1. Power Electronics Lecture-9 Power Transistors & GTO Dr. Imtiaz Hussain
Associate Professor
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2. Outline
Power Transistors
Power BJT
Power MOSFET
IGBT
GTO

3. Power Transistors
Power transistors are fully controlled semiconductor switches.
These are turned ON when the signal (voltage or current) is given to control terminal.
Types
Power BJT
Power MOSFET
Insulated Gate Bipolar Junction Transistor (IGBT)

4. Power BJT
The symbol of the Power BJT is same as signal level transistor.

5. Power BJT
The construction of the Power Transistor is  different from the signal transistor as shown in the following figure.
The n- layer is added in the power BJT which is known as drift region.

6. Power BJT
A Power BJT has a four layer structure of alternating P and N type doping.
In most of Power Electronic applications, the Power Transistor works in Common Emitter configuration.
In power switches npn transistors are most widely used than pnp transistors.
The thickness of the dirft region determines the breakdown voltage of the Power transistor.

7. VI Characteristics

8. VI Characteristics
The  VI characteristics of the Power BJT is different from signal level transistor.
The major differences are Quasi saturation region & secondary breakdown region.
The Quasi saturation region is available only in Power transistor characteristic not in signal transistors. It is because of the lightly doped collector drift region present in Power BJT.
The primary breakdown is similar to the signal transistor’s avalanche breakdown. 
Operation of device at primary and secondary breakdown regions should be avoided as it will lead to the catastrophic failure of the device.

9. Power MOSFET

10. Power MOSFET

11. Power MOSFET

12. Power MOSFET

13. IGBT IGBT is a new development in area of Power MOSFET technology.
This device combines into it the advantages of both MOSFET and BJT.
So an IGBT has high input impedance like a MOSFET and low on state power loss like a BJT.
Further, IGBT is free from second breakdown problem presented in BJT.

14. Cross-Sectional View of an IGBT
Metal
Silicon Dioxide
Metal

15. Cross-Sectional View of an IGBT

16. IGBT
 It is an FET integrated with a bipolar transistor in a form of Darlington configuration.

17. IGBT I-V Characteristics

18. IGBT
“IGBT Transistor” has the output switching and conduction characteristics of a bipolar transistor but is voltage-controlled like a MOSFET.
IGBTs are mainly used in power electronics applications, such as inverters, converters and power supplies, were the demands of the solid state switching device are not fully met by power BJTs and power MOSFETs.

19. Comparison Table

20. Antiparallel diode

21. GTO (Gate Turn-off Thyristor)
GTO can be turned-on by a gate signal, and it can also be turned-off by a gate signal of negative polarity.

22. GTO (Gate Turn-off Thyristor)
Applications of GTO: They are used in
Motor drives
Static VAR compensators (SVCs)
AC/DC power supplies with high power ratings

23. GTO (Gate Turn-off Thyristor)
Compared to a conventional SCR, the device has the following disadvantages
Magnitude of latching, holding currents is more. The latching current of the GTO is several times more as compared to conventional thyristors of the same rating. 
On state voltage drop and the associated loss is more.
Due to multicathode structure of GTO, triggering gate current is higher than that required for normal SCR.
Gate drive circuit losses are more.

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