Power BJT - Biploar Junction Transistor

Hello friends,

In one of my early articles i mentioned about BJT's known as bipolar junction transistors. Here i am talking about Power BJT's. There's a vast difference between BJT's and Power BJT's. Bipolar junction transistors are simply used in circuitry where we dont have to deal with delievering more power in short interval of time. Dont be confused. What i am trying to say is that simple BJT's are used in simple circuits where power in unit of time is of no issue. We can afford the electricity supplied to the circuit. In case of Power BJT's we require more energy in less time. Like in UPS we require more electricity to enable it to withstand for a larger duration of time. If we use simple bjt's then UPS which normally lasts for 1hour will last for 20 minutes.
This article will be of great help to students of electronics or computer science subjects. For others you can treat it as a general knowledge subject. The application of these power bjt's is very useful. Do you know that Power BJT's form an important unit of mobile battery chargers?

BJT (Bipolar Junction Transistor)

BJT has three terminals:

Power transistor of npn types are easy to manufacture and cheaper.

Used in high-voltage and high current application.


The base-emitter diode (forward) acts as a switch.
When v1>0.7 it lets the electrons flow toward collector, so we can control our output current (Ic) with the input current (Ib) by using transistors.


Curve I ? IB=0
Curve II? IB?0

Initial part of the curve II, characterized by VCE ? called saturation region. In this region transistor acts like switch.

Flat part of the curve ?with increasing VCE, almost IC is constant ?called active region. In this region transistor acts as amplifier.

Almost vertically rising curve is the breakdown region, which must be avoided at
all cost.

The load line IC=(VCC-VCE)/RC. The line joining A and B.

When transistor is ON, VCE=0, the IC=VCC/RC. This collector current is shown by
point A

When transistor is OFF, or in cut-off, Vcc appears across collector-emitter and
there is no collector current. This value is indicated by point B.


Forward current gain a=Ic/IE

The ratio of collector current (O/P) Ic and base current IB (I/P) called current gain.

Transistors work in 3 regions

Active: Always on --Ic=BIb
Saturation :Ic=Isaturation On as a switch
Off :Ic=0 Off as a switch

Transistors can be used as switches.
Transistors can either conduct or not conduct current.

Transistor Switching Example
When VBE is less than 0.7V the transistor is off and the lamp does not light.

When VBE is greater than 0.7V the transistor is on
and the lamp lights.

Transistor operation as switch means that transistor operates either in saturation region or in cut-off region and nowhere else on the load line.

As an ideal switch operate at A. At point B in cut-off state as an open switch.
When the control or base is reduce to 0, the transistor is turn-off and its operation is shift to B’ in the cut-off region. A small leakage current ICEO flow in the collector circuit when the transistor is off.

If VCE(S) is the collector –emitter saturation voltage, then the collector current
ICS is:The ratio of ICS to IB is called forced current gain and less than ß.

If the base current is less than IBS, the transistor operate in active region i.e. somewhere between the saturation and cut-off point.

If the base current is more than IBS , VCES is almost zero. This shows that
collector current at saturation remain substantially constant even if base current is increased.

Total power loss in the two junction of the
transistor is:
Under saturation condition VBES>VCES, means base emitter junction is forward bias. Ie VCB is –ve. Under saturation condition CJB is also FB. So at SC, BEJ &CBJ are FB.


When the base current is applied, a transistor does not turn on instantly because of the presence of internal capacitance.
When input voltage VB to base circuit is made –v2 at t0, junction EB is reverse
biased VBE=-V2, a transistor is OFF i.e. IB=IC=0 and VCE=VCC.

At time t1, input voltage VB is made +V1and IB rises to IB1.

After time t1, base emitter voltage VBE begins to rise gradually from -V2 and
collector current Ic begins to rise from zero and collector-emitter voltage start falling from initial value VCC.

After some time td called delay time, the collector current rises to 0.1ICS this
delay time is required to change the base emitter capacitance to VBES=0.7V. This
delay time td is defined as time during which the collector current rises from 0 to o.1Ics and collector –emitter voltage fall Vcc to 0.9Vcc.

After delay time td, collector current rises from 0.1Ics to 0.9Ics and VCE fall from 0.9VCC to 0.1 VCC in time tr. This time tr is known as rise time which depends upon transistor junction capacitance. The rise time tr is defined as the time during which collector current rises from 0.1 Ics to 0.9Ics. This shows the total turn on time ton=td+tr.


The safe operating area of a power transistor specifies the safe-operating limits of collector current versus collector emitter voltage.

For reliable operation of the power transistor, the collector current and voltage must always lie within this area.

Two types of safe-operating areas are specified by manufacturer:
FBSOA (Forward-biased safe-operating area)
RBSOA (Reverse-biased safe-operating area)
FBSOA ? belongs to the transistor operation when base-emitter junction is
forward biased to turn-on the transistor.


During turn-off, a transistor is subjected to high-current and high voltage with BEJ reverse biased.

Safe-operating area of transistor during turn-off is specified as

RBSOA specifies the limit of transistor operation at turn-off when the base current is zero or when the base emitter junction is reversed biased (with –ve base current). With increasing reverse bias, area RBSOA decreases in size.

font size=6 color=black>ADVANTAGES OF BJTs

•Have high switching frequencies.

•Turn-on losses are small.

•Controlled turn-on & turn-off characteristics.

•No commutation circuit required.

font size=6 color=black>DEMERITS OF BJT

•Drive circuit is complex.
• Has the problem of charge storage.
• Has the problem of second breakdown.
• Cannot be used in parallel due to

•Problems of negative temperature


Power Transistor Structure
Steady State Characteristics
Output Characteristics
Transfer Characteristics
Transistor as a Switch
Switching Characteristics
Safe Operating Areas
Advantages and Demerits of Power BJTS

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Author: jiaqing feng05 Nov 2009 Member Level: Bronze   Points : 0

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