What Is an IGBT?

IGBT stands for Insulated Gate Bipolar Transistor. It is a three-terminal power semiconductor device designed primarily for switching high voltage and high current.

The three terminals are:

• Gate (G)
• Collector (C)
• Emitter (E)

The gate is electrically insulated from the semiconductor structure, which means that the IGBT is controlled by voltage rather than by continuous gate current.

Why Was the IGBT Developed?

MOSFETs have excellent switching performance and very high input impedance, but their conduction losses can become significant at high voltage and current.

BJTs can provide useful conduction characteristics at high power, but they require base current and are more difficult to drive.

The IGBT combines important characteristics of both technologies.

IGBT Terminals

An IGBT has three main terminals:

       Gate
         │
         ▼
      ┌─────┐
      │ IGBT│
      └─────┘
       │   │
       │   │
  Collector Emitter

Gate

The gate controls the IGBT. Applying a suitable positive gate-to-emitter voltage turns an N-channel IGBT ON.

Collector

The collector is the high-voltage power terminal.

Emitter

The emitter provides the return path for the main current.

How an IGBT Works

An IGBT uses an insulated gate structure similar to a MOSFET. When the gate-to-emitter voltage exceeds the required level, a conductive channel is formed inside the device.

This allows the bipolar portion of the device to conduct current between collector and emitter.

Gate voltage
     │
     ▼
MOS gate structure
     │
     ▼
Conductive channel
     │
     ▼
Bipolar conduction
     │
     ▼
Collector → Emitter current

The result is a device that can be controlled using a relatively small gate-drive signal while handling substantial voltage and current.

IGBT as a Switch

An IGBT is normally used as a power switch rather than as a small-signal amplifier.

The basic states are:

• OFF: Little collector-emitter current flows.
• ON: Significant collector-emitter current flows.

Gate OFF
   │
   ▼
IGBT OFF
   │
   ▼
High VCE

Gate ON
   │
   ▼
IGBT ON
   │
   ▼
Low VCE

Gate Control

The IGBT is controlled by the voltage between gate and emitter:

VGE = VG - VE

For an N-channel IGBT, a positive VGE turns the device ON.

The exact gate-drive voltage should always be obtained from the manufacturer's datasheet.

Gate Threshold Voltage

The gate threshold voltage, usually represented by VGE(th), is the voltage at which the device begins to conduct under specified test conditions.

It should not be interpreted as the voltage required to fully turn the IGBT ON.

The gate should be driven at the voltage recommended by the manufacturer.

IGBT Switching

When an IGBT turns ON, the gate voltage rises and the collector-emitter voltage falls while collector current increases.

When the gate is driven OFF, the collector-emitter voltage rises and the collector current decreases.

The switching speed depends on:

• Gate resistance
• Gate-driver strength
• Gate charge
• Collector current
• Bus voltage
• Load characteristics
• Device construction

IGBT Turn-On

During turn-on, the gate driver supplies charge to the gate.

Gate voltage ↑
      │
      ▼
IGBT begins conducting
      │
      ▼
Collector-emitter voltage ↓
      │
      ▼
Collector current ↑

The switching transition produces energy loss, so the gate-drive and switching conditions are important in high-frequency applications.

IGBT Turn-Off

When the gate voltage is removed, the IGBT stops being actively controlled into conduction.

However, because an IGBT uses bipolar conduction, stored charge can remain inside the semiconductor.

This produces a characteristic current tail during turn-off.

Collector current

      │\
      │ \
      │  \
      │   \____
      │        \___
      └────────────────
             time

The current tail is one reason IGBTs generally cannot switch as fast as modern power MOSFETs.

IGBT Conduction Loss

Unlike a MOSFET, which is commonly described using RDS(on), IGBT conduction loss is often associated with its collector-emitter saturation voltage, VCE(sat).

A simplified conduction-loss estimate is:

Pcond ≈ VCE(sat) × IC

The actual VCE(sat) depends on collector current, gate voltage and temperature.

Example of IGBT Conduction Loss

Suppose an IGBT operates at:

VCE(sat) = 2.0 V
IC = 20 A

A simplified estimate is:

P = 2.0 × 20

P = 40 W

This is a substantial amount of heat and demonstrates why high-power IGBT circuits require careful thermal design.

IGBT Switching Loss

Every time an IGBT switches, voltage and current overlap for a short period. During this interval, electrical energy is converted into heat.

The switching losses increase with:

• Switching frequency
• Collector current
• Bus voltage
• Turn-on energy
• Turn-off energy

Datasheets may specify switching energies such as:

• EON
• EOFF

A simplified average switching-loss calculation is:

Psw ≈ (EON + EOFF) × f

IGBT Total Power Loss

A simplified total-loss calculation is:

Ptotal ≈ Pconduction + Pswitching

Additional losses may exist depending on the application, including diode losses and gate-drive losses.

