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There are three basic design types, each offering more power
protection than the preceding one:
OFFLINE-STANDBY TECHNOLOGY
During normal operation, the power flows straight through
the unit and hence only RFI filtering is usually provided.
When the input voltage fails or fluctuates outside of a
pre-set tolerance window, the UPS detects this and a relay
will close, allowing the UPS to start feeding battery power
via the inverter. The inverter is then switched on and either
a square, step or sinewave form output is supplied. Upon
the return of mains power, the output is switched back onto
mains and the inverter is turned off. Typically there will
be a break of between 4-10 ms during the transfer to and
from the battery mode.
Advantages:
• Low cost
• Silent operation (when in standby)
• Efficient
Disadvantages:
• Minimal power protection – only protects against
a small percentage of problems
• Poor output voltage regulation – fluctuations
such as sags and surges will be passed straight to the load
• Break transfer to battery mode
• No failsafe – UPS will drop the load if there
is a high start-up current, overload or inverter failure
LINE-INTERACTIVE TECHNOLOGY
A line-interactive UPS operates in a very similar
fashion to an offline UPS, except with the advantage of
better filtering and output voltage boost/reduce features.
Whilst not eliminating mains-borne interference, line-interactive
technologies reduce the impact of spikes, surges and sags
by ‘clipping’ the peaks and valleys, boosting
power or switching to battery back-up. As with offline UPS,
when the input voltage fails or fluctuates outside of a
pre-set tolerance window, the UPS detects this and a relay
will close allowing the UPS to start feeding battery power
via the inverter. The inverter, in a good line-interactive
UPS, will supply a sinewave output. Upon the return of mains
power, the output is switched back onto mains and the inverter
is turned off. As with offline UPS, typically there will
be a break on the transfer to and from battery mode, though
usually this will be shorter than with an offline UPS.
Advantages:
• Lower cost than online
• Gives better protection than offline
• Silent operation when in standby
• Efficient
Disadvantages:
• Fluctuations, such as spikes, can still be passed
straight to the load
• Break on transfer to battery mode
• No failsafe – UPS will drop the load if there
is a high start-up current, overload or inverter failure
ONLINE DOUBLE CONVERSION
TECHNOLOGY
An uninterruptible power supply using true online double
conversion technology provides the highest level of power
protection available. The UPS converts the 230V input AC
mains supply to DC power which is then used to charge the
battery. The DC current flow is then fed through an inverter
stage which reconstructs the 230V AC mains output. Because
the AC output is completely regenerated, it will be completely
free from any mains-borne interference such as spikes and
voltage variations. The output voltage and frequency is
controlled precisely, thus ensuring a clean and stable sinewave
power output. Online UPS are able to withstand large fluctuations
on the input voltage before transferring to battery power
(typically 276V-184V) thus eliminating unnecessary battery
discharges. Upon mains failure, transfer to battery power
is seamless – no break, i.e. NO TRANSFER TIME. Online
UPS also have various failsafe and self-diagnostic features
that will instantly transfer the load onto mains power if
there is a failure within the UPS hardware, or if the UPS
is overloaded.
Advantages:
• Continuous & total power functionig
• Failsafe/overload protection with static bypass
facility
• No break on mains failure
• Wide input voltage tolerance
Disadvantages:
• Because of the technology used, online usually costs
more than other types of UPS technology
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