MCTE 4334
Power Electronics
Power Transistors
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Contents
Bipolar junction transistor (BJT),
Metal-oxide semiconductor field-effect
transistor (MOSFET),
 Insulated gate bipolar transistor (IGBT).
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Basic Structure of BJT
NPN
PNP
I E  IC  I B
 F  hFE
IC

IB
Current Gain
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Basic Structure of BJT
I E  IC  I B
Current Gain
 F  hFE
IC

IB
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BJT Characteristics
Model of NPN
transistors
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Transfer Characteristics of BJT
Transistors’ role:
Switch: cutoff &
saturation region
Amplifier: active
region.
Saturation: the point above which any increase in base current does
not increase the collector current significantly.
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Transistor as a switch
At Steady State Conditions
VCE=VCB+VBE
In Saturation:
VCE<VBE;
VCE=VCE(sat)
Both junctions are forward biased.
IC=ICS;
IB=IBS=ICS/
Over drive factor:
ODF=IB/IBS
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At Transient Conditions
Switching characteristics
of a BJT due to CBJCapacitance:
td: delay time (charging)
tr: rise time (time const.)
ts: storage time
(removing charge)
tf: fall time (charging)
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BJT Applications and Limitations
Bipolar junction power transistors are applied to a variety
of power electronic functions, switching mode power
supplies, dc motor inverters, PWM inverters, and many
other functions too numerous to name.
•BJT is a current controlled device.
•The current gain is highly dependent on the junction
temperature.
•It requires continuous base current for conduction.
•Localize hot spot in base may damage the transistor,
called second breakdown, an energy dependent
phenomenon
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Switching Limits of BJT
•
•
•
•
•
Second Breakdown (SB)
Forward biased safe operating area
Reverse biased safe operating area
Power derating
Breakdown voltage
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Power Field Effect Transistor
The Field Effect Transistor, or simply
FET however, uses the voltage that is applied to
their input terminal, called the Gate to control the
current flowing through them resulting in the output
current being proportional to the input voltage.
As their operation relies on an electric field (hence
the name field effect) generated by the input Gate
voltage, this then makes the Field Effect
Transistor "VOLTAGE" operated device.
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Power Field Effect Transistor
The Field Effect Transistor has one major
advantage over its standard bipolar transistor
cousins, in that their input impedance, ( Rin ) is
very high, (thousands of Ohms), while the BJT is
comparatively low.
Bipolar Transistor
Field Effect Transistor
Emitter - (E) >> Source - (S)
Base - (B) >> Gate - (G)
Collector - (C) >> Drain - (D)
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Power Field Effect Transistor
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Power Field Effect Transistor
 The semiconductor "channel" of the Junction Field
Effect Transistor is a resistive path through which a
voltage VDS causes a current ID to flow. The JFET
can conduct current equally well in either direction.
 A voltage gradient is thus formed down the length of
the channel with this voltage becoming less positive
as we go from the Drain terminal to the Source
terminal.
.
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Power Field Effect Transistor
 The magnitude of the current flowing through the
channel between the Drain and the Source terminals
is controlled by a voltage applied to the Gate
terminal, which is a reverse-biased.
 The main difference between the JFET and a BJT
device is that when the JFET junction is reversebiased the Gate current is practically zero, whereas
the Base current of the BJT is always some value
greater than zero.
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Power Field Effect Transistor
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Power Field Effect Transistor
 The cross sectional diagram above shows an N-type semiconductor
channel with a P-type region called the Gate diffused into the N-type
channel forming a reverse biased PN-junction and it is this junction
which forms the depletion region around the Gate area when no
external voltages are applied. JFETs are therefore known as depletion
mode devices.
 This depletion region produces a potential gradient which is of varying
thickness around the PN-junction and restrict the current flow through
the channel by reducing its effective width and thus increasing the
overall resistance of the channel itself.
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Power Field Effect Transistor
 The cross sectional diagram above shows an N-type semiconductor
channel with a P-type region called the Gate diffused into the N-type
channel forming a reverse biased PN-junction and it is this junction
which forms the depletion region around the Gate area when no
external voltages are applied. JFETs are therefore known as depletion
mode devices.
