Electronic energy meters are replacing traditional electromechanical meters in many residential, commercial and industrial applications because of the versatility and low-cost afforded by electronic meter designs. Single- and multi-chip meter designs are helping meter manufacturers and their customers realize these benefits. Thanks to these continually evolving metering ICs, meter builders can implement many features that were impractical with the older mechanical designs.
For example, an electronic design can protect against meter tampering and theft of service. It also can measure and record energy usage at different times of the day, so utilities can bill customers for energy based on time of usage. An electronic energy meter also can enable automatic meter reading (AMR), whereby energy metering data is transmitted to the utility over an RF, power line or even infrared communications link. Furthermore, electronic meters pave the way for “submetering” of smaller operating units (for example, metering each apartment rather than just the building).
Improved accuracy and lower power consumption are other benefits of electronic metering. With a mechanical meter, the error in the basic energy usage measurement is on the order of 1%. But with an electronic implementation, it is possible to reduce that error to less than 0.1%. Moreover, running the mechanical meter with its continuously spinning dial may require hundreds of milliamps. That power consumption can be reduced to a couple milliamps in an electronic energy meter, producing big power savings for the utility.
Electronic meter designs also change the economics of manufacturing energy meters. A single hardware design may be customized for different customers and markets through changes in software. In addition, calibrating the finished meter at the factory is much easier with an electronic meter design.
Another consideration is the demand for mechanical-meter replacements that are as inexpensive as possible. In parts of the developing world where many new customers are being connected to the grid, the low cost of the electronic meter is its main attraction.
IC Development
Since the late 1990s, semiconductor vendors with mixed-signal and data conversion expertise have been developing ICs for electronic energy metering applications.
In varying degrees, these components have integrated the energy measurement, calculation and communication functions required to build electronic energy meters ranging from simple function, mechanical-meter replacements to advanced function all solid-state designs.
As in most areas of silicon development, the level of integration for these components grows with time, so that newer ICs offer more functionality and/or less cost. Consequently, the cost of electronic metering is coming down, which, in turn, is affecting the types of meters that are being built. As the metering ICs evolve, there is also a trend to higher accuracy, which is reflected in the energy measurement linearity of the new ICs.
The energy metering market is far from monolithic, so metering ICs target different applications. One way to differentiate these chips is by the number of phases that must be measured. Some ICs target single-phase applications, while others are crafted for multiphase (or poly-phase) applications. Within these categories, the chips also are distinguished according to whether they target residential, commercial or industrial applications. Another way to segment the energy metering ICs is according to the desired level of meter functionality.
The residential market accounts for the highest volumes of energy meter sales and a growing percentage of these meters are electronic. At the moment, many of these electronic meters are simple-function meters, consisting of an energy meter IC that drives a stepper-motor display or mechanical counter. Sometimes these meters are referred to as stepper-motor display meters or as electronic hybrids because they retain a mechanical component.
To perform energy-to-pulse conversion requires A/D converters, a voltage reference, a clock and some other signal-processing circuitry. A typical energy metering IC with pulse output integrates all of these functions on one die. In addition to the chip, the meter design requires an external power supply circuit and a number of passive components including a current sensor (Fig. 1). This element could be a current-sense transformer, shunt or possibly a Rogowski coil. For now, the hybrid or stepper-motor display energy meter represents the lowest-cost electronic energy meter.
However, for many applications, the cost of the meter is not its main attraction but rather its functionality. Advanced-function meters generally are distinguished by their use of an LCD display, which allows the meter to communicate a host of energy-related measurements beyond the basic watt-hour measurement. These meters are sometimes referred to as LCD display meters.
In countries with more established utility infrastructure, industrial and residential applications are adopting the advanced-function meters, driven largely by demands for multi-rate billing, AMR and other functions such as tamper protection. In terms of chip requirements, an advanced-function meter typically requires an analog front-end (A/D converters, reference, etc.) for measurement, a compute engine (such as an MCU or DSP) for calculations, memory, a real-time clock, an LCD driver and power supply circuitry. Other circuitry includes the current sensor and passives, plus a quartz crystal for timing and a battery for memory backup.
Until recently, semiconductor vendors only offered multi-chip solutions for building the LCD display meters. The most integrated designs required at least two chips — an energy metering IC plus an MCU with on-chip LCD driver. However, in the last year, at least two vendors — Texas Instruments and TDK Semiconductor — have introduced single-chip solutions for the LCD display meters (Fig. 2).
Both the simple-function and advanced-function energy metering ICs continue to evolve through improvements in performance and functional integration. In some cases, the lines separating simple function (stepper-motor display) and advanced-function (LCD display) meters are blurring. These trends can be seen in some of the recently introduced energy metering ICs.
