summer electronics lecture up to mid

2. Lecture 1
Power Electronics
Introduction
ENGR. JAHANGEER BADAR SOOMRO
Jahangir.soomro@iba-suk.edu.pk

3. Suggested Books
1. M.H Rashid “ Power Electronics Circuits
Devices & Applications 3rd Edition
2. NED Mohan “ Power Electronics Applications
and design”
3. Cyril Lander “Power Electronics”
4. B.K Bose “ Modern Power Electronics and AC
derives”
5. Dr. M.R Abro “Power Electronics And
Applications”

4. IEEE Journals
• There are 4 IEEE journals related to Power
Electronics.
1. IEEE journal on Power Electronics
2. IEEE Journal on Industrial application.
3. IEEE Journal On Industrial Electronics
4. IEEE Journal on Power Delivery

5. Quotes from IEEE Papers
“We now live in a truly global society. In the
highly automated industrial front with economic
competitiveness of nation, in future two
technologies will dominate:
Computers and Power Electronics – the former
providing intelligence as to “what to do” and the
latter “ the means to do it”

6. Quotes from IEEE Papers
• “Modern Computers, Communication and
Electronic Systems get their life blood from
power electronics”
• “Solid State Electronics brought 1st electronic
revolution , where as solid state power
electronics brought in the second electronics
revolution”

7. Energy Scenario Globally
87% percentage energy comes from burning fossil
fuels (coal, oil, natural gas)
6 % comes from Nuclear power plants.
Remaining from renewable energies.
According to one IEEE Journal Paper
Natural Uranium fuel lasts for approximately 50
years.
Oil for approximately 100 years.
Natural gas approximately 150 years.
Coal for approximately 200 years.

8. Energy Scenario Globally
• This period can be expanded by
1. Use electrical energy efficiently.
2. Improve Conversion Efficiency.
3. Explore Renewable Energies.
 If input is 100 KW of fuel energy, output is 15-
20 KW of useful work.
 Every KW of loss saved in the process drive. 6
KW of energy is saved on the front end.

9. Energy Scenario Globally
• The power stations burn the fossil fuels to
make our electricity and in that process a lot
of greenhouse gas is made, including carbon
dioxide and methane. This is why they are
called dirty sources of energy.
• Coal, oil and gas are non-renewable sources of
energy because we can only use what is
available and once they have been used up,
that's it!

10. Energy Scenario Globally
• The energy of the sun, wind, waves and
water, amongst these sources of energy
produce only very small amounts of
greenhouse gas once operating, if any at all -
now that's clean!
• They are also renewable which means they
can be used over and over again.

11. Energy Scenario Globally
• Renewable is a term applied to natural resources
and refers to those resources that can be
renewed or replenished in a short period of time.
• Renewable energy is also called “clean” or
“green” energy because it does not pollute the air
or harm the environment.
• As the demand for energy increases renewable
energy will play an important role in supplying
the worlds clean energy needs.

12. Types of Energy
Non renewable energy
cannot be generated
again and again, limited
sources
Petroleum
Natural Gas
Coal
Nuclear
Renewable energy can be
generated continuously
practically without decay
of source. e.g.,
 Solar
 Wind
 Hydropower
 Biomass
 Ocean energy
 Geothermal

13. Power Consumption
• Bulk of the power is used by motors globally.
Major loads are induction motor driving either
a fan , pump or compressor.
• Lighting consumes other good percentage of
power.
• If you save this power, significant over all
benefit can be achieved.

15. Control Speed of Fan
• Compare both fan regulators.
• Power electronic Fan control uses Power
semiconductor devices which enables smooth
control of power. It has very less heat
dissipation, high conversion efficiency, small
size, reliable.
Your Assignment
Find out different applications of power
electronics. You have to just list different
applications. Detailed analysis will be done in
the subject.

16. Power Electronics Definition & Goal
• “Power electronics is the technology
associated with efficient conversion and
control of electronic power by power
semiconductor devices.”
Goal of Power Electronics:-
• “To contribute the flow of energy from electric
source to electric load.

17. Power Electronics Definition & Goal
Interfacing is done by P.E Equipment. Depending upon nature of
source and load this power electronic equipment will change.
Source
P.E
Equipment
Load

18. Why Power Electronics is successful
Highly Efficient
Highly Reliable.
Size, Weight and cost should be low.
If it will be highly efficient, surely heat losses will
be minimum so cooling requirement comes
down. So temp: is low so we can package
various elements closely. So size comes down.

