![](media/image1.wmf)NEWS FLASH December 2003

## Focus: DC-to-DC Converters

*Jeannette Plante, Editor*

This issue of *E-Flash Spotlight* focuses on DC-to-DC converters used in
flight hardware. It first provides an overview of a work in progress
that will eventually be an electronic designer's and electronic parts
engineer's guide for successful use of these components in flight
hardware. Following that, other projects currently in progress will be
discussed that deal with analysis of the issues associated with DC-to-DC
converters and documented failures. Three studies done for the NEPP
program and one by the CALCE Center at the University of Maryland are
profiled. Finally, links for suppliers of space-grade DC-to-DC
converters are provided.

**How DC-to-DC Converters Work**

DC-to-DC converters are critical items in spacecraft power systems. They
accept power from the solar arrays or batteries at a range of voltages
depending on the power bus design and mission conditions (typically 24 V
to 36 V for many NASA satellites, around 70 V for some commercial
satellites, and as high as 125 V for the International Space Station)
and deliver voltage regulated outputs at the levels needed by various
digital and analog electronics. These converters are voltage-regulated,
switching power supplies that employ a variety of mature circuit
configurations and are implemented with mature electrical and electronic
parts. When the switching component inside of the converter, normally a
MOSFET, is turned on and off (at rates from 20 kHz to 500 kHz and
higher), voltages are produced at the output from energized capacitive
or inductive components in the output stage. This method allows good
current delivery with low amounts of loss inside the converter. The
ratio of output power to input power is called the efficiency.
Efficiency for most switching DC-to-DC converters used in space is
between 65% and 80% when the converter is used near its maximum rated
output power.

The input voltage of DC-to-DC converters changes with time for various
reasons, including transitions between light and dark periods in the
orbit and changes in the bus load conditions. Feedback circuits within
the converter provide regulation of the output voltage by comparing the
output voltage level to an internal reference voltage and driving the
difference to a low value by fine-tuning the duty cycle of the internal
switch. This is often done through a pulse width modulator (PWM)
circuit.

A number of circuit designs, or topologies, have been developed to
improve the output performance and durability of DC-to-DC converters in
a variety of electrical and environmental conditions. Filters on the
input and output help dampen signals that can result from
package-related capacitances and inductances and smooth output ripple.
Synchronization circuitry is provided to prevent multiple converters
that are working in parallel from generating a beat frequency^1^ through
coupling of unfiltered harmonics. Input filters are also used to improve
impedance matching. Transformers are used inside of DC-to-DC converters
to provide input-to-output isolation, and to achieve the desired output
voltages, including multiple voltage outputs. Careful selection of
converter topology, analysis and design filter circuits, and analysis
and design of power bus load management circuitry are required for
successful application of these converters in an electrical power bus
system.

**Which DC-to-DC Converters Are Needed**

The primary criteria for designing or buying a DC-to-DC converter are
the voltage input range, the output voltage, the efficiency, and the
power dissipation (sometimes in a W/in^3^ context in which
miniaturization is important). Spacecraft solar arrays, and their
back-up battery systems, have traditionally been designed to generate
output voltages of between 21 Vdc and 34 Vdc. Output voltages of ±15 V,
±12 V, 5 V, 3.3 V, and 1.5 V have been required from the converters.
Future applications are seeking voltages even lower. Power ratings below
10 W are commonly needed.

Individual integrated circuits called DC-to-DC converters are typically
low-voltage regulators. The switching converters used in NASA power
buses are typically achieved through a design and build that uses
discrete components, or can be by using a complex hybrid microcircuit (a
miniaturized circuit card within a package, containing magnetic coils,
resistors, capacitors, packaged ICs, and bare IC dies) or some
combination of all three. The IC solution is applicable in the
low-voltage, low-power, digital arena, while the latter highly complex
approaches are needed for higher power applications working at the
spacecraft bus level and containing filtering and synchronization
circuitry.

**How Hybrid Designs Enter Into Use**

DC-to-DC conversion has traditionally been implemented and has
successfully been achieved through "discrete" designs at the circuit
card level. While their performance may be very nearly matched to the
application requirements, they may *seem* large by modern electronic
packaging standards. Design of these circuits using hybrid microcircuit
manufacturing will greatly reduce the footprint of portions of the
system, may improve efficiency and may reduce common-mode noise.

Miniaturization in DC-to-DC converter hybrids is achieved mainly by
eliminating some of the IC packages (implementing chip and wire
technology) and by increasing the switching frequency which drives the
ability to use smaller magnetic components (these miniaturized DC-to-DC
converters are generally rated at an internal switching frequency of
around 500 kHz but designs have been found that go up to 1 MHz). The
chip and wire technology, the special substrate design and the thermal
management features contribute to large non-recurring engineering (NRE)
costs. The internal parts can be quite specialized for space hardware
builds which causes expansion of the lead times. Another serious
challenge associated with the hybrid microcircuit approach is that test
points can be lost and the opportunity for post-production changes or
repair is greatly diminished. The ability to assure the constituent
materials, the parts and the design, in a way that we have the most
experience, is diminished; this includes loss of 100% burn-in of each
internal part (a.k.a. the elements).

