The process of finding, selecting and qualifying commercial off-the-shelf Uninterruptible Power Supplies (UPSs) for military applications can be an exercise in frustration. Manufacturers' specifications and datasheets are written primarily for commercial business markets. They often do not reflect the wider spectrum of information needed to properly determine their applicability for use in a specific military application.

There are three industry standard UPS design topologies: off-line, line-interactive and on-line. For most military applications, the off-line and line-interactive designs are low-cost designs and are not acceptable. They were designed to provide basic battery backup. These types of units simply pass the utility power through the UPS until utility power is lost at which time they turn on and switch to battery-powered inverter operation.

In contrast, the advantages of a good on-line UPS are many. They provide a continuous duty inverter with a low distortion true sinewave output (3% THD typical). Their wide input voltage range marries well with a tight output voltage regulation--around ±2-3% typical. They also provide wide input frequency range while providing tight output frequency regulation. As the inverter is continuously providing the UPS output, there is no disruption of output power when the utility voltage is lost or returns. Meanwhile, battery mode operation time can be lengthened by adding more battery capacity. In some cases, the UPS may also be used as a frequency converter providing a 50, 60 or 400 Hz output independent from a 50, 60 or 400 Hz input. Online UPSs also eliminate the widest range of utility and generator source power problems. Figure 1 shows an on-line UPS and the sinewave input and output waveforms.

Construction Concerns

Most off-the-shelf UPS models available are not designed to meet the rigors of full Mil-Spec requirements. With modifications, some models can be made fairly rugged. As part of the engineer's initial investigations, care should be taken to verify the basics. The UPS market is very competitive and as such, some products available are not constructed using double-sided FR4 type laminated circuit boards. The manufacturer has cut costs by using inferior "chopper glass" board material. Even worse from a shock and vibration standpoint, they have used single-sided circuit boards without being plated through.

Circuit board, transformer, battery and heatsink mounting practices could be other areas of concern. Most good quality off-the-shelf on-line UPS products have been designed to withstand the normal shake and vibration sustained during shipment, which can be substantial. Continued trips across the desert in a Humvee (Figure 2) is another matter. The unit may require substantial modifications or even a mechanical redesign to meet more stringent military specifications. If the required quantities are large enough, the manufacturer may be willing to make the changes providing a timely and cost-effective UPS solution to the engineer's requirement. If not, there are other UPS companies offering build-to-order pre-ruggedized off-the-shelf products.

Environmental Concerns

Most domestic off-the-shelf UPS products available are tested to meet or exceed the UL1778 safety standard. As such, they are designed and intended for use in a protected environment. This means that they are not weatherproof, they do not come equipped with protective air filters, and importantly, they have not been tested by UL for operation in a wide temperature range--”only within 32° to 104°F (0° to 40°C) with a non-condensing humidity of 10% to 95%. Should the military application be inside a temperature-controlled shelter, this may not be a problem. The fact is that some well designed on-line UPS electronics, if provided enough cooling, would operate over a temperature range of 14° to 122°F or (-10° to 50°C).

For the standard Valve Regulated Lead Acid (VRLA) batteries typically used in off-the-shelf UPS units, it is another issue. Battery manufacturers state that if these batteries are operated or stored in temperatures above 90°F, they will have a noticeably shortened life expectancy. If operated in a continuous 122°F environment, the battery life could be shortened to less than one year, while at 75°F they would be expected to last three to five years. Battery charging above 104°F (40°C) also becomes a problem as the batteries should not be charged above this temperature. Some off-the-shelf UPS manufacturers offer wide temperature range battery options. The wide temperature range VRLA battery has been designed to operate over a -54° to +176°F (-65° to +80°C) range, but will still have a reduced life at temperature extremes. Charging is also not an issue at the elevated temperatures.