Power Electronics

Mixed-signal ICs Build Smart Energy Meters

The migration from electromechanical energy metering to solid-state energy metering is driven by demands for advanced functionality, higher accuracy and lower cost. Many of these benefits were discussed in a previous Analog Feedback article (see “ICs Ease Adoption of Electronic Energy Metering,” February 2005, Power Electronics Technology). That article also described specific energy-metering ICs used to design simple function electronic energy meters where data is output to a stepper motor display or mechanical counter. This article examines IC options for building advanced function energy meters.

In the advanced metering applications, the LCD display is the common element. The LCD display is critical, because unlike the stepper motor display, it can output multiple lines of data and show more than the basic kilowatt hour measurement. But beyond the LCD display, there are many variations in advanced energy-meter designs, as reflected in the different approaches supported by analog front end, microcontroller (MCU) and system-on-chip (SoC) energy-metering ICs.

Analog front end (AFE) chips perform the data conversion required to sense current and voltage on the power lines. In the advanced function meters, these AFEs are typically paired with microcontrollers, which then interface directly or indirectly with the LCD display. As the AFEs have developed over time, they've taken on performance of the energy calculations, unloading some of the number-crunching burden from the MCU. They've also added features to enhance measurement precision.

An example part from Cirrus Logic (www.cirrus.com) is the CS5461A, which was released last December. This device is a pin-compatible upgrade to the company's CS5460A power meter IC for LCD display meters. The CS5461A has all the functionality of its predecessor, but adds a temperature sensor that enables higher accuracy measurements over temperature. Energy data is measured with an error of ±0.1% over 1000-to-1 dynamic range.

Designed for single-phase or 3-phase meters, the CS5461A performs multiple measurements, including instantaneous current and voltage, instantaneous power, real power, apparent power, IRMS and VRMS. It also features ac and dc calibration and phase compensation. Housed in a 24-pin SSOP, this chip is priced at $1.09 each in lots of 100,000.

The company also plans to introduce another low-cost power meter IC. The CS5463 will measure instantaneous voltage, current and power, IRMS and VRMS, average real/apparent/reactive power, fundamental power, harmonic power and line frequency. This 24-pin SSOP device was expected in the first quarter of this year with pricing at $1.20 in quantities of 100,000.

Another company offering AFEs for energy metering is Analog Devices (www.analog.com). Its ADE7753 single-phase multifunction energy-metering IC, which was introduced two years ago, carries out numerous energy calculations on chip. These include reactive, active, and apparent energy, rms voltage and rms current. Active energy measurements achieve less than 0.1% gain error over a current dynamic range of 1000-to-1. Other capabilities include power line supervisory features, such as line period measurement, SAG, peak detection, peak level recording and missing zero crossing.

The company also offers a version of this AFE, the ADE7758, for 3-phase meters. In quantities of 1000, pricing is specified at $2.52 for the ‘7753 and $6.88 for the ‘7758.

Meanwhile, other vendors have been pursuing single-chip designs for advanced-function energy meters. These devices integrate all of the required silicon except for the 3-V or 5-V supply required to power the chip.

At the end of 2003, Texas Instruments (www.ti.com) introduced the MSP430FE427, a 16-bit low-power microcontroller that enables a single-chip energy meter design for LCD display meters. This device, which went into production in mid-2004, combines the functionality of an analog front end with MCU, memory, clock and LCD driver functions. At the same time, the MSP430FE427 reduces power consumption by adopting a 3-V supply rather than the 5-V supply seen in some energy-metering designs.

At the heart of the 64-pin QFP device is an embedded signal processor (ESP) — a DSP430 core — that measures current and voltage signals, performs energy calculations, and communicates with memory and other on-chip and external functions. Designed for single-phase energy meters, the chip measures 32 energy-related parameters.

The cost of the MSP430FE427 is said to be in the $2 to $3 range in high volume, and the company estimates that the total bill-of-materials cost for a complete meter design is in the $4 to $6 range.

Texas Instruments is developing variations of this chip for polyphase (3-phase) energy meters and for low-end LCD display energy meters. The version aimed at 3-phase meter designs (MSP430FE43X) is expected to sample at the end of 2005. The other model (MSP430FE41X) now in development will have a reduced feature set with less support for functions such as AMR.

