The STPM01 from STMicroelectronics incorporates all of the core circuitry required to implement fully electronic electricity meters. The STPM01 addresses a wide range of electricity metering requirements, operating as a standalone power meter in the simplest low-end equipment or as a peripheral in sophisticated microprocessor-based meters that support functions such as remote metering, reactive power measurement, multiple tariffs and tamper detection.
Jointly developed by ST and the Slovenia-based ISKRAEMECO, one of the world’s largest meter manufacturers, the STPM01 is a standard device modified from an ASIC solution built for ISKRAEMECO using ST’s advanced BCD6 smart power technology. This process technology allows all of the necessary analog, digital and power circuits to be integrated on a single chip.
The major functional blocks in the STPM01 include analog signal conditioning, two analog-to-digital converters and a hardwired dedicated digital signal processor (DSP). Also on-chip are an SPI interface to an external MCU, a block of 56 one-time programmable (OTP) bits for calibration and configuration, a voltage-to-frequency converter to drive a stepper motor display in low-end applications, and two current-limited, low-drop voltage regulators to power the analog (3 V) and digital (1.5 V) circuitry (see the figure). One of the distinguishing features of this chip is its ability to measure negative energy consumption such as the energy produced by a solar panel.
The chip also offers a high degree of design flexibility. For example, the STPM01’s configurable analog front-end allows the device to work with a variety of current sensors, including current transformers, micro-ohm shunts and Rogowski coils. In addition, the DSP performs real-time calculations of active, reactive and apparent power, RMS voltage and current, and line frequency, using innovative algorithms for which ISKRAEMECO has filed patent applications.
The SPI interface is used to control, configure and calibrate the device using the OTP block and to transmit the digital output data to an external MCU, which handles data collection and management in high-end applications. The computed power values also are converted internally into pulses to directly drive stepper motor displays.
The OTP block, which is accessible via a simple PC interface and software, allows fast and accurate calibration of the voltage and current sensors, phase correction and temperature drift. Additional features of the STPM01 include live and neutral wire current monitoring for tamper detection in secure energy meters; LED drivers to indicate power, tamper detection and no-load conditions; and a zero-crossing signal that can be used to avoid arcing when switching heavy inductive loads in industrial control applications.
The STPM01 offers high levels of accuracy, allowing full compliance with the IEC62052-11 and IEC62053-2X specifications for single-phase class 0.5 wattmeters, while a synchronization pin also supports 3-phase metering applications. The device operates over a wide range of mains voltages (40 V rms to 300 V rms) and line frequencies (45 Hz to 65 Hz), and can measure up to 200-A rms. Besides lowering component cost through integration, the STPM01 reduces manufacturing costs associated with calibration, thanks to the fast, tamper-proof OTP block.
The STPM01 is supported by two demo boards/reference designs: a measurement board that allows the STPM01to operate as a standalone solution for driving stepper motor displays, and a control board equipped with an ST7 MCU, LCD display and RTC that connects to the measurement board via the SPI to provide a complete solution for medium/high-end applications. Offered in a 20-pin TSSOP, the STPM01 is priced at approximately $2 in quantities of 1000. Further information on ST’s interface and metering ICs is available at www.st.com/interface and www.st.com/metering.
For more information on other energy meter ICs, see “ICs Ease Adoption of Electronic Energy Metering,” Power Electronics Technology, February 2005, powerelectronics.com/mag/power_ics_ease_adoption/index.html and “Mixed-Signal ICs Build Smart Energy Meters,” Power Electronics Technology, April 2005, powerelectronics.com/mag/power_mixedsignal_ics_build/.