While some commercial power supplies might encounter some tough operating environments on the factory floor or out in the weather, for example, military power supplies must operate reliably in a wide range of environments from deep space to undersea, from deserts to swamps and from the tropics to the arctic. As a result, a complete set of MIL-STDs have been developed to provide guidelines for the design and testing of units to accommodate any operational mission.
Extended temperature range: Commercial power supplies are generally developed with an operating temperature range of 0° to +70° C in mind. At a minimum, military power supplies must accommodate temperatures from -40° to + 85° C, with special requirements extending that range to -55° to +150° C in special cases. In power supplies, achieving these operating temperature ranges requires a focus on component selection and environmental stress testing.
Shock and vibration: Power supplies used in all types of military systems must stand up to significant shock and vibration from a wide range of sources. Products for use on naval vessels must meet the shock specifications of MIL-S-901 which can range up to 90gs. Vibration testing of electronic equipment like power supplies is governed by MIL-STD-810, with individual regimens selected for each product in accordance with its end use.
Thermal shock: Sudden extreme changes in temperature can significantly damage electronic systems including power supplies. MIL-STD-833 provides a number of test regimens for cycling an assembly or component through rapidly changing temperatures. Typical test cycles range from 0° to +100° C, up to a maximum of -65° to +150° C, with a minimum 6 second interval between limits.
Humidity, moisture and condensing atmospheres: Water in all forms can significantly damage unprotected electronic equipment like power supplies. MILSTD- 810 governs ruggedizing electronic equipment to deal with all types of water and related problems like fungus. Traditional solutions for dealing with condensing atmospheres involve potting the power supply assembly by filling its case with a non-conductive thermo-plastic material. This can add significant weight to each unit which can be a severe design penalty in military systems that fly or are mobile. Newer techniques employing innovative light-weight circuit board coatings and desiccants can pro vide the environmental isolation to protect electronic circuits from moisture with a lower weight penalty.
EMI and solar radiation: Military power supplies must be able to withstand radiation of all kinds — from electromagnetic interference from other systems to solar radiation from a sunspot event — and continue to operate. MIL-STD-461 governs a range of tests to ensure that power supplies can meet the noise rejection challenges that they will encounter.
Makers of military power supplies will continue to meet the reliability challenges imposed by the environmental and electrical system standards that are necessary when deploying military electronics systems in the field. In the future, the issue of reliability might become less important than the development of a prognostic capability to recognize impending power supply failures and notify the CPU embedded within the military system via a data link to prepare for corrective action. Such a system is not unlike those presently in modern automobiles, where the onboard engine management system detects a defect like a misfire in cylinder 3 and warns you with a check engine light as you drive down the freeway. However, just as the introduction of these systems in the auto industry has not slowed the drive for ever-more-reliable engines and components (in fact competitive pressures have greatly increased reliability), so the advent of failure detection systems for power supplies will not stop the quest for higher reliability power supplies.
Failure detection works well for systems that are easily recovered, for example, with power supplies that are easily removed and replaced, and systems designers should take this into consideration as part of their overall design process. But when lives, important operational capabilities, and significant amounts of money are at stake, knowing that your power supply is going to fail is not a comforting thing. There will always be a need in the military and in certain private industries for power supplies that can withstand rigorous power and environmental conditions to be there without fail when they are most needed.
This article was written by Ralph Livingstone, Chief Engineer, and Dave Newton, Design Engineer, Abbott Tech nologies (Sun Valley, CA). For more information, visit http://info.hotims.com/45600-401.