Power BJT
Power transistors
• Power diodes are uncontrolled devices- their turn-on and turn-off
characteristics are not under control.
• Power transistors - possess controlled characteristics i.e turned on
when a current is given to base, or control, terminal.
• The transistor remains in on-state as long as control signal is present.
When this control signal is removed, power transistor is turned off.
Power transistors are of four types
• Bipolar Junction transistors(BJTs)
• Metal-oxide semiconductor field effect transistors(MOSFETs)
• Insulated Gate Bipolar transistors(IGBTs)
• Static Induction Transistors(SITs)
Power BJT
• Power BJT is the power level counterpart of the normal signal level
BJT.
• It is used as a switch in power electronics.
• Power BJT is the four-layer structure which differs from the signal
level BJT which normally has three layers Emitter, Base and Collector
• The symbol for Power BJT is same as that for the normal signal-level
BJT.
Basic Structure of NPN Power BJT
• It has four layers.
• The first layer is a heavily doped emitter layer (n+).
• The second layer is moderately doped the base layer (p).
• The third region is lightly doped collector drift region (n-).
• The last layer is a highly doped collector region (n+).
• The width of this layer(n-) decides the breakdown voltage.
Contd..
• In most of Power Electronic applications, the Power Transistor works
in Common Emitter configuration.
• ie, Base is the input terminal, the Collector is the output terminal and
the Emitter is common between input and output.
• In power switches NPN transistors are most widely used than PNP
transistors.
• The characteristics of the device is determined by the doping level in
each of the layers and the thickness of the layers.
• The thickness of the drift region determines the breakdown voltage
of the Power transistor.
• A power transistor is a vertically oriented four layer structure of
alternating p-type and ntype. This is maximising the cross-section
area which results in higher current rating .
Input Characteristics of NPN Transistor
Power BJT steady state characteristics(VI
Characteristics)
Contd…
• The VI characteristics of the Power BJT is different from signal
level transistor.
• The major differences are Quasi saturation region & secondary
breakdown region.
• The Quasi saturation region is available only in Power transistor
characteristic not in signal transistors.
It is because of the lightly doped collector drift region present in
Power BJT.
• The primary breakdown is similar to the signal transistor’s
avalanche breakdown.
• Operation of device at primary and secondary breakdown
regions should be avoided as it will lead to the failure of the
device.
Contd…
• The primary breakdown shown takes place because of avalanche
breakdown of collector base junction.
• Large power dissipation normally leads to primary breakdown.
• The second breakdown shown is due to localized thermal runaway.
• Secondary breakdown is a failure mode in bipolar power transistors.
• In a power transistor with a large junction area, under certain
conditions of current and voltage, the current concentrates in a small
spot of the base-emitter junction. This causes local heating,
progressing into a short between collector and emitter. This often
leads to the destruction of the transistor.
• Secondary breakdown can occur both with forward and reverse base
drive.
• Except at low collector-emitter voltages, the secondary breakdown
limit restricts the collector current more than the steady-state power
dissipation of the device.
1.Cut-off region
• The BE and CB both junctions are reverse bias.
• The base current IB=0 and collector current IC is equal to the reverse
leakage current ICEO.
The transistor is now in the OFF state
• The region below the characteristic for IB=0 is cut-off region. In this
region, BJT offers large resistance to the flow of current. Hence it is
equivalent to an open circuit.
2. Active Region
• The BE junction is forward bias and CB junction is reverse bias.
• The collector current IC increase slightly with an increase in the
voltage VCE if IB is increased.
• The relation of IB and IC is, IC=βdcIB is true in the active region.
• If BJT uses as an amplifier or as a series pass transistor in the voltage
regulator, it operates in this region.
• The power dissipation is maximum.
3. Quasi-saturation region
• Quasi-saturation region is between the hard saturation and active
region.
• This region exists due to the lightly doped drift layer.
• When the BJT operates at high frequency, it is operated in this region.
• Both junctions are forward bias.
• The device offers low resistance compared to the active region. So,
power loss is less.
• In this region, the device does not go into deep saturation. So, it can
turn off quickly.
• Therefore, we can use for higher frequency applications.
4. Hard-saturation region
• The Power BJT push into the hard-saturation region from the
quasi-saturation region by increasing the base current.
• This region is also known as deep saturation region.
• The resistance offered in this region is minimum. It is even less
than the quasi-saturation region.
• So, when the BJT operates in this region, power dissipation is
minimum.
Contd..
• The device acts as a closed switch when it operates in this region.
• But it needs more time to turn off.
• So, this region is suitable only for low-frequency switching
application.
• In this region, both junctions are forward bias.
• The collector current is not proportional to the base current,
IC remains almost constant at IC(sat) and independent from the value
of base current.
• Application of Power BJT:
1. Switched Mode Power Supply (SMPS)
2. Power Amplifier
3. Relay and Drivers
4. AC motor speed controller
5. DC/AC inverter
6. The audio amplifier in the stereo system
7. Power control circuit
