IGBT Power Switching

A MOSFET front-end driving a bipolar back-end

The IGBT is what you get when you take a high-voltage power MOSFET, look at its terrible R_DS(on) at high blocking voltage (silicon MOSFET R_DS(on) scales roughly as V_DS², so a 1200 V Si MOSFET is hundreds of milliohms), and decide to fix that by replacing the drift region's resistive conduction with bipolar minority-carrier conduction. The result is one device with three terminals: gate, collector, emitter.

IGBT equivalent circuit
       C  collector
       │
       ●─ ── ── ── ── ── ── ── ┐
       │                       │
       │            ┌────────┐ │
       │            │   PNP  │ │   bipolar power transistor
       │            └──┬──┬──┘ │
       │               │  │    │
       │            ┌──┴──┘    │
       │     ┌──────┤ MOSFET   │   MOSFET drives the PNP's base
   G ──┴─────┤      └──┬───────┘
   gate      └─────────●─ ── ── ┘
                       │
                       ● E (emitter)

The MOSFET portion has a high-impedance insulated gate — same as any MOSFET, drawing negligible steady-state current. When V_GE exceeds the threshold V_th (4-7 V depending on device), the MOSFET turns on and supplies base current to the embedded PNP power transistor. That PNP, with its base now driven, conducts heavily through the device's drift region. The conducting drift region is flooded with minority carriers (conductivity modulation), which collapses its effective resistance and drops only V_CE_sat — 1.5 to 2 V — across the whole device regardless of how much current is flowing.

Compare that with a hypothetical 1200 V silicon MOSFET. R_DS(on) for a 1200 V silicon MOSFET would be on the order of 500 milliohms; at 100 A that's 50 V of drop, dissipating 5000 W in a single device. Unusable. The bipolar conduction inside the IGBT bypasses that limit at the cost of switching speed, and silicon manufacturers have been refining the trade since the early 1980s.

V_CE_sat and conduction loss

V_CE_sat is the IGBT's headline conduction parameter, analogous to MOSFET R_DS(on) but with very different scaling. Where a MOSFET drops V = I · R_DS(on) (linear in current), an IGBT drops V = V_CE_sat (roughly constant in current). Typical numbers:

IGBT module:    600 V / 200 A  V_CE_sat ≈ 1.7 V  →  P_cond = 1.7 V × 200 A = 340 W
                1200 V / 300 A  V_CE_sat ≈ 2.0 V  →  P_cond = 2.0 V × 300 A = 600 W
                1700 V / 1200 A V_CE_sat ≈ 2.2 V  →  P_cond = 2.2 V × 1200 A = 2640 W

Compare to Si MOSFET (impossible at these voltages):
                If R_DS(on) = 5 mΩ at 100 A → P_cond = (100 A)² × 5 mΩ = 50 W (great)
                If R_DS(on) = 200 mΩ at 100 A → P_cond = (100 A)² × 200 mΩ = 2000 W (impossible)

The crossover where IGBT beats MOSFET on conduction loss is roughly the I × R_DS(on) > V_CE_sat threshold. Below that current the MOSFET wins; above it, the IGBT wins. For silicon at 1200 V the crossover is around 5-10 A; above 50 A the IGBT is overwhelmingly better. SiC MOSFETs shift that crossover upward — they have much lower R_DS(on) at 1200 V (about 80 milliohms commercially as of 2024) so the IGBT advantage disappears at currents below about 30 A.

The turn-off tail current

The price of bipolar conduction is bipolar storage. During on-state the drift region is full of minority carriers — that's what makes V_CE_sat low. When you turn the device off, the MOSFET section stops supplying base current instantly, but the stored minority charge in the drift region has to recombine or be swept out before the device can block voltage. While that happens, current continues flowing through the device, forming the famous "tail current" of an IGBT turn-off waveform.

