ch-1Introduction to power electronics.pdf

4EL01 : Power Electronics
BY : PROF. A.A.DAIYA
1
BY: PROF A.A.DAIYA 7/19/2025

2

Ø Introduction to thyristor family devices
Ø SCR Characteristics
Ø Two Transistor model of SCR
Ø Ratings and data sheet
Ø Turn ON and Turn OFF methods:
Ø Gate circuit requirements, Isolation of gate with
pulse transformer and Opto-couplers
Ø Firing circuits: R Trigger, R-C Trigger and UJT
based triggering circuits
Ø Protections circuits for SCRs. Snubber Designs.
3

WHAT IS POWER ELECTRONICS ?
4

Electronics:
“deals with the study of semiconductor devices and circuits ”
Power:
“ deals with the generation, transmission and conversion of
electrical power”
Power Electronics
“deals with the use of electronics for the control and
conversion of electrical power”
5

History of power electronics
6

Ø Until 1956, the application of electronics
was confined to low power circuits
ØIn 1957 three terminal PNPN semiconductor
device called SCR was developed.
ØLater on many other power semiconductor
devices similar to SCR were developed.
ØProgress in power electronics today has been
possible primarily due to advances in power
semiconductor devices.
ØGradually various types of power
semiconductor devices were developed and
became commercial available in 1970.
7

Power
semiconductor
devices
Uncontrolled
turn-on & turn –off
e.g Power Diode
Controlled turn on
&
Uncontrolled turn-off
e.g. SCR, TRIAC
Controlled turn on
&
controlled turn-off
e.g.
GTO,MOSFET.BJT,MCS
Based on
controllability
8

Power semiconductor
devices
Uni directional
C o n d u c t s c u r r e n t i n o n e
direction only
e.g. SCR, GTO, MCT,BJT, IGBT
Bi-directional
C o n d u c t s c u r r e n t i n
bidrection
e.g. TRIAC, RCT
Based on conduction
of current
9

converters
Rectifiers
(AC to DC)
Choppers
(DC to DC)
Inverters
(DC to AC)
Cyclo-
converter
(AC ro AC
frequency
converter)
AC voltage
controllers
(AC regulators)
10

11
Thyristor is a general name given to a family
of power semiconductor switching devices, all
of which characterized by switching action
depending upon the PNPN regenerative
feedback.
Thyristor : Thyratron + Transistor
Thyristor is a solid state device like
transistor and has characteristics similar to
that of a thyratron tube.

SCR
Definition A SCR is a four layer, three terminal semiconductor
switching device which is used as a controlled
rectifier and switch in the electronic circuits.
Construction SCR is formed by four layers of P-type and N-type
materials arranged in an alternate manner
Terminals A SCR has three terminals named: anode (A),
cathode (K) and gate (G).
PN junctions A SCRr has three PN junctions.
Conduction SCR needs only a triggering pulse at the gate
terminal to make it conducting and thereafter it
remains conducting.
Need of turn-off
circuit
SCR needs an extra turn-off circuit in order to stop
the conduction.
Power rating Thyristor has capability to handle large power. Thus,
it is rated in kilowatts (kW).
12

13

14
The VI characteristics of SCR (silicon-controlled rectifier) is a
graph of anode current Ia on the y-axis and anode to cathode
voltage on the x-axis as shown in the graph.
Understanding VI characteristics is crucial for designing and
implementing SCRs in various applications.
The VI characteristics can be divided into three primary
Regions (Mode) of operation:
1. Reverse Blocking region
2. Forward blocking region
3. Forward Conduction region

15

16
Ø When the Cathode is made
positive with respect to anode with
gate current zero.
Ø The SCR becomes reverse biased.
Ø In this region characteristics is
similar to that of a diode.
Ø Junction J1 and J3 are reverse
biased and the middle junction J2
is forward biased, therefore only a
small leakage current flows.
Ø If the reverse voltage exceeds then
VBR, avalanche breakdown will
occur at junction J1 and J3
increasing current sharply which
may destroy SCR.
Ø In reverse blocking mode SCR
may be treated as an open switch.

