trillium said:
The task: power a highly dynamic load (takes 32A for 1ms, then 8A for
6ms, then 0 for 1ms, then starts all over) at 12V and 24V. Voltage can
vary 1V.
A suggested solution: for 24V, put in series two 12V/12.5A power
supplies (average load current works out to be 10A) and place a big cap
(like 200mF) across the load. For 12V, only one.
Would this work? Two possible problems I'm wondering about --
1) The power supply gets into current limitation for a little bit each
cycle; is that healthy?
Power supplies in series interfere in each other's feedback loop; given
the dynamic load, they may never settle.
This is a valid concern. It's generally bad to put supplies in series.
If you expect them to behave in current limit, you're dreaming.
They will never be exactly the same. You may be in the situation that
only one ever current limits. You won't be able to use foldback
limiting. There's probably a glitch filter in the limit that can put
both supplies in a frenzy. If you're only building one system, you only
have one customer to worry about. If you build a lot of them, you're
asking for trouble. It's unlikely that the current limit behavior you
need is in the spec. Using any component outside it's specification is
a recipe for disaster.
Some bean counter in your organization is gonna save a buck by changing
vendors. Or some bean counter at the supplier
is gonna change one of his vendors. And all of your careful evaluation
goes out the window and your replacement is on a plane to do an
emergency retrofit.
Put a filter between the supply and the load that guarantees you never
put the supplies into current limit. Make sure your caps are rated for
the repetitive peak current you are gonna put thru them. Once you've
met the specs on the supply, you can remote sense the output of the
filter. Yes, there will be phase issues and potential for oscillation.
Pay attention to your resistance budget. 1V 32A gives you 1/32 ohms
total resistance for all the caps, wire, connectors, etc. And that's
before you get to any capacitive droop.
Buy a supply that's rated for the peak and average currents you need.
The design problem is the same, it just goes inside the regulation loop
and is guaranteed to work.
mike
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