CONVERTERS.OCT–Interpoint–SC
Examine all options before specifying military converters
Even converters without full MIL-spec compliance can be viable depending
on the application
BY STEVE WHITNER Interpoint Corp. Redmond, WA
Design engineers specifying dc/dc converters for military and aerospace
systems are under pressure to use commercial parts in their products
wherever possible to cut costs. To make an intelligent decision, designers
and users must share some information about part quality and reliability.
Traditionally, designers choose hybrid dc/dc converters that fully comply
to MIL-STD-883 Class H requirements for their high-reliability systems.
Like compliant ICs, however, fully compliant dc/dc converters can be
several times more expensive than commercial-grade parts. Fortunately,
designers being squeezed by budget constraints can choose among several
other part grades, including non-compliant military-grade, standard
environmentally screened, unscreened, commercial hermetic, and commercial
non-hermetic (see photo). Quality and reliability differences exist among
the various product grades. The differences lie in the facilities the part
is built in, the control of processes used in manufacture, the level of
qualification the part passes, the levels of screening and testing that
each part sees, and the part's environmental and electrical ratings. Table
1 compares the grades available for high-reliability dc/dc converters.
Compliant devices Fully compliant hybrid dc/dc converters represent the
highest levels of reliability normally associated with standard products.
These devices must first be built in a facility that is certified under
MIL-STD-1772 and qualified to MIL-H-38534. This qualification assures an
overall high level of control throughout a supplier's organization over
the entire process of design, manufacture, and product maintenance. The
compliant standards define procedures for procuring and inspecting
components, workmanship, processes for documentation control, test
equipment calibration, screening equipment, inspection, handling
procedures, qualification methods, failure analysis, and overall quality.
The standards must be planned for, implemented, and maintained with
regular on-site inspection to verify that they are being upheld as
intended. In addition to being manufactured in a qualified facility, all
the processes used to manufacture fully compliant parts must also be
qualified (not all processes in a qualified facility are necessarily
qualified). This means that the processes follow a set of prescribed steps
and have a prescribed government certified paperwork trail that documents
the entire process. Part of this procedure ensures that all components
used to manufacture the converter receive extra testing and inspection,
known as element evaluation. MIL-H-38534 also specifies qualification
procedures for compliant parts. They consist of extended temperature
cycling, a 1,000-hour high-temperature life test, and constant
acceleration beyond the levels defined for normal screening. Qualification
procedures must be submitted to the qualifying authority (the Defense
Electronics Supply Center–DESC) which approves the plan and the final
report, and maintains the records. The electrical specifications for
compliant converters vary from model to model but all are required to
operate over an extended temperature range (-55 degrees to +125 degreesC),
and each must undergo a prescribed screening sequence. The screening
includes seven days of high-temperature burn-in, two levels of hermeticity
testing, temperature cycling, 5,000-g constant acceleration, and testing
over the full operational temperature range. For parts that are issued a
Standard Military Drawing number, the parts undergo additional testing and
have samples maintained at DESC.
Non-compliant parts Several suppliers offer lower-cost commercial parts
that are non-compliant products but are nevertheless based on fully
compliant models. Interpoint designates these products /SX grade, but the
nomenclature differs from supplier to supplier. This part grade is
normally built to the same design and base drawing set as compliant
products, is also built in a qualified facility, and sees the same
screening levels and electrical testing. Most differences relate to
paperwork and documentation. Not all components are element evaluated,
some processes may not be completed by fully certified operators, and some
qualification procedures may be documented internally rather than through
the government authority. Since these parts are built to the same general
specifications and in the same facilities as compliant products, many
designers view them as equal to fully compliant models in performance,
quality, and reliability. The costs, however, are 25% to 35% lower both
for the manufacturer and for the user.:
Standard screened parts
Another grade of commercial parts that can offer extremely reliable
performance at lower than compliant prices is standard, environmentally
screened parts. The distinction between this part and a non-compliant part
varies somewhat between suppliers, but several common features usually
apply. Normal screened products usually have the same operational ratings
as the compliant or non-compliant parts, and they see similar or identical
test procedures. For some suppliers and products, alternate cases are used
that allow a different sealing method to help lower costs. Most screening
procedures for standard screened parts are less severe than for compliant
or non-compliant parts: burn-in is often shortened from 160 to 96 hours,
temperature cycling may be slightly less severe, and acceleration levels
may be somewhat lower. Qualification for these parts is usually performed
to the vendor's own standards and may include slightly lower stress levels
than the ones used for compliant parts. Do these differences result in
lower reliability? There is not enough data to answer that question
definitively. The parts do have less design overhead for certain
mechanical and thermal stresses, so they see lower stresses in some parts of
the qualification and testing procedures. However, if the parts are used
within the operating limits defined for the part, they may be just as
reliable. It is important to note that if the screened parts are built in
a qualified facility, as many are, the same overall quality procedures
that add much of the value to a compliant part are also part of the
screened part. The standard environmentally screened level procedures do
add a specific economic value–they are typically about half the price of
their fully compliant equivalents.
