POWER SWITCHING DEVICES

POWER ELECTRONICS (for R18 Regulation)
B.Tech – III year-I sem
BY
K. Sadanandam,
Assistant Professor( c )
Department of EEE,JNTUHCEM
POWER ELECTRONICS 1

POWER ELECTRONICS 2
SYLLABUS
UNIT-I
UNIT-II
POWER SWITCHING DEVICES
Concept of power electronics, scope and applications, types of power converters;
Power semiconductor switches and their V-I characteristics - Power Diodes, Power
BJT, SCR, Power MOSFET, Power IGBT; Thyristor ratings and protection, methods
of SCR commutation, UJT as a trigger source, gate drive circuits for BJT and
MOSFETs
AC-DC CONVERTERS (PHASE CONTROLLED RECTIFIERS)
Principles of single-phase fully-controlled converter with R, RL, and RLE load,
Principles of single-phase half-controlled converter with RL and RLE load, Principles
of three-phase fully-controlled converter operation with RLE load, Effect of load and
source inductances, General idea of gating circuits, Single phase and Three phase
dual converters

POWER ELECTRONICS 3
UNIT-III
DC-DC CONVERTERS (CHOPPER/SMPS)
Introduction, elementary chopper with an active switch and diode, concepts of duty
ratio, average inductor voltage, average capacitor current. Buck converter - Power
circuit, analysis and waveforms at steady state, duty ratio control of output voltage.
Boost converter - Power circuit, analysis and waveforms at steady state, relation
between duty ratio and average output voltage. Buck-Boost converter - Power circuit,
analysis and waveforms at steady state, relation between duty ratio and average
output voltage
UNIT-IV
AC-DC CONVERTERS (INVERTERS)
Introduction, principle of operation, performance parameters, single phase bridge
inverters with R, RL loads, 3-phase bridge inverters - 120 and 180 degrees mode of
operation, Voltage control of single phase inverters –single pulse width modulation,
multiple pulse width modulation, sinusoidal pulse width modulation

POWER ELECTRONICS 4
UNIT-V
AC-AC CONVERTERS
Phase Controller (AC Voltage Regulator)-Introduction, principle of operation of single
phase voltage controllers for R, R-L loads and its applications. Cyclo-converter-Principle of
operation of single phase cyclo-converters, relevant waveforms, circulating current mode of
operation, Advantages and disadvantages
.

POWER ELECTRONICS 5
TEXT BOOKS:
1. M. H. Rashid, “Power electronics: circuits, devices, and
applications”, Pearson Education India, 2009.
2. N. Mohan and T. M. Undeland, “Power Electronics:
Converters, Applications and Design”, JohnWiley & Sons,
2007.
REFERENCES:
1. R. W. Erickson and D. Maksimovic, “Fundamentals of
Power Electronics”, Springer Science & Business Media,
2007.
2. L. Umanand, “Power Electronics: Essentials and
Applications”, Wiley India, 2009.

CONTENT
• Course Objectives
• Course Outcomes
• Concept of power electronics
• Scope and applications
• Types of power converters
• Power semiconductor switches
• V-I characteristics of Power Diodes, Power BJT, SCR, Power
MOSFET, Power IGBT
• Thyristor ratings and protection, methods of SCR
commutation,
• UJT as a trigger source, gate drive circuits for BJT and
MOSFETs
POWER ELECTRONICS 6

POWER ELECTRONICS 7
•To Design/develop suitable power converter for efficient
control or conversion of power in drive Applications
• To Design / develop suitable power converter for efficient
transmission and utilization of power in power system
applications.
• To develop skills to build, and troubleshoot power
electronics circuits
•Foster ability to understand the use of power converters in
commercial and industrial applications.
COURSE OBJECTIVES

POWER ELECTRONICS 8
COURSE OUTCOMES
At the end of this course students will demonstrate the
ability to
•Understand the differences between signal level and power level
devices.
•Acquire knowledge about fundamental concepts and techniques
used in power electronics
•Analyze controlled rectifier circuits.
•Analyze the operation of DC-DC choppers.
•Analyze the operation of voltage source inverters