17
Ø When the Anode is made
positive with respect to Cathode
with gate current zero.
Ø The SCR becomes Forward
biased.
Ø Junction J1 and J3 are
Forward biased and the middle
junction J2 is reverse biased,
therefore only a small leakage
current flows.
Ø In Forward Blocking mode,
even through the SCR is
forward biased, it does not
conduct.

18
Ø When anode to cathode voltage is
increased more than a critical
foraward break-over voltage
(VB0)SCR starts conducting.
Ø The voltage across the device
drops from several hundred volts
to 1-2 volts and very large current
starts flowing through the device
Ø The SCR is regarded as a closed
switch
Ø When a gate current is applied,
the thyristor turns-on before VBO is
reached
Ø Higher the gate current; lower the
forward break-over voltage.
Typical gate current are of order
of mA

Ø Latching Current (IL) : Once the SCR is conducting forward
anode current greater than the minimum value, called latching
current, the gate signal is no longer required, SCR will be latched
in its ON state.
Ø Holding current (IH): The SCR will return to its Forward
blocking state if the anode current falls below holding current (IH)
19

Latching Current (IL) Holding Current (IH)
The Latching current of a
d e v i c e d e f i n e d a s t h e
minimum ON state current
required to keep device in the
ON state after the gate signal
has been removed.
T h e h o l d i n g c u r re n t i s
defined as the minimum
value of anode current below
w h i c h t h e d e v i c e s t o p s
conducting and return to its
forward blocking (OFF) state.
This current is associated
with turn-on process.
This current is associated
with the turn-off process
Latching current is usually
slightly greater than the
holding current
Holding current is usually
lower than the latching
current
20

21
The two transistor model of SCR is defined as a representation of
an SCR using two interconnected transistors, one PNP and one
NPN, to explain its operation.
An SCR consists of four layers (PNPN) and can be split into two
transistors, making its function easier to understand.

It is observed from the figure that the collector current of
transistor T1 becomes the base current of transistor T2 and vice
versa.
22

23

24

25

From Equation, it can be analyzed that if , the value
of anode current Ia becomes infinite, that is, the anode current
suddenly attains a very high value, approaching infinity. In
other words, we can say that the device suddenly latches into
conduction (ON) state from the non-conduction (OFF) state.
This characteristic of the device is known as its regenerative
action.
26

This turn-on condition of the SCR can
be satisfied in the following ways:
Ø If the temperature of the device is very high, the
leakage current through it increase, which may
then satisfy the required condition to turn it on.
Ø When the current through the device is extremely
small, the alphas will be very small and the
condition for breakover can be satisfied only by
increasing the voltage across the device to VBO
Ø if a current Ig is injected into the base P in the
same direction as the current Ia across J2. Then
due to regenerative action SCR turns on. 27

1. Forward Voltage Triggering
2. Thermal Triggering (Temperature Triggering)
3. Radiation Triggering (Light Triggering)
4. dv/dt Triggering
5. Gate Triggering
I. D.C. Gate Triggering
II. A.C. Gate Triggering
III. Pulse Gate Triggering
28

1. Forward Voltage Triggering :
When anode-to-cathode forward voltage is
increased with gate circuit open, the reverse biased
junction J2 will have an avalanche breakdown at a
voltage called forward break over voltage VBO. At
this voltage, a thyristor changes from OFF state
(high voltage with low leakage current) to ON-
state characterized by a low voltage across it with
large forward current.
29

2. Thermal Triggering (Temperature Triggering):
when the voltage applied between the anode and
cathode is very near to its breakdown voltage, the
device can be triggered by increasing its junction
temperature.
3. Radiation Triggering (Light Triggering)
In this method, as the name suggests, the energy is
imparted by radiation. Thyristor is bombarded by
energy particles such as neutrons or photons. Light
activated silicon controlled rectifier (LASCR) and
light activated silicon controlled switch (LASCS)
are the examples of this type of triggering 30

