Power Electronics A to Z

About Power Electronics Concepts, Design & Analysis

IGBT – An Introduction

What is IGBT?:

  • 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.

Symbol:

  • The symbol is shown below. As shown, it has three terminals namely Emitter, Collector and Gate.   

IGBT_SYMBOL

  • 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.

 IGBT_Symbol_having_drain_Source_terminals

Read Further:
Structure of IGBT

V-I Characteristics:
IGBT_VI_Characteristics

  • 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
Transfer Characteristics:

IGBT_Transfer_Characteristics

  • 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.

Summary
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:

  • This power semiconductor device does not have the problem 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.

Read More:
Gate TurnOff Thyristor (GTO)
TRIAC
Comparison of MOSFET with BJT
Power Electronics Devices Classification
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Category: IGBT
  • ratan patil says:

    why there is diode symbol added in between collector and emitter

    June 18, 2014 at 2:58 pm

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