An “online, double-conversion uninterruptible power supply” is an uninterruptible power supply (UPS), which is always active, i.e. not just in case of a voltage drop in a power grid, to which the UPS is connected via its UPS input, but also when the power grid provides a nominal voltage. For compensating short power drops in the power grid, the UPS may comprise a capacitor between the rectifier and the inverter. For compensating longer power drops in the power grid, the UPS may comprise an internal battery or a connector for an external battery between the rectifier and the inverter.

Because the rectifier and the inverter are continuously in the current path between the power grid and an electrical consumer, no power transfer switch is necessary to switch the UPS “on” when there is a voltage drop in the power grid and to switch the UPS “off” when the power grid provides nominal voltage again like it is in case of an offline, standby or line-interactive UPS. When a power loss occurs in the power grid, the rectifier simply drops out of the circuit and the batteries keep the power at the UPS output steady and unchanged. When power is restored in the power grid, the rectifier resumes carrying most of the load and begins charging the capacitor and/or the battery. Hence, in an online, double-conversion UPS, the inverter is fed by a capacitor/the battery or by the rectifier connected to the power grid (AC mains).