An Insulated-Gate Bipolar Transistor (IGBT) is a three-terminal power semiconductor device that combines the high-input impedance and fast switching of a MOSFET with the low-conduction-loss and high-current capability of a bipolar junction transistor (BJT). If you are looking for the strict definition of IGBT, it is essentially a voltage-controlled switch optimized for high-power, high-voltage applications where standard MOSFETs would overheat from conduction losses.
What an IGBT Changes in a Real Circuit
Before the IGBT was commercialized, high-voltage switching was a compromise. You either used BJTs, which required massive, lossy base currents to stay turned on, or you used high-voltage MOSFETs, which suffered from crippling conduction losses. The IGBT changes the architecture of high-power circuits by allowing a low-power microcontroller or isolated gate driver to switch hundreds of amps at voltages up to 3.3kV using just a few milliamps of gate current.
Think of the gate terminal as a traffic cop who requires almost no physical effort to direct a massive flow of heavy trucks (current) from the collector to the emitter. By applying a positive voltage (typically +15V) to the gate relative to the emitter, the device turns on. Because the gate is insulated by a silicon dioxide layer, the steady-state gate current is virtually zero, drastically simplifying the drive circuitry compared to a BJT.
Worked Numeric Example: Sizing an IGBT for a 5 HP VFD
Let’s design a switch for a 5 HP (3.7 kW) 480V AC 3-phase motor using a Variable Frequency Drive (VFD). This requires calculating the DC bus voltage, selecting the part, and verifying the thermal design.
- Calculate DC Bus Voltage: A 480V AC line-to-line supply rectifies to a DC bus voltage of approximately 480V × 1.414 = 678V. Adding a 20% margin for grid transients and regenerative braking spikes, we need a minimum blocking voltage of 813V. The industry standard selection here is a 1200V IGBT.
- Determine Current Rating: The motor full load current (FLC) is roughly 7.5A. Accounting for inrush, starting torque, and switching spikes, we target a continuous current rating of at least 25A at 100°C case temperature.
- Select the Component: The Infineon IKW40N120H3 (1200V, 40A at 100°C) in a TO-247 package is a standard choice.
- Calculate Conduction Loss: At 25A, the collector-emitter saturation voltage (V_CE(sat)) is typically 2.0V. The conduction power loss per device is P = V_CE(sat) × I_C = 2.0V × 25A = 50W.
- Thermal Sizing: To keep the junction temperature (T_J) under 125°C in a 50°C ambient environment, the total thermal resistance from junction-to-ambient must be less than 1.5°C/W. Given the device's internal junction-to-case resistance is ~0.6°C/W, your heatsink and thermal interface material must provide a combined resistance of less than 0.9°C/W.
Where You Meet IGBTs in Practice
You will rarely find discrete IGBTs in low-voltage hobbyist circuits like 12V LED drivers or 5V buck converters. They dominate the high-voltage, high-current domain. According to All About Circuits, the primary applications include:
- Variable Frequency Drives (VFDs): Used in 3-phase inverter bridges to synthesize PWM waveforms for industrial AC motors.
- Induction Cooktops and Heaters: Switching at 20 kHz to 50 kHz in half-bridge or full-bridge topologies to drive resonant LC tanks.
- Solar String Inverters: Converting 600V-1000V DC from solar arrays into grid-tied AC. Modern units often use EconoDUAL half-bridge modules rather than discrete TO-247 parts.
- EV Traction Inverters: While Silicon Carbide (SiC) MOSFETs are taking over 800V architectures (like the Porsche Taycan), IGBTs remain the dominant, cost-effective choice for 400V EV platforms.
Comparison Matrix: IGBT vs. Power MOSFET vs. BJT
Choosing the right switch requires understanding the trade-offs between voltage, speed, and drive complexity. Use this matrix to guide your component selection.
| Parameter | Power MOSFET | IGBT | Power BJT |
|---|---|---|---|
| Control Method | Voltage (Gate) | Voltage (Gate) | Current (Base) |
| Optimal Voltage Range | < 250V | 400V to 3.3kV | < 800V |
| Conduction Loss at High V | High (R_DS(on) spikes) | Low (Constant V_CE(sat)) | Moderate |
| Max Switching Frequency | > 200 kHz | Typically < 50 kHz | < 10 kHz |
| Turn-Off Behavior | Fast, abrupt | Slower, exhibits "tail current" | Slow, prone to storage time |
Frequently Asked Questions About IGBTs
What is the difference between an IGBT and a MOSFET?
The primary difference lies in their conduction mechanisms and high-voltage performance. A MOSFET is a unipolar device (majority carriers only), meaning it switches incredibly fast but suffers from high on-resistance at voltages above 250V. An IGBT is a bipolar device that injects minority carriers into the drift region, which drastically lowers its forward voltage drop at high voltages. However, this minority carrier injection causes a "tail current" during turn-off, making IGBTs slower and unsuitable for high-frequency applications like MHz-range switch-mode power supplies.
Why do IGBTs need a negative gate voltage to turn off?
While an IGBT can technically turn off with 0V on the gate, high-power circuits apply a negative voltage (typically -5V to -15V) during the off-state to prevent parasitic turn-on. In a half-bridge configuration, when the bottom IGBT turns on, the high dv/dt (rapid voltage change) couples through the Miller capacitance of the top IGBT. This can induce a voltage spike on the gate that inadvertently turns the top device on, causing a catastrophic "shoot-through" short circuit. A negative gate bias provides a noise margin to prevent this.
Can I replace a Power MOSFET with an IGBT in my DIY SMPS?
In almost all cases, no. If you are building a high-frequency Flyback or LLC resonant converter operating above 50 kHz, swapping a MOSFET for an IGBT will likely destroy the circuit. The IGBT's "tail current" during turn-off will cause massive switching losses, leading to immediate thermal runaway. Furthermore, IGBTs lack the intrinsic body diode found in MOSFETs; they require a separate, fast-recovery anti-parallel diode to handle freewheeling currents, which adds complexity and cost to high-frequency designs.
What causes an IGBT to fail in an induction heater or VFD?
The most common failure modes are overvoltage, thermal runaway, and shoot-through. Overvoltage occurs when inductive kickback exceeds the 1200V breakdown limit (V_CES), punching through the silicon. Thermal runaway happens when the gate drive voltage sags below 12V; an under-driven IGBT operates in its linear region, drastically increasing V_CE(sat) and generating lethal amounts of heat. Finally, shoot-through occurs if dead-time between high-side and low-side switches is too short, allowing both IGBTs to conduct simultaneously and short the DC bus.
