Power solutions for military
01 July 2007
Custom power supply for a manned space vehicle
Military applications demand power solutions that are reliable but that do not have the lead time, expense or risk inherently associated with custom power solutions
It is not unusual for military systems designers to build their own power supplies from discrete components. Power supply design is perceived by many to be a low-tech, relatively low-cost task assignable to any design engineer, preferably junior level. It is also often thought that the schedule can be easily managed in-house as part of the overall system design. However, this is often not the case. Some, who have tried building their own, might view a custom power supply designed and built by a specialised power supply manufacturer as a more likely technical solution.
However, custom power solutions are usually expensive and risky, with long lead times, particularly for the military. They also involve greater risk and take longer to realise than non-military custom power solutions. Design risks in custom power solutions can often be traced to midstream changes or simply the unpredictable, which is inherent in traditional approaches. Typically, custom power design cycles can be six to nine months.
A frequently-quoted axiom claims that a buyer can expect a product to have high quality, fast delivery, and low price, but just not all three at the same time. Nevertheless, if the product is a custom military power supply, a component power solution often can be superior to a discrete power solution, completed within a shorter lead time, at lower cost and with less risk.
There are many manufacturers of custom power supply, but those whose designs are based on component power modules are relatively few. These manufacturers, particularly those specialising in the military COTS market, have some advantages over conventional custom power manufacturers. This is largely because modular power components (DC-DC converters) are manufactured in volume, undergo rigorous testing (including COTS level testing), and are used in many demanding applications, including military applications. However, there are other reasons.
Power components advantages
Although the military primarily uses input voltages of 28V for ground and airborne, 270V DC for airborne, plus AC, modular components come in a great range of different voltages, powers and physical sizes. Modular components used for COTS applications satisfy the other input characteristics as well, including low- and high-line conditions and the capability to handle voltage spikes, surges and excessive input ripple. Available output powers range from tens of Watts to kiloWatts, from single outputs to 40 outputs. Most high density DC-DC converter modules are built to stringent environmental requirements, and some standard bricks (those that are fully encapsulated) handle high Gforces. The building-block design approach is cost effective and offers a quick turnaround and reliable performance.
Some module manufacturers offer an accompanying array of accessories that help the design of a custom power system. Military COTS filter modules, for example, are available to complement the converter modules, offering EMI and transient compliance for both DC and AC systems. These compatible accessories help designers build solutions faster and with less risk and cost, considering there is no design time or contribution to what would otherwise be higher NRE (non-recurring engineering) cost.
Risk of obsolescence is an important problem for many military systems. A substantial segment of custom power manufacturing business comes when replacement or upgrade of the power system is needed for an old system. Custom military systems manufactured with component power modules often give designers cost effective options for such systems.
The replacement or upgrade is often made to be pin-compatible with the original, or even smaller, with the availability of fractional-sized bricks. One case involved replacement of an obsolete power supply and reduction of the weight of the Harpoon missile, which has a unique configuration. The designer could not change the missile, so the power solution had to fit in the space available. The retrofitted power supply was so much smaller and lighter that a piece of steel had to be inserted to maintain proper balance. In some new programmes, such as the Joint Strike Fighter or other new aircraft, the requirements are much more demanding, i.e. more power in a smaller package, no airflow and lighter weight.
Military customers need long-term support, as the life expectancies of military systems commonly extend to as long as 20 to 25 years. Some of the first modular DC/DC converters ever built are still in operation today and those designs are still being manufactured today. There is no need to redesign or requalify and the documentation can still be used.
Ease of EMC compliance
Another potential risk area in a custom military environment is the satisfaction of EMI/EMC requirements. Most component power manufacturers, especially those with COTS customers, make provisions for meeting such EMC compliance requirements as Mil-Std-461. While switching power supplies generates noise, in recent years the previous holder of the quiet crown, linear power supplies, have been replaced.
EMC filtering can be achieved using discrete components or filter modules. A common answer is the availability of an accessory module designed to provide the EMI/RFI filtering and transient protection required in military applications. Such modules meet the conducted emissions specifications of MIL-STD-461C/D and offer input transient, surge and spike protection to the most severe levels of MIL-STD-1275A and 704A.
COTS power components are routinely designed and manufactured to satisfy MILSTD-800 requirements for humidity, fungus, salt, fog, explosive atmosphere, acceleration, vibration and shock. Some undergo environmental testing as specified by MIL-S-900 and MIL-STD-202 and 100 per cent environmental stress screening as well. Operating temperature is one of the most important factors in determining overall module reliability. Therefore, its design must ensure efficient heat transfer to the system ambient. An exposed baseplate design allows heat removal, and a 10ºC decrease in baseplate temperature can increase MTBF by over 50 per cent. Some modules use a spin-fill process to assure complete, void-free encapsulation, making them suitable for the harshest military environments. In addition to providing mechanical rigidity, the encapsulant is thermally conductive eliminating hot spots and improving heat transfer to the baseplate.
In spite of starting with a known, proven product, the nature of a custom system is that it is unique. It needs experienced, knowledgeable power specialists and engineers with a track record. Many custom manufacturers have the skills and experience to offer effective complete solutions to meet unique military power requirements. Fewer can provide module -based custom solutions as well as having the requisite experience with military COTS and aerospace applications. Of the custom manufacturers who specialise in the design and manufacture of power supplies using power components, some are related by investment or organisation to a large power component manufacturer, with access to substantial resources, such as sales, technical and financial expertise. The COTS approach eliminates the need for full Mil-Std for many applications. The full MIL programmes still go to the larger companies that can support the specialised resources that are required. Recently, a small power manufacturer had an opportunity to design and build 60 power supplies for the Turkish Air Force and had to turn it down after determining that specialised manpower would be needed just to manage the paperwork.
With components, some significant part of the design is already done when the custom design begins. The custom manufacturer is concerned with thermal, packaging, filtering, input/output and management issues, but not the DC-DC conversion. DC-DC converters are the heart and soul of these custom power supplies, so the risk is mitigated when the component approach is taken.
KEITH NARDONE is senior manager, defense products, Vicor and MARK CONNOLLY is product marketing manager, custom systems, Vicor
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