Passive Standby (or Off-Line)

Entry-level Technology to Protect Workstations

Operating Diagram

Passive Standby (or Off-Line) technology is now the most commonly used technology for protecting PCs. In normal mode the PC is connected directly to the mains power supply, without any power conversion.

In general, the current is filtered in order to attenuate the most frequent forms of disturbance. In the event of a power problem (outage, voltage sags or spikes) the UPS transfers the load to the batteries to supply stabilised power.

Transfer Time

This question has been the subject of a considerable amount of debates: many people thought that the transfer time, that lasts a few milliseconds, was at the root of the problems encountered on the first generations of Off-line UPSs in the middle of the eighties.

This is a mistake, for two reasons:

• The typical transfer time for an Off-line UPS is 4 milliseconds. Thanks to its internal capacitors, the power supply on any PC can support outages of up to 10 milliseconds. Top-of-the-range hardware can support micro-breaks lasting about 100 milliseconds, or in other words 25 times longer than the transfer time of an Off-line UPS.

• The teething problems encountered by these products were due to under-sizing of inverter components, which could not withstand the step loads that had to be supplied instantaneously when the load was switched to the batteries.

Disturbance Tolerance

The principle of sequential operation (on mains/on battery) behind the Off-line UPS makes it unsuitable for use in situations where there are frequent disturbances: it is extremely likely that the UPS will keep switching to the batteries, without being able to recharge them. At the end of the battery backup time, the UPS will shutdown and it will not be possible to start it again, unless the disturbances cease.

Price/Volume Competitiveness

Passive Standby technology is very economical on account of the small number of components used.

All the products currently on the market are dependent on a low frequency (50 Hz) transformer, with a weight and cost that are prohibitive at ratings in excess of 2 kVA (or 2 000 VA).

The new Pulsar Ellipse and Ellipse Premium UPSs represent an innovation, with their high frequency technology, which reduces the size of the transformer to that of a matchbox. It is thus possible to deliver a very slim product (only 4 cm thick), making a pleasant change, in design terms, from the usual shoebox type offerings of the competition.