Unit i

Rama Kishore Bonthu
Associate Professor
E-mail: ramkishore.bonthu@gmail.com
POWER ELECTRONICS
Power Semi Conductor Devices :
Silicon controlled rectifier (SCR) was first introduced in 1957 as a power semi
conductor device. Since then, several other power semi conductor devices have been developed.
SCR --- Silicon Controlled Rectifier
LASCR --- Light Activated SCR
ASCR --- Asymmetrical SCR
RCT --- Reverse Conducting Thyristor
GTO --- Gate-Turn off Thyristor
SITH ---- Static Induction Thyristor
MCT ----- MOS controlled Thysristor
BJT ---- Bipolar Junction Thyristor
MOSFET ---- Metal-Oxide Semiconductor Field Effect Transistor
SIT --- Static Induction Transistor
IGBT --- Insulated gate bilpolar transistor

Rama Kishore Bonthu
Associate Professor
Based on (i)Turn-on and Turn-off characterstics (ii) gate signal requirements and (iii) degree of controllability, The
power semiconductor devices can be classified as under :
(a) Diodes : These are uncontrolled rectifieng devices. Their on and off states are controlled by power supply.
(b) Thyristors : These have controlled turned-on by a gate signal. After Thyristors are on, They remain in latched-in on
state due to internal regenerative action and gate loss control. These can be turned off by power circuit.
(c) Controllable Switches: These devices are turned-on and turned-off by the application of control signals. Ex: BJT,
MOSFET, GTO, SITH, IGBT, SIT and MCT.
Triac and RCT possess bi-directional current capability where as all other remaining devices (diode, SCR, GTO, BJT,
MOSFET, IGBT, SIT, SITH, and MCT) are unidirectional current devices.
TYPES OF POWER ELECTRONIC CONVERTERS:
A power Electronic Converter is made up of some power semiconductor devices controlled by integrated circuits. There
are six types of power electronic converters as under:

Rama Kishore Bonthu
Associate Professor
1. Diode Rectifiers: It coverts ac input voltage(1-φ or 3-φ) into a fixed dc voltage. We find diode rectifiers in electric
traction, battery charging, electro plating, electrochemical processing, power supplies, welding and uninterruptible power
supply (UPS) systems.
2. Ac to dc converters (Phase-controlled rectifiers): These convert constant ac voltage to variable dc output voltage.
These rectifiers use line voltage for their commutation, as such these are also called line-commutated or naturally
commutated ac to dc converters. These are used in drives, metallurgical and chemical industries, excitation systems for
synchronous machines etc.
3. DC to dc Converters (DC choppers) : These convert fixed dc input voltage to controllable dc output voltage. The
chopper circuits require forced or load commutation to turn-off the thyristors. Choppers find wide applications in dc
drives, subway cars, trolley trucks, battery driven vehicles etc.
4. DC to ac converters (Inverters): An inverter converts fixed dc voltage to a variable ac voltage. The output may be a
variable voltage and variable frequency. These converters use line, load or forced commutation for turning-off the
thyristors. Inverters find wide use in induction motor and synchronous-motor drives, induction heating, UPS, HVDC
transmission etc.
5. AC to ac converters: These convert fixed ac input voltage into variable ac output voltage. These are of two types :
(a) AC voltage controllers (AC voltage regulators): These convert fixed ac voltage directly to a variable ac voltage at the
same frequency. These are widely used for lighting control, speed control of fans, pumps etc.
(b) Cycloconverters: These convert input power at one frequency to output power at different frequency through one stage
conversion. These are used for low-speed large ac drives like rotary kiln etc.
6. Static Switches : The power semiconductor devices can operate as static switches or contactors. More beneficial than
circuit breakers.
POWER TRANSISTORS:
Power diodes are uncontrolled devices. Their turn-on and trun-off characteristics are not under control.
Power transistors, however, possess controlled characteristics. These are turned on when a current signal is given to base,
or control, terminal. When this control signal is removed, a power transistor is turned off.
Power Transistors are of four types as under:
(i) Bipolar Junction Transistors (BJTs)
(ii) Metal-Oxide-semiconductor field effect transistor (MOSFETs)
(iii) Insulated gate bipolar transistors (IGBTs)
(iv) Static Induction transistors (SITs)
POWER MOSFETS:
Power MOSFETs are of two types; n-channel enhancement MOSFET and p-channel enhancement
MOSFET. Out of these two types, n-channel enhancement MOSFET is more common in use because of higher mobility of
electrons. A power MOSFET has three terminals called drain (D), Source (S) and gate (G). The circuit symbol of n-
channel power MOSFET is as shown in below. Here arrow indicates the direction of electron flow. Power MOSFET, is a
voltage controlled device, is a unipolar device.