Market conditions and business decisions made by the hybrid DC-to-DC
converter suppliers have led to NASA projects buying hybrid converters
from companies who are building them both for the commercial sector as
well as for the high reliability market. These parts are advertised as
standard, commercial-grade products and are defined to a limited degree
in military specifications under the QML-38534 system. Leveraging of the
logistics and NRE costs of the high reliability production line with the
benefits of the higher volume commercial production line, have led the
manufacturers to attempt to align the design goals of their commercial
and high reliability customers. NASA is finding that the result can be a
reduction in the performance that NASA projects require.

**NASA's Issues With Commercially Available Hybrid Converters**

Some of the critical issues associated with the purchase of these
commercially available parts are:

-   A lack of knowledge by the user about, or input into, the electrical
    design.

-   Uncontrolled processes: traceability, qualification, screening,
    rework.

-   Quality and workmanship problems.

-   Long-lead times and competition for manufacturing floor priority.

-   Unavailability of build records and unpredictable progress through
    the production process.

-   Inability to modify the design at later stages of the program to
    resolve problems.

-   Insufficient functionality requiring design of external circuitry.

These issues are at the same time, related to design and application,
parts engineering, quality assurance and procurement.

The first problem bulleted above can be the root of very serious
performance problems. There is pressure on the manufacturer to make
customers compromise on performance in order to reduce the number of
different designs being produced. Critical features such as input and
output filters and rigorous ground isolation might be dropped to satisfy
the market majority's need to maximize efficiency, thereby exposing
space flight users to reliability concerns. Further, the design is
tweaked to provide peak performance zones that reduce the user's ability
to realize the nominal ratings outside of a "sweet spot". This has been
experienced by NASA designers in a significant reduction in efficiency
(from the advertised 70% to 90% to as little as 10% to 20%) when
operating at reduced output power and associated catastrophic damage to
external and internal components due to the changed load conditions.

**Application-Driven Issues**

To overcome these problems, users must design circuitry around the
hybrids, such as filtering, grounding and protection features, to
increase circuit performance. The miniaturization advantages begin to
disappear at this point while the project continues to carry the risks
associated with use of the hybrid components. In some cases the designer
is not completely aware of these deficiencies and does not adequately
mitigate the associated risks and problems. This may be from a lack of
experience with the part, access to a complete set of application notes
or a lack of rigorous application requirements from the project. Serious
performance problems or failures have recently occurred because of the
lack of sufficient application notes including:

-   Floating Case: Ground pins were left floating because it was assumed
    that the case would be installed in such a way as to electrically
    connect it to chassis ground. The thermal coupling material that was
    chosen was electrically non-conductive leaving the case ungrounded,
    which is warned against in the vendor's application notes. The
    result was an out-of-design condition that conflicted with the
    operation of the internal oscillator affecting the output signal and
    exposing an internal IC to voltage spikes. Though the new, higher
    input voltage was within the device's rating, it put the part into a
    mode that was not sufficiently radiation hard. Though the
    application note was there about grounding the case, elaboration on
    the consequences of an ungrounded case might have motivated the
    designers to ensure the rule was followed and to more rapidly
    identify the problem. Knowing the consequences of improper usage can
    trigger good safety as well as good quality practices at Integration
    and Test (I&T).

-   Input Filters: Input filtering application requirements are often
    inadequate. Though at least one manufacturer does discuss the need
    and proper use of input filters with their converters, no mention is
    made about the limit to how many converters can be supplied from a
    single filter. The consequences can be high noise, damaging
    oscillations at the input and reduced efficiency.

-   Variable Power Bus Loads: There are limits to the amount of current
    and voltage regulation that DC-to-DC converters can provide across a
    power bus that has dramatically changing load conditions. These load
    conditions might occur because subsystems have gone off-line for
    replacement or repair or due to failure. Without proper design and
    planning, these large changes in impedance matching conditions can
    lead to high currents causing damage to the converters or to
    collateral circuitry.

-   Synchronization: The synchronization (sync) pin is used to establish
    a leader-follower relationship among a group of converters to avoid
    generating beat frequencies due to the slight differences in
    internal switching frequencies among the individual converters in
    the group. To make this approach work, the sync pins must be
    connected and activated before the internal FET is operated. By
    turning all pins on at the same time, the internal frequencies of
    the follower units will not be properly controlled and a beat
    frequency will be propagated. If a "follower" unit's oscillator has
    a chance to compete with the "leader" the resulting signal can cause
    damage to the MOSFET.

