UPS True On-Line – Explained

UPS True On-Line

THE mission critical UPS Power Systems.
They hold the load without any break even a millisecond break, hence always ON-LINE.

What is the difference between a True On-Line and an Off-line system?

Primarily, the battery charger is replaced by a ‘rectifier/charger’ unit. This is in the True On-Line UPS

The rectifier/charger may be two separate units or a combined power unit.
When the mains supply is present, the battery charger charges the battery and the rectifier supplies the inverter with a stable DC voltage. In the absence of the mains supply, the charger shuts down. Also the battery will begin to discharge. The connection between the rectifier/battery and inverter. (often known as the DC busbar, or DC bus).

As part of its control function the rectifier/charger generally includes an input current limit feature to provide overload protection, and a DC over voltage shutdown mechanism to protect the battery / inverter and DC filter components.

Protection of Load

This UPS design, often referred to as a double conversion UPS, offers the greatest degree of critical supply integrity. Load is supplied with processed powered from the inverter. (well protected from input supply aberrations). The rectifier and inverter act as a barrier to mains borne noise and transient voltage excursions. In addition to providing a well-regulated output voltage.


If the input supply goes outside a preset voltage range or suffers a total failure, the inverter will continue to operate from battery power. The event is totally transparent to the load. No transfer operation involved. When operating from battery power the inverter supplies the same degree of supply regulation as when the main is present. (typical voltage range is +10% to -20%).

Mains must be restored. Especially before the battery reaches its end-of-discharge. If not, the voltage inverter shuts down. In some models, the static switch may attempt to transfer the load to the bypass line.

The result of the transfer action depends on:

  • the module’s bypass line connection
    Connected to the same mains supply as the module’s rectifier
  • Also if the bypass supply is live. (known as a split bypass system.)

All of this information doesn’t help if all you require is a reliable UPS Power System.

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