An Insulated Gate Bipolar Transistor (IGBT) is a three-terminal power semiconductor that merges the voltage-driven, fast-switching gate of a MOSFET with the high-current, low-conduction-loss output of a Bipolar Junction Transistor (BJT). If you are designing, repairing, or troubleshooting a circuit that switches hundreds of volts at tens of amps, the IGBT is the component doing the heavy lifting on the heatsink.
What an IGBT Changes in a Real Circuit
Before IGBTs became commercially viable in the 1990s, power engineers faced a frustrating compromise. If you needed to switch 400V at 30A, you either used a high-voltage MOSFET (which suffered from massive on-resistance and heat generation) or a BJT (which required continuous, wasteful base current to stay turned on and was painfully slow to switch off).
The IGBT changes this dynamic entirely by giving you the best of both worlds. Because the gate is insulated (just like a MOSFET), it draws virtually zero steady-state current from your gate driver IC. You can drive it directly from a low-power 3.3V or 5V microcontroller via a standard gate driver. However, once the device is fully enhanced, the current flows through a bipolar junction structure. This bipolar action causes conductivity modulation, which drastically drops the on-state voltage compared to a MOSFET of the same silicon die size.
Beginners often confuse IGBTs with SCRs (Silicon Controlled Rectifiers) or Thyristors. While both handle massive high-voltage currents, an SCR latches on once triggered and cannot be turned off via its gate; it requires the main current to drop to zero (commutation). An IGBT, like a MOSFET, turns off the instant you drop the gate voltage below its threshold, making it suitable for high-frequency Pulse Width Modulation (PWM).
The Conduction Loss Math: IGBT vs. High-Voltage MOSFET
To understand why you would specify an IGBT over a MOSFET in a 400V DC bus application, we need to look at the conduction loss math using real, bench-tested part numbers.
Let's assume you are switching a 400V, 20A induction motor phase using a 20kHz PWM signal. We will compare a standard high-voltage N-channel MOSFET against a standard IGBT, both in TO-247 packages.
Scenario A: High-Voltage Si MOSFET (Infineon IRFP460)
- Rating: 500V, 20A continuous
- On-Resistance ($R_{DS(on)}$): 0.27Ω (at 25°C, rising significantly at junction temps of 100°C+)
- Conduction Loss Formula: $P = I^2 \times R$
- Calculated Loss: $20^2 \times 0.27 = 108 Watts$
Dissipating 108W on a single TO-247 package requires an aggressive, actively cooled extruded aluminum heatsink. If ambient temperatures rise, the $R_{DS(on)}$ increases, pushing losses past 150W and likely causing thermal runaway.
Scenario B: Trench Si IGBT (Infineon IRG4PC50U)
- Rating: 600V, 55A continuous
- Saturation Voltage ($V_{CE(sat)}$): 1.8V (typical at 25°C)
- Conduction Loss Formula: $P = V_{CE(sat)} \times I$
- Calculated Loss: $1.8 \times 20 = 36 Watts$
By switching to the IGBT, you cut your conduction losses by roughly 66%. A 36W loss is easily managed with a standard passive finned heatsink and minimal airflow. This is exactly why Infineon and other semiconductor manufacturers optimize IGBT silicon specifically for the 300V to 600V range.
Where You Meet IGBTs in Practice
You will rarely find discrete IGBTs in low-voltage hobbyist projects (like 12V LED dimmers or 5V Arduino motor shields). They dominate in high-power, high-voltage infrastructure and appliances. If you are opening up any of the following equipment, you will find IGBTs (either as discrete TO-247 parts or encapsulated power modules) mounted to heavy thermal pads:
- Variable Frequency Drives (VFDs): The 3-phase inverter bridge that converts DC bus voltage into variable-frequency AC to control industrial motor speed.
- Induction Cooktops: The high-frequency (20kHz - 50kHz) resonant half-bridge that generates the alternating magnetic field in the cooking coil.
- EV Traction Inverters: While newer 800V EV architectures are moving to Silicon Carbide (SiC) MOSFETs, 400V EV battery systems still heavily rely on automotive-grade IGBT power modules.
- Solar String Inverters & UPS Systems: Used in the DC-to-AC conversion stages where grid-tie voltages (400V-800V DC buses) demand high blocking voltages.
