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Inverter Waveshape The inverters in most standby UPSs sold for microcomputer applications produce output current whose waveshape is described either as pseudosine, modified sine, or a stepped approximation of sine wave. This waveform has the same peak and root mean square (rms) voltage content as a sine wave and is ideal for driving the input bridge diodes of switching power supplies. This form of inverter output has certain advantages over inverter designs that deliver a pure sine wave, in particular, efficiency, reliability, and lower cost. However, sinewave output is more universally applicable. Some of the more sophisticated computer power supply designs include autoranging detection and switching circuits, or power fail detection circuits, and these can sometimes be confused by non-sinusoidal voltage waveforms. The only unacceptable inverter output waveform is a true square wave. A square-wave output may have the proper rms voltage, but its peak voltage level will be much lower than the switching supply expects. Low peak voltage causes the duty cycle of the compute power supply diodes to be extendedthis can stress the components to the point of failure. Battery Maintenance The most critical and most often overlooked component in any UPS is the battery. UPS batteries are made from the same materials and are generally of the same design as car batteries. As with a car battery, they can wear out just when they are most needed. UPS batteries in older or in contemporary low-cost designs can be tested by pulling the plug on the system when the computer is up and running and timing its operation until the UPS battery is drained Safer UPS designs include a test routine that simulates a power failure and runs the computer on the battery for a period of time. The test is triggered by pushing a test button or through UPS monitoring software that resides on the computer system and communicates with the UPS. More sophisticated designs use subtle measurement circuits to regularly test battery condition in the background and alert the network manager through front panel messages or through system-resident monitoring software. Replacing batteries in most small UPS (systems under 2 kilovolt-amperes) requires off-site service by trained technicians. Larger UPSs require on-site maintenance by trained service technicians. New designs allow the user to replace batteries on site. This feature can simplify maintenance logistics, reduce costs, and facilitate proper recycling of batteries, which pose a potential hazardous waste problem. Some designs also offer hot swap battery packs that are ideal for networks with high availability, total up-time requirements. Hot swap batteries can be replaced without bringing down the UPS or the system it supports. Power Conditioning Capacity The power conditioning capability of any UPS, online or standby, is only as good as the filtering circuitry built into the UPS. There are a limited number of UPS designs that provide superior filtering capabilities. These designs can be identified by the presence of a full output isolation transformer. These UPSs are slightly larger and heavier than those that use small autotransformers to step inverter output to normal line levels or boost low input lines without draining battery reserves. Autotransformers do not provide full output isolation. UPS Architecture Review There are two basic architectures for UPS systems: online and standby. Hybrid designs are a subset of the standby technology family but blend in some of the performance characteristics of online technology. Online UPSs The online UPS architecture supplies continuous power through the UPSs inverter. This architecture is illustrated in Exhibit 1-6-16. The inverter is typically fed either by the battery (during outages) or by the battery charger (during normal conditions). Online designs provide truly uninterrupted power with no transfer time. This was an important performance characteristic, especially for midrange and mainframe systems. However, in an online UPS, the power electronics function continuously. Online designs typically have half the MTBF performance of standby units, whose power electronics operate only when needed (i.e., when commercial power fails). The inherently higher cost and generally lower reliability of online units have led to a relative decline in their popularity.
Offline/standby UPSs These supply a computer with commercial AC through passive circuits, switching to their power electronics and batteries only when an AC line fault is detected. This architecture is illustrated in Exhibit 1-6-17.
Solid state power switches allow for instantaneous switching between line and battery. Most standby UPS designs include a line failure detection circuit that inhibits switching for 2 ms to 4 ms to confirm the outage is real and avoid activation of the UPS unnecessarily. Hybrid Designs These are a newer subset of the offline/standby class. They are labeled hybrid because they incorporate additional circuitry that provides useful performance advantages that blend the best characteristics of online and standby designs.
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