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user9253
Joined: 28 Mar 2008
Posts: 1922
Location: Riley TWP Michigan
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 Posted: Thu May 24, 2012 5:00 am Post subject: Schottky steering diodes in parallel |
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A search of the archives turned up this:
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.
Joe
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Joe Gores |
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retasker(at)optonline.net
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| Posted: Thu May 24, 2012 8:27 am Post subject: Schottky steering diodes in parallel |
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If the electrical characteristics of the two diodes were exactly the same, then your supposition would be correct.
The problem comes in when they are not exactly the same - which is all the time. For example, one diode may have a
0.65V drop at one amp while the other diode only has to pass 1/2 amp for the same voltage drop. So in this example,
with a total current of 1.5A, one diode handles 2/3 of the total current and the other handles 1/3.
Because the voltage drop changes very little for a change in current, it only takes a very small difference in
characteristics to cause a big mismatch in current sharing.
Dick Tasker
user9253 wrote:
| Quote: |
A search of the archives turned up this:
> 01/02/06
> http://www.opensubscriber.com/message/aeroelectric-list-digest(at)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.
Joe
--------
Joe Gores
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=373751#373751
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user9253
Joined: 28 Mar 2008
Posts: 1922
Location: Riley TWP Michigan
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| Posted: Thu May 24, 2012 9:21 am Post subject: Re: Schottky steering diodes in parallel |
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Thanks Dick Tasker,
You explained that very well. It looks like it would be better to use one diode rated for the current, rather than parallel smaller diodes.
Joe
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nuckolls.bob(at)aeroelect
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| Posted: Fri May 25, 2012 6:40 am Post subject: Schottky steering diodes in parallel |
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At 08:00 AM 5/24/2012, you wrote:
| Quote: |
A search of the archives turned up this:
> 01/02/06
>
http://www.opensubscriber.com/message/aeroelectric-list-digest(at)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 . . .
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