          ╱──────  V_CE rises to V_bus
   V_CE  ╱      ╲
   ────╱         ╲────────────
                       ╲___________
   I_C  ──────────╲     ╲_           ← TAIL CURRENT (100-500 ns)
                   ╲      ╲___________
                    ╲______________________
                                          ╲___ 0 A

   gate ↓ off
        ●─────────────────────────────────► time
        ↑
        gate driver pulled V_GE to 0 V here

   tail current carries V_bus → high V × I overlap → switching loss

The tail current's duration depends on the IGBT's specific design — fast-switching "non-punch-through" (NPT) IGBTs have shorter tails (~100 ns) but higher V_CE_sat; soft-switching "punch-through" (PT) IGBTs have lower V_CE_sat but longer tails (~500 ns). Modern field-stop / trench IGBTs balance both. Either way, the tail current limits switching frequency: at any given switching loss budget, you can either switch fewer times per second with each event being more lossy, or you can spend money on SiC.

Practically, big IGBT modules in EV traction or solar inverters switch at 8-15 kHz, balancing tail-current losses against the inductor sizing required for low PWM frequency. Train traction modules at 4500 V switch at 1-2 kHz because their tail currents are even longer. Industrial motor drives at 690 V often run 4-8 kHz.

Worked example — a 100 kW EV traction inverter

Consider a 400 V battery EV with a 100 kW peak motor. The inverter is a three-phase bridge of six 650 V / 600 A IGBT modules. At rated power, each phase carries 100 kW / (3 × 400 V × 0.95) ≈ 88 A average per phase. Peak phase current at acceleration could hit 600 A. Switching frequency 10 kHz.

Conduction loss per IGBT module at 88 A average:
  P_cond = V_CE_sat × I_avg × duty_avg = 1.7 V × 88 A × 0.5 = 75 W

Switching loss per IGBT module (datasheet E_sw at 600 A → 30 mJ at 10 kHz):
  P_sw = E_sw × f_sw = 30 mJ × 10 kHz × 0.5 = 150 W (scaled by avg load)

Per module: P_total ≈ 75 W cond + 150 W sw = 225 W
Six modules: 1350 W total
Inverter efficiency: 100 kW / (100 kW + 1.35 kW) = 98.7%

Same calculation with SiC MOSFETs:
  R_DS(on) 80 mΩ at 100°C → P_cond = (88 A)² × 80 mΩ × 0.5 = 310 W (worse!)
  But P_sw = 5 mJ × 10 kHz × 0.5 = 25 W (six times better)
  Six modules: 6 × (310 + 25) = 2010 W cond-dominant

Hybrid path — SiC at 800 V battery (Porsche Taycan):
  R_DS(on) 30 mΩ at 800 V class → P_cond = much smaller
  Plus SiC allows 30-50 kHz switching → smaller inductors

At 400 V battery with Si IGBT, the inverter hits 98.7% peak efficiency. SiC at the same voltage hardly helps because the current is so high that R_DS(on) × I² beats V_CE_sat × I. But 800 V SiC (Porsche, Hyundai E-GMP, Lucid, the Taycan platform) doubles the voltage and halves the current, so R_DS(on) × I² collapses and SiC's per-switch energy advantage wins. That is the entire reason 800 V architectures exist.

Module construction — IGBT chips bonded to a baseplate

An IGBT module is fundamentally a packaged assembly of multiple IGBT chips (with anti-parallel freewheeling diodes — usually fast silicon or SiC Schottky) soldered onto a Direct-Bonded Copper (DBC) substrate, with wire-bond interconnects between chips. The DBC consists of a thermally conductive ceramic (typically Al₂O₃ or AlN) sandwiched between two copper layers — top copper for circuit traces, bottom copper soldered onto a copper baseplate that bolts to a water-cooled cold plate.

