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Design Features
The Role of Hall Effect Sensors in Power Distribution Infrastructure
Rik Moens
Melexis
Power distribution units (PDUs) form an essential part of modern computing and data communications hardware. They provide multiple outputs for transferring electrical power with maximum efficiency, controlling the power capacity and safeguarding against the possible causes of supply interruption. With an ever increasing need from tech savvy consumers for higher data throughput and greater quantities of data storage capacity, as well as tough international legislation now governing CO2 emissions, the demands being placed on these units are proving challenging for engineering teams to satisfy.
Figure 1: Functional Block Diagram of PDU System
A large proportion of datacenters are now totally reliant on PDUs. As a result maximizing the levels of efficiency and reliability offered by them are both becoming major concerns. Being able to accurately monitor power consumption is now therefore vital for PDU manufacturers, so their products can meet with the ambitious power unit effectiveness roadmaps that search engine providers (like Google and Yahoo) have set. Various methodologies exist for the sensing of the current passing through each input/output in a PDU system. Many of these, however, have some sort of technical drawback.
Current sensor design considerations
A sensing system employed in the monitoring PDU current consumption needs to firstly be accurate. With space within a datacenter environment often limited, it should also only take up a small amount of board real estate. In addition, the sensor has to be able to deal with high voltage transients and it can at times be left exposed to inrush currents - as a result, a high degree of protection must to be included in its design.
Hall Effect sensors
The flow of an electrical current through a conductor generates a magnetic field around it and the field generated can be detected through the Hall Effect. The sensor output voltage relates directly to the current. Hall Effect sensors offer a non-contact technique for the monitoring of PDU electricity consumption which improves system longevity. Standard Hall Effect sensor solution implementations will unfortunately take up a considerable amount of board space and are relatively expensive too. The main problem witnessed with these sensing devices is that they only respond to magnetic fields that are perpendicular to the sensor surface. As these sensors rely on use of a looped ferrite core they will not only be bulky and expensive, but even slight inconsistencies in their construction will impinge on their overall accuracy. Also as there will be a certain amount of power dissipation through the packaging of these components there can still be issues in terms of its operational lifespan. Other issues that need to be taken into consideration include a tendency in such implementations for sensitivity to drift with temperature (due to heat dissipation that is proportional to the current to be measured), plus inrush current (as a high inrush current will saturate the ferrite core and result in a significant hysteresis when going back to zero current). This will clearly be compounded in datacenter environments, where servers are packed tightly together and large quantities of heat are constantly being generated. Even with highly efficient cooling mechanisms in place the effect of temperature on the accuracy of some sensing systems means that the quality of the data acquired is not as high as it should be. As the demands being put onto PDUs get tougher to satisfy using conventional sensing methodologies, the need for a more sophisticated approach heightens. This is encouraging a migration towards more innovative sensor implementations.
Figure 2: Standard Hall Sensor Structure
Figure 3: Triaxis Hall Sensor Structure
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