Insulated Gate Bipolar Transistor (IGBT) - Part 1

Everyone of us sometime or the other might have used a switching device in our application. Whether it be diode, power diode, bipolar transistor, mosfet, thyristor, when our application demanded ON/OFF condition we have used either of these devices. We tend to select either of them based on the application need. Application need meant current capability, switching frequency, reverse voltage rating, etc. But when and where to use what, is another point of discussion.

Among such devices, Insulated Gate Bipolar Transistor (IGBT) is one. IGBT is basically a power semiconductor device used in medium to high power applications. IGBT has 4 layers (P-N-P-N) which reminds of SCR configuration. The power .handling capability of IGBT can be increased by connecting several IGBTs in parallel. It is a minority carrier device which uses majority as well as minority carriers.

As discussed, IGBT is used as a switch in many applications. Switching operation meant device will be operating in saturation and cut-off state and not in linear region. IGBT switches at a faster rate and also has higher efficiency. We have to note that the switching frequency is always less than a power MOSFET.

IGBT combines the high input impedance characteristic of mosfet with current carrying capability of bipolar transistor. When we talk in terms of power handling capability, IGBT is preferred to Diode, BJT and FET and has less power handling capacity then thyristor.

How to allow reverse current in IGBT?

IGBT does not allow reverse conduction. In case, a reverse current is desired a freewheeling diode is used in parallel to IGBT.

What is the differentiating point in terms of forward drop when compared to Power FET?

As we all know, MOSFET behaves as a resistor. The voltage drop is proportional to current. Where as IGBT voltage drop behavior is on a logarithmic scale to current. So, while choosing either mosfet or IGBT, consideration must be given to current capability and blocking voltage specifications.

Operation of IGBT:

IGBT has 4 alternating layers.

Equivalent circuit of IGBT:

Structure of IGBT:

Advantages of IGBT:

• High power dissipation
• High reverse breakdown voltage
• Simple gate drive
• High current
• Low saturation voltage
• Combination of MOSFET and bipolar transistor in single device
• Rugged and tolerant to heavy loads

Disadvantages of IGBT:

• High voltage drop (but lower than mosfet)
• Low operating frequency because of current tail

Applications of IGBT:

• Switched mode power supply, UPS
• Traction motor circuit (Trains)
• induction heating
• Solar inverters