Rama Kishore Bonthu
Associate Professor
On p-substrate, two heavily doped n+
regions are
diffused. An insulating layer of silicon dioxide (SiO2)
is grown on the surface. Source and drain terminals are
embedded in the silicon layer, and in contact with n+
regions as shown above figure. A layer of metal is also
deposited on SiO2 layer so as to form the gate of
MOSFET in between source and drain terminals.
When gate circuit is open, junction between n+
region
below drain and p-substrate is reverse biased by input
voltage VDD . Therefore no current flows from drain to
source and load.
When gate is made positive with respect to source, an electric field is established as shown in above figure. Eventually,
induced negative charges in the p-substrate below SiO2 layer are formed thus causing the p layer below gate to become an
induced n layer. These negative charges, called electrons, form n-channel between two n+
regions and current can flow
from drain to source as shown by the arrow. If VGS is made more positive, induced n-channel becomes deeper and
therefore more current flows from D to S. This shows that drain current ID is enhanced by the gradual increase of gate
voltage, hence the name enhancement MOSFET.
In the above figure, the main disadvantage is that conducting n-channel in between drain and sources gives large on-state
resistance. This leads to high power dissipation in n-channel. So that above planar MOSFET construction is feasible only
for low-power MOSFETs.
The below diagram represents the construction of high power MOSFET. It is also known as planar diffused metal-oxide-
semiconductor FET(DMOSFET). On n+
substrate, high resistivity n-
layer is grown. The thickness of n-
layer determines
the voltage blocking capability of the device. On the other side of n+
substrate, a metal layer is deposited to form the drain
terminal. Now p-regions are diffused in the grown n-
layer. Further n+
regions are diffused in p-regions as shown. As
before, SiO2 layer is added, in that metallic source and drain terminals are embedded.
When gate circuit voltage is zero, and VDD is present, n-
- p junctions are reverse biased and no current flows from drain to
source. When gate terminal is positive with respect to source, an electric field is established and electrons from n-channel
in the p-
regions as shown. With gate voltage is increased, current ID also increases. Length of n-channel can be controlled
and therefore on-resistance can be made low if short length is used for the channel.
In the above figure source is negative and drain is positive. Therefore, electrons flow from source to n+
layer, then through
the n-channel of p-layer and further through n-
and n+
layers to drain. The current must flow opposite to the flow of
electrons as indicated in above figure.

Rama Kishore Bonthu
Associate Professor
PMOSFET Characteristics: The basic circuit diagram for obtaining static characteristics of n-channel PMOSFET, is
shown below. In that the source terminal is taken as common terminal, between in the input and output of a MOSFET.
Transfer Characteristics
Output Characteristics
a) Transfer Characteristics: This characteristic shows the variation of the drain current ID as a function of the gate-
source voltage VGS. VGST is the minimum positive voltage between gate and source to induce n-channel. Thus, for
threshold voltage below VGST , device is in the off state. VGST value is order of 2 to 3 volts.
b) Output Characteristics: It indicates the variation of drain current ID as a function of drain source voltage VDS, with
gate-source voltage VGS as a parameter. For low values of VDS, the graph between ID – VDS is almost linear. This indicates
that constant value of on-resistance RDS = VDS/ID . For given VGS, if VDS is increased, output characteristic is relatively flat.
A load line intersects the output characteristics at ‘A’ and ‘B’. Here ‘A’ indicates fully on condition and ‘B’ fully off-state.
PMOSFET operates as a switch either at ‘A’ or at ‘B’.
When PMOSFET is driven with large gate-source voltage VGS, PMOSFET is turned on, VDS.ON is small. Here PMOSFET
is acted as closed switch(turn-on), driven from cut-off, to active region and then to ohmic region. When PMOSFET is
turned-off, it takes backward journey from ohmic region to cut-off state.
Switching Characteristics:
The switching characteristics of a power MOSFET are
influenced to a large extent by the internal capacitance
of the device and the internal impedance of the gate drive
circuit. At turn-on, there is an initial delay tdr, during which
input capacitance charges to gate threshold voltage VGST.
Here tdn. is called turn-on delay time.
There is further delay tr, called rise time, during which gate
voltage rises to VGSP, a voltage sufficient to drive the
MOSFET into on state. During tr., drain current rises from
zero to full on current ID. Thus, the total turn-on time is ton
= tdn+ tr. The turn-on time can be reduced by using low-
impedance gate drive source.

