Power Transistors

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What is Power Transistor?

Power Transistor posses controlled characteristics. These are turned ON when current signal is given to base. The transistor will be in ON condition until control signal is present. When the control is removed, the power transistor is turned OFF. Power Transistor can be classified into four types,
  1. Bipolar Junction Transistor(BJT) 
  2. Metal Oxide Semiconductor Field Effect Transistor(MOSFET)
  3. Insulated Gate Bipolar Transistor(IGBT)
  4. Static Induction Transistor(SIT)


Bipolar Junction Transistors(BJT)

A BJT is a three-layer, two junction, npn or pnp semiconductor device. It can be classified into two types, 
  • PNP Transistor - Two p region sandwiched by one n region
  • NPN Transistor - One p region sandwiched by two n region
The term 'bipolar' denotes, the current flow in the device is due to the movement of both holes and electrons.Collector, Base, Emitter are the three terminals present in BJT. NPN Transistors are used widely in handling high-voltage and high-current applications as they are easy to manufacture and also cheaper.


BJT has two junctions namely, collector base junction(CBJ) and base emitter junction(BEJ).

Structure of Power BJT


The main difference between BJT and Power BJT is the presence of drift layer(N-) in Power BJT. Drift layer increases the voltage blocking capacity of the transistor. The width of drift layer decides the voltage blocking capacity. The disadvantage of drift layer is, it increase on state resistance in the device and thus increase the power loss. hence, it can be solved using vertically oriented structure of Power BJT. And also, this vertically oriented structure increase the cross sectional area for maximum forward current flow in Power BJT.

VI Characteristics

VI characteristics is classified into two sections,
  • Input characteristics
  • Output characteristics

Input Characteristics

A graph between base current and base emitter voltage gives input characteristics.

Output Characteristics

A graph drawn between collector current and collector emitter voltage gives output characteristics of a transistor.



Cut-off region

Here, both collector base junction and emitter base junction are reverse biased, When base current is zero, a small value of collector current(reverse leakage current) exists. Power transistor offer large resistance to the flow of current hence Power transistor acts as open switch in this region.  

Active region

Here, emitter-base junction is forward biased and collector-base junction is reverse biased. So Power transistor acts as amplifier and series pass transistor in the voltage regulator. When collector current increase, collector-emitter voltage also increase. This is because collector current depends on base current. Power dissipation is maximum at this region.

Quasi-saturation region

Here, both emitter-base junction and collector-base junction is forward biased. This region is formed due to the presence of drift layer. And it lies between active and hard saturation region. In this region,  the device operates at high switching frequency. Power transistor acts as switch in this region. And also it can turn off quickly.

Hard saturation region

Further increase in base current cause the device to operate in hard saturation region. In this region, both emitter-base and collector-base junction are forward biased. The main drawback in this region is, it takes more time to turn-off. So, it is applicable only for low switching applications. Here, collector current does not depend on base current. 

Applications
  • Power amplifier
  • Switch
  • Switch Mode Power Supply(SMPS)
  • Relay driver
  • Display driver
References

1] M.D. Singh and K.B. Khanchandani, "Power Electronics", Mc Graw Hill India, 2013.
2] Rashid M.H., "Power Electronics Circuits, devices and Applications", Prentice Hall, India, Third            Edition, New Delhi, 2004.
3] P.S.Bimbra, "Power Electronics", Khanna Publishers, Eleventh Edition, 2003.
 

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