Application Note
Designing a Power Supply
with Multiple Input Options
Some electronic systems such as field
communications equipment must be
able to operate from a variety of power
sources, for example, worldwide
universal AC (85-264V rms) or vehicle
battery power. These systems may also
require backup power in case the
primary source is interrupted.
The illustration shows a power supply
that can operate from the AC line,
from a 12V or 24V vehicle battery, or
from an internal 48V backup battery.
For AC line use, a front end module
accepts a universal AC input and
provides power factor correction. This
module feeds a high power converter
which supplies a highly regulated 48V
output to the point of load converters.
The 48V converter can also be
trimmed up to 52V to charge the
backup battery while the supply is
powered from the AC line.
The point of load converters have an
input range of 21–56V (36V nominal)
which allows them to accept 48V from
the AC line or the backup battery, or
24V from either of the two vehicle
battery sources. This input range
avoids the need for intermediate
conversion of the 24V battery input
and minimizes the number of modules
required for the 12V battery input.
Half the Modules Required
If the 12V battery input were converted
directly to 24V, six modules would be
required to provide sufficient power to
the point of load converters. Instead,
the 12V battery feeds a 12V out, 225W
converter array that provides half the
power needed (see shaded modules).
This output is placed in series with the
12V battery which provides the other
half of the required power.
This approach has several advantages.
The number of modules in the array is
reduced from six to three, which
reduces cost, simplifies layout, and
increases reliability. Overall efficiency
is dramatically increased, since half the
power is provided by the battery,
which is effectively a 100% efficient
source. Heat dissipation in the array is
cut in half, simplifying thermal
management.
To prevent backdriving, the outputs of
the intermediate converters and the
24V source are diode or’ed together.
Diodes should be selected to handle
maximum load current and voltage
with appropriate design margin. The
48V backup battery is not diode or’ed
to the bus so that it can be trickle
charged when the supply is powered
by the AC source.
In addition to the above, the DC-DC
converters should be sized to provide
regulation under worst case load
conditions. For example, if the final
loads could require a total of 75W
worse case, the preceding converters
must be able to supply 75W plus the
dissipated losses—in most cases, about
90W with 83% efficiency.
• • •
For assistance with this or any other
component-based power system
design, call Vicor Applications
Engineering at 1-800-735-6200.
+
48V Battery
(Backup)
–
+
24V Input
–
+ In
Gate Out
Gate In
+ Sense
Trim
– Sense
100W Module
21-56V In, 5V Out
– Out
Driver
Input Filter
Vicor Part Number
07818
*Derate output power
6W/V below 97 Vac.
VICOR CORPORATION
– Out
+ Out
– In
Universal
AC Input*
+ Sense
Trim
– Sense
– In
+ In
Gate Out
This power supply can operate from
the AC line, from a 12V or 24V vehicle
battery, or from an internal 48V
backup battery.
+ Out
Gate In
+ In
Booster
24V Out
@ 450W
Booster
Three 75W Modules = 225W
12V In, 12V Out
+ Out
Gate In
Gate Out
+ Sense
Trim
– Sense
– In
– Out
Driver
Booster
+
12V Input
–
Two 100W Modules = 200W
21-56V In, 12V Out
L1
+ Out
+In
+Out
Gate In
PC
Gate Out
P/OK
E/O
A/S
PR
+Sense
SC
–Sense
L2/N
– Out
–In
–Out
VI-HAM
Harmonic Attenuator Module
600W Module
375V In, 48V Out
+ In
+ Out
Gate In
For
Trim Up
Gate Out
+ Sense
Trim
– Sense
– In
– Out
75W Module
21-56V In, 15V Out
• 25 Frontage Road • Andover, MA 01810 • TEL: 800-735-6200 • FAX: 978-475-6715 • 12/96