Rama Kishore Bonthu
Associate Professor
As MOSFET is a majority carrier device, turn-off process is initiated soon after removal of gate voltage at
time t1. The turn-off delay time, td f, is the time during which input capacitance dishrages from overdrive
gate voltage V1 to VGSP. The fall time, tf is the time during which input capacitance discharges from VGSP to
threshold voltage. During tf, drain current falls from ID to zero. So when VGS≤VGST, MOSFET turn-off is
complete
Power MOSFETs are very popular in switched mode power supplies and inverters. They are, at present, available
with 500 V, 140 A rating's..
INSULATED GATE BIPOLAR TRANSISTOR (IGBT):
IGBT has been developed by combining into it the best qualities of both BJT and PMOSFET. Thus an IGBT possesses
high input impedance like a PMOSFET and has low on-state power loss as in a BJT. Further, IGBT is free from second
breakdown problem present in BJT. All these merits have made IGBT very popular amongst power-electronics engineers.
Basic structure:
Below figure illustrates the basic structure of IGBT. It is
constructed virtually in the same manner as a power
MOSFET. There is, however a major difference in the
substrate.
The n+
layer substrate at the drain in a PMOSFET is not
substituted in the IGBT by a p+
layer substrate called
collector C. Like a power MOSFET, an IGBT has also
thousands of basic structure cells connected
appropriately on a single chip of silicon. In IGBT, p+
substrate is called injection layer because it injects holes
into n-
layer. The n-
layer is called drift region. As in
other semiconductor devices, thickness of n-
layer
determines the voltage blocking capability of IGBT. The
p layer is called body of IGBT. The n-
layer in between
p+
and p regions serves to accommodate the depletion
layer of pn-
junction, i.e. junction J2.
Working:
When collector is made positive with respect to emitter, IGBT gets forward biased. With
no voltage between gate and emitter, two junctions between n-
region and p region (i.e. junction J2) are reverse
biased; so no current flows from collector to emitter.
When gate is made positive with respect to emitter by voltage VG, with gate-emitter voltage more than the
threshold voltage VGET of IGBT, an n-channel or inversion layer, is formed in the upper part of p region just
beneath the gate. This n-channel short-circuits the n region with n+
emitter regions.Electrons from the n+
emitter
begin to flow to n-
drift region through n-channel. As IGBT is forward biased with collector positive and
emitter negative, p+
collector region injects holes into n-
drift region. In short, n-
drift region is flooded with
electrons from p-body region and holes from p+
collector region. With this, the injection carrier density in n-
drift
region increases considerably and as a result, conductivity
of n-
region enhances significantly. Therefore, IGBT gets turned on and begins to conduct forward current IC.
Current IC, or IE, consists of two current components : (i) hole current Ih due to injected holes flowing from
collector, p+
n-
p transistor Q1, p-body region resistance Rby and emitter and (ii) electronic current Ie due to injected
electrons flowing from collector, injection layer p+, drift region n, n-channel resistance Rth, n+
and emitter.

Rama Kishore Bonthu
Associate Professor
This means that collector, or load, current IC = emitter current IE = Ih + Ie
Major component of collector current is electronic current Ie, i.e. main current path for collector, or load,
current is through p+
, n, drift resistance Rd and n-channel resistance Rch. Therefore, the voltage drop in IGBT in
its on-state is
VCE.on = IC.Rch + IC.Rd + Vj1
VCE.on = Voltage drop[in n-channel+across drift in n-
region + across forward biased p+
n-
junction J1]
IGBT Characteristics:
The above circuit shows the various parameters pertaining to IGBT characteristics. Static I-V or output
characteristic shows the variation of collector current IC as a function of collector-emitter voltage VCE, with gate-
emitter voltage VGE as parameter. When the device is off, Junction J2 blocks forward voltage, and in case reverse
voltage appears across collector and emitter, junction J1 blocks it. VRM is the maximum reverse breakdown
voltage.
The transfer characteristic of an IGBT is a plot of collector current IC versus gate-emitter voltage VGE. When VGE is
less than the threshold voltage VGET , IGBT is in the off-state.
When the device is off, junction J2 blocks forward voltage and in case reverse voltage appears across collector
and emitter, junction J1 blocks it.
Switching Characteristics:
Switching characteristics of an IGBT during
turn-on and turn-off are sketched in below.
The turn-on time is defined as the time
between the instants of forward blocking to
forward on-state. Turn-on time is
composed of delay time td, and rise time
tr, i.e. ton= tdn+ tr. The delay time is defined as
the time for the collector-emitter voltage to fall
from VCE to 0.9 V. Here VCE is the initial
collector-emitter voltage. Time tdn may also
be defined as the time for the collector
current to rise from its initial leakage current
ICE to 0.1IC. Here IC is the final value of
collector current.

Rama Kishore Bonthu
Associate Professor
The rise time tr is the time during which collector-emitter voltage falls from 0.9 VCE to 0.1 VCE It is also
defined as the time for the collector current to rise from 0.1 Ic to its final value I. After time ton, the collector
current is Ic and the collector-emitter voltage falls to small value called conduction drop = VCES, where
subscript S denotes saturated value.
The turn-off time is somewhat complex. It consists of three intervals : (i) delay time, tdf (ii) initial fall time, tf1
and (iii) final fall time, tf2 ; i.e. toff = tdf + tf1.+ tf2.The delay time is the time during which gate voltage falls from
VGE to threshold voltage VGET. As VGE falls to VGET during tdf, the collector current falls from Ic to 0.9 IC. At the
end of tdf, collector-emitter voltage begins to rise. The first fall time tfi is defined as the time during which
collector current falls from 90 to 20% of its initial value ic, or the time during which collector-emitter voltage rises
from VCES to 0.1 VCE.
The final fall time tt2 is the time during which collector current falls from 20 to 10% of or the time during
which collector-emitter voltage rises from 0.1 VCE to final value VCE.
Applications of IGBT
IGBTs are widely used in medium power applications such as dc and ac motor drives, UPS systems, power
supplies and drives for solenoids, relays and contactors. Though. IGBTs are somewhat more expensive than
BJTs, yet they are becoming popular because of lower gate-drive requirements, lower switching losses and
smaller snubber circuit requirements. IGBT converters are more efficient with less size as well as cost, as
compared to converters based on BJTs. Recently, IGBT inverter induction-motor drives using 15-20 kHz
switching frequency are finding favour where audio-noise is objectionable. In most applications, IGBTs will
eventually push out BJTs. At present, the state of the art IGBTs are available upto 1200 V, 500 A.
Comparison of IGBT with MOSFET:
The relative merits and demerits of IGBT over PMOSFET are enumerated below :
(i) In PMOSFET, the three terminals are called gate, source, drain whereas the corresponding terminals for
IGBT are gate, emitter and collector.
ii) Both IGBT and PMOSFET possess high input impedance.
iii) Both are voltage-controlled devices.
iv) With rise in temperature, the increase in on-state resistance in PMOSFET is much pronounced than it is in IGBT. So,
on-state voltage drop and losses rise rapidly in PMOSFET than in IGBT, with rise in temperature.
v) With rise in voltage rating, the increment in on-state voltage drop is more dominant in PMOSFET than it is in IGBT.
This means IGBTs can be designed for higher-voltage ratings than PMOSFETs.
In view of the above comparison, (a) PMOSFETs are available upto about 500 V, 140 A ratings whereas state of the art
IGBTs have 1200 V, 500 A ratings and (b) operating frequency in PMOSFETs is upto about 1 MHz whereas its value is
upto about 50 kHz in IGBTs.
THYRISTORS
As stated before, Bell Laboratories were the first to fabricate a silicon-based semiconductor device called thyristor. whole
family of semiconductor devices is given the name thyristor. Thus the term thyristor denotes a family of semiconductor
devices used for power control in dc and ac systems. One oldest member of this thyristor family, called silicon-controlled
rectifier (SCR), is the most widely used device. At present, the use of SCR is so vast that over the years, the word thyristor has
become synonymous with SCR.

