Electronic components derating (or de-rating) is the operation of a component at less than its rated maximum capability in order to prolong its life. The Derated parameter takes into account the case/body temperature, the ambient temperature and the type of cooling mechanism used.
Derating increases the margin of safety between the component’s rated values (design limits) and the actual applied stresses, thereby enhancing the reliability for the part under real life conditions. By applying derating to a component, its degradation rate, throughout its life, is reduced and the reliability and life expectancy are improved.
Intuitively, if a component or a system is operated under its rated values (design limit), it will be more reliable than if it is operated at or above the rated values.
A simple derating example can be seen in figure-1:
The purple line represents the component’s manufacturer maximum allowed stress for the parameter across component’s case temperature.
The black line represents the derated stress factor (The factor that the maximum vendor parameter rating is multiplied with to get the derated value) recommended by the derating standard across component’s case temperature.
In the example we can see that up to T.s1 temperature, the deration factor is constant and equals 0.50.
Beyond T.s1 and up to 150ᵒC the stress factor is degraded linearly from 0.50 to Zero.
T.s2 is the max. case temperature allowed by the standard for the component. Hence, the stress factor is clamped to Zero beyond T.s2.
We can see that at 25ᵒC the stress factor is 0.50. So, if for example, the rated value is 50V, the derated value shall be: 0.50 x 50V = 25V.
At 95ᵒC, the stress factor is degraded to 0.25. So, in our example the derated value shall be: 0.25 x 50V = 12.5V.
If we need a component to work under a voltage of 12.5V at a temperature of 95ᵒC, we should select a component rated at 50V (at least).
Several derating guidelines are in common use by military or other agencies, such as:
· MIL-STD-975, published by NASA
· MIL-STD-1547, published by the Department of Defense
· NAVSEA SD-18, published by the U.S. Navy
· ECSS-Q-30-11A, prepared and maintained under the authority of the Space Components Steering Board in partnership with the European Space Agency
· MSFC-STD-3012, prepared by NASA's Marshall Space Flight Center
In addition to the derating standards published by the military, many semiconductor manufacturers provide their own derating guidelines in the component’s datasheets.
Note: For the same component, the derating parameters in different standards may vary.