STMicroelectronics EVALSTPM-3PHISO energy-meter evaluation board

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The EVALSTPM-3PHISO evaluation board, from STMicroelectronics, helps to speed up design of three-phase AC wattmeters that meet international quality and accuracy standards using electromagnetic-immune shunt sensors and galvanic-isolation technology for robustness. It combines the STPMS2 metering front-end IC and the STISO621 digital isolator with customisable turnkey firmware running on an STM32 microcontroller to compute metrology and power-quality data. The sensing circuitry and PCB layout are optimised to ensure robustness against EMI and a strong signal-to-noise ratio, for optimal 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 onboard voltage divider and a shunt current sensor. It then oversamples the signal using a synchronised 4 MHz clock distributed by the microcontroller and multiplexes voltage and current sigma-delta bitstreams on a single output pin.

The firmware leverages digital filters for sigma-delta modulators (DFSDMs) integrated in the chosen STM32 microcontroller to convert the six bitstreams into 24-bit voltage and current values and computes all metrology data in real-time every 200 µs. It 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 leverages ST’s 6 kV thick-oxide galvanic-isolation technology to transfer data between isolated domains in a variety of industrial applications. It features two independent channels with Schmitt-trigger inputs that ensure high noise immunity and keeps pulse distortion below 3 ns.

The evaluation board meets EN 50470-x, IEC 62053-2x and ANSI12.2x standards for AC wattmeters. 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 three-phase active/apparent power measurement and accuracy Class 1 according to IEC 62053-21 on 3-phase reactive power measurement.

The product 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. It 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.

For more information: www.st.com/isolated-interfaces.