FEATURE COVER STORY
VERSATILE POWER CONVERSION FOR MILITARY & AVIATION APPLICATIONS
Steve Munns Mil/Aero product marketing manager at Linear Technology (now part of Analog Devices) discusses the company’s latest synchronous 4-switch buck-boost controller. With up to 98% efficiency it provides a versatile solution for a variety of military and aviation applications
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ustainability initiatives, green energy and environmental regulation are part of everyday life both in a domestic setting and in the wider world. Commercial aviation has been at the forefront of efforts to use lightweight materials, improved aerodynamics and more efficient engine technology to respond to commercial pressures and tighter emission targets. Electrical efficiency has become increasingly important as aircraft shift from traditional air bleed systems and hydraulics to electrically-powered actuators, pumps and compressors to drive a multitude of systems. On the Boeing 787, ice protection relies
on electro-thermal heater mats to replace hot air bleed from the engines, further reducing load and improving fuel economy and thrust. These developments have seen onboard electrical power demand escalate over three decades from around 300kW in the B767 to more than 1.4 megawatts in the B787, making the efficiency of power
Figure 1a:
environmental conditions. Furthermore, corporate policies for component selection sometimes limit the types of products available to those pre-approved by component engineering and re-use of proven standard circuit blocks is encouraged. It is therefore desirable to have products that lend themselves to multiple applications through innovative architectures and clever chip design such that fewer qualified or approved part types are in use. The problem is that even in a relative narrow area such as power conversion, applications and requirements vary widely. However, as we will see, some products such as the recently released LT8705 from Linear Technology are versatile enough to make them candidates in these scenarios.
conversion more critical than ever before. The military are also following the
same direction of travel, striving to make systems more energy efficient, reducing weight, size and heat generated, and thereby trimming down acquisition and operating costs.
14 APRIL 2017 | ELECTRONICS Selecting components for use in
military and aviation equipment includes all the usual design trade-offs and compromises, and in addition, is constrained by tough specifications for equipment survivability and rigorous qualification for use in harsh
Figure 1: Typical voltage stabiliser
HIGH EFFICIENCY POWER CONVERSION The LT8705 is a very high efficiency (up to 98%) synchronous buck-boost DC/DC controller that operates from input voltages above, below or equal to the regulated output voltage. Four feedback loops are integrated to regulate the input current, input voltage, output current and output voltage. The input current and voltage feedback loops prevent overloading of energy limited sources (i.e. solar cells) while the output current loop provides a regulated output current for a battery charger or current source. The LT8705 operates over a wide 2.8V to 80V input voltage range and produces a 1.3V to 80V output, using a single inductor with 4-switch synchronous rectification. The high input voltage simplifies transient protection as it can withstand the surge voltage requirements specified in MIL-STD-704 and DO-160. With the addition of a surge suppressor such as LT4363, this can be further enhanced to meet the surge requirements of MIL-STD-1275. Output power up to 250W can be
delivered with a single device, making it / ELECTRONICS
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