Common MOSFET Failures

A MOSFET can fail in several different ways. The most common failures are:

• Drain-source short circuit
• Gate-source short circuit
• Gate-drain short circuit
• Excessive drain-source leakage
• Open or damaged gate
• Increased RDS(on)
• Thermal damage
• Mechanical or package damage
• Intermittent operation

A failed MOSFET may appear completely shorted, completely open, or may still work electrically while exhibiting abnormal leakage or excessive heating.

Typical Symptoms of a Failed MOSFET

A MOSFET fault can produce several different symptoms depending on where the device is used.

• Blown fuse
• Power supply immediately shuts down
• Excessive input current
• Output voltage missing
• Output voltage too low
• Motor runs continuously
• Motor does not run
• Amplifier output stage fails
• Switching transistor becomes extremely hot
• Repeated failure of replacement MOSFETs
• Visible cracks or burn marks
• Short circuit between drain and source

MOSFET Terminal Identification

Before testing a MOSFET, identify its terminals correctly.

A typical discrete MOSFET has three terminals:

• Gate (G)
• Drain (D)
• Source (S)

However, the physical pin arrangement is not universal.

Always check the manufacturer's datasheet for the exact pinout.

       ┌─────────┐
Gate ──┤         │
       │ MOSFET  │
Drain ─┤         │
       │         │
Source─┤         │
       └─────────┘

Important Safety Rule

Before testing a MOSFET installed in a power circuit, disconnect the equipment from the mains or battery and discharge the capacitors safely.

Large capacitors in power supplies and amplifiers can retain dangerous voltages even after the equipment has been switched off.

Never perform resistance or diode-mode measurements on a powered circuit unless the measurement method specifically requires it and the equipment is designed for that procedure.

Tools Required

Basic MOSFET troubleshooting can often be performed with:

• Digital multimeter
• Test leads
• Insulated screwdriver or discharge equipment where appropriate
• Soldering equipment
• Desoldering tools
• Datasheet
• Current-limited power supply

An oscilloscope is extremely useful when diagnosing MOSFET switching problems because it allows the gate and drain waveforms to be examined.

Testing a MOSFET With a Multimeter

A digital multimeter can identify many common MOSFET failures.

For basic testing, the meter's:

• Resistance mode
• Diode-test mode

can be useful.

For reliable testing, it is usually better to remove the MOSFET from the circuit, or at least isolate one or more terminals.

Why In-Circuit Testing Can Be Misleading

When a MOSFET remains soldered into a circuit, other components can provide parallel paths between its terminals.

The multimeter may therefore measure:

• Resistors
• Diodes
• Transformer windings
• Other semiconductor junctions
• Capacitors

These components can produce readings that appear to indicate a MOSFET fault when the MOSFET itself is actually good.

For definitive testing, remove the MOSFET or isolate its terminals.

Test 1: Drain-to-Source Short

A very common MOSFET failure is a short circuit between drain and source.

Set the multimeter to resistance mode and measure between:

Drain ↔ Source

A MOSFET that measures extremely close to zero ohms in both directions may be shorted.

However, an in-circuit measurement can be misleading, so the device should be isolated before declaring it defective.

Test 2: Gate-to-Source Resistance

The MOSFET gate is insulated from the semiconductor channel.

Therefore, a healthy MOSFET normally shows very high resistance between:

Gate ↔ Source
 and: 
 
 Gate ↔ Drain ``` 
 A low resistance between the gate and either terminal is a strong indication of gate-oxide damage. 
 

Gate Leakage Test

Set the multimeter to a high resistance range if available and measure:

 G-S G-D ``` 
 A healthy discrete MOSFET should normally show very high resistance. 
 
 If the meter indicates a low resistance or a clear short circuit, the gate insulation may have failed. 
 

Test 3: Body Diode

A power MOSFET contains an intrinsic body diode.

This diode can be detected with the diode-test function of a multimeter.

For an N-channel MOSFET, the body diode is normally oriented from source toward drain.

 N-MOSFET Drain │ ├──── MOSFET │ Source ↑ Body diode direction ``` 
 The exact measurement polarity depends on the MOSFET orientation and the meter leads. 
 
