HP Pro UPS 500 240V ISS Technology Update, Volume 9, Number 4 - Page 1

2 HP ISS Technology Update Volume 9, Number 4 Keeping you informed of the latest ISS technology Best practices for selecting a UPS ...1 HP ProLiant servers offer protection against stealthy, malicious code 4 Setting HP Smart Array controller cache ratio 5 Meet the Expert-Reza Bacchus ...6 Recently published Industry-Standard Server technology papers 7 Contact us ...7 Best practices for selecting a UPS The primary function of an uninterruptible power supply (UPS) is to supply power to connected loads, such as computers, when the utility power fails or has poor quality. In either instance, the quality of UPS output waveform must be suitable for your computer's power supply. Most server power supplies contain input fault detection circuitry that verifies the quality of the UPS output waveform. Server power supplies also use ―active‖ power factor correction (PFC) to improve their operation. An active PFC power supply is especially affected by the quality of a UPS output waveform. This article describes the characteristics of UPS output waveforms and recommends best practices when selecting a UPS. UPS types There are three basic UPS topologies - standby, line-interactive, and double conversion --- but they all have a few things in common. Each consists of input circuitry, a battery bank and charger, as well as a DC/AC inverter. The input circuitry filters the source current and protects against power surges. The battery bank supplies power if utility power fails. And the DC/AC inverter circuit converts the DC power from the battery bank to AC power for the connected devices. Line interactive and double conversion UPSs include additional circuitry as described below. Standby (Offline) A standby UPS passes incoming utility power directly to the load through internal surge protection circuitry and a transfer switch. The battery remains charged and the DC/AC inverter is normally powered off. When the incoming utility voltage fails or falls below a predetermined level, the inverter turns on and the transfer switch shifts the load to the inverter. The transfer time can be as long as 10 milliseconds (ms) depending on how long it takes the standby UPS to detect the lost utility voltage. Since a standby UPS does not isolate the load from utility power during normal operation, it is not recommended for critical loads. 1