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ST simplifies energy metering apps with smart power tech

Posted: 22 Jun 2005 ?? ?Print Version ?Bookmark and Share

Keywords:electromechanical? iskraemeco? stpm01? microprocessor? asic?

Electric utility firms around the world are looking to replace the traditional electromechanical meter, which has remained essentially unchanged for over a century, by electronic meters. STMicroelectronics and Iskraemeco have jointly developed STPM01 device that addresses a wide range of electricity metering requirements, operating as a stand-alone power meter in the simplest low-end equipment or as a peripheral in microprocessor-based meters.

The STPM01 is a standard device modified from an ASIC solution built for Iskraemeco using ST's advanced bcd6 smart power technology, allowing all of the necessary analog, digital and power circuits to be integrated on a single chip. The major blocks include analog signal conditioning, two analog-to-digital converters, a hardwired dedicated DSP, an SPI interface to an external MCU, a block of 56 OTP (One Time Programmable) 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 supply the analog (3V) and digital (1.5) circuitry.

The flexible and configurable analog front-end allows the device to work with a variety of current sensors, including current transformers, microOhm shunts and Rogowski coils. The dedicated 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. In addition, the computed power values are also converted internally into pulses to directly drive stepper motor displays in the most cost-sensitive low-end applications.

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 performance in terms of accuracy, allowing full compliance with the IEC62052-11 and IEC62053-2X specifications for single-phase class 0.5 wattmeters, while a Synchronisation pin also supports three-phase metering applications. The device operates over a wide range of mains voltage (40-300VRMS) and line frequency (45-65Hz), and can measure up to 200ARMS. In addition to the lower component cost due to the high level of integration, the STPM01 also reduces hidden 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 STPM01 to operate as a stand-alone solution for stepper driving, 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.

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