Electronic Design

Modular DC-DC Converters Continue To Serve The Military

COTS applications take advantage of the enormous size and format range of these high-density converters.

Keith NardoneCompact, high-power-density dc-dc converters, introduced in 1984, quickly found homes in military applications due to their small size, high performance, and reliability. The converters (bricks) were packaged in a modular form factor that simplified power system design and reduced the time it took to bring a product to market.

A full quarter century after their introduction, bricks are still being specified for many applications, both commercial and military, but the attributes of bricks are especially well suited for military COTS applications. These high-density power components have earned acceptance in military and defense apps, where high performance, reliability, and low cost are critical.

The variety of available dc-dc converter modules is rather expansive, coming in a range of sizes and formats (e.g., full bricks, half bricks, quarter bricks, etc.) (Fig. 1). Highdensity converters—usually characterised by high-frequency operation, allowing them to achieve their small size, high power density, and efficiency— can be found in thousands of combinations of input voltage, output voltage, and power level.

Such a modular dc-dc converter, when combined with discrete components, can satisfy many unique power requirements. Because each module is a proven, prequalified performer, designers using these converters enjoy an advanced starting point toward a finished power supply.

The military commonly uses input voltages of 28Vdc for ground and 270Vdc for airborne and AC, but modular components offer just about any spec that’s need for a particular application. Modular components used for COTS applications satisfy 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 qualified to stringent environmental requirements, and some standard bricks (those that are fully encapsulated) handle high-g forces. The building-block design approach is flexible, as can be seen with a high-power-output design (Fig. 2) and a multiple-output design (Fig. 3). Such an approach is also cost-effective and offers quick turnaround and reliable performance.

Early designers had to supplement the capabilities of modular dc-dc converters by using discrete components to implement auxiliary power system features, such as ac-dc rectification, filtering, power factor correction, etc. That option is still open, but specialised accessory components have become increasingly available.

Together with the power components, these matched, compatible accessories—such as filters, holdup capacitors, heat sinks, and ac front ends—allow users to quickly assemble complete power systems. They can simply select and interconnect standard, modular parts to meet their design requirements.

Compatible front-end accessories, for example, provide a number of performance features such as input transient protection, EMI filtering, and inrush current limiting (Fig. 4). In addition, they have international agency approvals and can accommodate the wide range of input source voltages necessary to reach worldwide defense markets.

Military-targeted dc-dc converters typically satisfy U.S. Department of Defense definitions for nondevelopment items (NDIs) and commercial off-the-shelf (COTS) equipment. They meet key military performance specs for input power quality (MIL-STD-704, 1275, 1399), EMI levels (MIL-STD-461), environmental requirements (MILSTD- 810, 202), and component derating. Full encapsulation of the “brick” package enables the modules to meet severe environments of humidity, fungus, salt fog, explosive atmosphere, acceleration, vibration, and shock.

Bricks may not always be the first choice for some power designers. However, it’s proven technology, and they are a cost-effective, lowrisk, quick way to get to market solution for these power engineers.

TIME TO MARKET
Bricks are a good choice over a custom, discrete approach when time to market is an important criterion. Development time is shorter, less expertise is needed, and the modules represent an advanced starting point toward a final power solution. The power-supply designer can even obtain rapid delivery of small prototype quantities in the final form factor well before the system design is completed.

Every designer, whether designing commercial or military products, wants to get their products out there quicker. When it comes to the military, though, there’s often a higher value placed on that capability.

Designers of military electronic systems have shown a preference for older tried and true architectures. The reasons are not hard to find: demonstrated reliability, long lifetimes, compatible accessories, experienced applications support. Bricks and brick-based power solutions offer rich options for designers of military power solutions.

Furthermore, bricks are versatile, being applied to a large variety of end uses for the military and aerospace industries. Destinations include aircraft and ground vehicle and naval applications, underwater- type applications, and manpack- type applications.

Obsolescence is always a big issue in MIL and MIL-COTS apps. Typically, a military program takes several years to get going, and will serve several years in the field. As a result, support is going to be required.

Businesses driven by hot new technologies may well offer COTS products, too. But many of them are focused on the short term, including how long they will support their products. The proven brick technology with its stable manufacturers already have a very long track record, which gives confidence that they will likely be here for the long haul.

CUSTOM SOLUTION
Brick manufacturers offer multiple sizes, common platforms with common components used within those platforms, and a large mix of customer-selectable voltages and powers to satisfy unique requirements. What’s more, mass customisation is now a reality. An online suite of advanced powerdesign tools is in place, enabling designers to specify and verify the performance and attributes of custom solutions in real time.

For mission-critical applications, fault tolerance with bricks can be achieved by redundancy, i.e., the existence of at least one extra, or “redundant,” converter in the system. Such a system of converters is commonly referred to as an N+M array, where N converters are required to satisfy the power requirements and M additional modules provide redundancy.

All modules in the array must be able to supply undisturbed power in the event of shutdown or failure of one module, in spite of the sudden change in load current demanded of each. To satisfy these criteria, it’s essential that the individual converters share the load current, in order to minimise the dynamic response required of each. Bricks will typically load share automatically.

TAGS: Defense
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