One application of dc-dc converters is in an uninterruptible power supply (UPS). A UPS provides short-term emergency power to a critical load when the primary input power fails. A two-quadrant dc-dc half-bridge converter allows a battery to be charged by the primary input under normal conditions but can instantaneously switch to discharge the battery to the load, such that the load experiences a negligible interruption in power. In most UPS systems, battery power is only used for a few minutes, and is used to bridge small power interruptions in the primary source.
A simplified one-line diagram of a UPS is shown below in Figure 8. The system consists of a rectifier circuit to convert the three-phase ac input into dc and an inverter to convert the dc-link power back to the required ac waveforms to feed the load. The input of the rectifier and output of the inverter are typically filtered to adhere to both the grid-side and load power quality requirements. The dc-dc converter is connected to the dc-link of the UPS and is highlighted in bold. Under normal operating conditions, the bidirectional dc converter operates in buck mode and charges the battery at the specified charging current until the threshold battery voltage is met. In the event of a line outage, the rectifier no longer provides the dc-link power, and the dc converter operates in boost mode to maintain the dc-link of the inverter to the specified voltage. The passive circuit design and converter switching frequency of the dc converter are tailored to meet the battery ripple current thresholds and dc-link voltage requirements. The capacity of the battery is typically expressed in kilo-watt-hours (kWh) and is rated in accordance the load requirements. Refer to reference [2] for a more detailed diagram of a commercial UPS and for further information.
