Program management needs to be aware of the tradeoffs involved in
specifying input power so that system requirements can be met without placing
an excessive burden upon the power supply or power supply design activity.
There has been a trend towards requiring individual equipment to operate from
a large variety of power sources. This has increased the complexity of the
design approach, making the objectives of increased output power density and
reliability more difficult while simultaneously dissipating investment monies
and highly skilled manpower resources. Imposing the requirement to interface
with various prime power inputs and provide multiple regulated output voltages
reduce overall power density and can have a detrimental impact on
The tradeoffs for equipment for single-platform use are relatively simple.
The shipboard choice indicated by a formal tradeoff study almost always
results in the use of 60 Hz, 3-phase power at either 115 VAC or 440 VAC, the
latter being stepped down in the system to 115 VAC or some DC voltage, such as
155 VDC; i.e., the voltage obtained from off-line rectification of 3-phase 115
VAC. The aircraft choice almost always results in 400 Hz, 3-phase, at 115/200
VAC with consideration given to future 270 VDC operation. If local off-line
rectification is used, there is a need to ensure that the total contribution
from the equipment meets the 3% harmonic current requirement for shipboard
systems as found in MIL-STD-1399 SECTION 300.
C for a detailed discussion.
Selecting the best multi-platform power supply architecture is a fairly
complex task and beyond the scope of these guidelines, but the following
ground rules minimize the burden on the power supply:
(1) Optimize for aircraft power since the size and weight of the power
supply contribution to avionics usually has a greater impact on overall system
performance than it does for ships. One of the required input voltages for
multi-platform equipment would then be 400 Hz, 3-phase, 115/200 VAC with
strong consideration given to operating from 270 VDC.
(2) Design to interface with 60 Hz, 3-phase, 440 VAC or 115 VAC shipboard
power such that the system meets the 3% harmonic current requirement for
(3) Eliminate or minimize the need to interface with input power having any
The preceding ground rules should be applied by a program manager to help
minimize the burden placed upon power supplies by multiple input power
requirements until other approaches are developed or become mature.
Noninterruptable power is not a requirement set forth by any military
standard but is an equipment specification levied by the procuring agency. The
two most commonly specified prime power interrupt times, through which the
equipment is required to continue operating, are 10 to 300 microseconds and 20
to 150 milliseconds. At the present time, the requirement to operate through
the lesser interrupt does not impact the power supply