Honeywell_Trainee_Project_Report

Stress, De-rating and Reliability Prediction Library
for Aerospace Avionics Products
Anandhavel Nagendrakumar (1040910019)
Department of Electronics and Communication Engineering,
SRM University, Chennai
Abstract — The design and proper working of an aerospace
avionic product is always critical in the proper functioning of the
aircraft and the safety of the passengers on board. Hence each and
every electrical part must be “reliable” in all conditions. Every
part in an electrical design is subjected to a worst-case part stress
analysis performed at the anticipated part temperature
experienced during the assembly qualification test (typically 75*C).
Every part must meet the project stress de-rating requirements or
be accepted by a formal project waiver. Part failure rates are
proportional to their applied electrical and thermal stresses. By
predicting the stress through analysis, and applying conservative
stresses, the probability of mission success can be greatly
enhanced. Electrical circuits are analyzed to determine the
maximum stress on each part when all applied voltages or
currents are maximized and when all variations of other parts in
the circuit are set to that combination of minimum and maximum
values that produce worst-case maximum stress.
Index Terms — Electrical reliability, Stress De-rating, Parts
Stress Analysis (PSA), Reliability Prediction, Avionics.

I. INTRODUCTION
Reliability engineering is an engineering field that deals with
the study, evaluation, and life-cycle management of reliability:
the ability of a system or component to perform its required
functions under stated conditions for a specified period of
time. Reliability engineering is a sub-discipline within systems
engineering.
Reliability
is theoretically defined
as
the probability of failure, the frequency of failures, or in terms
of availability and maintainability. My project aims at
performing Part Stress Analysis (PSA), simulation in LTspice
and prediction using Relex on common functional blocks. To
start off the project, 3-4 Honeywell products were taken and
closely examined. The “isolated power supplies” were found to
be common across the products with little or no change. All
aircraft products have many sub-systems that use a similar range
of input voltage supplies. Microprocessors always require 1.9V,
driver circuits use 15V, etc. So all products have a DC-DC
converter from 28V (generated in all aircrafts) to different
voltage ranges such as 15V, 3.3V, 1.09V, etc.

(Pulse Width Modulation) controller to give 2 outputs, 15V and
-10V. The input to this circuit is 15V.
II. WORKING OF THE IDENTIFIED CIRCUIT
The circuit has an input of +15V. The maximum possible
supply can be +15.25V. The inductors, L5 and L6 are directly
connected to source and ground respectively. The inductors
remove any electromagnetic interference in the supply as well
as oppose any minute change in current. Hence this creates a
stable supply current to the circuit. Capacitors C13 and C14
form a filter to remove any high frequency components in the
supply. The most important component of the circuit is the
Texas Instruments manufactured IC UC2825A-EP. This is a
dual output PWM controller which alternatively turns on its
output ports in the frequency defined by the external timer
resistor and capacitor pair. The schottkey diodes D23, D24, D27
and D28 are used to regulate the voltages passing on to the load.
The MOSFETS Q8 and Q7 are NMOS gate drivers. They are
triggered at the gate by the outputs from the PWM controller. So,
Q8 and Q7 alternatively turn on. Both the drain pins are
connected to the Honeywell proprietary transformer along with
the filtered supply voltage of +15V. Capacitors C17, C18 and
C30 act as the filters in this scenario. When Q8 is turned on,
both the transformers step down the +15VDC to +7.5V AC. The
lower Schottkey diodes D35 and D36 rectify the AC to give
+7.5V DC. When Q7 is turned on, both the transformers step
down the +15VDC to +8.8V AC. The upper Schottkey diodes
D29 and D30 rectify the AC to give +8.8V DC.

The main function of the identified circuit is to deliver
power to the generator’s excitation winding during Start Mode,
which energizes the excitation winding of the generator to create
a magnetic field enabling the generator to behave as a motor. I
have taken the power supply which is controlled by a PWM

Fig. 2.1 – Full Circuit drawn in Pspice

1

III. PARTS STRESS ANALYSIS
Electrical Part Stress Analysis (PSA) depends is calculated
on each component on different parameters. The component
must pass the criteria in all the parameters. Similarly in a given
circuit, all components must individually satisfy the criteria.
PSA is generally calculated as follows,
Stress = Applied value of parameter/ Rated value of parameter.
If a component has a stress value above its de-rating percentage,
it is highlighted and sent back to the design board for better
stress enduring designs. The PSA was calculated for all the
components. Every component passed the de-rating criteria.

