With the proliferation of commercial off-the-shelf (COTS) products being offered for military use since the 1990s, it is all too easy to be lulled into believing that COTS products are always the most economical. That is not always true.
For more than a decade, the U.S. Department of Defense has stressed the need to use COTS components wherever possible as a way to reduce the total cost of ownership of military systems. However, because military systems are typically required to have a service life of 20 years or more while being subject to the extreme rigors of shipboard, airborne or mobile military applications, standard COTS products that appear to be satisfactory at first may not readily meet the longevity requirements and rugged performance specifications for the task at hand.
In the case of power supplies, ruggedized COTS products with some necessary modifications can be the ideal solution. They still deliver the cost benefits and streamlined development time of a standard COTS product but with the features and performance necessary for military applications.
Modified COTS power supplies from qualified vendors have delivered the value and performance required by all branches of the military, in a broad range of applications and systems. The right power supply vendor will have significant design engineering experience with power supplies for military applications and whenever necessary will take a partnered approach with armed forces managers, prime contractors and system integrators to ensure that the modified COTS power supply precisely meets all performance specifications, on budget and on schedule.
A preferred approach to designing cost-effective modified COTS AC-DC or DC-DC power supplies is to use standard DC-DC modules (COTS “bricks”), along with the necessary circuitry and mechanical and thermal design considerations to meet specifications. This approach significantly reduces non-associated costs, as well as reducing turnaround times.
The importance of choosing the right vendor for this approach cannot be overstated. For example, some power supply manufacturers advertise power densities in excess of 70 watts per cubic inch. Unfortunately, upon closer inspection, you’ll find that the manufacturer’s data also shows that the advertised power density is only achieved at moderately low temperatures and at a specific input and output voltage. Oftentimes, the advertised power density figures do not account for additional necessary components needed to meet the complete power system specification, and the result is an overall power system power density of between 10 to 15 watts per cubic inch — much lower than the advertised 70 watts.
When choosing the right power supply, there are several issues that need to be considered to ensure a reliable design that meets all specifications (as well as meeting all advertised performance parameters). Consulting with the vendor, you should be able to arrive at the answers to questions such as the following:
- Which standard module is best suited for the application at hand?
- What needs to be done to meet EMI requirements of MIL-STD-461?
- What needs to be done to meet hold-up and transient requirements of MIL-STD-704?
- What needs to be done to meet MIL- STD-1399 requirements?
- Should conduction or convection cooling be used to meet temperature requirements?
- Do the power supplies need to be paralleled?
- Is redundancy and N+I required?
- Which control circuits are necessary?
- And is built-in test (BIT) required?
In addition, the power supply vendor should be able to specify the inclusion and impact of such elements as protection circuitry, filtering, monitoring and control, environmental conditions, and the physical size and weight of the finished power supply in order to produce a reliable power supply meeting all system and environmental requirements.