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Paralleling power supplies - what current needs to be drawn?

Question asked by drkirkby on May 10, 2016
Latest reply on May 12, 2016 by drkirkby

I have a 6674A (option J06, 70 V, 30 A), and are thinking of buying another 6674A, although I'd rather have a standard model (60 V 35 A) or one of the lower voltage / higher current models.


I've looked at a Keysight document "10 Fundamentals You Need to Know About Your DC Power Supply" and gather one needs to be run in constant voltage, and the others in constant current. I'm a bit puzzled by this statement:


"The output load must draw enough current to keep the CC unit(s) in CC mode."

My load current is likely to change from about 1 A to 54 A @ 50 V, although for test purposes, I'd run at lower voltage and current (probably 25 V with a limit of 5 A).


Does this mean I'm basically stuck, and can't do this? I can add a bit more load resistance, and push the "no load" current to a few Amps, but I'm not going to be able to run much load current continuously, otherwise the power supplies will not have enough capability.


A few questions arise, which are not answered by that application note. These are:


1) Is it possible to run two similar, but not identical 6674A PSUs in parallel, given one is 60 V and the other 70 V? I only need 50 V.


2) Given two similar, but not identical 6674A (one 60 V 35 A, the other 70 V, 30 A), is there any advantage in running either the 60 V or 70 V unit in CC, and the other in CV? Or does it not matter?


3) What sort of load do I need to ensure the one running in CC mode is working as it should? (You may have seen from another post, my 6674A arrived damaged, so at this point I can't do any testing with it).


4) If power was to fail to one of the PSUs, (either the CC or CV), would the other get damaged as it tried to sink a lot of power?

I don't need particularly good regulation for this, so I am tempted to diode OR the outputs. With carefully adjustment of the voltages, it should be possible to get the currents about equal, or a bit more supplied from the higher current power supply. But if I can achieve better regulation, whilst not running into problems, that would be a bonus of course.