 Power Electronics is a field which combines Power (electric
power), Electronics and Control systems.
 Power electronics may be defined as the subject of
applications of solid state power semiconductor devices
(Thyristors) for the control and conversion of electric power.
 The power electronics subject deal about the conversion
and control of AC to DC ,DC to DC, AC to AC and DC
to AC
POWER ELECTRONICS 9
OVER VIEW OF POWER ELECTRONICS

POWER ELECTRONICS 10
AC INPUT
VARIABLE
DC OUT PUT
1-φ FULLY CONTROLLED RECTIFIER
3- φ AC
I/P
VARIABLE
DC OUT PUT
3-φ FULLY CONTROLLED RECTIFIER
FIXED DC
VARIABLE DC O/P
CHOPPER
Fixed
AC i/p
Variable
AC o/p
Ac voltage controller

1. COMMERCIAL APPLICATIONS: Heating Systems Ventilating, Air
Conditioners, Central Refrigeration, Lighting, Computers and Office
equipments, Uninterruptible Power Supplies (UPS), Elevators, and
Emergency Lamps.
2. DOMESTIC APPLICATIONS: Cooking Equipments, Lighting, Heating,
Air Conditioners, Refrigerators & Freezers, Personal Computers,
Entertainment Equipments, UPS.
3. INDUSTRIAL APPLICATIONS: Pumps, compressors, blowers and fans.
Machine tools, arc furnaces, induction furnaces, lighting control circuits,
industrial lasers, induction heating, welding equipments.
POWER ELECTRONIC APPLICATIONS

Power Semiconductor Switches
Power Diodes Power Transistors Thyristors
2 layer device 3 layer Device 4 layer Device
• Thyristor devices can convert and control large amounts of
power in AC or DC systems while using very low power for
control.

THYRISTOR FAMILY INCLUDES
1. Silicon controlled switch (SCR)
2. Gate-turnoff thyristor (GTO)
3. Triac
4. Diac
5. Silicon controlled switch (SCS)
6. Mos-controlled switch (MCT)

Power Diodes
Appearance
Symbol
CONSTRUTION OF POWER DIODE
KA

Typical power diode characteristics

Typical diode switching waveforms

1.Standard recovery
Reverse recovery time not specified, intended for 50/60Hz
2.Fast recovery and ultra-fast recovery
Reverse recovery time and recovered charge specified Intended for
converter applications
3.Schottky diode
A majority carrier device Essentially no recovered charge Model
with equilibrium i-v characteristic, in parallel with depletion region
capacitance Restricted to low voltage (few devices can block 100V
or more)
Types of power diodes

Power Transistors
Power transistors are classified as follows
1.Bipolar junction transistors(BJTs)
2.Metal-oxide semiconductor filed-effect transistors (MOSFETs)
3.Insulated-gate bipolar transistors(IGBTs)

BIPOLAR JUNCTION TRANSISTORS(BJTS)

Steady StateCharacteristics
CutoffRegion:When the base current (IB) is zero, the collector current
(IC) is insigniﬁcant and the transistor is driven into the cutoff region.
The transistor is now in the OFF state. The collector–base and base–
emitter junctions are reverse- biased in the cutoff region or OFF state,
and the transistor behaves as an open switch
In this region: IC= 0 and the collector–emitter voltage VCE is equal to the
supply voltageVCC

Saturation Region: When the base current is sufficient to drive the
transistor into saturation. During saturation, both junctions are
forward-biased and the transistor acts like a closed switch. In the
quasi saturation and hard saturation, the base drive is applied and
transistor is said to be on. In this region: IC = VCC /RC and VCE = zero
Active Region: In the active region, the collector–base junction is
reversed-biased and the base–emitter junction is forward-biased. The
active region of the transistor is mainly used for ampliﬁer
applications and should be avoided for switching operation.
The power BJT is never operated in the active region (i.e. as an
amplifier) it is always operated between cut-off and saturation.

PowerBJTasaSwitch
 The transistor is usedasaswitch therefore it is usedonly between
saturation and cutoff.

as long as VCE>VBE the Collector-Base junction is reverse biased
and transistor is in active region,
The maximum collector current in the active region, for
If IB > IBM → VBE↑, IC↑ and VCE falls below VBE. This continues
until Collector-Base junction is forward biased and the BJT goes into
saturation region
The collector current is

The ratio of IC to ICS is called forced β
The total power loss in the two functions is
The ratio of Ib to Ibs is called to overdrive factor ODF.