These problems can also arise with the discrete builds of DC-to-DC
converters. Both hybrid and discrete builds have experienced packaging
problems related to solder joints and stress relief. These problems are
closely related to packaging design and proper methods for attaching
packaged components to mechanically and thermally active elements in the
assembly such as the hybrid lid, the hybrid substrate, the printed
circuit board and the chassis. There has been a case where the hybrid
package was designed to use a new, high performance thermal material
that proved to be completely inadequate from a mechanical perspective.
Rigorous packaging analysis, assembly design and verification can reduce
these problems.

**Testing and Specification Issues**

Verification of the design's capability, the manufacturing facility and
the manufactured product, in the intended application, for some
specified amount of time, is the goal of qualification. Screening
provides the means to remove non-conforming product and infant mortals
from a production lot before the parts are sold to the buyer or
installed in the application. NASA has traditionally used both of these
reliability assurance strategies for procuring DC-to-DC converters. They
have leveraged off of the detailed written quality and test requirements
that can be found in the DoD's specification for hybrid microcircuits,
MIL-PRF-38534. NASA also uses guidelines given in MIL-STD-1580 for
destructive physical analysis, MIL-STD-883 for test and pre-cap visual
inspection and EEE-INST-002, which contains guidelines for qualification
and screening of space EEE parts.

Though these standards are readily available to us and NASA understands
and uses the information contained in them, there are shortcomings in
the requirements that are causing problems and the parts that are
purchased are not all described by detailed specifications. One very
significant shortcoming in the specifications is the lack of a
requirement to fully characterize the part over a wide variety of
operating conditions. Such a characterization would demonstrate the
results one can expect as the converter is used outside of the small
window of optimal performance conditions. The user can then determine if
the result is generally acceptable in their application, acceptable only
for short periods, never acceptable, or catastrophic. Some useful
application notes can also be achieved during extensive characterization
testing. The lack of performance characterization has led to exceeding
safe operating areas (e.g. for transformer wire insulation and capacitor
voltage ratings), leading to internal ringing, and voltage spikes
between V~out~ and ground.

Though NASA parts engineers can personally increase their attention to
these problems while following their procurement through the
manufacturer's facility, it is often difficult to discern changes to the
process, including those which effectively change the design, from what
was done to produce the units used in the original qualification
process. For example, a failure occurred when an IC was used as a
form/fit/function replacement for an obsolete part but was not an
adequate replacement. This sort of change cannot be "seen" in many
visual and electrical inspections. It ended up being detected during
radiation testing. NASA project budgets and schedules often depend on
leveraging qualification off of prior uses of the part; this can
"hamstring" the parts engineer, who finds it necessary to repeat
qualification tests at the lot level.

Another loophole in the specifications is the lack of attention to
element derating and no requirement to use established reliability parts
as the elements. Radiation testing is also not well specified, allowing
the testing to be done by irradiating particular elements at a time and
biasing the part with quiescent conditions or by using semiconductors
that are qualified at the element level. Though the entire packaged
hybrid carries a radiation tolerance rating, that rating may have been
insufficiently achieved. Characterization under vacuum and radiation
conditions can help identify critical problems early on in the stage
before spacecraft-level assembly. Again, this approach is time consuming
and costly and can be hard for projects to absorb.

Weaknesses in the manufacturer's data sheet and the military's
specification sheets drive the parts engineers to impose additional
requirements at the purchase order level. These may include:

-   Pre-cap visual inspection.

-   Traveler reviews.

-   Review of prior qualification records.

-   Progress reviews and reporting.

-   Delivery of data.

The vendor may or may not comply with all of these requirements. Also,
there are no penalties associated with lack of delivery to these
purchase orders. Procurements over the last few years have found that
the vendors can be inconsistent with loyalty to the delivery dates or
even to providing full disclosure about their schedule.

**Contracted Subsystem Procurements**

The alternative to buying hybrid DC-to-DC converters and adding on
discrete circuitry at the board level or to building the whole system
in-house is to treat the DC-to-DC converter procurement in the same way
as NASA would treat a subsystem through a subcontract. The advantages of
this approach are:

-   The ability to specify performance requirements over operating
    condition ranges (electrical, thermal, mechanical, and lifetime),
    and to negotiate design trade-offs with competing suppliers.

-   Increased accountability by the manufacturer to make delivery dates.

-   Quality stop-gaps such as critical design review (CDR) and
    manufacturing readiness review (MRR).

-   First article qualification rather than using old, insufficiently
    related qualification data.

-   Return policy.

