bulk-cap

Remedy ¶

Add a bulk reservoir capacitor where the rail enters, sized from the load step the rail has to absorb and from the regulator’s own stability requirement.

What it means ¶

Every named power rail should carry at least one capacitor. A named power rail is a net that an author gave a power symbol or asserted with a power flag.

Why engineers want it ¶

Decoupling is per-pin; bulk capacitance is per-rail. A rail with no capacitance at all sags on every load step, and LDO datasheets require an output capacitor for loop stability. This is the aggregate per-rail complement to decoupling-present.

Impact ¶

Rail droop under transients, regulator instability, resets that only reproduce under real load patterns.

Scope note ¶

Rail identity is a power symbol on the net, a power flag, or a global net that also reads as a rail, not pin directions, so a rail feeding only passives is still covered. A global label alone is not enough, because many boards draw signals with global labels, and on three public boards those signals were most of this rule’s findings (agni issue 1252). decoupling-present quantifies over power-input pins instead, and the two deliberately overlap on a rail that has power pins and no caps. Ground nets are excluded, by role or by name where no role was stamped. Every rail’s capacitors end ON ground, so “ground has no bulk cap” is not a defect. Unresolved external nets are skipped (the cap may live in an unread sheet). Distinguishing bulk from local decoupling by value is a datasheet-joined refinement (WS10).

A capacitor across a fitted zero-ohm link counts as on the rail, because the link joins two names for one rail (agni issue 1015). A 0 Ohm resistor often splits a converter’s output node from the rail it feeds. The resistor sizes the current path or lets a bench measure it, and the capacitors sit on the converter’s side. A link counts when its value is zero ohms. That includes a library that wrote the value as its part name, such as R_0R_22.4A_0603, which ingestion reads as 0 Ohm (agni issue 1115). The walk never crosses into a ground, and a series resistor of any other value still ends the rail. A pass names the link beside the capacitor.

Query structure ¶

select rails by fact, require a capacitor member.

select N in nets where (power_symbol(N) or power_driven(N) or (global(N) and rail(N)))
  and not ground_name(N) and not rail_capacitor(N)

Reads: component.class, net.attributes (global, power_driven, external), net.names (the ground-name skip), on_net, pin.electrical_type (a power symbol’s virtual pin). Tier R.