Example-1
The BJT is specified to have a range of 8 to 40. The load resistance in RE=11
ohm. The dc supply voltage is Vcc=200v and the input voltage to the base
circuit is VB=10v. If VCE(sat)=1.0v and VBE(sat)=1.5v. Find
a. The value of RB that results in saturation with a overdrive factor of 5.
b. The forced βf
c. The power loss Pt in the transistor

Switching Characteristics

Switching Times – turn on
Input voltage rises from 0 to V1
Base current rises to IB1
Collector current begins to rise after the delay time, td
Collector current rises to steady-state value ICS
This “rise time”, tr allows the Miller capacitance to charge
to V1 turn on time, ton = td + tr

Input voltage changes from V1 to –V2
Base current changes to –IB2
Base current remains at –IB2 until the Miller
capacitance discharges to zero, storage time, ts
Base current falls to zero as Miller capacitance charges
to –V2, fall time, tf turn off time, toff = ts + tf

ADVANTAGES OF BJT’S
•BJT’s have high switching frequencies since their turn-on and turn-off
time is low.
•The turn-on losses of a BJT are small.
• BJT has controlled turn-on and turn-off characteristics since base
drive control is possible.
•BJT does not require commutation circuits.
DEMERITS OF BJT
•Drive circuit of BJT is complex.
•It has the problem of charge storage which sets a limit on switching
frequencies.
It cannot be used in parallel operation due to problems of negative
temperature coefficient

Metal-oxide semiconductor filed-effect transistors(MOSFETs)
Unlike the devices discussed so far, a power MOSFET is a
unipolar, majority carrier, “zero junction,” voltage-
controlled device. Figures (a) and (b) below show the
symbol of an N- type and P-type MOSFETs

MOSFETConstruction

V - I Characteristics of MOSFET

The load line can be superimposed on the output characteristic to give
the operating point:
VDD=ID R+VDS
ID=(VDD/R)-(VDS/R)
At ID=0, VDD=VDS; at VDS=0, ID=VDD/R.

Insulated Gate Bipolar Transistor (IGBT)
Introduction
• BJT is a device with low power loss but long switching time
(especially at turn-off)
• MOSFET is a device with fast switching time( low turn ON &
turn OFF times) but have high power losses
• The draw backs of BJT & MOSFET can be overcome by
IGBT
• Hence the IGBT has low switching times as well as low power
loss

Silicon
Dioxide
Silicon
Dioxide
Body
Region
Buffer
Layer
Injecting
Layer
Drain
drift
region
Drain
Gate
Source
J1
J2
J3
n+
n+
P+
n+
P
Construction of IGBT
Insulated Gate Bipolar Transistor (IGBT) (contd..)

Parasitic SCR
G
E
R p
E
Structure of IGBT
Transistor model

Structure of IGBT
• IGBT is combination of MOSFET & BJT on a single chip
• It contains P+ layer forms as a drain also called as Injecting
layer ,next layer n+ layer also called as Buffer Layer
• In between Injecting layer and Buffer layers a P-N junction J1 is
formed

• It also consists of another two junctions termed as J2 and J3
• This IGBT has a parasitic Thyristor as shown in FIG.2 (Both
PNP and NPN)
• Turn ON of this Thyristor is undesirable and the body of IGBT
is designed to avoid the turn ON

• If IGBT does not have n+ layer, then the device is
called Non punch through IGBT (NPT-IGBT)
• If IGBT does not have n+ layer, then the device is
called Non punch through IGBT (NPT-IGBT)

• N - Channel IGBT
• P - Channel IGBT
Types of IGBT

Gate
Source
( Emitter )
Drain
( Collector )
N-Channel IGBT
Gate
Source
( Emitter )
Drain
( Collector )
P- Channel IGBT
Types of IGBT (contd..)
FIG.3
FIG.4

V - I Characteristics of IGBT
Drain Characteristics Transfer characteristics
POWER ELECTRONICS
46
I
VGS2
VGS1
VGS3
BVDSS VDS
ID
VGS(th)
VRM
0 0 VGS