However, these approaches have serious disadvantages too: cost and lead
time. Given the erratic lead times for the hybrid parts (6 months to 1
year, usually closer to or greater than the latter) and the
approximately 1 year needed for an in-house, discrete design and build,
the 1 year generally needed for a contracted procurement like this makes
each approach basically the same with regard to lead time. The costs can
be very different, though. Recent estimates for contracted builds have
ranged from \$200 K to \$3.2 M per system. This is a hard metric to
track because of the wide range of complexity of the systems that are
needed.

A variety of groups examined the issues that have caused NASA problems
with DC-to-DC converters and that have prevented NASA from avoiding
continued failures of converters in space hardware. Summaries of some of
that work and related efforts are provided in this issue. The NEPP
Information Management and Dissemination (IMD) Project has issued this
*E-Flash* to bring awareness to the topic at a time of increased
activity in this area. The features provided herein are intended to
capture and share information about the topic in general and about
ongoing activities.

**Note 1.** See this link for a definition of beat frequency:
<http://hyperphysics.phy-astr.gsu.edu/hbase/sound/beat.html>)

######### Features

**DC-to-DC Converter Buy for the SDO Program at NASA GSFC**

*Jeannette Plante, Editor*

Dennis Krus, the project parts engineer for the Solar Dynamics
Observatory (SDO), is working with the project's electrical power system
engineer, Amri Hernandez-Pellerano, characterizing DC-to-DC converter
hybrids for use both by SDO and by the Global Precipitation Management
(GPM) project. They have narrowed the need down to output voltage and
power ratings of:

1.  +3.3 V @ 20 W.

2.  +3.3 V @ 40 W.

3.  +5, +/-15 V @ 60 W.

4.  +5 V @ 30 W.

5.  +/-15 V @ 30 W.

Other requirements at this time are:

1.  Must include input filtering inside of the hybrid.

2.  5-year life expectancy.

3.  Class K construction.

4.  Radiation-hard elements.

The actual procurement work has not started, which will involve
formulating some focused specification requirements to overcome some of
the loopholes that exist in the military specifications for hybrid
microcircuits and to overcome some of the problems noted above.
Manufacturing capability and qualification history of available product
is currently being researched.

More insight into this parts engineering work or other SDO-related parts
issues can be obtained by contacting Dennis Krus at NASA GSFC,
301.286.1121, <dennis.krus.1@gsfc.nasa.gov>.

**Return of Solder Joint Failure Due to Flexing Lid in Interpoint
Converter**

*Jeannette Plante, NASA GSFC, Parts Branch*

The most recent GIDEP Problem Advisory issued on DC-to-DC Converters is
dated August 29, 2003 and was submitted by Crane Interpoint Corporation
(CAGE code 50821) for the part MFL2815D/883, which corresponds to the
military specification 5962-93193. The failure mode was identical to one
experienced in the MTR28 style at NASA GSFC. The problem was a failed
solder joint at the end of an inductor wire that resulted when stress
was transferred to the joint from the inductor wire, which was stuck to
the case lid. The case lid had flexed away from the case in a vacuum
environment due to the pressure differential between the inside of the
hermetically sealed part and the pressure of the vacuum outside of the
part. The solder joint failed due to creep-to-rupture. The failure was
not immediate but occurred after vacuum exposure. Crane Interpoint
issued a GIDEP on the MTR28 failure in March of 2000. The newer GIDEP
discusses their corrective action.

Two documents on the NEPP Web site discuss this flexing lid/breaking
bond issue:

1.  Finite Element Analysis Done for the Image Project:
    <http://nepp.nasa.gov/DocUploads/FA1E3FC1-14B8-449F-9B37B8E8A683AF06/IMAGE_FEA.pdf>.

2.  Probability of Success of Interpoint MTR28xxd Converters Used in
    Image:
    <http://nepp.nasa.gov/DocUploads/004B922E-920A-4348-9CFAF94AE14E7C4D/mtr28.pdf>.

Members of the GIDEP system can go to
[http://members.gidep.org](http://members.gidep.org/) to find other
alerts logged into the system. Affected products are as follows:

-   SMHF28XX, 5962-95559, -91614, -92144, -91601, -92139, Crane
    Interpoint.

-   11930-M111 (EMI Filter), 5962-94010, Crane Interpoint.

-   ATW2815D, 5962-91613, Lambda Advanced Analog.

-   AFL2815DW, Lambda Advanced Analog.

-   AT02815TF, Lambda Advanced Analog.

-   ATW2815D, Lambda Advanced Analog.

-   MI-J++-M+ Series, Vicor.

-   683-61700-001, Modular Devices Incorporated.

