MOSFET Avalanche Rating: What UIS Specs Mean for Real Spikes

A larger avalanche-energy number does not necessarily identify the more suitable MOSFET. One figure may be measured at a different current, starting temperature, or test inductance. A comparison that hides those conditions can reverse the practical conclusion.

The engineering cost is the missing analysis: where the spike’s energy goes, how often the event occurs, and how hot the device already is. Read the UIS entry as evidence tied to a test, then compare it with the intended power stage.

What does a MOSFET avalanche rating mean?

A MOSFET avalanche rating describes capability under stated breakdown-current, energy, temperature, and test-circuit conditions. UIS means unclamped inductive switching, where interrupted inductor current drives the device into avalanche. A single-pulse energy figure is not permission for arbitrary repetitive spikes. Compare the actual waveform and thermal history with the manufacturer’s applicable limits.

Avalanche can provide a path for current when drain voltage rises into breakdown. It is different from ordinary blocking below the rated limit, and different from body-diode forward conduction. Keep those three operating states separate when reviewing a fault or turn-off event.

What the UIS test deliberately makes happen

In a basic UIS sequence, the MOSFET conducts to build current in an inductor. The gate is then turned off. If the circuit provides no lower-voltage clamp path, inductor current drives drain voltage upward until avalanche carries current while the stored energy is dissipated.

The Nexperia AN10273 avalanche-ruggedness note describes single-shot and repetitive avalanche assessment. Its test discussion helps interpret the conditions rather than treating the energy entry as a free-standing property.

Read the initial junction temperature, avalanche current, inductance, gate conditions, and relevant voltage. Check whether the source remains connected during avalanche. The test arrangement affects how much energy is delivered to the device.

Build an energy account before comparing a headline

The energy initially stored in an ideal inductor is ½LI². For a hypothetical 100 µH inductance carrying 5 A, that is 0.5 × 0.0001 × 25 = 0.00125 J, or 1.25 mJ. This is stored energy, not automatically the complete MOSFET dissipation.

If the supply continues delivering energy during current decay, device energy can exceed that initial amount. Other circuit paths can instead take part of the energy. Determine the actual circuit before equating ½LI² with an avalanche specification.

Infineon’s Power MOSFET avalanche design guidelines distinguishes source-connected and decoupled test arrangements. Integrating the relevant instantaneous device voltage times current over the event provides a direct energy assessment when the waveforms are measured correctly.

A comparison table that keeps the conditions visible

Evidence needed beside an avalanche-energy number

Item	Why it matters	Question for the application
Initial junction temperature	Available thermal margin changes	Is the event cold, warm or following heavy load?
Avalanche current	Equal energy can occur at different peak stresses	Does the application stay within the relevant current boundary?
Pulse duration and inductance	Heat distribution and current decay differ	Is the waveform comparable?
Supply connection	The source may add energy during decay	Was this included in the calculation?
Single-shot or repetitive	Repeated events add thermal and reliability requirements	Is repetition explicitly supported?

Vishay’s AN-1005 avalanche design guidelines provides further context for test conditions and thermal considerations. Use the selected manufacturer’s actual limits and guidance when making a component decision.

Do not turn a single pulse into a repetitive allowance

Even a small energy per event can become meaningful average heating. If a hypothetical 1 mJ event occurred at 10 kHz, its energy-rate term would be 10 W. This arithmetic does not qualify that event for repetition; it merely shows why frequency cannot be omitted.

Repetitive operation can also involve limitations beyond average temperature. Apply the device’s specified repetitive-avalanche information or obtain clarification for the intended conditions. Do not infer permission simply because a cold single-pulse energy number is larger than the calculated pulse.

TI’s discussion of UIS and avalanche ratings emphasizes the test context behind datasheet entries. Comparing numbers without that context is especially risky across different manufacturers or product technologies.

