Any port that leaves the enclosure will eventually meet a charged human, a charged cable, or an industrial surge. Protection is cheap; the consequences of skipping it are not — field returns from a single ESD event pattern can cost more than every protection component in the design. Here is how TVS diodes and ESD protection ICs work and how to choose them without wrecking signal integrity.
How a TVS Diode Works (in Two Regimes)
A transient voltage suppressor is a silicon avalanche diode that is invisible at normal operating voltage and becomes a low-impedance shunt when a transient pushes it past breakdown. Two numbers describe both regimes:
- Working voltage (VRWM): the highest continuous voltage at which the diode stays essentially out of the circuit. Choose VRWM above your signal's worst-case steady-state — including logic High, PoE voltages, or automotive load-dump excursions.
- Clamping response: at ESD currents, the diode's dynamic resistance (Rdyn) sets how far the voltage rises above breakdown. Lower Rdyn means less residual stress reaching the protected IC — this spec matters more than the classical "clamping voltage at 1 A" tables for modern ESD-rated parts.
The Five Parameters That Decide Fit
- Reverse working voltage — as above, with margin for tolerances and transients on your own rail.
- Capacitance — the silent killer of high-speed ports. A 3 pF diode across a 5 Gbps USB 3 lane is a signal-killer; flow-through protection ICs for SerDes run at 0.3 pF per line. Match capacitance to the data rate: I2C/CAN tolerate tens of pF; USB 2.0 wants <1 pF; HDMI/Ethernet magnetics have their own specialized parts.
- ESD rating per IEC 61000-4-2 — ±8 kV contact / ±15 kV air is the usual floor for external ports; note this is a robustness rating, not a data-sheet guarantee against every strike.
- Surge/power rating (PPPM, 8/20 µs): relevant for power inputs, automotive ports, and anything that can see induced lightning surge, not just ESD.
- Leakage current: matters for battery-powered and high-impedance measurement inputs — a "leaky" protection diode can ruin a µA-level design.
Port-by-Port Quick Guide
- USB-C / USB 3.x: dedicated flow-through protection ICs with <0.5 pF per line, placed at the connector, with the protection ground return directly to the connector ground.
- Ethernet: protection lives behind the magnetics on the PHY side (and surge-rated parts on the cable side where the environment demands).
- CAN / RS-485: bidirectional TVS sized above the common-mode range (e.g., ±24 V working) with surge capability; remember the bus is long and coupled surges are normal.
- Power inputs: higher-power TVS (SMB/SMC class) plus a series element (fuse, PTC, or ideal-diode) so the diode has something to survive with.
Placement and Layout: Half the Protection Is Geometry
A TVS diode protects only what it reaches before. Place it at the connector, not at the IC; route the transient's path (connector → diode → ground) short and wide, and keep the protected line away from the unprotected segment. ESD current finds the lowest-inductance path — if the IC offers one, it wins, whatever the schematic says.
JTDZ Tech stocks TVS arrays, ESD protection ICs, and surge suppressors for USB, Ethernet, automotive, and industrial ports — with capacitance and clamping curves in the datasheets and traceable date codes. Tell us which ports you are protecting and we will quote parts matched to your data rates.
