EEE 453( Semiconductor Switch and Triggering Device)

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EEE 453
Power Electronics
EEE-D-63 A, B
Spring 2020
Md Jabed Hossain
Assistant Professor
Stamford University Bangladesh– Department
of Electrical and Electronic Engineering
Mobile: +88-01816 050766
Email: jabed@stamforduniversity.edu.bd
21/01/2020 Stamford University Bangladesh 1
EEE 453
Text Books
1. Power Electronics: Circuits, Devices & Applications by Muhammad H.
Rashid
2. Power Electronics by Muhammad H. Rashid
3. Power Electronics: Converters, Application and Design by Ned Mohan
4. Power Electronics by Daniel W. Hart
Simulation Tools
Pspice Simulation
Matlab Simulink
Course MaterialsEEE 453

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Course Contents
Power semiconductor switches and triggering devices: BJT, MOSFET, SCR,
IGBT, GTO, TRIAC, UJT and DIAC. Rectifiers: Uncontrolled and controlled
single phase and three phase. Regulated power supplies: Linear-series
and shunt, switching buck, buckboost, boost and Cuk regulators. AC
voltage controllers: single and three phase. Choppers. DC motor control.
Single phase cycloconverter. Inverters: Single phase and three phase
voltage and current source. AC motor control. Stepper motor control.
Resonance inverters. Pulse width modulation control of static converters.
Course OutlineEEE 453
Commonly Used Power and Converter Equation
Power ElectronicsEEE 453

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Definition of Power Electronics and Applications
Power electronics involves
the study of electronic
circuits intended to
control the flow of
electrical energy. These
circuits handle power flow
at levels much higher
than the individual device
ratings.
Classification of Power Electronics Circuits
The power electronics circuits can be classified into six types:
1. Diode rectifiers
2. Dc-dc converters(dc choppers)
3. Ac-dc converters(controlled rectifiers)
4. Ac-dc converters(controlled rectifiers)
5. Ac-ac converters(ac voltage controllers)
6. Static switches

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Power Diode Characteristics
The v-i characteristics show in figure can be
expressed by an equation known as Schockley diode
equation, and it is given under dc steady-state
operation by
/
1
Where
ID = current through the diode, A;
VD = diode voltage with anode positive with respect
to cathode, V;
IS = leakage (or reverse saturation) current, typically
in the range 10-6 to 10-15 A;
n = empirical constant known as emission coefficient,
or ideality factor, whose value varies from 1 to 2.
VT in Eq. is a constant called
thermal voltage and it is given by
Structure of Power Diode
Middle layer of lightly doped n– is known as a drift layer. The thickness of the drift layer depends
on the required breakdown voltage. The breakdown voltage increases with an increase in the
width of the drift layer. Resistivity of this layer is high because of the low level of doping. If the
width of the drift layer increased, then the on-state voltage drop increase therefore power loss is
more. The doping level of the drift layer is 1014 cm-3.
The operating principle of power diode is same as
the conventional PN junction diode. A diode
conducts when the anode voltage is higher than
the cathode voltage. The forward voltage drop
across the diode is very low around 0.5V to 1.2V.
In this region, the diode works as a forward
characteristic.
If the cathode voltage is higher than the anode
voltage, then the diode works as blocking mode.
In this mode, diode works according to the
reverse characteristic.

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Power Diode Characteristics
A very small amount of leakage
current flows in the reverse bias
(blocking mode). The leakage
current is independent of the
applied reverse voltage. The
leakage current flows due to the
minority charge carriers. When the
reverse voltage reaches the reverse
breakdown voltage, avalanche
breakdown occurs.
Reverse Recovery Characteristics
The current in a forward-biased junction diode is due to the net effect of majority and
minority carriers. Once a diode is in a forward conduction mode and then its forward
current is reduced to zero (due to the natural behaviour of the diode circuit or
application of a reverse voltage), the diode continues to conduct due to minority carriers
that remain stored in the pn-junction and the bulk semiconductor material. The minority
carriers require a certain time to recombine with opposite charges and to be neutralized.
This time is called the reverse recovery time

