Power
Dealing with the data centre’s appetite for power
Bob Cantrell, senior application engineer, Ericsson Power Modules, investigates how to make data centres more energy efficient
T
he cost of powering a data centre continues to be a large and growing concern for operators. Although the balance sheet can justify the cost of power consumed to perform useful work that generates revenue, a large amount of power is lost through electrical inefficiency. It can be felt in the heat radiated by servers and power modules, and heard in the buzz of air-conditioning or other cooling systems. More importantly, it is perceived as a financial burden that must be minimised. One area where improvements can be achieved is in the supply of power to the data centre’s servers. Today even a single server card can consume over 1kW. In a large data centre, which may contain hundreds of racks, power is supplied to the servers through a network of power modules that begins with a bulk AC/DC supply powered from the AC line provided by the utility company. This usually charges a bank of back-up batteries as part of an uninterruptible power supply (UPS) that may provide a high-voltage DC output, typically 380V, for distributing power to the data centre’s server racks. Alternatively, the UPS may contain an inverter that generates a 240V or 120V AC output for distribution to the racks.
Depending on the distribution strategy (AC or high-voltage DC), servers may contain an AC/DC power supply or DC/DC converter to provide a local 48V DC isolated power bus. An additional intermediate bus converter is often used to generate a semi-regulated 12V bus to power point-of-load (POL) DC/DC converters. A single server can contain several POL converters, positioned close to the processors and associated components such as application-specific integrated circuits (ASICs) or field- programmable gate arrays (FPGAs). In the future, some of the POL converters may be powered directly from the 48V DC bus – more of this later. If the total efficiency of the power distribution network is 80–90 per cent, then more than 10 per cent of the power supplied by the utility (and paid for by the data-centre owner) is dissipated as heat by these power modules. This unwanted heat must be removed to ensure that the environment remains at a stable temperature within the specified operating range, which is chosen to ensure adequate system reliability. For many data centres this is 20–22°C, which is usually maintained by
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air-conditioning, sometimes assisted by extra chillers. Another approach taken by some operators is to choose locations in cold, northern climates when building new data centres, so that the cooler outdoor ambient air can be used for cooling at a lower cost.
Considering the combined cost of
power-converter losses and the power to run the cooling system, a significant reduction in the utility bill could be achieved if the overall power supply efficiency could be improved by just a few points. If all data centres worldwide were to increase efficiency by just one per cent, electricity savings alone would equal nearly one billion euros.
The transition to digital power Conventional switched-mode power converters tend to reach a peak of efficiency just below their maximum load. The efficiency is slightly reduced closer to maximum load, but is significantly reduced at lower loads. These variations occur because the power supply’s behaviour is determined by components such as capacitors of fixed value. These are chosen to ensure that the feedback remains stable across a wide range of operating conditions, but cannot ensure uniform efficiency across the load range. The advent of digital power now offers a solution to overcome this limitation. Unlike conventional analogue power supplies, digital power supply characteristics are determined by firmware. The values of the registers that govern the operating parameters can be changed more quickly and easily compared to using components such as capacitors that have fixed values.
Early adopters of digital power have taken advantage of its enhanced flexibility to streamline testing and set-up, and to leverage economies of scale by creating platform power supplies that can be configured for a variety of applications or different end-user requirements or operating conditions, simply by loading different configuration files. Digital power functionalities can also help designers deal with the complexity of modern power distribution, such as the large number of different voltage rails required for multicore processors or FPGAs, and can respond to changes in line and load conditions as network traffic demand fluctuates.
Digital power modules are usually adjusted by a central controller, which communicates with the module over a power management bus (PMBus) connection as illustrated in Figure 1. PMBus is an industry standard developed from the system management bus (SMBus) specification, and defines a physical connection and protocol for exchanging data between power modules. In addition, digital power supplies can be stabilised using fewer on- board capacitors, which enables smaller power modules thereby liberating extra board real estate to host greater processing power and hence offer more, faster, and better services for end users.
The next step: software- defined power Software-defined power introduces real-time adaptability to digital power supplies. In a software- defined power architecture (SDPA), a software application hosted on the central controller takes over,
adjusting the power supply in real time in response to changes in load. It can also compensate for the effects of slower variations, such as component ageing.
Functionalities made possible by SDPA include dynamic bus voltage (DBV) adjustment, which optimises bus voltages as load conditions change, and adaptive voltage scaling (AVS) that responds to changes in the workload of individual processors to minimise power consumption.
Figure 1: Digital power modules controlled by instructions delivered across PMBus connection
proprietary solution. This type of approach has proved unsuccessful, as data-centre owners have historically placed higher value on reliable and consistent supply of modules, and highly competitive pricing.
Future trends: saving power and board space
Alongside changes in the way power modules are controlled, a new direct conversion strategy that leverages the latest advances in converter topologies and power semiconductors is also attracting the attention of power-system designers. Modules designed to be positioned at the point of load and convert directly from a higher DC bus voltage such as 48V to a voltage like 1.0V to power the latest processor cores can help designers reduce the number of intermediate converters in any given system, and thereby further optimise energy efficiency and reduce power dissipation. Direct conversion also typically should reduce the total board surface area occupied by the power solution, by allowing smaller modules and fewer power converters. In addition, distributing power at four-times higher
Conclusion
Power consumption has become a major concern for data-centre owners and operators, who are keen to evaluate which OEMs can deliver the most efficient solutions. Gaining some insight into how power is distributed to the hardworking computer servers can help with understanding how design improvements here can contribute to better overall efficiency, while at the same time supporting increased computer performance. Digital power leading to software-defined power, as well as other advances such as direct 48V-to-POL conversion, continue to bring new and greater opportunities to raise system energy efficiency and contribute towards controlling overall data-centre operating costs.
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voltage dramatically reduces i2R losses in conductors such as cables and PCB traces.
Securing market acceptance The data-centre industry is starting to adopt innovations such as digital power and software-defined power. The work of the AMP Group, which establishes comprehensive industry standards that allow interoperability between power modules from its three member manufacturers (Ericsson Power Modules, CUI Inc. and Murata Power Solutions), gives equipment buyers the assurance of a second-source option. Often, in the past, pioneers of new power technologies have sought to captivate customers by offering a new but
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