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failure modes description.
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@ -103,15 +103,19 @@ allowing re-use of modules and reducing the number of by-hand analysis checks to
\subsection{A detailed look at failure symptoms of two components: the op-amp and the resistor} \subsection{Determining the failure modes of components}
In order to apply any form of Failure Mode Effects Analysis (FMEA) we need to know the ways in which the components we are using can fail. In order to apply any form of Failure Mode Effects Analysis (FMEA) we need to know the ways in which the components we are using can fail.
Typically when choosing components for a design, we look at manufacturers data sheets,
which describe the range and tolerances, and can indicate how a component may fail/behave
under certain conditions or environments.
How base components could fail internally, its not of interest to an FMEA investigation. How base components could fail internally, its not of interest to an FMEA investigation.
The FMEA investigator needs to know what failure behaviour a component may exhibit, or in other words, its The FMEA investigator needs to know what failure behaviour a component may exhibit, or in other words, its
modes of failure. modes of failure.
A large body of literature exists which gives guidance for for determining component {\fms}. A large body of literature exists which gives guidance for for determining component {\fms}.
For this study FMD-91~\cite{fmd91} and the gas burner standard EN298~\cite{en298}. %
For this study FMD-91~\cite{fmd91} and the gas burner standard EN298~\cite{en298} are examined.
%Some standards prescribe specific failure modes for generic component types. %Some standards prescribe specific failure modes for generic component types.
In EN298 failure modes for generic component types are prescribed, or In EN298 failure modes for generic component types are prescribed, or
determined by a procedure where failure scenarios of all pins OPEN and all adjacent pins shorted determined by a procedure where failure scenarios of all pins OPEN and all adjacent pins shorted
@ -122,7 +126,7 @@ FMD-91 is a reference document released into the public domain by the United Sta
and describes {\fms} of common electronic components. and describes {\fms} of common electronic components.
FMD-91 entries include descriptions of internal failures along with {\fms}. FMD-91 entries include descriptions of internal failures along with {\fms}.
FMD-91 entries need, in some cases, some interpretation to be mapped to a clear set of FMD-91 entries need, in some cases, some interpretation to be mapped to a clear set of
component failure modes. component {\fms} suitable for use in FMEA.
% One is from the US military document FMD-91, where internal failures % One is from the US military document FMD-91, where internal failures
@ -141,8 +145,8 @@ component failure modes.
% I hope to have chapter 5 finished by the end of March, chapter 5 being the % I hope to have chapter 5 finished by the end of March, chapter 5 being the
% electronics examples for the FMMD methodology. % electronics examples for the FMMD methodology.
We look in detail at two common electrical components in this section and examine how In this section we look in detail at two common electrical components and examine how
two sources of information on failure modes view their failure mode behaviour. the two sources of information define their failure mode behaviour.
We look at the reasons why some known failure modes are omitted, or presented in We look at the reasons why some known failure modes are omitted, or presented in
specific but unintuitive ways. specific but unintuitive ways.
%We compare the US. military published failure mode specifications wi %We compare the US. military published failure mode specifications wi
@ -150,16 +154,17 @@ specific but unintuitive ways.
We then compare and contrast the failure modes determined for these components We then compare and contrast the failure modes determined for these components
from the FMD-91 reference source and from the guidelines of the from the FMD-91 reference source and from the guidelines of the
European burner standard EN298. European burner standard EN298.
\subsection{Failure mode determination for generic resistor}
%- Failure modes. Prescribed failure modes EN298 - FMD91 %- Failure modes. Prescribed failure modes EN298 - FMD91
\subsubsection{Resistor failure modes according to FMD-91}
\subsection{resistor} The resistor is a ubiquitous component in electronics, and is therefore a prime
example for examining its failure modes.
The resistor is a ubiquitous component in electronics, and is there fore a good
example for examining it failure modes.
FMD-91\cite{fmd91}[3-178] lists many types of resistor FMD-91\cite{fmd91}[3-178] lists many types of resistor
and lists many possible failure causes. and lists many possible failure causes.
For instance for {\textbf Resistor,~Fixed,~Film} we are given the following failure causes: For instance for {\textbf{Resistor,~Fixed,~Film}} we are given the following failure causes:
\begin{itemize} \begin{itemize}
\item Opened 52\% \item Opened 52\%
\item Drift 31.8\% \item Drift 31.8\%
@ -172,7 +177,7 @@ This information may be of insterest to the manufacturer of resistors, but it do
help a circuit designer. help a circuit designer.
The circuit designer is not interested in the causes of resistor failure, but to build in contingecy The circuit designer is not interested in the causes of resistor failure, but to build in contingecy
against symptoms of failure that the resistor could exhibit. against symptoms of failure that the resistor could exhibit.
We can determine these symptoms and map these failure causes to three symptoms, We can determine these {\fms} and map these failure causes to three symptoms,
drift (resistance value changing), open and short. drift (resistance value changing), open and short.
\begin{itemize} \begin{itemize}
@ -189,6 +194,8 @@ modes do not include drift.
If we can ensure that our resistors will not be exposed to overload conditions, drift or parameter change If we can ensure that our resistors will not be exposed to overload conditions, drift or parameter change
can be reasonably excluded. can be reasonably excluded.
\subsubsection{Resistor failure modes according to EN298}
EN298 ,the European gas burner safety standard,tends to be give more symptom centric failure modes than FMD-91, EN298 ,the European gas burner safety standard,tends to be give more symptom centric failure modes than FMD-91,
and requires that a full FMEA be undertaken, examining all characterisic failure modes and requires that a full FMEA be undertaken, examining all characterisic failure modes
of all components~\cite{en298}[11.2 5]. of all components~\cite{en298}[11.2 5].