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Power Diode Applications and Types
Applications:
Rectification
Freewheeling
Energy Feedback
AC to DC converters
DC to DC converters
DC to AC converters
Power Supply
Depending on the recovery characteristics and manufacturing
techniques, the power diodes can be classified into the
following three categories:
General Purpose PN junction Power Diodes
 These are used for the low frequency applications like
rectification.
 Voltage ratings are from 50V to 5kV.
Fast Recovery Diodes
 These diodes have low recovery time as compared to the
general purpose diodes.
 Can be used high frequency applications.
Schottky Diodes
 On state voltage drop across it is very low (0.3 V to 0.4 V).
 Large reverse leakage current and therefore a lower
breakdown voltage (so to 100V).
Power Bipolar Junction Transistors (BJT)
Structure of Power BJT
• A power transistor is a vertically oriented four
layer structure of alternating p-type and n-type.
The vertical structure is preferred because it
maximizes the cross sectional area and through
which the current in the device is flowing. This
also minimizes on-state resistance and thus power
dissipation in the transistor.
• The doping of emitter layer and collector layer is
quite large typically 1019 cm-3. A special layer
called the collector drift region (n-) has a light
doping level of 1014 cm-3.
• The thickness of the drift region determines the
breakdown voltage of the transistor. The base
thickness is made as small as possible in order to
have good amplification capabilities, however if
the base thickness is small the breakdown voltage
capability of the transistor is compromised.

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Power (BJT)
Features:
Larger size
Higher breakdown
voltage
Higher current carrying
and power handing
capability
Higher on state voltage
drops
High power application
Applications:
Switch Mode Power
Supplies(SMPS)
Power amplifiers
DC to AC converters
Relays
Power control circuits
Power (BJT) Circuits
BJT

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Power (BJT) Operation
Power (BJT) Operation
The circuit diagram of the NPN transistor is shown in the figure below. The forward biased
is applied across the emitter-base junction, and the reversed biased is applied across the
collector-base junction. The forward biased voltage VEB is small as compared to the
reverse bias voltage VCB.
Forward biased causes the electron in the n-type emitter to flow toward the base. This
constitutes the emitter current IE. As these electrons flow through the p-type base, these
electrons tend to combine with the holes. As the base is lightly doped and very thin,
therefore, only few electrons (less than 5%) combine with holes to constitute the base
current IB. The remainder (more than 95) cross over into the collector region to constitute
collector current IC.
Cut off ( Both BE & CB in RB)
Active (BE in FB & CB in RB)
Saturation ( Both BE & CB in FB)

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Bipolar Junction Transistors (BJT)
Steady-State Characteristics
The model of an NPN-transistor is shown in Figure 4.30
under large-signal dc operation. The equation relating the
currents is
I I I
The base current is effectively the input current and the
collector current is the output current. The ratio of the
collector current IC to base current IB is known as the
forward
current gain, βF:
β
I
I
The collector current has two components: one due to the
base current and the other is the leakage current of the
CBJ.
I I I
Power MOSFETs
Features of 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 as do BJTs.
However, MOSFETs have the problems of electrostatic discharge and
require special care in handling. In addition, it is relatively difficult to
protect them under short-circuited fault conditions.
The two types of MOSFETs are
(1) depletion MOSFETs and
(2) enhancement MOSFETs
IRFP 450 Power
MOSFET

