What is a bidirectional surge protection diode and how does it work?

Bidirectional TVS diodes clamp in both directions. What 600 W and 130 W parts really specify, and how standoff, clamping and capacitance decide the part.

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What is a bidirectional surge protection diode and how does it work?

A bidirectional surge protection diode clamps in both directions, and that symmetry is the whole point: it protects a line without quietly rectifying it. The distinction matters on any node whose polarity can reverse, such as a differential pair, an AC-side input, or a connector that can be mis-wired. A real part shows the trade: Nexperia’s PESD5V0S1BA is bidirectional, clamps at 14 V, leaks 5 nA, survives more than 30 kV of ESD contact discharge, and costs 35 pF of line capacitance to do it.

Last updated: October 11, 2026

Key Takeaways

A bidirectional TVS clamps in both directions. A unidirectional part clamps the reverse direction at its breakdown voltage but conducts the opposite polarity through a forward-biased junction at roughly a diode drop.
That forward path is an advantage when the node is a DC rail that must be clamped above and below, and a problem when the node carries a signal, because it rectifies part of the waveform.
Bidirectional availability is not limited to small parts: the same option exists across power classes, marked by the CA suffix on SMBJ and SM15T families.
The ratings that decide the choice are standoff voltage, clamping voltage at the surge current, leakage, and capacitance — in that order for power rails, with capacitance first for data lines.
ESD and surge are different tests. A part can carry an IEC 61000-4-2 level 4 ESD rating and a separate IEC 61000-4-5 surge rating, and both numbers belong in a review.

What the Bidirectional Arrangement Actually Changes

Two junctions facing opposite ways mean the device blocks until the voltage exceeds its breakdown in either polarity, then clamps. There is no forward-conduction shortcut on one side, because each direction is a breakdown path.

That single difference drives the application split. On a signal line carrying a bidirectional or differential waveform, a unidirectional device would clip one half of the signal at about 0.7 V and leave the protection asymmetric; a bidirectional device treats both halves the same way. On a DC rail, the reverse direction of a unidirectional part is often welcome — it clamps a negative transient at a diode drop, which for a supply input is exactly what a clamp should do.

Bidirectional parts are available at every useful power level, and the suffix is the marker: Vishay’s SMBJ series states it directly, telling readers to use the CA suffix for bidirectional devices with electrical characteristics that apply in both directions. The same convention runs through ST’s SM6T and SM15T families.

The Class Table, With Real Parts

Pick the class from the size of the event, then confirm the details. The parts below come from four different datasheets, and the spread is worth reading as one picture.

Device	Class	Standoff	Clamping / pulse	Leakage	Capacitance
Nexperia PESD5V0S1BA (bidirectional ESD diode)	130 W peak pulse	5 V	V_CL = 14 V; IEC 61000-4-5 surge 12 A	5 nA	35 pF typ, 45 pF max
Vishay SMBJxxCA (bidirectional)	600 W at 10/1000 µs	5.0 V to 188 V	Clamping per part number at the rated peak pulse current	Per datasheet, at rated standoff	Not specified — a surge device, not a line device
STMicroelectronics SM6TxxCA	600 W at 10/1000 µs, 4 kW at 8/20 µs	5 V to 188 V	0.2 µA at 25 °C; 350 W capability at 150 °C junction	0.2 µA at 25 °C, 1 µA at 85 °C	Not a line-protection part
STMicroelectronics SM15TxxCA	1500 W at 10/1000 µs, up to 10 kW at 8/20 µs	5 V to 188 V	Up to 1250 W at maximum junction temperature	0.2 µA at 25 °C	Not a line-protection part

The first row and the rest are doing different jobs, and the capacitance column is why. A 35 pF part belongs on a slow signal or a supply pin; the surge suppressors in the lower rows belong where an event arrives with energy rather than speed. Capacity to absorb energy and capacity to preserve a waveform are, to a first approximation, opposing requirements.

A Worked Comparison: Signal Line or Supply Line?

The decision usually comes down to one calculation, and it is the same one in both directions of the argument.

