DC-DC Converter Portal
Guide to Designers and EEE Part Engineers for the
Application of DC-DC Converters in Space Systems
This portal is published and maintained for and by NASA
GSFC for internal use and as a guide to electrical designers and EEE parts
engineers. The information in this
portal is for information only and is not intended to be a substitute for
contractual or programmatic requirements documentation. Questions about or inputs to this site should
be addressed to Jeannette Plante.
Extended Explanation
of Application Notes:
2. Filtering
3. Front End Oscillations – Changes in Vin and Zin
4. Synchronization and Beat Frequency
5. Thermal and Mechanical Packaging Design
6. Optocouplers
8. Testing to the Application Conditions
9. Establishing the Reliability for a Production Lot of DC-DC Converters
10. Government vs. Manufacturer Certification
11. Preventing Internal Packaging Defects
12. Rectifier Diode Testing In Situ
13. Application Scenarios Can Affect Radiation Tolerance of Internal Elements
14. Manufacturing Process Changes During Lot Production
16. Use of Single Lot Date Codes
17. Limits to the use of “Heritage” Qualification Data
18. Avoiding Damaging Feedback Signals
19. Floating Case
Supporting Pages:
Tally of problems and failures encountered for DC/DC Converters used in space programs
2. Packaging and Workmanship: Examples and Case Studies
3. DC/DC Converter Usage Database
(Account required – this link automatically shows you login screen and how to apply for an account. Accounts allowed for NASA and NASA contractors only).
4. DC/DC Converter Experts for Project Support
5. Article with background on DC/DC Converter issues (a primer)
6. DC/DC Converter Technology Evaluations by JAXA (link to presentation) and ESA.
7. Comparison of DC/DC Converter Needs throughout the Aerospace Industry
8. Military Specification Information for DC/DC Converters
Summary:
- The value given by a datasheet for Efficiency may be for conditions which do not apply to the user’s application (Vin, Vout, Load).
- Use of a converter outside of their nominal or design range (Vin, Vout, Load) can result in unpredictable or unstable performance.
Discussion:
Though often
recorded as a singular value on datasheets, Efficiency actually varies
significantly with output power, rapidly decreasing for output power below the
design’s optimum level. Input voltage
also effects efficiency with low input voltages giving better efficiency
numbers. The curve that describes how
the Efficiency depends on input voltage, output power, and the value of the
optimum output power, may or may not be published in the datasheet (See Figure
1.) Further, the converters are not
normally characterized for output voltages in regions where Efficiency is very
low (i.e. 10%); for high output power
Figure 1. Efficiency specifications from a typical datasheet
styles, this region can span 10’s of
2. Filtering
Summary:
- EMI filters are important for protecting the power bus from radiated noise generated by high frequency MOSFET switching inside of the converter.
- EMI filters must be designed (or selected) to match the applicable converter.
- The manufacturer should be consulted to be sure that external filters are selected and applied properly with their converter.
- When an EMI filter uses a common-mode choke, the input and output current windings should be separated from each other to prevent a short circuit between the two if the wire insulation is compromised.
Discussion:
High switching frequencies in switched
mode converters are the source of several types of noise, which can negatively affect
the circuit within the hybrid package as well as the system outside of the
package. EMI filters are commonly
employed both at the input and output stages of DC/DC converters to reduce
noise effects and for stability. To
maintain the effectiveness of these filters however, they must be placed as close to the converter as possible.
EMI filters
are designed by manufacturers for use with one converter. While some manufacturers may state that
filters may be shared among more than one converter, the filter sharing can
lead (and has lead) to converter
oscillations. Extensive analysis is required before multiple converters are
paralleled together.
Datasheets for
DC/DC converters without internal input filters may or may not adequately
instruct the user about how to design and apply filtering. Discuss filtering with the manufacturer and
ensure that the parts are characterized within the intended filter environment,
addressing circuit and system conditions, to avoid creating damaging
configurations (high noise, damaging oscillations at the input, and reduced
efficiency).
Damaged wire
insulation has been found to lead to shorting between the two windings of
common-mode chokes used in input filters.
Sufficient separation of these two wires
will remove this risk which has lead to blown fuses and the disabling of all
other hardware connected to the blown fuse.
