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Module 1 introduction
1. Power Electronics 17EC73
Subject Overview
This subject will lead students to understand basic theory of power
semiconductor devices and their importance in power electronics
systems
The subject helps students understand practical application of various
power semiconductor devices in power electronic systems.
This subject will lead students to understand the significance of
thyristors and its use in operation of AC-DC, DC-DC, DC-AC
conversion circuits and their applications so as to provide the basis for
further study of power electronics circuits and systems.
Subject helps students to understand concept of commutation and its
techniques.
The subject emphasizes students to test various power electronic
circuits using pspice.
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2. Module. 1
Introduction
Objectives:
Understand the construction and working of
various power devices,
Static and switching characteristics of Power
Devices.
Different types of power electronic circuits
Peripheral effects in power electronic converters
2
3. Outcomes
Describe the characteristics of different power devices and
identify the various applications associated with it.
Describe the working principle of different power
electronic converters.
3
4. Application of Power Electronics
In early days, control of the electric power was
achieved with electric machinery.
Power electronics have revolutionized the concept of
power control for power conversion and for control of
electrical motor drives.
Power electronics combine power, electronics, and
control.
4
5. Control deals with the steady-state and dynamic
characteristics of closed-loop system.
Power deals with static and rotating power equipment.
Electronics deals with the solid-state devices and
circuits for signal processing to meet the desired
control objectives.
5
6. Therefore, power electronics is defined as the
applications of solid-state electronics for control
and conversion of electric power.
Power electronics is based on switching of the power
semiconductor devices.
It covers a variety of switching circuits.
6
7. History of Power Electronics
The history of power electronics began with
introduction of the mercury arc rectifiers in 1900.
Devices which were based on the mercury arc valve
technology were used until 1950.
The first electronic revolution began in 1948 with the
invention of the silicon transistor at Bell Labs.
7
8. Most of today's advanced electronic technologies
are based on the transistor concept.
The next breakthrough was invention of Thyristor
(SCR) in 1956, which is a PNPN triggering
transistor.
The second revolution began in 1958 with
development of the commercial thyristor by GE.
That was the beginning of a new era of power
electronics.
8
9. Power electronic applications
Heat controls
Light controls
Motor controls
Power supplies
Vehicle propulsion system
HVDC(high voltage direct-current transmission)
10. Power Semiconductor Devices
Since the first thyristor was developed in 1957, there
have been tremendous advances in the power
semiconductor devices.
Until 1970, the conventional thyristors had been
exclusively used for power control applications.
Since 1970 many types of power semiconductor devices
were developed.
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14. Contd..
Line-frequency diodes: These PIN diodes with general-purpose rectifier type
applications, are available at the highest voltage (~5kV) and current ratings (~5kA) and
have excellent over-current (surge rating about six times average current rating) and
surge-voltage withstand capability.
They have relatively large Qrr and trr specifications.
Fast recovery diodes: Fast recovery diffused diodes and fast recovery epitaxial diodes,
FRED's, have significantly lower Q rrand trr (~ 1.0 sec).
They are available at high powers and are mainly used in association with fast
controlled-devices as free-wheeling or DC-DC choppers and rectifier applications.
Fast recovery diodes also find application in induction heating, UPS and traction.
Schottky rectifiers: These are the fastest rectifiers being majority carrier devices without
any Qrr..
However, they are available with voltage ratings up to a hundred volts only though
current ratings may be high.
Their conduction voltages specifications are excellent (~0.2V).
The freedom from minority carrier recovery permits reduced snubber requirements.
Schottky diodes face no competition in low voltage SPMS applications and in
instrumentation.
15. Silicon controlled rectifier(SCR)
The Silicon Controlled Rectifier is the most popular of
the thyristor family of four layer regenerative devices.
It is normally turned on by the application of a gate
pulse when a forward bias voltage is present at the
main terminals.
However, being regenerative or 'latching', it cannot be
turned off via the gate terminals specially at the
extremely high amplification factor of the gate.
16.
17.
18. There are two main types of SCR's.
Converter grade or Phase Control thyristors
Converter grade thyristors
These devices are the work horses of the Power Electronics.
They are turned off by natural (line) commutation and are
reverse biased at least for a few milliseconds subsequent to a
conduction period. No fast switching feature is desired of these
devices.
They are available at voltage ratings in excess of 5 KV starting
from about 50 V and current ratings of about 5 KA.