Rama Kishore Bonthu
Associate Professor
A thyristor has characteristics similar to a thyratron tube. But from the construction view point, a thyristor (a pnpn device)
belongs to transistor (pnp or npn device) family. The name `thyristor', is derived by a combination of the capital letters from
THYRatron and transISTOR.
TERMINAL CHARACTERISTICS OF THYRISTORS
Thyristor is a four layer, three-junction, p-n-p-n semiconductor switching device. It has three terminals ; anode, cathode and
gate. Basically, a thyristor consists of four layers of alternate p-type and n-type silicon semiconductors forming three junctions
J1, J2 and J3 as shown in fig.(a). The threaded portion is for the purpose of tightening the thyristor to the frame or heat sink
with the help of a nut. For large current applications, thyristors need better cooling ; this is achieved to a great extent by
mounting them onto heat sinks. Gate terminal is usually kept near the cathode terminal. Schematic diagram and circuit
symbol for a thyristor are shown respectively in Figs. (b) and (c). The terminal connected to outer p region is called anode
(A), the terminal connected to outer n region is called cathode(K) and that connected to inner p region is called the gate (G).
An SCR is so called because silicon is used for its construction and its operation as a rectifier (very low resistance in
the forward conduction and very high resistance in the reverse direction) can be controlled. Like the diode, an SCR is
an unidirectional device that blocks the current flow from cathode to anode. Unlike the diode, a thyristor also
blocks the current flow from anode to cathode until it is triggered into conduction by a proper gate signal between gate
and cathode terminals.
SCRs of voltage rating 10 kV and an RMS current rating of 3000 A with corresponding power-handling capacity
of 30 MW are available.
Static I-V Characteristics of a Thyristor:
The circuit diagram for obtaining static I-V characteristics of thyristor as shown above.The anode and cathode are
connected to main source through the load. The gate and cathode are fed from a source E., which provides positive
gate current from gate to cathode.
Here Va is the anode voltage across thyristor terminals A, K and Ia is the anode current. According to SCR I-V
characteristics, a thyristor has three basic modes of operation ; namely, reverse blocking mode, forward blocking
(off-state) mode and forward conduction (on-state) mode.

Rama Kishore Bonthu
Associate Professor
Reverse Blocking Mode:
When cathode is made positive with respect to anode with switch S open, Fig. 4.2 (a),
thyristor is reverse biased. Junctions J1, J3 are seen to be reverse biased whereas junction J2
is forward biased. A small leakage current of the order of a few milliamperes flows. This
is reverse blocking mode, called the off-state, of the thyristor. If the reverse voltage
is increased, then at a critical breakdown level, called reverse breakdown voltage VB R ,
an avalanche occurs at J1 and J3 and the reverse current increases rapidly. A large
current associated with VBR
gives rise to more losses in the SCR. This may lead to thyristor damage as temperature may exceed its
permissible temperature rise. It should, therefore, be ensured that maximum working reverse voltage
across a thyristor does not exceed VBR.. Reverse avalanche region is shown by PQ.
Forward Blocking Mode:
When anode is positive with respect to the cathode, with gate circuit open, thyristor
is said to be forward biased and junctions J1, J3 are forward biased but junction J2 is
reverse biased. In this mode, a small current, called forward leakage current, flows. In
case the forward voltage is increased, then the reverse biased junction J2 will have an
avalanche breakdown at a voltage called forward breakover voltage VBO. When forward
voltage is less than VBO, SCR offers a high impedance. Therefore, a thyristor can be treated as
an open switch even in the forward blocking mode.
Forward Conduction Mode :
In this mode, thyristor conducts currents from anode to cathode with a very small voltage drop across it. A thyristor is
brought from forward blocking mode to forward conduction mode-
by turning it on by exceeding the forward breakover
voltage or by applying a gate pulse between gate and cathode. In this mode, thyristor is in on-state and behaves
like a closed switch. Voltage drop across thyristor in the on state is of the order of 1 to 2 V depending on the rating
of SCR. This voltage drop increases slightly with an increase in anode current. In conduction mode, anode current
is limited by load impedance alone as voltage drop across SCR is quite small. This small voltage drop VT across
the device is due to ohmic drop in the four layers.
THYRISTOR TURN-ON METHODS
With anode positive with respect to cathode, a thyristor can be turned on by any one of the following techniques

