When specifying or purchasing an uninterruptible power supply, it's essential to choose the right topology, as getting it wrong can have a big impact on both performance and costs. However, there is no one topology that's right for every application, says Paul Norgate of Eaton Power Quality, who examines the three main options and provides useful guidance on how to decide between them

With only a very few exceptions, currently, UPS systems are based on one of three topologies: passive standby, which is sometimes called off-line; line interactive; and double conversion which is also referred to as on-line. The essential elements of each of these are shown in the accompanying diagram and we will shortly examine them in a little more detail. Before doing so, however, it is useful to look at the functions a UPS may be required to perform.

The most obvious function of a UPS system is to maintain power to connected equipment when power from the mains is interrupted. However, there are no fewer than eight other power problems that a UPS may be called upon to perform.

These are compensating for short-term low voltage conditions (power sag); eliminating supply transients (power surges); compensating for medium-term low voltage conditions (brownouts); dealing with medium-term overvoltage on the mains supply; eliminating electrical noise on the mains supply; compensating for supply frequency variations; eliminating switching transients (notches) and dealing with harmonic distortion of the supply.

Not all of these functions are required in every application, of course, and choosing a UPS that offers only those functions that are actually needed, instead of notionally playing safe by specifying a UPS that does everything, can be the key to making very useful savings. With this in mind, let's take a close look at the three UPS topologies.

In a passive standby UPS, when mains power is available, it is filtered to minimise power surges and supplied directly to the load, without any form of active conversion. At the same time, the battery is charged. If the mains power is interrupted, the UPS delivers stable power to the load from the battery, via the inverter. UPS systems of this type are inexpensive and can be a good choice for protecting, for example, individual PCs in SOHO (Small Office Home Office) and non-critical applications. They are not, however, generally considered suitable for more demanding applications.

This is partly because they provide only three of the nine possible protection functions (power failure, power sag and power surge) and partly because under brownout conditions they will use battery power, which will be exhausted after the planned back-up period. By contrast, other topologies will compensate for the low mains voltage without calling on power from the batteries and can therefore continue to operate indefinitely during a brownout.

For these reasons, the choice of UPS topology for critical applications such as data centres and other medium to large scale IT installations is invariably between line interactive and double conversion. At first sight, the line interactive topology looks very similar to the passive standby technology. The key difference, however, is the inclusion of circuitry that can either increase or reduce the supply voltage before delivering it to the load, without needing to call on battery power.

In modern line interactive UPS systems, the incoming mains voltage is continuously monitored by a microprocessor-based system and, if this voltage moves outside of a pre-determined range, compensation is applied instantly and automatically. This type of UPS can, therefore, provide not only the three protection functions available from passive standby systems, but also protection against undervoltage and overvoltage conditions.

Line interactive UPS systems are typically priced somewhere between passive standby systems and double-conversion systems, and are certainly worth considering for most applications, provided the frequency of the mains supply will be reasonably stable, and that the supply is not unduly contaminated with transients.

UPS systems based on double-conversion topology are usually seen as the top-of-the-range option and, in many respects, this is true. They comprise of a rectifier that takes power from the mains and converts it to DC. This is used to charge the batteries and to supply the output inverter, which converts the DC back to clean AC to supply the critical equipment. If the mains fail, the battery takes over the duty of feeding power to the inverter without any gap in the voltage during changeover.

Double-conversion topology very effectively isolates the load from anything untoward that may be going on with the mains power, and a double-conversion UPS can, therefore, provide all nine of the possible protection functions discussed earlier. In the most critical of applications, therefore, double-conversion is a natural choice, but that is not to say that it has no drawbacks. It is, for example, the most expensive of the three topologies and because power is converted twice on its way to the load, it is inherently less efficient than the other two topologies when operating with the mains supply present.

It is worth noting, however, UPS suppliers are now addressing this efficiency issue. Part of the solution, of course, is to improve the efficiency of the converter and inverter, but there is a limit to the gains that can be made in this way. This has led to the development of more innovative solutions, such as a system that effectively bypasses the UPS and feeds power direct from the mains to the load when the mains is healthy, but instantly reverts to full double-conversion mode should any problem occur.

The best systems of this type achieve efficiencies of around 99% when in bypass mode, and switch to double-conversion mode in less than two milliseconds, thereby ensuring there is no disturbance to even the most sensitive of loads.

As we've seen choosing the right topology is an essential first step in specifying a UPS installation. Making that choice is, however, not always as straightforward as it may at first appear. For this reason, there is much to be said for seeking advice from an experience UPS supplier that can offer all three technologies and that will not, therefore, be swayed by commercial bias but will offer dependable guidance based on the requirements of the application alone.