19. History of Power Electronics
• BRIEF HISTORY OF POWER ELECTRONICS
• The first Power Electronic Device developed was the Mercury Arc
Rectifier during the year 1900. Then the other Power devices like metal
tank rectifier, grid controlled vacuum tube rectifier, ignitron,
• phanotron, thyratron and magnetic amplifier, were developed & used
gradually for power control applications until 1950.
• The first SCR (silicon controlled rectifier) or Thyristor was invented and
developed by Bell Lab’s in 1956 which was the first PNPN triggering
transistor.
• The second electronic revolution began in the year 1958 with the
development of the commercial grade Thyristor by the General Electric
Company (GE). Thus the new era of power electronics was born. After
that many different types of power semiconductor devices & power
conversion techniques have been introduced.The power electronics
revolution is giving us the ability to convert, shape and control large
amounts of power.

20. Lecture 2 & 3
Power Diodes
ENGR. JAHANGEER BADAR SOOMRO
Jahangir.soomro@iba-suk.edu.pk

21. Power Semiconductor Devices
• Advances in Power Electronics is primarily due
to the advances in semiconductor devices.
• Power semiconductor devices are heart and
soul of modern Power electronics Equipment.
• They are used as Switches.

22. Properties of Ideal Switch
When the switch is off Is=0, there should not be
any current flowing through the switch when it is
open and it should be able to withstand any
voltage across it. In other words this switch
should be able to withstand any voltage from
minus infinity to plus infinity.
When it is on what should happen……………….?
the voltage across it should be zero and it should
be capable of passing any current through it.

23. Properties of Ideal Switch
Power dissipation that happens in switch when it
is on or off is zero because when it is off current
through it is zero and when it is on voltage
through it is zero so Pd=0.
When the switch is conducting or blocking. The
conduction loss and blocking loss should be zero.
Switch should be turned on and off
instantaneously. Turn on and turn off losses
should be zero Ton=0 and Toff=0 ( switching
losses should be zero).

25. Non ideal Switch
• Can you have this ideal characteristics in
practice………………………………………………????
• Difference should be low between ideal and non
ideal switch.
• So in non ideal switch off state( S is open) current
is non zero. A small current flows. And when it is
off it can not block any voltage. There is some
limit. Example MCB used at home have some
rating. So a semiconductor has some rating above
which it fails.

26. Non ideal Switch
• Also when switch is on, voltage across it is non
zero. There is also maximum current carrying
capability.
• So off state current is non zero, losses will happen
when is off/blocking. When the switch is on
voltage across it is non zero, there will conduction
losses.
• So both blocking and conduction losses are finite.
• Transition from on state to off state is not going
to be instantaneous. There are finite losses
during switching. Switching losses happen.

28. Non ideal Switch
• Every devices has maximum power dissipation limit.
All the devices are thermally unstable.
• As the power losses increases, junction temperature
will increase and if it increases certain value it will
fail, what we called thermal limit.
• So each device has safe operating area(SOA). The
operating point should lie in SOA. (see the figure). So
each device is thermally unstable because power
dissipation is finite during conduction period,
switching and blocking.
• You need to calculate or know heat sink
requirement/cooling requirement. Power supply
used in computer has cooling fan.

29. Various types of switches in Power
electronics.
1. Uncontrolled Switch:-
Because on and off is determined by state of the
circuit in which device is connected.
Diode is uncontrolled switch.

30. Various types of switches in Power
electronics.
2. Semi controlled Switch:-
It is because switch may be turned to one of its
state using controlled terminal and other state is
reachable through circuit only.
It is three terminal device.
SCR is an example.
You can turn on the device by supplying positive
gate current when the device is FB. (see figure)
Having turn on the device you can not turn it off
using gate. Hence the name semicontrolled.

31. Various types of switches in Power
electronics.
• 3. Fully controlled Switch:
Both on and off should be possible using
controlled terminal.
Example is BJT.
I can turn it on by supplying positive base
current and off if base current=0.

32. Diode
• 2 terminal device. Anode and Cathode.
• VAK should be positive. It is FB and diode
conducts.
• Low power silicon VAK= 0.7
• For a power diode approxmately VAK=1.5
• VI Characterstics ( You are familiar)
• PIV ( You are familiar)

33. Diode
• During on state there is going to be finite voltage
drop across diode (Vf) and Ia is current flowing
through diode. So on state loss or conduction
loss= Vf * Ia.
• If it is above a certain limit, you will mount on
heat sink. So for diode you will mount it on heat
sink. Small signal diodes does not need heat sink.
• In power electronics off state circuit , the way the
diode turns off is important.