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Power MOSFETs
Basic Operation
The gate-to-source voltage is set to zero volts by
the direct connection from one terminal to the other,
and a voltage VDS is applied across the drain-to-
source terminals. The result is an attraction for the
positive potential at the drain by the free electrons
of the n-channel and a current similar to that
established through the channel of the JFET. In fact,
the resulting current with VGS = 0 V continues to be
labelled IDSS, as shown in Fig.
In Fig, VGS has been set at a negative voltage such
as 1 V. The negative potential at the gate will tend
to pressure electrons toward the p-type substrate
(like charges repel) and attract holes from the p-
type substrate (opposite charges attract) as shown
in Fig.
Power MOSFETs
Depending on the magnitude of the
negative bias established by VGS, a
level of recombination between
electrons and holes will occur that will
reduce the number of free electrons
in the n-channel available for
conduction. The more negative the
bias, the higher the rate of
recombination.
The resulting level of drain current is
therefore reduced with increasing
negative bias for VGS as shown in
Fig. 5.25 for VGS = -1 V, -2 V, and so
on, to the pinch-off level of -6 V.

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Power MOSFETs
If VGS is negative, some of the electrons in the n-channel
area are repelled and a depletion region is created below
the oxide layer, resulting in a narrower effective channel
and a high resistance from the drain to source RDS If VGS
is made negative enough, the channel becomes
completely depleted, offering a high value of RDS, and no
current flows from the drain to source, IDS = 0. The value
of VGS when this happens is called pinchoff voltage VP.
An n-channel enhancement-type MOSFET has no
physical channel. 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 or equal to a value known as
threshold voltage VT, a sufficient number of electrons are
accumulated to form a virtual n-channel.
Silicon Controlled Rectifier (SCR)
Operation:
The SCR is a four-layer, three-junction and a three-
terminal device and is shown in fig. The end P-region is
the anode, the end N-region is the cathode and the inner
P-region is the gate. The anode to cathode is connected
in series with the load circuit. Essentially the device is a
switch. Ideally it remains off (voltage blocking state), or
appears to have an infinite impedance until both the
anode and gate terminals have suitable positive voltages
with respect to the cathode terminal. The thyristor then
switches on and current flows and continues to conduct
without further gate signals. Ideally the thyristor has
zero impedance in conduction state. For switching off or
reverting to the blocking state, there must be no gate
signal and the anode current must be reduced to zero.
Current can flow only in one direction.

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Imagine the SCR cut along the dotted
line, as shown in fig. a. Then we can
have two devices, as shown in fig.b.
These two devices can be recognized as
two transistors. The upper left one is
P-N-P transistor and the lower right
N-P-N type. Further it can be
recognized that the base of the P-N-P
transistor is joined to the
collector of the N-P-N transistor while the collector of P-N-P is joined to the base of N-P-N
transistor, as illustrated in fig. c. Now we can see that the two transistors are connected in such a
manner that the collector of Q1 is connected to the base of Q2 i.e. the output collector current of
Q1 becomes the base current for Q2. In the similar way the collector of Q2 is joined to the base of
Q1 which shows that the output collector current of Q2 is fed to Q1 as input base current.
These are back to back connections of transistors in such a way that the output of one
goes into as input of other transistor and vice-versa. This gives net gain of loop circuit
as β1 x β2 where β1 and β2 are current gains of two transistors respectively. As soon as a
small amount of gate current is given to the base of transistor Q2 by applying forward
bias to its base-emitter junction, it generates the collector current as β2 times the base
current. This collector current of Q2 is fed as input base current to Q1 which is further
multiplied by β1 times as ICl which forms input base current of Q2 and undergoes
further amplification. In this way both transistors feedback each other and the collector
current of each goes on multiplying. This process is very quick and soon both the
transistors drive each other to saturation. Now the device is said to be in on-state. The
current through the on-state SCR is controlled by external impedance only.

