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Power Hungry Server Farms

  1. May 4, 2015 #1
    1 Internet server farms demand a massive amount of power.
    2 The power grid can supply 3 phase.
    3 To increasing demands for data throughput rate both transistor size and voltage levels have been ever shrinking.

    The server farm market wants efficient and reliable power conversion from 3 phase mains to low voltage DC supply rails that are (at a guess) a couple of volts.

    How would you design the winning power conversion product?
     
  2. jcsd
  3. May 4, 2015 #2

    anorlunda

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    District heating. Build the farm in a dense city center in a cold region with long winters and no air conditioning in summer.

    Use water cooling for the server farm. Use the waste heat water to heat all the buildings and homes. Warm water return from the buildings melts the ice on streets and sidewalk. Then dump the water in the harbor, keeping it ice free through the winter. Shut down the city and send everybody on vacation during the hottest month.

    I once lived in Västerås Sweden, a city of 150,000 people. They did all those things by diverting 25% of the low pressure steam energy from the local power plant. When the power plant reached end of life, they went back to consuming electricity to do all that stuff.

    Using low grade heat (warm water below boiling temperature) is very difficult. District heating is almost the only way to use that heat productively. So my server farm design uses all that waste energy productively to to things that would otherwise have to consume electricity anyhow.
     
  4. May 5, 2015 #3

    tech99

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    How about transforming down to 48V AC, then a 3-phase rectifier to obtain fairly smooth 48V DC. At this point a battery back up can be connected. Then convert it to 48V 400 Hz AC by electronic commutation and distribute it to each rack, where there would be a small 400 Hz transformer with several 3V rectified outputs.
     
  5. Jun 2, 2015 #4
    The nice thing about 3 phase power into a transformer, is that it can output 6 phase with greatly reduced ripple. Rectified, is the ripple (13%) low enough to power servers directly, without filtering?
     
  6. Jun 3, 2015 #5

    tech99

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    I think 13% would probably be OK at input side of a 3V regulator. In any case, it is quite easy to filter this at 3 volts DC. Of course, every unit or circuit board in the rack would require a regulator followed by a small amount of noise filtering.
     
  7. Jun 3, 2015 #6

    f95toli

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    Servers are not (usually) powered directly. They are powered from UPS systems(batteries combined with e.g. diesel generators), which in the case of a farm can be quite large facilitates..
    There is, however, no reason why you can't charge the batteries using supply with relatively large ripple.
     
  8. Jun 3, 2015 #7

    Baluncore

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    By using power factor correction circuits, all available AC phases can be switched into one high voltage DC capacitor and battery bank. That will look like a resistive load to the grid. The HV DC can then be distributed to wherever the low voltage high current is needed. It can then be reduced efficiently to a couple of volts by using buck converters with synchronous rectifiers. That will give an overall efficiency of better than 90%.
     
  9. Jun 3, 2015 #8
    I like this idea. But doesn't switching place harmonics on the power lines?

    I'm not sure one large switching system is the answer. Perhaps several offset by a few microseconds? Or possibly timing switches so the low order harmonics cancel? (The higher order ones are more easily filtered.)
     
  10. Jun 3, 2015 #9
    So from what I'm reading, transform the voltage to a reasonable level. Reasonable would be defined as one where we could operated a high efficiency DC to DC switcher. (Remember the line voltage is likely coming in on a transmission line for a large farm, so some transformers will be needed anyway.) Then use power electronics to switch the current into a DC capacitor bank in such a way as to condition the line back to the power company (PC) to offset other PC loads. Attach the UPS here and distribute to various server racks. Have each rack provide a DC to DC switcher set and a backup. Distribute the DC lines to the boards which should each have their own linear supplies.

    I like Anolunda's idea for using cooling water to heat buildings and stuff. I also agree that the servers would do well in cool climates with lots of hydropower. (Northern Canada comes to mind.) Of course finding an area with that and a technically competent workforce will always be an issue. Lots of competent people seem to prefer at least some sunshine. :oldcool:
     
  11. Jun 3, 2015 #10

    Baluncore

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    Yes, but they are the harmonics of the PF correction circuit which operates at above 20kHz. Those harmonics are easily spread in time and attenuated by LC noise filters.

