This is just an Excerpt from a larger document, click here to view the entire document.Application of Derating Factors for Electronics
Derating guidelines (other than temperature) are typically stress ratios expressed as a percentage value. The stress ratio is the numeric ratio between the actual stresses determined from the circuit analysis divided by the stress rating of the part at the rated operating temperature.
To properly derate a specific part, a significant amount of detailed part construction information may be required. For example there are many different types of capacitors; Paper Film, Plastic Film, Mica, Glass, Ceramic, Tantalum Electrolytic, and Aluminum Electrolytic just to name a few. They are typically used in different types of applications and do not all exhibit the same failure modes and mechanisms. Individual construction differences can be significant. For example, the nature of one of the primary failure mechanism in aluminum electrolytic capacitors (i.e., breakdown of the dielectric by the electrolyte in the absence of applied voltage) is such that voltage derating by itself does not enhance the reliability of these parts. Similar technology difference examples could be given for many generic part types, especially solid state devices.
Derating guidelines can take a variety of forms. They are often published with different derating values for different environments. For example, the older military documents (References 6 and 7) typically gave three different sets of values in columns; one (the most severe derating) for a "Space" environment, a second (less severe) for "Aircraft" and a third (least severe) for "Ground." A more current military approach "Part Requirement & Application Guide" (SD-18) (Reference 1) still uses three columns but identifies the columns as "Protected", "Normal" and "Severe" environments. RIAC derating guidelines (see Reference 2 & 4) give two environment columns "Severe" and "Benign." The whole purpose of these multiple column approaches is to provide additional safety margins in critical applications or when the parts are subjected to extreme environmental conditions.
Transient conditions should also be taken into account. The derating analysis will not necessarily consider worst case conditions with regard to applied voltages or currents, part parameter values, or driving signals. However, when an undesirable stress condition is noted, worst case conditions should also be examined and the probability of worst case occurrence be further investigated.
To determine the derating conditions that should be applied to a specific product design, an analytical approximation, extracted from Reference 4, is included as Table 2. Table 2 has a set of factors that can be scored based on the challenge that the product design is expected to survive. For scores of eight or greater, the severe derating standards should be considered. Lesser scores indicate that the benign derating factors are appropriate.
Table 2. Part Derating Level Determination
For proven design, achievable with commercial parts/circuits
For high reliability requirements, special design features needed
For new design challenging the state-of-the-art, new concept
For easily accessible, quickly and economically repaired products
For high repair cost, limited access, high skill levels required, very low downtimes allowable
For non-accessible repair, or economically unjustifiable repairs
For routine safety program, no expected problems
For potential system or equipment high cost damage
For potential jeopardizing of life of personnel
For no significant design limitation, commercial practices
For special design features needed, difficult requirements
For new concepts needed, severe design limitation
For economical repairs, no unusual spare part costs expected
For potentially high repair cost or unique cost spares
For systems that may require complete substitution
Select score for each factor, sum the scores, and determine derating level or parameter.