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NAVSO P-3641: Navy Power Supply Reliability Design & Mfg
Guidelines (NAVMAT P4855-1)
Complete
List of Guideline Documents | Table
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2.0: EXECUTIVE SUMMARY

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2.1 INTRODUCTION

The purpose of this document is to define (1) guidelines for Navy
and contractor program managers and (2) design and
manufacturing fundamentals for power supply engineers,
that will result in power supplies which meet or exceed
the reliability requirements.

The information contained in Sections 3 and 4 applies primarily
to low-voltage power supplies delivering up to 5 kilowatts
and 300 VDC or less. Some topics, such as output power
density have been tailored for power supplies below
1,500 watts and outputs of less than 28 VDC. The unique
requirements of high-voltage power supplies, those whose
output voltage exceeds 300 VDC, are addressed in Section
5. While these guidelines do not directly address other power
conversion equipment, such as frequency changers, inverters,
uninterruptible power supplies, or static switches,
precepts contained herein, such as component deratings
and environmental stress screening, can be applied.
Consideration will have to be given to development
schedules, volume and weight, to take into account the
complexities of the other types of equipment.

Modern Navy system requirements dictate that most power supplies
in new equipment be of the switching-mode type. For this
reason, the switching-mode assumption is implicit
throughout the document.

2.2 PROGRAM MANAGEMENT

Program management must address those aspects of power supply
acquisition which are associated with (1) specifying the
power supply requirements and (2) meeting the
requirements, including reliability, within cost and
schedule targets. This involves constraints external to
design and manufacturing, including not only the
available resources but also realistic technical requirements
which acknowledge what can and cannot be done within both the
current state-of-the-art and the military standards,
specifications and policies.

Equally important is the selection of a design and manufacturing
activity capable of implementing the guidelines in this
document. Fundamental criteria in power supply source
selection should be the proposed, perceived and
demonstrated ability to comply with these
guidelines.

DEVELOPMENT SCHEDULE AND COST

Traditionally, the specification and design of the power supply
has been left until the host system has reached
substantial design maturity. The development of a
reliable custom power supply requires an average time of
18 months from specification release and design go-ahead
to first mature production units, using standard design and
construction techniques (military standard components,
non-proprietary circuitry); 24 months is not unreasonable for new
designs which involve invention. To reduce the schedule
below 18 months imposes shortcuts which are likely to
degrade reliability. Among the most common of these
shortcuts is postponement of environmental qualification
and reliability testing until installation in the host
system. By this time, redesign to improve reliability is
unlikely due to cost and schedule constraints.

The idea that power supplies are simple leads to insufficient
funding and buy-in by inexperienced and otherwise
unqualified firms. The non-recurring development cost of
a typical power supply is 5% to 10% of the total
electronic system development cost, excluding software.
Staffing may include some 6,600 man-hours in various
technical disciplines to reach reliability maturity.

Budgets, specifications, program plans and proposals which
violate the 18-month and/or 5% to 10% cost guidelines must be
considered high risk. It is incumbent on the Navy and the
contractor to question such power supply planning and
design to ensure that the reliability of the program is
not being compromised.

OUTPUT POWER DENSITY

Low-voltage power supplies on the order of 6 watts per cubic inch
represent conventional technology where high reliability
can be guaranteed using established design and
construction techniques. These power supplies are of the
switching-mode type; linear power supplies are limited
to about ½ watt per cubic inch. Exceeding a ½ watt per
cubic inch packaging density using linear techniques, or on
the order of 6 watts per cubic inch using switching-mode
techniques, can contribute to poor power supply
reliability in the fleet when the higher density is
achieved by current design practices rather than
advanced circuit and packaging technology. Factors restraining
higher power density are:

(1) Availability of approved components

(2) Isolation transformers

(3) Enhanced EMI requirements

(4) Holdup time requirements after prime power loss

(5) Cooling techniques.

Nevertheless, higher power density is, or will be, required in
current and future systems. Techniques exist and are being
improved upon to satisfy future system
requirements.

Output power density requirements exceeding 6 watts per cubic
inch should trigger an in-depth technical investigation and
design review to ensure that reliability is not being
compromised. Any reliability claims should be
substantiated by test data taken in environments equal
to or exceeding the anticipated system
environment.

OUTPUT POWER DENSITY DEVELOPMENT

As electronics continues the quest for miniaturization, the space
available for the power supply will continue to shrink,
demanding ever higher output power density . Maintaining
control of this necessary growth to ensure that
reliability is not compromised in the process must be a
major concern of program management. Factors that affect
the increase in output power density are advanced design
techniques and components.