**Phantom Application Notes for DC-to-DC Converters**

*Jeannette Plante, Editor*

Recent failures of DC-to-DC converters in NASA projects have not all
been attributable solely to design and manufacturing flaws in the hybrid
DC-to-DC converter components. Several problems can be traced to varying
electrical conditions that occur during integration and test, that are
outside of the safe or rated operating area of the part, or are a result
of how the part was installed. Though these problems are
operator-induced, the industry's stop-gap for these types of errors is
the issuance of application notes. These notes enable successful use of
the parts when they address known part deficiencies, idiosyncrasies, and
limitations that can cause unwanted, dangerous, or catastrophic results
in the circuit.

There are a number of application notes that have been dutifully and
clearly reported by the major manufacturers of DC-to-DC converters used
in space hardware. However, growing experience in the field is exposing
the need for more. Four recent failures illustrate four new application
notes that should be added to the corpus (these are discussed in the
lead article above: Floating Case, Filtering, Variable Output Loads,
Synchronization).

In mid-November 2003 a circuit design-parts engineering round table was
convened at NASA GSFC to discuss this and other issues related to the
lack of a centralized and complete collection of application notes for
DC-to-DC converters for space. This organization will be addressing this
need in a handbook for NASA power supply designers and parts engineers.

**Convening of DC-to-DC Converter Round Table at NASA GSFC**

*Jeannette Plante, Editor*

A meeting was held on November 14, 2003, between the power supply
designers of NASA GSFC Code 563 and the parts engineers and other
interested parties of NASA GSFC Code 562 to discuss issues with the use,
procurement, and assurance of DC-to-DC converters at NASA GSFC. The
discussion covered a wide variety of issues, some well known to the
group and others not so well known by one or both of the organization
represented. Broadly, the topics covered:

1.  Drivers for using hybrid DC-to-DC converters.

2.  How we attend to reliability.

3.  The variability of application conditions, some catastrophically
    dangerous to the converter and collateral parts or systems.

4.  The lack of a rigorous set of application notes.

5.  Cost, lead time, and assurance risks associated with hybrid
    microcircuit builds.

6.  Downfalls of in-house builds.

7.  Performance deficiencies in commercially available hybrid
    microcircuits.

8.  How we assure new buys and builds and procurement problems.

9.  Cost of managing discrete builds using contracts instead of purchase
    orders.

Much of the material contained in the lead article was generated as a
result of this round table meeting. The minutes of the meeting are
currently under review, looking toward what the next step will be with
regard to fact finding, technical research, and tools that can be
authored or built in the laboratory that can significantly decrease the
number of failures of DC-to-DC converters in NASA hardware.

**Preliminary Test Results for DC-to-DC Converter Noise and Transient
Turn-on Characteristics for NASA Applications**

*Ashok K. Sharma, NASA GSFC, Alexander Teverovsky, QSS Group, Inc.*

The reliability of DC-to-DC converters has long been a major concern for
NASA parts and application engineers. Currently available DC-to-DC
converters are complex hybrid devices that use a variety of active and
passive elements such as power MOSFETs, output diodes, pulse width
modulators, ASICs, stacked chip capacitors, optocouplers,
inductors/transformers, etc. These DC-to-DC converters are specified for
various input/output voltages, efficiencies, and power ratings ranging
from a few watts to over 100 watts. The applications of these parts
require operation at elevated temperatures and pulse stress conditions,
and therefore the quality/performance of the elements used in the
construction and assembly is crucial.

Failure modes of DC-to-DC converters experienced over the years have
been related to contamination, poor construction and assembly-related
defects, Schottky diode failures, optocoupler degradation, stacked chip
capacitor failures, and so on. A new failure mode of DC-to-DC converters
has been observed in a thermal vacuum chamber, which manifested itself
as an intermittent output interruption related to the level of vacuum
and temperature. Failure analysis showed that these interruptions were
due to the lid deformation caused by a difference between the internal
pressure of the hybrid and the external pressure created in the chamber.
Deformation of the lid was transferred to the solder joints through
adhesive coupling of the lid with internal elements of the hybrid.
During the thermal vacuum cycling, these deformations resulted in a
creep-to-rupture failure of the solder joints and caused interruptions
of the output voltage of the device.

Two other important issues related to the reliability of DC-to-DC
converters are noise and turn-on transients, which are design specific
and can be a serious concern in some applications. Switching in DC-to-DC
converters is a natural source of noise, which is an inherent feature of
all switch-mode designs. Under certain conditions, the turn-on
transients can create significant voltage spikes at the output of
DC-to-DC converters, which can potentially damage interconnected
microcircuits. The goal of this study was to perform preliminary testing
of noise and transient turn-on characteristics of DC-to-DC converter
part types from various suppliers.

The results from this work have not been released; for more information,
contact Ashok Sharma, <Ashok.K.Sharma.1@gsfc.nasa.gov>, or Alexander
Teverovsky, <Alexander.Teverovsky@gsfc.nasa.gov>.