34. Power Diode
• Power semiconductor diode
is the “power level” counter
part of the “low power
signal diodes”.
• The symbol of the Power
diode is same as signal level
diode. However, the
construction and packaging
is different.
34

35. Power Diode
• Power dides are required to carry up to several KA of current
under forward bias condition and block up to several KV under
reverse biased condition.
• Large blocking voltage requires wide depletion layer.
• This requirement will be satisfied in a lightly doped p-n
junction diode of sufficient width to accommodate the
required depletion layer.
• Such a construction, however, will result in a device with high
resistively in the forward direction. There fore almost 1.5 V.
• If forward resistance (and hence power loss) is reduced by
increasing the doping level, reverse break down voltage will
reduce.
35

36. Reverse Recovery Time of a diode

44. Reverse Recovery Time of a diode
• Reverse Recovery time is time required for the
diode to change from FB to RB.
• Forward Recovery time is time required to
change from RB to FB.
• Generally reverse recovery dominates over
forward recovery time because less minority
carrier move.

45. Switching Characteristics of Power Diodes
• Power Diodes take finite time to make transition from reverse bias
to forward bias condition (switch ON) and vice versa (switch OFF).
• Behavior of the diode current and voltage during these switching
periods are important due to the following reasons.
– Severe over voltage / over current may be caused by a diode switching
at different points in the circuit using the diode.
– Voltage and current exist simultaneously during switching operation of
a diode. Therefore, every switching of the diode is associated with
some energy loss. At high switching frequency this may contribute
significantly to the overall power loss in the diode.
45

46. Turn Off Characteristics
• The reverse recovery
characteristics shown is
typical of a particular type
of diodes called “normal
recovery” or “soft
recovery” diode.
• The total recovery time (trr)
in this case is a few tens of
microseconds.
46

47. Turn Off Characteristics
• This is acceptable for line frequency rectifiers (these diodes
are also called rectifier grade diodes).
• High frequency circuits (e.g PWM inverters) demand faster
diode recovery, other wise this reverse recovery current
can cause damage.
47

48. Types of Diodes
• Depending on the application requirement various
types of diodes are available.
– Schottky Diode
– Fast Recovery Diode
– Line Frequency Diode/general-purpose diode

49. Types of Diodes
 Schottky Diode
 Lets see an important video on Schottky diode
https://www.youtube.com/watch?v=bXEyCf1P0UU
 Used for high frequency and fast switching applications.
 Formed by joining n-type with metal such as gold, silver, platinum. So a metal to
semiconductor junction.
 It operates with only majority carrier and there is no reverse current in it.
 Since it operates with no minority carriers therefore applicable in high frequency
switching applications.
 These diodes are used where a low forward voltage drop (typically 0.3 v) is needed.
 These diodes are limited in their blocking voltage capabilities to 50v- 100v. It can not
block much reverse voltage. This is disadvantage.

50. Types of Diodes
Fast Recovery Diode
These diodes are designed to be used in high frequency
circuits in combination with controllable switches where a
small reverse recovery time is needed.
They are used in dc-dc or dc-ac where speed of recovery is
critical important.
At power levels of several hundred volts and several hundred
amperes such diodes have trr rating of less than few
microseconds.

51. Types of Diodes
Line Frequency Diode
The on state of these diodes is designed to be as low as
possible.
As a consequence they have large trr, which are
acceptable for line frequency applications.

52. Comparison between different types of Diodes
25rrt s 0.1 s to 5 srrt   a few nano secrrt 
52
Characteristics Gernal purpose
diode
Fast recovery diode Schottky diode
Working Voltage Up to 6000V &
3500A
Up to 6000V and
1100A
Up to 100V and
300A
Reverse recovery
time
High Low Extremely low
Turn off time high low Extremely low
Trr
Switching
frequency
– Low
(Max 1KHz)
– High
(Max 20KHz)
– Very high.
(Max 30KHz)
Vf
25rrt s 0.1 s to 5 srrt   a few nano secrrt 
0.7 to 1.2VFV  0.8 to 1.5VFV  0.4 to 0.6VFV 

53. Silicon Carbide Diodes
• Ulta low power loss
• Ulta fast switching behaviour
• Highly reliable ( no temperature influence on
the switching behaviour).
• Then why it not so common as compared to
silicon diode?????????????????????????????

