There are power supplies in virtually every military electronic system. These ubiquitous devices come in all sizes and power ratings. And just like their commercial counterparts, they are available in the AC/DC, DC/DC and DC/AC configurations that provide the appropriate electrical energy to operate the electronics.
For years, the difference between a military power supply and its commercial cousin has been reliability. This is primarily because a loss of power in a military system is not only inconvenient, it could result in a catastrophic failure and unnecessary loss of life. Military power supplies were designed with long MTBF (mean time between failure) ratings in mind, with a goal of delivering years of trouble-free operation.
Today, as in the past, reliability of military supplies remains paramount with MTBFs exceeding those of commercial counterparts, but now military systems are also being upgraded every two years just like consumer electronics. With each upgrade, the previous generation of power supplies becomes obsolete, so obsolescence and availability issues have become more likely, while old ideas of standards-based and systemcertified supplies make less sense. In addition, the ability to recognize a pending failure is becoming as important as long-term reliability.
The same trends that forced military users to adopt COTS standards for most computer equipment (e.g., rapid technology advances, component obsolescence, etc.) now apply to the power conversion industry. Nevertheless, there are still specific requirements that manufacturers of military power supplies must meet. These include detailed guidelines for selecting components that are part of each supply, a rigorous set of design rules to ensure manufacturability, and many specifications focused on the environment in which the supply will be used.
Parts Selection and Design
Parts selection is an important step in designing a military power supply and is clearly detailed in the U.S. Navy’s SD-18 “Parts Requirements and Application Guide.” All components used in the products must be qualified either by their manufacturer for use in military systems, or qualified for the application by the manufacturer of the power supply. This process is meant to accomplish two things. The first is to establish that a stable source of supply for the component exists and adequate quality control procedures are in place. The second is to prevent the usage of restricted materials that can degrade during normal usage. SD-18 also governs rules for derating components to help designers meet challenging specifications with more common commercial grade components.
While long life cycles may be a thing of the past, there remains a requirement that military electronics systems must be replaceable or at least repairable throughout their lifetime. This can place added demands on the military power supply manufacturer. For example, if a part used within a power supply is being discontinued by a component manufacturer, and an equivalent qualified part is not available from another source, the power supply maker must notify all previous buyers of the supply and offer a final build plan to allow them to order spares for future usage.
When it comes to design of military power supplies, NAVSO P3641 is the premier reference. First introduced in 1999, this comprehensive set of guidelines, subtitled “More Power for the Dollar,” details the best manufacturing practices for military power supplies. It includes provisions allowing the use of COTS (commercial-off-the-shelf) power supplies in applications like telecommunications, computing, or air traffic control where benign conditions do not exceed 0° to 70 C° limits. It also details requirements for designing and building power supplies for the more rugged -40° to +85° C conditions encountered in the field.
These guidelines suggest the use of metal clad printed circuit boards, surface mount packages for integrated circuits, gold plated interconnects to reduce the effects of corrosion, physical design parameters that impact cooling, electromagnetic interference management techniques including shielding and physical layout, and many more design details. It also gives detailed design guides for high and low voltage DC/DC converters, as well as AC/DC supplies, inverters and uninterruptable power supplies. In addition, environmental stress screening parameters and procedures, including highly accelerated stress screening and testing, are outlined.
Various branches of the military have published detailed sets of requirements that establish what will be expected of power supplies from all types of military systems deployed in the field. These Military Standards — or MIL-STDs — focus on performance issues as well as environmental conditions that can impact reliability.
Input voltage conditions for tactical military applications cover electromagnetic compatibility and input levels. The former is spelled out in MIL-STD-461 which details the amount of conducted RF energy the device must be able to withstand and still operate properly. This is typically 40 dB or more of attenuation from the internal power supply switching frequencies all the way out to several megahertz. Commercially available filters that meet FCC requirements for commercial noise suppression cannot achieve this level of signal rejection. So each military power supply application must be approached individually to construct filters and maintain proper impedance matching characteristics to eliminate radiated noise at the input.
Meeting the full range of input voltages required for military applications could be a challenge for off-the-shelf commercial products. For example MILSTDs generally detail three input voltage ranges for each device. The first would be normal operation; the second “abnormal” like when a malfunction or failure has occurred and protection devices are functioning to correct the situation; the third is emergency where the main generating equipment has failed and a limited independent source is powering vital systems. Examples for aircraft and vehicular standards are shown in the accompanying table.
Note that in all cases the minimum accepted input is lower for military power supplies than is generally found in specifications for COTS devices. This means that COTS supplies would shut down at higher voltages while military grade devices keep on powering systems. This is primarily because designers of military compliant devices tend to optimize performance of their DC/DC designs by trading off maximum output current for a wider input voltage range. High line ratings present another problem for COTS DC/DC converters, since their maximum rated line capability is less than applicable MIL-STDs. Here military compliant modules utilize clipping and pre-regulation schemes to prevent damage and allow for reliable operation across the entire full voltage range.
Harmonic noise and distortion can also be an issue with most commercial power supplies in certain environments. For example, in shipboard applications, MILSTD- 1399 specifies that single harmonic distortion be limited to 3 percent and total harmonic distortion to below 5 percent. Commercial units would require large passive components to meet these levels, whereas military designers accommodate these conditions as a matter of course.
Transient protection is required for virtually all military power supply input stages and varies from application to application. A typical specification can be found in the MIL-STD-1275B requirements for military vehicles. Here the input for a 28VDC DC/DC converter must be able to survive a spike of up to ±250V that is 50 μsec wide in a burst of 1 msec duration with 15 mJ maximum energy content per spike. Surges of up to 100V for 50 msec from a 0.5 Ohm source impedance repeated 5 times per second must also be accommodated. Most commercial units can meet neither specification, but well designed military units can. In addition military units must often be designed to survive lightning strikes and this would increase spike and surge protection requirements by several orders of magnitude.
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, Click Here