Rama Kishore Bonthu
Associate Professor
(a) Forward voltage triggering
(b) gate triggering
(c) dv/ dt triggering
(d) Temperature triggering
(e) Light triggering.
(a) Forward Voltage Triggering : When anode to cathode forward voltage is increased with gate circuit open,
the reverse biased junction J2 will break. This is known as avalanche breakdown and the voltage at which
avalanche occurs is called forward breakover voltage VBO. At this voltage, thyristor changes from off-state to
on-state. As other junctions J1, J3 are already forward biased, breakdown of junction J2 allows free movement of
carriers across three junctions and as a result, large forward anode-current flows. As stated before, this forward
current is limited by the load impedance. In practice, the transition from off-state to on-state obtained by exceeding
VB0 is never employed as it may destroy the device.
(b) Gate Triggering : Turning on of thyristors by gate triggering is simple, reliable and efficient. when turn-
on of a thyristor is required, a positive gate voltage between gate and cathode is applied. With gate current thus
established, charges are injected into the inner p layer and voltage at which forward breakover occurs is
reduced. The forward voltage at which the device switches to on-state depends upon the magnitude of gate
current. Higher the gate current, lower is the forward breakover voltage.
When positive gate current is applied, gate P layer is flooded with electrons from the cathode. This is because
cathode N layer is heavily doped as compared to gate P layer. As the thyristor is forward biased, some of these
electrons reach junction J2. As a result, width of depletion layer around junction J2 is reduced. This
causes the junction J2 to breakdown at an applied voltage lower than forward breakover voltage VBO. If
magnitude of gate current is increased, more electrons will reach junction J2, as a consequence thyristor
will get turned on at a much lower forward applied voltage.
Fig. (a) shows that for gate current Ig=0,
forward breakover voltage is VBO. For
Ig1, forward breakover voltage, or turn-on
voltage is less than VB0. For Ig2 > Ig1,
forward breakover voltage is still
further reduced. The effect of gate
current on the forward breakover
voltage of a thyristor can also be
illustrated by means of a curve as
shown in Fig.(b).
Typical gate current magnitudes are of the order of 20 to 200 mA.
Once the SCR is conducting a forward current, reverse biased junction J2 no longer exists. As such, no gate
current is required for the device to remain in on-state.
latching current may be defined as the minimum value of anode current which it must attain during turn-on
process to maintain conduction when gate signal is removed.
Once the thyristor is conducting, gate loses control. The thyristor can be turned -off only if the forward
current falls below a low-level current called the holding current, Thus holding current may be defined as the
minimum value of anode current below which it must fall for turning-off the thyristor.

Rama Kishore Bonthu
Associate Professor
(c)dv/dt Triggering : With forward voltage across the anode and cathode of a thyristor, the two outer junction of J1, J3
are forward biased, but inner junction J2 is reverse biased. This junction J2 has the characteristics of a capacitor due to
charges existing across the junction. If forward voltage Va is suddenly applied, a charging current ic through junction
capacitance Cj may turn on the SCR. Then charging current is
If the rate of rise of forward voltage dVa/dt is high, the charging current would be more. This charging current plays the
role of gate current and turns on the SCR even though gate signal is zero.
(d) Temperature Triggering : During forward blocking, most of the applied voltage appears across
reverse biased junction J2. This voltage across junction J2 associated with leakage current may raise the
temperature of this junction. With increase in temperature, leakage current through junction J2 further increases.
This cumulative process may turn on the SCR at some high temperature.
(e) Light Triggering:
For light-triggered SCRs, a recess (or niche) is made in the inner p-layer. When this
recess is irradiated, free charge carriers (holes and electrons) are generated just like
when gate signal is applied between gate and cathode. If the intensity of this light
thrown on the recess exceeds a certain value, forward-biased SCR is turned on. Such a
thyristor is known as light-activated SCR (LASCR). Light-triggered thyristors have
now been used in high-voltage direct current (HVDC) transmission systems.
In these several SCRs are connected in series-parallel combination and their light-
triggering has the advantage of electrical isolation between power and control circuits.
SWITCHING CHARACTERISTICS OF THYRISTORS:
During turn-on and turn-off processes, a thyristor is subjected to different voltages across it and different currents
through it. Here, first switching characteristics during turn-on are described and then the switching
characteristics during turn-off.
Switching Characteristics during Turn-on:
A forward-biased thyristor is usually turned on by applying a positive gate voltage between gate and cathode.
There is, however, a transition time from forward off-state to forward on state. This transition time called
thyristor turn-on time, is defined as the time during which it changes from forward blocking state to final on-state.
Total turn-on time can be divided into three intervals ; (i) delay time td, (ii) rise time tr and (iii) spread time tp,
(i) Delay time td : The delay time td is measured from the instant at which gate current reaches 0.9 I to the
instant at which anode current reaches 0.14. Here Ig and Ia are respectively the final values of gate and
anode currents. The delay time may also be defined as the time during which anode voltage falls from Va to 0.9Va where
Va = initial value of anode voltage. Another way of defining delay time is the time during which anode current rises from
forward leakage current to 0.1 Ia whereIa = final value of anode current. With the thyristor initially in the forward
blocking state, the anode voltage is OA and anode current is small leakage current as shown in below.