54. SILICON CONTROLLED RECTIFIER
Lecture 4
ENGR. JAHANGEER BADAR SOOMRO
Jahangir.soomro@iba-suk.edu.pk

55. • One of the most important type of power semiconductor
device.
• Compared to transistors, thyristors have lower on-state
conduction losses and higher power handling capability.
• However, they have worse switching performances than
transistors.
• Name ‘thyristor’, is derived by a combination of the capital
letters from THYRatron and transISTOR.
• It is semi controlled switch.
Introduction
55

56. • Thyristors are four-layer pnpn power
semiconductor devices.
• These devices switch between conducting and
nonconducting states in response to a control
signal.
• Thyristors are used in timing circuits, AC motor
speed control and switching circuits.
Introduction
56

57. Thyristors
• Bell Laboratories were the first to fabricate a silicon-
based thyristor.
• Its first prototype was introduced by GE (USA) in 1957.
• Later on many other devices having characteristics
similar to of a thyristor were developed.
• These semiconductor devices are SCR, SCS, Triac, Diac,
PUT, GTO, e.t.c.
• This whole family of semiconductor devices is given
the name thyristors. 57

58. Why not germanium controlled
rectifier
• The device is made of silicon because leakage
current is very small in silicon as compared to
germanium. since device is used as switch, it
will carry leakage current in off condition
which should be as small as possible.
• It got its name because it is silicon device and
is used as rectifier and that rectification can
be controlled.

62. Thyristor/ SCR
• SCR is a three terminal, four layers solid state
semiconductor device, each layer consisting
of alternately N-type or P-type material, i.e;
P-N-P-N,
• It can handle high currents and high voltages,
with better switching speed and improved
breakdown voltage .
A K
G
62

63. Thyristor/ SCR
• Thyristor can handle high currents and high voltages.
• Typical rating are 1.5kA & 10kV which responds to 15MW
power handling capacity.
• This power can be controlled by a gate current of about 1A
only.
• Thyristor acts as a bistable switch.
– It conducts when gate receives a current pulse, and
continue to conduct as long as forward biased (till device
voltage is not reversed).
– They stay ON once they are triggered, and will go OFF only
if anode current is too low or when triggered off.
63

64. Thyristor/ SCR Operation
• Refer VK mehta book.
• When the anode voltage is made
positive with respect to the cathode,
junctions J1 and J3 are forward biased
and junction J2 is reverse biased.
• The thyristor is said to be in
the forward blocking or off-state
condition.
• A small leakage current flows from
anode to cathode and is called the off-
state current.
64

65. Thyristor/ SCR Operation
• If the anode voltage VAK is increased to a
sufficiently large value, the reverse biased
junction J2 would breakdown.
• This is known as avalanche breakdown and the
corresponding voltage is called the forward
breakdown voltage VBO.
• Since the other two junctions J1 and J3 are
already forward biased, there will be free
movement of carriers across all three junctions.
• This results in a large forward current and the
device is now said to be in a conducting or on-
state.
• The voltage drop across the device in the on-
state is due to the ohmic drop in the four layers
and is very small (in the region of 1 V).
65

66. Thyristor/ SCR
66

68. Thyristor Operating modes
Thyristors have three modes :
• Forward blocking mode:
Only leakage current flows,
so thyristor is not
conducting.
• Forward conducting mode:
large forward current flows
through the thyristor.
• Reverse blocking mode:
When cathode voltage is
increased to reverse
breakdown voltage ,
Avalanche breakdown
occurs and large current
flows.
68

69. Important terms
Latching Current IL
• This 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.
• If a gate current greater than the threshold gate current is applied until
the anode current is greater than the latching current IL then the
thyristor will be turned on or triggered.
Holding Current IH
• This is the minimum anode current required to maintain the thyristor in
the off-state.
• To turn off a thyristor, the forward anode current must be reduced
below its holding current for a sufficient time for mobile charge carriers
to vacate the junction.
• Generally the value of holding current is 1/1000 of the rated anode
current.
• http://aueeestudents.blogspot.com/2015/01/what-is-difference-
between-holding.html
69

