Portable power units like this frequency and phase converter provide mobile communications capability anywhere in the field.
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.

This article was written by Michael A. Stout, Vice President of Engineering, Falcon Electric (Irwindale, CA). For more information, contact Mr. Stout at This email address is being protected from spambots. You need JavaScript enabled to view it., or visit http://info.hotims.com/34453-200.

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