Rama Kishore Bonthu
Associate Professor
As gate current begins to flow from gate to cathode with the application of gate
signal. during delay time td, anode current flows in a narrow region near the gate
where gate current density is the highest.The delay time can be decreased by
applying high gate current and more forward voltage between anode and cathode.
The delay time is fraction of a microsecond. The Above figure represents
Distribution of gate and anode currents during delay time. In that circles
represents conducting area of cathode (i) during td (ii) after tr. (iii) after tp.
(ii) Rise time tr: The rise time tr. is the time taken by the anode current to rise from 0.1 Ia to 0.9Ia. The rise time is also
defined as the time required for the forward blocking off-state voltage to fall from 0.9 to 0.1 of its initial value OA.
However, the main factor determining t,. is the nature of anode circuit. For example, for series RL circuit, the rate of rise
of anode current is slow, therefore, tr is more. For RC series circuit, di/dt is high, tr is therefore, less.
(iii) Spread time tp : The spread time is the time taken by the anode current to rise from 0.9 Ia to Ia. It is also
defined as the time for the forward blocking voltage to fall from 0.1 of its value to the on-state voltage drop (1 to
1.5 V). During this time, conduction spreads over the entire cross-section of the cathode of SCR. After the
spread time, anode current attains steady state value and the voltage drop across SCR is equal to the on-state
voltage drop of the order of 1 to 1.5 V, shown above. Total turn-on time of an SCR is equal to the sum of delay time,
rise time and spread time.
Switching Characteristics during Turn-off:

Rama Kishore Bonthu
Associate Professor
This dynamic process of the SCR from conduction state to forward blocking state is called commutation process
or turn-off process.
Once the thyristor is on, gate loses control. The SCR can be turned off by reducing the anode current below
holding current. If forward voltage is applied to the SCR at the moment its anode current falls to zero, the device
will not be able to block this forward voltage as the carriers (holes and electrons) in the four layers are still
favourable for conduction. The device will therefore go into conduction immediately even though gate signal is not
applied. In order to obviate such an occurrence, it is essential that the thyristor is reverse biased for a finite
period after the anode current has reached zero.
The turn-off time tq of a thyristor is defined as the time between the instant anode current becomes zero and the instant
SCR regains forward blocking capability. The turn-off time is divided into two intervals ; reverse recovery time t„
and the gate recovery time tgr. i.e, tq = trr + tgr.
At instant t1, anode current becomes zero. After t1, anode current builds up in the reverse direction with the same di/dt
slope as before t1. The reason for the reversal of anode current after t1 is due to the presence of carriers stored in the
four layers. The reverse recovery current removes excess carriers from the end junctions J1 and J3 between the
instants t1 and t3.
At instant t2, when about 60% of the stored charges are removed from the outer two layers, carrier density
across J1 and J3 begins to decrease and with this reverse recovery current also starts decaying. At instant t3,
when reverse recovery current has fallen to nearly zero value, end junctions J1 and J3 recover and SCR is able to
block the reverse voltage.
At the end of reverse recovery period (t3- t1), the middle junction J2 still has trapped charges, therefore,
the thyristor is not able to block the forward voltage at t3. these trapped charges must decay only by
recombination. This recombination is possible if a reverse voltage is maintained across SCR. The time for the
recombination of charges between t3 and t4 is called gate recovery time tgr. At instant t4, junction J2 recovers and the
forward voltage can be reapplied between anode and cathode.
The thyristor turn-off time tq is applicable to an individual SCR. In actual practice, thyristor (or thyristors)
form a part of the power circuit, The turn-off time provided to the thyristor by the practical circuit is called
circuit turn-off time tc. It is defined as the time between the instant anode current becomes zero and the
instant reverse voltage due to practical circuit reaches zero.
THYRISTOR PROTECTION:
A thyristor must be protected against all abnormal conditions. SCRs are very delicate devices, their protection against
abnormal operating conditions is, therefore, essential.
(i) di/dt protection: When a thyristor is forward biased and is turned on by a gate pulse, conduction of
anode current begins in the immediate neighbourhood of the gate-cathode junction. Thereafter, the current
spreads across the whole area of junction. However, if the rate of rise of anode current, i.e. di/dt, is large as
compared to the spread velocity of carriers, local hot spots will be formed near the gate connection on account
of high current density. This localised heating may destroy the thyristor. Therefore, the rate of rise of anode
current at the time of turn-on must be kept below the specified limiting value. The value of di/dt can be
maintained below acceptable limit by using a small inductor, called di/dt inductor, in series with the anode
circuit.
(ii)dv/dt protection: With forward voltage across the anode and cathode of a thyristor, the two outer
junctions are forward biased but the inner junction is reverse biased. This reverse biased junction J2, has the
characteristics of a capacitor due to charges existing across the junction. In other words, space-charges exist in the
depletion region around junction J2 and therefore junction J2 behaves like a capacitance.