70. Important characteristics
Reverse Current IR
• When the cathode voltage is positive with respect to the
anode, the junction J2 is forward biased but
junctions J1 and J3 are reverse biased. The thyristor is said
to be in the reverse blocking state and a reverse leakage
current known as reverse current IR will flow through the
device.
Forward Breakover Voltage VBO
• If the forward voltage VAK is increased beyond VBO , the
thyristor can be turned on. But such a turn-on could be
destructive. In practice the forward voltage is maintained
below VBO and the thyristor is turned on by applying a
positive gate signal between gate and cathode.
70

77. • We will derive expression for anode current
Mathematical analysis of two transistor model

78. Mathematical analysis of two transistor model
• relationship between IC and IB is

79. Mathematical analysis of two transistor model

80. Mathematical analysis of two transistor model

81. Mathematical analysis of two
transistor model

82. • Do you expect a thyristor to turn ON if a positive
gate pulse is applied under reverse bias condition (i.
e cathode positive with respect to anode)?
• Answer: The two transistor analogy of thyristor
shown in Fig 4.2 (c) indicates that when a reverse
voltage is applied across the device the roles of the
emitters and collectors of the constituent transistors
will reverse. With a positive gate pulse applied it may
appear that the device should turn ON as in the
forward direction. However, the constituent
transistors have very low current gain in the reverse
direction. Therefore no reasonable value of the gate
current will satisfy the turn ON condition (i.e.∝1 +
∝2 = 1). Hence the device will not turn ON.

83. Thyristor turn-ON methods
• Thyristor turning ON is also known as Triggering.
• With anode is positive with respect to cathode, a thyristor
can be turned ON by any one of the following techniques :
– Forward voltage triggering
– Gate triggering
– dv/dt triggering
– Temperature triggering
– Light triggering
83

84. 1. Forward Voltage Triggering
• When breakover voltage (VBO) across a thyristor is exceeded
than the rated maximum voltage of the device, thyristor turns
ON.
• At the breakover voltage the value of the thyristor anode current
is called the latching current (IL) .
• Breakover voltage triggering is not normally used as a triggering
method, and most circuit designs attempt to avoid its occurrence.
• When a thyristor is triggered by exceeding VBO, the fall time of the
forward voltage is quite low (about 1/20th of the time taken
when the thyristor is gate-triggered).
• This method is not preferred because during turn on of
thyristor, it is associated with large voltage and large current
which results in huge power loss and device may be
damaged.
84

85. 2. Gate Triggering
• Turning ON of thyristors by gate triggering is simple and
efficient method of firing the forward biased SCRs.
• Whenever thyristor’s turn-ON is required, a positive gate
voltage b/w gate and cathode is applied.
• The pulse remains for some time untill the anode current
has increased to a certain value known as latching
current or pick up current.
• Forward voltage at which device switches to on-state
depends upon the magnitude of gate current.
– Higher the gate current, lower is the forward
breakover voltage . 85

86. Gate Triggering
• Turning ON of thyristors by gate triggering is simple and
efficient method of firing the forward biased SCRs.
86

87. Thyristor Gate Control Methods
• An easy method to switch ON a SCR into conduction is to apply
a proper positive signal to the gate.
• This signal should be applied when the thyristor is forward
biased and should be removed after the device has been
switched ON.
• Thyristor turn ON time should be in range of 1-4 micro
seconds, while turn-OFF time must be between 8-50 micro
seconds.
• Thyristor gate signal can be of three varieties.
– D.C Gate signal
– A.C Gate Signal
– Pulse
87

88. Thyristor Gate Control Methods
D.C Gate signal: Application of a d.c gate signal causes the flow
of gate current which triggers the SCR.
– Disadvantage is that the gate signal has to be continuously
applied, resulting in power loss.
– Gate control circuit is also not isolated from the main
power circuit.
88

89. Thyristor Gate Control Methods
A.C Gate Signal: In this method a phase - shifted a.c voltage derived from
the mains supplies the gate signal.
– Instant of firing can be controlled by phase angle control of the gate
signal.
89

90. Thyristor Gate Control Methods
Pulse: Here the SCR is triggered by the application of a positive pulse of
correct magnitude.
– For Thyristors it is important to switched ON at proper instants in a
certain sequence.
– This can be done by train of the high frequency pulses at proper
instants through a logic circuit.
– A pulse transformer is used for circuit isolation.
90

