- 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
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