Data centers are experiencing a 16% annual increase in energy consumption. Due to the growing demand for cloud computing, social media, multimedia, and other computing services, government agencies and commercial enterprises will have to spend approximately $8 billion annually to power and cool data centers. Server systems and cooling account for the majority of this energy consumption.
Traditional server power architectures are reaching the technological limits of energy efficiency. Some strategies to improve efficiency might require redesigning the entire data center power infrastructure to reduce energy consumption, but this solution is prohibitively expensive. Redesigning the data center building and infrastructure can easily cost tens of millions of dollars.
Companies like IDT have addressed the problem by focusing on the overall energy efficiency of the server system, without significantly altering the data center infrastructure or building. IDT's high-efficiency power architecture for servers, known as coolRAC (RAC stands for Resonant AC), is a viable solution capable of delivering significant energy savings.
Overall server efficiency is measured from the AC power input to the processors on the motherboard, and throughout the rack. Server platform architectures typically have an overall energy conversion efficiency of 80% and often utilize high-cost components. This means, simply put, that around 20% of the energy consumed is lost as heat. With solutions like IDT's coolRAC, this conversion can be improved to achieve efficiencies exceeding 90%, thereby reducing energy losses and related cooling requirements. Since coolRAC utilizes the existing electrical infrastructure of data centers, this technology can be easily retrofitted to existing centers without the need for costly building upgrades.
Rack-mounted servers with multiple processors and minimal downtime typically consume more than 70% of a data center's energy. For example, a large data center with 50,000 servers can consume more than 12 MW of power. By improving server efficiency with IDT's coolRAC technology, energy savings of more than 1 MW can be achieved directly. Based on a conservative estimate of $0.10 per kWh, this equates to direct savings of more than $800,000 annually in energy costs. Furthermore, it contributes to increased data center throughput. coolRAC technology utilizes a standard input current (220/480 V three-phase AC) and a high-frequency AC (HF) distribution.
CoolRAC Concept with High-Frequency AC Distribution:
To achieve the maximum effect on overall system efficiency, the first step is to address the efficiency of the point-of-load (POL) converters. In current architectures, buck converters have proven to be the simplest, most efficient, and most cost-effective POL converter topology. However, modern digital integrated circuits require the POL converter to supply a DC current with an output voltage of approximately 1 V. Consequently, a buck converter with a 12 V DC input voltage will have a high conversion rate and, therefore, will not be able to achieve high efficiency for a 1 V DC output voltage. The importance of improving point-of-load efficiency is shown in Tables 1 and 2 by comparing the efficiency and power losses for different architectures at specific stages.
To reduce the conversion rate and increase POL efficiency, the input bus voltage at the load point must be decreased. Figure 1 shows a dramatic increase in POL efficiency with no change other than reducing the input bus voltage in steps from 12 V to 5 V.
Efficiency could be further increased by using silicon switches with a lower voltage rating. Devices with lower voltages would achieve, at a comparable cost, even higher efficiency levels than those shown in Figure 1. This is due to the lower price per silicon area, or in other words, the reduced drain-source conduction resistance (Rdson) per silicon area. Furthermore, depending on the application, a customer might choose to save on silicon costs if high efficiency is not critical, while still significantly improving efficiency compared to a 12 V input bus.
Given the industry trend toward reducing input supply voltages in modern digital integrated circuits, such as CPUs, GPUs, ASICs, and memories, reducing the input voltage at the load point has become a primary concern for highly efficient POL converters. It appears that for today's industrial applications, a 5 VDC POL input bus voltage is the optimal solution for server systems. This low input voltage significantly reduces the voltage conversion rate and increases energy efficiency. Furthermore, power components with a nominal voltage of 5 V are readily available on the market. The 96% peak efficiency for a 5 VDC to 1.2 VDC buck converter, shown in Figure 1, was demonstrated using silicon switches with a nominal voltage of 30 V, and it would be possible to increase it even further with devices rated at 12 V.
However, supplying the input current to the load point at lower voltages increases losses in the distribution infrastructure, making a 5 VDC distribution system impractical in most cases. Sophisticated backplane board designs, including connectors, backplane printed circuit boards, and expensive busbars, are required to achieve a modest 2% power loss in a traditional 12 VDC distribution architecture for high-end server systems. Changing the distribution voltage to 5 V will increase power losses more than fivefold.
IDT's coolRAC technology delivers a low voltage to the point of load with maximum energy efficiency; thus solving the fundamental conflict of maximizing efficiency for the POL converter (which requires a LOW voltage distribution) and minimizing losses in the power distribution system (which requires a HIGH voltage distribution).
Figure 2 compares the most common 12 VDC distribution architecture with IDT's coolRAC solution. It shows a redundant system with N front-end power supplies feeding K processing units.
Clearly, both architectures consist of an equal number of power conversion stages. Simply shifting the synchronous rectifiers (SRs) from the power supplies to the motherboards allows for a reasonably high distribution voltage (e.g., 50 V peak-to-peak). This minimizes losses in the distribution media. The distribution AC voltage, defined by the transformer turns ratio, is highly flexible, enabling an optimal and scalable solution for server systems with varying power requirements.
Obviously, redundant AC AF distribution architecture presents greater challenges for designers than traditional DC distribution systems, such as generator synchronization and the current sharing required for functions like hot-swappable power supply redundancy. This is the main reason why the market has not generally adopted the AC AF distribution concept.
For many years, experts have recognized the advantages of the AF AC distribution topology, but its implementation has been impractical due to the challenges of controlling multiple AC generators in parallel and developing an efficient and reliable control algorithm for synchronous rectifiers. However, recent advances in analog-to-digital conversion and digital control techniques have made this topology an attractive option in the market.
Given the fundamental differences between alternating and direct current, AC AF distribution offers the additional advantage, compared to DC distribution, of being able to use reactive power components for regulation, in addition to active power dissipation components. By using resonant topologies in the described coolRAC architecture, hot-plug and redundancy functions can be provided without the need for OR or inrush current control circuits.
Of course, all these advantages have only been possible after developing new digital and analog control algorithms and implementing them in new integrated control circuits to make this new AF AC distribution architecture a viable and cost-effective system.
Conclusion:
The electrical power requirements of data centers continue to rise to meet the growing demand for processing capacity. Since servers consume the majority of the power in a data center, improving server efficiency would have the greatest impact on saving energy, reducing cooling requirements, and lowering costs.
IDT's coolRAC intermediate bus architecture is an example of a server power solution that increases efficiency, reduces heat losses, and consequently lowers energy consumption without requiring major changes to the data center design. Further improvements to silicon devices, such as 12V MOSFETs for POL converters, will enable even greater overall system efficiency.
Each year, more than eight million servers are installed worldwide, and it is estimated that the total annual energy consumption of server systems exceeds 60 billion kilowatts. As data center managers become more aware of their companies' growth needs and costs, server energy consumption will become a key factor in determining the most efficient and cost-effective system.
