SALIENT FEATURES OF POWER TRANSISTORS

• The power capability of a transistor is limited by three major factors First, there is a maximum reverse voltage that the collector can withstand.

• As the reverse voltage applied to co]lector base junction is increased beyond certain limit, the transistor gets damaged either due to

                                                1.Avalanche effect (or)

                                                2.Punch through effect

• The second factor that limits the maximum power capability of transistor is the decrease in current gain with increased current.

• The third factor that establishes a limit to the maximum power output of a transistor is the safe amount of heat that the material or junction can withstand.

• Another way of stating this is maximum power dissipation of the transistor.

• Another important factor be considered in power transistors is its frequency response because of power gain that falls off rapidly (above certain point).

• An important characteristic in establishing the frequency behaviour of a transistor is the time required for a signal to travel from emitter to collector. This, in-turn, depends upon the mobility of the carriers within the silicon.

• It is not possible to apply signals whose frequency changes are very rapid. Because the carriers are unable to transport the charges from emitter to collector at these rapid change of frequency.

• Electrons move almost twice as fast as holes and hence NPN transistors have higher frequency response than PNP transistors.

• Another factor that limits the high frequency response is the capacitance between section of a transistor.

• The higher the frequency, the lower the impedance of the shunting capacitor and greater its shunting effect on the applied signal.

Power transistors are classified depending upon:



        1.Capability of dissipating power

        2.Current carrying capacity 

        3.Frequency of operation

• Transistors which can dissipate power in the range of 1 to 10 watts are classified as lower transistors.

• A medium power transistor is one which can dissipate power in the range of 10 watts to 100 watts.

• High power transistors are transistors that can dissipate above 100 watts.

Depending upon continues current carrying capacity transistors are classified as

1.Low current transistors

                   Those which can carry current less than 3 amperes continuously

2.Medium current transistors

               Those capable of carrying currents from 3 amperes to 10 amperes.

3.High current transistors

                   Those capable of handling current exceeding 10 amperes.

• Silicon power transistors having voltage rating less than 60 volts are called low- voltage type. Medium voltage transistors have voltage rating from 90 to 300 volts.

• Beyond this range comes high voltage transistor.

• Low frequency transistors are those which operate below 3K Transistors which can operate in the frequency of 3 to 15 KHz are middle frequency transistors and transistors capable of operating at higher frequencies efficiently are high frequency type.