Use a TVS for high-energy transient clamping in many power-switch applications, and use a Zener where energy is lower or as part of a controlled clamp—but select from data-sheet pulse behavior rather than the product name. Both devices can operate in reverse avalanche. TVS diodes are usually optimized and specified for surge absorption; small Zeners are often intended for regulation, references, or modest clamps.
The important quantities are standoff voltage, breakdown range, clamp voltage at the actual pulse current, dynamic resistance, pulse duration, repetition rate, temperature, capacitance, and failure mode.
Set the protection window
The clamp should not conduct significantly at the highest normal drain voltage, including supply tolerance and regenerative events. At fault current, its maximum clamping voltage plus trace-induced overshoot must remain below the MOSFET rating with engineering margin. Those two requirements create a window; sometimes no catalog part fits and the circuit topology must change.
Never choose solely by matching the printed TVS voltage to the MOSFET voltage rating.
Compare TVS and Zener characterization
A TVS data sheet commonly specifies peak pulse power or current using defined waveforms, along with standoff, breakdown, and clamping voltages. A small Zener may specify steady dissipation and test current but offer limited surge detail. Larger avalanche diodes blur the naming distinction, so use the guaranteed curves and package thermal path.
Unidirectional and bidirectional TVS parts also behave differently for reverse polarity. Confirm the required current path.
Calculate energy and repetition
For a simple inductor, ½LI² is a starting point. Add supply-delivered energy during the clamp interval and parasitic inductance. Compare the actual pulse shape with the data-sheet derating method, then calculate average power at the repetition rate. Allow the junction to cool according to transient thermal impedance instead of assuming each pulse starts at ambient.
The analysis in protecting MOSFETs with inductive loads connects clamp choice to current decay.
Choose clamp location and topology
A drain-to-source TVS directly limits VDS but carries the load’s clamp current. A diode-plus-TVS across a coil can keep high current away from the MOSFET loop. An RCD snubber shapes repetitive leakage-inductance energy. A drain-to-gate Zener or active clamp turns the MOSFET partially back on to control drain voltage, transferring energy into the MOSFET and driver network.
For an active clamp, protect both positive and negative VGS and confirm linear-mode SOA.
Include parasitics and temperature
Lead and trace inductance adds L·di/dt above the suppressor’s intrinsic clamp voltage. Place the device close, use short wide connections, and avoid routing clamp current through the gate-source reference. TVS leakage rises with temperature; breakdown and clamping values also vary. Capacitance can load a fast switch or communication node.
Check MOSFET dV/dt protection when edge coupling and false turn-on accompany the drain transient.
Define safe failure behavior
A severely overloaded suppressor may fail short, open, or degrade. Add a fuse or current limit so a shorted TVS does not turn into a fire hazard. Monitor repeated transient events if they signal a failing load or connector. Consider whether a clamp failure leaves the MOSFET, wiring, and user in a safe state.
Bench validation should capture clamp current, drain voltage, gate voltage, pulse duration, and temperature over repeated worst-case events.
Turn the answer into a robust MOSFET design
For TVS or Zener for MOSFET Drain Protection?, create an operating-state table before choosing a device or protection value. Include off, startup, steady on, PWM transitions, shutdown, reverse current, load connection, controller reset, brownout, open load, overload, and short circuit. For each state, write the expected VGS, VDS, current, duration, repetition, and starting temperature. This exposes short but severe events that an average-current calculation hides.
Next, compare the table with guaranteed data-sheet limits and characterized curves. Use voltage margin for supply tolerance and ringing; use RDS(on) at the actual gate voltage and hot junction; use total gate charge and Miller charge for driver sizing; and use DC or pulsed SOA for intervals where voltage and current coexist. Include package, PCB copper, interface material, airflow, connector, and source-return inductance in the model. A transistor cannot be evaluated separately from its commutation path and gate loop.
Build a staged test plan that begins with limited available energy. Capture gate-to-source voltage at the pins, drain-source voltage, current, and temperature while changing one stress at a time. Repeat enough cycles to reveal cumulative heating and recovery behavior. Define pass limits before looking at the waveform, preserve oscilloscope files, and record device lot, ambient, probe method, and firmware revision. If measured peaks approach an absolute maximum, redesign for margin rather than accepting the result because one prototype survived.
Practical validation checklist
Validate at the worst credible supply voltage, load current, switching frequency, ambient temperature, and fault condition. Measure gate-to-source voltage at the device pins, drain-to-source voltage with adequate bandwidth, current, case temperature, and—when power or pulses are significant—estimated junction temperature. Account for probe ground inductance and avoid interpreting ringing created by the measurement setup as circuit behavior.
Compare every result with the data-sheet safe operating area, absolute maximum ratings, transient thermal impedance, and temperature-dependent curves. Test startup, shutdown, load connection, stalled or shorted load, controller brownout, and repeated operation. The guides to reading MOSFET safe operating area and MOSFET thermal calculation provide useful companion checks.
Frequently asked questions
Is every TVS diode a Zener diode?
They use related avalanche behavior, but TVS devices are packaged and characterized for transient suppression; selection must follow their specified use.
Can I put a TVS directly across drain and source?
Often yes, if polarity, standoff, clamp voltage, pulse current, energy, repetition, and layout are all suitable.
Should the clamp voltage equal the MOSFET rating?
No. Maximum clamp plus parasitic overshoot needs margin below the MOSFET rating.
Can a gate Zener protect the drain?
A gate clamp protects VGS. A drain-to-gate active clamp can influence VDS, but it is a different circuit requiring driver and SOA analysis.
