UPS Systems are devices providing continuity of power in the event of a power grid anomaly. They can also provide other degrees of power protection. The levels of power protection obtained depend on the technology utilised.

There are 3 UPS System technologies and these are offline, line interactive and online double conversion. These are designated VFD, VI and VFI according to the UPS standard EN62040. V stands for Voltage, F is Frequency. D means Dependent and I means Independent. The nomenclature is comparing the output power waveform of the UPS to the input. For example, VFD, the output Voltage and Frequency are Dependent on the input. In VI topology the output Voltage is Independent of the input voltage (and implies Frequency Dependent). VFI both the output Voltage and Frequency are Independent of the input.

VI/VFD UPS System Building Blocks

Line Interactive (VI) UPS in Normal Mode

The red lines show the power flow path. The surge and filter provide surge protection and EMI filtration. The charger charges the battery and keeps it charged. The relay is a safety device to prevent “backfeed” operation. In normal operation this relay is closed.

The AVR TX is the Automatic Voltage Regulator Transformer. This is the only difference between a line interactive (VI) and offline (VFD) UPS System. It “bucks” the voltage if it becomes too high, or “boosts” if it becomes too low. Without this, a small under or over voltage would necessitate the UPS reverting to battery power. The AVR avoids this and extends battery life.

In offline or line interactive units in normal mode what goes in comes out. If the incoming voltage varies so will the output. If there is waveform distortion on the input, there will be waveform distortion on the output.

Battery Operation

Line Interactive (VIX) UPS System in battery mode

Should the input voltage go out of tolerance, that is, it becomes too high or low for the AVR or fails altogether then the unit will revert to battery operation. The backfeed relay opens immediately open to prevent the inverter output voltage connecting to the input. The battery provides power to a DC Boost circuit which converts the low level DC into a high level DC bus voltage. The inverter uses this to create an output voltage waveform. The switch then changes position to connect the output to the inverter.

There is always a small break in the output voltage when the UPS has to revert to battery operation. Although specifications will say typically 4-6ms, in reality it depends where on the incoming mains cycle the power is lost. It is wise to assume at least a full half cycle loss of 10ms, and even consider a full cycle, e.g. 20ms.

In low cost systems the inverter waveform is not a sinewave. It is a square wave, which is surprisingly good at powering a multitude of devices for short periods. Items which use Switch Mode Power Supplies (SMPS) are particularly suited to this type of power waveform. Anything that contains inductive devices such as motors, pumps or transformers will have an issue with non sine wave waveforms. Since the units are non-sine the designation letter is “X”, hence a line interactive square wave system is a VIX UPS System.

Higher specified units can have a more expensive inverter which outputs a sine wave which in most instances is superior to what you obtain direct from the mains supply. Such line interactive sine wave systems are termed VIS UPS Systems.

Online Double Conversion VFI UPS Systems

Online Double Conversion VFI UPS System in normal operation

These systems work in a different way to line interactive systems. They are powering the load directly from the inverter at all times. The input is fed through surge protection / filtration and the backfeed relay as is the same as line interactive. The charger is also present. The difference now is that the mains supply feeds an AC Boost Circuit. This converts the incoming AC to a high DC level to supply the inverter bus. The inverter is on and supplies the output via the switch. The term double conversion comes from the fact the AC input power is converted to DC and this DC is then converted back to AC.

With the output being powered from the inverter there are also other advantages for online technology over line interactive. The output voltage is fixed and unchanging. It is adjusted usually via the LCD to the required voltage. For example you may require 220V, 230V or 240V. Another function is that the frequency can be adjusted. This allows the UPS to be used in “frequency converter” mode, allowing a 50Hz input and a 60Hz output and vice versa.

As can be seen the output voltage and frequency are truly independent and hence the term VFI UPS Systems.

Battery Operation

Online Double Conversion UPS System in battery mode

If the mains fails the backfeed relay opens and the DC Boost circuit starts up and powers the inverter DC bus. Due to the high capacitance on the bus any lag in switching from AC Boost to DC Boost does not affect the inverter operation. As a consequence the output is unchanged when switching from mains power to battery power – there is no break.


The bypass line serves two functions. It allows the UPS internal systems to be switched off whilst maintaining power to the load. It also is activated if the UPS develops a fault, or the UPS is overloaded. As such, the bypass adds a higher degree of fault tolerance to the overall system.

In some systems the bypass line can be enabled or disabled in settings. Disabling the bypass provides two benefits. The unit is unlikely to be left in standby mode, and the load is never presented with unconditioned power. In the first instance, since bypass will provide power to the load, some users are unaware that they do not have UPS support and the load fails when an outage occurs. The UPS will usually periodically alarm to warn of this, but it is a common occurrence. Secondly, although bypass lines can have some tolerance built in, the unconditioned input power is transferred straight through to the load. Disabling the bypass will ensure that only high grade power is provided to the load at all times.

Pros & Cons

The drawback of VFI technology is the efficiency is not as high as line interactive technology, as there are obviously conversion losses. This can be mitigated somewhat by operating the unit in an ECO mode. However this basically means the UPS will operate as a line interactive unit thereby reducing the overall power protection effectiveness. Another issue is the unit requires forced cooling, which of course causes an audible distraction. This makes VFI UPS Systems only suitable for locations where background noise is not an issue.

Due to the fact the load is always powered from the inverter VFI technology offers the highest levels of power protection, significantly superior to line interactive technology. Since the inverter is designed to be active at all times, VFI technology is also useful for long runtime applications where additional battery packs are added.

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