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MOTHERBOARD VOLTAGE REGULATION EFFICIENCY
So far we have identifed several efficiency bottlenecks: The high power draw of the CPU and the VGA, and the (in)efficiency of the AC/DC power supply. There is yet another rarely discussed botleneck. This is the Voltage Regulator Module on the motherboard itself.
Looking specifically at a single CPU desktop system, AC power is delivered to the PSU, which then coverts it into three main DC lines: +12V, +5V and +3.3V for delivery to the components. The DC current gets to the motherboard in two ways, the 20 or 24 pin main ATX connector and the dual 12V connectors (or quad 12V in the case of dual CPU boards). In the most current P4 and A64 motherboards, all the power to the CPU is obtained via the dual 12V lines. The VRM, whose components are usually arrayed close to the CPU, performans the function of converting the 12VDC input from the PSU into the ~1.4VDC (and other) voltages needed for the CPU. By now you should not be surprised that there are losses through the VRM.
The efficiency of VRM circuits on motherboards ranges considerably. The very best server motherboards desgined for critical enterprise or military applications can reach past 90% efficiency. The cost of the components for such circuitry is extremely high, as is the overall cost of the board.
Efficiency curve of high end server board.
(Graph courtesy of Andy Watts, Test Automation at Intel)
Typical retail motherboards range closer to 75~80%. Intel's own retail offerings tend towards the high end of this range, possibly up to 85% for entry level server boards. In comparison, budget products may offer even less than 75% efficiency.
Efficiency curve of good quality Intel retail board.
The bottom line is price. Higher efficiency can be had for a price, and that price gets steeper the higher you go. Also, it is difficult to design out the bottom end drop-off in efficiency when the VRM is optimized for high power delivery (>100W). So it is not realistic to have high efficiency at Pentium M level power draw (barely 20W) and still have high efficiency ramping up past a hundred watts. In other words, even if you could get a P4 desktop-type CPU to idle really low, the power range might be too high for VRM linearity so that the overall power draw at idle would be little improved. For mainstream desktop and server systems, efficiency losses in the VRM will hit hard when trying to reach the idle levels proposed in the new Energy Star spec. It will be interesting to see whether any motherboard makers make any moves to improve VRM efficiency at this time.
LOW IDLE JUST FEASIBLE TODAY w/ SOME DESKTOP SYSTEMS
When you add up the efficiency losses in the PSU and on the motherboard, how much AC power does it take to power up, say, the Intel P4-2.8C (Northwood) that is powering the system I am working on now?
Let's assume that the processor demands the theoretical maximum of 80W when I am working on large graphics while multitasking with 6~8 other programs. Assuming 80% efficiency in the VRM, this means 100W DC enters the board. For an 80 Plus certified PSU with 80% efficiency, 125W AC input is needed to ensure 80W of power at the CPU. This matches the highest peaks seen on my AC power meter when the system is worked very hard. The system efficiency is about 64%; conversely the total energy loss to heat is 36% or a third. Obviously, the motherboard is a significant bottleneck to improved efficiency in this desktop computer.
It's difficult to calculate what happens with my system at idle for many reasons, but I can measure it with the AC power meter: 74W. The PSU I am using might not pass 80 Plus, but it would come within a hair. This is a 1GB ram system with a low power dissipation Matrox P650 video card and no extras except an additional hard drive, which I know draws about 6W AC at idle. So the lowest idle I can achieve here with just one desktop HDD is about 68W. Scaling back to 512mb ram might bring it down to the low 60s, but it would still be higher than the idle max of the proposed draft Energy Star spec.
An AMD Athlon 64-3500+ (90nm core) CPU system running in the lab with a single hard drive and otherwise similar equipment draws only 51W in idle with Cool 'n' Quiet enabled and 64W without. This is a substantially faster processor than the P4-2.8 for most applications. But lest you take away the lesson that this processor is somehow intrinsically superior, just wait. Competition between the big silicon part makers is such that we've seen one surge ahead technologically, only to be caught up and passed in the next surge by the other. As mentioned earlier, Intel has brought dynamic clock / voltage throttling at idle to their desktop processors, and there is no going back. Both major processor companies will be producing more efficient processors with smarter thermal management in the coming years.
Even today, 50~60W AC idle is clearly within reach for a certain segment of systems. By the start of 2007, we can certainly expect to see greater progress on the low power, high efficiency front, with technology and thinking from the energy-miserly mobile sector permeating more thoroughly through all the processor divisions. Higher efficiency PSUs will also be more widely available with increasing efficiency competition among PSU makers spurred on partly by rising Intel power supply design guide specs and recommendations.
IS 2007 TOO LATE?
Without looking at all the specs for the other categories of computers, we'd suggest that unless there was a serious hue and cry among the attendees, the specification has no teeth. From discussions with many experienced people who were there at March 15, the consensus seems cautiously optimistic. Yes, there were grumbles and complaints, and perhaps some things will have to be tweaked. But there are nearly two years before the new spec becomes effective, time enough to prepare for anyone who is interested enough. A great deal of computer technology sees the light of day in such a span of time.
It is a brave new world the Energy Star team is building, and we wish them all the encouragement. We hope for everyone concerned that the proposed specifications are not watered down. The target of 20~25% initial product compliance with Energy Star 2007 is a good one. If everyone is a star, then, most people would say, no one is a star. Allow the exceptionally efficient products to be recognized, and let them be examples for others to match. This is the whole point of a voluntary program ¬ó to encourage high performance. Make it too easy and it becomes meaningless.
What is SPCR's interest in the new Energy Star program, in Intel's evolving PSU design spec, in Ecos Consulting's 80 Plus program or in the prospect of more power efficient CPUs? The answer is simple: Low heat and high efficiency are the cornerstones for quiet computing. Less heat, less need for fan airflow, less noise: On a certain level, it really is as simple as that. The efforts to produce exceptionally efficient computers for Energy Star approval will create the foundations for quiet, ergonomic computers. Clever marketing people will not miss such an opportunity. If some exceptionally quiet, attractive and powerful computers are not among the Energy Star approved products of 2007, I promise to eat my crystal ball. In the meanwhile, if you don't want to wait that long, SPCR remains dedicated to silent computing with today's systems and components.
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My thanks to the many individuals who helped me gather the information for this article:
Craig Hershberg, Energy Star, EPA; Dan Snyder, PR Manager, Intel; Brian Griffith, Power Delivery Architect, Intel; Andrew Watts, Test Automation, Intel; Chris Calwell, VP Policy & Research, Ecos Consulting; Peter Ostendorp, Research, Ecos Consulting; Vincent Chang, US Manager, Seasonic; Teresa de Onis, Desktop Branding Manager, AMD; Vic Bhagat, Product Marketing Manager, AMD
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