Check spikes with a measurement that can support the calculation

Use suitable voltage and current probes, bandwidth, and common-mode capability. Keep the measurement loop appropriate to the event. A probe artifact can exaggerate a spike, while insufficient bandwidth or range can hide one.

For energy integration, voltage and current timing alignment matters. A delay mismatch between probes can distort instantaneous power during a fast transition. State the integration interval and verify that it captures the intended avalanche event rather than unrelated switching intervals.

The Tektronix floating-measurement guide explains restrictions of earth-referenced measurement arrangements. Never remove protective earth to reach a floating node. Start with an appropriately limited-energy procedure, and avoid deliberate uncontrolled avalanche testing as a substitute for the manufacturer’s qualification.

Keep clamping and device selection connected

An external clamp may limit drain voltage and take part of the energy, but its tolerance, dynamic clamping voltage, pulse capability, and layout need review. The TVS surge-protection guide addresses the clamp role. A nominal TVS voltage does not by itself establish the peak voltage at the MOSFET terminals.

Likewise, the MOSFET versus BJT switching comparison is a different selection task. A transistor technology name cannot replace an inductive-energy account. In larger systems, including those described in PV inverter semiconductor operation, consider the surrounding control and fault sequence.

Include the clamp’s return connection in the sketch. A local path and a remote path can expose the transistor to different transient voltage because connection inductance participates in the event. The voltage measured at a distant connector need not equal the voltage at the device terminals. Keep the measurement location with any claimed clamping result.

A practical review before accepting the spike

Record the source of inductance, interrupted current, energy paths, event frequency, initial temperature, and voltage/current waveforms. Include tolerances and relevant abnormal conditions. Compare those requirements with the device’s specific current, energy, thermal, and repetitive-operation boundaries.

Distinguish normal switching, occasional faults, and repeated retry sequences. A fault that initially seems rare may repeat automatically if a controller keeps restarting into the same load condition. Determine the number of events and the cooling interval in that sequence. The device can begin later pulses at a higher temperature than the first, even when each commanded pulse looks identical.

When two replacement candidates differ, compare the work required to qualify each one. A higher voltage rating, different clamp, or revised layout may change the energy distribution and switching behavior. Preserve those changes in the review so an apparent component improvement is not separated from the circuit conditions that made it work.

If a failed MOSFET is being investigated, multimeter fault checks may locate a short after discharge, but cannot establish the original cause. Preserve the switching conditions and examine the clamp and gate drive before replacing the device.

Read the next datasheet as a conditional specification

Keep the exact part number and document revision, then find the test diagram and notes associated with the avalanche entry. Clarify missing repetitive or temperature information before relying on it. The defensible conclusion is that a defined event fits a supported operating condition, with an understood energy path and margin.

FAQ

Does a larger EAS number mean a better MOSFET?

Not by itself. Avalanche energy is tied to test current, starting temperature, inductance, pulse shape, and test arrangement. Two headline values measured under different conditions may not be directly comparable. Determine the actual event and its repetition, then use the selected manufacturer’s relevant limits and guidance. Also check ordinary conduction, switching, and thermal requirements rather than choosing a transistor on avalanche energy alone.

Is avalanche energy always one half L times I squared?

That expression gives the energy initially stored in an ideal inductor. It equals the device’s absorbed energy only under suitable circuit assumptions. A connected supply may add energy during decay, while clamps or other paths may take some away. Identify the complete current path and test arrangement. Correctly measured device voltage and current can be integrated over the event for a more direct energy assessment.

Can a single-pulse avalanche rating be used every switching cycle?

Do not infer that permission. Repetition introduces average heating, thermal history, and potentially other device limitations. Use explicit repetitive-avalanche information and the manufacturer’s applicable guidance. Even if one pulse is smaller than a cold single-shot energy entry, its current, temperature, waveform, and repetition may differ substantially. Review the complete operating condition and any clamp before accepting repetitive avalanche as part of normal operation.