**IGBT:

**

• IGBT stands for Insulated Gate Bipolar Transistor, which is the latest device in power electronics. IGBT is a three terminal power semiconductor switch used to control the electrical energy.
• It is obtaining by combining the properties of BJT and MOSFET. BJT has lower on-state losses for high values of collector current. But the drive requirement of BJT is little complicated.
• The gate circuit of MOSFET and collector emitter circuits of BJT are combined together to form a new device. This device is called IGBT.
• Thus IGBT has advantages of both the BJT and MOSFETs. It shares many of the appealing features of power MOSFET, such as ease of gate-drive, peak current capacity and ruggedness.
• The fig. shows the symbol of IGBT. The symbol clearly indicates combination of MOSFET and BJT.
• The IGBT has 3 terminals: Gate (G), collector (C) and emitter (E). Current flows from collector to emitter whenever a voltage between gate and emitter is applied.
• The IGBT is said to have turned 'on'. When gate emitter voltage is removed, IGBT turns-off. Thus gate has full control over the conduction of IGBT.
• When the gate to emitter voltage is applied, very small (negligible) current flows. This is similar to the gate circuit of MOSFET. The on-state collector to emitter drop is very small like BJT.

Characteristics:

V-I Characteristics:

• The V-I characteristics curves are drawn for different values of VGS.
• When VGS > VGS(threshold) the IGBT turns-On.
• In this figure VGS4 > VGS3 >VGS2 > VGS1.
• By keeping VGS constant, the value of VDS is varied and corresponding values of ID is noted down.
• As shown, the V-I characteristics of IGBT is similar to BJT.

Transfer Characteristics:

• The transfer characteristics of IGBT and MOSFET are similar.
• The drain current versus gate-source voltage is called transfer characteristic. This characteristic relates variation in output current with respect to variation in input voltage.
• Transfer characteristics are reasonably linear over most of the range of drain currents. The characteristics becomes nonlinear when gate to source voltage VGS(th).
• The IGBT is in the Off-state if the gate-emitter potential(VGE) is below the threshold voltage(VGE(threshold)).
• For gate voltages greater than the threshold voltage, the transfer curve is linear.
• The maximum drain current is limit by the maximum gate-emitter voltage.

Switching Characteristics:

• The gate to source voltage is normally negative. This voltage is made positive to turn-on the IGBT. When VGS > VGS(th), the collector current starts increasing.
• Turn-on delay, td(on) is the delay when gate drive is applied and iC starts increasing. When iC increases to its full value, collector emitter voltage starts falling. 'tri' is the rise time of collector and tfv is the fall time of voltage. Thus, turn-on time of IGBT is: ton= td(on) + tri + tfv.
• The turn-off of the IGBT is initiated by reducing the gate voltage. When gate voltage falls to the value equal to vGS1, vCE starts rising. vGS1 is the voltage where IGBT comes out of saturation.
• Turn-off delay, td(off) is the delay time when gate voltage is reduced and VCE starts increasing, When VCE reaches to supply voltage, iC starts reducing.
• iC reduces fast till vGS reaches to vGS(th). This fast decay in iC is basically due to internal MOSFET. Then vGS goes to zero and becomes negative. But iC keeps on flowing for sometime, This is internal BJT current. This current flows due to stored carriers in the drift region. Hence, turn-off time of IGBT is higher than IGBT is higher than IGBT. The turn-off time of IGBT will be.
  $t_{off}= t_{d(off)}+t_{rv}+t_{fi_1}+t_{fi_2}$
  Here, trv: voltage rise time,$t_{fi_1}: is\ MOSFET\ current\ fall\ time; \ t_{fi_2}: is\ BJT\ current\ fall\ time.$

IGBT ratings:

• Maximum collector-emitter voltage (VCES): This rating should not be exceeded even on instantaneous basis in order to prevent avalanche break down of the drain-body p-n junction. This is specified at a given negative gate emitter voltage or a specified resistance connected between the gate and the emitter.
• Maximum continuous collector current (IC): This is the maximum current the IGBT can handle on a continuous basis during ON condition. It is specified at a given case temperature with derating curves provided for other case temperatures.
• Maximum pulsed collector current (ICM): This is the maximum collect or current that can flow for a specified pulse duration. This current is limited by specifying a maximum gate-emitter voltage.
• Maximum gate-emitter voltage (VgES): This is the maximum allowable magnitude of the gate-emitter voltage (of both positive and negative polarity) in order to: Prevent break down of the gate oxide insulation and restrict collector current to ICM.
• Collector leakage current (ICES): This is the leakage collector current during off state of the device at a given junction temperature.
• Gate-emitter leakage current (IGES): Usually specified at vCE= 0V & vgE= vgES.
• Collector emitter saturation voltage (VCE(sat)): This is specified at a given junction temperature, gate-emitter voltage and collector current.
• Gate-emitter threshold voltage (vgE(th)): It is specified at a low collector emitter voltage and collector current.
• Forward Transconductance (gfs): This is again specified at a low value of vCE.
• Input, output and transfer capacitances (Cies, Coes& Cres): These are gate-emitter, collector-emitter and gate-drain capacitances of the device respectively, specified at a given collector-emitter voltage. Variation of these parameters as functions of vCE are also supplied.
• Maximum total power dissipation (Ptmax): This is the maximum allowable power lass in the device (both switching and conduction) on a continuous basis at a given case temperature.

Merits of IGBT:

• Voltage controlled device. Hence drive circuit is very simple.
• On-state losses are reduced compared to BJT, MOSFET and SCR.
• Switching frequencies are higher than thyristors.
• No commutation circuits are required as compared to SCR.
• Gate have full control over the operation of IGBT.
• IGBTs have approximately flat temperature coefficient.
• The drive of IGBT is simple and driving current requirement is also very small compared to BJT, since there is no direct current flow in the gate.
• The voltage and current ratings of the IGBTs are better compared to MOSFETs.

Demerits of IGBT: