As the number of servers and other computer equipment in data centers drastically increases over time, being able to efficiently power multiple equipment becomes critical. To provide power to the large number of equipment in data centers, power distribution units (PDUs) are often used. Similar to a power strip, PDUs distribute the power at its input to its multiple outlets. Each outlet can be used to power a different server or other type of computer equipment. The current drawn from each outlet is measured and, along with the measured voltage, is used to calculate the power drawn from each outlet.

To maximize the number of outlets that can be implemented in a given form factor, it is imperative for the current sensing circuits of the outlet to be compact. Rogowski coils and current transformer current sensors are often relatively large, which in turn, leads to more area being occupied per outlet. An alternative option is to use shunt current sensors, which are relatively smaller than current transformers and Rogowski coils; however, shunts inherently do not have isolation, so they would require extra circuitry that increases the solution size if isolation is required. In addition, the shunt temperature would increase at higher currents. Due to the increase in temperature from operating at high currents and the heat generated by the servers themselves, the accuracy of the shunts could also drift as well since the shunt resistance drifts across temperature.

To address the limitations of the other potential PDU current sensors, a Hall-effect current sensor, such as the TMCS1100, can be used. The TMCS1100 is a galvanically isolated Hall-effect current sensor capable of DC or AC current measurement with temperature stability. The TMCS1100 Hall-effect sensor enables isolated, compact current sensing for PDU applications as well as other end equipment that may require compact current sensing, such as power quality meters.

For maximizing the accuracy of the power measurements, a high-precision ADC, such as the ADS131M08, should be used to measure the output of the TMCS1100 current sensor. The ADS131M08 device is an eight-channel, simultaneously-sampling, 24-bit, 2nd order delta-sigma (ΔΣ), analog-to-digital converter (ADC) that offers wide dynamic range. Using an ADC for the sensing and a separate microcontroller for the calculations provides flexibility when mapping channels. As an example, if it is desired to sense seven outlet currents and one input voltage, the ADS131M08 can support this by connecting one voltage sensing circuit to one ADC channel and seven current sensing circuits to the other seven channels. With a fixed function device that calculates the metrology parameters, typically only a maximum of four currents can be supported. As a result, at least two fixed function devices would be needed to sense one voltage and seven currents. In comparison, only one ADS131M08 device would be needed for this same scenario. Reducing the number of devices needed for PDUs that have a large number of sockets further reduces solution size and cost.

The processing in this design is done by the MSP432P4111, which acts as the metrology calculation microcontroller. This device has an Arm® 32-bit Cortex®-M4F CPU with Floating-Point Unit and Memory Protection Unit, a real-time clock, port mappable GPIOs, an AES encryption and decryption accelerator, and a CRC calculation module.

This application report describes how to use the TMCS1100 Hall-effect current sensor, ADS131M08 precision delta-sigma ADC, LM27762 charge pump, and a metrology calculation microcontroller to design a Class 1 energy measurement system. The results for an example implementation is also shown. The Table 1-1 table shows the key system specifications of this example implementation.