4. dv/dt Triggering
We know that with forward voltage across the anode
and cathode of a device, the junctions J1 and J3 are
forward biased, whereas the junction J2 becomes
reverse biased. This reverse biased junction J2 has the
characteristics of a capacitor due to charges existing
across the junction. If a forward voltage is suddenly
applied, a charging current will flow tending to turn
the device ON.
Therefore, if the rate of change of voltage across the
device is large, the device may turn-on even though
the voltage appearing across the device is small. 31

5. Gate Triggering
This is the most commonly used method for
triggering SCRs. By applying a positive signal at the
gate terminal of the device, it can be triggered much
before the specified break over voltage.
For gate triggering, a signal is applied between the
gate and the cathode of the device. Three types of
signals can be used for this purpose.
II. A.C. Gate Triggering
32

In this type of triggering, a d.c. voltage of proper
magnitude and polarity is applied between the gate
and the cathode of the device in such a way that the
gate becomes positive with respect to the cathode.
When the applied voltage is sufficient to produce the
required gate current, the device starts conducting.
Drawback of . D.C. Gate Triggering :
Ø both the power and control circuits are d.c. and there
is no isolation between the two.
Ø In this method continuous d.c. signal has to be applied,
at the gate causing more gate power loss. 33

II. A.C. Gate Triggering :
a.c. source is most commonly used for the gate signal
in all application of thyristor control adopted for a.c.
applications.
Advantage:
Ø This scheme provides the proper isolation between the
power and the control circuits.
Disadvantage:
Ø The gate drive is maintained for one half cycle after
the device is turned ON, and a reverse voltage is
applied between the gate and the cathode during the
negative half cycle.
Ø A separate transformer is required to step down the
a.c. supply, which adds to the cost. 34

This is the most popular method for triggering the
device. In this method, the gate drive consists of a
single pulse appearing periodically.
A pulse transformer is used for isolation.
Advantages:
Ø there is no need of applying continuous signals and
hence, the gate losses are very much reduced.
Ø Electrical isolation is also provided between the main
device supply and its gating signals
35

The static characteristics gives no indication as to the speed at
which the SCR is capable of being switched from the forward
blocking voltage to the conducting state and vice-versa. However,
the transition from one state to the other does not take place
instantaneously, it takes a finite period of time.
Dynic turn on characteristics describe its behavior during turn-on
process.
36

The total turn-on time ton
of the SCR is subdivided
into three distinct periods,
called
I. delay time,
II. rise time and
III. spread time.
37

I. delay time (td)
This is the time between the instant at which the gate
current reaches 90% of its final value and the instant
at which the anode current reaches 10% of its final
value.
The gate current has non-uniform distribution of
current density over the cathode surface. Its value is
much higher near the gate but decreases rapidly as the
distance from the gate increases. It shows that during
td, anode current flows in a narrow region near the
gate where gate current density is the highest
38

II. Rise Time (tr):
This is the time required for the anode current to rise
from 10 to 90% of its final value.
This time is inversely proportional to the magnitude of
gate current and its build up rate.
Thus, tr can be minimized if high and steep current
pulses are applied to the gate.
During rise-time, turn-on losses are the highest due to
high anode voltage Va and large anode current Ia
occurring together in the thyristor.
39

III. Spread-time (ts):
The spread time is the time required for the forward
blocking voltage to fall from 0.1 to its value to the on-
state voltage drop (1 to 1.5 V). After the spread time,
anode current attains steady-state values and the
voltage drop across SCR is equal to the on-state
voltage drop of the order of 1 to 1.5 V
40

Turn-on Time (ton):
This is the sum of the delay time, rise-time and spread
time.
ton = td + tr + ts
Ø This is typically of the order of 1 to 4 µs.
Ø depends upon the anode circuit parameters and the
gate signal wave shapes.
Ø The width of the firing pulse should, therefore, be
more than 10 µs, The amplitude of the gate-pulse
should be 3 to 5 times the minimum gate current
required to trigger the SCR.
41

