Stress Derating for Electronic Design
Derating, which can be performed manually and summarized in a spreadsheet format, is often a time consuming and laborious task for PCB designers, and eventually causes calculation errors. Derating, a technique of either reducing stress on a particular component, or increasing the strength of a component by replacing it with another which has a higher rated value, is best handled by fiXtress’s stress derating module.
This module uses the stress inserted by the user, in order to check if the components’ ‘rating’ values meet the derating criteria. The derating criteria can be defined in different tables for every type of component in an organized manner, based on the component’s temperature, and they include Standard, Military, Industrial and customized derating curves.
The component’s stress is required to be in the green area, rather than in the yellow or red area.
The stress parameters used include Power, Voltage, Current and temperature. Over-stressed and over-designed components are reported, and a recommendation is offered for optimal rating. In other words, the applied value (power, voltage, current) of a particular component is multiplied by its derated factor value, which also depends on temperature. In the case that, after derating, the ‘applied value’ exceeds the maximum rating assigned to it, the stress derating analysis will offer a recommendation of what needs to be done to reduce the stress on the component. This can include either replacing the component with a superior one (increasing the rating value), relocating the component on the PCB to a cooler area, and if the component is temperature-related, possibly adding a fan or heat-sink in order to reduce the temperature.
For example, below you can see the stress parameters of a resistor. The maximum power, as defined by the component datasheet is 5W, and although the applied value is 4W, the derating curve states that it needs to be 51.7% of the maximum rating.This will allow dissipating only 2.5862W. If we divide 4W with 2.5862W we will arrive at the conclusion that the component is overstressed by 154.7%. Since this constitutes 80% of the rating, the software will mark the relevant row with an asterisk, indicating that this is a critical item that needs to be corrected. If the power dissipation is above 100% of the rating, two asterisks will be used.