The largest converters for HVDC transmission are built with
series-parallel combination of these devices.
Conduction voltages are device voltage rating dependent and
range between 1.5 V (600V) to about 3.0 V (+5 KV).
These devices are unsuitable for any 'forced-commutated'
circuit requiring unwieldy large commutation components.
19. Contd..
Inverter grade thyristors:
Turn-off times of these thyristors range from about 5 to 50
μsecs when hard switched.
They are thus called fast or 'inverter grade' SCR's. The SCR's
are mainly used in circuits that are operated on DC supplies
and no alternating voltage is available to turn them off.
Commutation networks have to be added to the basic
converter only to turn-off the SCR's. The efficiency, size and
weight of these networks are directly related to the turn-off
time, tq of the SCR. The commutation circuits utilize
resonant networks or charged capacitors.
Asymmetrical, light-activated, reverse conducting SCR's
Quite a few varieties of the basic SCR have been proposed for
specific applications. The Asymmetrical thyristor is
convenient when reactive powers are involved and the light
activated SCR assists in paralleling or series operation.
20. Other Devices Of the Thyristor Family
DIAC
TRIAC
GTO
LASCR
MCT
20
21. Power MOSFETS
The Power MOSFET technology has mostly reached maturity and is the most
popular device for SMPS, lighting ballast type of application where high
switching frequencies are desired but operating voltages are low.
Being a voltage fed, majority carrier device (resistive behavior) with a
typically rectangular Safe Operating Area, it can be conveniently utilized.
The capacitances are the main hindrance to operating the MOSFETS at
speeds of several MHz
The resistive characteristics of its main terminals permit easy paralleling
externally also.
At high current low voltage applications the MOSFET offers best conduction
voltage specifications as the RDS(ON) specification is current rating dependent.
However, the inferior features of the inherent anti-parallel diode and its
higher conduction losses at power frequencies and voltage levels restrict its
wider application.
22.
23. The IGBT
It is a voltage controlled four-layer device with the advantages of
the MOSFET driver and the Bipolar Main terminal.
Punch-through IGBTs are available up to about 1200 V.
NPT IGBTs of up to about 4 KV are available.
Its switching times can be controlled by suitably shaping the drive
signal.
This gives the IGBT a number of advantages: it does not require
protective circuits, it can be connected in parallel without difficulty,
and series connection is possible without dv/dt snubbers.
29. Power semiconductor switching devices can be classified on
the basis of:
Uncontrolled turn on and off (diodes)
Controlled turn on and uncontrolled turn off (SCR)
Controlled turn on and off (BJT, MOSFET, GTO, IGBT)
Continuous gate signal requirement (BJT, MOSFET, IGBT)
29
31. Characteristics and Specification of
Switches
There are many types of power switching devices.
Each has its own advantages and disadvantages for an
application.
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32. Ideal Switches
In the on-state: carry high forward current,
low forward voltage drop, and low resistance
In the off-state: withstand a high voltage, low
leakage current, and high resistance
During turn-on and turn-off process
instantaneously turn on and off
32
33. Types of Power Electronic CKTs
For control of electric power or power conditioning,
the conversion of electric power from one form to
another is necessary.
Switching characteristics of the power devices permit
this conversion.
33
34. Power electronics circuits can be classified into six types:
Diode rectifiers
Ac-dc converters (controlled rectifier)
Ac-ac converters (ac voltage controllers)
Dc-dc converters (dc choppers)
Dc-ac converters (inverters)
Static switches
34
38. Design of Power Electronics
Equipment
The design is divided into four parts:
Design of power circuits
Protection of power devices
Determination of control strategy
Design of logic and gating circuits
38
39. In the chapters that follow, we will describe various types of
power electronic circuits.
In analysis, the power devices are assumed to be ideal
switches.
The effect of circuit resistance and source inductance is
ignored.
Ignoring these parameters will simplify the design steps, but
it is very useful to understand operation of the circuit and
establish the control strategy.
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40. Peripheral Effects
Operations of power converters are mainly based on
the switching of power semiconductor devices.
As a result, converters introduce current and voltage
harmonics into the supply system and on the output of
the converters.
40
41. These can cause problems of distortion of the output
voltage, harmonic generation into the supply system,
and interference with the communication and
signaling circuits.
Therefore, it is normally necessary to introduce filters
on the input and output of a converter system to
reduce the harmonic level.
