PSU redundancy

Discussion in 'General Electronics Chat' started by RogueRose, Jun 19, 2015.

  1. RogueRose

    Thread Starter Member

    Oct 10, 2014
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    I'm curious as to various options for a project. Let's say that a device needs 400w and I have 200w PSU's. I want some redundancy so would it be possible to run 3 200w PSU's in parallel so that if one of them drops off then I am still covered by the extra PSU? I would probably try to make the coverage more like 3 200w psu's on a 350w device, so that the supplies aren't running at 100% all the time.

    Now is is possible to run 2 supplies and if one starts acting up, the other one turns on and takes over. IDK how difficult that would be.

    I have a number of computers that have 2 PSU's (each is capable of 100% load) that seem to run constantly and I don't know if the power is being split 50/50 across them or if one PSU is doing all the work and the other is in standby mode.

    IMHO, I would think that running them 50/50 or 33/33/33 would be best as if power needs to transfer instantly the PSU doesn't have to power up. IDK if that makes sense at all but that is why I am asking.

    Thanks for any help or suggestions here!
     
  2. pwdixon

    Member

    Oct 11, 2012
    488
    56
    Many systems use redundant PSUs, most simply connected via two diodes. Load sharing is a slightly different proposition and needs some attention to the PSU specs but it's certainly do-able.
     
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  3. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
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    Diodes are all you need, it matters not that one supply may carry more load than the other during normal operation. Add some form of voltage sensing to each supply before the diodes so that you will know if one has failed; this could be as simple as a relay with its coil connected across each output. Each power supply should be able to supply the full current required so your 3x200W arrangement for a 400W load would be a bad idea. If two supplies failed then so would the 100% overloaded 3rd.
     
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  4. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    What you are asking about is called n+1 operation, where n=2 in your case with three supplies. Some supplies are designed for this, and have a bidirectional analog or digital control pin that is bussed among all supplies. When working properly, this forces each supply to supply load/n current. This is the most complex arrangement, but it increases long-term reliability over simple diode-ORing because while no supplies are running at 100% (usually at or neat their peak efficiency), no supply is running at maximum internal temperature.

    Supplies designed and built from the ground up for stand-alone operation usually don't have a 3rd wire current share bus. In this case, if the supply has a remote sense input you can add an external current sense shunt and create a current share bus using chips designed specifically for this control function. Unitrode (now TI) has some. If there is no remote sense input, your only option is diode-ORing. If there is a voltage adjust feature, you can adjust this so the two supplies are not so close together that they oscillate, fighting over who is driving the load. A minor variation is called droop-share, where the diodes act as ballast resistors. In this case you intentionally adjust the supplies to be as close to the same output voltage as possible, and the diodes act as summing resistors into the load.

    Bottom line, everything depends on the features and operating characteristics of the supplies you have.

    ak
     
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  5. RogueRose

    Thread Starter Member

    Oct 10, 2014
    189
    4

    Thanks so much for all the responses!!

    Well the reason I'm looking at a setup like the 3, where 1 is redundant is because the device may be needing something like 4,500-5,000 watts and the largest PSU that I can find that is affordable and will fit in the application is about 2500 watts. Or I could run 5 1,200watt PSU's which would give a little margin, but that isn't the best idea IMHO.

    What made me think about this is that in computer systems, hard drives are often run redundantly where one drive is a "parity" drive, which is basically allows for a single drive failure and the system to continue. It's possible to have as many parity drives as there are main drives do if there are 5 spanned drives (like 5 parallel PSU's) you could have 1-5 parity drives, each allowing for more drive failure before the system crashes.
     
  6. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    What manufacturer/model supplies are you considering?

    ak
     
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