Switching Characteristics of IGBT

The Insulated Gate Bipolar transistor is the new power semiconductor device which has the advantages of MOSFET and BJT. The IGBT is widely used in power electronics application where higher switching and lower loss across the device is the design consideration. The device has  high switching and high input impedance.

The IGBT(Insulated Gate Bipolar Transistor) has different characteristics during turning on and turning off. The graphical presentation of turn on and turn off of the IGBT can be graphically presented and it is known as switching characteristics of IGBT. 

The IGBT cannot be instantly turned on by applying the gate pulse , and IGBT takes certain time from its transition  from forward blocking to forward conduction mode. The time duration between the forward blocking to forward conduction is known as turn on time of IGBT.

When IGBT is turned on, the collector current starts rising from its initial leakage current and consequently the collector-emitter voltage falls. The collector-emitter voltage falls from VCE to 0.9VCEs when the collector current rise to 10% of IC (0.1 IC). The turn on time is defined as the time duration of  collector current rising from its initial leakage current to 10 % of the collector current. 

After reaching collector current at 10 % of IC, the collector current starts rising and when it reached IC, the device is turned on. In the same time, the VCE voltage drops from 0.9 VCE to 0.1 VCE during the rise of collector current from 10% to 100% IC. Thus, the turn on time of IGBT has two time components: Delay time(ton) and Rise time(tr).Thus;

A typical Switching Characteristics of an IGBT is as shown below.  

Now let us discuss  turn off time of the IGBT. When IGBT is in conduction state, it is not possible to bring the IGBT from conduction state to blocking state immediately. The time duration between  the application of turn off voltage to actual turning off IGBT is called turn off time. The turn off time has three intervals Delay time(tdf), Initial fall time(tf1) and final fall time( tf2).

The collector current reduces from IC to 0.9 IC as gate voltage falls from VGE to threshold voltage VGET. Thus, the delay time is the time interval during which gate voltage falls from VGE to threshold voltage VGET.  As gate voltage falls to VGE during tdf, the collector current drops from IC to 0.9IC. After lapse of delay time, collector-emitter voltage starts rising.

The first fall time tf1 is defined as time during which collector current drops from 90% to 20% of its final value IC. Thus, it is the time during which collector-emitter voltage increases from VCES to 0.1VCE.

The final fall time tf2 is  time during which collector current drops from 20% to 10% of IC or in other words, the time during which collector-emitter voltage increases from 0.1VCE to final value VCE.