@ -226,15 +233,24 @@ i.e.
$$ fm(R) = \{ OPEN, SHORT \} . $$ $$ fm(R) = \{ OPEN, SHORT \} . $$
\subsection{op-amp} \subsection{Failure modes determination for generic OP-AMP}
The op-amp is a differential amplifier and is very widely used. \begin{figure}[h+]
\centering
\includegraphics[width=200pt]{./lm258pinout.jpg}
% lm258pinout.jpg: 478x348 pixel, 96dpi, 12.65x9.21 cm, bb=0 0 359 261
\caption{Pinout for an LM358 dual OP-AMP}
\label{fig:lm258}
\end{figure}
The op-amp is a differential amplifier and is very widely used in nearly all fields of modern electronics.
They are typically packaged in dual or quad configurations---meaning They are typically packaged in dual or quad configurations---meaning
that a chip will typically contain two or four amplifiers. that a chip will typically contain two or four amplifiers.
For the purpose of example, we look at For the purpose of example, we look at
a typical op-amp designed for instrumentation and measurement, the dual packaged version of the LM358~\cite{lm358}. a typical op-amp designed for instrumentation and measurement, the dual packaged version of the LM358~\cite{lm358}
(see figure~\ref{fig:lm258}).
\subsubsection{FMD-91 Op-AMP Failure Modes} \subsubsection{ Failure Modes of an OP-AMP according to FMD-91 }
%Literature suggests, latch up, latch down and oscillation. %Literature suggests, latch up, latch down and oscillation.
For OP-AMP failures modes, FMD-91\cite{fmd91}{3-116] states, For OP-AMP failures modes, FMD-91\cite{fmd91}{3-116] states,
@ -244,10 +260,10 @@ For OP-AMP failures modes, FMD-91\cite{fmd91}{3-116] states,
\item Opened $V_+$ open\% \item Opened $V_+$ open\%
\end{itemize} \end{itemize}
These are internal causes of failure, more of interest to the component manufacter Again these are mostly internal causes of failure, more of interest to the component manufacturer
than a designer looking for the symptoms of failure. than a designer looking for the symptoms of failure.
We need to translate these failure causes within the OP-AMP into symptoms. We need to translate these failure causes within the OP-AMP into {\fms}.
We can look at each failure cause in turn. We can look at each failure cause in turn, and map it to potential {\fms}.
\paragraph{OP-AMP failure cause: Poor Die attach} \paragraph{OP-AMP failure cause: Poor Die attach}
The symptom for this is given as a low slew rate. This means that the op-amp The symptom for this is given as a low slew rate. This means that the op-amp
@ -255,12 +271,12 @@ will not react quickly to changes on its input terminals.
This is a failure symptom that may not be of concern in a slow responding system like an This is a failure symptom that may not be of concern in a slow responding system like an
instrumentation amplifier. However, where higher frequencies are being processed instrumentation amplifier. However, where higher frequencies are being processed
a signal may be lost. a signal may be lost.
We can map this failure cause to a failure symptom, and we can call it $LOW_{slew}$. We can map this failure cause to a {\fm}, and we can call it $LOW_{slew}$.
\paragraph{No Operation - over stress} \paragraph{No Operation - over stress}
Here the OP\_AMP has been damaged, and the output may be held HIGH LOW, or may be effectively tri-stated Here the OP\_AMP has been damaged, and the output may be held HIGH LOW, or may be effectively tri-stated
, i.e. not able to drive circuitry in along the next stages of te signal path: we can call theis state NOOP (no Operation). , i.e. not able to drive circuitry in along the next stages of the signal path: we can call this state NOOP (no Operation).
%
We can map this failure cause to three symptoms, $LOW$, $HIGH$, $NOOP$. We can map this failure cause to three symptoms, $LOW$, $HIGH$, $NOOP$.
\paragraph{Shorted $V_+$ to $V_-$} \paragraph{Shorted $V_+$ to $V_-$}
@ -273,10 +289,11 @@ This failure cause will mean that the minus input will have the very high gain
of the OP-AMP applied to it, and the output will be forced HIGH or LOW. of the OP-AMP applied to it, and the output will be forced HIGH or LOW.
We map this failure cause to $HIGH$ or $LOW$. We map this failure cause to $HIGH$ or $LOW$.
\paragraph{Collecting failure symptoms from FMD-91} \paragraph{Collecting OP-AMP failure modes from FMD-91}
We can define an OP-AMP, under FMD-91 definitions to have the following failure mode symptoms. We can define an OP-AMP, under FMD-91 definitions to have the following {\fms}.
$$fm(OP-AMP) = \{ HIGH, LOW, NOOP, LOW_{slew} \} $$ $$fm(OP-AMP) = \{ HIGH, LOW, NOOP, LOW_{slew} \} $$
\subsubsection{Failure Modes of an OP-AMP according to EN298}
EN298 does not specifically define OP\_AMPS failure modes; these can be determined EN298 does not specifically define OP\_AMPS failure modes; these can be determined
by following a procedure for `integrated~circuits' outlined in by following a procedure for `integrated~circuits' outlined in
@ -286,13 +303,6 @@ We can examine these failure modes by taking a typical instrumentation op-amp, s
and examining and examining
these conditions. these conditions.
\begin{figure}
\centering
\includegraphics[width=200pt]{./lm258pinout.jpg}
% lm258pinout.jpg: 478x348 pixel, 96dpi, 12.65x9.21 cm, bb=0 0 359 261
\caption{Pinout for an LM358 dual OP-AMP}
\label{fig:lm258}
\end{figure}