Why Does TVS Diode VBR Change With Temperature?

The breakdown voltage, or VBR, of a TVS diode changes with junction temperature because avalanche breakdown is a semiconductor-junction process. In many commonly used avalanche TVS devices, VBR has a positive temperature coefficient: it tends to rise as the junction becomes hotter. The exact slope is device-specific, so a generic percentage should not be applied when the datasheet does not provide a coefficient or curve.

The engineering problem has two boundaries. At the lowest temperature and highest normal operating voltage, the TVS must remain off with adequate leakage margin. At the highest junction temperature and specified surge current, the voltage reaching the protected circuit must remain below its allowable limit. A room-temperature VBR value alone does not prove either boundary.

What do VRWM, VBR, and VC tell you?

VRWM is the reverse working stand-off voltage. It defines a voltage region in which the TVS is expected to remain in its off state under the specified conditions. VBR is measured at a stated breakdown test current. VC is the clamping voltage at a specified pulse current, waveform, and test condition.

Each parameter answers a different question:

• VRWM checks whether normal voltage, tolerance, ripple, and permitted continuous overvoltage can cause unwanted conduction.

• VBR describes the transition into the avalanche region and includes production tolerance as well as temperature movement.

• VC checks the residual voltage during a surge and must be read with the associated IPP and pulse waveform.

• IR checks whether leakage near the working voltage can affect standby current, bias points, or thermal behavior.

A higher VBR at elevated temperature is not automatically safer. The device may begin strong avalanche action later, increasing the transient voltage presented to the load. At the same time, off-state leakage can increase with temperature. Both effects belong in the review.

How should temperature be included in the protection window?

Use the part-specific VBR temperature coefficient or curve when it is available. Keep minimum and maximum conditions separate instead of calculating one typical value and treating it as a guaranteed boundary.

A practical review sequence is:

1. Establish the highest continuous voltage at the TVS location, including source tolerance, charging voltage, ripple, and permitted abnormal operation.

2. Check the cold condition and device tolerance to confirm that the TVS does not conduct during normal operation.

3. Correct the breakdown boundary for the highest credible junction temperature.

4. Read maximum VC near the expected surge current and applicable waveform.

5. Add the overshoot caused by package and PCB inductance to the protected-pin voltage budget.

6. Compare the result with both the absolute maximum rating and the functional limit of the downstream circuit.

If only 25°C values are published, do not invent a temperature coefficient. Ask the supplier for characterization data or compare samples under controlled hot and cold conditions. The result should identify the sample count, pulse shape, starting temperature, and measurement points.

What can be learned from three 24 V TVS models?

The ASIM SMA04J24V, SMB06J24V, and SMC15J24V all have a 24 V VRWM and a VBR range of 26.7 to 29.5 V. Each lists a maximum VC of 38.9 V, but the associated IPP values are 10.3 A, 15.5 A, and 38.6 A respectively. Their packages and peak pulse power classes are different.

This comparison shows that devices in the same voltage class can support different specified pulse currents. It does not prove that their full VBR-versus-temperature curves are identical. It also does not mean that 38.9 V is the clamp voltage at every current or for every waveform.

A temperature-sensitive design review still needs the following information:

• the VBR temperature curve or coefficient for the selected model;

• the actual pulse shape, duration, and peak current at the TVS branch;

• power derating with ambient temperature and pulse duration;

• PCB copper, pad geometry, and interval between repeated pulses;

• downstream voltage limits in powered, standby, and fault states.

The model comparison is useful for candidate selection. Final approval has to follow one model through all operating extremes.

Why does the cold condition matter?

For a device with a positive VBR temperature coefficient, breakdown voltage can be lower when the junction is cold. If the system voltage already sits close to the stand-off boundary, startup overshoot, charging voltage, or ripple may produce unexpected TVS current.

The symptom may be subtle. A battery-powered product may show higher standby current. A supply may enter current limit only during cold startup. A fuse or electronic protection switch may trip intermittently. These failures can disappear when the board is returned to room temperature.

Cold verification should use the real supply environment whenever possible. A clean bench supply set to nominal voltage does not reproduce cable inductance, charger tolerance, slow overvoltage, or the transient behavior of an industrial or vehicle supply.

How should the hot condition be tested?

Putting an unpowered board in a chamber and measuring one breakdown point is not enough. Run the product in its highest realistic power mode, allow the local board temperature to stabilize, and then apply the required surge or a documented development pulse.

Record at least these signals and conditions:

• voltage directly across the TVS;

• current through the TVS branch;

• peak voltage at the protected converter, MOSFET, or interface pin;

• leakage and product function before and after the pulse;

• pulse shape, polarity, count, interval, and sample identity.

Probe technique matters. A long oscilloscope ground lead adds loop inductance and ringing. That ringing can be mistaken for a temperature-driven increase in clamp voltage. Use a short ground spring, a suitable differential probe, or a coaxial measurement connection as the bandwidth and voltage require.

How do tolerance and temperature interact?

The published VBR range already represents a test-current condition and manufacturing spread. Temperature movement is another dimension; it should not replace the original tolerance. For a lower-bound check, begin with the appropriate minimum production value and apply the cold behavior. For a clamp-side check, use the maximum values or curves specified for the hot condition.

This distinction is important when the normal voltage sits near VRWM. A calculation based only on typical VBR can make the apparent window much wider than the guaranteed one. It is equally risky to compare a minimum VBR with a typical VC from a different test condition.

Common questions

Do all TVS diodes have a positive VBR temperature coefficient?

No universal assumption is safe. Many avalanche TVS devices in common voltage ranges show a positive coefficient, but low-voltage junction structures and different processes can behave differently. Use the data for the exact part.

Does VC rise by the same percentage as VBR?

Not necessarily. VC also depends on pulse current, dynamic resistance, junction heating, waveform, and package or PCB inductance. VBR movement cannot be used as a direct percentage conversion for VC.

Can a higher VRWM solve high-temperature leakage?

It may reduce leakage during normal operation, but it also raises the breakdown and clamp window. If the protected circuit has limited voltage headroom, moving to a higher voltage class can reduce protection.

What if the datasheet has no temperature curve?

The device can be screened as a candidate, but a wide-temperature design is not fully verified. Obtain supplier data or run a controlled sample comparison before releasing the selection.

A useful TVS review places the cold unwanted-conduction boundary and the hot residual-voltage boundary on the same voltage-window worksheet. That makes temperature a design input instead of a note beside the room-temperature table.