The term "on-line" UPS is used
differently by various manufacturers. Most UPS makers reserve the term for true
double-conversion UPS's because they keep their inverter circuitry constantly
on-line. Some manufacturers, however, also use the term for line-interactive
products.
The name double-conversion UPS arises from the operation of the
device. It first converts line voltage into battery-compatible low-voltage DC
using circuitry much like a battery charger, but rated for higher power and
continuous operation. This direct current is then converted by an inverter back
to the equivalent of standard utility power. A pack of batteries, connected
between the two circuits, acts as a reservoir and keeps the inverter operating
should the power line fail.
The double-conversion UPS is the only truly uninterruptible system. It never
switches because its inverter is always connected to your computer. But the
double-conversion design does more than eliminate the brief switching
interruption. The battery reservoir completely isolates your computer from the
vagaries of the power line. It protects against all forms of power irregularity
including surges, spikes, over-voltages, sags, brownouts, and blackouts. The
double-conversion UPS doesn't switch transformer taps when line voltage surges
or sags, but adjusts its own solid-state voltage regulators to cope with under-
and over-voltages. Consequently the double-conversion UPS maintains a closer
tolerance on its output voltage. Typically, a double-conversion UPS delivers an
output within 2% of its nominal rating.
On the other hand, the double-conversion UPS is inherently more complicated
and costly. A single transformer doesn't suffice because a double-conversion
unit regularly operates its battery charger and inverter at the same time. The
double-conversion UPS consequently has required two heavy and costly
transformers in the past, though new engineering developments are reducing
transformer size and cost (see below).
The added expense of the double-conversion UPS has a big benefit: it improves
the overall quality of power delivered to your computer. It acts as your
computer's own generating station only inches away from the machine it serves,
keeping it safe from the polluting effects of lightning and load transients.
Dips and surges can never reach the computer. Instead, the computer gets a
genuinely smooth, constant electrical supply exactly as it expects and was
designed to utilize.
The latest generation of on-line UPS's trim their price penalty through
clever engineering. They eliminate the large twin power transformers by simply
rectifying line voltage, regulating it, then inverting it back to AC. To match
the low-voltage batteries to the line-voltage bus, this design uses a DC-to-DC
voltage converter, which operates at high frequencies (like a PC switching power
supply), and thus needs only small transformers. In this design, the batteries
are actually out of the circuit except during a power failure, although there's
actually no switching delay because they remain constantly connected. By keeping
the load off the batteries, this design may enhance battery life (but actual
units are too new on the market to tell for sure). Then again, the batteries
provide no extra buffering of power anomalies. In testing, we found the new
on-line design to be quite effective and cost competitive with line-interactive
units.
The true double-conversion UPS provides an extreme measure of surge and spike
protection (as well as eliminating sags) because no direct connection bridges
the power line and the protected equipment--spikes and their kin have no pathway
to sneak in.
