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NAVSO
P-3641: Navy Power Supply Reliability Design & Mfg
Guidelines

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3.6 Input Power
Specifications
Multiple-Platform
Tradeoffs

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 density.

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 aircraft use.

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 shipboard systems.

(3) Eliminate or minimize the need to
interface with input power with any other
characteristics.

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.