Fig. 4.1 – Full Circuit using LTC3721-1 in LTspice

The output is exactly the same as how the original circuit
might have behaved. One problem with this model though, is
that the current and voltage parameters of each component will
vary from the actual circuit. This is due to the change in IC
which may draw more or less current as compared to
UC2825A_EP. So, this schematic should only be used for
analyzing the outputs. Another change in the circuit is the use of
100Ω resistors R1017 and R1018. They are not needed by the
LTC3721-1, but needed for LTspice. The coils L2 and L8 form
a parallel inductive pair. In practical situations, the wire would
carry some resistance. But in the simulation a pure inductive
loop is formed. Hence a pair of 100Ω resistors is used to remove
the pure inductance. If a 10Ω resistance is used, the output
voltage in the 2nd transformer will follow the output voltage in
the 1st transformer very closely. But the output will be delayed
by a few milliseconds. To get an instantaneous output, a 100Ω
pair is used. This results in a small loss in the output of the
second transformer compared to the first.

Table 3.1 – PSA for capacitors

IV. SIMULATION AND RESULTS
LTspice IV is freeware computer software implementing a
SPICE simulator of electronic circuits, produced by
semiconductor manufacturer Linear Technology (LTC).
LTspice IV provides a schematic capture and waveform viewer
with enhancements and models to speed the simulation of
switching regulators.
LTspice IV is node-unlimited and 3rd party models can be
imported. Circuit simulations based on transient, AC, noise and
DC analysis can be plotted as well as Fourier analysis. Heat
dissipation of components can be calculated and efficiency
reports can also be generated. LTspice IV is used within LTC,
and by many users in fields including radio-frequency
electronics, power electronics, digital electronics, and other
disciplines.
An IC which is not entirely similar to the UC2825A_EP but
has the required dual output is used. The components required
for the operation of the 2825 series will not be present in the
new circuit. Additional components required for the proper
functioning LTC3721-1 will be used. For easy distinguishing,
the new components will be labeled with reference designator
numbers greater than thousand.

Fig. 4.2 – Voltages at both the ends of the Primary coils of the transformer

Fig 4.3 – Rectified outputs at upper (GREEN) and lower (BLUE) test points

2

ΠN = No-defect process factor
ΠW = Wearout process factor

V. RELIABILITY PREDICTION
A prediction of reliability is an important element in the
process of selecting equipment for use by buyers of electronic
equipment. Reliability is a measure of the frequency of
equipment failures as a function of time. Reliability has a major
impact on maintenance and repair costs and on the continuity of
service.
Relex is a reliability software that helps in faster, efficient
and a more graphical way of getting the reliability information.
Various standards can be applied to the circuits in question. The
outputs obtained can be further processed for other uses. Relex,
has been recently acquired by PTC, and functionality integrated
into Windchill.
For any company interested in predicting field reliability
performance, finding a prediction technique that provides a high
degree of fidelity to observed field data is essential. With the
discontinuance of military handbook Mil-Hdbk-217, reliability
prediction of electronic equipment and the limited
environmental applications of Telcordia SR-332, reliability
prediction for electronic equipment, Reliability Analysis
Center’s (RAC) PRISM is a favored software tool as an
improved methodology in predicting the field reliability of
aviation systems. The PRISM tool is not as strict as the MIL217. It is an excellent tool for commercial avionics reliability
prediction. But flux dependent components like inductors and
transformers have to adhere to the MIL-217 standard.

B. Calculations and Result
Verifying the MIL- Standard prediction above,
The factors required are,
πT , Thermal Factor
πQ, Quality Factor
πE, Environmental Factor
λb, Base Failure rate
λp=λb*πT*πQ*πE Failures / 106 Hours
For Ground Profile,
From MIL-HDBK-217
transformer,

Handbook,

for

our

required

λb= 0.022 for low power pulse tranmsformer
πQ= 1 for MIL - spec
πE= 1 for ground benign
Hot Spot temperature can be estimated as follows:
THS = TA + 1.1 (ΔT)

(2)

where:
THS = Hot Spot Temperature ("C)
TA = Inductive Device Ambient Operating Temperature ("C)
ΔT = Average Temperature Rise Above Ambient ("C)
TA = 40°C; ΔT = 0°C