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Characteristics of (SCR)
When the anode voltage is made positive with respect to the cathode, the junctions J1 and J3 are
forward biased. The junction J2 is reverse biased, and only a small leakage current flows from anode to
cathode. The thyristor is then said to be in the forward blocking, or off-state, condition and the leakage
current is known as off-state current ID. If the anode-to-cathode voltage VAK is increased to a sufficiently
large value, the reverse-biased junction J2 breaks. This is known as avalanche breakdown and the
corresponding voltage is called forward breakdown voltage VBO. Because the other junctions J1 and J3
are already forward biased, there is free movement of carriers across all three junctions, resulting in a
large forward anode current. The device is then in a conducting state, or on-state. The voltage drop
would be due to the ohmic drop in the four layers and it is small, typically, 1 V. In the on-state, the
anode current is limited by an external impedance or a resistance, RL.
The anode current must be more than a value known as latching current IL to maintain the required
amount of carrier flow across the junction; otherwise, the device reverts to the blocking condition as the
anode-to-cathode voltage is reduced. Latching current IL is the minimum anode current required to
maintain the thyristor in the on-state immediately after a thyristor has been turned on and the gate
signal has been removed.
Characteristics of (SCR)
if the forward anode current
is reduced below a level
known as the holding
current IH, a depletion region
develops around junction J2
due to the reduced number of
carriers and the thyristor is
in the blocking state.
Holding current IH is the
minimum anode current to
maintain the thyristor in the
on-state. The holding current
is less than the latching
current.

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Insulated Gate Bipolar Transistor ( IGBT )
The operation of the IGBT is explained as follows
OFF STATE :
• When gate-emitter voltage less than the threshold voltage, inversion layer is
not created.
• The forward voltage between collector to emitter reverse biased across
junction J2 and only leakage current flow.
ON STATE:
• When gate - emitter voltage greater than threshold voltage inversion layer is
created.
• Due to this inversion layer conduction channel is created, therefore flow of
current is possible.

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Gate Turn-off Thyristors (GTO)
A GTO is turned on by applying a short positive pulse and turned off by a short negative pulse to
its gate.
The GTOs have these advantages over SCRs:
(1) elimination of commutating components in forced commutation, resulting in reduction in
cost, weight, and volume;
(2) reduction in acoustic and electromagnetic noise due to the elimination of commutation
chokes;
(3) faster turn-off, permitting high switching frequencies; and
(4) improved efficiency of converters
In low-power applications, GTOs have the following advantages over bipolar transistors:
(1) a higher blocking voltage capability;
(2) a high ratio of peak controllable current to average current;
(3) a high ratio of peak surge current to average current, typically 10:1;

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(4) a high on-state gain
(anode current and gate
current), typically 600; and
(5) a pulsed gate signal of
short duration.
Operation:
A positive gate pulse turns
the device ON.
if GTO is already ON and a
negative pulse is applied to
the gate terminal, the device
will turn off
Characteristics

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TRIAC
A TRIAC can conduct in both
directions and is normally
used in ac-phase control. It
can be considered as two SCRs
connected in antiparallel with
a common gate connection as
shown in Figure
Application of TRIAC:
1. Lamps control
2. Speed control of fans
3. Chopper
4. AC phase control
TRIAC
Because a TRIAC is a
bidirectional device, its
terminals cannot be designated
as anode and cathode. If
terminal MT2 is positive with
respect to terminal MT1, the
TRIAC can be turned on by
applying a positive gate signal
between gate G and terminal
MT1. If terminal MT2 is negative
with respect to terminal MT1, it
is turned on by applying a
negative gate signal between
gate G and terminal MT1.

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DIAC
A DIAC, or “diode for alternating
current,” is also a member of the
thyristor family. DIAC is used for
triggering TRIAC. It is just like a
TRIAC without a gate terminal.
The cross section of a DIAC is
shown in Fig. Its equivalent circuit
is a pair of inverted four-layer
diodes. MT2 and MT1 are the two
main terminals of the device.
There is no control terminal in this
device. A DIAC can be switched
from off-state to the on-state for
either polarity of the applied
voltage.
DIAC
Operation:
When the terminal MT2 is positive enough to break the junction N2-P2, the
current can flow from terminal MT2 to terminal MT1 through the path P1-N2-P2-
N3. If the polarity of terminal MT1 is positive enough to break the junction N2-P1,
the current flows through the path P2-N2-P1-N1. A DIAC may be considered as
two series-connected diodes in opposite direction.
Characteristics:
When applied voltage in either polarity is less than the avalanche breakover
voltage VBO, the DIAC is in the off-state (or nonconducting state) and a very
small amount of leakage current flows through the device. However, when the
magnitude of the applied voltage exceeds the avalanche breakover voltage VBO,
the breakdown takes place and the DIAC current rises sharply, as shown in
Figure. Once the current starts to flow, there is an on-state voltage drop V due to
the load current flow.