Back to top
More on locating the filter
close to the converter: Code 562 has found that
designers have on occasion misperceived that the EMI filter’s function is to
eliminate random noise from entering the converter at the input. Though this may be true, more importantly it
is used to keep large switching noise, present in the front end of the
converter from propagating to the power bus, both directly and via radiation
(EMI). Therefore the design (or
selection) of the filter is closely tied to the converter style it is to be
used with. To further ensure good
filtering, the filter/converter pair must be physically close together so that
an EMI radiation source is not created between the two. Back to top
More on wire-to-wire shorts
in the common-mode choke:
One vendor
uses a common mode choke (also known as a balin) in the input filter in which
the current-feed passes through one winding and the current-return passes
through the other winding. If these two
windings are in contact, then their insulation is the only barrier to a short
circuit. Three instances of failure were
encountered between the years 2000 and 2003, due to a short between the two
windings of the common mode choke in the input filter. This failure resulted in a very low ohmic
short (nominally one ohm) which will interrupt a fuse disabling the converter
and all other systems attached to that fuse.
A well space separation of these windings can remove this potential failure
mode. Back to top
3. Front End
Oscillations – Changes in Vin
and Zin
Summary:
- System conditions can cause damaging front end oscillation in DC-DC converters both due to extremes in input voltage (high and low) and due to changes in input impedance.
- Input impedance can change significantly due to system conditions. Ranges of input impedance should be considered when bench testing to establish acceptable worst case conditions.
Discussion:
Oscillations
can occur at the input to the DC-DC converter due to the negative input
impedance of the regulator circuit. The
DC-DC converters are constant output power devices, so when the input supply
voltage is high (as when the solar cells are charging the batteries) less
current is required to supply the load, and when the input supply voltage is
low (when the spacecraft is in the shadow half of the orbit and drawing from
the batteries) more current is required.
This is the definition of negative input impedance and it is a characteristic
of the DC-DC converters. If appropriate
values of capacitance and inductance (input filter and connecting PCB trace)
are connected to the input terminal of the DC-DC converter, an oscillator will
be created. These oscillations can
produce voltage levels and heat that can damage the internal components of the
DC-DC converter.
Bench testing
the DC-DC converter may not be sufficient to guarantee that it will function
properly in the application environment because the impedance of the test
fixture may not be identical to the impedance of the application. In particular, fixtures with long power
supply leads will provide additional inductance, which may produce oscillations
or even eliminate the oscillations. Load
conditions can also change when subsystems have gone off-line for replacement
or repair, or due to failure.
With regard to
variable bus loads, there are limits to the amount of current and voltage
regulation that
Also See: Helpful
discussion about design features which address negative resistance conditions.
4.
Synchronization and Beat Frequency
Summary:
- DC-DC converters used in parallel can generate a “beat frequency” if they are not correctly synchronized and power sequenced.
- The proper configuration for synchronizing parallel converters is to assign one as a “teacher” and the others as “students”, the “students” having their synch pins connected to the oscillator output pin of the “teacher”. The “teacher” unit must be then turned on before the “students”.
Discussion:
DC/DC
converters have an internal clock to control the switching circuitry; these
clocks are designed to run at close to the same frequency in similar DC/DC
converters. In practice, the frequencies
differ slightly and may drift. If power
requirements call for two or more DC/DC converters to be connected in parallel,
these frequencies will heterodyne and produce noise at the sum and difference
of the oscillator frequencies, and their harmonics. This noise is very difficult to
eliminate. The preferred method for
paralleling the DC/DC converters is to use the “teacher – student” option. This is accomplished by choosing one as the
“teacher” and feeding its internal oscillator output to the “sync” terminals of
the “students.” The “teacher” DC/DC
converter should be turned on first, and after the oscillator has stabilized, then the “students” should be turned on. If a “student” unit’s oscillator has a chance
to compete with the “teacher” the resulting signal can cause damage to the
MOSFET. Care must be taken when
paralleling converters to insure proper current sharing. Back to top
5. Thermal and
Mechanical Packaging Design
Summary:
- Innovations used to provide improved heat dissipation may or may not be appropriate for space use.
- Innovations used to provide improved heat dissipation may affect the package’s mechanical performance.
Discussion:
Thermal
management will become more critical as the DC/DC converter packages continue
to shrink while maintaining or increasing their power ratings. Thermal analysis is critical and necessary at
the part and board level to assure that the individual elements and assembly
materials will not be overheated or aged prematurely. Manufacturers are already addressing this
issue by introducing new packaging materials and heat spreaders into their
products. Careful examination of these
new design features must be done to confirm that they actually perform the
function for which they are intended and that no other negative effects are
generated.