Rama Kishore Bonthu
Associate Professor
If the rate of rise of forward voltage dVa/dt is high, the charging current i will be more. This charging current
plays the role of gate current and turns on the SCR even when gate signal is zero. Such phenomena of turning-
on a thyristor, called dv/dt turn-on must be avoided as it leads to false operation of the thyristor circuit.
For controllable operation of the thyristor, the rate of rise of forward anode to cathode voltage dVa/dt
must be kept below the specified rated limit. Typical values of dv/dt are 20 — 500 V/µsec.
Design of Snubber Circuits:
A snubber circuit consists of a series combination of resistance Rs and capacitance
in parallel with the thyristor as shown. W h e n switch S is closed, a sudden
voltage appears across the circuit. Capacitor Cs behaves like a short
circuit, therefore voltage across SCR is zero. With the passage of time, voltage
across Cs builds up at a slow rate such that dv/dt across Cs and therefore across
SCR is less than the specified maximum dv/dt rating of the device.Before SCR is
fired by gate pulse, Cs charges to full voltage Vs. When the SCR is turned o n ,
c a p a c i t o r d i s c h a r g e s t h r o u g h t h e S C R a n d s e n d s a c u r r e n t
e q u a l t o Vs/ (resistance of local path formed by C3 and SCR). As this resistance is quite low, the turn-on di/dt
will tend to be excessive and as a result, SCR may be destroyed. In order to limit the magnitude of discharge
current, a resistance Rs is inserted in series with Cs as shown inabove Figure.
Thyristor Turn-off Methods:
The turn -off of a thyristor means bringing the device from forward-conduction state to forward-
blocking state. The thyristor turn-off requires that (1) its anode current falls below the holding current and (ii) a
reverse voltage is applied to thyristor for a sufficient time to enable it to recover to blocking state,
Commutation is defined as the process of turning-off a thyristor.
Once thyristor starts conducting, gate loses control over the device, therefore, external means may have to be adopted to
commutate the thyristor.
CLASS A COMMUTATION : LOAD COMMUTATION
For achieving load commutation of a thyristor, the commutating components L and C are connected as shown below.
Here R is the load resistance. For low value of R, L and C are connected in series with R. For high value of R, load R
is connected across C. The essential requirement for both the circuits is that the overall circuit must be
underdamped.
When these circuits are energized from dc, current waveforms as shown are obtained, It is seen that current i first
rise to maximum value and then begins to fall. When current decays to zero and tends to reverse, thyristor T is turned-off on
its own at instant A.
Load commutation is possible in dc circuits and not in ac circuits. Class A, or load, commutation is also called resonant
commutation or self-commutation.
CLASS B COMMUTATION : RESONANT-PULSE COMMUTATION

Rama Kishore Bonthu
Associate Professor
In class-B, or resonant-pulse, commutation, source voltage Vs charges
capacitor C to voltage Vs. Main thyristor T1 as well as auxiliary
thyristor TA are off. Positive direction of capacitor
voltage vc and capacitor current is are marked. When T1 is turned on
at t = 0, a constant current I0 is established in the load circuit. Here,
for simplicity, load current is assumed constant. Uptill time t1, vc=
Vs, ic= 0, i0 =I0 and iT1=I0. For initiating the commutation of
main thyristor T1, auxiliary thyristor TA is gated at t = t1. With TA on, a
resonant current i0 begins to flow from C through TA, L and back to C.
This resonant current, with time measured from instant t1, is given by
Minus sign before Ipsin w0 t is due to the fact that this current flows
opposite to the reference positive direction chosen in circuit.
After half a cycle of ic from instant t1, ic = 0, vc= –Vs, and iT1=I0.
After π radians from instant t1, i.e. just after instant t2, as ic tends
to reverse, TA is turned off at t2. Resonant current ic now builds up
through C, L, D and T1. As this current ic grows opposite to forward
thyristor current of T1, net forward current iT1 = I0 – ic begins to
decrease. Finally, when ic in the reversed direction attains the value I0,
forward current in T1 (iT1 = I0 –I0 = 0) is reduced to zero and the device
T1 is turned off at t3. As thyristor is commutated by the gradual build up
of resonant current in the reversed direction, this method of commutation
is called current commutation, class-B commutation or resonant-pulse
commutation.
After T1 is turned off at t3 , constant current I0 flows from Vs to load throughC, L and D. Capacitor
begins charging linearly from - Vab to zero at t4 and then to Vs at t5. As a result, at instant t5, when vc= Vs, load
current io = ic = I0 reduces to zero as shown.
It is seen from the waveform of ic that main thyristor T1 is turned off when
Main thyristor T1 is commutated at t3. As constant load current Io charges C linearly from - Vab, at t3 to zero at t4,
SCR T1 is reverse biased by voltage vc for a period (t4 - t3) = tc.
Circuit turn-off time for main thyristor,
the magnitude of reverse voltage Vab across main thyristor T1, when it gets commutated, is given by
Vab = Vs cos ωo(t3-t2)
CLASS C COMMUTATION COMPLEMENTARY COMMUTATION: In this type of commutation, a thyristor
carrying load current is commutated by transferring its load current to another incoming thyristor.Below Figure
illustrates an arrangement employing complementary commutation. In this figure, firing of SCR T1 commutates T2