V - I Characteristics of IGBT
• The junction blocks reverse voltage. An IGBT without n+ buffer
layer has higher reverse blocking capability
• The reverse blocking voltage V RM on the V-I characteristics
• The junction J2 blocks forward voltage when IGBT is OFF
• BVDSS denotes the breakdown voltage in the forward direction
• Transfer characteristics of IGBT is similar to that power
MOSFET

SCR(silicon Controlled Rectifier

Threaded Stud
Anode ( aluminium)
Gate terminal Welded
to P region
J1
J2
J3
Cathode ( aluminium)
Construction of SCR

Anode
Gate
Cathode
J1
J2
J3
N
N
P
P
Anode
Gate
Cathode
Construction of SCR

Constructional of SCR
• SCR – is a Four layer, Three terminal, Three
P-N junction device
• Outer P region is Called Anode
• Outer N region is called Cathode
• Inner P region is called Gate

Switch-1
Switch-2
VDC

SCR Switching Characteristics

A
G
K
N
PP
P
N
N
Schematic Diagram
Two Transistor Analogy of SCR
Two transistor model of SCR is obtained by
splitting the two middle layers into two separate
parts.

• Emitter E of T1 is Anode, Emitter E of
T2 is Cathode
• Gate G is positive wrt Cathode.
P
P
N N
P
N
A
G
k
Two transistor Model
• Transistor T1 - PNP and Transistor T2 -NPN
Two Transistor Analogy of SCR (contd..)

Two transistor Model
J2
α1 , β 1
I a
I g
α2 , β2
A
K
G
IC2
I B1 = IC2
IB2
IC1
IK

During the OFF state of a transistor,
The collector Current, IC = α I E + I CBO
where
α – is the common base current gain
I CBO - Common base leakage Current
For transistor – 1
IC1 = α1 I E + I CBO1
Similarly for transistor - 2
IC2 = α2 I E + I CBO2

Total current I a - is sum of collector currents of two transistors.
I a = Ic1 + Ic2
When gate is triggered , I K = Ia + I g
I k = -----------------------------------------------------
( α1 I g + I CBO1 + I CBO2 )
( 1- (α1 + α2 )

V-I Characteristics of SCR
Vs
G
K
A
E
ES
Load
The Thyristor is connected as shown in circuit diagram

When Anode is made positive with respect to
Cathode and the switch is in open position the
Thyristor is said to be forward bias
During Forward Bias
A
G
K
J1
J2
J3
N
P
P
N
Forward Leakage
Current
Junction J1 , J 3 are forward bias and J2 is in
reverse bias
A small forward leakage current flows from Anode to
Cathode. At this position the Thyristor behaves like a “
Forward Blocking or OFF- State Condition ”

If the Anode to Cathode voltage is increased to sufficiently
large voltage , the junction J2 will breakdown. This is known
as “ Avalanche Breakdown ”
The voltage at which the junction- J2 gets break down is called
“ Forward Break Over Voltage ”
During Forward Bias (contd…)

During Reverse Bias
When Cathode is made positive with respect
to Anode and the switch is in open position
the Thyristor is said to be “ Reverse Bias ”
Junctions J1 , J 3 are reverse bias and J2 is in forward bias
A
G
J1
J2
J3
N
P
P
N
K
S
Reverse Leakage current

POWER ELECTRONICS
63
A small reverse leakage current flows from cathode to anode of
the order of few milliamperes or microamperes will flow. This is
known as “ Reverse Blocking or OFF- State Condition ” of
Thyristor.
If the reverse voltage is increased , then at a critical voltage
the Junctions J1 ,J 3 gets breakdown and the current increases
rapidly
Hence, a large magnitude of current flows through the
Thyristor and causes Thyristor gets damaged
During Reverse Bias (contd..)

