Power Supplies for Datacom Applications

Before the advent of packet switching1, circuit-switched networks processed only voice-band traffic using large numbers of analogue amplifiers, equalisers, and rotary solenoid relays. The -48 V power supplies for these systems were compatible with the large-capacity lead-acid battery systems necessary to meet high up-time requirements.

 

Accentuate the Positive: The Move to +48 V 

The mains-powered energy source was typically unregulated, resulting in system distribution voltages ranging from -75 V at mains high line to -36 V at the end of the backup battery’s charge cycle. Systems frequently overcharged the lead-acid batteries of the day, resulting in the escape of acidic vapours. The crude charge-control systems, humidity, and poor ventilation in the battery rooms of that era, exacerbated corrosion, so engineers preferred to use a negative distribution voltage because it helped to minimise and localise corrosion products.

 

New architectures

Since the advent of packet switching and the conversion of telecom providers into datacom companies, central offices (COs) have provided voice, video and internet communications through common digital resources. The changes in network architecture resulting from this trend, in combination with technology advances in power-subsystem components, have motivated shifts in power-subsystem design: in particular, a switch from negative distribution voltages to positive. 

New battery designs and a trend toward better environmental controls in battery rooms have contributed to this trend. For example, the majority of new facilities and central office upgrades at datacom companies such as AT&T (US), Deutsche Telekom (Germany), Orange SA (France), and Verizon (US) will now operate from +48 V distribution feeds.

This polarity shift allows datacom companies to take advantage of economies of scale for power-distribution hardware already in heavy use by server-class data-processing systems in other sectors. This, however, is just the most visible shift in power-distribution strategies that datacom companies are exploiting. The output voltage range of datacom AC to +48 V converters is substantially narrower than their AC to -48 V predecessors. The narrower range of distribution voltage brings with it several key benefits.

 

Figure 1: The evolution of power-conversion modules: (Left to right); Vicor VI-200 full brick; Mini (1/2 brick);  IBC (1/8 brick);  Full and half VI Chips and SiP (System in Package) plus ChiP (Converter housed in Package)

 

SELV savings

With tighter output-voltage tolerances, the high-line limit for telecom +48 V distribution rails is 60 V, which allows them to qualify as SELV (safety extra low voltage) systems. SELV power distribution systems are less expensive to design and construct than equivalent systems that allow higher voltages because they do not require additional personnel safety features. In addition to hardware savings, installation and maintenance technicians do not require certification for higher-voltage non-SELV circuits, so labour costs for these functions are typically lower as well. SELV systems are also more compatible with high-density design than systems that operate at higher voltages because they require smaller creepage and clearance distances. 

Within the CO, density is a keen concern. In the days of switched circuits, power consumption was only a couple of hundred watts per square metre, whereas today that figure is several kilowatts. Moreover, consumer demand is driving network traffic at a compound annual growth rate of 23%, projected until 2017 and, given the incremental cost of central office real estate, maximising throughput per unit floor area becomes a priority².

 

Narrow-range distribution

Narrow-range 48 V power distribution allows system designers to specify more efficient and more compact converters for downstream loads such as line cards and CPUs. Efficiency for its own sake may not drive hardware selection decisions but, by enabling higher density, energy-efficient converters are compelling for several reasons. Key among these is that they allow designers to drive power and functional densities to the practical limits of current technology, minimise the cooling load, and maintain high reliability. 

Early 48 V SELV power distribution systems maintained their output voltages to ±20%. Current designs typically exhibit tighter output ranges with tolerances of ±10% or better. As a result, for some technologies, the same conversion topology that can source +48 V distribution systems can also provide 54 V outputs and stay within the 60 V SELV high-line limit. Operating the power distribution apparatus at 54 V reduces I2R losses by 26% compared to 48 V power feeds. The higher distribution voltage also enables the use of Power over Ethernet within the facility for on-site communication, remote sensors, and physical-plant security functions.

 

High power density

As power-conversion topologies, switching-device designs, and power packaging technologies have advanced, per-package power capacity and overall power density have also increased (Figure 1 and Table 1). The raw power density, however, does not tell the whole story, particularly for 48 V input/1.x V output devices for CPU applications. These power converters eliminate a conversion stage by replacing both an intermediate bus DC-DC and the CPU’s point-of-load (PoL) regulator, often in a smaller footprint than the original PoL. This approach allows designers to route the 48 V supply on the CPU board, reducing PCB I₂R losses by a factor of 16 or more.

An example of this approach is the Vicor 48 V VR12.5 reference design, which does away with multi-phase conversion topologies, thereby reducing component count while maintaining full compatibility with Haswell power requirements. The architecture also significantly reduces bulk capacitance, increasing power density further. The reduction in component count and lower energy-storage requirements allows designers to locate the power train closer to the processor. This reduces losses and parasitic inductances in those processor supply traces that carry the board’s highest currents and exhibit the largest current dynamics.

 

Table 1: Power density for various power converters with 48 V inputs and 12 V or 1.x V outputs  

 

Conclusion

Datacom applications demand high-density low-loss power subsystems. Tight tolerance +48 V (nominal) power distribution allows sys-tems to qualify as SELV designs, reducing installation and operating costs. Narrow-range 48 V rails facilitate high-efficiency power converters to power line cards and processors. These power components help power-subsystem designers minimise power losses and attain maximum power density.

 

About the Author

Chester Firek works as the Director of Marketing at Vicor Corporation since April 1995. He earned his Diploma in Associate of Science in Electrical and Electronics Engineering at Northern Essex Community College. He then acquired his Bachelor's Degree in Marketing at the Northeastern University.

 

References

1. Roberts, Lawrence: The Evolution of Packet Switching, November 1978
2. Cisco Visual Networking Index: Forecast and Methodology, 2012- 2017, Cisco Systems, May 2013

 

This article originally appeared in the Bodo’s Power Systems magazine.