A new evaluation board from STMicroelectronics speeds design of 3-phase AC Watt meters that meet the most stringent international quality and accuracy standards using low-cost, electromagnetic-immune shunt sensors and advanced galvanic-isolation technology for superior reliability and robustness.
The EVALSTPM-3PHISO evaluation board combines the high-accuracy STPMS2 metering front-end IC and the advanced STISO621 digital isolator, with customizable turn-key firmware running on an STM32 microcontroller to compute metrology and power-quality data. The sensing circuitry and PCB layout are optimized to ensure robustness against EMI and a strong signal-to-noise ratio, for high-accuracy measurement and post-processing computation.
The STPMS2, a two-channel 24-bit second-order sigma-delta modulator, measures voltage and current for each phase through an on-board voltage divider and a shunt current sensor. It then oversamples the signal using a synchronized 4MHz clock distributed by the microcontroller and multiplexes voltage and current sigma-delta bitstreams on a single output pin. Three STPMS2 are used in the 3-phase system to collect voltage and current data from each phase.
The provided firmware leverages digital filters for sigma-delta modulators (DFSDMs) integrated into the chosen STM32 microcontroller to convert the six bitstreams into 24-bit voltage and current values and compute all metrology data in real-time every 200µs. The firmware also implements a virtual com port that provides access to internal parameters for reading metrology data, modifying the internal configuration, and calibrating the board.
The STISO621 dual-channel digital isolator is the first in a new series of ICs that leverage ST’s 6kV thick-oxide galvanic-isolation technology to transfer data between isolated domains in a variety of industrial applications. The STISO621 features two independent channels with Schmitt-trigger inputs that ensure high noise immunity and keeps pulse distortion below 3ns. With a maximum data rate of 100Mbit/s, 6000V impulse withstand voltage (VIOTM), and 1200V maximum repeating isolation voltage (VIORM), the STISO621 ensures faster data transmission, longer lifetime, and higher reliability than conventional optical isolators.
ST’s metering evaluation board meets EN 50470-x, IEC 62053-2x, ANSI12.2x standards for AC watt meters. The application provides active wideband, active fundamental, reactive, and apparent power and energy data, both per-phase and cumulative, reaching accuracy Class 0.5 according to IEC 62053-22 on 3-phase active/apparent power measurement and accuracy Class 1 according to IEC 62053-21 on 3-phase reactive power measurement. It performs RMS and optional distortion (THD) calculations for each voltage and current signal, as well as DC measurements, and line period, phase shift, and phase-voltage delay for each phase.
The EVALSTPM-3PHISO can be also used as a reference for multi-phase industrial metering applications beyond utility metering, such as electric-vehicle charging, servers, and solar inverters.