 This depletion region produces a potential gradient which is of varying
thickness around the PN-junction and restrict the current flow through
the channel by reducing its effective width and thus increasing the
overall resistance of the channel itself.
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Power Field Effect Transistor
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Power Field Effect Transistor
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Power MOSFETs
Voltage controlled device
Requires only a small input current
Switching speed is very high, in
nanosecond
Suitable for low power high
frequency converter.
No second breakdown problem like
BJT
Due to electrostatic charge, special care in handling is
required.
Difficult to protect against short circuited faults.
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Power MOSFETs
 The most common type of insulated gate FET which is
used in many different types of electronic circuits is
called the Metal Oxide Semiconductor Field Effect
Transistor or MOSFET for short.
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Power MOSFETs
 MOSFET is a voltage controlled field effect transistor
that differs from a JFET in that it has a "Metal Oxide"
Gate electrode which is electrically insulated from the
main semiconductor N-channel or P-channel by a thin
layer of insulating material usually silicon dioxide
(commonly known as glass).
 This insulated metal gate electrode can be thought of as
one plate of a capacitor. The isolation of the controlling
Gate
makes
the
input
resistance
of
the MOSFET extremely high in the Mega-ohms ( MΩ )
region thereby making it almost infinite.
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Power MOSFETs
 As the Gate terminal is isolated from the main current
carrying channel "NO current flows into the gate"and
just like the JFET, the MOSFET also acts like a voltage
controlled resistor were the current flowing through the
main channel between the Drain and Source is
proportional to the input voltage.
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Types of MOSFETs
• MOSFETs are three terminal devices with
a Gate, Drain and Source
1. Depletion Type - the transistor requires the
Gate-Source voltage, ( VGS ) to switch the device
"OFF". The depletion mode MOSFET is equivalent
to a "Normally Closed" switch.
2. Enhancement Type - the transistor requires a
Gate-Source voltage, ( VGS ) to switch the device
"ON". The enhancement mode MOSFET is
equivalent to a "Normally Open" switch.
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Types of MOSFETs
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Depletion type MOSFETs
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Depletion type MOSFETs
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Enhancement type MOSFETs
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Output Characteristics
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Switching Characteristics
Equivalent Circuit
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The Insulated Gate Bipolar Transistor
IGBT
Insulated gate bipolar
transistor:
Cross sectional view.
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The Insulated Gate Bipolar Transistor
IGBT
The Insulated Gate Bipolar Transistor also
called an IGBT for short, is a cross between a
conventional Bipolar Junction Transistor, or
“BJT” and a Field Effect Transistor, or “FET”
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The Insulated Gate Bipolar Transistor
IGBT
The insulated gate bipolar transistor, (IGBT) uses
the insulated gate of the MOSFET with the output
performance characteristics of a conventional
bipolar transistor, (hence the second part of its
name).
The advantage gained by the IGBT is that it offers
greater power gain than the bipolar type together
with the higher voltage operation and lower input
losses of the MOSFET.
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The Insulated Gate Bipolar Transistor
IGBT
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Equivalent Circuits IGBT
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Output and Transfer Characteristics
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Features of IGBT
•
•
•
•
•
•
It is the combination of BJT and MOSFET
High input impedance, like MOSFET
Low on-state losses, like BJT
No second break down problem
Voltage controlled device.
Switching speed is faster than BJT but less than
that of MOSFET
• Switching circuits is simple and low switching
losses.
• IGBTs are replacing applications where BJT and
MOSFETs were predominantly used as switches.
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Series and parallel Operation
The required equalizing network is similar to that of diodes in
series or parallel combination.
BJTs have a –ve temperature coefficient, and MOSFET has +ve
temperature coefficient.
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Summary
•
•
•
•
Characteristics of BJTs, Operating zones.
Switching characteristics, criteria, ODF, etc..
Power losses in BJT, turning times.
MOSFET types, advantages and
disadvantages over BJT.
• Switching Characteristics of MOSFETs
• IGBT and its advantageous features.
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