Online UPS are the preferred choice for mission-critical applications as they ensure 'no-break switchover' in the event of any mains power supply failure and an automatic internal bypass for safe failure to mains in the event of a fault.
EN/IEC 62040-3 defines three generic Static UPS topologies. Each topology is then further classified based on its output waveform and dynamic performance:
- VFI - Voltage and Frequency Independent: more generally known as 'Online UPS' or 'Double Conversion', where the UPS output is completely independent of any voltage and/or frequency abnormalities on the incoming mains supply. The highest classification for an Online UPS is VFI-SS-111. There are two main approaches to Online UPS - transformer-based or transformerless. It is known as 'Double Conversion' because of its two voltage conversion stages: rectifier (AC to DC) and inverter (DC to AC).
- VI - Voltage Independent: typically referred to as 'Line Interactive UPS', where voltage fluctuations are stabilised by built-in regulation devices. Output frequency tracks the input frequency when the mains supply is present.
- VFD - Voltage and Frequency Dependent: more commonly referred to as 'Offline UPS' or 'Standby UPS', where the output of the UPS output tracks the voltage and frequency of the input mains supply. When the mains supply fails, there's a brief (4-8msec) break in supply while the inverter switches on and powers the load from the batteries.
What Power Problems Do The Different UPS Topologies Protect Against?
| Power Problem | Online (VFI) | Line Interactive (VI) | Offline (VFD) |
| Sags/Brownouts | Yes | Yes | - |
| Surges | Yes | Yes | - |
| Spikes/Transients | Yes | Yes | Yes |
| Electrical Noise | Yes | Yes | Yes |
| Harmonics | Yes | - | - |
| Frequency Variation | Yes | - | - |
| Mains Failure | Yes | Yes | Yes |