Insulated Gate Bipolar Transistor is known as IGBT. The IGBT is a combination of two power electronics devices that is MOSFET and BJT. MOSFET has high-speed switching characteristic and BJT has characteristic of low ON-state resistance. The IGBT combines both of this characteristic. That’s why it is widely used power electronics switch in all fields.
The equivalent circuit of IGBT
The IGBT includes an equivalent circuit of MOSFET as well as a transistor. So, it has properties of both of the elements. As mentioned in the below figure, the IGBT has MOSFET at the input side and the transistor at the output side. Because of this type of circuit, the IGBT has advantages of MOSFET and transistor. The resistor connected in the equivalent circuit is known as the drift resistor.
IGBT has three terminals; Collector, Emitter, and Gate. The Gate terminal is not connected with internal structure. The Gate is insulated. Capacitance produces between the Gate and semiconductor layer.
P+ layer connects with the collector terminal. This layer is also known as the injection layer. N- layer is known as a drift layer. There are two major junctions.
Working of IGBT
Gate connect with the battery (VG) and an emitter terminal. The GATE terminal is positive with respect to the emitter terminal.
the battery (VCC) connected between the collector and emitter. Here, the collector terminal is positive with respect to the emitter terminal.
For the Gate voltage (VG) is zero. When the collector is positive with respect to the emitter, the junction J1 is in forward bias and junction J2 is in reverse bias. Because of J2 is in reverse bias, the current cannot flow through the collector to emitter.
Now, Gate voltage starts to increase. A channel (-Ve charge carrier) forms between the N+ layer and P layer. Because of this channel, the breakdown occurs and current start flowing in the circuit.
In the circuit, if the Gate voltage (VG) increases, the gate current (IG) and VGE increase. Now, as the VGE increases, collector current increases, and VCE decrease. Here, the input current is Gate current IG and output current is the collector current IC. Input voltage is VGE and output voltage is VCE.
In IGBT, we will discuss two types of characteristics. The first characteristic transfer characteristic. The transfer characteristic is associated with the output current (IC) and the input voltage (VGE). So, the transfer characteristic is a graph of IC and VGE. This is given in the below figure.
As we have seen in the working, as we increase VGE voltage, IC current will increase. VGE voltage at least given higher than the threshold voltage. Then only, Gate will trigger and IC will flow through the circuit. For silicon structured IGBT, VGE threshold voltage is around 1 V.
The output characteristic is a graphical relation between the output current IC and output voltage VCE. For a different value of VGE, we draw this characteristic. For VGE=0, a very small amount of current produce due to the minority charge carrier. Now, if increase VGE and for different value of VGE, there will be a flow of current is increased as shown in the below figure.
- Easy to turn ON and OFF
- IGBT is a voltage-controlled device. Therefore, the driver circuit is simple.
- Low on-state voltage drop. Hence, low on-state power dissipation.
- Switching frequency higher than that of a power BJT.
- Does not need snubber circuit for its protection.
- It has a flat temperature coefficient of resistivity.
- Switching frequency is not as high as that of the MOSFET.
- The problem with latch-ups.
- At the time of turn OFF, it takes place excessive power dissipations due to the “current tail” present in the turn-off characteristic.
- Switching mode power supply (SMPS)
- UPS system
- AC motor controller
Advantages of IGBT over BJT
- IGBT is a voltage-controlled device. So, the driver circuit is simple.
- IGBT can easily be connected in parallel.
- Second break-down does not take place.
- IGBT can switch at a higher frequency than BJT.
- IGBTs need not snubber circuit for their protection.
Advantages of IGBT over MOSFET
- In IGBT, on-state voltage loss is less compared to the MOSFET.
- On-state power dissipation is less.
- IGBT can handle higher power than MOSFET.