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EETE JULAUG 2012

DESIGN & PRODUCTS POwER SUPPlIES Comparing uninterruptible power supply topologies By Mike Elms Today, uninTErrupTiBlE powEr supply (upS) systems are typically employed to support data centres, communications hubs and other applications using sensitive iCT equipment. These installations’ continuous availability is usually critical to their own- ers’ survival, so a clean, uninterrupted power supply becomes a business-critical requirement. under these circumstances, on line upSs are understandably the most popular choice. They deliver processed, clean power under all conditions and transfer to bat- tery without power interruption if the mains fails. However there are still many applications with less critical requirements; these can benefit from the lower costs of an off line UPS solution. This article therefore compares on line and off line UPS topolo- gies, together with a third variant – line interactive topology. as rising energy costs sustain the demand for better energy effi- ciency, this article also reviews Eco mode upS operation and its contribution to improved puE. applications that use upSs vary greatly in size as well as nature. Small desk top or instrumentation devices may run from a portable unit with a capacity of up to 250 VA, whereas a large data centre serving a significant enterprise could call for multiple units delivering up to 5 MVa or more. On line UPS topology and advantages Fig. 2: Off line UPS topology. irrespective of their size or topology, all upSs depend on a bat- tery, a means of charging it and a method of converting its dC output to aC for the critical load during a mains failure. an on line upS performs these functions with a series of power blocks between the mains supply and the critical load - see figure 1. The first of these is a rectifier/charger which float charges the battery while the mains is present. it also supplies a stable dC voltage to the second block - an inverter which feeds the critical load. During a mains failure the floating battery seamlessly takes over the task of supplying dC to the inverter, to which it is permanently connected. Therefore, the event remains truly invisible to the criti- cal load unless the battery becomes discharged. This seamless power continuity is a key attraction for critical applications that may not be capable of riding out a switching delay. another major advantage arises from the on line upS’s dual conversion topology. The input rectifier removes all the incom- ing mains spikes, transients and noise by converting it into dC. The inverter then feeds the load with a clean new aC waveform which is tightly regulated by the UPS. The rectifier also provides power factor correction, which reduces high frequency harmonics and the current drawn from the mains. The on line upS acts as a true power firewall, protecting sensitive instrumentation and ICT equipment from mains-borne problems while improving the load presented to the mains. Off line topology By contrast an off line UPS, during normal operation, feeds raw mains to the critical load through a bypass line and static switch - see figure 2. The load is therefore exposed to poor voltage Fig. 1: On line UPS operation. regulation and possibly noise and transients from the mains. if the mains transgresses preset limits, the static switch transfers Mike Elms is Technical Sales Manager for uninterruptible power the load to the upS’s inverter and battery – a changeover break Supplies limited - www.upspower.co.uk – he can be reached at that can last from 2ms to 10ms. Many operators of data centres sales@upspower.co.uk and other sensitive equipment consider off line UPS systems 20 Electronic Engineering Times Europe July/August 2012 www.electronics-eetimes.com


EETE JULAUG 2012
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