The resulting total power must be removed through the thermal system.

IGBT Thermal Management

Thermal management is critical in high-power IGBT applications.

The junction temperature must remain below the manufacturer's maximum specified value.

The thermal path can be represented as:

IGBT Junction
      │
     θJC
      │
      ▼
Package Case
      │
     θCS
      │
      ▼
Heatsink
      │
     θSA
      │
      ▼
Ambient

The approximate junction temperature can be calculated from the total thermal resistance and power dissipation.

IGBT Heatsinks

High-power IGBTs commonly require heatsinks.

The required heatsink depends on:

• IGBT power dissipation
• Ambient temperature
• Maximum junction temperature
• Thermal resistance
• Cooling method
• Airflow

Forced-air cooling may be required for high-power applications.

IGBT Applications

IGBTs are particularly useful in applications where high voltage and moderate switching frequency are required.

• Motor drives
• Industrial inverters
• UPS systems
• Welding machines
• Solar inverters
• Induction heating
• Electric vehicle power electronics
• Industrial power supplies
• Variable-frequency drives
• High-power converters

IGBTs in Motor Drives

Industrial motor drives frequently use IGBTs in three-phase inverter bridges.

          DC BUS
            │
      ┌─────┴─────┐
      │           │
    IGBT        IGBT
      │           │
      ├── Motor ──┤
      │           │
    IGBT        IGBT
      │           │
      └─────┬─────┘
            │
           GND

PWM signals control the IGBTs to generate the required motor voltage and frequency.

IGBTs in Inverters

IGBTs are commonly used in inverter bridges to convert DC into controlled AC power.

Typical applications include:

• Solar inverters
• UPS systems
• Industrial drives
• Welding equipment
• Backup power systems

IGBT vs MOSFET

IGBT vs BJT

When Should You Choose an IGBT?

An IGBT can be an excellent choice when:

• The DC bus voltage is relatively high.
• High current must be controlled.
• Switching frequency is moderate.
• High power efficiency is required.
• A voltage-controlled gate is desirable.
• The application involves motor drives or inverters.

For very high switching frequencies and lower voltages, a MOSFET may often be more appropriate.

IGBT Gate Driver

An IGBT requires an appropriate gate-driver circuit.

The driver must provide:

• Suitable gate voltage
• Enough source current
• Enough sink current
• Appropriate switching speed
• Reliable turn-off
• Suitable isolation where required

High-power bridge circuits frequently use isolated or dedicated high-side and low-side gate drivers.

Gate Resistor

A gate resistor controls the rate at which the IGBT gate is charged and discharged.

Changing the gate resistance can affect:

• Switching speed
• Switching losses
• EMI
• Gate ringing
• Voltage overshoot

The appropriate value depends on the IGBT, gate driver and circuit layout.

IGBT Dead Time

When two IGBTs form the upper and lower switches of a half-bridge, they must not be turned ON simultaneously.

       DC+
        │
      IGBT
        │
        ├── Output
        │
      IGBT
        │
       DC-

A short delay known as dead time is normally introduced between turning one device OFF and turning the other device ON.

This helps prevent shoot-through current.

IGBT Shoot-Through

Shoot-through occurs when the upper and lower switches of a bridge are simultaneously conducting.

DC+
 │
IGBT ON
 │
 │ ← Very high current
 │
IGBT ON
 │
DC-

This can destroy the IGBTs extremely quickly.

Proper gate-driver design, dead time and fault protection are therefore essential.

IGBT Protection

High-power IGBT systems may require several protection mechanisms.

• Overcurrent protection
• Short-circuit protection
• Overvoltage protection
• Gate-voltage protection
• Overtemperature protection
• Desaturation detection
• DC-bus protection

IGBT Short-Circuit Protection

A short circuit can produce extremely high collector current.

IGBT gate drivers designed for industrial applications may include desaturation detection to identify abnormal collector-emitter voltage while the device is commanded ON.

Fast protection is important because an IGBT may only tolerate a short short-circuit duration.

IGBT Freewheel Diode

Many inverter and motor-drive circuits require a diode to provide a path for inductive current when an IGBT is switched OFF.

        IGBT
          │
          ├──── Load
          │      │
          │     Diode
          │      │
          └──────┘

Some IGBT modules include anti-parallel diodes internally.

When selecting an IGBT module, always check whether the required diode is already integrated.

IGBT Modules

For high-power applications, several IGBTs may be integrated into a single power module.

An IGBT module may contain:

• Two IGBTs
• Four IGBTs
• Six IGBTs
• Anti-parallel diodes
• Temperature sensors or other monitoring features

Three-phase inverter modules commonly contain six switching devices.

IGBT Module Cooling

Power modules require careful mechanical and thermal installation.

