What is IGBT..How it Works?
What is IGBT..How it Works ?
Insulated Gate Bipolar Transistor:
The Insulated Gate Bipolar Transistor also called an IGBT for short, is something of a cross between a conventional Bipolar Junction Transistor, (BJT) and a Field Effect Transistor, (MOSFET) making it ideal as a semiconductor switching device.
The IGBT transistor takes
the best parts of these two types of transistors, the high input
impedance and high switching speeds of a MOSFET with the low saturation
voltage of a bipolar transistor, and combines them together to produce
another type of transistor switching device that is capable of handling
large collector-emitter currents with virtually zero gate current drive.
Typical IGBT
The Insulated Gate Bipolar
Transistor, (IGBT) uses the insulated gate (hence the first part of its
name) technology of the MOSFET with the output performance
characteristics of a conventional bipolar transistor, (hence the second
part of its name). The result of this hybrid combination is that the
“IGBT Transistor” has the output switching and conduction
characteristics of a bipolar transistor but is voltage-controlled like a
MOSFET.
IGBTs are mainly used in power electronics applications, such as
inverters, converters and power supplies, were the demands of the solid
state switching device are not fully met by power bipolars and power
MOSFETs. High-current and high-voltage bipolars are available, but their
switching speeds are slow, while power MOSFETs may have high switching
speeds, but high-voltage and high-current devices are expensive and hard
to achieve.
The advantage gained by the insulated gate bipolar transistor device
over a BJT or MOSFET is that it offers greater power gain than the
bipolar type together with the higher voltage operation and lower input
losses of the MOSFET. In effect it is an FET integrated with a bipolar
transistor in a form of Darlington configuration as shown.
Insulated Gate Bipolar Transistor
We can see that the insulated gate bipolar transistor is a three
terminal, transconductance device that combines an insulated gate
N-channel MOSFET input with a PNP bipolar transistor output connected in
a type of Darlington configuration. As a result the terminals are
labelled as: Collector,Emitter and Gate. Two of its terminals (C-E) are associated with a conductance path and the third terminal (G) associated with its control.
The amount of amplification achieved by the insulated gate bipolar transistor is
a ratio between its output signal and its input signal. For a
conventional bipolar junction transistor, (BJT) the amount of gain is
approximately equal to the ratio of the output current to the input
current, called Beta.
For a metal oxide semiconductor field effect transistor or MOSFET, there
is no input current as the gate is isolated from the main current
carrying channel. Therefore, an FET’s gain is equal to the ratio of
output current change to input voltage change, making it a
transconductance device and this is also true of the IGBT. Then we can
treat the IGBT as a power BJT whose base current is provided by a
MOSFET.
The Insulated Gate Bipolar Transistor can
be used in small signal amplifier circuits in much the same way as the
BJT or MOSFET type transistors. But as the IGBT combines the low
conduction loss of a BJT with the high switching speed of a power MOSFET
an optimal solid state switch exists which is ideal for use in power
electronics applications.
Also, the IGBT has a much lower “on-state” resistance, RON than an equivalent MOSFET. This means that the I2R drop
across the bipolar output structure for a given switching current is
much lower. The forward blocking operation of the IGBT transistor is
identical to a power MOSFET.
When used as static controlled switch, the insulated gate bipolar
transistor has voltage and current ratings similar to that of the
bipolar transistor. However, the presence of an isolated gate in an IGBT
makes it a lot simpler to drive than the BJT as much less drive power
is needed.
An insulated gate bipolar transistor is simply turned “ON” or “OFF” by
activating and deactivating its Gate terminal. A constant positive
voltage input signal across the Gate and the Emitter will keep the
device in its “ON” state, while removal of the input signal will cause
it to turn “OFF” in much the same way as a bipolar transistor or MOSFET.
IGBT Characteristics
Because the IGBT is a voltage-controlled device, it only requires a
small voltage on the Gate to maintain conduction through the device
unlike BJT’s which require that the Base current is continuously
supplied in a sufficient enough quantity to maintain saturation.
Also the IGBT is a unidirectional device, meaning it can only switch
current in the “forward direction”, that is from Collector to Emitter
unlike MOSFET’s which have bi-directional current switching capabilities
(controlled in the forward direction and uncontrolled in the reverse
direction).
The principal of operation and Gate drive circuits for the insulated
gate bipolar transistor are very similar to that of the N-channel power
MOSFET. The basic difference is that the resistance offered by the main
conducting channel when current flows through the device in its “ON”
state is very much smaller in the IGBT. Because of this, the current
ratings are much higher when compared with an equivalent power MOSFET.
The main advantages of using the Insulated Gate Bipolar Transistor over
other types of transistor devices are its high voltage capability, low
ON-resistance, ease of drive, relatively fast switching speeds and
combined with zero gate drive current makes it a good choice for
moderate speed, high voltage applications such as in pulse-width
modulated (PWM), variable speed control, switch-mode power supplies or
solar powered DC-AC inverter and frequency converter applications
operating in the hundreds of kilohertz range.
A general comparison between BJT’s, MOSFET’s and IGBT’s is given in the following table.
IGBT Comparison Table
Device Characteristic | Power Bipolar | Power MOSFET | IGBT |
Voltage Rating | High <1kV | High <1kV | Very High >1kV |
Current Rating | High <500A | Low <200A | High >500A |
Input Drive | Current 20-200 hFE | Voltage VGS 3-10V | Voltage VGE 4-8V |
Input Impedance | Low | High | High |
Output Impedance | Low | Medium | Low |
Switching Speed | Slow (uS) | Fast (nS) | Medium |
Cost | Low | Medium | High |
We have seen that the Insulated Gate Bipolar Transistor is
semiconductor switching device that has the output characteristics of a
bipolar junction transistor, BJT, but is controlled like a metal oxide
field effect transistor, MOSFET.
One of the main advantages of the IGBT transistor is the simplicity by
which it can be driven ON or OFF or in its linear active region as a
power amplifier. With its lower on-state conduction losses and its
ability to switch high voltages without damage makes this transistor
ideal for driving inductive loads such as coil windings, electromagnets
and DC motors.
1 comments
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