't use Fuhjyyus. Maybe you should try looking at the dates next time you reply to a thread.
And actually read the thread you respond to. It was already pointed out the caps weren't the correct ESR, what caps should've been used and why they weren't used.
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