Ø From Characteristics, it is noted that during rise-
time, the SCR carries a large forward current and
supports an appreciable forward voltage. This may
result in high instantaneous power dissipation
creating local internal hot-spots which could
destroy the device. It is, therefore, necessary to
limit the rate of rise of current. Normally, a small
inductor, called di/dt inductor is inserted in the
anode circuit to limit the di/dt of the anode current.
The shadow area under the power-curve in
characteristics represents the switching loss of the
device. This loss may be significant in high-
frequency applications.
42

1. The latching current of a thyristor circuit in Figure is 50 mA.
The duration of the firing pulse is 50 µs. Will the thyristor get
fired?
43

As the SCR is triggered, the current will rise exponentially in
the inductive circuit.
Since the calculated circuit current value is less than the given
latching current value of the SCR, it will not get fired.
44

2. If the latching current in the circuit shown in Figure is 4 mA,
obtain the minimum width of the gating pulse required to
properly turn-on the SCR.
45

The circuit equation is
The minimum width of the gating pulse required to properly turn-on the
SCR is 4 ms.
46

Once the SCR starts conducting an appreciable forward current,
the gate has no control on it and the device can be brought back
to the blocking state only by reducing the forward current to a
level below that of the holding current. Process of turn-off is also
called as commutation.
If a forward voltage is applied immediately after reducing the
anode current to zero, it will not block the forward voltage and
will start conducting again, although it is not triggered by a gate
pulse. It is, therefore, necessary to keep the device reverse biased
for a finite period before a forward anode voltage can be
reapplied.
47

The turn-off time of the thyristor is defined as the minimum time
interval between the instant at which the anode current becomes
zero, and the instant at which the device is capable of blocking
the forward voltage.
The total turn-off time toff is divided into two time intervals
Ø reverse recovery time (trr)
ØGate recovery time (tgr).
toff = trr + tgr.
48

49

Ø At the instant t1, the anode forward current becomes zero.
Ø During the reverse recovery time, t1 to t3, the anode current
flows in the reverse direction.
Ø At t3, junction J1 and J3 are able to block a reverse voltage.
However, the thyristor is not yet able to block a forward
voltage because carriers, called trapped charges, are still
present at the junction J2.
Ø During the interval t3 to t4, these carriers recombine. At t4,
the recombination is complete and therefore, a forward
voltage can be reapplied at this instant.
Ø The SCR turn-off time is the interval between t4 and t1. In an
SCR, this time varies in the range 10 to 100 µs.
Ø In practical applications, the turn-off time required to the
SCR by the circuit, called the circuit turn-off time tq, must be
greater than the device turn-off time toff by a suitably safe
margin. 50

Thyristor having large turn-off time (50– - 100 µs) are called as
slow switching or converter grade Thyristors,
and those having low turn-off time (10 - – µs ms) are called fast
switching or inverter type Thyristors.
In high frequency applications, the required circuit turn-off time
consumes an appreciable portion of the total cycle time and
therefore, inverter grade thyristors must be used
51

In a thyristor, the gate is connected to the cathode through a PN
junction and resembles a diode. Therefore, the V-I characteristic
of a gate is similar to a diode but varies considerably in units.
The gate characteristics of SCR include the safe limits for gate
voltage and current, which are crucial for proper operation.
The circuit which supplies firing signals to the gate must be
designed:
(1) To accommodate these variations.
(2) Not to exceed the maximum voltage, and power capabilities of
the gate
(3) To prevent triggering from false signals or noise, and
(4) To assure desired triggering.
52

53

Ø curves ON and OM corresponds to
t h e p o s s i b l e s p r e a d o f t h e
characteristic for SCRs of the same
rating.
Ø Region OA is the maximum gate
voltage that will not trigger any
device. The gate must be operated in
this region whenever forward bias is
applied across the thyristor and
triggering is not necessary.
Ø OL and OV are the minimum gate-voltage and gate current to trigger SCR.
Ø OQ and OP are the minimum permissible gate-voltage and gate current
Ø Pg curve and must be contained within the maximum and minimum limits
of gate voltage and gate current.
Ø The maximum value of this series resistance is given by the line HE, where
E is the point of intersection of lines indicating the minimum gate voltage
and gate current. The minimum value of gate source series resistance is
obtained by drawing a line HC tangential to Pg curve.
54

