Power BJT

Power BJT

September 08, 2019 0

Here present the article about the power BJT , and construction, structure, I-V characteristic and application.

➸Power BJT(Bipolar Junction Transistors)
  • Power transister are used to handle large current and large voltages.
  • the power handling capacity of the power transistors is very large.They have to dissipate a large power in the form of heat.
  • The size of power transistors is larger than that of the low power transistor.This is necessary for dissipating more power.
  • Sometimes power transistors are mounted on heatsinks. The heatsinks are made of metal and they increase the effective area of the power transistors.
  • The increased area allows easy transfer of heat to take place from the device to atmosphere.
 Construction of Power BJT:
  • The power bipolar junction transistor is supposed to block a high voltage in the off-state and have high current carrying capacity in the on-state.
  • In order to have these characteristics the power bipolar junction transistor(BJT)must have a very different structure than a small low power BJT.
  • Due to the modified structure the I-V characteristics and the switching behaviour of power BJT will be significantly different than its logical counterpart.
  • The base drive circuits will be different, and protection circuits will have to be included alongwith a power BJT.
Power BJT Structure:
  • A power transistor has a vertically oriented four-layer structure of alternating p-type and n-type layers with doping similar to the n-p-n transistor .
  • The power transistor has three terminals collector,base and emitter.
  • The n-p-n transistors are more widely used than the p-n-p transistors.
types of BJT :

There are two types of power BJT as shown in the figure above: 

1.NPN transistor :-

 the transistors in which base is positive and collector and emitter are both negative. NPN transistors are more commonly used and preffered over PNP transistors.
2.PNP transistor:
the transistors in which base is negative and collector and emitter are positive

Why is the vertical structure used?

  • The reason for using the vertical structure is that it increases the cross-sectional area through which the device current flows.
  • This will reduce the on-state resistance and hence the on-state power dissipation taking place in the transistor.
  • The vertical structure also minimizes the thermal resistance of the transistor(resistance offered to the flow of heat)and reduces the power dissipation.

I-V characteristic

The I-V characteristic of Power BJT divides into four regions.
  1. Cut-off region
  2. Active region
  3. Quasi-saturation region
  4. Hard saturation region
 

1. Cut-off region:

The BE and CB both junctions are reverse bias. The base current IB=0 and collector current IC is equal to the reverse leakage current ICEO. The region below the characteristic for IB=0 is cut-off region. In this region, BJT offers large resistance to the flow of current. Hence it is equivalent to an open circuit.

2. Active region:

The BE junction is forward bias and CB junction is reverse bias. The collector current IC increase slightly with an increase in the voltage VCE if IB is increased. The relation of IB and IC is, IC=Ξ²dcIB is true in the active region.
If BJT uses as an amplifier or as a series pass transistor in the voltage regulator, it operates in this region. The dynamic resistance in this region is large. The power dissipation is maximum.

3. Quasi-saturation region:

Quasi-saturation region is between the hard saturation and active region. This region exists due to the lightly doped drift layer. When the BJT operates at high frequency, it is operated in this region. Both junctions are forward bias. The device offers low resistance compared to the active region. So, power loss is less. In this region, the device does not go into deep saturation. So, it can turn off quickly. Therefore, we can use for higher frequency applications.

4. Hard-saturation region:

The Power BJT push into the hard-saturation region from the quasi-saturation region by increasing the base current. This region is also known as deep saturation region. The resistance offers in this region is minimum. It is even less than the quasi-saturation region. So, when the BJT operates in this region, power dissipation is minimum. The device acts as a closed switch when it operates in this region. But it needs more time to turn off. So, this region is suitable only for low-frequency switching application. In this region, both junctions are forward bias. The collector current is not proportional to the base current, IC remains almost constant at IC(sat) and independent from the value of base current.
Doping levels:
  •  the doping levels in each of the layers.The thickness of the different layers will have a significant effect on the characteristics of the device. 
  • the emitter layer is heavily doped.The base is moderately doped.The(n-)region is known as the collector drift region and it is lightly doped.
  • The(n+)region next to the drift region has doping level similar to that of emitter.This(n+)region serves the collector region.
  • The base region(p)is moderately doped and it is thin as compared to the drift region.

Applications of power BJT :

some of the typical application areas are:
  1. as series pass transister in regulated power supply
  2. the final audio amplifier in the stereo system
  3. power amplifiers 
  4. switched mode power supplies(SMPS)
  5. relay and display drivers
  6. AC motor speed controllers
  7. DC / AC Inverters.

You can also read this :
 πŸ‘‡ πŸ‘‡ πŸ‘‡ πŸ‘‡ πŸ‘‡πŸ‘‡ πŸ‘‡

https://electricalmafia.blogspot.com/2019/03/what-is-diode.html

 https://electricalmafia.blogspot.com/2019/03/what-is-led.html

 https://electricalmafia.blogspot.com/2019/03/how-is-principal-of-dc-generator.html

 https://electricalmafia.blogspot.com/2019/03/marx-generator.html
Electricalmafia

0 Comments: