Compact Multichannel Current Sensing

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1 Introduction

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.

Table 1-1 System Specifications For Example Implementation

FEATURES	DESCRIPTION
Selected current sensor	TMCS1100 Hall-effect current sensor
Selected ADC	ADS131M08
Selected microcontroller	MSP432P4111
Number of voltage and current channels	1 voltage and 3 current channels (ADC has 8 channels but only 4 channels of ADC used in this implementation)
Accuracy class	Class 1
Tested current range	0.1–20 A
Selected reference for ADC	Internal reference option for the ADS131M08 device
ADS131M08 Clock(CLKIN)	8,000,000 Hz derived from the 8.000 MHz crystal that is connected to the XTAL1 and XTAL2 pins of the ADS131M08 device
ADS131M08 Delta-sigma modulation clock frequency	4,000,000 Hz (= CLKIN / 2)
SPI Clock	8,192,000 Hz derived from 16.384-MHz crystal of the MSP432 (To support this frequency, the LaunchPad™ crystal was changed from 48 MHz to 16.384 MHz)
Oversampling ratio (OSR)	512
Digital filter output sample rate	7812.5 samples per second
Phase compensation implementation	Software
Phase compensation resolution	0.0090° at 50 Hz or 0.0108° at 60 Hz
Selected CPU clock frequency	48 MHz
System nominal frequency (f_NOM)	50 or 60 Hz (selectable in software)
Metrology parameters measured	1-cycle root mean square (RMS) voltage for detecting sags, swells, and interruptions 10 or 12 cycle RMS voltage, RMS current, fundamental RMS voltage, fundamental RMS current Voltage underdeviation and voltage overdeviation Phase to phase angle Active, fundamental active, reactive, fundamental reactive, apparent , fundamental apparent power and energy Power factor Line frequency with zero crossing indication
Update rate for measured parameters	1-cycle for sag/swell RMS voltage readings; 10 cycles(when using 50-Hz nominal frequency) or 12 cycles(when using 60-Hz nominal frequency) for other parameters
Additional boards used for tested	TIDA-00163 Isolated UART to RS-232 board for communication from LaunchPad to PC GUI MSP-EXP432P4111 LaunchPad with its 48-MHz crystal replaced with a 16.384-MHz crystal
Board power supply	3.3-V output from MSP-EXP432P4111 LaunchPad

2 Implementation Block Diagram

The Block Diagram depicts the block diagram of the example TMCS1100 PDU implementation tested in this application note. This implementation uses the TMCS1100 Hall-effect current sensor, ADS131M08 delta-sigma ADC, the LM27762 charge pump, a metrology microcontroller, and an isolated UART to RS-232 board.

Figure 2-1 Block Diagram.

In this implementation, there are four ADC channels that are used. One of these channels senses the inlet voltage. For sensing the voltage, a voltage divider divides the Mains voltage to a range that can be sensed by an ADC. The choice of voltage divider resistors for the voltage channel is selected to ensure the mains voltage is divided down to adhere to the normal input ranges of the ADS131M08 device. Since the ADS131M08 ADCs have a large dynamic range and a large dynamic range is not needed to measure voltage, the voltage front-end circuitry is purposely selected so that the maximum voltage seen at the inputs of the voltage channel ADCs are only a fraction of the full-scale voltage. By reducing the voltage fed to the ADS131M08 voltage ADC, voltage-to-current crosstalk, which actually affects metrology accuracy more than voltage ADC accuracy, is reduced at the cost of voltage accuracy, thereby resulting in more accurate energy measurements at lower currents.

The other three channels of this device are used to sense the current of the different outlets. Each current channel uses the TMCS1100 Hall-effect current sensor to translate the sensed current into a voltage sensable by the ADS131M08 ADC. To fit within the input voltage range of the ADS131M08, level shifting is necessary. This level shifting is accomplished by using the LM27762 charge pump to create 2.5-V and –2.5-V voltage rails that power the TMCS1100.

The ADS131M08 uses a crystal connected to its XTAL1 and XTAL2 pins to generate an internal clock, f_CLKIN. The ADS131M08 internally divides this clock by two and uses this divided down clock as the delta-sigma modulation clock, f_M. The sampling rate of the ADS131M08 is therefore defined as
f_s = f_M / OSR = f_CLKIN / (2 × OSR). Whenever there are new samples available, the ADS131M08 asserts its DRDY pin to notify the microcontroller that new samples are available. The microcontroller would then use one of its SPI interfaces and its DMA to get the voltage and current samples from the ADS131M08 device. The microcontroller uses the new voltage and current samples for the calculation of the metrology parameters, such as the power and RMS readings.

The ADC connections to the microcontroller are brought out the LaunchPad connector of the design, which allows for different microcontrollers to be used as the metrology microcontroller by connecting the corresponding microcontroller LaunchPad to the LaunchPad connector of the design. For this specific implementation, the MSP432P4111 device was used as the metrology microcontroller by connecting the MSP-EXP432P4111 LaunchPad to the LaunchPad connector of the design.

For calibrating and testing the design, a PC GUI was used. The PC GUI communicates to the design through an isolated RS-232 connection created by the TIDA-00163 board. The TIDA-00163 board connects to a set of UART transmit (pin P2.5 on the MSP432) and receive (pin P2.3 on the MSP432) pins pin from the MSP432. The board isolates the signals from these pins and then translates the isolated signal to RS-232 signal levels. The resulting RS-232 signals are sent to the RS-232 connector of the TIDA-00163, which the PC is connected to.