High end computing systems and data communication networks are becoming increasingly reliant on the use of power distribution units (PDUs). PDUs are basically multiple output modules through which power is delivered directly to server racks. With ever greater concerns about energy consumption, it is critical that these items of hardware offer the means to constantly access data on the current flowing in and flowing out, so that both power efficiency and operational reliability are assured.

When monitoring PDU current consumption, the sensing system utilized needs to, first and foremost, have a high degree of accuracy. It will also have to contend with high voltage transients and inrush currents, so possession of fully effective protection mechanisms is vital. The limited space available in the target application must also be considered, with highly compact solutions clearly being favorable. 


Figure 1: Basic Structure of a PDU


Several different measurement techniques can be utilized to ascertain the current that is passing through each input or output of a PDU. Many of these, however, have technical drawbacks that engineers need to be aware of. Fundamental electromagnetic theory states that an electrical current passing through a conductor will generate a magnetic field around it. In turn, the strength of the magnetic field generated by such a current can be measured via application of the Hall Effect. Although Hall Effect sensors provide an accurate non-contact technique for examining PDU current, generally devices based on this technology are bulky and take up too much board space. The price tag associated with these devices is also quite high. The fundamental issue is that they only respond to magnetic fields perpendicular to the sensor’s surface and thus need a looped ferrite core through which to detect the current. In addition to making the sensor solution costly and cumbersome, even small inconsistencies in the looped ferrite core’s construction means that it will have an effect on the exactitude of the measurements. Power dissipation through the packaging must also be taken into account, as this can shorten the operational lifespan as well as potentially causing a drift in the device’s sensitivity. This can prove problematic in data communications implementations, such as server farms, where electronics hardware is densely packed together with large quantities of heat being generated.


Figure 2: Hall Effect Current Sensor Based on Conventional Technology




Figure 3: Triaxis Hall Sensor from Melexis


Melexis has reacted to industry demands for more sophisticated current sensing technologies to be incorporated into PDUs. Its family of Triaxis magnetic sensors, which use the company’s proprietary technology, can be implemented into current measurement applications – either sensing current directly from a PCB trace (usually 5A to 50A) or a bus bar (somewhere between 50A to 1000A). These innovative devices can accurately sense the generated field (and thereby the current) while dispensing with the need for a ferrite loop. This is thanks to the patented IMC™ (Integrated Magnetic Concentrator) ferromagnetic film incorporated into their design – which effectively magnifies sensitivity levels. This, with the support of sophisticated algorithms, enables the magnetic flux density parallel to the sensor to be evaluated in a rapid and precise manner. Triaxis sensors’ analog outputs are able to provide a 4µs response time – resulting in a 200kHz bandwidth (approximately four times what conventional Hall Effect sensors are capable of). In addition, the non-linearity these devices exhibit is only 0.5 % (between four and six times better than conventional Hall Effect sensors). Temperature drift will, in addition, be markedly reduced. Since no ferrite core is required, package dimensions are reduced, thus saving board space.  Their programmability makes them highly versatile, allowing them to be adapted to a wide range of different current ratings.


Incorporation of PDUs into power infrastructure is becoming increasingly commonplace, but engineers face considerable challenges when deploying them. They have to safeguard against voltage transients, ensure protection from inrush currents, maintain low bill of materials costs, maximize longevity and keep board real estate requirements to a minimum. Advanced sensing solutions such as Triaxis are leading to more streamlined implementations that exhibit greater accuracy and reliability while taking up less space.