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

4.2 DESIGN FOR RELIABILITY

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The design of highly reliable power supplies begins with:

(1) Verified circuit designs

(2) High reliability components

(3) Conservative derating practices

(4) Low junction/hot-spot temperatures

(5) Printed wiring boards and connectors instead of
point-to-point and/or wiring harnesses.

CIRCUIT DESIGN CONSIDERATIONS

Peak Instantaneous Transients and Subtleties

The most predominant power supply failure modes are caused by
peak instantaneous transients and subtle factors within and
external to the power supply. Sneak circuits and
built-in wear-out mechanisms must be considered in the
design and packaging of the power supply system. Sneak
circuits are addressed in Department of the Navy
document NAVSO P-3634.

Hostile environments such as temperature, shock, vibration,
altitude and humidity accelerate failure modes.

Training engineers, designing with margin, incorporating
protective circuits, performing worst-case analysis and
conducting comprehensive testing to prove the design are
musts to achieve power supply reliability and
performance longevity.

Use of MIL-HDBK-217

Many designers consider only static conditions and do not address
transient conditions such as transistor load line
variations and various peaks and surges as well as items
such as capacitor core temperature.

Be cautious of a simple and inexpensive switching-mode power
supply with a high predicted MTBF determined in accordance
with MIL-HDBK-217. See Reference 30. The expected MTBF
of power supplies designed and manufactured in
accordance with the guidelines presented herein will
exceed predictions per MIL-STD-756, Method 2005, using
MIL-HDBK-217.

These predictions can be used to identify internal component
reliability concerns and also identify the lower limit of
expected MTBF performance. These predictions should not
be used as a basis for setting contractual reliability
requirements, but should focus on operational mission
and system expectations.

Points to Consider

The following is a list of key points to consider when designing
and evaluating a switching-mode power supply
design:

(1) Incorporate voltage transient protection on the input power
lines.

(2) Include a controllable soft-start circuit to relieve the
component stresses during turn-on.

(3) Build an internal housekeeping power supply to isolate
sensitive circuits from the hostile power line, improve human
safety and allow control of the power-up and power-down
cycles.
(4) Tailor the turn-on and turn-off load
lines to minimize peak power in the semiconductors
during power supply turn-on/turn-off intervals.

(5) Incorporate a crossover interlock circuit in the power stage
that is connected across the power line so that two
devices can never conduct simultaneously across the
power line.

(6) Incorporate input EMI filtering having characteristics
compatible with the negative resistance of the power supplies
and variable input source impedances so that the
combination is stable.

(7) Incorporate a fast-attack current limiting circuit to protect
the power devices when the control logic "glitches" due
to random noise or other abnormal circumstances.

(8) Design the power inductors, transformers and magnetics to
keep them out of saturation during peak load and transient
conditions.

(9) Sequence the turn-on/turn-off logic in an orderly and
controllable manner.

(10) Analyze and measure worst-case peak currents, peak power,
peak voltage and ripple currents in all devices and under
all worst-case static and dynamic conditions. Compare
the measured results with the rated limits of all
components.

(11) Perform worst-case thermal and hot-spot analyses.

(12) Design printed wiring boards and packaging for the best heat
transfer. Plan the circuit so that it will not be
subject to common-mode and differential-mode noise.
Circuit design engineers must be responsible for the
electrical and printed wiring board layouts.

(13) Package magnetic devices and other heavy-current-carrying
conductors with thermal interfaces adequate to meet
hot-spot temperature requirements.

(14) Avoid ground loops and potential cross-talk and interaction
by developing a grounding technique that is appropriate
to the application.

(15) Analyze and measure loop stability to ensure that there is
adequate phase and gain margin under all line, load,
temperature and component tolerance variations.

RELIABILITY REQUIREMENTS FOR COMPONENTS

The ground rules for selecting components are:

(1) Microcircuits are in accordance with MIL-M-38510 Class B
Standard Military Drawing (SMD) or are procured to Source
Control Drawings, Specification Control Drawings and
Selected Item Drawings that provide the same or a higher
reliability level. Specification control drawings are
preferred over source control drawings.