Simple-function Energy Metering ICs
Last fall, Cirrus Logic introduced the CS5466, a power meter IC for residential single-phase measurements. Aimed at the stepper-motor display applications, this device generates the pulsed output required for the stepper motor display. It achieves a fully integrated design that requires no external MCU. With its 24-bit fourth-order delta-sigma A/D converter, the chip can achieve a high degree of precision. Energy data is measured with an error of ±0.1% over a current dynamic range of 1000-to-1. Packaged in a 24-pin SSOP, this device is priced at $1.44 each in quantities of 10,000.
In April, Analog Devices introduced two ICs for the stepper-motor display meter. The ADE7760/7761 are extensions of an existing ADI product (the AD7751), but include features to thwart meter tampering and theft of utility services. These tamper-proofing functions were previously associated with the advanced-function meter designs.
Certain tamper conditions, referred to as “earth fault” and “missing neutral,” result in unbalanced currents on the phase and neutral inputs of the meter. These conditions occur when the inputs to the meter are swapped so that the meter runs more slowly and the customer is billed for less service. The ADE7760/7761 monitors current on both the phase and neutral inputs and compares the results. When the difference is greater than 6.25%, the chip chooses the higher of the two readings for billing.
The ADE7761 has the additional capability of billing the energy based on one current input if the voltage input and the return current are missing. During the manufacturing of the meter, the ADE7761 can be calibrated for this “missing neutral mode” by adjusting a dc analog input. The ADE7760/7761 measure energy data with <0.1% error over a 500-to-1 dynamic range. A reference design for the ADE7761 is shown in Fig. 1.
Packaged in 20-lead SSOPs, the ADE7760 and ADE7761 are priced at $2.68 and $2.83 each, respectively, in quantities of 1000.
Advanced-function Energy Metering ICs
In December, Cirrus Logic released the CS5461A, 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 residential or 3-phase industrial meters, the CS5461A performs a variety of measurements on chip, including instantaneous current and voltage, instantaneous power, real power, apparent power, IRMS and VRMS. It also features ac and dc calibration and phase compensation. The CS5461A works with an external microcontroller, communicating with the MCU over a bidirectional serial interface. Housed in a 24-pin SSOP, this chip is priced at $x.xx in quantities of 100,000.
The company also plans to introduce another power meter IC that performs a host of advanced power measurements at low cost. The CS5463 will measure instantaneous voltage, current and power; Irms and Vrms; average real/apparent/reactive power; fundamental power; harmonic power; and line frequency. The company plans to introduce this device in the first quarter of this year with unit pricing at $1.20 in quantities of 100,000. Packaging is in a 24-pin SSOP.
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 power supply required to power the chip.
At the end of 2003, Texas Instruments introduced the MSP430FE427, a 16-bit low-power microcontroller that enabled a single-chip energy meter design for LCD display meters (Fig. 2). This device, which went intro production in mid-2004, combined the functionality of an analog front end with MCU, memory, clock and LCD driver functions. At the same time, the MSP430FE427 reduced power requirements by operating form a 3-V supply rather than the 5-V supply typical in previous energy metering designs.
At the heart of the TI chip 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 a total of 32 energy-related parameters.
Packaging is a 64-pin QFP. 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 in development, the MSP430FE41X, will have a reduced feature set with less support for functions, such as AMR, and will offer lower cost.
Another vendor, TDK Semiconductor, also has introduced single-chip energy meter designs based on the company's “single-converter” technology, which allows multi-phase 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. The chip derives its timing from an external 32-kHz external crystal and included battery back up support (Fig. 3). Packaging is a 100-lead exposed pad 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.
In November 2004, TDK Semiconductor followed its introduction of the TDK 71M6513 with a version developed for single-phase application. The TDK 71M6511 shares many of the features present in the company's 71M6513 multiphase metering chip. Like its predecessor, the ‘71M6511 is capable of measuring a 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.
Companies offering single-chip solutions for the advanced function energy meters claim these energy metering ICs reduce the cost differential between these stepper-motor display and LCD display meters. Over time, high-volume production and die shrinks are expected to further reduce the cost of the silicon for advanced function meters. Meanwhile, the cost of LCD displays is expected to fall as well, making them more cost competitive with stepper motor displays and mechanical counters. As the cost differential between simple and advanced function energy meters shrinks, it is expected that more applications will opt for the advanced functionality.
Although integration is an undeniable trend in the development of energy metering ICs, pursuit of greater measurement accuracy is another. Consequently, some vendors are focusing their chip development on optimizing the performance of the analog front end in the energy metering IC. These efforts are expected to further expand the measurement capabilities of the electronic energy meter.
*Details on the advanced-function energy metering ICs will follow in an upcoming edition of Analog Feedback.