Rama Kishore Bonthu
Associate Professor
and subsequently, firing of SCR T2 would turn off T1.
When T1 is turned on at t=0, current through R1 is i1=Vs/R1 and through R2 is ic=Vs/R2, so that thyristor T1 current iT1 =
i1+ic = Vs(1/R1 + 1/R2) begins to flow. Capacitor C begins charging through R2 from vc=0.
The charging current through the circuit Vs, C and R2 is given by

Rama Kishore Bonthu
Associate Professor

Rama Kishore Bonthu
Associate Professor
CLASS D COMMUTATION : IMPULSE COMMUTATION:
In this class D, or impulse commutation, T1 and TA are called main and auxiliary thyristors respectively.
Initially, main thyristor T1 and auxiliary thyristor TA are
off and capacitor is assumed charged to voltage Vs with
upper plate positive. When T1 is turned on at t = 0,
source voltage Vs is applied across load and load
current I0 begins to flow which is assumed to
remain constant. With T1 on at t = 0, another
oscillatory circuit consisting of C, T1., L and D is
formed where the capacitor current is given by
When ωo t = π, ic = 0. Between 0 < t < (π/ωo), iT1 = I0 +Ip sin ωo t. Capacitor voltage changes from + Vs to - Vs co-
sinusoidally and the lower plate becomes positive. At ω0t=π, ic = 0, iT1 = I0 and vc= -Vs.
At t1, auxiliary thyristor TA is turned on. Immediately after TA is on, capacitor voltage Vs applies a reverse voltage across
main thyristor T1 so that vT1 = -Vs at t1 and SCR T1 is turned off and iT1= 0. The load current is now carried by C and TA.
Capacitor gets charged from - Vs to Vs with constant load current I0. When vc = Vs, ic = 0 at t2, thyristor TA is turned off.
During the time TA is on from t1 to t2, vc = vT1, ic = -I0 and i0 = I0 . For main thyristor T1, circuit turn-off time is tc, as shown
in Fig.

Rama Kishore Bonthu
Associate Professor
With the firing of thyristor TA, a reverse voltage V is suddenly applied across T1 ; this method of commutation is
therefore, also called voltage commutation. With sudden appearance of reverse voltage across T1, its current is
quenched. As an auxiliary thyristor TA is used for turning-off the main thyristor T1, this type of commutation is also
known as auxiliary commutation.
When thyristor TA is turned on, capacitor gets connected across T1 to turn it off, this type of commutation is therefore,
also called parallel-capacitor commutation.
CLASS E COMMUTATION : EXTERNAL PULSE COMMUTATION
In this type of commutation, a pulse of current is obtained from a separate
voltage source to turn off the conducting SCR. The peak value of this current
pulse must be more than the load current. In the circuit, Vs is the voltage of the
main source and VI is the voltage of the auxiliary supply. When thyristor T1 is
conducting and load is connected to source Vs. When thyristor T3 is turned on
at t = 0; V1, T3, L and C form an
oscillatory circuit. Therefore, C is charged to a voltage + 2V1 with upper plate positive at 𝑡 = √𝐿𝐶 and as
oscillatory current falls to zero. Thyristor T3 gets commutated. For turning off the main thyristor T1, thyristor T2 is
turned on. With T2 on, T1 is subjected to a reverse voltage equal to Vs-2V1 and T1 is turned off. After T1 is off,
capacitor discharges through the load
CLASS F COMMUTATION : LINE COMMUTATION
This type of commutation is also known as natural commutation. Here, the thyristor carrying the load current is
reverse biased by the ac source voltage and the device is turned-off when anode current falls below the holding current
(assumed nearly zero).
In this figure, thyristor T is fired at firing angle equal
to zero, i.e. when ωt = 0, vs = 0. During the positive
half-cycle, v0 = vs and waveshape of load current i0 is
identical with the waveshape of vo for a resistive load.
At ωt = π, vs = 0, v0 = 0 and i0 = 0; therefore T gets
turned off at this instant. From ωt = π to ωt = 2π,
T is reverse biased for a period tc = π/ω sec, longer
than the thyristor turn-off time tq. Here tc is called the
circuit turn-off time.
Another method of classification of thyristor commutation technique is as tinder
(1) Line commutation class F
(2) Load commutation : class A
(3) Forced commutation class B, C and D
(4) External-pulse commutation : class E.
In line, or natural, commutation, natural reversal of ac supply voltage commutates the conducting thyristor. As stated
before, line commutation is widely used in ac voltage controllers, phase-controlled rectifiers and step-down
cycloconverters.
In load commutation, L and C are connected in series with the load or C in parallel with the load such that overall load
circuit is under damped. Load commutation is commonly employed in series inverters.
In forced commutation, the commutating components L and C do not carry load current continuously. So class B, C and D
commutation constitute forced commutation techniques. As stated before, in forced commutation, forward current of the
thyristor is forced to zero by external circuitry called commutation circuit. Forced commutation is usually employed in dc
choppers and inverters.

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