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Outlook 2014: Power electronics moves forward on many fronts

Power electronics and power management technology rank among the most diverse in terms of breadth of applications and markets.

Power electronics technology is a critical ingredient in the efficient supply, distribution, conditioning and control – or what the industry has begun to label as the electrification – of applications and platforms across an extensive range of uses, from communications, defence and security systems to avionics and industrial automation and control equipment. Also important is the ability to achieve ultra low power consumption for a variety of devices.

The industry is meeting electrification challenges with a variety of advances: from increasing power output and switching performance in radar systems to optimising power efficiency in systems where energy must be sourced from small batteries or harvesting techniques, to the complete re-architecture of mechanical systems to full electrical control. These advances have required innovations in areas including material and process technology, device packaging and IC design.

Some of the biggest advances have come from innovations in materials and process technology. Silicon remains the prevailing substrate solution for most high power devices, especially in the cost driven insulated gate bipolar transistor, diode and thyristor markets. But performance demands in other applications have pushed the industry to next generation wide bandgap semiconductors, including silicon carbide (SiC) and gallium nitride (GaN).

SiC was initially applied to high power VHF and UHF band pulsed radar systems. The technology delivers higher breakdown field strength and improved thermal conductivity. This improves such performance characteristics as higher temperature operation, higher power densities, voltage capability and operating frequencies.

Microsemi introduced its first two commercial SiC products in 2008. These RF transistors for high power VHF and UHF band pulsed radar applications were a departure from silicon based bipolar junction transistor and LDMOS RF power transistor solutions that increased power levels by using complex push-pull circuit designs. The single ended design of SiC RF power transistors simplified impedance matching and enabled them to be housed in smaller, more reliable hermetically sealed packages.

The next milestone was a new class of SiC based static induction transistors (SITs). By up-scaling transistor cell size significantly, SITs further increased pulsed output in the UHF band to improve radar system detection range and sensitivity. Using this technology, today's latest SiC-based devices now deliver 750W of peak power with 17dB of power gain and a typical drain efficiency of 70% when operating at 1030/1090MHz. This is the pinnacle of power output in a single ended device covering this band for applications including air traffic control and collision avoidance equipment, as well as commercial secondary surveillance radar systems.

SiC material and technology is also used in other types of high power systems in a broadening range of industrial applications. For instance, SiC based Schottky diodes are found in systems where power density and higher performance and reliability are important. Also available are SiC based power modules with extended temperature ranges for use in high power, high voltage systems requiring high performance and reliability.

The use of aluminium nitride substrates enables isolation from the heat sink, which improvers heat transfer to the cooling system.

Thermal management is similarly important; heat buildup can have devastating effects in applications such as avionics systems that must be protected from mid air lightning strikes. Here, the latest power electronics packaging techniques are enabling transient surge protection (TVS) devices to meet increasingly stringent lightning protection specifications.

It has been estimated that lightning hits aircraft approximately once every 1000 flight hours. Protecting avionics from these strikes is particularly important for fly by wire systems that move primary flight control commands over an aircraft's data bus and power wiring. It has become more challenging with the latest carbon composite aircraft skins. This has led to new standards for TVS diodes, which provide critical protection by going into avalanche breakdown within no more than a few nanoseconds after a strike, clamping the transient voltage and routing its current to ground.

It is difficult for axial leaded TVS devices to comply with these standards because heat accumulates within their semiconductor device stacks by conduction along the leads and by convection through the casing. A relatively high thermal resistance from diode (p-n) junction to leads or ambient can be expected, particularly from multiple p-n junctions in the centre of the TVS package's stacked die design. It won't diffuse efficiently to the heatsink or to ambient within the rapid test pulse train's timescale, which can lead to high junction temperatures, impaired performance and even failure.

A better TVS construction is one in which only one or two semiconductor dice of large area are connected to a large contact/thermal pad. The latest solutions feature a top contact formed by a copper clip exiting the package that acts as the second electrical contact and provides an additional thermal path. Junction to heatsink thermal resistance is 0.2°C/W, enabling the TVS device to undergo the industry standard multistroke test sequence with minimised heat accumulation near the p-n junctions.

The latest TVS devices bring this same construction and electrical and thermal performance characteristics to a broader range of 15 and 30kW configurations at about 20% lower cost than their predecessors, reducing the cost of voltage surge protection significantly.

A third area of innovation is low power operation, in applications such as industrial wireless sensors. The move to wireless sensor networks eliminates data communications wiring, but still requires power sources. Batteries are expensive to replace when they wear out, especially when sensors are installed in hard to reach locations. Now, energy harvesting sensor nodes, combined with ultra low power radio transceivers, enable a variety of short range industrial wireless sensor networks that don't require battery replacement.

Proper transceiver implementation is essential. Running sensors from energy harvesting sources is best done with supplies of less than 2V, which means short range radio transceivers must be designed for 1.1V operation or lower, while still delivering the necessary performance. They should also use a current profile without excessive peaks to fit supply impedance. A transceiver's peak current consumption must be low to reduce power supply constraints. A third element is the radio transmitter's power amplifier (PA), which must deliver adequate output impedance plus enough receiver sensitivity to reduce the power radiated for a given range.

Today's transceivers have been optimised for both supply voltage and peak power consumption. Housed in 2 x 3 mm chip scale packages, they meet today's small footprint requirements and feature standard two wire and SPI interfaces for control and data transfer using any standard microcontroller.

An A/D converter in the microcontroller connects to the ultra low power analogue front end. By implementing transceivers with the right features and considering other important system design issues (such as the optimal carrier frequency to improve PA power consumption, and protocol stacks that reduce payload transport times and associated power consumption), today's short range wireless sensor networks can perform duty cycled spot measurement and other functions without needing their batteries to be changed.

Power electronics technology continues to advance on many fronts. Innovations in such areas as materials and process technology, packaging techniques and low power IC design will drive further improvements across systems with diverse power needs. These innovations and industry trends will continue to grow in importance as we push systems to do things that require more power, better thermal management and improved power efficiency.

Microsemi
Microsemi offers a comprehensive portfolio of semiconductor and system solutions for communications, defence and security, aerospace and industrial markets. Products include high performance, radiation hardened and highly reliable analogue mixed signal integrated circuits, FPGAs, SoCs and ASICs; power management products; timing and voice processing devices; RF solutions; discrete components; security technologies and scalable anti-tamper products; Power over Ethernet ICs and midspans; as well as custom design capabilities and services.

Russ Garcia is executive vice president of worldwide marketing for Microsemi.

Author
Russ Garcia

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