The Selection Decision Tree
Choosing the right power switch is a function of bus voltage, switching frequency, and continuous current. Use the decision matrix below to narrow down your semiconductor chemistry and topology.
| Application Voltage | Switching Frequency | Continuous Current | Best Semiconductor Choice |
|---|---|---|---|
| < 150V | > 100 kHz | < 100A | Si MOSFET (e.g., IRF3205) |
| 200V - 600V | 5 kHz - 50 kHz | 20A - 150A | Si IGBT (e.g., IRG4PC50U) |
| 600V - 1200V | > 50 kHz | < 40A | SiC MOSFET (e.g., C3M0075120K) |
| > 1200V | < 2 kHz | > 200A | IGCT / Thyristor Module |
Gate Drive Rules and Shoot-Through Prevention
An IGBT is only as good as its gate driver. A common bench mistake that leads to catastrophic failure (often literally blowing the TO-247 package apart) is treating the IGBT gate exactly like a low-voltage logic MOSFET.
The Miller Plateau and Negative Turn-Off Bias
When switching high voltages rapidly, the $dV/dt$ across the collector-emitter terminals couples through the Miller capacitance ($C_{gc}$) back into the gate. If your gate driver only pulls the gate down to 0V (GND), this capacitive coupling can spike the gate voltage above the threshold ($V_{GE(th)}$), turning the IGBT back on while the high-side switch is also on. This is called shoot-through, and it creates a dead short across your DC bus.
The Fix: Always use a gate driver that provides a negative turn-off bias. Driving the gate to -5V to -8V during the off-state provides the necessary noise margin to absorb the Miller current spike without accidental turn-on. Drivers like the Infineon EiceDriver series or Texas Instruments UCC21520 natively support split-rail outputs for this exact purpose.
Tuning the Gate Resistor ($R_g$)
Never drive an IGBT gate directly from the driver IC without a series gate resistor. The $R_g$ resistor controls the $di/dt$ and $dV/dt$ slopes. * Too low $R_g$: The IGBT switches too fast, causing severe voltage ringing on the bus due to parasitic trace inductance ($V = L \times di/dt$), which can exceed the 600V breakdown rating and avalanche the die. * Too high $R_g$: The IGBT spends too much time in the linear (active) region during transitions, massively increasing switching losses and overheating the silicon. Start with the manufacturer's recommended $R_g$ value (usually between 10Ω and 47Ω for discrete parts) and verify the collector-emitter voltage waveform on an isolated oscilloscope channel to check for overshoot.
Frequently Asked Questions
Can I parallel IGBTs to increase current capacity?
Yes, but with strict caveats. Unlike MOSFETs, which have a positive temperature coefficient for $R_{DS(on)}$ (meaning they naturally current-share as they heat up), IGBTs have a negative temperature coefficient for their saturation voltage at lower currents. This means a hotter IGBT will try to hog more current, leading to thermal runaway. To parallel IGBTs successfully, you must use devices from the same manufacturing batch, mount them on a shared, highly conductive heatsink to ensure thermal coupling, and use individual gate resistors for each device to balance the switching transients. For most DIY and bench applications, buying a single higher-rated IGBT module from onsemi is vastly safer than paralleling discretes.
Do I need an anti-parallel diode?
In almost all motor drive and inverter applications, yes. When switching inductive loads, you need a freewheeling path for the inductive kickback current. Standard IGBTs do not have an intrinsic body diode like MOSFETs do. You must either specify an IGBT that comes with a co-packaged reverse diode (often denoted by a 'D' in the part number suffix, or sold as a 'Co-Pack'), or place a fast-recovery or SiC Schottky diode in anti-parallel on the PCB.
Why not just use Silicon Carbide (SiC) for everything?
SiC MOSFETs offer lower switching losses and can operate at much higher frequencies and temperatures than silicon IGBTs. However, they are significantly more expensive (often 3x to 5x the cost per amp) and require highly specialized gate drivers that can handle tighter threshold voltages and higher common-mode transient immunity (CMTI). For a 20kHz, 400V industrial drive, the silicon IGBT remains the undisputed king of cost-to-performance.
