The bipolar power transistor is a three layer NPN or PNP device within the working range, the collector current IC is a function of the base current IB, a change in the base current giving a corresponding amplified change in the collector current for a given collector emitter voltage VCE. The ratio of these two currents is of the order of 15 to 100.
Symbol with current Directions
Common emitter Characteristics for NPN Transistor
Consider an NPN power transistor biased in common emitter configuration.
Using the above circuit a set collector characteristic curves can be generated, that show how the collector current IC, varies with the collector to emitter voltage VCE for the specified values of base current IB.
Assume that VBB is set to produce a certain value of IB and VCC is zero. For this condition both the emitter base junction and base collector junction are forward biased because the base is approximately at 0.7V while the emitter & collector are at 0V. IB is through base emitter junction due to low impedance path and IC is zero. When both the junctions are forward bias and the transistor is in saturation region of its operation.
Linear Region or Active Region
As VCE is increased, VCE increases gradually as the collector current increases. Ideally when VCE exceeds 0.7V , the base collector junction becomes reverse biased and the transistor goes into the active or linear region of its operation. Once the base collector junction is reversed biased, IC levels off and remains essentially constant for a given value of Ib as VCE continue to increase. Actually, IC increases very slightly as VCE increases due to widening of the base collector depletion region.
Break Over Region
When VCE reaches a sufficiently high voltage, the reverse biased base collector junction goes into breakdown & the collector current increases rapidly. This collector region is known as breakover region.
When IB = 0, the transistor is said to be in cut-off region although there is a small collector leakage current indicated.
A family of characteristic curves is produced when IC vs VCE is plotted for several values of IB as shown.
Power Application of BJT’s
Transistor can act as open or close switch depending on the base current.
If base current is zero, then collector current is very small leakage current, transistor under these conditions acts as open switch.
If such amount of base current is supplied which drives the transistor into saturation state then the transistor acts as a closed switch.
In order to maintain control, the base current should be just sufficient to keep the device in saturation. At turn-ON initially; the base current should be high so as to give a fast turn on. Any change in collector current must be matched by a change in base current. At turn-off the base current should be reduced at a rate that collector current can follow so as to avoid secondary breakdown. In the off-state, a small reverse IB is maintained to avoid serious collector current.
Power Losses in Power Transistor
The power loss in a transistor is a function of the product of collector emitter voltage and the collector current. As a switch the power losses of transistor are small, because
- In the open position leakage current is small
- In the close position the saturation voltage is small.
Typical safe operation Area
To exploit the transistor fully without over heating during switching, safe operation area characteristics can be used.
When switching between the two states occurs, it is essential that voltage and current values must at all times during the switching period be within the rectangular area.
The highest instantaneous power losses that can be tolerated being progressively restricted for longer switching times.
The switching loss of transistor can be high because during the switching both the voltage across and current through the transistor can be high. A high switching frequency can mean the predominant loss is that due to switching. The exact switching loss is a function of the load circuit parameters as well as the form of the base current change.
Darlington Arrangement of Power Transistor
The current gain of a transistor can be improved if base drive current is obtained from another transistor this is known as Darlington arrangement.
Overall gain of current of 250 is possible but with a longer switching time.
Comparison of Power BJT & Thyristor
- The transistor can switch considerably faster than thyristor, typically a switching time of less than 1µs being possible.
- The base current requirements are comparatively more to the gate current requirements of thyristor. E.g. a 30A thyristor may require 0.1A pulse to turn ON while a 30A BJT may require 2A base current continuously during the ON period.
- The overload ratings of transistor are much less than that of thyristor.
- Unlike the thyristor, the transistor has little ability to withstand a reverse voltage.