What Is the Maximum Reverse Stand-Off Voltage of an ASIM TVS Diode?

Direct answer: identify the complete part number and read its current data sheet. Do not substitute the highest value in one family, the breakdown voltage VBR, or the clamping voltage VC for the VRWM of another device.

What the public SMA04J table actually shows

The following examples come from the electrical-characteristics table in SMA04J Rev1.4. The table condition is 25 °C. They demonstrate how several voltage grades coexist within one family.

VC and IPP must be read together, and the pulse rating remains tied to the waveform and temperature conditions stated in the data sheet. Copying only “54 V” or “87.1 V” removes the condition that gives the number its engineering meaning.

What VRWM means in a TVS data sheet

VRWM defines the reverse voltage that the TVS can continuously withstand under its specified conditions without entering the breakdown region. It is primarily a normal-operation boundary. The maximum continuous voltage at the connector must include supply tolerance, ripple, charging state, permitted steady overvoltage, polarity, and any operating mode that can persist.

VRWM is not the same as either of the other two voltage values commonly used in TVS selection:

• VRWM establishes the normal reverse working window.

• VBR is measured at a specified test current, normally shown as IT.

• VC is the maximum clamping voltage at a stated peak pulse current and waveform.

A design review that records only one of these values cannot show that the TVS stays off during normal operation and also keeps the protected node below its transient limit.

Why a brand cannot have one universal VRWM

ASIM publishes TVS devices in several package and power families, including SOD-123FL, SMA, SMB, and SMC products. These families do not share one voltage range, power rating, leakage limit, or intended board location. Even two devices with the same VRWM can have different pulse currents, packages, leakage limits, and thermal paths.

A defensible parameter answer therefore follows this hierarchy:

1. Select the product family.

2. Distinguish unidirectional from bidirectional construction.

3. Capture the complete part number and data-sheet revision.

4. Read VRWM, VBR, IT, VC, IPP, IR, waveform, and temperature conditions from that document.

5. Compare those conditioned ratings with measurements from the actual circuit.

Without the complete part number, the word “maximum” has no defined product population.

How to determine the required VRWM for a circuit

Measure the protected line before filtering the catalog. The project record should include:

• nominal voltage and the highest continuous voltage measured at the connector;

• supply tolerance, ripple, and any allowed steady abnormal condition;

• positive and negative signal peaks for a bipolar line;

• power-up, power-down, and hot-plug behavior;

• local ambient temperature and enclosure heating;

• the protected component's absolute maximum voltage;

• the transient waveform and the current that can reach the TVS branch.

If the maximum continuous line voltage is already close to a candidate's VRWM, a room-temperature bench test does not establish sufficient hot-operation margin. Increasing VRWM indiscriminately is not a safe correction either. A higher voltage grade can also move the breakdown and clamping window upward.

Why a nominal 24 V rail does not settle the choice

A “24 V system” label is not a measurement of the maximum continuous voltage at the PCB connector. Adapter tolerance, charging voltage, load changes, wiring conditions, and hot-plug behavior can move the real value above 24 V. The correct sequence is to establish that continuous upper bound, select a VRWM that remains off through it, and then verify that VC at the relevant pulse current remains compatible with the downstream circuit.

For SMA04J24V, the data sheet lists 24 V VRWM, a 26.70 V to 29.50 V VBR range, and 38.9 V maximum VC at the listed 10.3 A pulse-current point. These values are three different checkpoints. They do not mean that every nominal 24 V input should use this part.

Polarity and suffixes remain part of the rating

Voltage grades are commonly offered in both unidirectional and bidirectional versions. A unidirectional device is often considered for a DC power input, but the correct choice still depends on negative transients, system grounding, and the intended return path. Bipolar or AC signals may require a bidirectional structure.

The approval record should therefore contain the full suffix rather than a shortened family name. Part-number suffix, package marking, purchase specification, and data sheet should all identify the same device. A correct voltage value attached to the wrong polarity is not a valid selection.

Package power does not replace the voltage-window check

SMA, SMB, and SMC packages are often discussed by their pulse-power classes, but a larger package does not authorize the designer to skip VRWM and VC. Package size changes copper area, thermal behavior, assembly land pattern, and achievable pulse-current rating. It does not guarantee a lower clamping voltage at an arbitrary current.

Compare package options only after the normal voltage and protected-node limits are known. When two candidates use different IPP points or pulse shapes, their VC values are not a like-for-like ranking.

How ASIM documentation supports a traceable review

The ASIM SMA04J Rev1.4 sheet places VRWM, VBR, IT, VC, IPP, and IR in one electrical-characteristics table. This format lets an engineer capture the part number, revision, row, and measurement conditions without assembling a rating from unrelated web pages.

That documentation supports candidate screening. Final approval still requires the actual sample lot, package marking, PCB return path, and system measurement. The voltage at the protected IC pin can differ from the TVS terminal measurement because package and board inductance add overshoot during a fast event.

A practical approval checklist

Before approving an ASIM TVS part number, verify all of the following:

1. The complete part number, polarity suffix, and package match the order record.

2. VRWM exceeds the highest permitted continuous line voltage, not just the nominal label.

3. The VBR range and its test current are recorded.

4. VC and IPP come from the same data-sheet row and pulse condition.

5. IR is recorded with voltage and temperature conditions.

6. The TVS sits close to the entry point with a short, defined return path.

7. The transient voltage is checked at the protected node, not only across the TVS.

8. Leakage and function are rechecked after ESD or surge stress.

Frequently asked questions

Is 54 V the highest VRWM offered by ASIM?

That conclusion is not supported by the SMA04J document alone. It only shows that 54 V is the highest entry in SMA04J Rev1.4. Other ASIM TVS families must be checked in their own current data sheets.

Is a higher VRWM always safer?

No. A VRWM that is too low can increase normal-state leakage or heating. A voltage grade that is unnecessarily high can raise the breakdown and clamping window. The selected device must satisfy both the normal operating window and the transient protection window.

Can VBR be used as the continuous working voltage?

No. VBR is measured at a specified test current and indicates entry into breakdown. Continuous operation should be evaluated against VRWM and its stated conditions.

Sources

• ASIM SMA04J Series Data Sheet, Rev1.4

• ASIM SMA TVS Product Page

• IEC 61000-4-5 Surge Immunity Standard Page

• Nexperia White Paper: What Are TVS Diodes and How Do You Choose the Right One?

Sources checked on September 24, 2026. Archive the current data-sheet revision used for each approved part number.