Voltage
Current
0
Ig3 Ig2 Ig1
Ig =0
Holding Current
Latching Current
forward conduction
state ( on state )
forward leakage Current
Forward Blocking
Reverse leakage
Current
Reverse Blocking
V - I Characteristics of SCR
9EE604.2&3POWER ELECTRONICS 64

SCR Ratings
SCR Current Ratings
1- Maximum Repetitive RMS current Rating
• Average on-state current is the maximum average current
value that can be carried by the SCR in its on state.
• RMS value of non sinusoidal waveform is simplified by
approximating it by rectangular waveform.
• This approximation give higher RMS value, but leaves slight
safety factor.
65

• Average value of pulse is
• Form factor is
66
•After approximating, the RMS value of the current can be found
from
IRMS =√
Iave = Im to
T
f0 =
IRMS
Iave
Im t0
T
2

 Knowing the form factor for given waveform, RMS current can
be obtained from IRMS=fo(IAVE)
 Maximum repetitive RMS current is given by
IT(RMS)=fo(IT(AVE))
 Conduction angle verses form factor
67
Conduction angle (θ) Form factor (fo)
20° 5.0
40° 3.5
60° 2.7
80° 2.3
100° 2.0
120° 1.8
140° 1.6
160° 1.4
180° 1.3

1.Conduction Angle
• Duration for which SCR is on. It is measured as shown
68

2. Surge Current Rating
Peak anode current that SCR can handle for brief duration.
3. Latching current
Minimum anode current that must flow through the SCR in order
for it to stay on initially after gate signal is removed.
4. Holding Current
Minimum value of anode current, required to maintain SCR in
conducting state.
69

SCR Voltage Ratings
1. Peak repetitive forward blocking voltage
Maximum instantaneous voltage that SCR can block in
forward direction.
2. Peak Repetitive Reverse Voltage
Maximum instantaneous voltage that SCR can
withstand, without breakdown, in reverse direction.
3. Non-repetitive peak reverse voltage
Maximum transient reverse voltage that SCR can
withstand.
70

SCR Rate-of-Change Ratings
1.di/dt rating
Critical rate of rise of on-state current. It is the rate at which
anode current increases and must be less than rate at which
conduction area increases.
To prevent damage to SCR by high di/dt value, small inductance
is added in series with device.
2.dv/dt rating
Maximum rise time of a voltage pulse that can be applied to the
SCR in the off state without causing it to fire. Unscheduled firing
due to high value of dv/dt can be prevented by using RC snubber
circuit.
72

Gate Parameters
1.Maximum Gate Peak Inverse Voltage
Maximum value of negative DC voltage that can be applied without
damaging the gate-cathode junction.
2.Maximum Gate Trigger Current
Maximum DC gate current allowed to turn on the device.
3.Maximum gate trigger voltage
DC voltage necessary to produce maximum gate trigger current.
4.Maximum Gate Power Dissipation
Maximum instantaneous product of gate current and gate voltage
that can exist during forward-bias.
5. Minimum gate trigger voltage
Minimum DC gate-to-cathode voltage required to trigger the SCR.
6.Minimum gate trigger current
Minimum DC gate current necessary to turn SCR on.
73

What is Commutation?
The process of turning off an SCR is called commutation.
It is achieved by
1. Reducing anode current below holding current
2. Make anode negative with respect to cathode
Types of commutation are:
1. Natural or line commutation
2. Forced commutation
74
Methods of SCR commutation

1. Natural or line commutation
Natural commutation can be observed in AC voltage controllers,
phase controlled rectifiers and cyclo converter

 Class A: Self commutated by a resonating load
 Class B: Self commutated by an LC circuit
 Class C: C or L-C switched by another load carrying SCR
 Class D: C or L-C switched by an auxiliary SCR
 Class E: An external pulse source for commutation
 Class F: AC line commutation
Forced Commutation Methods

Class A: Self commutated by a resonating load

Class B: Self Commutated by an L-C Circuit

Class C: C or L-C switched by another load carrying SCR

Class D: L-C or C Switched by an Auxiliary SCR

Class E: External Pulse Source for Commutation

SCR R-triggering circuit

SCR RC-triggering circuit

SCR trigger circuits using UJT oscillator
Circuit A Circuit B

Gate drive circuits for BJT

Gate drive circuits for MOSFET

CONCLUSION
•This unit discussed about the fundamental details of power
semiconductor switches and their V-I Characteristics, it helps
students to learn usage and application of switches into modeling
of various convertors and controllers.

POWER SWITCHING DEVICES

Recommended

Recommended

More Related Content

What's hot

What's hot (20)

Similar to POWER SWITCHING DEVICES

Similar to POWER SWITCHING DEVICES (20)

Recently uploaded

Recently uploaded (20)

POWER SWITCHING DEVICES