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In the event of a power interruption, a
switching function is required to bring the
load back on to the inverter, and mains
need to be quickly isolated to prevent
power feedback into the distribution
system. No matter how fast or reliably
this switching occurs, there is still a chance
it will fail when it is needed most.
There has been discussion in the industry
about simplifying the AC distribution
scheme by eliminating the step-down in the
power distribution unit and delivering higher
voltages to the IT equipment. One exam-
ple, illustrated in Figure 3, takes 277V AC
output from the UPS and delivers it directly
to the server power supply. This approach
may have potential in the future but is not
viable today for a number of reasons.
First, by connecting loads phase-to-neutral,
additional harmonic currents may be intro-
duced, offsetting some of the anticipated
efficiency gains. Second, because of the
elevated voltages at the rack, arc flash
concerns for IT personnel working in the
rack will increase. Finally, there are no
commercially available power supplies at
this input level. All in all, this architecture
needs further development before it can
be considered a true alternative.
Consequently, 48V DC power may prove to
be the best alternative to 480V/208V AC
power distribution for organizations seeking
to optimize reliability and efficiency today.
Figure 3. 480V AC to 277V AC data center power system configuration.
Figure 2. In eco-mode, incoming power bypasses the inverter to increase UPS system efficiency.
Figure 2. In eco-mode, incoming power bypasses the inverter to increase UPS system efficiency.
PDU PSU
Transformer
Rectifier
Bypass
Battery
Double Conversion UPS
Server
480V AC 480V AC 208V AC 12V DC
AC
DC
DC
DC
Load
Inverter
PDU PSU
Transformer
Rectifier
Bypass
Battery
Inverter
Double Conversion UPS
Server
Figure 1. Typical 480V AC to 208V AC data center power system configuration.
480V AC 480V AC 208V AC 12V DC
AC
DC
DC
DC
Load
Figure 3. 480V AC to 277V AC data center power system configuration.
PDU
PDU
PSU
Rectifier
Bypass
Battery
Double Conversion UPS
Server
480V AC 480V/277V AC 277V AC 12V DC
AC
DC
DC
DC
Load
Inverter
Figure 4. -48VDC power as typically implemented in telecommunications central offices.
PSU
Switch
12V DC
DC
DC
Load
Battery
480V AC 48V DC 48V DC
Rectifiers
(N+m)
Figure 5. A row-based DC UPS minimizes the amount of copper and floor space required for installation in the data center.
PSU
Battery
PDU
Row-Based DC UPS Server
480V AC 48V DC 12V DC
DC
DC
Load
Rectifiers
(N+m)
Figure 2. In eco-mode, incoming power bypasses the inverter to increase UPS system efficiency.
PDU PSU
Transformer
Rectifier
Bypass
Battery
Double Conversion UPS
Server
480V AC 480V AC 208V AC 12V DC
AC
DC
DC
DC
Load
Inverter
PDU PSU
Transformer
Rectifier
Bypass
Battery
Inverter
Double Conversion UPS
Server
Figure 1. Typical 480V AC to 208V AC data center power system configuration.
480V AC 480V AC 208V AC 12V DC
AC
DC
DC
DC
Load
Figure 3. 480V AC to 277V AC data center power system configuration.
PDU
PDU
PSU
Rectifier
Bypass
Battery
Double Conversion UPS
Server
480V AC 480V/277V AC 277V AC 12V DC
AC
DC
DC
DC
Load
Inverter
Figure 4. -48VDC power as typically implemented in telecommunications central offices.
PSU
Switch
12V DC
DC
DC
Load
Battery
480V AC 48V DC 48V DC
Rectifiers
(N+m)
Figure 5. A row-based DC UPS minimizes the amount of copper and floor space required for installation in the data center.
PSU
Battery
PDU
Row-Based DC UPS Server
480V AC 48V DC 12V DC
DC
DC
Load
Rectifiers
(N+m)
In recent years, manufacturers have
increased component efficiencies and
developed new operating modes, such
as the “eco-mode” available on some UPS
systems, to improve efficiency.
Figure 2 shows AC UPS operation when
in eco-mode. Efficiency is improved by
bypassing the conversion in the inverter;
however, this introduces the potential for
reduced system reliability. Critical loads
are no longer isolated from the mains or
subject to the tight voltage regulation
normally provided by the inverter, and
complex synchronization circuits are
needed to ensure reliable power transfer.