Re: Load exceeding supply current

trillium wrote: 
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.

The average current for the above pattern is 10 amperes, so you need a supply with greater capacity than that, or it is going to be in current limit all the time.

A 30mF cap will sag 1 volt in 1 millisecond while delivering the whole 32 amperes. but if the supply can deliver 10 of those amperes, then 22 mF will deliver the remaining 22 amps during that millisecond while the voltage sags by a volt. It will recharge back to full voltage with the excess current capacity during the rest of the cycle. Current limit is not a problem for the regulator as long as it is designed to be in current limit, continuously. Current limiting is just regulation with a different condition controlling the pass device, except that the current is high. But any supplies are not designed to handle the temperature rise of continuous current limit operation.

Coupling the big capacitor to the supply through a big inductor will let the cap carry more of the peak current (requiring a larger capacitor) but will smooth out the current drawn from the supply (and also waste some voltage with its resistance). A bigger supply is probably cheaper than this solution.
.

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