Workmanship
and poor packaging design has also lead to many failures of these devices. Very close attention to material selection
and assembly through pre-cap visual and traveler review is critical. Back to top
More on packaging and workmanship problems including examples.
Summary:
- Some styles of DC-DC converters use optocouplers in the feedback loop for stage isolation and others styles use a transformer.
- The double-heterojunction LEDs used in the optocouplers are preferred over the amphoterically-doped type for spacecraft applications due to their resistance to displacement radiation damage from proton and neutron exposure.
Discussion:
Some DC-DC
converter designs use optocouplers for isolation. The feedback loop in many converters uses an
optocoupler to feed information across the isolation barrier. There are currently two types of optocouplers
available on the market. The division is
based on the LED manufacturing technology, amphoterically-doped and
double-heterojunction LEDs. The double-heterojunction
LEDs are preferred for spacecraft applications because recent publications
indicate a factor of ten to one hundred improvement in
resistance to displacement radiation damage from protons and neutron
exposure. Refer to the radhome.gsfa.nasa.gov
website for detailed information.
Transformers can also be used to bridge the isolation barrier, and avoid
the problems encountered with using optocouplers. Back to top
Summary:
- Supporting and protecting circuitry required in all power supply applications, that are not included within the DC-DC converter hybrid, can significantly reduce the miniaturization advantages the project may have assumed when they chose to use the hybrid devices, over a discrete design.
Discussion:
Though
hybridized DC-DC converters are often brought to a design for their perceived
advantages of miniaturization and relatively low non-recurring engineering
costs, careful examination of the application needs might prove this to be a
faulty assumption. Additional filtering
and thermal management needs may result in the adding of components and
hardware around the hybrid converter and increasing the overall board volume
and mass. Back to top
8.
Testing to the Application Conditions
Summary:
- Testing by the manufacturer may not be comprehensive because the manufacturer will probably not attend to application worst-case conditions (if a source control drawing is not used), including radiation and thermal-vacuum conditions.
- Users must characterize candidate units to be sure that they perform adequately and are stable under worst case and nominal application conditions.
- If application conditions (nominal and worst case) change over the life cycle of the mission, characterization and acceptance test data should be reviewed to confirm that the part is still considered qualified given the new conditions.
- Characterization and acceptance test conditions, used by the designer, should be documented to support later analysis by the project or research by users who might be leveraging off of prior use of the part.
- Electrical testing to a source controlled specification is a good strategy for confirming receipt of compliant devices. Use of generic (to the datasheet) testing for acceptance, rather than to application specific criteria can be done only under special conditions.
- When acceptance testing is done by the manufacturer using databook conditions it is highly recommended that the designer confirm that the flight model parts perform correctly across the required application range.
Discussion:
As discussed
in Note 1 above, the manufacturer’s datasheet may or
may not explicitly guide the user about the behaviors that can be expected from
a particular DC-DC converter, given the variety of circuit conditions that can
be applied to the part. It also may not
warn the user about the consequences of, or indications of, misapplication
thereby hampering troubleshooting or placing collateral hardware at risk. It is then important to fully characterize
candidate devices in the application in which they are intended to function and
to develop detailed and performance verification specifications. By conducting this type of analysis and
recording the requirements, the types of tests to use, the special conditions
used by the test (temperatures, times, voltages, currents), and the
accept/reject criteria used to determine failure, can
be applied appropriately and completely.
Writing a
source control drawing is a method by which individual organizations (NASA,
NASA GSFC, Projects, Companies) can define their own requirements, which may
not be included in the datasheet (procurement, testing, performance,
traceability, reliability, shipping, etc.), and can be used as leverage to
reject delivery of non-conforming product.
Source control drawings are useful for specifying electrical test
conditions and criteria that are not included in the datasheet.
Cost
and lead time constraints will
impact a project’s ability to utilize source control drawings in their
procurements. The project may not have
the performance criteria completely defined by the time that the procurement
activity must be started (12 – 24 months between start of review and delivery
of product is not unusual for this part type) and manufacturers have been known
to add cost charges for changing their electrical test programs from databook
settings. In this rather common
scenario, styles whose datasheets provide a sufficiently large performance
range are characterized by the power supply designer. If the converter is found to be satisfactory
in the design, the electrical behavior of the part in the application is
considered to be “correlated” to the electrical testing done by the
manufacturer using the databook values.