### Preliminary Report on DC-to-DC Converters Temperature Cycling Test Results

*Ashok K. Sharma, NASA GSFC, Alexander Teverovsky, QSS Group, Inc.*

Several types of DC-to-DC converters and EMI filters were subjected to
multiple temperature cycling over the range from -65 ºC to +150 °C.
Interim electrical measurements were performed at three temperatures
(-55 °C, 20 °C, 125 °C) after 20, 90, 300, and 1,000 cycles. All EMI
filters passed the electrical tests after the thermal cycling, but only
one DC-to-DC converter withstood 1,000 thermal cycles. Electrical
measurements of the EMI filters did not show any significant variations
in electrical characteristics.

The results from this work have not been released because of
vendor-sensitive material; for more information, contact Ashok Sharma,
<Ashok.K.Sharma.1@gsfc.nasa.gov>, or Alexander Teverovsky,
<Alexander.Teverovsky@gsfc.nasa.gov>.

######## Related NEPP/NEPAG Reports

**Hybrid DC-to-DC Power Converter Survey and Analysis for NEPAG by NASA
LaRC Team**

*Jeannette Plante, Editor*

A report being prepared for the NASA EEE Parts Assurance Group (NEPAG)
on DC-to-DC converters is nearing completion. A team of NASA and
contractor engineers has studied market offerings and reliability
benchmarks in order to determine NASA's best strategies for meeting
their needs for future missions.

> The report examines the following:

-   Availability of space and military specified and qualified product
    > and the size of that market.

-   The number of standard parts available for space system use and the
    > number of vendors producing standard, space-grade parts.

-   The most common styles of DC-to-DC converters that have been used by
    > NASA programs.

-   NASA application performance needs and the availability of product
    > on the market to meet those needs.

-   NASA's most urgent needs and problems with DC-to-DC converters.

-   Obstacles to growth in the high reliability DC-to-DC converter
    > marketplace.

-   Current R&D efforts being directed at NASA's DC-to-DC converter
    > needs.

-   Industry-Government partnering opportunities.

-   Related papers and publications.

For more information on this work, prior to publishing, contact Mike
Sampson, <msampson@pop300.gsfc.nasa.gov>.

**Computation of Reliability by "Virtual" Method of Plastic Encapsulated
DC-to-DC Converters**

*K. Ghosh and F. P. McCluskey, University of Maryland (CALCE)*

**Introduction**

The use of solid-state hi-power electronic components and modules is
becoming more widespread in a variety of applications. This is true both
in large power systems, as electrical control and distribution of power
replaces mechanical control and distribution and in smaller,
point-of-use power supply applications. The use of solid-state
electronic components and modules has the potential to significantly
improve the efficiency and performance of power systems while reducing
their cost. Critical to the widespread incorporation and success of
these solid-state modules, however, is understanding and controlling
their reliability under the harsh environmental (high temperatures,
humidity, salt spray) and operational loading conditions (high voltages,
currents and power dissipation levels) that are typical in power
electronics applications. Such knowledge is essential for maximizing
performance and minimizing life cycle cost.

Accelerated tests have long been the preferred method for assessing the
reliability of power electronic modules and systems. However, this
method is a time-consuming and costly approach for determining the
relative merits of different design options or of choosing between
commercially available sub-systems. In addition, the reliability of many
commercially available modules and sub-systems has improved to the point
where standardized qualifications are uncovering fewer potential
problems, and, thus, are less cost-effective. Finally, accelerated
testing, if done improperly, can induce failure mechanisms that are not
representative of field use. A new method of reliability assessment,
physics-of-failure, has been developed to address these shortcomings of
traditional accelerated testing. It is a methodology that assesses the
reliability of electronics upfront during the design phase, based on
models of the fundamental mechanisms by which they fail. The CALCE
physics-of-failure (PoF) approach to reliability assessment includes
virtual qualification and physical verification. Using this approach,
the reliability of power electronic components, modules, and sub-systems
can be assessed quickly at a minimum cost. University of Maryland
researchers have developed and demonstrated methodologies and tools for
performing up-front physics-of-failure reliability assessment of both
plastic-encapsulated and ceramic encapsulated power electronic modules.

In this study, a virtual qualification was performed on a small DC/DC
power converter, and then validated with tailored accelerated testing.
The aim of this approach was to validate the virtual qualification with
experimental data and then predict the life of the components under
field load conditions. In the accelerated validation testing of the
converter, a performance baseline was determined, environmental stress
was applied, the performance was reassessed for shifts, and, finally,
root cause analysis was conducted on any unit that exhibited significant
reductions in performance. The results of the testing were compared
against the virtual qualification to confirm that the virtual
qualification predicted similar time to failure in test by similar
failure mechanisms.

