IGBT

The IGBT combines a MOSFET's easy gate with a bipolar transistor's low conduction drop at high voltage. Learn V_CE(sat), switching energies and tail current, loss and thermal calculations, modules, short-circuit protection and IGBT vs SiC.

Above a few hundred volts, a silicon MOSFET's resistance grows rapidly. The insulated-gate bipolar transistor fixes that with conductivity modulation: a MOSFET gate controls a structure that injects minority carriers, so the on-state drop stays near 1.5–2 V even at 1200–6500 V ratings.

IGBTs power industrial drives, traction, UPS systems, solar and wind inverters and induction heating. They switch slower than MOSFETs (typically up to 20–50 kHz), which is fine for motor drives.

An analogy

An IGBT is like a valve where a small pilot valve (the MOSFET gate) opens a much larger flow path assisted by pressure (minority carriers). It opens easily and flows freely, but takes a moment to close fully — the 'tail'.

Best of both worlds

Gate like a MOSFET: voltage-driven, almost no steady gate current (typically ±15 V drive). Conduction like a bipolar transistor: a roughly constant voltage drop plus a small slope, V_CE ≈ V₀ + r·I. At high currents this beats a MOSFET's I·R_DS(on).

Tail current and switching energy

At turn-off, stored minority carriers must recombine, so collector current decays with a 'tail' while voltage is already high. That makes turn-off energy E_off significant and limits frequency. Datasheets give E_on and E_off at reference conditions; scale them with voltage and current.

Modules and anti-parallel diodes

IGBTs are packaged as half-bridge or six-pack modules with fast anti-parallel diodes (IGBTs have no useful body diode). Diode reverse-recovery adds to the opposite IGBT's turn-on loss. Modules mount on heat sinks with thermal interface material.

Short-circuit protection and gate drive

IGBTs survive a hard short circuit for ~10 µs. Desaturation detection senses V_CE rising while the gate is on and shuts down softly to avoid overvoltage from L·di/dt. Negative gate bias (−8 to −15 V) prevents Miller-induced turn-on, and active clamping limits overshoot.

IGBT vs SiC MOSFET

SiC MOSFETs have no tail current and much lower switching loss, and at light load their resistive drop beats an IGBT's knee voltage — improving EV drive-cycle efficiency by several percent. IGBTs remain cheaper and very robust, and dominate at the highest powers (traction, HVDC VSC).

Formulas

• Conduction loss: P_cond = V_CE(sat) · I_avg (≈ (V₀ + r·I)·I·D)
• Switching loss: P_sw = (E_on + E_off) · f_sw · (V/V_ref) · (I/I_ref)
• Junction temperature: T_j = T_heatsink + P · (R_θJC + R_θCS)

Worked example: IGBT loss in an inverter leg

Given: I = 100 A while conducting; V_CE(sat) = 1.8 V; D = 0.5; E_on + E_off = 10 mJ at operating point; f_sw = 10 kHz

Find: Conduction, switching and total loss.

• P_cond = 1.8 × 100 × 0.5 = 90 W
• P_sw = 10e-3 × 10e3 = 100 W
• Total = 190 W

Answer: 190 W per IGBT

Worked example: Junction temperature

Given: P = 190 W; R_θJC = 0.15 K/W, R_θCS = 0.05 K/W; Heat-sink = 70 °C

Find: T_j.

• T_j = 70 + 190 × (0.15 + 0.05) = 70 + 38 = 108 °C

Answer: 108 °C (within a 150 °C rating, with margin)

Common mistakes

• Wrong: Running IGBTs at MOSFET-like frequencies (100 kHz+). — Right: Tail-current switching loss explodes; keep IGBTs to ≤ 20–50 kHz or use SiC/GaN.
• Wrong: Driving the gate to 0 V for off-state in a bridge. — Right: Use negative gate bias or an active Miller clamp to prevent parasitic turn-on.

Applications

• Industrial VFDs: Six-pack IGBT modules switch at 2–16 kHz to drive motors from a few kW to MW.
• Rail traction: 3.3–6.5 kV IGBT modules power locomotive and metro inverters.
• Renewables: Central solar inverters and wind converters use IGBT modules at hundreds of kW to MW.

Key takeaways

• IGBT = MOSFET gate + bipolar conduction; best at high V and I.
• Losses: V_CE(sat)·I·D + (E_on + E_off)·f.
• Tail current limits frequency to tens of kHz.
• Protect with desat detection and negative gate bias; SiC is displacing IGBTs where efficiency matters.

Going further

• Power cycling and lifetime: Repeated junction temperature swings (ΔT_j) fatigue bond wires and solder layers. Coffin–Manson models predict cycles-to-failure; traction designs limit ΔT_j per load cycle.
• Thermal networks: Transient thermal impedance Z_th(t), modelled as Foster/Cauer RC networks, predicts T_j ripple at low output frequency (e.g. a motor at standstill), which is often the real limit.
• Reverse-conducting and RB-IGBTs: RC-IGBTs integrate the diode on the same die; reverse-blocking IGBTs block both polarities for matrix and current-source converters.