Re: Diodes in parallel
From: Don Bruder (dakidd_at_sonic.net)
Date: 02/05/05
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Date: Sat, 05 Feb 2005 20:55:19 GMT
In article <4205296A.1BE5D30F@rica.net>,
John Popelish <jpopelish@rica.net> wrote:
> Don Bruder wrote:
> >
> > In article <4205195E.59FC5115@rica.net>,
> > John Popelish <jpopelish@rica.net> wrote:
> >
> > > Jack// ani wrote:
> > > >
> > > > Hi there,
> > > >
> > > > Can I increase the current rating of my psu which is having 1N4007
> > > > diode bridge rectifier, to 2amps by placing similar diodes in parallel
> > > > to each diode in the bridge rectifier? Actually my pcb is not having
> > > > enough room for 1N540X series diode, but I can easily put 1N4007 diodes
> > > > in parallel!
> > > >
> > > > Thanks for any help
> > >
> > > Whether the power supply can provide more amperes is not just
> > > dependent on the diodes. But to address the diode question,
> > > paralleling with a similar type will allow a bit higher current
> > > without destroying the diodes, but not twice as much. The diodes will
> > > not share the current exactly and each will heat the ambient air near
> > > he other, so each will get hotter for its share of the current than if
> > > it were operating at that current without its partner near by.
> > >
> > > Even alone, a 1 amp diode is not normally expected to deliver 1 amp
> > > average, except under the most ideal of conditions (leads heat sunk to
> > > 25 C a short distance from the body, for example).
> >
> > John...
> > This fellow's question brings to mind a similar concept I had over on an
> > automotive group - Alternators are fairly tough little beasties - Most
> > of the guts of them are all but indestructible short of physical damage.
> > But a very common failure mode for them is for the diode trios to barf
> > if/when they're asked to try to charge a mostly-dead battery.
> >
> > I had the thought not too long ago that paralleling multiple diodes per
> > "trio position" should give at least some added "anti-fry" protection in
> > terms of how dead a battery the alternator can charge without burning
> > itself (or more specifically, its diode trios) up.
> >
> > Since it's such a common failure mode, and cheap to repair (compared to
> > "buy a new/reman alternator and replace as a unit") the benefit would
> > potentially be huge to the person doing such a mod to their alternator.
> >
> > Now, it sounds like you're saying that rather than improving the
> > situation, such a setup would likely be an actively BAD thing?
>
> I didn't say anything about a bad thing. I said that two 1 amp diodes
> in parallel and also in close proximity should not be expected to
> deliver 2 amps. They will deliver somewhat more than one diode would
> be capable of, just not twice as much.
OK, clarification noted. So does it make sense to expect that
"unloading" any single diode by splitting the load across (as per my
previous example) two more diodes wired in parallel with it would have a
positive effect as far as "diode-death prevention" is concerned?
Basically, I'm working under the concept of "A draw that would kill a
"lone" diode should (generally speaking, and for the sake of discussion,
ignoring the possibility/effects of uneven load division/thermal runaway
that you mention later on) be insignificant when three diodes are
splitting the load."
Do I need a new concept?
> > Is the "extra heating" issue still a significant problem in a forced-air
> > environment such as what I'd have in my particularly alternator? (diode
> > trios in it are heat-sinked, and are the first thing that the air, being
> > actively pulled in through the back of the alternator by its built-in
> > fan, encounters)
>
> Forced air definitely redefines close proximity.
Kinda figured it would :) Seems to me that heat would become (at least
for practical purposes) a non-issue in such a situation.
> > It would seem to me, at least at first glance, that "stacking" multiple
> > diode trios (let's say to three diodes per phase and polarity, for a
> > total of 18 discrete diodes (versus the standard 6 -- two, one for each
> > polarity, on each of three phases)) in this beast would be beneficial,
> > if only in terms of "Well, that one over there cooked, but I'm still
> > doing fine, and so is the one next to me" style backup - should one
> > fail, the remaining two for that phase pick up the slack. (Of course,
> > with such an arrangement, if there's an overload big enough to blow one
> > of the diodes, I think it unlikely that the other two would survive as
> > they were forced to pick up even more of the load that managed to burn
> > out one of the three already...)
>
> Blown diodes are normally short circuits, so they will not take turns
> as you describe.
Hmmm... not a concept that I had thought about - That would tend to
throw a bit of a wrench in the works, wouldn't it? Again, though, it
comes back to the whole "Shouldn't the load that would blow a single
diode be seen as effectively insignificant by three similarly
rated/heat-sinked/etc diodes wired in parallel?" - If the diode is
unloaded enough by paralleling multiple "copies" of itself in its
position in the circuit that even under ridiculaous loading, no single
diode of the group should fail in the first place, it's pretty much
irrelevant that a given diode "usually fails shorted" when it dies, no?
> A better way to protect the alternator might be to
> add a resistor between the regulator and the armature winding, so that
> the excitation current was limited to a value slightly less than the
> design maximum. Since the alternator normally uses way less than the
> maximum possible excitation, this would affect its operation only at
> lowest speed and during overload situations.
>
> I am working on a new regulator for my motorcycle that includes an
> adjustable current limit for the excitation, but for a completely
> different reason.
May I be so bold as allow my curiousity to run wild long enough for me
to enquire about what this completely different reason is?
>
> > Which doesn't even start to address the whole "Since three of them are
> > sharing the load (however unevenly they might be portioning it out
> > amongst themselves) there should be little or no reason for one to fail
> > in the first place" concept...
>
> There would be less heat in any one of the parallel diodes than if
> only one were carrying the whole current. The problem is that the
> hottest diode tends to have the lowest forward drop and hog most of
> the current, making sharing worse. So all sets need similar heat
> sinking.
Ahhh, yes... Good ol' thermal runaway.
Makes good sense, now that I consider it. That would seem to indicate
that one would want to carefully match the diodes being used to
construct the "trio set" - A task with the potential to be anywhere from
"trivial - I did it three times while sleeping last week" to "outright
impossible", I imagine...
-- Don Bruder - dakidd@sonic.net - New Email policy in effect as of Feb. 21, 2004. Short form: I'm trashing EVERY E-mail that doesn't contain a password in the subject unless it comes from a "whitelisted" (pre-approved by me) address. See <http://www.sonic.net/~dakidd/main/contact.html> for full details.
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