UPS by design

It is widely believed that there are only two types of UPS systems, namely standby UPS and on-line type UPS. These two terms are often incorrectly applied to many UPS systems on today's market. Many misunderstandings about UPS systems are cleared up when the different types of UPS topologies are properly identified.

There are a variety of design approaches when building UPS systems, each of which have distinct performance characteristics.

Standby UPS

The standby UPS is the most common UPS type used for PCs. The transfer switch is set to choose the filtered AC input as the primary power source and switches to the battery/inverter as the backup source in case of the failure of the primary source (AC). In the case of power failure, the transfer switch must operate to switch over to the battery/inverter backup power source. The inverter only starts when the power fails, hence the name "standby". Benefits of this model include high efficiency, small size, and low cost.

Line interactive UPS

The line interactive UPS is the most common UPS used for small business, Web, and departmental servers. In this design, the battery-to-AC power converter (inverter) is always connected to the output of the UPS. Battery charging is provided by operating the inverter in reverse during times when the input AC power is normal. When the input power fails, the transfer switch opens and the power flow is from battery to the UPS output. The fact that the inverter is always connected to the output provides additional filtering and yields reduced switching transients when compared with the standby UPS.

The inverter also provides regulation, operating to correct brownout conditions which would otherwise force the UPS to switch to battery operation. This allows the UPS to operate at sites with very poor power.

High efficiency, low cost, and high reliability coupled with the ability to correct low or high line voltage conditions make this the dominant type of UPS in the 0.5-5kVA power range.

Standby on-line hybrid

The standby on-line hybrid is the topology used for most UPSes under 10kVA that are labelled "on-line". The standby converter from the battery is switched on when an AC power failure is detected, just like in a standby UPS. Additionally, the small battery charger resembles a standby-type UPS. This UPS will exhibit no transfer time during an AC power failure. The most misunderstood part about this topology is the belief that the primary power path is always "on-line" when in fact, the power path from the battery to the output is only half "on-line" (the inverter), while the other half (the DC converter) is operated in the standby mode. This design is sometimes fitted with an additional transfer switch for bypass during a malfunction or overload.

Standby-ferro UPS

The standby-ferro UPS was once the dominant form of UPS in the 3-15kVA range. This design depends on a special transformer that has three windings (power connections). The primary power path is from AC input, through a transfer switch, through the transformer, and to the output. In the case of a power failure, the transfer switch is opened, and the inverter picks up the output load.

In the standby-ferro design, the inverter is in the standby mode, and is energised when the input power fails and the transfer switch is opened. The transformer has a special "ferro-resonant" capability, which provides limited regulation and output waveform "shaping". The isolation from AC power transients provided by the ferro transformer is as good or better than any filter available, but the ferro transformer itself creates severe output voltage distortion and transients, which can be worse than a poor AC connection.

The design has very low efficiency combined with instability when used with some generators and newer power-factor corrected computers. This has caused its popularity to decrease significantly.

Double conversion on-line UPS

This is the most common type of UPS above 10kVA. Double conversion on-line UPS is the same as the standby UPS except that the primary power path is the inverter instead of the AC mains.

In the design of double conversion on-line operation, failure of the input AC does not cause activation of the transfer switch, because the input AC is not the primary source, but is rather the backup source. Therefore, during an input AC power failure, on-line operation results in no transfer time.

The on-line mode of operation exhibits a transfer time when the power from the primary battery charger/battery/inverter power path fails. This can occur when any of the blocks in this power path fail. The inverter power can also drop out briefly, causing a transfer, if the inverter is subjected to sudden changes in the load, or if the inverter experiences an internal control "glitch".

Both the battery charger and the inverter convert the entire load power flow in this design, which causes undesirable heat and results in reduced efficiency.

The design provides nearly ideal electrical output performance. However, the constant wear on the power components reduces reliability over other designs and the energy consumed by the electrical power inefficiency is a significant part of the life-cycle cost of the UPS.

Delta conversion on-line UPS

This UPS design is a new technology that was recently introduced to eliminate the drawbacks of the double conversion on-line design and is available from 5kVA upwards. Like the double conversion design, the delta conversion on-line UPS always has the inverter supplying the load voltage. However, the additional delta converter also contributes power to the inverter output. Under conditions of AC failure or disturbances, this design behaves identically to the double conversion on-line. During steady-state conditions, the delta converter allows the UPS to deliver power to the load with much greater efficiency than the double conversion design.

The delta conversion on-line UPS provides the same output characteristics as the double conversion on-line design. In addition, the delta conversion on-line UPS offers reduction in energy losses and costs by approximately a factor of 4. It is the only core UPS technology today protected by patents and it is therefore not likely to be available from a broad range of UPS suppliers for some time.


Different UPS types are appropriate for different applications, and there is no single UPS type that is ideal for all applications. There are significant differences in UPS design between available products on the market, with theoretical and practical advantages for different approaches. Nevertheless, the basic quality of design implementation and manufactured quality are often dominant in determining the ultimate performance achieved in the customer application.

This article is printed courtesy of American Power Conversion.