A. Process Grade Factor
The reliability prediction of a particular component also has
to take into account the process followed in brining that
component into the circuit. This is a model which takes into
various factors like Parts process factor, Design process factor,
Manufacturing process factor, System Management process
factor, Induced process factor, No-defect process factor and
Wear-out process factor. Each process factor has a set of
questions and a certain weight age for each question. The total
points are calculated to bring about the Π factor for each process.
Relex has these questions in built and automatically calculates
the PGF for prediction. Given below is the calculation model for
failure rate using process grade factor,

Therefore, THS = 40 + 1.1 (0) = 40°C

πT = exp(

(

-

))

(3)

Ta
ΔT

40

T_HS+273

313

1/(T_HS+273)

0.003194888

1/(T_HS+273) - 1/298

-0.00016082

Multiply by -0.11

1.76898E-05

Divide by 8.617 ×10^(-5)

0.205289742

Therefore, πT

1.227880782

πQ

1

πE

1

λb

Where, λp = Predicted failure rate of the system
λIA = Initial assessment of the failure rate. This failure rate is
based on new component failure rate
Each of the following model factors represents a failure
cause:
ΠP = Parts process factor
ΠD = Design process factor
ΠM = Manufacturing process factor
ΠS = System Management process factor
ΠI = Induced process factor

0

Ths

λP= λIA (ΠP*ΠIM*ΠE+ ΠD*ΠG+ ΠM*ΠIM*ΠE*ΠG+ ΠS*ΠG+ ΠI+
ΠN+ ΠW) + λSW
(1)

40

0.022
0.027013377

3

Failure rate Transformer =

λb

0.022

Failure rate Transformer =
For Inductor,

For, inductor,

πQ

πE
λb
Failure rate Inductor =

3.68E-05
0.0541

0.000221

Total Failure rate =

0.00003

0.00003

Failure rate Inductor =

1

6

λb

1

1

πE

πQ

Total Failure rate =
Now, for airborne profile.
For transformer,

0.162080263

0.324603

Hence, the software calculations and theoretical calculations
coincide.

πQ

1

πE

6
Table 5.1 – Failure rate of all profiles

Operating Profile
Standard used

Ground

Airborne

PRISM

Failure rate in
fpmh
Total Failure Rate
in fmph
Total MTBF in
Hrs

MIL-STD-HDBK

PRISM

MIL-STD-HDBK

0.13542900

0.05414100

0.18645700

0.32656200

0.18957000

0.51301900

5275096

1949246
my project with them and providing me a sense of the corporate
environment. I express my sincerest gratitude to the entire
Reliability, Maintainability, System, Safety (RMSS) team which
is a part of the Mechanical Center Of Excellence (MCOE) under
the Aerospace Strategical Business Unit (AERO SBU) for their
support and help throughout the project.

VI. CONCLUSION
A parts stress analysis library of 2000+ components has
been created. An estimation showed that the Reliability,
Maintainability, System, Safety (RMSS) team saved 15hrs per
100 components in doing PSA using the PSA library. A very
commonly used circuit – isolated power supply, which is used
across many products, has been simulated in 2 different models.
One model is used for measuring the parametric values of the
components. But it splits the circuit for simulation into 2
different simulation platforms. The other model simulates the
entire circuit with a different IC. It can be used for analyzing the
outputs. The manual calculations and simulated values have
been compared. No design changes has been suggested as each
and every component in the circuit is required for the proper
functioning of the circuit. No extra component is required by the
circuit to make it function better.. Reliability prediction has been
done for the given circuit and found it to be very reliable. All
the objectives of the project have been met.

REFERENCES
[1] G Rene L. Bierbaum, Thomas D. Brown, and Thomas J.
Kerschen,
“Model-Based
Reliability Analysis”,
IEEE
Transactions on Reliability, VOL. 51, NO. 2, JUNE 2002.
[2] Sherif Yacoub, Bojan Cukic, and Hany H. Ammar, “A ScenarioBased Reliability Analysis Approach for Component-Based
Software”, Transactions on Reliability, VOL. 53, NO. 4, DEC
2004.
[3] Huairui Guo and Haitao Liao, “Methods of Reliability
Demonstration Testing and Their Relationships,” IEEE
Transactions on Reliability, VOL. 61, NO. 1, MAR 2012.

ACKNOWLEDGMENT
I take this opportunity to thank each and everyone who is
responsible in making this project possible. I hereby express my
sincere gratitude to the Faculty and Management of SRM
University, Chennai, for their kind encouragement bestowed up
on me to complete this project. I deeply thank the management
and the employees of Honeywell Technology Solutions Lab,
Bangalore for giving me the wonderful opportunity to complete

4

Honeywell_Trainee_Project_Report

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