Table of Contents
A TVS diode is selected for the voltage that reaches the protected circuit during the specified transient, not merely for the voltage printed in its part description. Its normal working limit, breakdown region, and clamping voltage describe different operating conditions.
Before comparing devices, define the protected node, its normal voltage range, the transient source, and the permissible stress at the IC pin. Without those inputs, choosing a “higher-wattage” or “lower-voltage” TVS can create a new problem rather than solve the original one.
Establish the Normal Voltage and the Transient Requirement
Include supply tolerance, signal swing, expected ground offset, and permitted operating excursions when defining the normal voltage range. The protection device must avoid unacceptable conduction or leakage throughout that range.
Then identify the required transient test or credible internal switching event. Record the waveform, source conditions, coupling arrangement, polarity, repetition, and operating temperature. A voltage applied by a test generator is not automatically the voltage or current experienced by the TVS after the rest of the circuit is included.
For an industrial equipment interface, the test plan should distinguish surge, ESD, and any other required immunity tests. Nexperia’s TVS selection white paper explains why pulse duration, energy, polarity, and temperature matter. An ESD capability statement should not be relabeled as proof of a different surge test.
Read Standoff, Breakdown, and Clamping as Separate Values
TI’s surge-diode selection report explains the key parameters and the way clamping changes with current. Use the selected device’s own definitions and test conditions when filling out the comparison.
| Parameter | Selection question |
|---|---|
| Reverse standoff voltage, VRWM | Is normal operation within the specified low-leakage region? |
| Breakdown voltage, VBR | At what stated test current is breakdown characterized? |
| Clamping voltage, VC or VCL | What voltage is specified at the relevant pulse current and waveform? |
| Peak pulse current, IPP | Can the device tolerate the required pulse under the stated conditions? |
| Leakage and capacitance | Will protection disturb the normal power or signal function? |
Do not compare the IC’s absolute maximum directly with VRWM and declare the design protected. Assess the voltage actually reaching the pin, including the TVS’s clamping behavior and the interconnect contribution. Absolute maximum is a damage-avoidance limit, not a preferred operating target; apply the design margin and transient guidance required for the protected component.
If no suitable device leaves room between normal operation and permissible transient stress, reconsider the protection architecture. Additional impedance, filtering, a different clamp technology, or a different protected component may be necessary. A catalog substitution cannot resolve an impossible voltage budget.
Compare Waveforms Before Comparing Power Ratings
Two pulse-current or power figures are not interchangeable when their pulse durations differ. The Nexperia white paper explicitly discusses 8/20-µs and 10/1000-µs waveforms and temperature-dependent capability. Keep the waveform attached to every number in the selection record.
The MMBZ16VZLS-Q datasheet provides a concrete example. It lists a rated peak pulse current of 7 A for an 8/20-µs waveform and 0.8 A for a 10/1000-µs waveform. Its characteristic table gives a maximum clamping voltage of 43 V at 7 A with the 8/20-µs pulse, versus 26 V at 0.8 A with the 10/1000-µs pulse, at 25°C.
Those entries describe different test conditions on the same device. They must not be combined into a fictitious 7-A, 26-V specification. The example illustrates how to read a datasheet; it is not a recommendation for a particular supply rail or communication port.
When sourcing through the Nexperia catalog, retain the exact part and the full rating conditions. A shortened description such as “16-V TVS” cannot serve as an acceptance specification.
Account for Signal Loading and the Current-Return Path
A protection component may meet the transient requirement but add too much capacitance or leakage during normal operation. Check the relevant capacitance conditions and the interface’s signal budget. A device appropriate for a slow control line may be unsuitable for a faster link even when their nominal voltage ranges appear similar.
Choose unidirectional or bidirectional behavior from the actual positive and negative voltage conditions. Package polarity and pin assignment also need explicit review; a similar footprint does not ensure the same protection network.
The layout is part of the circuit. Current flowing through interconnect inductance adds voltage according to V = L × di/dt. As a simplified illustration, 10 nH carrying a current change of 1 A/ns contributes 10 V. These are hypothetical values, not a measurement of a particular board or a complete surge model.
That relationship explains why a remote clamp and a long return loop can undermine an otherwise reasonable component choice. Inspect the path from connector to protection device and back through the intended return. Avoid routing transient current through a sensitive signal reference merely because it is labeled “ground.” The final arrangement must follow the equipment’s grounding and safety design.
For an RS-485 interface, evaluate the TVS together with the transceiver’s common-mode limits, termination, biasing, and any isolation. Do not optimize each part independently and assume the assembled port will inherit all their individual headline ratings.
Qualify the Assembled Protection Circuit
Define the pass criteria before testing. Depending on the applicable requirement, acceptable behavior may involve continued operation, controlled recovery, or a specified temporary disturbance. Permanent damage is not the only possible failure: corrupted communication, a latched state, or an unintended output may also violate the equipment requirement.
Use the required test setup and qualified personnel, and capture conditions sufficiently to reproduce the result. Record the pulse, polarity, coupling, temperature, board configuration, protected-node waveform, and functional outcome. If measurement bandwidth or probing limits the observation, keep that limitation in the report.
For a proposed alternate, compare the complete voltage-current behavior and test conditions, not just the family name, package, or nominal watts. Preserve the board-level qualification evidence with the approved ordering code. That gives purchasing a defensible substitution boundary and prevents a plausible-looking protection diode from becoming an untested change at an exposed port.
Frequently Asked Questions (FAQ)
Is a TVS diode's standoff voltage its clamping voltage?
No. Standoff voltage concerns normal operation and specified leakage. Clamping voltage is specified at a particular transient current and waveform and can be substantially higher.
Can two TVS ratings be compared using peak watts alone?
Only after checking the pulse waveform, duration, temperature, repetition conditions, and clamping behavior. A peak-power figure without those conditions is not a complete comparison.
Does a transceiver's ESD rating remove the need for a surge assessment?
No. ESD and surge tests use different stress conditions. Evaluate the complete port against its required tests, including protection components and the PCB current-return path.
