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Defence & DSEi Special Breaking free


Kai Johnstad explains how high-density modules that meet military PSU mandates will help design engineers break free of the restraints they often confront in designing for military projects


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esigners of today's military and aerospace systems face many similar challenges to their colleagues working in communications infrastructure, or in IT systems; while the components and subsystems available to them enable them to build ever-more capable systems, there remain a host of critical problems to solve. High-end, latest generation microprocessors deliver massive processing power, but consume equally large amounts of electrical power at increasingly inconvenient voltage and current levels. To take one high-profile example, the


Typhoon (Eurofighter) has both radar and infrared target detection and tracking systems on board, capable of track-while- scan operation while handling multiple targets. Add to that, capabilities for aiming weaponry according to where the pilot looks and voice-commanded systems and even designers far removed from military projects will infer the use of many power- hungry processor cores to process all of the data.


Many of the same forces that have


prompted server-blade and cellular- basestation designers to adopt successive generations of distributed power architecture – leading to the latest generation, factorised power – are mirrored in military systems. Minimising weight and fitting


increasingly-powerful hardware into confined spaces are perennial concerns for military design teams. Never more so than in the ongoing design efforts, in multiple companies and countries, to turn out more and more sophisticated unmanned aerial vehicles – UAVs, or “drones”. BAE Systems' twin-engined Mantis, for example, is a completely autonomous aircraft, able to take off, fly a mission of up to 30 hours, and land under internal control. The complete aircraft weighs only 1000kg so every gram matters in the weight of the on-board systems, and that concern is reflected in the requirements documentation for every on-board module.


Total available power UAVs also highlight yet another constraint on the military designer; total available power. Much discussed in the press as a consequence of its deployment in Afghanistan is the Predator, made by General Atomics. A remotely-piloted vehicle (RPV) rather than a self-piloting aircraft, the Predator carries multiple sensor and weapons systems in addition to its flight control. The main engine that powers the craft develops a maximum power of 86kW; clearly, the proportion of that figure available for the electrical power load must be strictly limited. Within projects such as these, the power system designer can do little about the wattage of the systems themselves; but – just as much as the consumer-product engineer striving to turn out a design as “green” as possible – the pressure is on to waste as little energy as possible, and to make power conversion and distribution as efficient as possible. This is especially true in the case of man-portable systems, where all power is from batteries and long operating life is essential. Requirements for low weight, small size and maximum efficiency converge in such designs. Away from high-profile, clean-sheet


projects, pressure on defence budgets is leading to a continuing programme of upgrades; radars are re-fitted with uprated phased-arrays, and infra-red cameras are rejuvenated with higher-resolution sensors; surveillance and signal-intelligence systems gain more inputs, backed by increased data-processing resources. All of this activity, in both new-build and upgrade areas, has meant that for power systems providers, military markets have produced continuous growth over the last decade, and have not experienced a downturn as a result of the recession. A significant contributing factor has been the continuing trend of using COTS components and subsystems. While military systems still require long-lifetime


24 July/August 2011 Components in Electronics


commitments from their suppliers, they are not immune to demands for quick time-to- market. Designs are needed on shorter timescales than before, while still meeting the relevant standards.


Standards requirements Power supply designs, as much as any other military-equipment system, must conform to standards in areas such as conducted and radiated EMC emissions and susceptibility. An extensive array of standards governs this area, for example MIL-STD-461 (461F, in its latest revision) and MIL-STD-1275 (1275D, in its latest revision), which is of particular interest as it governs EMC for 28V systems in military vehicles. 1275D imposes some strenuous requirements in terms of transients that connected systems must survive. With all of these, sometimes contradictory, factors in play, Vicor's VI Chips represent an example of a solution that needed minimal adaptation to extend their use from the commercial into the military environment. They use integrated- circuit-style packaging to yield modules that are small and lightweight (15g) with very high power densities. They are equally adaptable to aerospace, surface vehicle or portable applications and a completely moulded construction makes them robust and capable of operating reliably in harsh environments.


Meeting the needs of a change currently under way in some aircraft systems – namely, the move to distribution, around the airframe, of higher-voltage bused power at 270Vdc, obtained by rectification of alternator output, is a MIL-COTS Bus Converter Module (BCM). Using a Sine Amplitude Converter (SAC) architecture from Vicor, it accepts the 270V nominal input and outputs up to 235W at 30.0 – 41.25 V or 270W at 38.3 - 55.0 V with an (isolated) conversion efficiency of up to 96%. The very small package equates to a power density of over 919 W/in3. Designers can use it alongside the MIL- COTS PRM (Pre-Regulator Module) and VTM Current Multiplier. The former is a similarly-packaged, high-efficiency DC/DC buck/boost device that produces a regulated, adjustable output, again with very high efficiency; the VTM fixed- conversion-ratio point-of-load current multiplier is an isolated Sine Amplitude


Converter module that operates from a 26 to 50 Vdc primary bus – provided by the PRM - to deliver power efficiently to applications that require low voltages and high currents.


The very high efficiency of these devices means that under normal circumstances, they only require minimal heatsinking up to their rated power. At these power levels, the application itself will dissipate significant heat which its designers will allow for; but flexibility is gained if the power supply does not contribute substantially to that load, and if it does not have to be fixed to a cold-wall. Long- outdated is the assumption that in a military design there is usually a mass of metal nearby that can conduct away heat: in contrast, today's systems may be deployed inside a composite structure that has excellent insulation properties. The MIL-COTS PRM operates from a wide input range of 16-50 Vdc (13.5-50 Vdc after startup), meeting many of the ground vehicle and airborne requirements of MIL-STD-1275 and MIL-STD-704: many of the surge-voltage-withstand characteristics embodied in those standards are within the nominal operating range of the modules. A DC-DC converter comprising a PRM and VTM (or multiple VTMs) adds low-noise operation and very fast transient response to the density and efficiency benefits. One isolated VTM will supply up to 100 A of output current and output voltages from 1 to 50 Vdc. The transient response performance is due in part to the elimination of bulk capacitance at the point-of-load; this further increases overall system power density due to the reduced board space, and removing bulky components from the circuit board simplifies ruggedised system design. Stepping from the IC-style packaging to the familiar “brick” and fractional-brick outline further extends the options open to the power architect, adding functional blocks such as filters; for example, a PRM- plus-integrated-filter module can provide a design with compliance to MIL-STD-1275 and -704, and to MIL-STD-461.


Vicor | www.vicor.com Kai Johnstad is Sr. Product Marketing Manager, Transportation, Aerospace, and Defence Products, Vicor


www.cieonline.co.uk


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