by Gerald Hovdestad, Behlman Electronics
Commercial-off-the-shelf power equipment can be an option for situations that normally demand specialized designs optimized for harsh conditions.
A lot of DOD and industrial equipment is so specialized that it must be custom-developed – it just can’t be bought off-the-shelf. Ironically, there is a large array of commercial products that would be candidates for such uses but for one thing: They aren’t compatible with the power available. That goes even for relatively common devices such as displays, servers, printers, memory, and sensors.
Power compatibility becomes a serious issue because there are a wide array of power specifications such as MIL-STD-1399, MIL-STD-704, MIL-STD-1275, DO160, and numerous others. These specifications continually undergo revisions, making the compatibility problem exponentially more challenging. When a manufacturer claims compatibility with a specification such as MIL-STD-704, a knowledgeable prospective customer typically asks, “Which version?”
Consider, also, that there are a number of voltage levels and configurations used for distributing both dc and ac power. Common dc power voltages include 12, 28, 48, and 270 V. AC lines can be 50, 60, or 400 Hz and distributed in single and multiple phases, using wye and delta arrangements, with voltages ranging from 115 to 440 V.
One way of managing this diversity is to use flexible power conditioning equipment to permit the use of high-reliability COTS (commercial off-the-shelf) components. The use of commercial of-the-shelf components is a possibility if input power can be stabilized, removing voltage and frequency fluctuations, as well as momentary transients and dropouts. For example, COTS power supplies are an option to condition the “dirty power” to produce clean, regulated 115 Vac 60 Hz or 400 Hz, without 180 Vac transients or 50-msec or 200-msec dropouts or a stable 28 Vdc.
For the most critical requirements, power supplies can be configured in a redundant N+1 configuration so a single failure won’t affect the mission. The redundancy might take the form of individual plug-in units or redundant components housed in a single supply.
COTS power supplies have steadily grown physically smaller with better reliability. COTS power modules can now be had in the 2 to 4-kW range with MTBF as high as 300,000 hours, both calculated and demonstrated. Power devices such as Behlman’s DCR2U Critical Mission Power Supply are now configurable to accept dc inputs (28 V, 48 V or 270 V) or ac inputs (single or three phase, 47-440 Hz or wide frequency), and provide up to 4 kW out of up to four dc voltages, from 3.3 to 48 V, all in a 2U, 30-lb package. Where even smaller size is required, 6U or 3U VPXtra power supplies are available that weigh 2 to 4 lb and produce up to 1,500 W of dc from an ac source. These can be paralleled where there is a need for higher power.
A flexible input power source can drastically reduce system development time. For example, a major prime contractor came to Behlman with a COTS solution for an important development program, but had a major power problem. The U.S. Navy needed a dipping sonar for a shipboard application. It had been under development for several years with no clear end point. The prime manufacturer noted that its existing product was already qualified and would meet all the specifications without any further work, but for one thing: This sonar was designed to work on aircraft and wasn’t compatible with shipboard power.
Available power was MIL-STD-1399 115-V single-phase or 300 Vdc. The existing sonar required three-phase 400-Hz 200-V power per MIL-STD-704. To solve the problem, Behlman was able to combine three standard COTS frequency converters producing approximately 5 kW and package both the Behlman power supplies and the dipping sonar in a water-cooled enclosure. The Behlman supplies were conduction-cooled and mounted to a water-cooled plate. The dipping sonar required air-cooling, so the plate had a heat exchanger which cooled the air, allowing the sonar to work as designed.
The net result was that the Navy had a working radar in less than six months using this COTS approach. It would have taken two years of effort for a completely new R&D program using non-COTS equipment.
Whether the power is for a T&M bench in a lab, support of production on a manufacturing floor, or for the most sophisticated electronics on military vehicles, some COTS power supplies are versatile enough to provide rapid, cost-effective solutions.