High-power designs continue to be developed for both military and commercial applications. As original equipment manufacturers (OEMs) strive to miniaturize the footprint of the final design, the components designed to handle high power levels in those systems must also shrink. Some components can handle higher power levels due to material advances, better thermal management, clever circuit layouts, or a combination of these factors. As engineers work to develop such advanced highpower components, they are taking advantage of robust and reliable electronicdesign- automation (EDA) software, test methods, and instrumentation to ensure top performance.
For example, new generations of military and aviation systems as well as stand-alone commercial power amplifiers (PAs) and portable radios are progressing to higher frequencies and power densities. In doing so, they are driving the need for components that can handle high-power levels. "Avionics and radar-system updates to expand the pulse width and duty factoras well as upgrades to reduce the part count and increase reliabilityare driving the need for RF/microwave power transistors and other components to operate at increased power levels," observes Mike Mallinger, Director of Business Development, WBG, Microsemi Corp. "RF power devices are being developed with higher power densities, higher efficiencies, and better internal thermal-resistance capabilities so that OEM vendors can save size and costs in their heatsink assemblies," adds Leonard Pelletier, Applications Support, Freescale Semiconductor. "This is especially valuable for the newer, tower-top amplifier requirements." In support of these trends, Microsemi recently released the 0405SC-2200M siliconcarbide (SiC) transistor for ultra-high-frequency (UHF) radar systems operating from 406 to 450 MHz. This device provides 2200 W peak operating at 125 V drain bias with a pulse width of 300 s, 6 percent duty (Fig. 1). Designed for UHF weather and long-range, over-the-horizon radar applications, this transistor is part of a series that includes 100-, 500-, 1000-, and 1500-W alternatives. (For more on the 0405SC-2200M transistor, see p. S46 in this month's Defense Electronics.)
In May, Freescale announced the MRFE6VP6300H RF power field-effect transistor (FET) with enhanced ruggedness (Fig. 2). Pelletier reports that this transistor was designed to withstand the extremely high voltage-standing-wave-ratio (VSWR) conditions (up to 65.0:1) common to many carbon-dioxide (CO2) laser drivers and magnetic-resonanceimaging (MRI) applications. It also is well suited for use in broadcast and RF heating and lighting applications. The company also offers the MRF6VP- 41KH RF power FETa 50-V, 500- MHz device that is capable of 1-kW continuous-wave (CW) output power. It is one of Freescale's largest, highestpower- density devices.
CREATIVE SOLUTIONS
The need for optimal thermal management requires some creative solutions. At Anaren, designers are using substrates with higher thermal conductivity. In addition, Martin Stensgaard, Plant Manager of Anaren Ceramics, reports, "We also develop innovative approaches in order to maximize heat transfer from components and in the final application. We use several kinds of high-frequency simulators and thermal simulation tools."
Recently, Anaren announced its newest line of Xinger-III brand hybrid and directional couplers. Based on proprietary multilayer stripline technology, the new Xinger components are designed using high-frequency laminates with enhanced thermal capabilities. As a specific example for higher-frequency use, the model X3C19E2-20 is a 20-dB directional coupler that can handle 225 W CW input power from 1400 to 2700 MHz in a 0.56-x-0.20-in. package. The firm also offers a line of resistive products that handle high power and are particularly well suited for defense and instrumentation applications. (For more on the Xinger-II products, see the July 2010 Cover Feature of Microwaves & RF, p. 97.)
Mini-Circuits also offers a range of high-power directional couplers. For instance, its ZGDC6-362HP+ directional coupler spans 380 to 3600 MHz handling to 250 W CW. It typically exhibits 0.20 dB insertion loss and 30 dB return loss. This coupler targets high-power applications like transmitters, base stations, and high-power device characterization. The company also offers bidirectional couplers for use in applications like Universal Mobile Telecommunications System (UMTS) communications equipment, power leveling, and VSWR measurements. The ZABDC20-182H+, for example, is a bidirectional coupler that handles power to 100 W from 700 to 1800 MHz with typical VSWR of 1.08:1. A family of in-line multi-couplers from Merrimac, called the Multi-Mix PICO SH series, provide a power divider/combiner function. It is aimed at wireless applications, such as PAs and signaldistribution processing. Merrimac's Multi-Mix manufacturing process is based on fluoropolymer composite substrates, which are fusion bonded into a multilayer structure. In the case of the SH series, maximum input power can be specified at 10 or 100 W CW with 1.35:1 typical VSWR and 0.25 dB typical insertion loss from 1.8 to 2.0 GHz.
Investment in materials science is paying off for high-power designs. For instance, Microsemi's designers are using wide-bandgap materials because they can operate at higher bias levels and power densities. Examples include SiC and gallium nitride (GaN). Mallinger notes, "In addition, we are working to update the thermal conductivity of the transistor packages. The design improvements are based on the RF performance data that we have taken on existing and new devices as well as automated junction temperature and load-pull measurements. Evolutionary work continues on the fundamental chip and end item designs utilizing our more mature Si BJT technology."
