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If a replacement transistor burns repeatedly, the transistor is usually the victim, not the cause. Stop fitting “stronger” parts and test the circuit for voltage spikes, excessive current, partial turn-on, inductive kickback, inadequate drive, thermal overload, wiring errors, or operation outside the device’s safe operating area (SOA).
The correct diagnosis depends on whether the failed part is a power MOSFET, BJT, IGBT, or a transistor inside an integrated driver. Mains circuits, large batteries, high-voltage capacitors, and high-energy motor drives can be lethal; use appropriate isolation and measurement equipment or have a qualified technician perform the tests.
First identify the transistor and when it fails
Record the exact marking, package, manufacturer, and datasheet. Confirm the device type and polarity: drain-gate-source for a MOSFET, collector-base-emitter for a BJT, and the corresponding terminals for an IGBT. Do not assume that parts in the same TO-220 or TO-247 package share a pinout. Check whether a tab is electrically connected to the drain or collector.
Also record the supply voltage, load type, switching frequency, gate or base-drive voltage, protection parts, heatsink arrangement, and the time between power-up and failure. A transistor inside a regulator, motor-driver IC, or gate-driver module may be the visible casualty of a fault elsewhere.
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| Failure timing or symptom | Investigate first |
|---|---|
| Immediate failure on power-up | Wrong pinout or polarity, shorted load, gate/base overvoltage, failed driver, missing current path, or shoot-through |
| Failure after seconds or minutes | Conduction loss, switching loss, inadequate cooling, excessive continuous current, or DC-SOA violation |
| Failure only with a motor, relay, solenoid, or transformer | Startup or stall current, inductive kickback, avalanche, ringing, or failed clamp |
| Two devices in a bridge fail together | Cross-conduction, insufficient dead time, false turn-on, or driver timing trouble |
A device that tests short after it fails does not identify the original mechanism. Electrical overstress can leave a part visibly cracked, burned, or holed, or internally shorted with little external evidence. See Infineon’s overview of EOS symptoms and causes: Electrical Over-Stress (EOS) review.
The six checks that explain most failures
1. Voltage and transient peaks
Compare the actual peak drain-source or collector-emitter waveform with the absolute-maximum rating. The DC supply value is not enough: wiring inductance, transformer leakage, diode recovery, and switching edges can add a large turn-off spike. Leave practical margin below the rating rather than selecting a device whose rating merely equals the supply.
Measure turn-off overshoot, ringing, repetitive avalanche, gate overshoot, and negative gate voltage. TI discusses parasitic inductance and abrupt load disconnection as sources of damaging spikes in this switching-transient guide. Use a properly rated differential probe or isolated method; never connect a grounded oscilloscope probe across a floating or mains-referenced switching node.
2. Current, including short transients
A load marked “5 A” or “10 A” may draw far more during motor startup or locked-rotor operation, capacitor charging, transformer energization, lamp cold-start, solenoid pull-in, or a short circuit. Measure steady, startup, stall, PWM-peak, and charging current with a current probe, suitable shunt, or rated current-sense instrument. A basic multimeter can miss the event that destroys the transistor.
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3. Power dissipation
When fully enhanced, a MOSFET’s approximate conduction loss is:
Pconduction = I2 × RDS(on)
For a BJT or IGBT, use approximately VCE(sat) × IC. During partial turn-on, the dangerous instantaneous loss is VDS × ID or VCE × IC; both voltage and current are substantial at once.
Hard-switching loss can be estimated as:
Psw ≈ ½ × VDS × ID × (tr + tf) × switching frequency
This is only a first estimate. Measured voltage-current waveforms include parasitic inductance, diode recovery, gate resistance, ringing, and real load behavior. Infineon explains waveform-based switching-loss analysis in its power-loss and thermal guide.
