Optional DC Surge Filter PCB — GDT/MOV/TVS + Common-Mode Chokes

1) Purpose & Electrical Targets

Use case: Protect the 24 V DC output bus (to lights, radios, routers, etc.) against lightning-like surges, ESD, and fast transients conducted in on long leads — and keep high-frequency gunk from escaping the box. Designed to live at the enclosure wall, bonded to the internal bond bar, and feed the outside via your SB50 or similar.

Nominal system: 24 V LiFePO₄ bus (≈27.0–28.4 V when charging). Continuous current: 10–20 A (pick a choke size accordingly). Transient goal: improve survival for surges modeled on IEC 61000-4-5 combo-wave impulses (1.2/50 µs – 8/20 µs), not a formal certification.

Design philosophy: Let the GDT gulp big, fast energy to chassis; let MOV + TVS clamp what’s left line-to-line; add a common-mode choke so both conductors look “heavy” to RF; finish with shunt caps (to chassis and across the rails). Keep all surge returns short & wide into the bond bar.

2) Schematic (Text) & Topology

Order (outside → inside):

[Field harness] ──► ESD ring
│
├─► J1 (2-pin power in, +V_OUT / 0V_OUT)
│
├─► GDT (3-electrode: +→CHASSIS, –→CHASSIS, common chamber)
│
├─► MOV (low-V DC) from + to − // big differential clamp
│
├─► TVS (SMCJ33A) from + to − // fast differential clamp
│
├─► LCM (common-mode choke, 2-line, 12–20 A rating)
│
├─► C_Y1: +→CHASSIS (Y safety cap, few nF)
├─► C_Y2: −→CHASSIS (Y safety cap, few nF)
├─► C_DM_small: +↔− (0.1–1 µF MLCC)
├─► C_DM_bulk: +↔− (2.2–10 µF film or MLCC bank)
│
└─► J2 (2-pin power to “clean side” bus inside)

• GDT: 3-electrode so both rails share the same gas chamber (better symmetry, lower residual). Choose ≈90–150 V sparkover for a 24 V bus.
• MOV: low-voltage DC type with MCOV around 38–40 V DC so it sleeps at 28–29 V but clamps hard on surges.
• TVS: SMCJ33A (33 V standoff, ~53 V clamp) is a solid pick for 24 V rails; bump to SMCJ36A if your float runs hot.
• CMC: power-line common-mode choke sized for your continuous current (12–22 A families listed below).
• Y caps: real Class-Y safety capacitors from rails to chassis (2.2–10 nF each) to shunt common-mode noise with safe leakage.
• DM caps: 0.1–1.0 µF MLCC right after the choke, with 2.2–10 µF film/MLCC bulk as needed.

3) Bill of Materials (Example Parts)

Choose current ratings to match your loads. Through-hole surge parts handle heat and lead length gracefully; keep TVS/MOV loops very tight.

4) PCB Layout — The Non-Negotiables

• Segregate halves: everything before the CMC is DIRTY (field), after is CLEAN (internal bus). Mark the “fence” in silkscreen.
• Shortest return to chassis: pour a big CHASSIS copper zone to the mounting holes. GDT and any line-to-chassis shunts should dump into that pour with via-stitching.
• Creepage/clearance: for ~30 V DC in PD2, keep ≥1.0–1.5 mm on the clean side; add milled slots under GDT/MOV pads to elongate creepage.
• Place parts in path order: J1 → GDT → MOV → TVS → CMC → Y caps → DM caps → J2.
• Minimize loop area: put MOV and TVS pads shoulder-to-shoulder across the rails.

5) Example Netlist (Human-Readable)

NETS:
N_IN+ // field + from J1
N_IN- // field - from J1
N_OUT+ // clean + to J2
N_OUT- // clean - to J2
CHASSIS // copper pour to mounting holes / enclosure

DEVICES:
J1: 2-pin input terminal (field side)
J2: 2-pin output terminal (clean side)

GDT1: 3-electrode GDT, pins: L1(→N_IN+), L2(→N_IN-), G(→CHASSIS)

MOV1: MOV between N_IN+ and N_IN-
TVS1: TVS between N_IN+ and N_IN-

LCM1: common-mode choke, series in + and - between IN and OUT
pins: IN+→N_IN+, OUT+→N_OUT+, IN-→N_IN-, OUT-→N_OUT-

CY1: Y2 cap from N_OUT+ to CHASSIS
CY2: Y2 cap from N_OUT- to CHASSIS
CDM1: 0.1 µF MLCC from N_OUT+ to N_OUT-
CDM2: 1.0 µF MLCC from N_OUT+ to N_OUT-

6) Assembly Order (Bench-Friendly)

1. Mechanical first: mount holes in the kiosk’s steel panel so this PCB sits 2–5 cm from the cable gland/bulkhead. Scrape paint under bolt landings for metal-to-metal.
2. Solder SMD (TVS, SMD GDT, MLCCs) → Through-hole (MOV, CMC, Y caps, terminals). Keep GDT legs short; avoid stress.
3. Install braid/strap from the board’s CHASSIS pad to the kiosk bond bar — short & wide.
4. Torque terminals and tug-test.
5. Inspect creepage; add conformal coat between high-stress pads if dusty/humid (do not coat GDT vents).
6. Label edges: “DIRTY (field)” and “CLEAN (internal)”.

