What is Online UPS?
We are already aware of the word UPS that simply represents “Uninterrupted Power Supply”. If don’t, let me remind you, in your old personal computer there is a power backup unit that provides a power supply in absence of mains power. When an incoming power failure occurs then only backup batteries connect to the inverter circuit to generate AC power by using DC power. But “Online UPS” is slightly different from this conventional uninterrupted power supply unit. In an Online UPS, the backup batteries are always connected to the inverter circuitry (DC to AC converter circuit), as of result, it is not necessary to have a power transfer switch or a relay unit (power transfer switch is responsible to connect backup batteries to inverter circuit only on the absence of the main power). In Online UPS when mains power failures, the rectifier circuit (AC to DC converter circuit) just drops out of the circuit and the batteries keep the flow of power steady and unchanged.
Online UPS working diagram
When mains power comes back, the rectifier circuit resumes carrying most of the load and also starts charging the backup batteries. In most Online UPS, charging current is limited by the high power rectifier circuit to prevent overcharging or heating to backup batteries. The most important significance of the Online UPS over other UPS is its ability to provide an “Electrical Firewall” or “Electrical Surge Protector” between the incoming utility power and sensitive electronic equipment at the load end.