If the design and construction can be controlled enough to produce
physically identical and high quality parts in subsequent lots, the original
“correlation” becomes the basis for acceptance testing done by the manufacturer
using databook conditions rather than using application conditions, which may
be different. It is still highly
recommended that the designer confirm that each of the parts intended for use
in the flight hardware be tested throughout the range of application conditions
including environments that may not be considered by the manufacturer such as
vacuum. (Also See Lid Flexing
Failure in Vacuum)
The features
that the parts engineer attempts to control and the tests that they impose to
ensure traceability between the characterized units and the flight units
include the following:
- Element
Traceability. All devices, by part type, manufacturer
and revision, are identical between the prototype and the flight units, and from unit to unit within a production
lot. This especially applies to
devices which are chosen for radiation hardness.
- Element
Qualification. All internal devices are screened and
qualified (and preferably bought) to the nearest applicable military
specification and are made by qualified sources (QPL or QML). DPA is performed by the user on finished
lots to confirm that undesirable parts and packaging methods have not been
used.
- Internal
Packaging Design. Substrate and assembly materials are
space grade and have been applied in configurations that preclude failure
in rugged environments. This
includes adequate spacing, stress relief in bonds and wires, efficient and
clean use of epoxies and staking, judicious placement of components,
adequate separation of choke windings. Also, see MIL-STD-883,
Method 2010 and MIL-STD-750,
Methods 2072 and 2073. Continually
review process travelers to identify possible problems with the internal
packaging design or processing.
- External
Package Review. Confirm that the package material, lead
feedthroughs and plating are all suitable for space use and that the thermal design is valid for space.
- Change
Notification. Confirm that any manufacturing process
changes are reported and can be evaluated for impact. Confirm that rework is controlled and
does not create multiple versions with a single lot date code. Continually review travelers to look for
process changes.
- Radiation
Testing. Confirm that the radiation performance
is sufficient given the die/revision being used. This includes Single Event Effect
evaluation as well as Total Ionizing Dose evaluation.
- Non-destructive
Testing (Screening). Ensure that non-destructive testing used
to “weed out” weak or defective units from the delivery lot, is performed
per NASA guidelines.
- Destructive
Testing (Qualification). Ensure that destructive testing used to
demonstrate performance margins in relevant environments, is performed per
NASA guidelines. Back to top
9. Establishing
the Reliability for a Production
Summary:
- There are no practical methods available to NASA at this time to independently and accurately establish the reliability (i.e. failure rate, life expectancy, MTBF, etc.) of any of the commercially available DC-DC converters.
Discussion:
Parts
Engineers use a wide variety of techniques to provide insight and oversight for
DC-DC converter purchases for flight hardware.
The majority of the requirements negotiated with and imposed on the
vendors attend to packaging and part selection concerns in order that safety
margins are designed in for the individual internal components and the
mechanical features (different types of joints, platings etc.) to improve the
long term performance of the part in rugged environments (temperature,
temperature cycling, vibration, ionizing radiation, etc.). Though extended life in these rugged
environments is a goal, the ability to quantify that lifetime is not generally
available because hundreds to thousands of individual parts would be needed to
run the destructive testing which determines at what times the infant mortality
region of the life curve ends, normal lifetime begins and then the wearout
region begins. Though 1000 hour life
tests on relatively small samples is often done to demonstrate relative
longevity of a particular style, design or build lot, this test will not
provide a number which describes reliability if sufficient number of samples
are not tested. Therefore, greater
attention may be made to quality assurance efforts and traceability analyses,
in order to strengthen the case for using a part for which reliability cannot
be quantified. Back to top
10. Government
vs Manufacturer Certification
Summary:
- Part numbers that are not listed on QML-38534 may not be sold with all of the quality controls and test data required by NASA projects.
Discussion:
The hybrid
military specification family that can be used to buy a number of catalog parts
that are preferred for space use is MIL-PRF-38534. Not every catalog part number can be bought
with the requisite quality controls applied, even if they are made by a
manufacturer who is certified to mark some
of their parts with military part numbers.
(Further, not every MIL-PRF-38534 part number may correspond with the
level and quality of reliability required by a particular NASA program). Datasheets and advertising materials must be
studied carefully when one purchases DC-DC converters that are indicated as
high quality, high reliability, “military grade”, “space grade”, “QML-like” or
“Class K Equivalent” but are not traceable to mil-spec qualification and
screening. Use of a source controlled
drawing or specification is highly recommended when procuring converters that
are not military qualified to a sufficiently high level (or at all), to be sure
that all tests, inspections and controls are applied. When characterization indicates application
conditions that are highly variable from the nominal conditions in the
datasheet or military slash sheet, use of a source control drawing is again recommended
to impose and record the additional requirements or the different accept/reject
criteria. Procedure 562-PG-8700.2.7
Preparation of S311 Specifications and Standards provides the boilerplate
for NASA GSFC source control drawings for EEE Parts. Follow this link for an
extended explanation of the MIL-PRF-38534 specification.
