- IGBT is a three terminal power semiconductor switch used to control the electrical energy.
- Both Power BJT and Power MOSFET have their own advantages and disadvantages.
- BJTs have lower conduction losses in on state condition, but have longer turn off time.
- MOSFETs have higher on state conduction losses and have lower turn on and turn off times.
- The combination of BJT and MOSFET monolithically leads to a new device called Insulated Gate Bipolar Transistor.
- The other names of this device are GEMFET (Conductivity Modulated FET), COMFET (Conductivity Modulated Field Effect Transistor), IGT (Insulated Gate Transistor), bipolar mode MOSFET, bipolar MOS Transistor.
- It has superior on-state characteristics, good switching speed and excellent safe operating area.
- The symbol is shown below. As shown, it has three terminals namely Emitter, Collector and Gate.
- There is a disagreement in the engineering community over the proper symbol and nomenclature of the IGBT symbol.
- Some prefer to consider the IGBT as basically a BJT with a MOSFET gate input and thus to use the modified BJT symbol for the IGBT as shown above.
- Some prefer to consider drain and source rather than collector and emitter as shown below.
Structure of IGBT
- 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
Click here for the Switching Characteristics of IGBT
- The transfer characteristics of IGBT and MOSFET are similar.
- 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.
The main advantages of the IGBT are:
- Good Power handling capabilities
- Low forward conduction voltage drop of 2V to 3V, which is higher than for a BJT but lower than for a MOSFET of similar rating.
- This voltage will increase with the temperature. This property makes
the device easy to operate in parallel without danger of thermal instability.
- High speed switching capability.
- Low gate current.
- Relatively simple voltage controlled gate driver.
Some other important features of the IGBT:
- Absence of secondary breakdown.
- So it has large Safe Operating Area (SOA) and low switching losses
- Only small snubbers are required.
- Absence of body diode in IGBT. (Remember that Power MOSFET has the parasitic diode)
- Separate diode must be added in anti-parallel when reverse conduction is required.
Gate TurnOff Thyristor (GTO)
Comparison of MOSFET with BJT
Power Electronics Devices Classification
Please write your comments below… Your comments are highly appreciated…
If you like the post, then please subscribe to get new posts directly to your mail id…