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4. Junction temperature
The silicon can exceed its temperature limit before the package feels extremely hot. Use the thermal chain:
TJ = TA + P × RθJA
or, when case temperature is known, TJ = TC + P × RθJC. Datasheet current and power figures often assume a specified case temperature, ideal heatsinking, or a short pulse. In the real assembly, derate for ambient temperature, enclosure airflow, PCB copper, mounting pressure, thermal-pad flatness, interface material, and whether an insulating washer is required. Check whether the tab must remain electrically connected.
5. Gate or base drive
For a MOSFET, measure voltage directly between gate and source at the device pins, not merely the logic rail relative to ground. Verify that the datasheet specifies RDS(on) at your actual drive voltage (for example 10 V, 4.5 V, 2.5 V, or 1.8 V). Threshold voltage only marks the beginning of conduction under a test condition; it is not a fully-on specification.
- Check for a floating gate and provide the required pull-up or pull-down.
- Confirm the driver can source and sink the required gate current.
- Look for excessive gate resistance, Miller-plateau delay, gate overshoot, or negative spikes.
- For high-side devices, verify the driver’s common-mode range and bootstrap or isolated supply.
For a BJT, measure actual base current, forced beta, saturation voltage, stored charge, and turn-off drive. A switching BJT can remain stressed while stored base charge is removed. TI covers base drive, saturation, and turn-off behavior in this BJT switching article.
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- Comprehensive Transistor Assortment: This kit offers 24 values with 840 pieces with 35 pieces of each type, including 2n2222/2n2907/2n3904/2n3906/BC327/BC337/A1015/C1815/S8050/S8550 transistors. It offers a wide variety of NPN and PNP transistors to meet all your circuit design and maintenance requirements
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6. Safe operating area
Use the exact device’s SOA graph for the actual voltage, current, pulse duration, case temperature, and operating mode. A “100 A” headline rating does not mean the part can safely dissipate 100 A at tens of volts.
SOA is critical when a MOSFET acts as an electronic load, current limiter, inrush limiter, linear regulator pass element, or soft-start device. A low-RDS(on) switching MOSFET may have poor DC or long-pulse linear-mode SOA. onsemi describes hot-spot and thermal-runaway behavior in its power-MOSFET thermal-management paper.
MOSFET-specific failure mechanisms
Inductive kickback and avalanche
When current through a motor, relay, solenoid, transformer, or long cable is interrupted, stored magnetic energy forces the switching node upward. The MOSFET may avalanche, dissipating that energy as heat. Repetitive-avalanche ratings are conditional on pulse energy, repetition rate, temperature, and the datasheet test circuit; they are not permission for unlimited avalanche.
Use a protection path chosen for supply voltage, load current, stored energy, repetition rate, and required release speed:
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- A flyback diode suits many low-side DC loads but slows current decay.
- A TVS or zener clamp permits faster release at a controlled voltage.
- An RC snubber, RCD clamp, or active clamp can suit converters and transformers.
- Keep the clamp physically close to the high-current loop.
Infineon identifies avalanche, back-EMF, and excessive gate voltage among common MOSFET-burning causes in this troubleshooting note.
Gate ringing and false turn-on
Drain-voltage transients couple through gate-drain capacitance and parasitic inductance. If ringing crosses the threshold, a supposedly off MOSFET can turn on partially. Symptoms include both bridge devices heating, shoot-through, intermittent destruction, EMI, and sensitivity to probe-ground lead length.
- Probe gate-to-source with a spring ground connection.
- Use a dedicated driver and a gate-source pull-down or pull-up.
- Optimize, rather than blindly increase, gate resistance.
- Use Kelvin-source routing and separate power and gate returns.
- Reduce edge speed, add dead time, or use a Miller clamp or negative turn-off drive where justified.
Too much gate resistance lengthens transitions and increases switching loss; Infineon explains this trade-off in its gate-ringing guidance.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.BJT and IGBT considerations
A BJT needs enough base current to reach the intended saturation voltage at the real collector current. An overly large base resistor, weak driver, or inadequate turn-off path can leave it in a high-dissipation region. Check forced beta, collector-emitter voltage during turn-off, stored charge, and switching frequency.
