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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

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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.

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PeterW May 11, 2016 6:40 AM

1) Identical model numbers are required to ensure current sharing is performed correctly, also, because frequency response/operational bandwidth is equivalent and stable.

2) To ensure successfully parallel operation, both supplies require the capability to attain the same output voltage level.

3) Operating two (2) supplies in parallel, now with the capability of supplying ~60A, at the 1amp level is a miss application of the product(s).

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drkirkby May 12, 2016 9:39 AM

I did say the other PSU would be the same model (6674A), but not necessarily the same option. I'm wondering if the option J06 (70 V, 30 A) was set up in constant voltage mode, and a standard (60 V 35 A) 6674A in constant current mode, whether one could get 50 V @ 60 A.

The point about the load being as low as 1 A is that this is a class AB amplifier that could be switched in and out. On it would draw 54 Amps, but off it would be bypassed and probably take an Amp or less. When bypassed, I would not be too bothered about the exact voltage.

I know there are better PSUs than the 6674A for my application (e.g. N8940A) but these need a 3 phase mains supply, which I don't easily have. I would prefer a single phase unit.

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PeterW @ drkirkby on May 12, 2016 10:04 AM

1) The additional "option" results in an instrument that is not identical to a 6674A. The model you have is a "6674A-J06".

2) Most AB amplifiers that I'm familiar with are generally powered from a CV power supply, not a CC power supply.

3) Loading/un-loading characteristics of a CV power supply are dramatically different than that of a CC power supply. CC power supplies have a response time which is highly load dependent and often 10-100 times slower that a CV supply.

BTW, the 6674A-06 has been unavailable for a large number of years.

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drkirkby May 12, 2016 2:11 PM

Thank you PeteW.

Currently I have just one amplifier, which will take about 27 A @ 50 V. It seems if I want to add a second, I'll need to look for another PSU type, as the J06 is a bit rare, and in any case I don't want a 70 V unit.

I will probably have to look for a 3-phase model, and get all 3 phases which are fed to my house, into my lab/garage. Here in the UK, having 3 phases at a domestic property is quite rare, but I do have 3 phases to the house. It would however needs some wiring work to get all 3 where needed.

Cheers,

Dave.

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PeterW May 11, 2016 6:40 AM
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1) Identical model numbers are required to ensure current sharing is performed correctly, also, because frequency response/operational bandwidth is equivalent and stable.

2) To ensure successfully parallel operation, both supplies require the capability to attain the same output voltage level.

3) Operating two (2) supplies in parallel, now with the capability of supplying ~60A, at the 1amp level is a miss application of the product(s).
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drkirkby May 12, 2016 9:39 AM
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I did say the other PSU would be the same model (6674A), but not necessarily the same option. I'm wondering if the option J06 (70 V, 30 A) was set up in constant voltage mode, and a standard (60 V 35 A) 6674A in constant current mode, whether one could get 50 V @ 60 A.

The point about the load being as low as 1 A is that this is a class AB amplifier that could be switched in and out. On it would draw 54 Amps, but off it would be bypassed and probably take an Amp or less. When bypassed, I would not be too bothered about the exact voltage.

I know there are better PSUs than the 6674A for my application (e.g. N8940A) but these need a 3 phase mains supply, which I don't easily have. I would prefer a single phase unit.
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- PeterW @ drkirkby on May 12, 2016 10:04 AM
  Mark Correct
  Correct Answer
  1) The additional "option" results in an instrument that is not identical to a 6674A. The model you have is a "6674A-J06".
  2) Most AB amplifiers that I'm familiar with are generally powered from a CV power supply, not a CC power supply.
  3) Loading/un-loading characteristics of a CV power supply are dramatically different than that of a CC power supply. CC power supplies have a response time which is highly load dependent and often 10-100 times slower that a CV supply.
  
  BTW, the 6674A-06 has been unavailable for a large number of years.
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drkirkby May 12, 2016 2:11 PM
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Thank you PeteW.

Currently I have just one amplifier, which will take about 27 A @ 50 V. It seems if I want to add a second, I'll need to look for another PSU type, as the J06 is a bit rare, and in any case I don't want a 70 V unit.

I will probably have to look for a 3-phase model, and get all 3 phases which are fed to my house, into my lab/garage. Here in the UK, having 3 phases at a domestic property is quite rare, but I do have 3 phases to the house. It would however needs some wiring work to get all 3 where needed.

Cheers,

Dave.
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Paralleling power supplies - what current needs to be drawn?

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