In 1992 the Department of Defense (DOD) was exploring ways to reduce the procurement costs and delivery times of equipment purchases having to meet military standards. The development, documentation and certification costs of the equipment was astronomical, as at that time all of the equipment purchased had to comply with one or more rigorous military standards. In response, the DOD implemented the commercial-off-the-shelf (COTS) initiative.
In the early years of COTS the amount of actual commercial-off-the-shelf equipment purchased by the DOD was limited because there was a cultural reluctance to purchase equipment that did not meet the associated military standards. There was an unwillingness to modify or relax the specifications within the standards. However, over time COTS opened the door to allow the DOD and their contractors to explore the possibilities of modifying existing off-the-shelf products to meet less demanding applications.
Today, the COTS initiative has been so successful that engineers incorporate COTS equipment into military systems wherever possible. There is now a willingness to amend or even eliminate unnecessary portions of military specifications to accept COTS equipment. However, in most cases not all of the military standard elements are eliminated, requiring the equipment manufacturer to make necessary product modifications to comply. After the required modifications, classifying the equipment as true COTS is misleading and the acronym modified-commercial-off-the-shelf (MCOTS) should really be used.
In the uninterruptible power supply (UPS) and power conversion industries, like most other electronic equipment, the modifications typically required are in the areas of ruggedization and environmental concerns. The environmental issues could include widening the equipment’s operational temperature range, as well as reducing conducted and emitted radio frequency interference (RFI) and electro-magnetic interference (EMI) emissions. A third environmental area relates to where the equipment is to be used. Will it be used in a protected indoor, or unprotected, outdoor environment? Will it be used in the desert or near the ocean? Is it an aircraft, ground, or shipboard installation? Unlike the days prior to COTS, the level of modification required and specified military standard compliance is often determined by the end-use of the equipment. The extent of product modification required may range from a simple circuit change or a complete redesign and repackaging of existing electronics. What follows is an illustration of the various facets that might have to be addressed in developing an MCOTS product.
Case in Point
A large military contractor was developing a mobile advanced communications system. The system, which was to consist of a large array of advanced RF and satellite communications equipment, would be housed inside a shelter enclosure that was permanently mounted to the frame of a HMMWV (High Mobility Multipurpose Wheeled Vehicle, a.k.a. Humvee). To power the vast array of equipment, the contractor required a custom 12 kVA power system having a unique set of operational requirements. The power system had to operate from any domestic, international or military single or three-phase power source over a voltage range of 85 VACto260VAC, 45Hzto450Hz. In the event of a loss of power from an onboard generator or external utility power source, the system had to provide up to 15 minutes of backup power from an onboard battery source. The entire system had to be ultra-rugged and pass the Army’s Munson Road Test. Further it had to survive continuous off-road use. To reduce the amount of ruggedizing required, the power system was to be installed inside shock absorbing equipment racks.
To accommodate the differing input phase configurations and voltages the system needed a special three-channel voltage conversion module having galvanic isolation on each channel. If available, local utility power was to be used as an alternate source. A three-channel voltage conversion module was needed to meet this requirement and to distribute the input current demands enough to keep them within reasonable limits.
The system would have to operate reliably from questionable third-world AC utility and generator power sources having differing frequencies. It was imperative the connected communications equipment operate from a well-regulated 120 VAC 60 Hz domestic power source. This dictated that three 4 kVA, self-contained, online UPS/frequency conversion modules be used. As only two of these modules demanded battery backup, two external battery banks had to be supplied. All of these various system elements had to fit into 36.75 inches of rack space height.
Given the level of specialized power system equipment needed, together with all of the other requirements, it seemed hard to believe that any COTS or MCOTS products could be found to satisfy the prime contractor’s needs. However, after carefully reviewing the requirements, it was determined that the electronics from existing online UPS products could be adapted and repackaged to meet the requirements.
Solving the Problem
The unit would have to be packaged into a rugged 5.5 inch high, rackmount chassis. Three of the newly designed modules would make up the bulk of the required electronics and function as the three required UPS/frequency conversion modules. As the third module would act as a frequency converter and voltage regulator for the HVAC system, it was not required to provide battery backup. However, two additional rugged 3.5 inch high rackmount battery banks would be designed to house the required backup batteries to support the modules requiring UPS functionality. A 12 kVA, three channel, voltage conversion system element would have to be a completely new design, and it had to fit into the remaining 8.75 inches of rack space. Due to the galvanic isolation requirement, that would prove to be difficult, but not impossible.
Because of the shock and vibration concerns typically encountered in an off-road Humvee environment, great care had to be taken in the mechanical and electrical design areas. All of the power electronics that were repackaged into the systems used numerous high mass components, batteries, and inductors. That meant they had to be housed and mechanically mounted or supported at both ends of the component. The various enclosures were fabricated from heavy gage 5052H3 aluminum, and circuit boards with high mass components were mounted directly to the chassis at multiple axis points. Smaller components had to be affixed to the circuit boards using adhesive. Rugged military standard power and interface connectors had to be used, and self-clinching nutserts and hardware were used throughout the designs. All wiring had to be changed from PVC insulation to Teflon and wire bundles were further insulated and protected using fiberglass reinforced tubing. Finally, all external switches and circuit breakers had to be changed to meet military specifications.
The operational temperature specification was from -20°C to 50°C. This narrow temperature range was allowable because the shelter had an internal HVAC system. It’s important to remember that with power electronics, long term reliability depends on proper airflow design. The typical efficiency of an online UPS can vary from 85% to 90% depending on the mode of operation.
For the entire 12kVA system, that could result in having to manage up to 1200-1800 watts of heat, or up to 400-600 watts per UPS/frequency converter module. The same electronics in a commercial environment would be designed for use from 0°C to 40°C and require cooling fans that deliver an airflow rate of 55cfm. For the military version, air filters were added in the inlet and exhaust openings, and the airflow had to be increased to 200 cfm.
As a further improvement, and to assure the heat was removed, the heatsinks cooling key power components were mounted in line with the airflow to assure a forced laminar airflow was directed through the heatsinks and across components mounted on circuit boards. The enclosure was used as a plenum to assure heated air was exhausted outside the enclosure. To help promote proper preventive maintenance, the air filter housings were designed to assure they were accessible and easy to remove and service.
Special attention was also paid to the design and installation of the battery module. Temperatures above 35°C start to shorten the life of valve regulated, VRLA lead acid batteries. Should VRLA batteries be subjected to continuous temperature at 50°C, their life is shortened from many years to several months. By comparison, the same batteries used in a cold -25°C environment would not suffer any reduction in service life and the life would be increased. However, they will suffer from a reduced capacity of up to 20%. This would yield a shorter battery backup runtime than expected.
In conclusion, there are numerous factors that need to be taken into consideration when undertaking an MCOTS development project. The amount of design work on the project can be substantial. However, using circuit board level electronics from existing proven products can significantly shorten the time from development to production, resulting in robust systems that can be delivered on time and under budget.