Unscreened parts A third level of commercial part is one that is often
designated “unscreened.” The nomenclature can be deceptive, and it
illustrates the difference between parts built in a military-aerospace
environment and those built in a completely commercial context.
Non-screened parts for a military/aerospace supplier may be built in a
fully qualified facility, undergo the same inspection systems as higher
level parts, and ordinarily see at least 24 hours of burn-in and
hermeticity testing. Because the parts see less initial stress, users
will see more infant mortality in their systems than with any of the
screened units, but the quality and reliability levels are still high. At
Interpoint, analysis of the distribution of failures for non-screened
parts shows that after a few field months the converters reach their
constant failure rate region and their reliability levels are as high as
those for fully screened parts.
Hermetic and non-hermetic Two other high-reliability part grades encompass
commercial converters based on high-reliability designs, but with some
kind of design modification specifically aimed at reducing cost. The
variation here is enormous from vendor to vendor, with two general
standards–one that applies to parts that are packaged in metal, hermetic
cases and a second that applies to non-hermetic plastic packaged and epoxy
encapsulated parts. Hermetically packaged parts are probably derived from
a design that approaches military reliability standards. To lower costs,
the supplier usually lowers the rated operating temperature range to allow
use of less expensive components, and relaxes electrical characteristics
to allow use of less expensive components. In some cases, the parts may be
built on a separate non-certified line which can help reduce the overhead
associated with maintaining compliant facilities. It is important to note,
however, that non-qualified facilities owned by high-reliability companies
may match the quality levels of certified facilities. Non-hermetic
converters, packaged in either metal or plastic cases, generally fall in a
separate category. As with plastic packaged ICs, non-hermetic converters
can be expected to have somewhat lower long-term reliability than their
hermetic counterparts. However, there are differences among non-hermetic
converters as well. In general, products built in facilities with military
quality, screening and test programs can be expected to produce more
reliable parts with than facilities that normally not meet those standards
and. merely apply extra screening in an attempt to increase reliability.
Making the decision With all of these choices in converters, how does
the designer choose one over another? Generally, compliant and
non-compliant-grade parts are designed with slightly greater overhead for
mechanical and thermal stress. These parts are likely to provide longer
average life in applications that expect rapid and frequent thermal
excursions beyond the rated operating temperature, or that see very high
levels of vibration or mechanical shock. However, for high-reliability
uses that do not expect radical environmental extremes, standard screened
units rated for high-temperature operation provide reliability consistent
with mission-critical applications. These parts are far and away the most
common product grade in commercial and military avionics, missiles,
spacecraft, and satellite systems, with a long history of extended
operation in harsh environments with very low failure rates. Hermetic and
non-hermetic commercial parts made by modifying and cost-reducing military
quality grades are suited for more benign environments or in applications
where performance is slightly less critical. Table 2 summarizes the
screening procedures for the various dc/dc converter grades.
Photo CAPTION:
A number of options are available to designers specifying dc/dc
converters for high-reliability applications, including (clockwise from
top) fully MIL-STD-883 compliant, non-compliant, standard screened,
non-screened, hermetic commercial, and non-hermetic commercial.
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