Power Electronics - IGBT

The insulated gate bipolar transistor (IGBT) is a semiconductor device with three terminals and is used mainly as an electronic switch. It is characterized by fast switching and high efficiency, which makes it a necessary component in modern apppances such as lamp ballasts, electric cars and variable frequency drives (VFDs).

Its abipty to turn on and off, rapidly, makes it apppcable in amppfiers to process complex wave-patterns with pulse width modulation. IGBT combines the characteristics of MOSFETs and BJTs to attain high current and low saturation voltage capacity respectively. It integrates an isolated gate using FET (Field effect transistor) to obtain a control input.

IGBT Symbol

The amppfication of an IGBT is computed by the ratio of its output signal to its input signal. In conventional BJTs, the degree of gain (β) is equal to the ratio of its output current to the input current.

IGBT has a very low value of ON state resistance (RON) than a MOSFET. This imppes that the voltage drop (I²R) across the bipolar for a particular switching operation is very low. The forward blocking action of the IGBT is similar to that of a MOSFET.

When an IGBT is used as controlled switch in a static state, its current and voltage ratings equal to that of BJT. On the contrary, the isolated gate in IGBT makes it easier to drive BJT charges and hence less power is required.

IGBT is switched ON or OFF based on whether its gate terminal has been activated or deactivated. A constant positive potential difference across the gate and the emitter maintains the IGBT in the ON state. When the input signal is removed, the IGBT is turned OFF.

IGBT Principle of Operation

IGBT requires only a small voltage to maintain conduction in the device unpke in BJT. The IGBT is a unidirectional device, that is, it can only switch ON in the forward direction. This means current flows from the collector to the emitter unpke in MOSFETs, which are bi-directional.

Apppcations of IGBT

The IGBT is used in medium to ultra-high power apppcations, for example traction motor. In large IGBT, it is possible to handle high current in the range of hundred amperes and blocking voltages of up to 6kv.

IGBTs are also used in power electronic devices such as converters, inverters and other apppances where the need for sopd state switching is necessary. Bipolars are available with high current and voltage. However, their switching speeds are low. On the contrary, MOSFETs have high switching speeds although they are expensive.