    There would be redundancy, with more than two AC to HV DC units per server rack.

    The advantage of HV DC is that HV swamps the PN junction voltage, reduces current through MOSFET/IGBT switches and reduces HV ripple since E = ½ C V2.

    The buck topologies that convert HV DC to a couple of volts DC are limited in efficiency by output diode voltage drop. Synchronous diodes made from MOSFETs resolve that inefficiency. The HV side needs low current switch semiconductors while the LV side needs LV high current semiconductors.

    This is not new technology. It has been in use for 10 years in some fields.
     
  12. Jun 3, 2015 #11
    Do you have a name for this sort of high voltage DC converter. I looked up HV DC converters and they seemed to only go from low to high. I need something to google please.

    I'm curious about the circuitry.
     
  13. Jun 3, 2015 #12

    jim hardy

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    I like Baluncore's approach. It makes use of a subtlety that's not immediately obvious -

    To make 3 volts DC you rectify some higher voltage. The voltage drop across your rectifiers is an unavoidable loss.
    Using the best rectifiers around might make that only 1/4 volt loss.
    Mosfets can rectify with only a few millivolts.loss by turning them on and off at the right times, ie synchronizing them with the incoming line...

    A search on "Synchronous Rectification" produces lots of references
    http://scholar.lib.vt.edu/theses/available/etd-09152003-180228/unrestricted/Ch2.pdf
     
  14. Jun 3, 2015 #13

    Baluncore

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    Power Factor Correction. Search 'smps pfc'
    http://en.wikipedia.org/wiki/Switched-mode_power_supply#Power_factor

    See; LT8312. Boost Controller with Power Factor Correction.
    http://cds.linear.com/docs/en/datasheet/8312f.pdf [Broken]


    “Synchronous Converters” employs synchronous rectification;
    http://en.wikipedia.org/wiki/Active_rectification
    http://en.wikipedia.org/wiki/Switched-mode_power_supply#Voltage_converter_and_output_rectifier

    Also see;
    The LTC4357 “Positive High Voltage Ideal Diode Controller” can be used to replace Schottky power diodes in redundant power supply selection.
    http://cds.linear.com/docs/en/datasheet/4357fd.pdf
     
    Last edited by a moderator: May 7, 2017
  15. Jun 4, 2015 #14
    I was thinking something more like this. The higher voltage would limit copper losses. The big DC bus could be unconditioned (in its primary, user side path anyway) to avoid conditioning losses and use IGBT switchers for rack power. IGBTs have a higher gate capacitance which will limit the switch frequency, but the skin effect and other effects limit switcher speed for high currents anyway. That means spending more on inductors (and likely capacitors since we are likely looking at a resonance mode switcher), but the efficiency savings would likely be worth it.

    There would be safety issues as well. A high voltage, high current DC power bus is a dangerous combination. It would need to be armored. But the power industry has been dealing with these problems for decades.
     
  16. Jun 4, 2015 #15

    Baluncore

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    The IXYS Sixpack IGBT Module, VWI 6-12P1, is a 3PH 'H'-bridge.
    It looks like it was designed to be used to convert HV DC to three phase AC for variable speed drives.
     
  17. Jun 4, 2015 #16

    jim hardy

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    Thanks Baluncore

    you showed me a whole new approach. That LTC4537 is an eye opener.

    Hanging out at PF one learns a LOT and that's good for the psyche. Thanks for sharing your expertise.


    old jim
     
  18. Jun 4, 2015 #17

    Baluncore

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    The karma is reciprocated.
     
  19. Jun 4, 2015 #18

    The Electrician

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    If you liked the LTC4357, you'll love this: http://www.linear.com/product/LT4320
     
  20. Jun 4, 2015 #19

    jim hardy

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    Wow.

    I try to learn something every day. Those ideal rectifiers are a week's worth !

    What a natural step from switching regulators. Interesting possibilities with synchronous alterators....

    Thanks, guys..
     
  21. Jun 5, 2015 #20

    anorlunda

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    I expect that the very big server farm owners would assign some of their best engineers to address this problem. Does anyone here know some of the things they have adopted? rejected?
     
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