Power reqmts for new imac

in Current Mac Hardware edited January 2014
Anyone know what Apple is using to power this new beast ? There is a ton of caps showing through the back cover, and my guess is that they are using a 24V converter and a ton of discrete components to make it all happen ... Seems like a lot of room for improvement.


  • Reply 1 of 8
    stoostoo Posts: 1,490member
    Those capacitors won't be the PSU ones. They'll be the on board power farm to make sure that the components get nice clean power. Any PSU capacitors would be kept well away from the user.
  • Reply 2 of 8

    Originally posted by Stoo

    Those capacitors won't be the PSU ones. They'll be the on board power farm to make sure that the components get nice clean power. Any PSU capacitors would be kept well away from the user.

    What you say is true, the little silver box is the main power "brick", and then the on board power farm is directly converting to the CPU, the memory, the airport express, and the other stuff that needs nice clean power. There are much better and much more modern ways of doing this is all I am implying.
  • Reply 3 of 8
    stoostoo Posts: 1,490member
    Then why does everyone in the technology industry use piles of capacitors to clean power for ICs?
  • Reply 4 of 8
    Bulk capacitors are used on circuit boards to supply current during transient load changes that occur faster than the power supply loop filter can react. No power supply can respond instantly to a quickly changing load, so bulk capacitors are generally sprinkled across the board to handle these spikes in current demand until the main supply can catch up. If a particular voltage rail must supply 15A and the loop filter takes 10us to react to a load step, you will need enough capacitance to bridge that gap to prevent the voltage from drooping.

    Smaller capacitors are there to suppress noise generated by other components (in the MHz-GHz range), not from the power supply itself (in the KHz range). High frequency components require large numbers of "decoupling" or "bypass" capacitors to decrease the impedance of the PDS (Power Distribution System) at high frequencies. This is required not only to suppress noise, but also to create low-inductance paths for signal return currents and to provide protection from EMI (Electro-Magnetic Interference). A large device like the G5 processor likely has a few bulk capacitors in the vicinity as well as at least 20-30 0.1uF capacitors directly underneath for decoupling. Additional caps to suppress EMI are even more important in a product like the iMac, which does not have a nice metal case to act as a Faraday cage and keep the emissions contained.

    I haven't seen the picture to which you are referring, but you may also be seeing the individual power rail converters that step the intermediate voltage from the main power brick down to the voltages required by the various devices on the board. The design I'm currently working on has 7 rails: 5V, 3.3V, 2.5V, 1.8V, 1.5V, 1.3V & 1.2V. There will be a supply for each rail and each supply will have a few relatively large caps.

    This is all just speculation, of course, as I haven't seen the picture you are talking about.
  • Reply 5 of 8
    powerdocpowerdoc Posts: 8,123member
    The power alimentation of the Imac G5 is very small for heat and place reasons. As Fawkes explained, it certainly not sufficiant for quickly changing load, thus more capacitors are needed with a small alim than a very big one like the one of the powermac.
  • Reply 6 of 8
    I just saw a picture of the guts... those caps are definitely part of individual power supplies for the low voltage rails. I count 7 individual voltage rails there, although there might be more hidden. Each one will need a few large electrolytic caps to function.

    Those are nice big discrete converters with big high and low-side FETs--the sort of thing you would do if you needed very high efficiency. The higher the efficiency, the less power you lose to heat, and I expect this was of major concern when designing the iMac!
  • Reply 7 of 8
    Kind of off topic, but another thing I notice looking at those internal pictures is that I can't see a way for Apple to have avoided the 'chin', even if they had used an external power supply, as some have suggested.

    In addition to the speakers--which I suppose could have been external too--there also appears to be a cooling unit for the G5, and it certainly needs to be at the air intake; it, more than just about anything else in the case, requires cool, fresh air. Had that been located anywhere else in the case, it would have experienced a heat-rise of ~10-15°C. If the chin weren't there, I'm not sure where else this could have fit.

    Here's the picture, by the way...
  • Reply 8 of 8
    kcmackcmac Posts: 1,051member
    Look here

    Says the new iMac uses 180 watts vs. 170 on the eMac. Don't know about the older iMac.
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