What is an IGBT? How is it different from a MOSFET?
Open a motor drive, an inverter welder or an induction cooker and you'll see a big black module or a few TO-247 parts bolted to the heatsink. Most aren't MOSFETs but IGBTs: the switch for high voltage and high current.
What does it do? An IGBT (Insulated Gate Bipolar Transistor) is a switch whose input is driven by voltage through an insulated gate like a MOSFET, and whose output conducts like a bipolar transistor. When on, a VCE(sat) of ≈1.5–2.5 V remains, changing little with current; a MOSFET's loss grows with the square of the current, I² × RDS(on). At 600–1700 V and tens to hundreds of amps the IGBT runs cooler. The price is speed: at turn-off a 'tail current' flows until the stored charge clears, so IGBTs usually run at 2–20 kHz. It has no inherent body diode; modules add a fast anti-parallel diode to every IGBT. The gate turns on with +15 V and is held off with −5…−15 V so noise can't turn it on.
⏱ Slow motion: what the driver IC outputs and what the circuit does (the highlighted slice is the moment shown on the schematic)
At turn-off IC doesn't drop to zero at once (tail current) while VCE is already rising: that overlap is the turn-off loss. That's why IGBTs usually run below 20 kHz while MOSFETs run at hundreds of kHz.
Tap the dashed parts for what they do, how to measure them and how they usually fail. Numbered points are scope measurements.
Phase 1/3: Conducting
The driver applies +15 V gate-to-emitter. The IGBT turns fully on; the winding current flows DC bus (+) → winding → collector → emitter → bus (−) and slowly rises. About 1.5–2.5 V of VCE(sat) remains across the IGBT, almost regardless of current.
Gate driver: HIGH (≈12 V, switch on)
1. Gate–emitter (VGE)
+15 V on, −8 V off. The negative voltage stops noise coupled through the Miller capacitance from falsely turning the IGBT on. Measured against the emitter; in a bridge the upper IGBT's emitter is floating.
On the scope
Multimeter+15 V / −8 V
🔍 How to spot it on a board
•Large modules bolted to the heatsink with screw or pin terminals marked C, E, G or numbered (e.g. a dual half-bridge, a six-pack three-phase module)
•Discrete TO-247 parts: codes often start with 'G' or come from series like 'IHW', 'FGH', 'GT' (e.g. FGH40N60, IHW30N120)
•Gate driver optocouplers (HCPL-3120 etc.) nearby, and a small isolated supply for each IGBT
🔧 How to measure it
•First switch off and confirm with a meter that the DC bus has discharged: it can stay above 500 V for minutes.
•Diode mode: red on E, black on C ≈0.3–0.5 V (the anti-parallel diode); OL the other way. G–E OL both ways.
•Turn-on test: briefly charge G positive relative to E with a 9 V battery, C–E then reads low; shorting G–E brings it back to OL.
•Running, scope the G–E pulses (+15 / −8 V); the upper switch floats, so use a differential probe.
⚠ Common faults
•C–E short (most common): the module blows; always check the fuses, gate resistors and driver too.
•G–E reads a few ohms or a short: the gate oxide is punctured; the driver side may be damaged as well.
•If it keeps blowing: don't fit a new module before checking motor/cable insulation, the negative gate supply, dead time and the DC bus capacitors.
🔒 This content is in the repair pack
The repair pack unlocks all interactive content in one go: diagrams and current flows, the Repair Workshop, simulations, concepts, IC guides, the Component Library and comparison guides. One sample in every section is free.