Electronic Design

Not All Uninterruptible Power Supplies Are Equal

When it comes to purchasing uninterruptible power protection, the consumer is faced with a market full of hype and misleading claims. For example, one 500-VA uninterruptible power supply (UPS) can cost just $50, and another as much as $400, with both proclaiming to meet your power protection needs. How can you pick the best one?

Buying a UPS is very much like buying an insurance policy. As with insurance, knowing the right questions to ask, combined with a basic knowledge of what one is buying, is all that's necessary to make a good decision. The questions to ask are simple: What types of insurance (power protection) are needed, and how much insurance (power protection) is enough?

In many cases, UPS manufacturers state only what their particular UPS will protect against. Then they play off of consumers' lack of information by not telling them what the UPS won't protect against. This missing information is critical to making a meaningful decision about your specific power protection requirements. Understanding the level of protection that a UPS offers can usually be determined by its design topology. The UPS industry has three basic topologies: offline, line-interactive, and online.

The offline UPS is typically the least expensive topology available. It provides the basic functions of surge protection and battery backup. In rare cases, it may provide a true sinewave output during inverter operation, but it typically has a square-wave, step-wave, or quasi-sinewave output. This design has an inherent switchover transfer time of about 4 to 20 ms between loss and return of the utility voltage. It offers no output-voltage regulation when the utility voltage is present.

In locations susceptible to brownout conditions, the offline UPS is not a good choice, as the battery may discharge completely and no longer offer backup protection. Its low-cost electronics don't support extended battery packs for extended battery operation. Also, many offline UPS models can't detect a defective battery and, upon utility loss, they may drop the load without warning.

The line-interactive UPS typically costs more than the offline topology. It provides the same basic functions of surge protection, battery backup, and output waveforms. However, the line-interactive UPS adds a tap-switching transformer to provide basic voltage regulation (8% to 12% is typical) while operating from the utility line. It thereby provides reasonable brownout protection. This design also has an inherent switchover dropout. Some models support extended battery packs, but most don't. Again, many models do not adequately detect a defective battery.

Normally, the online UPS costs more than other topologies, while offering superior protection. The inverter powers the connected equipment during both utility and battery operation, so there is usually no switchover dropout. This design offers the best level of surge protection and output voltage regulation (±2% to 3% is typical). It offers the added benefit of output frequency regulation, which is necessary for use with generators. Some models may even be implemented as international 50- and 60-Hz frequency converters. The output waveform is typically a true sinewave. Most models support extended battery packs, providing up to several hours of backup. Defective battery detection is usually better than the other topologies.

As an added note, some models of all three topologies provide the ability to start the UPS without a utility voltage present, providing portable remote power. All typically have an RS-232 computer interface and support the use of remote monitoring and shutdown software.

As with insurance, being underinsured may be as bad as no insurance at all. Take the time to research your specific requirements, and don't be afraid to call a UPS manufacturer's technical support line and ask questions.

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