7) Bring-Up Tests (Quick but Meaningful)

• Continuity & polarity: IN/OUT not shorted; GDT open at rest (megaohms).
• Leakage: At 28.4 V, verify tiny Y-cap leakage (a few microamps with 2.2–10 nF parts).
• ESD pre-check: With an ESD gun, zap the field-side shell and pins (±8 kV). Watch the clean-side ripple — TVS should tame differential spikes; CMC+Y caps should squash common-mode.
• Surge sanity: If you can rent a generator, step up 250→500→1000 V combo-wave L-L, then L/− to chassis, and log clamp levels & temperatures.

8) Picking Exact Values (With Reasoning)

• GDT sparkover: 90–150 V. Lower fires earlier (lower residual), higher gives more margin against bus kicks.
• MOV MCOV: ~38–40 V DC so it sleeps at 28–29 V yet clamps hard.
• TVS: SMCJ33A for 24 V rails; SMCJ36A if float approaches ~29 V.
• CMC: 12 A class for small kiosks, 20–22 A class for heavier outputs; pick low DCR parts.
• Y caps: 2.2–10 nF Class Y2; 4.7 nF is a conservative default.
• DM caps: 0.1 µF + 1.0 µF X7R; add 2.2–4.7 µF film if low-kHz damping is needed.

9) Board Size, Stack-Up, & Thermal

• Board: ~100×60 mm, 2 oz copper both sides; fat pours for +/− rails.
• CMC thermal: choose low-DCR parts and leave air gap; datasheets show ΔT vs current.
• MOV life: surges age MOVs; use a 20 mm disk for harsher sites or plan replacements after big hits.

10) Mounting in the Kiosk (Shield-Conscious)

• Mount immediately inside the steel wall so the CHASSIS pad is 10–30 mm from a bonded bolt into the bond bar.
• Field harness enters via a metal gland; J1 faces that gland.
• Keep J2 leads to your internal busbars short.
• Panel feedthrough filters are the “pro” swap-in later — same wiring order, better HF insertion loss at the wall.

11) Full Example BOM (Ready to Shop)

Alternates: Bourns 10D/14D MOV families, Littelfuse TMOV (for AC mains), additional parallel TVS for extreme edges.

12) Validation Playbook

• Conducted EMI sniff: LISN or current probe from 150 kHz–30 MHz on the field cable; A/B with filter engaged.
• Surge ladder: 250 → 500 → 1000 V combo-wave L-L, then L/− to chassis; record clean-side peaks & MOV/GDT temps.
• Abuse test: Switch a chunky inductive load on the field side and log the clean side. TVS handles the kick; CMC+Y caps kill CM spray.

13) Why Each Piece Is the Right Tool

• 3-electrode GDT: big energy to chassis with low capacitance and symmetric firing.
• Low-V MOV + SMCJ TVS: MOV swallows energy; TVS cleans the fast edge.
• CMC + Y caps: classic way to crush common-mode noise while passing DC.
• Feedthrough vs PCB: panel feedthroughs give best HF performance; this PCB emulates that by bonding surge shunts right at the wall.

14) Fabrication Notes (So You Get Good Boards)

• Copper: 2 oz both sides; generous pours; wide necks into CMC pins.
• Slots: mill creepage slots under GDT/MOV toward chassis metal.
• Silk: mark current direction (DIRTY→CLEAN), testpoints TP_IN+/− and TP_OUT+/−, and a big CHASSIS symbol at the braid land.
• Finish: ENIG or thick HASL; keep TVS/MOV loops tight and low-inductance.

15) Step-by-Step Install in Your Existing Kiosk

1. Mount on M5 standoffs near the gland; one screw also clamps the braid straight to the bond bar.
2. Wire the field cable to J1 (DIRTY).
3. Wire the internal bus to J2 (CLEAN), short to the busbars.
4. Power up battery-only; flip a noisy test load on the field side while scoping the clean side.
5. Close the lid and confirm your usual “door-closed RF drop” — the shield still works with the board installed.

16) Practical Limits & Extensions

• This PCB massively improves conducted surge tolerance and conducted EMI. It is not a certification for IEC/EMC or HEMP — but it follows the proven stack: surge → clamp → filter → bond.
• Add series differential chokes after the CMC if you need more low-frequency damping.
• Parallel a second TVS or upsize the MOV to 20 mm for harsher sites.
• Future rev: swap to panel feedthrough EMI filters on the DC rails for even higher HF insertion loss.

17) Sources & Pointers

• Surge standard & waveforms: IEC 61000-4-5 overview.
• GDTs: Bourns 3-electrode families (2038 / GDT35 / 2052) datasheets & app notes.
• MOVs: Littelfuse LV UltraMOV low-voltage series; Bourns MOV selection guides.
• TVS: Littelfuse SMCJ33A / SMCJ36A datasheets.
• Common-mode chokes: Würth WE-CMB / WE-CMBHV families; Coilcraft & TDK selection guides.
• Y-class capacitors: TDK Y2 film capacitor datasheets; safety primers on leakage and ratings.
• Feedthrough filters: Knowles / Spectrum Control panel feedthrough notes on placement at the barrier.
• Creepage/clearance: IPC/industry primers (PD2, CTI, altitude notes) and PCB layout guidance.
• Trade-offs: TVS vs MOV vs GDT application summaries (e.g., TI surge protector selection notes).