 The datasheet symbol should be used to confirm the diode orientation. 
 

Typical Body-Diode Measurement

In diode mode, a healthy body diode normally shows a forward-voltage reading in one direction and an open-circuit indication in the opposite direction.

The exact forward-voltage reading depends on the MOSFET and test current.

Do not expect every MOSFET to produce the same numerical value.

Body Diode Short Circuit

If the meter indicates a near-short in both directions between drain and source, the MOSFET may have failed.

Before replacing it, confirm that no external circuit component is creating the same reading.

Test 4: Gate Charging Test

A useful additional test for many MOSFETs is to temporarily charge the gate.

The gate acts approximately like a capacitor.

A simple test can be performed with an isolated MOSFET:

1. Discharge the gate by shorting gate to source briefly.
2. Measure the drain-source condition.
3. Apply a suitable positive voltage between gate and source for an N-channel MOSFET.
4. Measure drain-source conduction again.
5. Discharge the gate once more.
6. Check whether the device returns to its original state.

The exact gate voltage should remain within the MOSFET's specified VGS limits.

N-Channel MOSFET Gate Test

For an N-channel MOSFET, applying a suitable positive voltage to the gate relative to the source can turn the MOSFET ON.

 Gate voltage ↑ │ ▼ MOSFET turns ON │ ▼ Drain-source resistance decreases ``` 
 After the gate is discharged, the MOSFET should turn OFF. 
 
 This simple test can help identify a MOSFET that cannot be controlled through its gate. 
 

P-Channel MOSFET Gate Test

For a P-channel MOSFET, the gate must be driven sufficiently negative relative to the source to turn it ON.

The polarity is therefore opposite to an N-channel MOSFET.

Always remain within the manufacturer's maximum VGS rating.

Gate-Source Short

A gate-source short is one of the easiest MOSFET failures to identify.

If the gate and source measure close to zero resistance, the gate oxide has probably failed.

A gate-source short prevents the MOSFET from being controlled normally.

Gate-Drain Short

A gate-drain short is similarly abnormal.

The gate should normally be electrically isolated from the drain.

A low-resistance measurement between gate and drain usually indicates MOSFET failure.

Drain-Source Leakage

A MOSFET can fail without becoming a complete drain-source short.

Excessive leakage can cause:

• Unexpected current flow
• Heating while OFF
• Battery discharge
• Incorrect switching behavior
• Reduced efficiency

Detailed leakage testing should be performed according to the MOSFET's datasheet specifications.

Increased RDS(on)

A MOSFET may remain functional but develop excessive ON resistance.

This can cause:

• Excessive heating
• Voltage drop
• Reduced efficiency
• Current limiting
• Repeated thermal shutdown

For accurate testing, measure the voltage across the MOSFET while a known current flows through it.

Then calculate:

 RDS(on) = VDS / ID 

Example of RDS(on) Testing

Suppose a MOSFET carries:

 ID = 10 A 

and the measured drain-source voltage while fully ON is:

 VDS = 0.15 V 

Then:

 RDS(on) = 0.15 / 10 RDS(on) = 0.015 Ω RDS(on) = 15 mΩ 

This value can then be compared with the expected RDS(on) at the actual gate voltage and operating temperature.

MOSFET Overheating

Excessive MOSFET temperature is a common troubleshooting symptom.

Possible causes include:

• Excessive load current
• Insufficient gate voltage
• Weak gate driver
• High RDS(on)
• Excessive switching frequency
• Slow switching
• Incorrect dead time
• Poor heatsinking
• Poor PCB thermal design
• Excessive voltage spikes
• Damaged MOSFET

MOSFET Gets Hot Even With Low Current

If a MOSFET becomes very hot at a relatively low load current, check the gate drive first.

A MOSFET that is only partially enhanced can have much higher RDS(on) than expected.

For example, a MOSFET specified for low resistance at 10 V gate drive may become significantly more resistive when driven with an inadequate voltage.

Measure the actual VGS while the circuit is operating.