11.
Preventing Internal Packaging Defects
Summary:
- Internal packaging defects are a leading cause of DC-DC converter failures. These can be avoided through the use of multiple inspections before the lid is installed and by understanding the processes to be used through manufacturing traveler reviews.
Discussion:
DC-DC
converters are hybrid devices and by definition are made using various assembly
processes involving a variety of organic and non-organic materials which
require various processing conditions.
Given the current state-of-the-art, it is still very common to find
workmanship-related defects in build lots, regardless of the supplier. A pre-cap inspection per MIL-STD-883,
Method 2017 is recommended in all cases.
Test Method 2017 is specific to Hybrid Microcircuits and
element-specific visual inspection methods are called out within 2017 as
follows:
·
MIL-STD-883, TM2010: Internal Visual (Monolithic),
Test Method Standard for Microcircuits.
·
MIL-STD-750D,
TM 2072.6: Visual Inspection (Transistor), Test Method Standard for
Semiconductor Devices.
·
MIL-STD-750D, TM
2073: Visual Inspection (Semiconductor), Test Method Standard for
Semiconductor Devices.
·
MIL-STD-883, TM
2032.2: Visual Inspection of passive elements, Test Method Standard for
Microcircuits.
The build
travelers should be reviewed during pre-cap inspection to verify that
sufficient instructions were provided to ensure a high quality assembly.
12. Rectifier Diode Testing In Situ
Summary:
- Particular rectifier diodes must be tested in the converter circuit using low level loading (≤10%) to screen units with a unique latent failure mode. This test must be performed during the manufacturing process to provide the circuit conditions and to allow for replacement of the diode, if it fails, before final lidding.
Discussion:
Though the
military specification requires screening of all internal components
(elements), experience has shown that particular rectifier diodes used in
snubber circuitry can have a latent failure mode that is only detectable when
installed in the circuit. To avoid this
failure, manufacturers have employed special testing steps and replacement of
defective diodes prior to lidding. The
snubber circuit is inserted to protect a switching device from overvoltage due
to accumulated energy in the wiring inductance of the circuit, when the switching
device is turned off. The worst case
application condition is when the DC-DC converter is operating at 10% or less
of its rated loading; in this case, the duty cycle of the switching circuit is
so small that the accumulated energy cannot be effectively removed unless the
snubber circuit is designed to address this low power operation condition. The robustness of the switching circuit and
snubber circuit can be tested by exercising the DC-DC converter with low loading
conditions. Back to top
13. Application
Scenarios Can Affect Radiation Tolerance of Internal Elements
Summary:
- Careful design of the electrical conditions used when performing radiation tolerance testing is required to capture anomalous behavior that may not be predicted or experienced for nominal conditions.
Discussion:
The radiation
tolerance of individual active components has been found to vary over
electrical and thermal application conditions.
Because the variety of application conditions can be so numerous for
analog integrated circuits, published radiation tolerance data must be reviewed
for applicability to the part as it is being used in the converter. Testing the parts while they operate in the
actual converter of interest is the best way to predict the overall systems
performance in a radiation environment.
Experience has shown that non-catastrophic, anomalous behavior within
the circuit can result, or be manifested, in a reduction in radiation tolerance
in one or more of the active components because the operating conditions of
those components has been changed.
Also See Floating Case. Back to top
14. Manufacturing Process Changes During
Summary:
- Production travelers must be reviewed and close oversight must be practiced when procuring DC-DC converters to ensure that significant changes to the production process are not made either breaking traceability between the characterized prototype lot or between individuals with a single lot date code.
Discussion:
Several
problem alerts published in the last decade have addressed problems that
occurred following a manufacturer’s departure from its qualified manufacturing
process. These changes may or may not
have been recorded for later review and impact assessment. Always obtain copies of the build travelers
for the lot that is being procured for review during pre-cap visual inspection
and throughout the procurement, to confirm that the instructions match the item
actually produced. Changes in materials
and internal elements can have a significant impact on the part’s in-service
performance and life expectancy. Also,
carefully review manufacturer’s build travelers for any rework performed and
documented. Back to top
Summary:
- It is critical to be aware of supply chain conditions such as plant transfers and internal element obsolescence, in order to assess risk to on-time delivery.
