Streamlining power systems for servers

In computing systems, the need for more efficient systems has become paramount, especially in servers and mainframes, where the high power density per square foot of office space has demonstrated the limited capacity of office buildings to meet this large demand. With the rapid improvement in power delivery, new opportunities must be found within the system to reduce energy consumption. These opportunities lie where most people are not lookingnot in a single subsystem, but in the interaction between subsystems.

Theyre taking over

Growth of the Internet has created the need for server farms. Large clusters of servers collocated at critical junctures where multiple trunk lines provide very high-speed access to communications. These buildings are often in expensive rent areas, which translates into the cost per square foot of floor space charged for housing the servers. Historically, power has been proportional to the operating frequency of the CPU, thus the power demand has increased 2 (double frequency) * 4 (processors) * 5 (reduction of rack space per server) = 100 times during this short time.

This concentration of power often provoked the realization that typical buildings are not designed to provide the high power demanded to power a room full of these dense servers. Also lacking are redundant power supplies to protect against blackouts or brownouts. Finally, the building airconditioning is insufficient to cool the thermal load found in these server clusters.

One design focus has been on the layout of the server itself, its card spacing and chassis, and whether they allow efficient airflow for cooling. The AdvancedTCA card cage has offered improvements and challenges. The denser and larger blades in the 8U x 280mm format boost performance in server and communications switching centers. But thermal management can be a challenge for the cards, especially where the board spacings are tight. Cooling 200W per slot while cramming boards and modules in the smallest amount of rack space requires careful attention to power efficiency. Every percentage in lost efficiency will be dissipated as heatand we have a stack in reducing, not increasing, the number of fans required.

In a typical server system, even if the server power is 48Vdc, there is always an AC power source. This primary power source is often neglected when looking for the sources of power loss and heat. Today, there are standards on how much distortion can be introduced onto the power grid, making it necessary to add power factor correction to the input of the bulk regulator or AC/DC power system.

The next stage is usually a traditional isolated DC/DC converter that may be housed in a standard brick-type converter, followed by several POL converters distributed throughout the server and located close to the critical circuits. The power of each of these circuits has also increased as more memory was added to the computer systems. More complex chipsets were added to enable increased parallel operation of, and faster I/O to, the processors.

Who consumes power?

In a typical server processor, the V_core converter processes 130A * 1.3V = 169W of power at approximately 88 percent efficiency. This creates a loss of 23W that requires an input of 192W. If the efficiency is increased by 2 to 90 percent, the resulting input is reduced by only 5W. But if the converter is designed to require less or no cooling, then the resulting power savings can eliminate a fan required to blow 200 linear feet per minute (LFM) against static pressure of 0.25-inch H₂O. This savings can be 2A to 4A at 12V or 20W to 50W.

In the past, when the V_core converter efficiency was only 75 percent, the power loss would have been 56W each times 2 for a total of 112W, which could be reduced to only 46W using the latest technology. But now the power of the fan is approximately equal to the losses of the converter.

Thus, designing the V_core converter with a technology that eliminates the need for a dedicated fan can save more power than if the converter were operating at 100 percent efficiency. Such a solution will make use of high-efficiency MOSFETs and a multiphase controller. A comparison of the converter efficiency from three years ago and today demonstrates how the solution can eliminate the need for cooling through a combination of enhanced efficiency and improved MOSFET packaging that reduced thermal density by 50 percent over the previous solution at the new higher current.

Less heat, more power

Whether this savings is accomplished by reducing the amount of air or eliminating the fan entirely, the opportunity exists to significantly reduce the power consumption for a typical server. There can be as many as seven fans in a single 1U server, each drawing up to 1A.

More fans have been added because the back pressure of the new higher-density servers requires more power to move the same airflow described in reference 13, which describes processor cooling. If the total power dissipated within the server is 500W, then each fan is cooling approximately 70W of power and consuming 12W to accomplish this. It becomes clear that simply adding more airflow has reached the point of diminishing return. More efficient thermal design is now as important as more efficient power conversion.

If we now step back and look at the entire power train, we can see the impact of fan motors in a broader view. Beginning with the electronic loads totaling 280W typicalnot worst casethe fan losses are still among the largest. At 88 percent efficiency, there is little room to reduce these losses. Eliminating a fan can reduce losses by 12W. Improving the DC/DC converter with a new bus converter can reduce the losses of that section by 12W. Improving the efficiency of the PFC section and offline rectifier can reduce the losses by another 41W. This accounts for about 50 percent of the losses at full load.

At light load, the fan represents a much greater percentage of the total power losses, thus the fan is turned off whenever possible in many desktop and notebook computers. In servers, variable-speed fan drivers can improve the light load efficiency when used in conjunction with the power converter to match cooling capacity to power losses. Variable-speed motors can also be used to reduce the audible noise by raising the frequency above 20kHz.

Past improvements in power-conversion efficiency have reduced the total losses from AC prime power to the CPU input, but future improvements will demand overall solutions based on simultaneously improving the thermal, mechanical and power-conversion systems to achieve new levels of energy utilization.

- Carl Blake

Technical Marketing Manager, Switch-mode Power Supply Products

International Rectifier