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DIAC
If a DIAC is connected to an ac
sinusoidal supply voltage, as
shown in Figure the load current
will flow only when the supply
voltage exceeds the breakover
voltage in either direction.
Unijunction Transistor (UJT)
The unijunction transistor
(UJT) is commonly used for
generating triggering
signals for SCRs. A basic
UJT-triggering circuit is
shown in Figure 9.47a. A
UJT has three terminals,
called the emitter E, base-
one B1, and base-two B2.
Between B1 and B2 the
unijunction has the
characteristics of an
ordinary resistance.

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Operation:
When the dc supply voltage VS is applied, the
capacitor C is charged through resistor R
because the emitter circuit of the UJT is in
the open state. The time constant of the
charging circuit is ζ1 = RC. When the emitter
voltage VE, which is the same as the capacitor
voltage VC, reaches the peak voltage VP, the
UJT turns on and capacitor C discharges
through RB1 at a rate determined by the time
constant ζ2 = RB1C. ζ2 is much smaller than
ζ1. When the emitter voltage VE decays to the
valley point VV, the emitter ceases to conduct,
the UJT turns off, and the charging cycle is
repeated. The waveforms of the emitter and
triggering voltages are shown in Figure
Characteristics:
The static emitter characteristic (a curve showing the relation between emitter
voltage VE and emitter current IE) of a UJT at a given inter base voltage VBB is
shown in figure. From figure it is noted that for emitter potentials to the left of
peak point, emitter current IE never exceeds IEo . The current IEo corresponds
very closely to the reverse leakage current ICo of the conventional BJT. This
region, as shown in the figure, is called the cut-off region. Once conduction is
established at VE = VP the emitter potential VE starts decreasing with the
increase in emitter current IE. This Corresponds exactly with the decrease in
resistance RB for increasing current IE. This device, therefore, has a negative
resistance region which is stable enough to be used with a great deal of
reliability in the areas of applications listed earlier. Eventually, the valley point
reaches, and any further increase in emitter current IE places the device in the
saturation region, as shown in the figure. Three other important parameters for
the UJT are IP, VV and IV and are defined below:

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Peak-Point Emitter Current. Ip : It is
the emitter current at the peak point. It
represents the minimum current that is
required to trigger the device (UJT). It
is inversely proportional to the
interbase voltage VBB.
Valley Point Voltage VV : The valley
point voltage is the emitter voltage at
the valley point. The valley voltage
increases with the increase in interbase
voltage VBB.
Valley Point Current IV The valley point
current is the emitter current at the
valley point. It increases with the
increase in inter-base voltage VBB.
Programmable Unijunction Transistor(PUT)
The programmable unijunction transistor
(PUT) is a small thyristor shown in
Figurg a. A PUT can be used as a
relaxation oscillator, as shown in Figure
b. The gate voltage VG is maintained from
the supply by the resistor dividers R1 and
R2, and determines the peak point voltage
VP. In the case of the UJT, VP is fixed for
a device by the dc supply voltage.
However, VP of a PUT can be varied by
varying the resistor dividers R1 and R2. If
the anode voltage VA is less than the gate
voltage VG, the device can remain in its
off-state. If VA exceeds the gate voltage by
one diode forward voltage VD, the peak
point is reached and the device turns on.