At Freescale, designers are conscious of the inherent tradeoffs in any highpower designs, "In high-power RF die layout designs, there is always a tradeoff between a tight layout for increased power density and a spreadout layout for improved thermal cooling capability. We find that by optimizing the internal active channel area of the LDMOS die for improved control of the impact ionization rates, we can create the greatest amount of RF performance improvements," explains Pelletier. Although they may be easy to forget among the more engaging parts of a design, connectors also play a role in maintaining the power-handling ability of a completed system. Companies like Amphenol RF, Southwest Microwave , and ARRA, for instance, all offer SMA connectors for highpower designs.
Creativity also is required on the activecomponents side. Mark Schrepferman, Director of Marketing, High Performance Solutions at Peregrine Semiconductor Corp., explains that Peregrine uses its UltraCMOS sapphire substrate to achieve even voltage distribution across a large stack of devices. "By utilizing device stacking, single device breakdown under high power is avoided. In addition, we use thermal modeling and design techniques to manage device reliability under high power, and device improvements for lower RON and lower insertion loss reduce dissipated power. Lastly, Peregrine engineers utilize innovative package and contacting technology to manage heat and reduce RF losses." The firm is offering its PE42510A and PE42650A UltraCMOS switches for high-power wireless applications including military radios (Fig. 3). For example, the PE42510A SPDT and PE42650A SP3T high-power RF switches offer 50-W output power at 1-dB compression from 30 to 2000 MHz. (For more on Peregrine and military radios, see p. S26 in this month's Defense Electronics.) Despite the design, material, and product advances, technical challenges remain to achieve high-power operation. The fundamental challenge is to achieve high power handling at high frequencies with a reasonable cost. Other challenges include maintaining good power gain and high drain efficiency across the operating frequencyin other words, not sacrificing performance for power handling. Given the difficult nature of these challenges, many wonder how component manufacturers are managing to accomplish such tasks. In most cases, the answer is a tightly held secret. Yet Freescale's Pelletier does offer a glimpse by saying, "We are always in the process of discovering new ways to improve upon the overall RF performance capability of the LDMOS die structure and we have been working on this technology since the early 1980s. Surprisingly, as the cellular market evolveswith new modulation formats and better errorcorrection methodologiesthere are still plenty of ways to optimize the LDMOS die to keep pace with these newer developments."
EDA AND TEST
Electronic design software, test methodologies, and test equipment seem to have kept pace with the needs of highpower design. Whether these tools are developed by in-house teams or thirdparty vendors, they are highly valued by many high-power designers. For thermal management, for instance, Microsemi reports that it uses proprietary thermalsimulation software to simulate thermal dissipation and distribution performance at the chip design level. The company's teams then use an infrared (IR) microscope to measure the actual junction temperature and thermal distribution of the power transistor under full operation. The ANSYS finite-analysis program is used to model the thermal response.
Freescale looks to put some of its simulation functionality into its customers' hands. "We have online mean-time-tofailure (MTTF) calculators for all of our Freescale devices," notes Pelletier. "This tool allows our customers to enter in the exact operating conditions of the device. It then gives a customized MTTF prediction under those specific conditions."
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Mentor Graphics helps designers working on high-power designs by approaching the problem from two angles. To enable fast analysis of the power-distribution networks (PDNs) that supply the multiple voltage levels to the components through the printed-circuit board (PCB), Mentor developed its HyperLynx PI (Power Integrity) product (Fig. 4). In addition, the company offers its FloTHERM product for thermal analysis to evaluate proper heat dissipation for the electronic product without the need to build and test physical prototypes.
The Mentor Graphics products evolved as power requirements increased while product complexity and competitive pressures drove the need for virtual versus physical prototyping. "We have customers that are designing and analyzing up to 30 PDNs jigsawed into a single PCB," says John Isaac, Director of Market Development for the firm's Systems Design Division. "Some of those same customers are producing racks of PCBseach dissipating 300 to 400 W of powerand others are putting high-power dissipating PCBs in small-form-factor products."
In addition to products from Mentor Graphics, engineers are using a number of different software packages for high-power designs including COMOSDesignSTAR, ANSYS, Ansoft HFSS (), Agilent's Advanced Design System (ADS), AWR's Microwave Office, and TCAD.
To test their designs to ensure that they conform to high-power operation, engineers have a number of different choices that range from high-end test equipment to customized in-house setups. "RF performance is measured using the in-house-designed test fixture with the product operating under the full set of specified conditions, frequency, pulse width, and duty factor biased at 125 V and then driven over the dynamic range. Tests will include load mismatch tolerance and a range of drive levels and bias conditions. The test setup includes the appropriate bias supplies, RF sources, and power-measuring equipment," reports Mallinger. For example, the engineers at Anaren use a customized setup of Lambda power supplies, Agilent dataloggers, and computers.
Freescale has a policy to production test its devices under the exact same test conditions that its customers will use. So Freescale's engineers generate a full-power, actively modulated signal and measure all of the vital RF performance parameters. "This takes a very expensive set of test equipment consisting of signal generators, network analyzers, spectrum analyzers, power meters, and power supplies. We use a wide selection of instrumentation from several market-leading microwave testequipment vendors," says Pelletier.
Undoubtedly, the real estate available for high-power designs will continue to shrink. And system requirements will continue to push for higher power densities from both active and passive circuits. The good news is that component designers seem to have the methods in place and the tools they need to meet the needs of next-generation high-power designs.