How do you check a MOSFET? Begin with the circuit symptoms, confirm the device part number and pinout, remove power, discharge stored energy, and check the gate, body diode, and drain-to-source path. A digital multimeter can identify many shorted or gate-damaged devices. Powered waveform checks are needed when the MOSFET passes static tests but still overheats, switches slowly, or fails under load.
This guide presents a fault-finding workflow rather than one isolated meter trick. It helps separate a defective MOSFET from a gate-driver problem, shorted load, failed diode, poor solder joint, or thermal design issue.
Start with the failure symptoms
| Symptom | Possible MOSFET-related cause | Other circuits to check |
|---|---|---|
| Fuse opens immediately | Drain-source short or shoot-through | Rectifier, load, opposing switch, controller timing |
| Load never turns on | Open device, damaged gate, insufficient VGS | Gate resistor, driver supply, controller enable, wiring |
| MOSFET becomes hot | High RDS(on), slow switching, linear operation | Gate-drive level, switching frequency, cooling, load current |
| Intermittent shutdown | Leakage or temperature-dependent damage | Protection circuit, current sense, solder joints, supply transients |
| Drain-voltage spikes | Slow or poorly controlled commutation | Layout inductance, clamp, snubber, flyback path |
The symptom determines the most useful test. A dead-short check is appropriate after a fuse failure. Gate-drive and thermal measurements matter more when the circuit runs but the device overheats.
Safety before checking the device
- Disconnect mains, batteries, and external power sources.
- Discharge bulk capacitors and confirm the remaining voltage with a meter.
- Use ESD precautions when touching the gate terminal.
- Do not use resistance or diode mode on an energized circuit.
- Use correctly rated differential probes and safe isolation methods for powered high-voltage measurements.
High-voltage supplies, inverters, chargers, and motor drives can retain dangerous energy after power is removed. Follow the equipment service procedure when the circuit exceeds your training or test equipment rating.
Identify the exact MOSFET
Read the complete marking and find the manufacturer’s datasheet. Confirm N-channel or P-channel polarity, package pinout, maximum gate voltage, body-diode orientation, and whether the part includes internal gate protection or multiple dies.
Do not assume that every TO-220 or surface-mount package uses the same lead order. The metal tab is often connected to drain, but that is not universal. A wrong pinout can make a healthy device appear defective.
Perform a visual and mechanical check
Inspect the package for cracks, discoloration, melted molding, lifted pads, and damaged leads. Examine nearby gate resistors, driver ICs, current shunts, diodes, snubbers, and decoupling capacitors. A MOSFET often fails because another part or an abnormal load created excessive voltage, current, or heat.
Check the solder joints under magnification. A fractured source joint can create excessive common-source inductance. A damaged gate connection can leave the gate floating. Carbonized board material may remain conductive even after the MOSFET is replaced.
Quick unpowered multimeter checks
Discharge the gate
Short gate to source through an ESD-safe conductive path before each test. The insulated gate behaves like a capacitor and can hold enough charge to turn the channel on after the circuit is disconnected.
Check gate insulation
Measure gate-to-source and gate-to-drain resistance in both directions. A standard unprotected MOSFET should normally show very high resistance. A persistent low value suggests gate-oxide damage, contamination, or an in-circuit path. Devices with internal gate resistors or Zener protection may read differently, so compare the result with the datasheet or a known-good part.
Check drain to source
With gate shorted to source, measure drain-to-source resistance in both directions. One direction may show the body diode in diode mode. A near-zero reading in both directions is a strong sign of a shorted device when the MOSFET is isolated from the circuit.
Check the body diode
For a common N-channel MOSFET, diode mode normally shows a forward drop with the red probe on source and black probe on drain. Reversing the probes should block when the gate is discharged. A common P-channel device shows the opposite diode direction.
Decide whether to test in circuit or out of circuit
In-circuit testing is useful for a fast short check and for comparing identical phases. It can be misleading because transformer windings, shunts, body diodes, snubbers, drivers, and other MOSFETs create parallel paths.
If the reading is unclear, lift the gate and one power terminal or remove the device. Record the orientation before desoldering. An out-of-circuit test provides the clearest static result.
Compare identical devices or phases
Multi-phase motor drives, server supplies, and synchronous converters often contain repeated channels. Compare diode-mode and resistance readings from equivalent positions. A large difference can quickly identify the suspect phase.
