IGBT

The Insulated Gate Bipolar Transistor combines the isolated gate of a MOSFET with the high current handling of a BJT. It dominates medium to high power applications like traction motor drives and solar inverters.

Core Concept

An IGBT is triggered by a positive gate voltage forming a channel like a MOSFET. This channel provides base current to a PNP bipolar structure causing conductivity modulation in the drift region.

Conductivity modulation dramatically lowers the on state voltage drop to around 2V even for 1200V devices like the FGA25N120. This allows high voltage and high current operation simultaneously.

During turn off the IGBT exhibits a current tail due to minority carriers clearing out of the base region. This tail current causes switching losses and limits operation to around 20 kHz.

Key Formulas

Total power loss combines conduction and switching losses. Pcond equals Vce saturation multiplied by average collector current.

Given: Ic = 50 A, Vce_sat = 2.0 V, Duty Cycle = 0.5
Formula: Pcond = Ic * Vce_sat * Duty Cycle
Steps: Pcond = 50 * 2.0 * 0.5
Final Answer: 50 W

Exam Tip: Latch up is a critical failure mode in IGBTs caused by the parasitic thyristor turning on. Always ensure the device operates within its Safe Operating Area SOA.

Performance Parameters

• Voltage rating: 600 V to 6500 V
• Current rating: up to 3000 A modules
• Switching frequency: 5 kHz to 50 kHz
• On state voltage Vce sat: 1.5 V to 3.0 V
• Input impedance: Extremely high

Quick Revision

• Voltage controlled input
• Bipolar current conduction
• Low conduction loss at high voltages
• Tail current limits switching speed
• Prone to destructive latch up if overloaded
• Exam trap: Assuming IGBTs can switch as fast as MOSFETs