Today's Message Index:
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1. 07:40 AM - Re: Schottky steering diodes in parallel (Robert L. Nuckolls, III)
2. 08:15 AM - Re: Civility (Robert L. Nuckolls, III)
Message 1
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| Subject: | Re: Schottky steering diodes in parallel |
At 08:00 AM 5/24/2012, you wrote:
>
>A search of the archives turned up this:
>
> > 01/02/06
> >
> http://www.opensubscriber.com/message/aeroelectric-list-digest@matronics.com/2983404.html
> > . . . . .You can wire multiple diodes in parallel but don't depend
> > on this technique to "double" current rating of the steering diode.
> > Bob . . .
>
>Does the above statement apply only to multiple diodes contained in
>one package such as a bridge rectifier?
>If two discrete diodes are connected in parallel, would the total
>current capacity be almost doubled? If not, then why? It seems to
>me that each diode would carry half of the total current. If one of
>the diodes were to carry more than half of the current, then the
>voltage drop across that diode would increase. Thus the load would
>then draw current from the other diode which has a higher voltage output.
> Your explanation of diode physics will be appreciated.
other folks have pretty much covered it already
but there are cases where paralleling is helpful
but not to the extend of exact doubling.
Back in the good ol' days the germanium power
rectifier was a really whippy-do thing followed
very shortly by the silicon device. In the beginning
there was a lot of variation in some performance
values even in the same lot of finished devices
for reverse voltage standoff.
A guy from some semiconductor house visited us techs
at Hughes one day and asserted that rectifiers they sold
were all done on the same line. The were first graded for
the highest voltage rating, I think it was 400 volts.
As soon as they had their 400v quota . . . or ran
out of passable units, they would grade for 200v, then
100v and finally 50 volts. On a good day, every device
in the batch might pass 400v but still bear a 50v rating
just cause they needed parts with that number on it to
fill the requests.
The other side of the coin speaks to forward conduction
characteristics which gave he designer a heads up
on heat sink requirements. The lower the voltage drop
the less heat was generated. Of course, this also
impacted the ability of two devices in parallel to
precisely share total current between the two devices.
I think processes have improved quite a lot over the
years (worked at Hughes in '63). In particular, devices
that come from closely spaced locations in the
same production lot can be paralleled with a high degree
of sharing.
The Schottky diode I sell is actually a dual junction
device with both diodes wired in parallel. Did some
early development tests and found that the pair did
indeed parallel well. Also found that most devices
were much better than published worst-case data. I
did not rate my product with the assumption that a
user would NEED the full capability of both diodes
working together . . . but the testing I did confirmed
my confidence in offering a robust product that is
not intended to work up to the edge of falling over
the cliff.
Diodes are cheap. Unless one is inclined to measure
and confirm the willingness of two devices to pull
their fair share in tandem it's good policy just to buy the
next bigger device and not worry about it. Nobody runs
their prop bolts or strut brackets anywhere close to
limits . . . there are few good reasons to run diodes
any differently.
Bob . . .
Message 2
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>
>If you consider my actions in this matter uncivil, I will adjust my
>actions accordingly.
>
> Cordially,
>--
>Raymond Julian
>Kettle River, MN
I tried a reply to your direct mail; for some reason, my DNS server
seemed not to know of your email domain address. Hence this
more public reply . . .
Not at all my friend. I was commenting on no particular
posting . . . only acknowledging the fact that the
subject discussions had run far afield and stirred up
excited responses.
It's interesting that we should be having this exchange
on an open forum. I'm presently a too close observer of what
happens when one person starts a snowball of excited and
misinformed constructions rolling down hill generating what
has become a career threatening situation . . . all for
the stupidest of reasons. This has happened in an "institution
of higher learning" where we expect our children to emerge
with high standards of honorable conduct! I always thought
airplane drivers were a cut above most other fraternities;
a week at OSH readily reenforces that notion.
My expectations for myself and my wishes for others
is that we be good critical reviewers of simple-ideas
and capable teachers for how those ideas assemble into
recipes for success. For want of a nail, the war was
lost. For want of a well understood and properly applied
idea, the system is lost. This Nirvana of elegant solutions
is difficult find to in a blizzard of irrelevant or
erroneous information. I envision this List to be the
great filter through which only elegant solutions will
pass.
I think we strive to conduct ourselves accordingly.
If I ever have any issues with you personally, I will bring
them up with you personally in what I hope is a gentlemanly
manner. I would only expect a similar courtesy from you.
Fly comfortably my friend.
Bob . . .
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