Increasing switching frequencies from 20-40 kHz to 100-500 kHz or
higher is presently the most significant technique used
to increase power density. This reduces the size of the
magnetics and filter capacitors. Higher power field
effect transistors and newer devices are replacing
conventional bipolar devices. Power hybrid microcircuits
and monolithic integrated circuits for power supply
functions are becoming available for military applications.
Capacitor volume continues to be a burden; however, improved
versions of both aluminum electrolytic and tantalum
capacitors are becoming available. Packaging techniques
have a direct bearing on increasing output power
density. Examples include rectangular capacitors and
both flexible and multi layer printed wiring
boards.

Selection and conservative application of components in power
supplies is critical to ensuring a highly reliable design.
Maximum usage of military- qualified standard components
is recommended. However, rapid advances in power supply
design technology and availability of new, higher
capability, high- reliability components have resulted
in making some of the standards obsolete. Military
qualification and standardization of new power supplies has not kept
pace with switching-mode power supply design technology.
Application of new technology to power supply design and
production can result in meeting the power density
requirements while in fact increasing the
reliability.

Examples of qualifiable components for power supply applications
include higher power Schottky diodes, bridge diode
assemblies, highly integrated regulator circuits, analog
optical isolators, metalized polypropylene capacitors,
printed wiring board connectors, isolated-case power
transistors and power field effect transistors. Until
such are qualified as military standard, their use requires
the exercise of existing government procedures for nonstandard
component approval. The use of nonstandard components in
switching-mode power supplies should not be prohibited,
provided there are at least two independent
sources.
INPUT POWER SPECIFICATIONS

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 reliability.

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.

See Appendix 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 shipboard systems.

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

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 significantly.

STANDARDIZATION

Presently, nearly every power supply applied within a Navy system
was or is being uniquely packaged. To a large extent,
this is unnecessary and often occurs because the
procuring activity is unaware of the design and
manufacturing impact of even small differences between
power supply specifications. Power supply
standardization is an approach to achieving high reliability with an
affordable expenditure of resources. Standardization can
make available a family of high reliability power supply
designs that would reduce the cost and expedite the
acquisition of Navy electronic systems. By limiting the
proliferation of unique power supplies, logistic support
could also be significantly improved.

Standardization would (1) partition power supplies in a manner
which allows for commonality among a majority of equipment
applications; (2) provide a rigorous quality program to
ensure interchangeability and reliability: (3) establish
basic requirements compatible with the majority of Navy
electronic system applications and environments; (4)
reduce recurring development costs and ease the logistic
support burden by extensive intersystem commonality of a
limited number of power supplies; (5) provide established
mechanical packaging having flexibility for growth; and (6) develop
functional specifications to avoid dependence on
specific technology.

DESIGN REVIEWS

Design reviews can be powerful tools for the procuring agency to
(1) maintain visibility of the status and progress of the
power supply development to meet the specified
requirements and (2) initiate corrective action if
warranted. However, in order for a design review to be
effective, it must be timely, expertly staffed and have
an effective agenda.

The best intentions of program managers, with regard to power
supplies in their systems, are for naught if they are unable
to evaluate and therefore manage the status of the
system at a level of technical detail adequate to
identify noncompliance to the design and manufacturing
guidelines outlined in this publication.

Design reviews should be timed to the completion of major
milestones of the power supply development. Rather than
establishing firm dates for the design reviews at
program start, some flexibility in scheduling should be
provided such that the activities associated with the
milestones are indeed completed. The Navy or prime
contractor may take appropriate action if a significant schedule
slippage is indicated.

The quality and effectiveness of the design review is directly
dependent upon the technical qualifications of the review
team members. Technical incompetence can lead to
misunderstanding, irrelevant questions and issuance of
unnecessary action items. The Navy program manager
should enlist the assistance of a team of power supply
electrical, thermal and structural specialists, to assist in
the design review. A design review checklist can be a
significant aid to the review team in ensuring that
every important issue is addressed. Detailed sample
checklists are included in this publication as a guide
(Appendices A and B).

An agenda should be generated and agreed upon by the Navy and the
contractor well in advance of the design review. The
agenda should delineate the items to be covered and
should include the format in which the information will
be presented. Once the design review date is established
and an agenda agreed upon, a design review package
should be provided to the procuring activity prior to the review in
order that the members of the review team may become
familiar with the task ahead and the material to be
discussed.

Three and sometimes four design reviews are key program
management tools in the material acquisition cycle: the
Preliminary Design Review (PDR), the Critical Design
Review (CDR), the Production Readiness Review (PRR) and
at Navy option, the Preproduction Reliability Design
Review (PRDR).

Additional design reviews may be conducted to reduce design
risk.

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Last Modified on :
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