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Derating for Electronic
Components
Derating is a technique usually employed in electrical power
and electronic devices, wherein the devices are operated at less
than their rated maximum power dissipation, taking into account
the case/body temperature, the ambient temperature and the type
of cooling mechanism used. In this article, we will briefly
explain the theoretical background of derating and how it is
applied.
Derating increases the margin of safety between part design
limits and applied stresses, thereby providing extra protection
for the part. By applying derating in an electrical or
electronic component, its degradation rate is reduced. The
reliability and life expectancy are improved.
Intuitively, if a component or system is operated under its
design limit, it will be more reliable than if it is operated at
or above the design limit. Theoretically, the benefit of
derating can be explained using load-strength interference
theory.
Load-Strength Interference
Usually, failure happens when the applied load exceeds the
strength. Load and strength should be considered in a general
way. For electronic parts, "load" might refer to voltage, power
or an internal stress such as junction temperature. "Strength"
might refer to any resisting physical properties.
Electronic components of a given type are not identical. They
have strength variability. This variability results from the
differences between raw materials and between manufacturing
processes. Even for components made from the same materials and
by the same processes, differences still exist due to noise
factors such as microscopic material defects or variations
within a single manufacturing process. Therefore, the strength
of a component is considered to be a random variable. The load
applied to electronic parts, such as power, temperature or
humidity, is also a random variable. Thus, statistical
distributions are usually used to describe the load and
strength.
Two factors are used to analyze the interference of load and
strength distributions. These factors are safety margin (SM) and
loading roughness (LR).[1]
Safety margin is defined by:
where
L
and
S
are the mean values of the load and strength distributions, and
σL
and
σS
are the standard deviations of the load and strength
distributions. SM is the relative separation of the mean values
of load and strength.
Loading roughness is defined in
terms of the standard deviation of the load by:
Figures 1-3 show three examples of different relationships
between safety margin and loading roughness.
Load and strength distributions that are non-overlapping, as
shown in Figure 1, indicate a highly reliable scenario, with
narrow distributions, low loading roughness and a large safety
margin.
Figure 1: Non-overlapping load and strength distributions
with high SM and low LR
Figure 2 illustrates low loading roughness and low safety
margin. In this situation, extreme load will cause failures of
weak parts.
Figure 2: Load and strength distributions with low SM and low
LR
Figure 3 is the situation with low safety margin and high
loading roughness. This is an undesirable case because an
extreme stress will cause a large proportion of the population
to fail.
Figure 3: Load and strength distributions with Low SM and
High LR
From the above figures, we can see that there are several
possible ways to improve the population reliability: by
increasing the safety margin or by decreasing the width of the
load and/or strength distribution. Often, increasing the safety
margin by increasing the mean strength is expensive; thus,
curtailing the load distribution may be preferable because of
low expense. Derating is actually one method of load
distribution curtailing.
Derating Standards
Several derating guidelines have been issued by military or
other agencies, and the following are some examples:
- MIL-STD-975, published by NASA, focuses on selection of
parts used in the design and construction of space flight
hardware as well as mission-essential ground support
equipment.
- MIL-STD-1547, published by the Department of Defense, is
targeted to aid in the design, development and fabrication
of electronic systems with long life and/or high reliability
requirements while operating under the extreme conditions of
space and launch vehicles.
- AS4613, published by the U.S. Navy, sets forth derating
requirements for the reliable application of electronic and
electromechanical parts.
- NAVSEA TE000-AB-GTP-010, published by the U.S. Navy,
contains derating requirements and part selection and
application information on the ten most commonly used
electrical and electronic parts.
- ECSS-Q-30-11A, prepared and maintained under the
authority of the Space Components Steering Board in
partnership with the European Space Agency, contains
derating requirements applicable to electronic, electrical
and electromechanical components.
- MSFC-STD-3012, prepared by NASA's Marshall Space Flight
Center, sets requirements for electrical, electronic and
electromechanical parts selection, management and control
for space flight and mission-essential ground support
equipment for Marshall Space Flight Center programs.
In addition to the derating standards published by the military,
many semiconductor manufacturers such as Freescale and Hitachi
provide their own derating guidelines, which can be explored in
the datasheets or application notes of electronic parts.
For the same component, the derating parameters in different
standards may be different. An example using MIL-STD-975M for a
resistor is given below. The software package used is
ReliaSoft's Lambda
Predict.
Example
Table 1 gives the properties of the resistor and the derating
specifications. Figure 4 is the derating output from Lambda
Predict.
Table 1: MIL-STD 975M derating specs for resistor, fixed
carbon
|
Standard |
MIL-STD 975M |
|
Component |
Resistor, fixed carbon
|
|
Parameter |
Power |
|
Derating Specs |
60% up to 70C
and further decreasing to 0 at 110C |
Figure 4: Lambda Predict 2 Derating Example for Resistor,
Fixed Carbon
In Figure 4, the green line is the derating curve given by
MIL-STD-975M. The blue dot shows the resistors working
temperature and power ratio under the defined working condition.
For the case shown, the resistor is properly derated since the
power ratio is below the derating line for the resistors working
temperature. However, if the dot were above the derating curve,
the stress under the working condition would not meet the
derating requirement given by MIL-STD-975M. This would imply
that the resistor would be likely to fail; therefore, changing
to a higher-rated resistor or adjusting the operating condition
might be needed.
References
[1] OConnor, Patrick D. T., Practical
Reliability Engineering, 4th ed., West Sussex, UK: John
Wiley & Sons, 2002.
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