IGBT module cross-section
   ┌─────────────────────────────────────────┐
   │           Plastic housing               │
   │   ┌───┐ ┌───┐ ┌───┐                    │
   │   │IGBT│ │IGBT│ │IGBT│   wire bonds    │
   │   └───┘ └───┘ └───┘                    │
   ├─────────────────────────────────────────┤  ← Top copper (DBC)
   ├─────────────────────────────────────────┤  ← Al₂O₃ or AlN ceramic
   ├─────────────────────────────────────────┤  ← Bottom copper (DBC)
   ├─────────────────────────────────────────┤  ← Solder
   ├─────────────────────────────────────────┤  ← Copper baseplate
   └─────────────────────────────────────────┘
                ↓ thermal interface ↓
              Cold plate (water-cooled)

The thermal resistance from junction to coolant is the module's most important figure of merit after V_CE_sat. A modern half-bridge automotive IGBT module has R_th_jc (junction-to-case) of about 0.1 K/W; at 600 W of total loss this gives a 60 °C rise from coolant to silicon. Coolant at 65 °C means junction at 125 °C — the standard maximum for silicon. SiC modules tolerate 175-200 °C junctions; new sintered-silver attach replaces solder to remove the thermal bottleneck. Module construction is where the difference between a 50,000-cycle and a 500,000-cycle thermal lifetime is engineered.

IGBT vs Si MOSFET vs SiC MOSFET

Property	Si MOSFET	Si IGBT	SiC MOSFET
Voltage class typical	30 V – 1500 V (mostly ≤200 V)	600 V – 6500 V	650 V – 3300 V
Current per device	10 A – 500 A	30 A – 3600 A	50 A – 1000 A
On-state drop at rated current	I × R_DS(on)	V_CE_sat ≈ 1.5-2 V	I × R_DS(on), 3-5× lower than Si MOSFET
Switching frequency	50 kHz – 1 MHz	1 – 20 kHz	30 – 200 kHz
Gate drive	0-12 V, 4-6 A peak	0-15 V, 4-15 A peak	-5 / +18 V, 4-15 A peak
Tail current	None	100-500 ns	None
Sweet spot	Low-V, high-freq, low-I	High-V, high-I, medium-freq	High-V, high-I, high-freq
Cost ($/A at 1200 V)	Impractical	Baseline	2-4× baseline
Typical use	Buck converters, ATX power supplies	EV inverters (≤400 V), solar, traction, drives	800 V EV inverters, premium solar, MV converters

Where IGBTs live

• EV traction inverters (Si IGBT era). Toyota Prius generations 1-4 (1997-2022), Nissan Leaf, Chevrolet Volt/Bolt, BMW i3, early Tesla Model S and Roadster — all 400 V / 100-200 kW inverters built around six 600 V or 650 V IGBT modules switching at 8-15 kHz. The IGBT was the only viable choice; SiC didn't reach commercial volumes until ~2018.
• Solar string inverters. 60 kW to 250 kW commercial-rooftop and utility-scale solar inverters use 1200 V or 1700 V IGBT modules in three-phase bridge plus separate boost converter. SMA Sunny Tripower, Huawei SUN2000, and similar lines. Switching at 8-16 kHz to keep output filter inductors small. Higher-end products are moving to SiC.
• Industrial motor drives. ABB ACS800, Siemens Sinamics, Schneider Altivar — variable-frequency drives from 1 kW to 5 MW. The mid-power workhorse where IGBTs at 1200 V dominate and switching frequencies of 4-8 kHz are standard. Drives below 1 kW use discrete IGBTs or moving to Si MOSFETs; drives above 500 kW use water-cooled IGBT stacks.
• Railway traction. Modern locomotives and high-speed trains use 3300 V or 6500 V IGBT modules in three-phase inverters driving large induction or PMSM traction motors. Switching at 0.5-2 kHz; tail currents are very long and switching loss is the dominant design constraint. Bombardier MITRAC and Mitsubishi/ABB traction blocks are typical.
• HVDC transmission. Voltage Source Converter (VSC) HVDC schemes use stacks of IGBT modules — typically Modular Multilevel Converters (MMC) with hundreds of submodules per pole — to convert AC to DC at hundreds of kilovolts and gigawatt power levels. Each submodule is a small half-bridge IGBT switching at 100-500 Hz.
• Welding inverters. Stick welders, MIG welders, and TIG welders use IGBTs for the high-current secondary switching. Hundreds of amperes at a few volts, switching at 20-50 kHz to keep the transformer small. Lincoln Electric, Miller, Fronius IGBT-based welder lines dominate.

