by Michael Stout
Like most personal computing products, the uninterruptible power supply (UPS) remains unchanged in its basic job of battery backup. However, it has increased in its level of sophistication, performance, and features, and costs a fraction of what it did in years past.
Changes include improvements in cost, high-power semiconductor devices, and the incorporation of microelectronics, which have resulted in a reduced parts count. Combine that with the cost advantages of offshore manufacturing, and the UPS luxury status of old has evolved into an affordable commodity for the masses.
This holds true independently of the UPS design topology. A basic entry level "off-line UPS" with surge protection and a 280-watt output that can back up one personal computer, can now be purchased for less than $50. A midrange "line-interactive UPS" with a 280-watt output, sophisticated RS-232 serial port, shutdown and monitoring software, and a SNMP/HTTP option card interface is available for less than $150. The most significant change involves the high-performance "online UPS" that continuously regenerates a new 700-watt output power and offers the highest level of equipment protection. Shipped with advanced RS232 communications, network ready shutdown and monitoring software, and an SNMP/HTTP option port, this UPS can now be purchased for less than $390.
Not all UPS design topologies are equal in their levels of protection and performance. The good news is that whatever level of protection is necessary, it will cost less and provide more features than it would in the past. The bad news is that personal computer demands — not military, governmental, or scientific applications — have driven the UPS market for the last two decades.
Most UPS products have not been designed mechanically or electronically to meet the demands of most military applications. Therefore, the engineer should always be aware of the differences and benefits of the three basic UPS design topologies. Prospective users should choose a UPS that most closely meets the project requirements, and subject it to extensive qualification testing. If packaging or shake and vibe are concerns, some UPS companies offer products at an assembled and tested circuit board level, thereby virtually eliminating these concerns while offering the engineer much greater flexibility.
A true understanding of the differences between UPS design topologies is essential. To that end, it is helpful to do a brief overview of the three basic UPS topologies.
Many engineers may not be aware that the off-line and line-interactive UPS designs simply pass the utility power directly through the UPS during the time the utility is present. These UPS designs are doing nothing more during normal utility operation than a surge protected plug strip. It is only when utility power is lost that they switch to their internal AC inverter to provide backup power. Should the engineer select an off-line UPS for a location with extended brownout or low-utility-voltage conditions, it would not solve the intended problem. However, selecting a line-interactive UPS would solve this problem due to an additional internal boost circuit that increases the UPS output voltage during low voltage brownout conditions.
Another problem area actually comes from the UPS itself. Most off-line and many line-interactive UPSs, while running on battery, have inverter outputs that are not a true sine wave, but are referred to as square wave, modified square wave, or quasi-sine wave. These types of output may be incompatible with some equipment.
For most military projects the online UPS offers the level of performance, features, and best benefit-to-cost ratio. With the online design, the incoming AC utility power converts continually to a direct current (DC) that is cleans easily by filtering out most unwanted power problems. The cleaned DC feeds to a continuous duty true sine wave inverter that regenerates a totally new, tightly regulated AC power, devoid of all incoming utility power problems.
When the utility power shuts off, the battery simply takes over as the power source without any interruption at the UPS output. Therefore, the online UPS offers a much greater level of protection. Many newer online UPS models offer the additional advantage of providing international frequency conversion. In the United States, utilities generate AC power at 60 Hertz (Hz) or 60 cycles per second. In Europe and many other places throughout the world, utilities generate power at 50Hz. Some military generators and most aircraft generate AC power at 400Hz. An online UPS equipped with this feature can power equipment designed for the U.S. while operating from the differing AC line frequencies. Since offline and line-interactive UPSs pass the utility power directly through, they will not work at all in this application.
The online UPS has a wide input voltage range (±25 percent typical) with a tightly regulated output voltage (±2 percent typical). This makes the online UPS ideal for use in generator-powered applications where sensitive equipment would not normally operate directly from the generator, or any other application where equipment had a sensitivity to line voltage fluctuations. Because the online's inverter is designed to provide output power continuously, whether it is operating from utility or battery, it may be connected to larger battery packs, extending the battery backup time to many hours. This is not the case with most offline and line-interactive designs.
When purchasing a UPS, the engineer should definitely beware. The market is flooded with very cheap products from China. Many have not been submitted to Underwriters Laboratories, or equivalent organization for a safety evaluation and subsequent UL Listing. Many would not meet their requirements if they were submitted. In some cases, UL has recently found electronic equipment on the market illegally bearing their UL Listing mark. This equipment had never been submitted to UL for testing. The safety of the electronic equipment should be the engineer's first concern.
UPS communications and the associated remote monitoring and control are primary concerns when qualifying the UPS for a military application. Most UPS manufacturers offer products with what they refer to as an RS-232 interface. Again the engineer has to beware. Some interfaces are only contact closures, others are true RS-232, and some are a combination of both. The interface is typically made available through a DB-9 connector, but again in most cases the DB-9 pinout does not meet the RS-232 specification and is proprietary to the individual UPS manufacturer or product line.
Many UPS manufactures offer models that support an option card referred to as an SNMP/HTTP interface card. The option interfaces the UPS to a standard Ethernet 10BaseT LAN, WAN, or directly to the Internet. This makes the UPS a TCP/IP-addressable device and facilitates the remote monitoring and control of a UPS from virtually anywhere in the world. Most offer a "traps" feature where the remote monitoring workstation IP address enters into the UPS so it can automatically notify the network administrator of an abnormal power event or UPS failure. The UPS may connect directly to the Internet via a DSL line having fixed or static IP addresses, or to most proprietary military and government networks.
One notable advance in this industry is the incorporation of input power factor correction into some online UPS models. The UPS can now correct the power factor of any equipment connected to it, yielding a relatively high overall system efficiency and reducing neutral current where the design incorporates several different switch mode power supplies. This feature can also be a curse. Most power factor correction circuits are designed to operate only from a 47-53Hz line frequency and would be damaged if powered from a 400Hz source. The engineer must discuss the specific application with the UPS manufacturer prior to selecting a UPS with input power factor correction.
The small UPS can be the solution to many design and power problems facing the military design engineer if the engineer is knowledgeable enough to associate the UPS's attributes to project requirements. The UPS industry has a great deal to offer the military design engineer. However, with the constant changes in the UPS industry, now is the best time for design engineers to discuss military requirements with the manufacturer.
Michael Stout is engineering manager of Falcon Electric Inc. in Monrovia, Calif. Readers may contact him by phone at 877-232-5266, or on the World Wide Web at http://www.falconups.com.