Choosing the optimal UPS topology

When considering different topologies of uninterruptible power supplies (UPSs), it's important to weigh the needs of your environment and the equipment you intend to support, while also balancing your budget. What is UPS topology? Find out more about the different UPS types and how to select the optimal solution.

UPS Topology 101

When choosing the best uninterruptible power supplies (UPSs), for a particular application, one size doesn't necessarily fit all. Certain UPS topologies are better suited for your power infrastructure, and determining the best fit depends on a number of different factors. Recognising the different UPS design types is a key step to ensuring that your critical equipment and processes are properly safeguarded against power disturbances. The three primary UPS topologies are standby (also called offline), line-interactive and online. While all three can meet input voltage requirements for the equipment that will be plugged into the UPS, there are differences in how the result is achieved, as well as the frequency and duration of demands on the battery. 

Considerations for line-interactive vs offline vs online UPSs

Selecting the best UPS topology depends largely on three factors: your environment, your budget and the level of sensitivity of the equipment you want to protect. Because modern IT power supplies are built to withstand a wide range of power quality issues, a variety of UPS topologies provide some degree of protection against power loss and disturbances in the electrical system. 

Standby UPS topology

The most cost-effective topology available, a standby or offline UPS provides surge suppression and battery back-up in the event of an input voltage sag or power outage. This type of system allows equipment to run off of utility power while providing continuous surge protection and, then switches to battery mode during voltage sags or outages. Standby topology is best suited for small office/home office and point-of-sale applications, providing solid protection for equipment such as LCD monitors, docking stations, modems and routers. Small size, simple design, low cost, ease of use and high efficiency are amongst the main benefits of this topology. See Eaton standby UPSs. 
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Line-interactive UPS topology

With a highly reliable and efficient design, line-interactive UPS topology is the most widely used to support loads under 10 kVA. Ideal for distributed IT equipment such as servers, switches and storage, these UPSs actively regulate voltage by either boosting or decreasing utility voltage as needed before allowing it to pass to the protected load. Available in a wide range of shapes and sizes, line-interactive UPS models boast high energy efficiency, reliability, small size, moderate cost and the ability to correct low or high voltage conditions, making them attractive for many applications. Explore Eaton's line-up of line-interactive UPSs.
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Double-conversion online UPS topology

With the ability to filter even the fastest power quality disturbances, online UPS topology offers optimal protection for highly sensitive loads and are ideal for use in environments with poor power quality. Unlike the other topologies, an online UPS provides zero transfer time to battery. Because its design is more complex, the online UPS price is usually greater than that of standby or line-interactive UPSs. By re-creating the voltage output from raw utility power, online UPS re-create the voltage output from raw utility power by converting it from AC to DC and back to AC, making them best suited for highly sensitive IT equipment, PLC power supplies, ice cube relays and other sensitive electronics. Discover Eaton's double-conversion online UPS offering.
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The difference is in the design

While every UPS includes an inverter – which is responsible for converting the DC power into AC power – each topology operates a bit differently. 

Standby UPS

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By design, standby UPSs turn on the inverter only when battery back-up is needed during a loss of utility power or severe voltage sag. When this occurs, the unit experiences a brief delay in switching to battery – normally ~10 ms or less – which can be easily tolerated by equipment such as monitors, docking stations, and modems/routers. However, this topology is not recommended for high-end workstations, servers, switches and storage loads because in battery mode, the inverter in a standby UPS produces a simulated sine wave voltage output (also known as a square wave). Because some modern computers and IT power supplies cannot operate properly with this square wave voltage, a line-interactive UPS with sine wave output is a better choice for these types of loads.

Line-interactive UPS

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In line-interactive UPSs, the UPS passes filtered utility voltage to the load when operating normally, and only switches to battery mode due to bad input frequency or when the input voltage falls outside of the correctable range. This design also provides some line filtering and often regulates voltage by adjusting transformer taps as the input voltage varies. Some line-interactive UPSs offer pure sine wave output in battery mode, which provides wider compatibility with the power factor corrected (PFC) IT power supplies found in today's servers, switches and other IT equipment. As a result, the topology provides appropriate protection for most distributed IT and business loads, which are designed to tolerate the UPS's transfer time of ~10 ms or less. However, some lower-cost line-interactive models rely on an inverter with a simulated sine wave output like standby units, which is better suited for less sensitive home office equipment and peripherals.

Online UPS

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In an online UPS operating in double-conversion mode, power flows through a rectifier and an inverter, which charges the batteries and converts incoming utility power from AC to DC. Also, a separate rectifier from the powertrain charges the batteries in most double-conversion UPS <10 kVA.  In this way, power is continuously provided to connected equipment, filtering power anomalies with zero transfer time between utility power and the battery mode. Since the inverter is always running to provide output to the load, this topology provides a wide tolerance for utility input voltage variations, preventing the unit from going to battery mode as often. Double-conversion UPSs are also able to filter out sub-cycle voltage anomalies that are not caught by the built-in surge protection of standby and line-interactive 

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How UPS topology affects battery life

The difference between online, off-line and line-interactive UPSs is also evident in the topologies' battery lifespans. It's important to note that battery lifespan is affected by a number of different factors, including frequency of use and extremely cold or hot environments. Although a lead-acid (VRLA) UPS battery is typically rated for a three- to five-year lifespan, frequent transfers to battery and high ambient temperatures can reduce expected battery life and increase the need for recurrent battery changes.

Because standby UPSs often switch to the battery when installed in environments with poor utility voltage, battery replacement may be required more frequently than if using a line-interactive UPS. The more the voltage fluctuates, the more the offline UPS resorts to the battery.

In line-interactive UPSs, the use of an automatic voltage regulation (AVR) transformer to correct abnormal voltages prevents excessive battery use during undervoltage and overvoltage conditions. Some line-interactive UPSs also feature a more advanced battery charging and management scheme than standby UPSs such as Eaton's ABM technology, which helps extend battery life and provides early warning of battery failure. In addition, certain line-interactive UPSs are able to provide longer runtime with the addition of external battery packs.

Online UPSs only draw power from the batteries only when the utility voltage is extremely low, high or when a complete power loss occurs, which may decrease the need for costly and frequent battery replacements. Most online models have similar battery management schemes as higher-end line-interactive units, which further extends battery life. Additionally, an online UPS can provide long periods of runtime by adding extra battery packs like some line-interactive designs.