Common failure modes

• Latch-up under short-circuit. A parasitic thyristor inside the IGBT structure can latch if collector current overshoots during a short-circuit fault. Once latched, the gate loses control and the device conducts uncontrollably until something else fails. Modern IGBTs include layer modifications and short-circuit ratings (typically 10 μs withstand); gate drivers use desaturation (DESAT) protection to detect the fault and turn the gate off within 5-7 μs.
• Excessive dV/dt at turn-off. Tail currents combined with stray inductance produce voltage overshoots above V_CE_max during turn-off. The standard mitigation is a snubber capacitor across the DC bus, a turn-off gate resistor sized to slow dV/dt to under 5 kV/μs, and active clamping that turns the gate slightly on if V_CE goes past the bus voltage.
• Thermal cycling fatigue. The wire bonds and solder joints inside IGBT modules see thermal expansion as load varies. Over hundreds of thousands of power cycles the bonds fatigue and the solder cracks, eventually causing the bond to lift or the solder to delaminate. Mitigation: copper-clad bonding wires (lower thermal expansion mismatch), sintered-silver chip attach instead of solder. Tesla's switch to sintered-silver in the Model 3 traction module is widely cited as a reliability breakthrough.
• Cosmic-ray-induced single-event burnout. Above about 1200 V, atmospheric neutrons can create local hot spots inside the silicon that punch through the drift region and short the device. Failure rate scales with applied voltage. Mitigation: derate the DC bus below the device's V_CES rating (typical 0.5-0.65 of V_CES for guaranteed FIT rates), and choose specifically rated 'cosmic-ray-tolerant' parts for HVDC and traction.
• Reverse-recovery dV/dt on the anti-parallel diode. The freewheeling diode inside the module experiences reverse recovery when the opposite IGBT in the bridge turns on. A diode with too much reverse-recovery charge produces a large di/dt and dV/dt spike that can latch the IGBT it is paired with. Fast soft-recovery diodes or SiC Schottkys eliminate this.

A short history — Becke and Wheatley to Tesla and CRRC

The IGBT was invented in 1979-1982 by Hans Becke and Carl Wheatley at General Electric (US patent 4,364,073). Early devices suffered badly from latch-up and had very long tail currents. Manufacturable IGBTs reached the market in the mid-1980s, replacing Darlington bipolar power transistors and gate turn-off thyristors (GTOs) in motor drives and inverters. By the early 1990s, "punch-through" IGBT structures cut V_CE_sat below 2 V at 1200 V and made the device the dominant high-power switch for medium-voltage industrial applications. Trench-gate and field-stop architectures of the 2000s and 2010s pushed switching losses down by half and allowed higher current density per chip.

The 2010s explosion of EV traction made IGBT modules the centrepiece of every electric drivetrain. Toyota's Prius hybrid powertrain alone shipped millions of IGBT modules through the decade. Tesla's Model S and Model X (2012-2020) used Infineon and Mitsubishi IGBT modules until the 2019 Model 3 Performance switched to SiC. China's BYD, NIO, and dozens of EV startups all run on Si IGBT inverters today; CRRC's locomotives use 6.5 kV IGBT stacks. The current direction of travel is hybrid Si-IGBT-plus-SiC-diode modules at 400 V battery, full SiC at 800 V — but Si IGBT will remain the workhorse below 50 kW and above 1700 V for many years.