MOSFET Does Not Turn ON

If the MOSFET does not turn ON, possible causes include:

• No gate-drive signal
• Insufficient VGS
• Incorrect gate polarity
• Damaged gate oxide
• Incorrect source reference
• Faulty gate driver
• Broken PCB trace
• Incorrect MOSFET pinout

Always measure the gate voltage relative to the source, not simply relative to ground.

Why VGS Must Be Measured Relative to Source

The MOSFET responds to:

 VGS = VG - VS 

For a high-side MOSFET, the source terminal can move significantly above ground.

Therefore, measuring the gate voltage relative to ground can give a misleading result.

MOSFET Does Not Turn OFF

If a MOSFET remains ON, possible causes include:

• Gate permanently driven
• Gate-source short
• Faulty gate driver
• Gate resistor failure
• Excessive Miller coupling
• Incorrect driver reference
• Drain-source short inside the MOSFET

Disconnecting the gate driver and testing the MOSFET separately can help determine whether the fault is in the MOSFET or the surrounding circuit.

MOSFET Fails Immediately After Replacement

Replacing a failed MOSFET without finding the original cause can result in another immediate failure.

Check:

• Gate-driver circuit
• Gate resistor
• Gate-source protection
• Drain voltage spikes
• Snubber circuit
• TVS protection
• Load current
• Shorted load
• Transformer or motor windings
• Dead-time control
• PCB layout

In switch-mode power supplies, the MOSFET is often only one part of the failure.

Testing the Gate Driver

A MOSFET can be perfectly good but still fail to switch correctly because the gate driver is defective.

Check:

• Driver supply voltage
• Gate signal amplitude
• Gate signal frequency
• Gate pulse width
• Gate resistor
• Gate-source voltage
• High-side driver operation
• Driver output current

An oscilloscope is particularly useful for examining the actual gate waveform.

What a Good Gate Waveform Looks Like

A properly driven MOSFET should receive a clean gate waveform appropriate for the application.

 VGS ┌──────────┐ │ │ ──────┘ └────── ``` 
 The actual waveform may include controlled rise and fall times and can vary considerably depending on the switching topology. 
 
 Problems such as excessive ringing, slow transitions or insufficient gate voltage can indicate a gate-drive or layout problem. 
 

Gate Ringing

Gate ringing can be caused by parasitic inductance and capacitance in the gate-drive loop.

Excessive ringing can:

• Increase switching losses
• Cause false turn-on
• Increase EMI
• Exceed the MOSFET's maximum VGS
• Cause unreliable switching

A suitable gate resistor and careful PCB layout can help control ringing.

Drain Voltage Spikes

Inductive circuits can generate voltage spikes when the MOSFET switches.

These spikes can exceed the MOSFET's VDS rating even when the nominal supply voltage is much lower.

Possible solutions include:

• TVS diode
• RC snubber
• RCD clamp
• Improved PCB layout
• Reduced switching speed
• Appropriate MOSFET voltage rating

MOSFET Failure in Motor Circuits

Motors are inductive loads and can produce significant voltage transients.

Common causes of MOSFET failure in motor controllers include:

• Motor stall current
• Inductive voltage spikes
• Insufficient MOSFET voltage rating
• Insufficient current rating
• Poor cooling
• Inadequate gate drive
• Insufficient dead time

Protection and proper freewheel-current paths are essential.

MOSFET Failure in SMPS Circuits

In switch-mode power supplies, MOSFET failures can result from:

• Drain-voltage overshoot
• Transformer leakage inductance
• Excessive current
• Gate-drive failure
• Insufficient dead time
• Switching losses
• Overheating
• Controller malfunction

If a primary-side MOSFET repeatedly fails, the controller, snubber, current sense circuit and transformer should also be investigated.

Testing a MOSFET in a Power Supply

When troubleshooting a power supply:

1. Disconnect power.
2. Discharge the capacitors safely.
3. Identify the MOSFET terminals.
4. Check drain-to-source resistance.
5. Check gate-to-source resistance.
6. Check gate-to-drain resistance.
7. Check the body diode.
8. Compare the readings with a known-good device if available.
9. Inspect the gate-drive circuit.
10. Inspect nearby components for damage.