An IGBT combines a MOS gate with bipolar conduction. Verify gate voltage, short-circuit withstand time, turn-off overshoot, desaturation protection, and its SOA. Do not substitute a MOSFET, BJT, or IGBT solely because the package and nominal current look similar.
Layout and installation faults
- Keep the high-current loop short and wide; place the diode, TVS, or snubber close to the switch and load loop.
- Put ceramic and bulk bypass capacitors close to the switching path.
- Keep gate traces away from the drain node and use a dedicated gate-return path.
- Avoid solderless breadboards for high-current or fast-switching circuits.
- Check vias, solder joints, thermal-pad attachment, connectors, and carbonized PCB material.
- Verify the exact datasheet pinout, body-diode direction, polarity, tab connection, and part marking.
- Use traceable parts; counterfeit, remarked, salvaged, or damaged devices may not meet the printed rating.
A safe diagnostic sequence
- Stop installing replacements. Disconnect power and inspect the failed device, driver, load, diode, snubber, PCB copper, connectors, and wiring.
- Test unpowered. Check supply-to-ground resistance, load resistance, clamp components, gate-driver output, pull resistors, complementary devices, and solder bridges.
- Use current limiting. Start a low-voltage circuit with a low bench-supply limit or suitable series limiter. Increase it only after current and temperature behave normally.
- Measure load current. Capture steady, startup, stall, PWM peak, and capacitor-charging current; do not rely on the nameplate running current.
- Measure drive waveforms. For a MOSFET, probe gate-to-source at the pins. Look for inadequate amplitude, slow edges, ringing, false turn-on, incomplete turn-off, and excessive Miller time. For a BJT, measure base current and collector-emitter voltage.
- Measure the switching node safely. Capture peak voltage, turn-off spikes, ringing, diode recovery, current-voltage overlap, and bridge dead time with a correctly rated differential or isolated probe.
- Compare with the datasheet. Check voltage, gate/base limits, continuous and pulsed current, SOA, junction temperature, avalanche energy, drive-voltage conditions, gate charge, capacitance, and thermal resistance.
- Correct the cause first. Redesign the clamp, improve drive, add current limiting, alter dead time or edge speed, improve cooling, select a linear-mode-rated device, change topology, or replace the failed driver/controller.
Choosing a replacement
Prioritize measured voltage margin and SOA, then correct drive-voltage specifications, package and thermal capability, conduction loss, switching loss and gate charge, avalanche or short-circuit capability, and traceable availability. A lower-RDS(on) part can have higher gate charge and capacitance, overloading the existing driver or slowing its transitions.
A larger heatsink only addresses thermal dissipation. It cannot cure gate overvoltage, drain spikes, shoot-through, wrong wiring, inductive kickback, switching loss, or an unsuitable SOA. A higher-rated transistor helps only if its package, drive requirements, capacitances, SOA, and thermal resistance fit the circuit.
Quick Recap
When the transistor is not the real fault
| Observed symptom | Likely areas |
|---|---|
| Hot while supposedly fully on | Insufficient drive, high actual current, high temperature, or excessive RDS(on) |
| Works with a resistor but not a motor or relay | Inductive transient, startup current, or inadequate clamp |
| Works at low voltage but fails at rated voltage | Overshoot, ringing, insulation, or SOA margin |
| Fails after an inrush pulse | Linear-mode thermal stress or unsuitable pulse SOA |
| Only one board component fails | It may be the first component to shunt the overstress, while the driver, supply, layout, or load caused it |
Information needed for a reliable diagnosis
- Exact transistor part number and package
- Schematic or clear, high-resolution board photographs
- Supply voltage, tolerances, and possible surges
- Load type, steady current, startup or stall current, and inductance
- Gate/base-drive voltage and switching frequency
- Failure timing and whether it occurs at turn-on or turn-off
- Flyback, TVS, snubber, current-sense, fuse, and dead-time details
- Heatsink, thermal interface, airflow, and enclosure temperature
- Measured gate, current, and switching-node waveforms
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