TVS Diode Failure Modes: Short, Open, and Drift After Surges

One board stops powering up after an event; another still runs but has lost its intended protection path. A shorted shunt and an open protection path can explain those different outcomes. Normal operation alone cannot establish that a TVS remains healthy.

The comparison below is a hypothetical repair review, not a report of customer failures. Follow the evidence from symptom to isolated component, then back to the source and current path that produced the stress.

What are the main TVS diode failure modes?

A TVS can fail short, become open, or drift outside its specified electrical behavior after excessive stress. Symptoms depend on the surrounding source, load, and protection network. A short can suppress the rail; an open can leave normal operation apparently unchanged. Confirm the isolated part and investigate the original stress before approving a replacement.

Vishay’s Failure Modes and Fusing of TVS Devices identifies those different outcomes. It also discusses how continued source current can change the condition after the initial event. The failed component is therefore one clue in a sequence, not a complete explanation of the sequence.

Start the review with two different symptom paths

In the first hypothetical path, the input rail measures low resistance and the supply reaches its current limit. Isolating the TVS removes that low-resistance path. That supports a shorted protection component, but still leaves the source of the damaging event unresolved.

In the second path, normal voltage reaches the load, but the isolated protection device no longer shows the expected controlled characteristic. The load functioning does not restore the missing clamp. An open shunt can be electrically unobtrusive during normal operation.

Record the event history, supply state, fuse condition, and visible damage before changing the assembly. A photograph and initial measurements can preserve evidence that disappears when a part is removed or a fuse is replaced.

Trace the current after a short

A shorted TVS can draw continued current from the normal source. Its magnitude depends on source impedance, current limiting, wiring, fuses, and other parts. Do not assume that the initial surge ends all subsequent stress.

As a deliberately simplified hypothetical example, 12 V across an effective 2 Ω path gives 6 A and 72 W total resistive dissipation. A real current-limited supply can behave very differently. The example illustrates why follow-on current requires its own review rather than predicting a particular board’s fault current.

Check what interrupts that path and whether nearby tracks, connectors, or components were affected. A replacement TVS will encounter the same unresolved abnormal source if the underlying condition remains.

Compare the failure evidence with normal test behavior

Observations that should not be conflated

Observation	Useful hypothesis	Required distinction
Rail low resistance	Shorted shunt or another board fault	Isolate paths before blaming the TVS
Bidirectional part open in diode mode	Instrument below breakdown	This can be normal, not proof of open failure
Changed leakage or breakdown	Degraded characteristic or wrong test conditions	Compare defined current, voltage and temperature
Board works after TVS removal	Removed shunt was loading the rail	Protection has not thereby been restored

The Fluke diode-test guide explains directional checks and parallel paths. Use the exact device identity and meter capability when deciding what an open display means.

Look for drift that an ordinary startup misses

Excess leakage can be more noticeable at temperature or on a high-impedance node than at a robust power rail. Changed breakdown behavior also needs defined current and temperature to interpret. A visual inspection and successful power-up can miss those conditions.

ST’s TVS FAQ technical note explains leakage, breakdown, clamping, and pulse-current terms. Keep the intended operating point attached to the test rather than using a generic pass/fail label.

Compare isolated measurements with the relevant source limits and any pre-event baseline. A typical curve is not a guaranteed acceptance boundary. If the product’s repair criteria call for replacement after an event, a plausible static reading does not override that requirement.

Reconstruct why the original protection was overstressed

Investigate pulse duration, available current, repetition, operating temperature, and normal-voltage mismatch. A prolonged abnormal supply can impose a different stress from the short pulse behind the part’s peak-power rating. Automatic retries can turn one fault into repeated loading.

The TI surge-diode selection report provides the source-and-clamp framework. The Vishay SMBJ source illustrates waveform-specific pulse and voltage entries. Use the actual selected product rather than attaching those example limits to every TVS.

Also inspect the return path and placement. A protection part may survive one installation but encounter different stress after a wiring or layout change. Preserve those differences in the investigation.

Review nearby damage before replacing the shunt

Check the protected load, input circuitry, fuse or interruption element, and any damaged interconnect. Use the TVS protection explanation to identify the intended shunt path and TVS versus Zener to avoid installing a different function in its place.

For interfaces, USB ESD protection adds signal and array considerations. A large power rectifier discussed in the TO-247 guide may be a neighboring component but is not a TVS substitute.

A replacement checklist that follows the evidence

Confirm the exact manufacturer part, polarity, standoff, clamp at the expected current, waveform, temperature allowance, package, and placement. Do not raise the voltage blindly to stop the replacement from conducting; that can remove protection from the load.

Restore the defined protection network and verify the repaired system under suitable controlled conditions. Record what was replaced, what caused the stress, and which acceptance checks passed. Leaving the TVS out can be a diagnostic step, but it is not evidence of a completed protection repair.

Read the next datasheet with the failure sequence in view

Ask whether the candidate supports the actual event and follow-on condition, and what limits the energy if it becomes conductive. Keep pulse survival separate from normal leakage and breakdown checks. The repair conclusion should explain both the failed component and the circuit conditions that will face its replacement.

FAQ

Does a TVS always fail short?

No. A short is a common outcome, but open and electrically degraded conditions are also possible. Continued source current after the initial event can further change the damaged component. Confirm the actual isolated behavior and investigate the surrounding circuit. Do not design or repair on the assumption that every TVS produces one guaranteed resistance or that a working load proves the protection remains intact.

If the board works after removing the TVS, is the repair complete?

No. That can help identify a shunt that was loading the rail, but removing it also removes its intended protection path. Investigate why the part was overstressed, inspect surrounding damage, and install a correctly specified replacement under the product’s repair criteria. Then verify the repaired protection network. Normal operation without the component does not establish acceptable behavior during the next transient event.

Can an open diode-mode reading prove an open TVS failure?

Not by itself. A healthy bidirectional TVS can appear open in both directions because the meter cannot reach breakdown. A unidirectional part also requires interpretation against its expected polarity and the instrument’s capability. Account for parallel circuit paths and identify the exact part. Use suitable controlled characteristic tests when needed, while recognizing that static measurements do not establish full surge performance.