55

ØSCR turn-on time can be reduced by using gate current of
higher magnitude.
ØIt should be ensured that pulse width is sufficient to allow the
anode current to exceed the latching current. In practice, the
gate pulse width is usually taken as equal to or greater than
SCR turn-on time, ton.
If T is the pulse width as shown
then
Duty Cyle =
56

57

3. For an SCR, the gate cathode characteristic is given by a
straight line with a gradient of 16 volts per amp passing
through the origin, the maximum turn-on time is 4 µs and
the minimum gate current required to obtain this quick
turn-on is 500 mA. If the gate source voltage is 15 V, (a)
Calculate the resistance to be connected in series with
the SCR gate.
(b) Compute the gate power dissipation, given that the
pulse width is equal to the turn-on time and that the
average gate power dissipation is 0.3 W. Also, compute
the maximum triggering frequency that will be
possible when pulse firing is used
58

59

Turn-off Methods
Natural Commutation
Forced Commutation
1. Class-A
2. Class-B
3. Class-C
4. Class-D
60

Ø The simplest and most widely used method of commutation.
Ø makes use of the alternating, reversing nature of a.c. voltages
to effect the current transfer.
Ø We know that in a.c. circuits, the current always passes through
zero every half cycle. As the current passes through natural
zero, a reverse voltage will simultaneously appear across the
device. This immediately turns-off the device. This process is
called as natural commutation.
Ø no external circuit is required for this purpose.
Ø This method may use a.c. supply voltages.
Ø In rectifier circuits SCR turns off naturally
61

Ø In case of d.c. circuits, for switching off the thyristors, the
forward current should be forced to be zero by means of some
external circuits. The process is called forced commutation.
Ø The external circuits required for it are known as commutation
circuits.
Ø The components (inductance and capacitance, auxiliary SCR)
which constitute the commutating circuits are called as
commutating components.
Ø A reverse voltage is developed across the device by means of a
commutating circuit OR forward current through the SCR is
made zero to turn off the device.
Ø This method is used in designing chopper and inverter circuits.
62

Forced Commutation
Current Commutation
current commutation involves
reducing the current through
the thyristor below its holding
current, causing it to turn off.
.
Voltage Commutation
voltage commutation refers to
turning off a thyristor by
applying a reverse voltage
across it.
63

Ø This is also known as resonant commutation. This type of
commutation circuit using L-C components-in-series-with the
load are shown in Figure.
Ø In this process of commutation, the
forward current passing through the
device is reduced to less than the level of
holding current of the device. Hence, this
method is also known as the current
commutation method.
Ø This type of commutation circuits are
most suitable for frequency above 1000
Hz.
Ø This commutation circuit is used in series
inverter
64
BY: PROF A.A.DAIYA
7/19/2025

65

Ø In this method, the LC resonating circuit is across the SCR and
not in series with the load.
Ø In this circuit the SCR after getting
ON for sometime, automatically gets
OFF. So it is called self commutated.
Ø The main application of this process
is in d.c. chopper circuits,
Ø In this Class B commutation method,
the commutating component does not
carry the load current.
66

Ø Initially when supply Edc is provided to the circuit and no gate
pulse is provided to the SCR, so SCR is OFF.
Ø It causes the capacitor to get charged. The current in the circuit
flows through commutating components L and C and the load,
this is shown figure: and it charges capacitor up to the voltage
Edc. As the capacitor does not have a path to discharge thus it
will hold the charge +Edc.
67

Ø When thyristor T is triggered, the circuit current flows in two
directions:
Ø (1) The load current IL:
flows through the path Edc+ – T – RL – Edc- –
Ø (2) Commutating current Ic :
flowths through path C+  – L  – T  – C– -
68

Ø When the capacitor C becomes completely discharged, it starts
getting charged with reverse polarity. Due to the reverse voltage,
a commutating current IC starts flowing which opposes the load
current IL
Ø When the commutating current IC is greater than the load current
IL, thyristor T becomes turned OFF.
69
BY: PROF A.A.DAIYA
7/19/2025

From this discussion, we can conclude that the device gets on and
off for some period of time and this is a continuous process.
70

