Smart Meter Design Considerations Detailed at Smart Grid Electronics Forum

October 26, 2011 by Jeff Shepard

Communications, metrology, security and power are the four primary considerations when designing next-generation smart meters. Those topics discussed by authors from Allegro MicroSystems, Fairchild Semiconductor, Pulse Electronics, Maxim Integrated Products and Texas Instruments during the Smart Meters and Energy Measurement session at this week’s Smart Grid Electronics Forum.

The session started with Andreas Friedrich, Technical Director with Allegro Microsystems’ Sensor Technical Centre in Europe discussing, "Improving Efficiency in Smart Grid Applications with Fully Integrated Current Sensing ICs." He presented a series of design solutions for applications ranging from under 50A to over 200A. These sensors combine the company’s BiCMOS process technology with Hall sensor technology in new packages to enable greater than 120-kHz output bandwidth and fast output response time and a 20 to 40X reduction in output noise for high-accuracy and resolution.

"Meeting Next Generation Smart Meter Power Conversion Design Challenges" was the focus of the presentation by Christopher Siegl, Member of Technical Staff with Fairchild Semiconductor. After a brief review of common design approaches, Siegl presented a new approached based the company’s "ESBC" switch combined with a newly-developed high-voltage package. The new device is a "marriage of bipolar high-voltage with low-voltage MOSFET that negates the weaknesses of both devices while taking advantage of their strengths." The result is higher breakdown voltage, minimum switching losses and easy drive requirements.

Next, Siegl discussed the newly-developed high-voltage D2Pak that achieves 6.4mm of creepage and clearance. The modifications included removing the center pin, changing the lead frame, shortening the metal back tap and optimizing the landing pad. A wide-input (150Vac – 1,000Vac) 10W metering power supply was described that delivers a flat efficiency curve down to 1-2 W, depending on the input voltage.

Glenn Roemer, Field Applications Engineer with Pulse Electronics described the company’s "Sidewinder" current sensors based on Rogowski coil technology. He compared with performance of Rogowski coils with shunt resistors, current transformers and Hall-effect sensors across a wide range of characteristics including; linear amplitude and phase, wide range (5 decades), wide bandwidth, no dc saturation, low temperature coefficient, high electrical isolation, low power consumption, output voltage, low cost, light weight and flexible size and shape. He determined that the Rogowski approach was as good as or better than all alternatives except for two areas: shunt resistors are lower in cost and both shunt resistors and current transformers produce higher output voltages.

A complete smart meter reference design was presented by Anil Telikepalli, Business Director for Industrial Power with Maxim. The six elements in the reference design included: 3G-PLC communications based on the MAX2991/MAX2992 devices. Metrology accuracy of 0.2% was achieved over a 2000:1 dynamic range using the 71M6541F. Encrypted security with tamper detection based on the MAXQ1050. The MAX17498 off-line controller with an integrated step-down regulator to deliver +12 and +3.3 Vdc power rails. Timekeeping accuracy to 5ppm was accomplished with the MEMS-based DSWEW1M. And an RS485 interface with 15kV isolation using the MAX13256 and MAX13412 devices. The result is a region-specific reference design with extensive software support.

Jerry Steele, Strategic Applications Manager with Texas Instruments, closed the session with a discussion of "Current Sensing for Power Monitoring: Magnetic, MOSFET or Shunt?" curing which he compared several of the technical advantages of the various approaches. For magnetic sensing, he concluded that the advantages include that it is a non-intrusive method it is lossless and easy to power, while the disadvantages are cost and size. Looking at MOSFET current sensing, he observed that high-ratio senseFETs are becoming hard to find and it is also hard to find the min/max specifications needed to design good current sensing applications.

Steele concluded his talk with a comparison of today’s state-of-the-art shunt current sensors and next-generation devices under development. Today’s devices have a typical voltage drop of 25mV which results in 1.25W of power loss, while next generation devices will reduce the voltage drop to 10mV resulting in lower losses, less heat generation and smaller solutions. In addition, next-generation devices will feature an increased dynamic range. Today, the dynamic range is specified with current at 20%, next-generation devices will reduce that to only 10% resulting in a substantial increase in the dynamic range.

More news and information regarding the latest developments in Smart Grid electronics can be found at Darnell’s SmartGridElectronics.Net.