fet-vdss-below-switched-rail

A MOSFET sits on a rail at or above its datasheet drain-source breakdown voltage.

Remedy

Fit a FET whose drain-source rating clears the rail with margin for switching overshoot, checking it against the rail’s transient peak rather than its nominal.

What it checks

A MOSFET sitting on a power rail whose voltage is at or above the part’s datasheet drain-source breakdown rating (VDSS). Both numbers are vendor values where the rail’s voltage comes from a datasheet, and the finding cites each one it used.

For hardware engineers

VDSS is the voltage a FET can block across drain and source while it is off. Past it the part stops being a switch. It avalanches, conducts when it is meant to block, and often fails short.

The short is what makes this worth an error rather than a warning. An open failure disconnects the load, which is visible and usually harmless. A shorted high-side switch hands the full rail straight to whatever it was protecting, so a part chosen to protect the load becomes the thing that destroys it. A 30V-rated FET on a 48V rail is not marginal; it is outside the envelope the vendor will stand behind.

Rating a switch above the rail is also not the whole story in a real design, since inductive kickback and hot-plug transients push the drain well above the nominal rail, and that is why designers derate. This rule only catches the case where the part is under-rated against the steady-state rail, which is the unambiguous half.

Where the rail voltage comes from, and why the message says so

Two sources, preferred in this order:

  1. A driving part’s datasheet output. If something on the rail declares an output voltage in its spec, that is a vendor value. It earns its own citation, and the review’s data-trust gate weighs it alongside the FET’s.
  2. The net’s name. +5V means 5V by convention. That is a design convention, not a document, so it gets no citation.

The message names which was used. 5V read from a datasheet and 5V inferred from a net name are not equally trustworthy, and a report that flattened them would be overstating what it knows.

Precision limit worth knowing

VDSS is a rating on the drain-source pair specifically, but the engine’s pin-role vocabulary has no drain or source, only power, ground, and the two diode terminals. So the rule cannot tell which of the FET’s nets actually sits across those terminals.

It therefore compares against every rail the part touches, and reports the highest. Rails are the right filter rather than every net: a gate-drive net is not a rail, so the obvious false pairing is excluded structurally rather than by luck.

The residual case is a gate deliberately tied to a rail (an always-on FET, or a P-FET gate pulled to its source). There the binding limit is VGSS, not VDSS, and VGSS is usually far lower, so the condition is typically still a defect, but this rule would name the wrong parameter for it. WS3-117 (FET pin roles in the naming lexicon) is what makes this exact.

For software engineers

A join between two projections that were read independently before: the part’s breakdown rows (FetBreakdownLimits) and the rail’s voltage, resolved from either a driving part’s OutputVoltageLimits or RailMaxVoltage’s name-derived nominal.

Where several rows are comparable the rule takes the lowest breakdown (a part is endangered at its weakest rating) against the highest rail voltage. Any other pairing under-reports.

DatasheetProv carries one citation or two depending on the rail’s evidence, and the plural citation field from WS3-028 exists for that: the data-trust gate rates a finding by its weakest citation, so a rail voltage from a low-confidence extraction correctly drags the whole finding to Provisional.

When it stays silent

  • No seeded datasheet set. The rule reads the params tier, so check.Available gates it to not-applicable without --params. Unevaluable, never clean.
  • The FET is unseeded, or its spec carries no breakdown row, so skip, not pass.
  • The rail’s voltage is unknown: no driving part declares an output and the name carries no voltage token. A rail named VSYS yields no number, and the rule does not guess one.
  • Ground. Excluded explicitly: it is a rail by the engine’s definition but carries no voltage to compare.

Fixing a finding

Either the FET is under-rated for where it sits, or it is on the wrong rail. Check the intent before swapping the part: the rule reports the FET because that is the part with the rating, but a switch wired onto a rail it was never meant to see is a netlist error, not a part-selection one.