Do not immediately install a new MOSFET until the cause of the original failure has been investigated.

Testing a MOSFET With a Current-Limited Supply

After replacing a MOSFET, a current-limited laboratory power supply can reduce the risk of catastrophic failure during initial testing.

The current limit should be appropriate for the circuit under test.

This technique is especially useful during prototype development and repair.

Visual Inspection

Always inspect the MOSFET before electrical testing.

Look for:

• Cracked package
• Burn marks
• Discoloration
• Melted plastic
• Damaged leads
• Loose mounting
• PCB carbonization
• Burned gate resistor

Physical damage does not always occur when a MOSFET fails, so a visually perfect device can still be electrically defective.

Check Surrounding Components

A failed MOSFET can damage other components at the same time.

Check:

• Gate resistors
• Gate-driver IC
• Diodes
• TVS devices
• Snubbers
• Current-sense resistors
• Fuses
• Inductors
• Transformers
• Load components

The failure may have propagated through the circuit.

How to Distinguish MOSFET Failure From Circuit Failure

The best approach is to separate the device from the surrounding circuit.

 Circuit fault? │ ▼ Remove / isolate MOSFET │ ▼ Test MOSFET separately │ ┌────┴────┐ ▼ ▼ Bad Good │ │ ▼ ▼ Replace Investigate MOSFET surrounding circuit 

This prevents other components from producing misleading multimeter measurements.

Common Multimeter Mistakes

• Testing a powered circuit in resistance mode
• Ignoring the MOSFET pinout
• Testing only drain-to-source
• Ignoring gate leakage
• Assuming every MOSFET has the same body-diode orientation
• Testing the device only while it is installed in the circuit
• Using VGS(th) as the expected ON voltage
• Declaring a MOSFET good without testing it under load

When a Multimeter Is Not Enough

A multimeter is excellent for identifying obvious shorts and open circuits, but it cannot completely characterize a MOSFET.

For advanced troubleshooting, an oscilloscope may be needed to inspect:

• Gate waveform
• Drain waveform
• Switching transitions
• Voltage spikes
• Ringing
• Dead time
• Driver operation

A component tester can also be useful for basic semiconductor identification, but its measurements should not replace the manufacturer's datasheet specifications.

Systematic MOSFET Troubleshooting Procedure

1. Identify the circuit topology.
2. Disconnect power.
3. Discharge stored energy safely.
4. Inspect the MOSFET and PCB visually.
5. Identify the exact MOSFET part number.
6. Find the manufacturer's datasheet.
7. Confirm the pinout.
8. Check drain-source resistance.
9. Check gate-source resistance.
10. Check gate-drain resistance.
11. Check the body diode.
12. Remove or isolate the MOSFET if measurements are inconclusive.
13. Test the surrounding components.
14. Check the gate-driver circuit.
15. Check the load for shorts or excessive current.
16. Check voltage spikes and switching waveforms when necessary.
17. Replace the MOSFET with an appropriate equivalent.
18. Power the circuit using suitable current limiting where possible.
19. Monitor gate voltage, drain voltage and temperature.
20. Verify normal operation under the intended load.

Quick MOSFET Fault Table

Key Points

• A shorted drain-source junction is one of the most common MOSFET failures.
• The insulated gate should normally show very high resistance to drain and source.
• The body diode can be checked with a multimeter's diode-test function.
• In-circuit testing can produce misleading measurements.
• For definitive testing, isolate or remove the MOSFET.
• Always verify the MOSFET pinout before testing.
• VGS must be measured relative to the source.
• A MOSFET can overheat even when its current is below the advertised maximum.
• Insufficient gate voltage can cause excessive RDS(on).
• A failed MOSFET may damage its gate driver and surrounding components.
• Repeated MOSFET failures usually indicate an unresolved circuit problem.
• Inductive loads can produce destructive voltage spikes.
• An oscilloscope is valuable for diagnosing switching and gate-drive problems.
• Thermal management must be considered when diagnosing overheating.
• Always investigate the original cause before installing a replacement MOSFET.