Power distribution units (PDUs) play a key role in today’s computing and networking sectors. This multiple output, rack-mounted electronic hardware allows the efficient transfer of power throughout datacenters and other communication hubs (Fig. 1). Applications for PDUs are growing, fuelled by the need for ever greater levels of computing power, larger data storage capacities and higher bandwidth levels. Industry analyst firm TechNavio predicts that the annual global PDU market for datacenters will be worth about $770 million by 2014, as existing equipment is upgraded, made more energy efficient, and their system reliability enhanced.
With a significant proportion of modern power infrastructure now being provided through PDUs, the degrees of reliability and efficiency that they are able to offer is critical. Stringent legislation that has been put in place to curb carbon emissions means a great deal of pressure is being directed onto datacenter engineering staff.
There are various methodologies being employed for the sensing of the current consumption for each input/output in a PDU system. However, many of these have serious technical challenges associated with them.
Sensors are electronic components meant to translate a physical dimension (pressure, heat, light, etc.) into an electrical signal (an electrical potential), so that it can be measured or processed. For current sensors the physical dimension is the electrical current. The sensor output is an electrical potential proportional to the current flowing in the bus bar, PCB or nearby cable (Fig. 2). Hall Effect current sensors do not measure the current directly. Instead, they are sensitive to the magnetic field generated by (and proportional to) the current. The measurement principle is therefore fully non-intrusive and electrically isolated from the measured current.
Current Sensing Challenges
Sensing systems being utilized for monitoring PDU current consumption need to be able to deal with high voltage transients. Likewise, these systems potentially can be left vulnerable to inrush currents. As a result, a high degree of protection needs to be factored into the sensor system’s design. As space within a datacenter environment is at a premium, a small footprint is also desirable. Furthermore, system accuracy needs to be maximized.
From basic electromagnetic theory, it is known that the flow of an electrical current through a conductor generates a magnetic field around it. The field generated can be detected through the principles of the Hall Effect. The voltage drop across the Hall elements of the chip is proportional to the applied current. This voltage drop is amplified and various corrections are applied (for instance, to compensate for offset and sensitivity drifts). The final output is either analog or digital (PWM). For the analog output, there is typically an offset of 2.5V (at zero current), then the output rises or drops linearly by +/-2V in proportion with the applied current. In advanced linear Hall sensors many parameters, including offset and sensitivity, are user-programmable.
Hall Effect sensors offers a non-contact technique to monitor PDU electricity consumption in order to improve system longevity (Fig. 3). Conventional Hall Effect sensor solutions unfortunately take up considerable board space and also are relatively expensive. The main problem with these sensing devices is that they only respond to magnetic fields that are perpendicular to the sensor surface.
As these sensors rely on the use of a looped ferrite core they are not only bulky and expensive, but even slight inconsistencies in their construction will impinge on their overall accuracy. Also, as a certain amount of power dissipation through the packaging of these components occurs, there can be operational lifespan issues.
The Triaxis current sensors (Fig. 4) from Melexis present a non-intrusive solution that can be used to measure the current directly from a PCB trace (normally 5 A to 50 A) or a bus bar (normally 50 A to 1000 A). What differentiates these devices is their ability to sense the field generated in a PCB trace or bus bar without need of a ferrite toroid or steel lamination stacks. This is due to the patented IMC™ (Integrated Magnetic Concentrator) ferromagnetic film which is deposited onto the chip. When used in conjunction with complex mathematical algorithms, it allows the density of magnetic flux parallel to the sensor to be precisely and rapidly determined. This proprietary technology intrinsically result in magnifying their magnetic sensitivity.
The analog output of a Triaxis sensor can support a response time of 4 µs. This translates into a 200 kHz bandwidth compared with 50 kHz for conventional Hall Effect sensors. While there will normally be 2-3% non-linearity with conventional devices due to the ring’s construction and some hysteresis effects, Triaxis devices have just 0.5 % non-linearity. In addition, the temperature drift will also be reduced considerably if a Triaxis device is used instead of a conventional Hall Effect sensor.
The contactless sensing solution outlined here, based on proprietary Triaxis technology, is highly suited to accurate and reliable monitoring of PDU input/outputs (Fig 5). It has the strong linearity and responsiveness needed to obtain meaningful results, as well as allowing the implementation of effective temperature compensation. As no ferrite core is required, it fits into a package, thus saving board space.
Non-intrusive sensing solutions that can offer intrinsic isolation and protection against high transient voltages or inrush currents have a clear advantage over the more tradition approaches (Fig. 6). The IMC technology employed by the Triaxis sensor devices makes it possible to amplify the magnetic field incident on the sensor chip, thus enhancing sensitivity levels and minimizing noise.