There are many Uninterruptible Power Supply (UPS) products. This UPS tutorial will help you understand the three basic UPS design types in order to select the best one for your application.

A UPS, also known as battery backup, provides varying degrees of power protection against blackouts and other power disturbances – depending on the UPS type (topology). If a manufacturer is honest, the design type and capabilities will be clearly stated.


These are the 3 UPS types:

STANDBY (Offline), Low Protection Level

LINE-INTERACTIVE, Middle Protection Level

ONLINE, High Protection Level


Standby (Offline) UPS

The standby UPS solves a minimal number of power quality problems and is only appropriate for non-critical applications.

Most standby UPS products provide a sinewave output to equipment when operating normally from the utility line. When a standby UPS switches to its internal DC-AC inverter, it may provide a square wave, modified square wave or quasi-sinewave, not a pure sinewave. In many cases equipment may appear to operate normally on these waveforms, but over time may be damaged by them. If you decide only minimal protection is needed, it is always best to select a standby UPS that states it has an inverter with a true sinewave output.

You should also be aware that a standby UPS is not capable of accepting additional battery packs for increased battery runtime. To keep the cost down and prevent overheating, its inverter is designed to only operate as long as the internal battery capacity allows.


Line-Interactive UPS

The line-interactive UPS offers the same bare bones surge protection and battery backup as the standby type, except it has the added feature of minimal voltage regulation while the UPS is operating from the utility source. This UPS design came about due to the standby UPS’s inability to provide an acceptable output voltage to the connected equipment during “brownout” conditions.

A brownout happens when the utility voltage remains excessively low for a sustained period. Under these conditions, the standby UPS will go to battery operation. If the brownout is sustained long enough, the UPS battery will become fully discharged and turn the power off to the connected equipment. In this scenario, the UPS will not turn back on until the utility voltage returns to normal.

To prevent this from happening, a voltage regulating transformer was added and the line-interactive UPS was born. This feature helps as low voltage utility conditions are common. The down side for this design is that most of the units available have to switch to battery momentarily when making transformer voltage adjustments which reduce battery life over time. If you choose a line-interactive UPS, it is wise to select a model with a true sinewave output.


Online UPS

The online UPS provides the highest and best level of power protection for critical applications. It typically costs more, but like all electronic equipment, the cost is coming down as technology advances.

The true advantage of the online UPS is its ability to provide an electrical firewall between the incoming utility power and your sensitive electronic equipment. While the standby and line-interactive designs leave your equipment connected directly to the utility power with minimal surge protection, the online UPS provides an electronic layer of insulation from power quality problems. This is accomplished inside the UPS in several tiers of circuits.

First, the incoming AC utility voltage is passed through a surge protected rectifier stage where it is converted to Direct Current (DC) and is heavily filtered by large capacitors. This stage removes line noise, high voltage transients, harmonic distortion and all 50/60 Hertz frequency related problems. The capacitors also act as an energy storage reservoir giving the UPS the ability to “ride-through” momentary power interruptions. The battery is also connected to this stage and takes over as the energy source in the event of a utility loss. This makes the transition between utility and battery power seamless, without an interruption. Additionally, an online UPS can provide extended runtimes by adding extra battery banks.


Bottom Line

When considering a UPS solution, be aware of the UPS design and corresponding level of protection. Ask the question: Can I take a risk with my applications and devices? For non-critical applications, a standby or line-interactive UPS may be sufficient. For critical applications, an online UPS is the optimal solution.

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