UPS topologies: single-conversion versus double-conversion
The basic design of a UPS is referred to as its topology. Two common UPS topologies are
single-conversion and double-conversion. A single-conversion UPS (Figure 4A) is a relatively simple
design. In normal (offline) operation, a single-conversion UPS routes AC input voltage straight through
to the output while charging the internal batteries. When the AC input is out of range or lost, the UPS
goes online and uses an inverter to produce the AC output from the battery power. Because of its
simple design, the single-conversion UPS is very efficient during normal operation, but it must provide
battery operation until utility power is completely restored or until a generator is brought up to a
stable condition.
A double-conversion UPS (Figure 4B) includes a rectifier. In normal operation, the rectifier processes
the AC input voltage and routes the DC to the inverter that produces the output. In online operation,
the batteries provide the input for the inverter as in a single conversion system.
Figure 4
. Common UPS topologies
Example A: Single-Conversion UPS
The double-conversion design is more complex and is less efficient, but it can ensure a high-quality
output regardless of the condition of the input power. While utility power is absent, the double-
conversion UPS can process and use power from a generator that is still stabilizing. Therefore, the
double conversion UPS can return to normal operation sooner and reduces the period of time when IT
operations are dependent on battery operation.
DC-AC
Inverter
Input
from
AC mains
Charging
Circuit
Output
to
PDPs
Example B: Double-Conversion UPS
Rectifier/
Charging
Circuitr
DC-AC
Inverter
y
Input
from
AC mains
Output
to
PDPs
Normal operation
Battery (stored energy) operation
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