Take a 50 Ω line and a device capacitance C. The time constant introduced is R × C: about 50 Ω × 35 pF = 1.75 ns for the bidirectional ESD diode above, and about 0.18 ns for a 3.5 pF line-protection part such as ST’s USBLC6-2. On a 480 Mb/s USB 2.0 link, where a bit is roughly 2 ns long, the first number is comparable to the bit period and the second is not. That is the entire reason low-capacitance ESD diodes exist as a separate product family: the same clamp is not usable on both kinds of node.

Now run the same reasoning on the supply side, where capacitance is irrelevant. A 24 V rail with tolerance to 28 V needs a standoff above that, so a 30 V class part is the starting point, and the clamping voltage at the rated peak pulse current then has to stay below the absolute maximum of everything downstream. If it does not, the protection has simply moved the failure. This is a model using datasheet capacitance and a stated 50 Ω line, not a measurement; substitute the real impedance of your trace or cable.

Boundary Conditions Worth Checking

Four conditions decide whether a bidirectional device behaves the way the schematic implies.

Standoff must clear the highest normal voltage in both polarities, or the device conducts during normal operation and shows up as leakage or distortion. Clamping must sit below the downstream absolute maximum at the surge current, not at some nominal current. Leakage matters on battery-backed and high-impedance nodes, and it is specified at a temperature — the PESD5V0S1BA quotes 5 nA, while the SM6T family quotes 0.2 µA at 25 °C rising to 1 µA at 85 °C, which is a different design budget entirely. And the standards are not interchangeable: IEC 61000-4-2 describes ESD, IEC 61000-4-5 describes surge, and a part that carries level 4 ESD plus a 12 A surge rating has demonstrated both, while many ESD diodes have not.

FAQ

Are TVS diodes directional?

It depends on the type. A unidirectional TVS clamps its breakdown polarity and conducts the opposite polarity as a forward-biased diode; a bidirectional TVS clamps in both directions because it contains two opposing junctions.

What is the difference between a bidirectional and a unidirectional TVS diode?

Symmetry. The bidirectional part presents the same behaviour to positive and negative transients, while the unidirectional part behaves differently in each direction. Use bidirectional where the waveform itself has both polarities, or where a symmetrical clamp matters.

How do I test a bidirectional TVS diode?

Out of circuit, a diode test shows conduction in both directions at the breakdown voltage rather than a single forward drop, and a high resistance below that. An in-circuit reading is only meaningful if the surrounding impedance is isolated.

Can I use a bidirectional TVS on a high-speed data line?

Only if its capacitance is small enough for the line. A 35 pF part on a 50 Ω line introduces a time constant around 1.75 ns, which is comparable to a bit period at USB 2.0 speeds; line-protection diodes of a few picofarads exist for exactly that reason.

Do ESD and surge ratings cover the same event?

No. IEC 61000-4-2 covers electrostatic discharge and IEC 61000-4-5 covers surge, and they describe different energies and waveforms. Check both if both events are possible at the node.

Official Resources

References

Nexperia, PESD5V0S1BA — Bidirectional ESD protection diode — source of the 130 W peak pulse power, 14 V clamping voltage, 5 nA leakage, greater than 30 kV ESD rating, 12 A IEC 61000-4-5 surge current, 5 V standoff and 35 pF typical capacitance in SOD323. assets.nexperia.com — PESD5V0S1BA datasheet (PDF)
Vishay, SMBJ5.0A thru SMBJ188A — Surface-Mount TRANSZORB Transient Voltage Suppressors, document 88392 — source of the 600 W peak pulse power at 10/1000 µs, the 5.0 V to 188 V standoff range, the CA suffix convention for bidirectional devices, the SMB (DO-214AA) case and the AEC-Q101 ordering options. vishay.com — SMBJ datasheet (PDF)
STMicroelectronics, SM6TxxA, SM6TxxCA — 600 W TVS in SMB — source of the 600 W and 4 kW pulse ratings, 5 V to 188 V standoff range, 0.2 µA and 1 µA leakage figures and the 150 °C junction limit. st.com — SM6T series datasheet (PDF)
STMicroelectronics, SM15TxxA, SM15TxxCA — 1500 W TVS in SMC — source of the 1500 W (10/1000 µs) and up to 10 kW (8/20 µs) ratings used for the power-class comparison. st.com — SM15T series datasheet (PDF)

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