Power transistors are finding increasing popularity in low to medium power applications.A power transistor has low current gain and requires continuous base drive during on-state conditions but doesn’t require forced commutations.Circuitry power transistors can be used in high switching frequency,permitting size reduction of electromagnetic components and can provide current limit protection by base drive circuit.Power transistor cannot withstand reverse voltage and application is limited to dc voltage for inverters and choppers.
A transistor is a three layer pnp or npn semiconductor device having two junctions.This type of transistors is know as bipolar junction transistor(BJT).The structure and symbol of npn transistor is shown below.
The three terminals of the device are called the collector (c), the base (B) and the emitter (E). The collector ans Emitter terminals are connected to the main power circuit, the base terminal is connected to control signal.
Transistors can be operated in the switching mode. If base current IB is zero transistor is in an off state and behaves as a switch.On the other hand,if the base is driven hard,ie if the base current IB is sufficient to drive the transistor into saturation,then the transistor behaves as a closed switch.
The transistor is a current driven device.The base current determines whether it is in on state or off state.To keep the device in on state,there should be a sufficient base current.
Transistor with high voltage and current ratings are known as power transistors.The current gain of power transistors(IC/IB)can be as low as 10.For eg. base current of 10A is required for 100A of collector current.
Power transistors switch on and off much faster than thyristors.They may switch on in less than 1μs and turnoff in less than 2μs.Therefore power transistors can be used in applications where frequency is as high as 100kHz.These devices are very delicate.They fail under certain high voltage and high current conditions.They should be operated within its specified limits,known as safe operating area(50A).
The characteristics depict the relation between collector current(IC) and collector to emitter voltage(VCE) for different values of base current.These characteristics is of npn transistor.These characteristics have three regions ie active region,saturation and cut off region.
The input characteristics depicts the relationship between base current IB and emitter to base voltage for different values of collector to emitter voltage VCE.
The input characteristics is shown in the figure below
The V-I characteristics(output characteristics of power npn transistor is shown in figure below
As in the case of lower power BJT,this characteristics depict the relationship between collector current(IC) and collector to emitter voltage(VCE) for different values of base current IB.
It is seen that these characteristics have some special features very different from those for lower power BJT.These features are as follows.
1. For substantial values of collector current,there is maximum value of collector emitter voltage which the device can sustain,it is denoted as BVsus in above figure.If IB= 0 the maximum voltage which can be sustained by the device increases to BVCEO .(The voltage (VCE)when the base is open circuited).The voltage BVCBO is the breakdown voltage when the emitter is open circuited.
2. The primary breakdown is due to the avalanche breakdown of C-B junction.In this region the current and the power dissipation can be very high.Therefore this region should be avoided.
3. In the region marked second breakdown,The C-E voltage decreases substantially and the collector current is high.This region is due to thermal runaway.A cumulative process occurs in this region and the device gets destroyed.In this breakdown,power dissipation is not uniformly spread over the entire volume of the transistor but is rather restricted to highly localized areas.Therefore the chances of the device getting destroyed are high.
4. A quasi saturation(between saturation and active region)region exists.This region is due to the lightly doped drift collector region.