More 11/750 PSU Qs

From: Andreas Freiherr <Andreas.Freiherr_at_Vishay.com>
Date: Wed Apr 17 04:53:41 2002

Lee,

the point is:

> running only one in "voltage source" mode, the other(s) in
> "current source" mode effectively.
>
> This is how any group of paralell power supplies works.

And this requires the over-current protection to not shut down the
individual supply, but instead have it contribute as much as can safely
be done. On the other hand...

> If the outputs are overcurrent protected then a faliure of too many
> supplies should shut down the whole thing. Trouble is it often
> doesn't work that way.

This will be inhibited by the above-mentioned load-sharing feature, so
you need some kind of "communication" between the supplies in order to
be able either to share load or to completely shut down all others
instead of running them in current-limiting mode after some have failed.

> Use enough supplies so that one faliure is tollerable. you can
> then hot swap the dud supply.

This would even take us further, into redundancy. Nice idea! The only
thing that needs preparation for this may be mechanics: you wouldn't
want to try unscrewing thick power wires and risking them to touch any
other metal parts inside a running system...

My remark...

> However, this setup may get difficult if loading varies widely.

... was referring to the effect of individual supplies going back and
forth between current-limiting and voltage-limiting mode, which can
indeed make them perform very poorly. I take this for agreed. With a
constant load, you could avoid this problem by properly designing the
system.

> Usually it isn't a problem as long as there is no minimum
> load requirement for the individual supplies.

Yes, if minimum loading would be required, things would get worse. In
this case, you would need a way of shutting down one or more of the
bunch completely in order to comply with the minimum loading requirement
for the others. We get to a point where a microcontroller makes sense to
control this, and I have seen this in real systems.

So, I think the bottom line is: running PSUs in parallel is something
that really can be done, but there are several bells and whistles you
need to take into account.

--
Andreas Freiherr
Vishay Semiconductor GmbH, Heilbronn, Germany
http://www.vishay.com
Received on Wed Apr 17 2002 - 04:53:41 BST

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