Discussion:
Supply chain
concerns can have significant impact on converter design and construction. Supply chain issues can be difficult to
identify and assess. Examples of well
recognized supply chain issues are:
a. Factory move to a
new location affecting the consistency and quality of the product
b. Loss of a part or
material supplier resulting in a substitution, without the benefit of a
re-qualification (see Note 14 above).
c. Supplier mergers
and acquisitions that result in retention of two manufacturing lines and one
workforce. (The workforce knowledgeable
about how to run the acquired line is not retained and the retained workforce
does not have sufficient knowledge about how to run the new line.)
d. Large high volume,
commercial orders occupying the production line, block or corrupt the
manufacturing flow of the separate, smaller NASA orders (when one production line is used for both flight and
commercial parts.)
These issues
can be researched through conversations with the supplier, consulting the trade
press, industry and NASA information websites, and through some distributor’s
parts databases. Back to top
16.
Use of Single
Summary:
- Use of single lot date codes is important for assuring homogeneous performance (and reliability) throughout the flight lot. The single lot date code standard applies to internal elements and materials as well as the completed units.
Discussion:
Throughout
industry, several definitions exist for a single lot date code. For NASA’s purposes, lot date codes are used
to identify units that contain the same elements, were made using identical
processes, and were side-by-side during all manufacturing processes. We also prefer that internal elements
(semiconductor dice, capacitors, resistors, coils, etc.) are of a singular lot
date code among their counterparts in a single lot of finished DC/DC
converters. This enables NASA to isolate
and analyze problem or suspect units.
Experience has shown, however, that a single lot date code has been used
for parts that were processed together, as well as for parts taken from that
“batch” and reprocessed in a different way on a different day. When problems arose with the reprocessed
parts, it was difficult to identify which ones were the reworked items. Careful attention to lot code designation
with the manufacturer and use of serial numbering is highly recommended. Back to top
17.
Limits to Using “Heritage” as a Substitution for
Summary:
- DC-DC converters do not fall into the category of parts which can be qualified by similarity.
- A new Source Control Drawing is valid via its pass/fail criteria and test plan for parts shown to meet those requirements. Parts which have new or additional requirements must have new part numbers or a new specification which describes the additional performance requirements.
Discussion:
It is common
practice to use qualification data accumulated for a particular part lot as a
substitute for qualification data for a later lot, if the two part designs are
identical or sufficiently similar, and if the calendar time between the build
date of the qualified lot and the candidate lot is less than two years. This practice is reasonable in an environment
where production practices and lot control are static and highly
repeatable. This is not the current environment
of the DC-DC converter industry, and so this practice should not be applied to
qualifying DC-DC converters for space use.
Qualification by similarity or heritage is not permitted. Similarly, a part that has already been
described in the S-311 specification system may be used in such a different way
in a second application that a new, unique specification is needed to address
the new electrical performance requirements.
This requires the DC-DC converter be re-qualified and appropriate
changes made to the specification to add additional part numbers and the
additional performance or test requirements that correspond with those new part
numbers. Back to top
18. Filtering
High Frequency Feedback
Summary:
- Delayed high frequency components in the feedback signal can create positive feedback causing noise at the output and possibly damage to the converter due to overheating.
- The manufacturer’s design must be checked to be sure that high frequency signals are eliminated from the feedback loop by doing the following: Generate a Bode plot for each converter at the minimum, nominal and high loads expected for the application (or using the converter datasheet limits), with the input voltages at nominal, low and high levels. The phase margin should be greater than 45 degrees and the gain margin should be greater than 12dB.
Discussion:
The output of
the DC-DC converter is stabilized by negative feedback. The finite delay caused by the length of the
feedback path may cause the high frequency components of the feedback signal to
appear in phase, in effect becoming positive feedback. The presence of this positive feedback
produces high frequency noise on the output and overheating of the internal
components. The DC-DC converter should
be characterized by the manufacturer to verify that the high frequency
harmonics are eliminated from the feedback loop. Bode plotting should be performed for each
converter at the minimum, nominal and high loads expected for the converter,
with the input voltages at nominal, low and high levels. The phase margin should be greater than 45
degrees and the gain margin should be greater than 12dB. Back to top
Summary:
- Manufacturer application notes do not always provide failure scenarios when disallowed configurations are used.
Discussion:
Ground pins on a DC-DC converter were left floating in a flight
hardware build 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). Back to top