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Figure 4. -48V DC power as typically implemented in telecommunications central offices.

Figure 5. A row-based DC UPS eliminates the long copper runs and is optimized for the data center.

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

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

Load

Figure 3. 480V AC to 277V AC data center power system configuration.

PDU

PSU

Rectifier

Bypass

Battery

Double Conversion UPS

Server

480V AC 480V/277V AC 277V AC 12V DC

Load

Inverter

Figure 4. -48VDC power as typically implemented in telecommunications central offices.

PSU

Switch

12V 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

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

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

Load

Figure 3. 480V AC to 277V AC data center power system configuration.

PDU

PSU

Rectifier

Bypass

Battery

Double Conversion UPS

Server

480V AC 480V/277V AC 277V AC 12V DC

Load

Inverter

Figure 4. -48VDC power as typically implemented in telecommunications central offices.

PSU

Switch

12V 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

Load

Rectifiers

(N+m)

Today, telephone central offices

(exchanges) are still powered by -48V DC.

These facilities exhibit levels of availability

as high as 9-nines, as confirmed in a study

of 23,000 DC power systems over 10 years

by NTT Facilities

In most telecommunications installations, the

48V DC power system is deployed as three

distinct elements as shown in Figure 4:

1. 480V AC to -48V DC modular

power system

2. Battery banks for extended run time

(at least 8 hours)

3. Load distribution cabinets (BDFB/PDU)

These elements are connected with large

copper bus bars and wires routed around

the facility to distribute power directly to

-48V power supplies at the point of use. The

power supplies step the voltage down to

12V and lower DC voltages for internal uses.

It is easy to see the benefits of this

approach when applied to the data center.

Downstream of the AC/DC rectifiers, the

power is completely isolated from the

mains and is considered “safety extra low

voltage” (SELV) per IEC/UL 60950 and can

be maintained live by trained personnel.

In addition, there is no need to derate

capacity to account for phase balancing or

harmonics, which are not a factor with DC

power. Personnel safety is also improved

when servicing equipment in the rack

because arc flash is not a concern at -48V.

However, unlike telecommunications cen-

tral offices, data centers are not designed

with large copper bus bars to distribute

DC power to racks. Fortunately, with new

row-based DC topologies, these large

copper runs are no longer necessary. In

this row-based configuration, power is

converted from AC to DC very close to

the point of use, which decreases the

conductor size and cost [see Figure 5].

Data centers are entering a new stage of

maturity where the emphasis is on reliably

delivering more computing capacity to

the organization at lower cost and with

reduced power consumption. Row-based

DC power represents a practical and

affordable solution for reducing data

center complexity, increasing efficiency

and enabling growth.

Historical Background

48V DC power has a long history in tele-

communication networks. It is inherently

simple and reliable with few conversion

stages to the point of use. In Alexander

Graham Bell’s day, 48V DC was chosen as

the standard for two reasons:

1. DC power was felt to be more reliable

than AC because it could be directly

connected to backup batteries during

grid outages;

2. 48V was considered the optimal

tradeoff between transmission

distance and human safety because

it is considered safe to touch during

maintenance or accidental exposure.

A Practical Approach to DC in the Data Center