Comparison does not prove that the lower-reading device is defective. Layout, connected magnetics, and controller circuitry may differ. Use the comparison to choose where to isolate and test next.
Check the gate-drive circuit
A healthy MOSFET cannot work correctly without suitable gate-to-source voltage. In a powered test, measure VGS directly between the gate and source pins. Do not measure gate relative to a distant board ground when the source node moves.
- Confirm the off-state voltage is low enough to prevent unintended conduction.
- Confirm the on-state voltage matches the RDS(on) test condition or the device’s logic-level rating.
- Observe rise time, fall time, Miller plateau, ringing, and negative spikes.
- Check the gate resistor, driver output, bootstrap supply, and driver return path.
A threshold voltage of 2 V does not mean that the MOSFET is fully enhanced at 2 V. Threshold is measured at a small drain current. Use the guaranteed on-resistance data at the actual gate voltage.
Check the drain waveform
Measure VDS with a correctly rated probe. When off, the device must stay below its blocking-voltage rating with appropriate margin. During turn-on and turn-off, inspect overshoot and ringing. When on, an unexpectedly high VDS can indicate weak gate drive, high junction temperature, excessive current, or a damaged channel.
Use a short probe ground spring for low-side measurements. A long ground lead can create apparent ringing that is not present at the device.
Check current and temperature
Measure or calculate RMS current rather than relying only on average current. Estimate conduction loss using IRMS2 × RDS(on) at the expected junction temperature. Add switching, diode, output-capacitance, and gate-drive losses where applicable.
Monitor case or board temperature after the system reaches thermal equilibrium. Compare equivalent devices. A hot MOSFET may be the result of poor heatsink contact, insufficient copper, blocked airflow, or an unexpected switching condition rather than an internally defective part.
Common checks and their limitations
| Check | What it can reveal | What it cannot prove |
|---|---|---|
| Continuity test | Hard drain-source or gate short | Rated-voltage leakage or switching speed |
| Body-diode test | Diode direction and obvious junction failure | Low RDS(on) or healthy gate oxide |
| Gate-charge meter test | Basic channel turn-on and turn-off | Performance at rated current and temperature |
| Powered VGS check | Driver amplitude and timing | Complete device safety margin |
| VDS waveform | Overshoot, ringing, and on-state drop | Junction temperature without loss and thermal analysis |
When a MOSFET passes the checks but the circuit still fails
- The driver supply may collapse during switching.
- The microcontroller or PWM controller may remain disabled.
- The load, transformer, inductor, diode, or opposing MOSFET may be shorted.
- Dead time may be too short, causing cross-conduction.
- The replacement MOSFET may have the wrong pinout, gate requirement, or dynamic behavior.
- Board contamination or a damaged gate resistor may create intermittent operation.
- The device may leak only at high voltage or fail only after heating.
Replacement checks before power-up
Do not replace a failed MOSFET using voltage and current ratings alone. Confirm polarity, package pinout, VDSS, VGS(max), RDS(on) at the available gate voltage, gate charge, output capacitance, body-diode behavior, safe operating area, avalanche capability, and thermal interface.
Power the repaired circuit through a current-limited source where practical. Monitor gate and drain waveforms before applying full load. Replacing the MOSFET without finding the original cause often leads to a second failure.
Frequently asked questions
Can a MOSFET look normal on a meter and still be bad?
Yes. It may have excessive leakage at high voltage, increased on-resistance, weak avalanche capability, temperature-dependent damage, or poor dynamic switching. These faults require powered or specialized testing.
Does a shorted MOSFET always show zero ohms?
A catastrophic drain-source short often reads near zero in both directions, but circuit paths and meter lead resistance affect the value. Isolate the device before making a final judgment.
Should I replace the gate driver when the MOSFET fails?
Check it rather than replacing it automatically. A failed MOSFET can damage the driver, and a damaged driver can destroy the replacement. Verify driver supply, output resistance, gate waveform, and off-state control.
Summary
To check a MOSFET efficiently, connect the symptom to a structured sequence: make the circuit safe, identify the device, inspect the surrounding components, discharge the gate, check gate insulation and the body diode, isolate ambiguous readings, then verify VGS, VDS, current, and temperature under controlled power. This workflow finds both failed MOSFETs and the circuit conditions that caused the failure.
