Derating

Running a part below the limit its datasheet states, by a factor the design picks rather than one the vendor prints. A 10 V rail with a 1.25 derate factor asks for a capacitor rated 12.5 V, not 10 V.

Neither number in that comparison is exact. A rail sits a few percent off its nominal value, a supply overshoots on a load step, a part’s rating was characterised at 25°C and falls as the board warms, and both sides drift with age. Sitting exactly on the rating means any one of those ordinary movements takes the design out of spec.

It is constantly confused with the absolute maximum rating, and the two sit at opposite ends of the same axis. An absolute maximum is the vendor’s ceiling, printed in the document, and a fact about the part. A derate factor is the designer’s policy about how far below a limit to sit. Nothing in a datasheet states it, and two teams reading the same page can derate differently and both be right.

The factor also depends on what is being derated. Around 20% is the common convention for a ceramic capacitor’s voltage. Sizing that same capacitor for stable capacitance under DC bias usually wants 2x or more, which is a different concern with the same shape, and one no rule in this catalog checks yet.

Rules that read it. cap-voltage compares a capacitor’s rated voltage against its worst rail times a fixed 1.25, and states the arithmetic in the finding so the factor is visible rather than assumed. intent-rail-current-margin does the same for current with a factor you declare, and leaves the harder failure to intent-rail-current-capacity, because a supply that cannot meet the peak at all is one defect rather than two. fet-vdss-below-switched-rail covers the transient case, where inductive kick and hot-plug push a drain well above the nominal rail.

Derating only makes sense against a limit the vendor actually stated, so a rule derates from a transcribed parameter with its citation attached, never from a number somebody remembered.

Where the course teaches it: The numbers in chapter 3 introduces the derate factor on a capacitor’s voltage rating, and chapter 7 is the datasheet contract a factor is applied to.