(2) Diodes and transistors are JANTX (or JANTXV) or are procured
to Source Control Drawings, Specification Control
Drawings and Selected Item Drawings that provide the
same or a higher reliability level. Specification
control drawings are preferred over source control
drawings.

(3) Other discrete components are P-level or higher Established
Reliability (ER) components or equivalent.

(4) Magnetic components should be equivalent to MIL-T-27
insulation Class S or better, with the addition of a 100%
thermal-shock screen.

(5) Wet slug tantalum capacitors, except type CLR 79 per
MIL-C-39006 /22 and /25, are not allowed in the design of
militarized power supplies since they are prone to leak,
resulting in corrosion and power supply failure.

There are special criteria for improving reliability for selected
types of components. These are as follows:

(1) Aluminum Electrolytic Capacitors

These devices are susceptible to external contaminants during the
assembly of the power supply, primarily by the cleaning
agents. The failure mechanism is an internal short from
chloride contamination that is a long- term, time and
temperature process. Specifying epoxy end-seals
minimizes or eliminates these external contaminants inside
the capacitors.

(2) Power Semiconductors

Avoid the use of packages with electronically hot cases and use
isolated cases where practical. Hot case devices require
well thought out isolation techniques and additional
assembly steps that are susceptible to error.

DESIGN DERATING

The failure rate increases as a function of the applied stresses
imposed by the initial design decisions. Depending upon
the specific component type, construction and materials
used, these failure-prone stresses may include voltage,
current, or power dissipation. However, predominant
among all others is temperature (case hot-spot, junction
temperature, ambient temperature). Derating is simply the
practice of designing equipment using components whose allowable
maximum application stresses are constrained to some
percentage of the Absolute Maximum Rating (AMR), thus
taking advantage of the lower failure rate which
results.

Absolute Maximum Rating

AMRs on parameters are derived by component manufacturers as
guidance for designers in determining whether their component
applications are compatible with anticipated worst-case
stress conditions in their equipment. An AMR is usually
based on one of the following:

(1) The stress point beyond which device performance parameters
are not specified or controlled.

(2) The stress limit beyond which permanent degradation of
parameters may begin to occur.

Voltage and Power Derating

Operating voltages and power dissipation levels are derated, for
particular applications, to ensure that the components
will operate at the required reliability levels under
specified environmental conditions. Voltage and power
derating are separate and independent procedures.
Voltage derating is done to reduce the possibility of
electrical breakdown, whereas power derating is done to maintain the
component material below a specified maximum
temperature.

The first step in the process of derating is to establish the
operating voltages and the second step is to adjust the
power dissipation level. The voltage derating of passive
components prevents voltage breakdown, flashover and
corona effects at the atmospheric pressure (altitude) to
which the components are exposed. These effects are
dependent upon voltage gradients, configuration of
terminals and the nature of the dielectric path. The operating
voltage of active components, such as semiconductors, is
dependent upon the breakdown characteristics of the
semiconductor material.

After the operating voltages are established, the power
dissipation level is determined. The degree of heat transfer from
a heat-producing component and the immediate ambient
temperature surrounding the component, will determine
the surface temperature or junction temperature, core
temperature and hot-spot temperatures at a particular
power level. The junction temperature should not exceed
+110ºC under worst-case conditions.
Table 4-1 defines the minimum
derating criteria that are recommended for the design of
power supplies. TE000-AB-GTP-010 and NAVAIR AS-4613 may
in some instances require more stringent derating. Each
application should be considered individually whenever
design penalties result. The parameters of the values
listed under "DERATED TO % RATING" are shown in
"DERATING PARAMETER."

COMPONENT OPERATING TEMPERATURES

Lower operating temperature is synonymous with more reliable
operation. The absolute value of operating temperature for
all components in a power supply should be determined
both analytically and
empirically.

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Last Modified on :
Friday, 07-Mar-03
14:20:11