Real-time Power Monitoring Spreading to Many Applications
Performance AC power measurement is most commonly thought of as an application for electric utility meters or industrial power quality meters. However, companies in a broad range of industries are starting to recognize the value of adding power monitoring capabilities to their commercial, industrial, infrastructure and consumer products. These products include power hungry server centers, industrial machinery with large reactive loads, connected lighting, and IoT (Internet of Things) modules.
Performance AC power measurement is most commonly thought of as an application for electric utility meters or industrial power quality meters. However, companies in a broad range of industries are starting to recognize the value of adding power monitoring capabilities to their commercial, industrial, infrastructure and consumer products. These products include power hungry server centers, industrial machinery with large reactive loads, connected lighting, and IoT (Internet of Things) modules.
Real-time power monitoring can be used within a system to improve robustness, adjust performance and improve predictive thermal management over varying power conditions. In addition, it can monitor system efficiency and detect problem conditions which can lead to malfunction and also indicate needed maintenance. Power monitoring also allows products to display power usage to the end user as a tool for reducing energy use and utility bills in smart energy schemes.
Based on technology used in the electric utility meters, power monitoring ICs—such as Microchip Technology’s MCP39F511 two-channel and MCP39F511N three-channel single-phase power monitoring ICs—offer high performance and similar utility grade accuracy, while employing a flexible feature set to address the needs of a wide variety of applications. These power monitoring ICs replace either low accuracy microcontroller (MCU) based solutions, or multichip solutions based on 16- or 24-bit analog-to-digital converters (ADCs) and, again, an MCU. In either case, these solutions require the necessary programming skills and metrology knowledge to develop the appropriate power measurement including domain expertise and knowledge that may not readily be available in companies outside the metering industry.
Microchip’s power monitoring ICs (MCP39F511 and MCP39F511N) address this broad spectrum of power monitoring needs by integrating high-performance 24-bit delta-sigma ADCs with 94.5 decibels of SINAD (signal-to-noise and distortion ratio) with a 16-bit calculation engine. The result is a power monitoring solution capable of just 0.1% error across a wide 4000:1 dynamic range for the MCP39F511, which can monitor a single electrical load with temperature input, or 0.5% error across a similar 4000:1 dynamic range for the MCP39F511N, which can monitor two electrical loads. The high performance and accuracy is comparable to a utility meter, meaning that what is measured is what is billed. The high level of accuracy and the integrated programmable gain amplifiers (PGAs) also make the parts suitable with smaller, low-power shunt sensors.
Figure 1 – MCP39F511N Functional Block Diagram (click here to zoom)
The 16-bit calculation engine uses a robust coherent sampling algorithm to phase lock the sampling rate to the line frequency on the voltage channel input with an integer number of samples per line cycle, and reports all power output quantities at a user selectable 2N number of line cycles. The power quantities include active, reactive, and apparent power as well as active and reactive energy, RMS current and voltage, power factor, line frequency and zero-crossing of the voltage waveform. More explicitly, the MCP39F511 and MCP39F511N offer signed power numbers for active and reactive power, import and export registers for active energy, and a four-quadrant reactive power measurement. Import power or energy is considered positive (power or energy being consumed by the load), and export power or energy is considered negative (power or energy being delivered by the load). This can be useful for designs involving reactive loads or power generation by a renewable source, such as solar panels.
Figure 2 – The Power Circle and Triangle (S = Apparent, P = Active, Q = Reactive)
In addition to these power calculations, event alarms are also built-in for over current, over power and over temperature events, as well as voltage sag and surge events. The voltage sag and surge events are critical to power supply operation thus these events are designed to provide a very fast interrupt, less than a line cycle, if their condition occurs. The ability to record minimum and maximum values is also provided, which can be useful for reviewing if the product has been operating within specified conditions. In short, the engine provides the most useful power quantities and power tracking, mitigating the need for in-depth metrology knowledge and extensive firmware development.
Calibration is another point of contention when implementing power monitoring. Calibration compensates for gain error, component tolerances and overall noise in the system. The MCP39F511 and MCP39F511N provide an on-chip calibration algorithm that allows simple system calibration to be performed quickly. The excellent performance of the ADCs mentioned earlier allows for a single-point calibration and a single calibration command to achieve accurate measurements, which is all that is required for most systems. Additional calibration points are also available for AC offset, phase compensation, and DC offset, if so desired. Calibration can be done by either using the predefined auto-calibration commands, or by writing directly to the calibration registers. Users who do not need the highest accuracy can opt for a blanket calibration. Here, a prebuild run of products is characterized to determine the mean calibration values. This set of values is then used to calibrate in production and eliminates the need for individual product calibration, providing a significant savings in production time, cost and equipment. Since a mean set of values is being used, the error is larger, ranging from 1.5-2.5% error. The error is dependent on component tolerances, such as whether 0.1% resistors or 1% resistors are being used, good board design and production quality.
Companies who are constantly looking for creative ways to gain the competitive edge and to keep retail prices up should look at the benefits of adding power monitoring capability in their next generation of product. Power monitoring can improve system robustness and performance and aid the end user in making smarter energy decisions. Although power monitoring does add to the complexity of a product, the use of a dedicated power monitoring IC greatly facilitates development and reduces time to market for a high performance solution.
