Recent attempts to specify equipment for multi-platform operation have led
to input power specifications for a single equipment that have specified five,
or six input electrical interfaces.
This places a burden on the power supply that makes it difficult, if not
impossible, to meet the primary objective of this guideline reliable power
supplies while striving to meet other needs such as increased output power
Tradeoff studies seeking the best approach for multiple-platform use of an
equipment or system are not straightforward.
Modern power supply technology is based on DC-to-DC converter techniques
that use an internal switching frequency dependent on the current state of
component and circuit technology. This internal frequency is usually in the
range of 20 kHz to 500 kHz and in some designs it is higher. Since DC-to-DC
converters require a DC input, the easiest way to obtain this DC is by line
rectification of 3-phase power (direct rectification of the AC line voltages
with no isolation). This eliminates the size and weight of line-frequency
transformers from the power supply.
Direct line rectification of shipboard 3-phase, 440 VAC results in
approximately 590 VDC which is too high for active power supply components
that operate in the 20 kHz to 500 kHz range.
Direct line rectification of shipboard 3-phase, 115 VAC results in
approximately 155 VDC. This is the logical DC voltage for shipboard use and is
highly compatible with existing and developing component and circuit
technology. Direct line rectification of shipboard 3-phase, AC power can
create excessive current harmonics on the AC bus. There is a need to ensure
that appropriate means for meeting the 3% harmonic requirement of MIL-STD-1399
Section 300 are provided. Direct rectification of aircraft 3-phase, 115/200
VAC results in approximately 270 VDC. This is the logical DC voltage for
The difficulty of designing to a 1.74 ratio between line rectification of
aircraft and shipboard standard AC voltages, when added to the additional
spread due to tolerances and transients on aircraft and shipboard power, is
one factor in considering the best power supply architecture. How to meet the
3% harmonic current specification for shipboard applications is another
factor. Selecting the best multi-platform power supply architecture is a
fairly complex task and beyond the scope of these guidelines, but some ground
rules should be followed to 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
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 with any
These ground rules can be applied by a program manager to help minimize the
burden placed on power supplies by multiple input power specifications until
other approaches are developed or mature.