IGBT-Insulated Gate Bipolar Transistor Details:
IGBT is a short form of Insulated Gate Bipolar Transistor, combination of Bipolar Junction Transistor (BJT) and Metal oxide Field effect transistor (MOS-FET). It’s is a semiconductor device used for switching related applications.
As IGBT is a combination of MOSFET and Transistor, it has advantages of the both transistors and MOSFET. MOSFET has advantages of high switching speed with high impedance and on the other side BJT has advantage of high gain and low saturation voltage, both are present in IGBT transistor. IGBT is a voltage controlled semiconductor which enables large collector emitter currents with almost zero gate current drive.
As discussed, IGBT has the advantages of both MOSFET and BJTs, IGBT has insulated gate same as like typical MOSFETs and same output transfer characteristics. Although, BJT is current controlled device but for the IGBT, the control depends on the MOSFET, thus it is voltage controlled device, equivalent to the standard MOSFETs.
A standard BJT’s pin out includes Collector, Emitter, Base and a standard MOSFET pin out includes Gate, Drain and Source. But in the case of IGBT transistor Pins, it is the Gate, which is coming from the N-channel MOSFET and the Collector and Emitter are coming from the PNP transistor.
In the PNP transistor, collector and Emitter is conduction path and when the IGBT is switched on it is conducted and carry the current through it. This path is controlled by the N channel MOSFET.
In case of the BJT, we calculate the gain which is denoted as Beta ( ), by dividing the output current by the input current.
In the above image, symbol of IGBT is shown. As we can see, the symbol includes Transistor’s collector emitter portion and the MOSFET’s gate portion. The three terminals are shown as Gate, collector and Emitter.
When in conducting or switched ‘ON’ mode the current flow from collector to emitter. Same thing happens for the BJT transistor. But in the case of IGBT there is Gate instead of base. The difference between Gate to Emitter voltage is called as Vge and the voltage difference between collector to emitter is called as Vce.
The emitter current (Ie) is almost same as the collector current (Ic), Ie = Ic. As the current flow is relatively same in both collector and emitter, the Vce is very low.
IGBT I-V Curve and Transfer Characteristics
In the above image, I-V characteristics are shown depending on the different gate voltage or Vge. The X axis denotes collector emitter voltage or Vce and the Y axis denotes the collector current. During the off state the current flowing through the collector and the gate voltage is zero. When we change the Vge or the gate voltage the device goes in to the active region. Stable and continuous voltage across gate provides continuous and stable current flow through the collector. Increase of Vge is proportionally increasing the collector current, Vge3 > Vge2 > Vge3. BV is the breakdown voltage of the IGBT.
This curve is almost identical with BJT’s I-V transfer curve, but here Vge is shown because IGBT is a voltage controlled device.
Advantages of IGBT:
1. Easy to turn on and off.
2. It is a voltage controlled device. Therefore driver circuit is simple and cheap.
3. Low on state voltage drop. Therefore low on state power dissipation.
4. Switching frequency higher than that of a power BJT.
5. Does not need snubber circuit for its protection.
6. It has a flat temperature coefficient of resistivity. So it is easy to connect IGBTS in parallel with each other.
Disadvantages of IGBT:
1. It has an asymmetric blocking capacity. It cannot block high reverse voltages.
2. Switching frequency is not as high as that of a power MOSFET.
3. Problem of latch UPS.
4. Excessive power dissipation can take place at the time of turn-off due to the “current tail” present in the turn-off characteristics.
Applications of IGBT:
1. Switching mode power supplies (SMPS).
2. UPS systems (IGBT based inverters).
3. AC motor controllers.