The transistor can switch considerably faster than thyristor, typically a switching time of less than 1µs being possible.Overall gain of current of 250 is possible but with a longer switching time. 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. The exact switching loss is a function of the load circuit parameters as well as the form of the base current change.ĭarlington Arrangement of Power Transistor A high switching frequency can mean the predominant loss is that due to switching.
The switching loss of transistor can be high because during the switching both the voltage across and current through the transistor can be high. The highest instantaneous power losses that can be tolerated being progressively restricted for longer switching times. 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. To exploit the transistor fully without over heating during switching, safe operation area characteristics can be used. In the close position the saturation voltage is small.In the open position leakage current is small.As a switch the power losses of transistor are small, because The power loss in a transistor is a function of the product of collector emitter voltage and the collector current. In the off-state, a small reverse IB is maintained to avoid serious collector current. At turn-off the base current should be reduced at a rate that collector current can follow so as to avoid secondary breakdown. Any change in collector current must be matched by a change in base current. At turn-ON initially the base current should be high so as to give a fast turn on. In order to maintain control, the base current should be just sufficient to keep the device in saturation. If such amount of base current is supplied which drives the transistor into saturation state then the transistor acts as a closed switch. If base current is zero, then collector current is very small leakage current, transistor under these conditions acts as open switch. Transistor can act as open or close switch depending on the base current. When IB = 0, the transistor is said to be in cut-off region although there is a small collector leakage current indicated.Ī family of characteristic curves is produced when IC vs VCE is plotted for several values of IB as shown. This collector region is known as breakover region. When VCE reaches a sufficiently high voltage, the reverse biased base collector junction goes into breakdown & the collector current increases rapidly. Actually, IC increases very slightly as VCE increases due to widening of the base collector depletion region. 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. 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. Characteristics Curve of Common Emitter Linear Region or Active RegionĪs VCE is increased, VCE increases gradually as the collector current increases. When both the junctions are forward bias and the transistor is in saturation region of its operation. IB is through base emitter junction due to low impedance path and IC 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.
Saturation RegionĪssume that VBB is set to produce a certain value of IB and VCC is zero. 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. Power Transistor Common Emitter Configuration Structure PNP NPN Transistors Construction Common emitter Characteristics for NPN TransistorĬonsider an NPN power transistor biased in common emitter configuration. Symbol with current Directions PNP NPN Transistors Current Directions The ratio of these two currents is of the order of 15 to 100. 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.
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