The electromagnetic interference can be reduced by
grounded shielding
41
42. The following figure shows the block diagram of a
generalized power converter.
42
43. Part 2
Power Transistors
Introduction
Power transistors have controlled turn-on and turn-off
characteristics.
They are used as switching elements, are operated in the
saturation region, resulting in a low on-state voltage drop.
43
44. The power transistors can be classified broadly into five
categories:
Metal oxide semiconductor field-effect transistors
(MOSFETs)
Bipolar junction transistors (BJTs)
Insulated-gate bipolar transistors (IGBTs)
Static induction transistors (SITs)
44
45. A transistor can be operated as a switch. Practical
transistors differ from ideal devices.
The transistors have certain limitations and are restricted
to some applications.
The characteristics of the gating circuit are key elements
in achieving the desired output and the control
requirements of any power converter.
Power electronics is increasingly used in applications that
require gate-drive circuits with advance control, high
speed, high efficiency, and compactness; therefore, gate
drive integrated circuits (ICs) are becoming commercially
available.
45
46. Bipolar Junction Transistors
A bipolar transistor is formed by adding a second p- or n-region
to a pn junction diode.
With two n-regions and one p-region, two junctions are formed
and it is known as an NPN transistor.
With two p-regions and one n-region, it is known as a PNP
transistor.
46
47. The three terminals are named collector, emitter, and base.
A bipolar transistor has two junctions, a collector-base
junction (CBJ) and a base-emitter junction (BEJ).
The base and collector currents flow through two parallel
paths, resulting in low on-state collector-emitter resistance,
RCE(ON).
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48. Steady-state Characteristics
Figure(a) shows a common-emitter configuration for an
NPN transistor, which is generally used in switching
applications.
The typical input characteristics of base current, IB against
base-emitter voltage, VBE are shown in Figure(b).
Figure(c) shows the typical output characteristics of
collector current, IC against collector-emitter voltage, VCE.
48
49. There are three operating regions of a transistor: cutoff,
active and saturation.
(1) Cut off region
the transistor is off
the base current is not enough
to turn it on
both junctions are reverse biased.
(2) Active region
the transistor acts as an amplifier
base current is amplified by a gain
the collector-emitter voltage decreases with the base
current.
the CBJ is reverse biased, BEJ is forward biased. 49
50. (3) Saturation region
base current is sufficiently high
collector-emitter voltage is low
transistor acts as a switch
both junctions (CBJ and BEJ) are forward biased.
The transfer characteristics, which is a plot of VCE against IB is
shown in Figure.
The model of an NPN transistor is also shown in Figure under
large-signal dc operation.
50
58. Switching Limits
(i) Second breakdown (SB)
It is a destructive phenomenon, results from the current
flow to a small portion of the base, producing localized
hot spots.
If the energy in these hot spots is sufficient, the excessive
localized heating may damage the transistor.
SB is caused by a localized thermal runaway, resulting
from high current concentrations.
The current concentration may be caused by defects in the
transistor structure.
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61. 61
(iv) Breakdown voltages
A breakdown voltage is defined as the absolute
maximum voltage between two terminals with the
third terminal open, shorted, or biased in either
forward or reverse direction.
At breakdown the voltage remains relatively constant,
where the current rises rapidly.
62. Power MOSFETs
A power MOSFET is a voltage-controlled device and
requires only a small input current.
The switching speed is very high and the switching times
are of the order of nanoseconds.
Power MOSFETs find increasing applications in low-
power high-frequency converters.
MOSFETs do not have the problems of second
breakdown phenomena.
MOSFETs have the problems of electrostatic discharge
and require special care in handling.
It is relatively difficult to protect them under short-
circuited fault conditions.
62
64. If VGS is positive, an induced voltage attracts the electrons from the
p-substrate and accumulates them at the surface beneath the oxide
layer.
If VGS is greater than threshold voltage, VT, a sufficient number of
electrons are accumulated to form a virtual n –channel, as shown by
shaded lines in the Figure, and the current flows from the drain to
source.
The enhancement-type MOSFETs are generally used as switching
devices in power electronics.
MOSFETs require low gate energy, and have a very fast switching
speed and low switching losses.
The input resistance is very high, 109 to 1011 Ω.
However, MOSFETs suffer from the disadvantage of high forward
on-state resistance, and hence high on-state losses, which makes
them less attractive as power devices, but they are excellent as gate
amplifying devices for thyristors.