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Programmable Unijunction Transistor(PUT)
voltage VD, the peak point is reached and the device turns on. The peak current
IP and the valley point current IV both depend on the equivalent impedance on
the gate
! !
and the dc supply voltage Vs. In general, Rk is limited to a value below 100.
VP is given by
"
!
!
#
Home Work: Example 9.6
Diode Rectifiers
A rectifier is a circuit that converts an ac signal into a unidirectional signal. A rectifier
is a type of ac–dc converter.
Performance Parameters:
Harmonics:
In an electric Power System, a harmonic is a voltage or current at a multiple of the
fundamental frequency of the system, produced by the action of non-linear loads such
as rectifiers, discharge lighting, or saturated magnetic devices.
Ripple:
Ripple (specifically ripple voltage) in electronics is the residual periodic variation of the
DC voltage within a power supply which has been derived from an alternating current
(AC) source. This ripple is due to incomplete suppression of the alternating
waveform after rectification. Ripple voltage originates as the output of a rectifier or
from generation and commutation of DC power.
Ripple (specifically ripple current or surge current) may also refer to the pulsed current
consumption of non-linear devices like capacitor-input rectifiers.

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Diode Rectifiers
The performances of a rectifier are normally evaluated in terms of the following
parameters:
The average value of the output (load) voltage, Vdc
The average value of the output (load) current, Idc
The output dc power,
$%& %& %&
The root-mean-square (rms) value of the output voltage, Vrms
The rms value of the output current, Irms
The output ac power
$'& () ()
Diode Rectifiers
The efficiency (or rectification ratio) of a rectifier, which is a figure of merit and
permits us to compare the effectiveness, is defined as
*
$%&
$'&
It should be noted that j is not the power efficiency. It is the conversion efficiency
which is a measure of the quality of the output waveform. For a pure dc output,
the conversion efficiency would be unity.
The output voltage can be considered as composed of two components: (1) the dc
value and (2) the ac component or ripple. The effective (rms) value of the ac
component of output voltage is
'&
!
()
!
%&
+

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Diode Rectifiers
The form factor, which is a measure of the shape of output voltage, is
,,
()
%&
The ripple factor, which is a measure of the ripple content, is defined as
,
'&
%&
the ripple factor can be expressed as
, !
()
!
%&
+
, -./
01
!
1
+
= ,,! 1
+
Diode Rectifiers
The transformer utilization factor is defined as
23,
$%&
where Vs and Is are the rms voltage and rms current of the transformer secondary,
respectively. The input power can be determined approximately by equating input
power with the output ac power. That is, the power factor is related by
$,
$'&
Crest factor (CF), which is a measure of the peak input current Is(peak) as compared
with its rms value Is, is often of interest to specify the peak current ratings of devices
and components. CF of the input current is defined by
4,
"5'

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Diode Rectifiers
Diode Rectifiers
Full-wave Rectifier Circuit with a Center-tapped Transformer:
A full-wave rectifier circuit with a center-tapped transformer is shown in Figure
a. During the positive half-cycle of the input voltage, diode D1 conducts and
diode D2 is in a blocking condition. The input voltage appears across the load.
During the negative half-cycle of the input voltage, diode D2 conducts while
diode D1 is in a blocking condition. The negative portion of the input voltage
appears across the load as a positive voltage. The waveform of the output
voltage over a complete cycle is shown in Figure b. Because there is no dc
current flowing through the transformer, there is no dc saturation problem of
transformer core. The average output voltage is
%&
2
2
7 )89:;<=<
2 )
?
!
@
0.6366 )

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Diode Rectifiers
Diode Rectifiers
Performance Parameters:

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Diode Rectifiers
Diode Rectifiers

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Half Wave Rectifiers RL Load

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Output Voltage and Current:
Half Wave Rectifiers with RL Load and Battery

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Diode Rectifiers
Diode Rectifiers

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Diode Rectifiers

EEE 453( Semiconductor Switch and Triggering Device)

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