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Old 9th January 2004, 03:32 AM   #1
dqswim is offline dqswim  United States
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Default Schottky rectifier cricuits

..... after building 2 (and parts) Pass amps its my first post in the this forum

What I currently looking for help on is what kind of voltage safety margin should be designed into the rectification stage of any of these projects? currently I'm assembling pieces along the lines towards the bosoz-x, and of course aleph-x like everyone else

I thought that the voltage safety margin on the rectifiers would be somewhat standard or at least easy to look up, and this becomes quite important when looking at the Schottky rectifiers which tend to the low voltage end. A look through 'The Art of electronics' and this forum and the web didnt prove all that enlightening

The current ratings are easier to understand and I can get a visual of more or less what happens with large capacitance using ducan amps psud2 , but the voltage on the diodes never seems to peak, I would expect some line variation in a home (don't have a clue what effect this has, i understand that the line voltage is RMS but I dont know what kind of load the diodes see) obviously the transformer passes through a ratio what is on the mains

Many on this board seem to aim for the far reaches of safety on this issue but as you get higher voltage rated rectifiers you increase the diode voltage drop and I would guess you increase capacitance

For a proposed circuit that had 36v secondaries (36 *1.4 = 51V ) Nelson Pass mentioned that a 50v schottky would be the best??

http://www.diyaudio.com/forums/showt...ght=#post96298

I also noticed user Till who both used the mbr10100 (100v 8a) schottky's in the Bosoz and reported workable results, bzls circuit gives 60v secondary (60 * 1.4 = 85v)

http://www.diyaudio.com/forums/showt...ht=#post228691

And just for reference the Pass Z4 recommends a bridge with a multiple of 4 safety on the voltage (referneced to 1.414 * sec.) and the bosoz has about a 2.5 voltage safety margin

I had more references but i dont need to knock everyone over the head with anymore,

Summary

1. What kind of diode rectifier safety margin is needed? Do the rectifier diodes see much more voltage than the psu capacitors?

2. Aren't there some ill effects of using much higher Voltage and Ampere rated diodes than needed (or is it just price???)


looks like i have more questions than answers or ideas for today

-dave
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Old 9th January 2004, 03:53 PM   #2
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Some margin is generally built into the manufacturer's spec,
so a 50 V rectifier exposed to 51 volts is not likely to be a
problem. I have actually seen 80 volts on the Gate-Source
of a Mosfet rated at 20 V before it broke, for example.

In general, you can go ahead and use much higher rated
components. There is usually a small trade off, again for
example, a 400 V Mosfet usually has a lower transconductance
than a 50 V, all other things being equal (which they aren't).
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Old 9th January 2004, 04:04 PM   #3
sully is offline sully  United States
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Quote:
Originally posted by Nelson Pass
Some margin is generally built into the manufacturer's spec,
so a 50 V rectifier exposed to 51 volts is not likely to be a
problem. I have actually seen 80 volts on the Gate-Source
of a Mosfet rated at 20 V before it broke, for example.

In general, you can go ahead and use much higher rated
components. There is usually a small trade off, again for
example, a 400 V Mosfet usually has a lower transconductance
than a 50 V, all other things being equal (which they aren't).
For shottky's, I'd go with the higher margin. The reverse leakage, especially at elevated temperatures, can really start dissipation and thermal runaway.

80 volts G-S? Wow, I was always scared of the 10 volts and 20 volt tests I did in a hybrid production line on the IR/IXYS/Siliconix products.

Cheers, John
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Old 9th January 2004, 05:45 PM   #4
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It takes a fearless attitude to do this work.
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Old 9th January 2004, 09:00 PM   #5
dqswim is offline dqswim  United States
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Join Date: Nov 2003
Location: il
Thanks for tips Nelson, I might start with a timidly fearless attitude until I know what to be fearful of, and then I'll replace what i break and try again.

And to think I just started grad. school in the direction of single molecule wires and rectifiers. Maybe some fearless experiments are in order....haha....watch out (but don't wait). I'll be sure to let you know if I find or make anything good.
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