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65. Steady-state Characteristics
The MOSFETs are voltage-controlled devices and have very high
input impedance.
The gate draws a very small leakage current on the order of
nanoamperes.
The transconductance, which is the ratio of drain current to gate
voltage, defines the transfer characteristics and is a very
important parameter.
65
67. Figure shows the output characteristics
of an n channel enhancement
MOSFET. There are three regions of
operation:
(1) cutoff region, where VGS ≤ VT;
(2) pinch-off or saturation region,
where VDS ≥ VGS – VT;
(3) linear region, where VDS ≤ VGS –
VT .
In the linear region, the drain current,
ID varies in proportion to the drain-
source voltage, VDS.
Due to high IDS and low VDS, the
power MOSFETs is operated in the
linear region for switching actions.
67
69. In order to keep the value of VDS low, the VGS must be higher so
that the transistor operates in the linear region.
The steady-state switching model, which is the same for both
depletion-type and enhancement-type MOSFETs, is shown in
Figure.
RD is the load resistance. A large resistance, RG (megaohms) is
connected between the gate and source to establish the gate voltage
to a defined level.
Rs limits the charging current through the internal capacitances of
the MOSFET.
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Steady-state switching model of MOSFETs
71. IGBTs
An IGBT combines the advantages of BJTs and
MOSFETs.
An IGBT has high input impedance (like MOSFETs), and
low on-state conduction losses (like BJTs).
There is no second breakdown problem as with BJTs.
The silicon cross section of an IGBT is shown in Figure
which is identical to that of an MOSFET except for the p+
substrate.
However, its performance is closer to that of a BJT than
an MOSFET.
This is due to the p+ substrate, which is responsible for the
minority injection into the n-region.
71
74. In the NPT structure, carrier lifetime is kept more than that of a PT
structure, which causes conductivity modulation of the drift region and
reduces the on-state voltage drop.
An IGBT is a voltage controlled device similar to a power MOSFET.
Like a MOSFET, when the gate is made positive with respect to the
emitter for turn-on, n carriers are drawn into the p-channel near the
gate region; this results in a forward bias of the base of the NPN
transistor, which thereby turns on.
An IGBT is turned on by just applying a positive gate voltage to open
the channel for n carriers and is turned off by removing the gate
voltage to close the channel. It requires a very simple driver circuit.
It has lower switching and conducting losses while sharing many of
the appealing features of power MOSFETs, such as ease of gate drive,
peak current, capability and ruggedness.
An IGBT is inherently faster than a BJT. However, the switching speed
of IGBTs is inferior to that of MOSFETs.
74
75. The symbol and circuit of an IGBT switch are shown in Figure.
The three terminals are gate, collector and emitter.
The typical output characteristics of iC versus vCE are shown in
Figure(a) for various gate-emitter voltage, vGE.
The typical transfer characteristics of iC versus vGE is shown in
Figure(b).
The parameters and their symbols are similar to that of
MOSFETs.
75
Symbol and circuit for an IGBT
76. 76
Typical output and transfer characteristics of IGBTs
•The current rating of a single IGBT can be up to 6500 V, 2400 A and the switching
frequency can be up to 20 kHz.
•IGBT are finding increasing applications in medium-power applications such as dc
and ac motor drives, power supplies, solid-state relays and contractors.
•As the upper limits of commercially available IGBT ratings are increasing, IGBTs
are finding and replacing applications where BJTs and conventional MOSFETs
were predominantly used as switches.
77. Summary
Differences between Static and Switching/Control
characteristics.
Classification of Power Devices based on control
characteristics.
Define Forced beta, threshold voltage, Over Drive Factor
Relevant equations
77
79. Activity
Design and simulate Power Electronic Converters
using Pspice software.
Obtain the switching characteristics of MOSFET and
IGBT using Pspice.
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80. Probable questions
What is power electronics and mention its applications.
List the different types of power electric circuits with neat
waveforms
What are peripheral effects? What are the remedies for
them?
Give symbol, characteristics and features of the following
devices. i. GTO ii. TRIAC
iii .MCT iv. DIAC v. IGBT
Explain the control characteristic TRIAC, MCT, IGBT with
neat waveforms. Mention and explain the classification of
power semiconductor switching devices on the basis of
control characteristics.
What is a static power converter? Name the different types
of power converters and mention their functions.
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