**Virtual Qualification**

Virtual qualification (VQ) is a simulation-based process that models the
stress history experienced by the proposed product when it is subjected
to its anticipated life cycle, to assess whether the anticipated
reliability is achievable. The process can be used for identifying and
ranking failure sites based on likelihood of occurrence. Physical
verification then follows using accelerated stress testing to verify
that the simulation adequately captures the physical design and the
dominant failure mechanisms.

The parts assessed were 32 mm x 20 mm 8-pin DIP cavity packages
consisting of a diallyl phthalate shell filled with thermally conductive
flexible gray silicone. Embedded in the silicone was a double-sided
surface mount printed wiring board. The devices had an input voltage of
48 V DC. The manufacturer qualified the devices for operation over a
temperature range of --40 °C to +75 °C without derating. The devices are
single output, 3 watt power systems with 100% surface mount technology
(SMT), including the magnetics. calcePWA software was used in this study
to model the printed circuit board of the DC-DC converter. Component
data for the simulation was obtained from the manufacturers, while the
board was modeled based on layer stack-up information determined by
cross-sectioning. The model is illustrated in Figure 1.

![](media/image2.png){width="2.6625in" height="3.6041666666666665in"}

![](media/image3.png){width="2.4125in" height="3.1131944444444444in"}

**Figure 1.** calcePWA Model of the Converter PCB

Table 1 indicates that the DC/DC converter should survive 20 years life
in its intended application environment, which was 5 temperature cycles
per day from 25 °C to 50 °C with a 30-minute dwell at high temperature.
It also provides the time-to-solder joint failure in test for the
components most likely to fail in this manner. The test condition was 24
cycles per day from -55 °C to 125 °C with a 15 minute dwell at high
temperature. The inductor and leadless capacitors are considered to be
most at risk for early failure in test.

**Accelerated Testing**

Multiple test configurations were used to carry out a full set of
experiments. Each setup was capable of testing multiple units
simultaneously. A block diagram illustrating the fixturing for
accelerated test characterization is shown in Figure 2.

Five samples, soldered to the board, were used in each test. The output
ends of the wires were connected to a data acquisition unit. A power
supply unit was supplying the 48 V input. Three different experiments
were conducted. Two were life tests at 12 5°C and 11 0°C at half load,
and the third was temperature cycling between --50 °C to +150 °C with 10
°C/min ramp. Half load means that the load resistance across the output
of the converters was drawing half the maximum current. The temperature
cycling test was done unpowered.

Two out of five samples had failed by 405 unbiased thermal cycles, one
at 210 cycles and one at 405 cycles. Solder joint failure at the
inductor was observed to be the failure mechanism. This corresponded to
the virtual qualification prediction. This observation and those
following life testing matched the virtual qualification predictions.

**Conclusion**

Virtual qualification based on physics-of-failure has proven to be an
effective method to evaluate the robustness and reliability of DC-DC
converters from manufacturers in a cost-effective and timely manner. The
technique indicated that under field load conditions the critical parts
are expected to survive more that 30 years, while the predictions under
test loading were validated by the results of the accelerated testing.

Four thousand, four hundred hours of life testing at 110 °C and 3,911
hours of life testing at 125 °C were completed. Eight-hundred fifty
cycles of temperature cycling was completed. Samples from the 125 ºC
life test failed at 725, 1,902, 3,187, 3,191 and 3,911 hours. The
failure criterion was considered to be Vout crossing the 5 ± 0.05 V
tolerance limit when tested at room temperature. Failure analysis
revealed evidence of temperature overstress on the backside, under the
transformer as shown in Figure 3; this is the location of the highest
temperature in the corresponding calcePWA model.

**Table 1.** Component Solder Joint Lifetime (Field: 5 Cycles Per Day;
Accelerated: 24 Cycles Per Day)

![](media/image4.wmf){width="4.016666666666667in"
height="2.3583333333333334in"}

Figure 2. Block Diagram of the Electrical Setup

![](media/image5.jpeg){width="2.0in" height="1.3222222222222222in"}

![](media/image6.jpeg){width="2.8756944444444446in"
height="1.3791666666666667in"}

Figure 3. Failure Site Due to Life Test and Verified by calcePWA
Software

For more information, contact Patrick McCluskey, <mcclupa@eng.umd.edu>

**Phase and Gain Response of a Closed-Loop-Control High Voltage DC-DC
Converter at Cryogenic Temperatures**

*Scott Gerber, ZIN Technologies, Malik Elbuluk, The University of Akron,
Richard Patterson, NASA Glenn Research Center, Ahmad Hammoud, QSS Group,
Inc., NASA Glenn Research Center, Cleveland, Ohio*

**Background**

The phase and gain response of a 1 kW, 80-110 V/500 V
closed-loop-controlled high-voltage full-bridge DC-to-DC converter was
evaluated at room temperature and at various temperatures down to ‑185
°C. The converter design was based on that of a beam power supply for a
space electric propulsion system, with the added capability of being
able to operate over a very wide, low-temperature range. This cryogenic
beam supply was previously tested for efficiency, output voltage
regulation, switching behavior and transient load testing. Recent
availability of a Venable frequency response system has made phase and
gain response testing of the supply a reality.