Ø In this method, the main thyristor (SCR T1) that is to be
commutated is connected in series with the load. An additional
thyristor (SCR T2), called the complementary thyristor is
connected in parallel with the main thyristor
Ø Circuit operation can be explained in Four modes.
71

Mode 0: [Initial-state of circuit]
Initially, both the thyristors are OFF. Therefore, the states of
the devices are –
T1  OFF
T2  OFF,
Ec = 0
72

Mode 1:
When a triggering pulse is applied to
the gate of T1, the thyristor T1 is
triggered. Therefore, two circuit
current, namely, load current IL and
charging current IC start flowing.
Their paths are:
IL: Edc+  —R1—  T1  —Edc-–
IC: Edc+  —R2 — C+ — C– - 
—T1—  Edc-–
Capacitor C will get charged by the
supply voltage Edc with the polarity
shown in Figure. The states of circuit
components becomes
T1  ON
T2  OFF
Ec = Edc 73

Mode 2:
Ø When a triggering pulse is applied to
the gate of T2, T2 will be turned on.
As soon as T2 is ON, the negative
polarity of the capacitor C is applied
to the anode of T1 and
simultaneously, the positive polarity
of capacitor C is applied to the
cathode. This causes the reverse
voltage across the main thyristor T1
and immediately turns it off.
Ø Charging of capacitor C now takes
place through the load and its
polarity becomes reverse. Therefore,
charging path of capacitor C
becomes Edc+  —R1  —C+
—C-– —T2 – Edc-–
Hence, at the end of Mode
2, the states of the devices
are
T1  OFF,
T2  ON,
Ec = – Edc
74

Mode 3:
Mode 3: Now, when thyristor T1 is
triggered, the discharging current of
capacitor turns the complementary
thyristor T2 OFF. The state of the
circuit at the end of this Mode 3
becomes
T1  ON
T2  OFF
Ec = Edc
Therefore, this Mode 3
operation is equivalent Mode 1
75

76

Ø Figure shows the Class D
commutation circuit. In this
commutation method, an
auxiliary thyristor (T2) is
required to commutate the main
thyristor (T1), Here, inductor L
is necessary to ensure the
correct polarity on capacitor C.
Thyristor T1 and load resistance
RL form the power circuit,
whereas L, D and T2 form the
commutation circuit
Ø Circuit operation can be
explained in modes.
77

78

Mode 0: [Initial-state of circuit]
When the battery Edc is connected and in the absence of gate
triggering pulse both the thyristors are in the off state so no
current flows. Hence, initially, the state of the circuit
components becomes–
T1  OFF
T2  OFF,
Ec = 0
79

Mode 1:
Initially, SCR T2 must be triggered
first in order to charge the capacitor
C with the polarity shown.
This capacitor C has the charging
path Edc+  —C+ —C- –
—T2  —RL —Edc– -.
As soon as capacitor C is fully
charged, SCR T2 turns-off.
Hence at the end of Mode 1
becomes,
T1  OFF
T2  OFF
Ec = Edc
80

Mode 2:
When thyristor T1 is triggered, the
current flows in two paths:
(a) Load current IL flows through:
Edc+ — T1  —RL 
—Edc-–
(b) Commutation current flows
through: (Discharging capacitor)
C+  —T1 — L  —D
— C-– .
After the capacitor C has completely discharged, its polarity will
be reversed, i.e., its upper plate will acquire negative charge and
the lower plate will acquire positive charge. Reverse discharge of
capacitor C will not be possible due to the blocking diode D. at
the end of Mode 2
T1  ON , T2  OFF , Ec = - Edc
81

Mode 3:
When the thyristor T2 is triggered,
thyristor T1 gets OFF. Therefore,
at the end of Mode 3, the state of
circuit component becomes
T1  OFF, T2  ON
Again, capacitor C will charge to
the supply voltage with the
polarity shown when Capacitor
charges to +Edc at that time SCR
T2 gets OFF. Therefore, thyristors
T1 and T2 both get OFF, which is
equivalent to Mode 1 operation.
82

83

84

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