Inrush & EMI Control

Derive the maximum slope from I_inrush,max = C_load·dV/dt, then add 10–20% margin for cold starts. Map the slope to gate R/C or controller current limit and verify PG timing. Finally, sweep VIN and temperature to ensure UV/OV filters do not chatter during the soft-start ramp under worst-case cable and load conditions.

Gate R/C sets a predictable dV/dt and low EMI but can lengthen startup and stress upstream UV filters. Current-limit mode clamps I_inrush regardless of C_load, yet raises MOSFET dissipation and thermal risk. Use gate shaping for moderate capacitance; prefer current-limit for large or variable C_load. Avoid combining aggressive limits that trap the device in foldback.

Probe the switch node with a short ground spring and run FFT to find the dominant mode during transition. Confirm with a near-field H-probe over the loop. Capture both steady-state and hot-plug events. Use f_r to target R·C ≈ 1/(2π·f_c) with f_c ≈ 0.5–0.7·f_r, then validate energy and thermal limits at the intended event rate.

Begin with ±3–6% depth at 10–40 kHz modulation. This usually lowers peak emissions without significant phase-margin erosion. Always re-run Bode plots and load-step tests because modulation smears energy and can degrade gain/phase near crossover. If margin narrows below target, reduce depth or shift the modulation rate away from compensation poles or sampling notches.

Place the RC snubber directly between the switch node and its quiet return, minimizing loop area and via count. Keep it on the same layer as the MOSFET and catch diode, away from signal traces. Avoid sharing the return via with high di/dt currents. Short, wide traces beat long narrow ones; verify improvement with near-field scans.

Spread-spectrum reduces spectral peaks but does not dissipate localized ring energy. High-C_load hot-plug often excites a hard resonance that needs real damping. Use SS to lower emissions and an RC snubber to absorb energy at the switch node. Validate by comparing peak amplitude, ring decay time, and temperature rise across representative plug/unplug cycles.

Cap the compensation (±% window), reduce loop gain at light load, and filter sense leads with a small RC close to the controller. Add a fail-safe clamp so open or reversed sense does not drive overshoot. Test with load steps, open-sense injection, and long harnesses at temperature extremes to confirm stability and overshoot limits.

Use copper’s temperature coefficient (≈0.0039/°C) to bound resistance drift and add SKU tolerance from supplier data. A practical starting window is ±15% when SKUs or ambient vary widely. Validate by sweeping current across −20/25/85 °C with the longest harness, logging load-end voltage error. Tighten limits once production measurements show narrower dispersion.

Build a mini-matrix: C_load (10–470 µF) × cables (0.5–3 m). Capture I_pk, 10–90% rise, PG order, and switch-node FFT with SS on/off and snubber before/after. Accept only configurations showing I_pk ≤ budget and ≥6 dB peak reduction. Archive CSV and screenshots to seed chamber runs and speed failure triage.

Gate firmware states that disable snubbers, expand compensation windows, or turn off spread-spectrum behind PG conditions. Require a passing near-field pre-scan signature before releasing PG to the system. Log FAULT if SS depth or limiter settings are out of policy. This prevents shipping images that raise emissions or destabilize the power path.

Use proxies: availability of an external C_ss pin, programmable gate-current sources/sinks, and hot-swap controllers with defined fault timers. App notes describing inrush waveforms are strong evidence. If dV/dt is not specified, measure with a standard C_load and cable set. Favor devices whose gate-drive current and fault handling stay within thermal limits.

Log timestamp, VIN, Vout, I_pk, 10–90% rise, ringing frequency, SS depth/rate, snubber state, PG/FAULT, pass/fail, operator, and temperature. Attach scope screenshots and raw waveforms for edge cases. Use consistent units and naming so replacements compare directly across brands. This enables regression dashboards and traceable sign-offs during audits.