Design Log DEC 064 ctl poe c17 final part selection kemet c0805c223k2racauto 22nf 200v x7r 0805 - thepinkmile/Enigma-NG GitHub Wiki

DEC-064 - CTL PoE C17 Final Part Selection: Kemet C0805C223K2RACAUTO (22nF 200V X7R 0805)

Field Value
Decision ID DEC-064
Status Confirmed
Date 2026-05-09
Author Izzyonstage & Copilot (session e39f3cc4)
Amends DEC-062 (C17 MPN "update pending" resolved); DR-CTL-18 package and voltage updated

Context

DEC-062 updated C17 value from 10nF to 22nF but deferred MPN selection. Worst-case Vclamp = Vin×D/(1−D) = 36×0.667/0.333 = 72V. A 100V-rated 22nF X7R MLCC in 0402 package was initially specified in DR-CTL-18; two problems emerged:

  1. Package constraint: 22nF at 100V+ cannot fit in 0402 - 0603 is the minimum practical package for this voltage/capacitance combination in X7R.
  2. DC bias derating: At 72V/100V = 72% of rating, X7R capacitance degrades by approximately 25-35%, leaving effective capacitance of only 15-18nF - significantly below the 22nF target and approaching the 19.9nF minimum from DR-CTL-18.

A 200V-rated 0805 part resolves both issues: at 72V/200V = 36% of rating, DC bias derating is only ~5%, giving effective capacitance of ~21nF - compliant with DR-CTL-18.

Decision

C17 selected: Kemet C0805C223K2RACAUTO (22nF, 200V, X7R, 0805, ±10%, AEC-Q200). DigiKey: 399-17630-1-ND; Mouser: 80-C0805C223K2RAUTO; JLCPCB: C3843023.

Package and voltage rating in DR-CTL-18 updated from 100V 0402 to 200V 0805.

The Open Mode (J-series, FO-CAP) variant (C0805J223K2RACAUTO) was considered - it provides fail-open behaviour as a reliability enhancement - but is not stocked at JLCPCB. The standard C-series hard-termination part (C0805C223K2RACAUTO) satisfies all electrical requirements; Open Mode is a desirable reliability enhancement but not an electrical necessity for this application.

Rationale

  • 200V rating eliminates meaningful DC bias derating and ensures the part operates within its characterised range at all PoE operating points.
  • 0805 package is a standard JLCPCB machine-placeable size with excellent availability.
  • Kemet C-series X7R is a well-characterised commodity MLCC; AEC-Q200 automotive qualification maintained.
  • JLCPCB LCSC number C3843023 confirmed as in-stock.

Files Changed

  • design/Electronics/Controller/Design_Spec.md: DR-CTL-18 package/voltage updated (100V 0402 → 200V 0805). BOM C17 row updated (TBD → C0805C223K2RACAUTO with supplier PNs and corrected spec).
  • design/Electronics/Consolidated_BOM.md: CTL C17 row updated (C0402C103K1RACAUTO → C0805C223K2RACAUTO with supplier PNs).