Important considerations include:

• Heatsink size
• Thermal interface material
• Mounting pressure
• Thermal resistance
• Airflow
• Baseplate condition

Poor mounting can create significant thermal resistance even when the heatsink itself is large enough.

Testing an IGBT With a Multimeter

A multimeter can identify some obvious IGBT failures, particularly short circuits.

Before testing:

1. Disconnect the equipment from power.
2. Discharge high-voltage capacitors safely.
3. Identify the IGBT terminals.
4. Check the datasheet.
5. Preferably isolate the IGBT from surrounding circuitry.

In-circuit measurements can be misleading because other components may provide parallel current paths.

Checking an IGBT for a Short

Check resistance between:

Gate ↔ Emitter
Gate ↔ Collector
Collector ↔ Emitter

The insulated gate should normally show very high resistance to the power terminals.

A low-resistance gate-to-emitter or gate-to-collector measurement may indicate gate-oxide damage.

A very low resistance between collector and emitter in both directions can indicate a failed IGBT.

IGBT Body Diode

Many IGBT circuits use an anti-parallel diode, either externally or integrated inside the module.

The diode can be checked using the diode-test function of a multimeter when its terminals are accessible.

The exact diode orientation depends on the particular device or module, so consult the datasheet.

Common IGBT Failures

• Collector-emitter short circuit
• Gate-emitter short circuit
• Gate-collector short circuit
• Excessive leakage
• Overheating
• Gate-drive failure
• Overvoltage damage
• Short-circuit damage
• Thermal cycling damage

Why IGBTs Fail

Common causes include:

• Excessive collector current
• Excessive collector-emitter voltage
• Overtemperature
• Insufficient cooling
• Gate overvoltage
• Gate-driver malfunction
• Shoot-through
• Inductive voltage spikes
• Incorrect dead time
• Short circuit

How to Select an IGBT

Important selection parameters include:

• Collector-emitter voltage (VCE)
• Collector current (IC)
• VCE(sat)
• Gate-emitter voltage rating
• Gate charge
• Turn-on energy
• Turn-off energy
• Switching frequency
• Short-circuit capability
• Thermal resistance
• Maximum junction temperature
• Package or module configuration

IGBT Voltage Rating

The IGBT's collector-emitter voltage rating must exceed the maximum voltage that can occur in the circuit.

The designer must consider:

• DC-bus voltage
• Supply tolerance
• Switching overshoot
• Inductive spikes
• Snubber performance

Selecting the voltage rating requires adequate margin rather than simply matching the nominal bus voltage.

IGBT Current Rating

The collector-current rating must be evaluated together with temperature and switching conditions.

Consider:

• Continuous current
• Peak current
• Motor startup current
• Short-circuit current
• Ambient temperature
• Cooling system

VCE(sat)

VCE(sat) is the collector-emitter saturation voltage when the IGBT is conducting under specified conditions.

A lower VCE(sat) generally reduces conduction losses.

However, switching characteristics and other parameters must also be considered.

IGBT Switching Frequency

IGBTs are generally better suited to moderate switching frequencies than very high-frequency applications.

The appropriate frequency depends on the specific IGBT technology.

Always consult the manufacturer's switching-loss data.

IGBT Advantages

• Voltage-controlled gate
• Very high input impedance
• Suitable for high-voltage operation
• Suitable for high-current applications
• Good power-handling capability
• Common in industrial inverter systems
• Simple gate-drive concept

IGBT Disadvantages

• Slower than many MOSFETs
• Turn-off current tail
• Switching losses increase at high frequency
• Requires careful thermal management
• Gate oxide can be damaged by excessive voltage
• Requires protection against short circuits and overvoltage

IGBT vs MOSFET: Practical Rule

As a general starting point:

Lower voltage + high frequency
          │
          ▼
       MOSFET

Higher voltage + moderate frequency
          │
          ▼
        IGBT

This is only a general guideline. Modern semiconductor technologies overlap considerably, so the actual device datasheets should be compared for the specific application.

Key Points

• IGBT stands for Insulated Gate Bipolar Transistor.
• An IGBT has gate, collector and emitter terminals.
• The gate is voltage controlled and electrically insulated.
• IGBTs combine MOS gate control with bipolar conduction.
• VCE(sat) is important for conduction-loss calculations.
• Gate charge and switching energy determine switching losses.
• IGBTs are widely used in motor drives and high-power inverters.
• Many IGBT modules include anti-parallel diodes.
• Dead time is essential in half-bridge and full-bridge circuits.
• IGBTs require careful thermal management.
• Gate-driver protection is important in high-power applications.
• Desaturation protection can be used to detect severe overcurrent conditions.
• IGBTs generally operate at lower switching frequencies than high-speed MOSFETs.
• Always check the manufacturer's datasheet before selecting or replacing an IGBT.