**Test Setup**

Testing of the cryogenic beam supply was performed in a liquid nitrogen
cooled chamber manufactured by SUN Systems. A Venable frequency response
system was used to measure phase and gain response over a frequency
range of 10 Hz to 1 MHz. Response measurements were recorded at chamber
temperatures of 25 °C, ‑55 °C, ‑100 °C, -150 °C, and -185 °C. Due to
limited availability of the frequency response system, initial testing
was limited to a single operating point of 100 V input voltage, 500 V
output voltage, and a 600 Ω load (half load ≈ 415 W).

**Results and Discussion**

The closed-loop controller was constructed from metal film resistors;
solid tantalum, npo, ceramic, and mica capacitors; and CMOS devices
(timer, logic, and operational amplifier). The controller utilized a
Type I compensation network. Previous testing utilized a 0.01 μF
compensation capacitor. After the initial response measurements showed a
phase margin of approximately 45º and a gain margin of 14 dB at 25 °C,
additional compensation was added by utilizing a 0.02 μF compensation
capacitor. Response measurements for this configuration showed a phase
margin of approximately 60º and a gain margin of 30 dB at 25 °C. Table 1
lists the measured phase and gain margins of the controller at each test
temperature. As the temperature was decreased to ‑100 °C, both phase and
gain margins were reduced to their minimum values of 45° and 16 dB,
respectively. As temperature was further decreased down to ‑185 °C, the
phase and gain margins returned to values close to those obtained at 25
°C. At all temperatures the controller maintained adequate phase and
gain margins for controller stability.

**Table 1.** Measured Phase and Gain Margins of the Controller at
Various Test Temperatures

  ----------------------- ----------------------- -----------------------
  **Temperature (°C)**    **Phase Margin (°)**    **Gain Margin (dB)**

  25                      60                      30

  -55                     60                      27

  -100                    45                      16

  -150                    48                      17

  -185                    53                      35
  ----------------------- ----------------------- -----------------------

The phase and gain vs. frequency plots for the controller at test
temperatures of 25 °C and ‑185 °C are shown in the full report.

**Conclusion**

A 1 kW, 80-110 V/550 V closed-loop controlled, full-bridge DC-to-DC
converter, designed to operate from 25 °C to ‑185 °C using commercially
available components, was evaluated in terms of its frequency response
over temperature. Testing was performed at a single nominal operating
point of 100 V input voltage, 500 V output voltage, and a 60 0Ω load
(half load ≈ 415 W). Phase and gain response measurements were made over
a frequency range of 10 Hz to 1 MHz. The converter's phase margin varied
from a maximum of 60° at 25 °C to a minimum of 45° at ‑100 °C, and the
gain margin varied from a maximum of 35 dB at ‑185 °C to a minimum of 16
dB at ‑100 °C. Measured phase and gain margins at ‑185 °C were similar
to those measured at 25 °C. At all temperatures the controller maintains
adequate phase and gain margins for controller stability. The results
from this work indicate that control circuits can be designed and
operated at very low temperatures well beyond normal operating
temperatures.

The full report, including the charts and schematics not shown here and
the references can be found by "Phase and Gain Response" in the search
box and clicking on "Search Alphabetically" at the Web site:
<http://nepp.nasa.gov/index_nasa.cfm/477/>. For more information about
low temperature testing of DC-to-DC converters contact Dr. Richard
Patterson, <Richard.L.Patterson@grc.nasa.gov>

**Vendor Links**

**International Rectifier:**
<http://www.irf.com/product-info/govt-space/dcdcconverters.html> and
<http://www.irf.com/product-info/govt-space/rrart2800.pdf>

**VPT, Inc.:**
[www.vpt-inc.com/space_power_products](http://www.vpt-inc.com/space_power_products)

**Crane Interpoint:**
[http://www.interpoint.com](http://www.interpoint.com/)

**Modular Devices Incorporated:**
[http://www.mdipower.com](http://www.mdipower.com/)

**Editorial Staff**

**NEPP Program Managers:**

Mike Sampson, NASA, GSFC, 301-286-3335,
<Michael.J.Sampson.1@gsfc.nasa.gov>

Ken Label, NASA, GSFC, 301-286-9936, <klabel@pop700.gsfc.nasa.gov>

**Editor:**

Jeannette Plante, Dynamic Range Corp., GSFC, 301-286-7437,
<jfplante@pop500.gsfc.nasa.gov>