When current in an inductive load is interrupted, the inductor forces voltage upward until another path carries the current. If no external clamp conducts first, a MOSFET may enter avalanche and absorb the stored energy. Datasheets often provide a single-pulse unclamped inductive switching rating, but repetitive field operation introduces additional conditions that a one-time test does not represent.
What the single-pulse test demonstrates
In a typical unclamped inductive switching test, current is built in an inductor and the MOSFET is turned off. Drain voltage rises to breakdown and the device dissipates energy while current decays. The rating is tied to specified starting junction temperature, inductance, current, gate drive, and test circuit. It demonstrates ruggedness under that defined event, not unlimited energy absorption under arbitrary waveforms.
Compare the application with all test conditions. A different avalanche current changes current density and peak power. A higher initial temperature reduces thermal margin. A different clamp voltage changes the decay time and energy distribution.
Why repetition changes the problem
Repeated events add average avalanche power equal to event energy multiplied by repetition frequency. The junction may not return to its original temperature between pulses, so each event begins hotter. Thermal cycling also stresses die attach, clips, bond structures, and package interfaces. Even when each pulse is below the single-event energy, the combination can exceed a repetitive safe operating boundary.
- Event energy and peak avalanche current
- Avalanche duration and clamp voltage
- Pulse frequency, duty cycle, and burst pattern
- Initial junction temperature before each event
- Board, case, and ambient thermal impedance
- Expected lifetime number of events
Separate abnormal events from normal control
If avalanche occurs only during a rare connector interruption or load dump, the design may be evaluated as a limited number of independent events with sufficient cooling between them. If avalanche is used every PWM cycle to clamp a solenoid, it is a repetitive operating mode. The latter needs a device with suitable repetitive data or manufacturer guidance, plus a thermal and lifetime assessment.
An external flyback diode, TVS, Zener network, or active clamp can move energy away from the MOSFET or control the clamp voltage. The trade-off includes current decay time: a low clamp voltage protects the switch but may release a solenoid slowly, while a higher clamp produces faster release and higher electrical stress.
Validate the waveform and temperature
Measure drain voltage and current at the device and calculate instantaneous avalanche power. Integrate the event to obtain energy, then evaluate the pulse train with transient thermal impedance or an electrothermal model. Test at the hottest initial condition and the maximum expected repetition rate. Monitor parameter drift and leakage during endurance testing rather than looking only for catastrophic failure.
Document an explicit operating boundary
A production specification should state maximum current, supply voltage, clamp voltage, energy per event, frequency or event count, ambient condition, and allowed initial temperature. If any of those variables are not controlled, add circuit protection or select a device with greater characterized margin.
Avalanche capability is useful, but it should be engineered as an operating condition. Treating a single-pulse number as a repetitive guarantee can turn a device that passes initial testing into a long-term reliability risk.
Technical reference: Nexperia, Power MOSFET single-shot and repetitive avalanche ruggedness rating.
