Looking Under the Hood of a Military Power Supply
- Friday, 01 February 2013
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.