Another vendor, TDK Semiconductor (www.tdksemiconductor.com), also has introduced single-chip energy-meter designs based on its “single-converter” technology, which allows multiphase measurements to be taken with a single A-D converter. In August 2004, the company introduced its TDK 71M6513, a power meter system-on-chip, which the company developed for multiphase energy-meter designs.

Among this device's features are a 21-bit delta-sigma A-D converter, six analog inputs, digital temperature compensation, a precision voltage reference and a 32-bit computation engine. Offered in a 100-lead LQFP, this chip measures watt hours with a high degree of accuracy. One option provides <0.1% error over a 2000-to-1 dynamic range, while another option offers <0.5% error. The TDK 71M6513 operates from a 3.3-V supply. Pricing on this device starts at $4.95 each in quantities of 10,000.

Last November, TDK Semiconductor introduced a single-phase version of its power meter chip. The TDK 71M6511 shares many of the multiphase part's features — the ‘71M6511 is capable of measuring a wide variety of energy-related parameters and provides tamper protection and reverse current measurement. Now sampling in a 64-pin LQFP package, the ‘71M6511 is priced at $2.60 each in quantities of 10,000.

General-purpose microcontrollers provide another development path for advanced energy-meter designs. These MCUs may include peripherals, such as LCD drivers, EEPROM and analog front-end functions. This variation in the level of functional integration addresses the range of energy-metering requirements encountered in different regions of the world.

(For more on MCUs targeting energy-meter applications, see the online version of this article at www.powerelectronics.com.)

Going forward, designers can expect to see continued development of both single-chip energy-metering solutions and designs based on general-purpose MCUs paired with AFE chips or discrete AFEs. Each approach permits tradeoffs in integration, flexibility, functionality, metering accuracy, power consumption, cost and other factors. Naturally, the advantages of each design style will depend on the specific metering requirements. But in general, all of these analog front ends, MCUs and SoCs support the continuing migration away from electromechanical meters to more versatile solid-state designs.

MCUs Provide Flexibility In Energy Meter Designs

While ASICs now provide the basis for single-chip energy meter designs, general-purpose microcontrollers still have a role to play in these applications because of their inherent flexibility. There are a variety of approaches to implementing electronic energy meters with microcontrollers (MCUs). In some MCU-based designs, analog front-end chips provide the energy measurement and data conversion functions, while in other cases on-chip peripherals and external op amps are used to create the analog front end. The approach chosen by the designer may depend on application requirements for energy measurement accuracy and dynamic range, the types of metering functions that are required, demands for design flexibility, and cost considerations.

A number of MCU vendors have introduced general-purpose MCUs with sets of peripherals that make them well suited for energy metering. Microchip Technology (www.microchip.com) is one vendor that offers several families of MCUs applicable in the metering applications. These MCUs include the PIC18F8490, an 80-pin flash microcontroller with LCD driver and 12 channels of 10-bit A-D conversion. The PIC18F8490 can directly drive four 48-segment columns.

Another MCU offered by Microchip is the 16F877, a 40-pin 8-bit CMOS flash microcontroller with 8 channels of 10-bit A-D conversion and on-chip EEPROM—a plus for tamper protection. The 16F777 is a similar type of MCU without the EEPROM and with additional A-D channels. This 40-pin device can be combined with an AFE chip, display driver, and a 7-segment LCD display to complete a meter that is deemed an upgrade to one with a mechanical counter. Meanwhile, Dallas Semiconductor/Maxim (www.maxim-ic.com) has developed the MAXQ3120, a 16-bit microcontroller that includes two 16-bit A-D converters on chip, two UARTS (one configured for IR communication), an LCD controller and a 16 X 16 multiply-accumulate unit on a single die. The MCU also provides enough data RAM and program flash to implement a multirate, multifunction electric meter. This part will be available at the end of July 2005.

Another vendor, Atmel (www.atmel.com) introduced the AVR family of 8-bit RISC microcontrollers last year. This MCU family generally targets energy meter designs, but one member in particular, the ATmega406 is particularly suited to these applications. Housed in a 48-pin LQFP, the ATmega406 integrates an LCD driver and a 10-channel 12-bit sigma delta A-D converter. This chip is priced starting at $2.75 each in quantities greater than 100,000. A reference design using this device in a single-phase power/energy meter with tamper protection is available at the company’s website (see application note AVR465).

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