The power bipolar junction transistor (BJT) blocks a high voltage in the off state and high current carrying capacity in the on-state. The power handling capacity is very high.
Application of Power BJT:
- Switched Mode Power Supply (SMPS)
- Power Amplifier
- Relay and Drivers
- AC motor speed controller
- DC/AC inverter
- As series pass transistor in the regulated power supply
- The audio amplifier in the stereo system
- Power control circuit
Construction of Power BJT:
The power BJT has three terminals Collector (C), Emitter (E) and Base (B). It has a vertically oriented four-layers structure. The vertical structure uses to increase the cross-sectional area.
There are two types of BJT; n-p-n transistor and p-n-p transistor. Out of these two types, the n-p-n transistors widely use compare to the p-n-p transistor.
It has four layers. The first layer is a heavily doped emitter layer (n+). The second layer is moderately doped the base layer (p). The third region is lightly doped collector drift region (n-). The last layer is a highly doped collector region (n+).
The drift layer (n-) increase the voltage blocking capacity of the transistor due to the low doping level. The width of this layer decides the breakdown voltage. The disadvantage of this layer is that the increase on state voltage drops and increase on state device resistance, which increases power loss.
The power handling capacity of the power transistor is very large. So, they have to dissipate power in the form of heat. Sometimes, heatsink uses to increase effective area and therefore increase power dissipation capacity. the heatsink made from metal.
The I-V characteristic of Power BJT divides into four regions.
- Cut-off region
- Active region
- Quasi-saturation region
- Hard saturation region
In the structure of BJT, there are two junctions; Emitter junction (BE) and Collector junction (CB).
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=βdcIB 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.
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