91. 3. Temperature Triggering
• If the temperature of the thyristor is high, it results in
increase the leakage current ICBO1 and ICBO2 and
leakage current is strong function of temperature. They
approximately double for every 10 °C rise in
temperature. If they increase collector current as well as
α1 & α2 raise and in case (α1 + α2) approach one, anode
current increases and device goes into conduction
mode.
• This type turn on is not preferred as it may result in
thermal turn away and hence it is avoided.
91

92. 4. Light Triggering
• In this method light particles (photons) are made to
strike the reverse biased junction, which causes an
increase in the number of electron hole pairs and
triggering of the thyristor.
• For light-triggered SCRs, a slot (niche) is made in the
inner p-layer.
• When it is irradiated, free charge carriers are
generated just like when gate signal is applied b/w
gate and cathode.
• Pulse light of appropriate wavelength is guided by
optical fibers for irradiation.
• 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 is mostly used in high-
voltage direct current (HVDC) transmission systems.
92

93. 5. dv/dt triggering
• With forward voltage across anode & cathode of a thyristor, two
outer junctions (A & C) are forward biased but the inner junction
(J2) is reverse biased.
• The reversed biased junction J2 behaves like a capacitor because of
the space-charge present there.
• As p-n junction has capacitance, so larger the junction area the
larger the capacitance.
• If a voltage ramp is applied across the anode-to-cathode, a current
will flow in the device to charge the device capacitance according
to the relation:
• If the charging current becomes large enough, density of moving
current carriers in the device induces switch-on.
• This method of triggering is not desirable because high charging
current (Ic) may damage the thyristor.
93

96. Thyristor Switching characteristics
• Also called thyristor dynamic characteristics or on-off
characteristics of scr.
• The switching characteristics are important particularly
at high-frequency, to define the device velocity in
changing from conduction state to blocking state and
vice versa.
• Losses occurring in the device during switching from
ON state to OFF state and OFF state to ON state is
known as Switching Losses.
• The device’s switching characteristics tells us about the
switching losses, which is very important parameter to
decide the selection of device.
• At high frequency, the switching losses are more.

98. • (a) Rate of rise of anode voltage dv/dt: It is
the slope of the line showing anode voltage
VAK between time t0 and t1. Rapid rising of
the voltage produces a transient current
across junction J2. This causes false
triggering. Typical value of dv/dt rating is
100-300 volts per microseconds.
Dynamic Characteristics of Thyristor

99. • (b) Rate of rise of Anode current di/dt:
When gate pulse is applied the conduction
of anode current starts near the gate
connection and spreads from there across
the whole area or the junction. If di/dt is
large, than a local hot spot will be formed
due to the high current density and may
result in failure of device. Typical value of
di/dt rating is 100-500 amps per μs.
Dynamic Characteristics of Thyristor

101. • Rate of rise of re-applied voltage dv/dt: It is
shown as slope of the line between time t8 and t9.
Turn-off time increases with the increase of re-
applied dv/dt. Re-applied dv/dt is more crucial
than initial applied dv/dt. This is because the
current carriers at the junction take finite time to
recombine naturally and cause the blocking state.
Dynamic Characteristics of Thyristor

102. Turn ON Time of SCR
• A forward biased thyristor can be turned on by
applying a positive voltage between gate and cathode
terminal. But it takes some transition time to go from
forward blocking mode to forward conduction mode.
This transition time is called turn on time of SCR and
it can be subdivided into three small intervals as delay
time (td) rise time(tr), spread time(ts).
• Rise time inversely proportional to magnitude of gate
current and its build up rate. Thus tr can be reduced if
high and step pulses are applied to gate.

104. Turn off mechanism

107. THYRISTOR RATING &
LOSSES OF THYRISTOR
Lecture 5
ENGR. JAHANGEER BADAR SOOMRO
Jahangir.soomro@iba-suk.edu.pk

113. Thyristor protection and cooling
system of thyristor.
Lecture 6
ENGR. JAHANGEER BADAR SOOMRO
Jahangir.soomro@iba-suk.edu.pk

125. • COOLING SYSTEM OF THYRISTOR

130. Gate Protection of Thyristor

132. TYPES OF THYRISTOR
Lecture 7
ENGR. JAHANGEER BADAR SOOMRO

165. Your assignment
• Compare and contrast between SCR and
TRIAC.
• What are disadvantages and limitation of
TRIAC.
• Different application where QUDRAC(diac-
triac) together are used (Hint vk mehta book)
• Last date is 1 october.

172. GATE TURN OFF THYRISTORS(GTO)

177. Turn on and Turn off time of SCR
• GTO can be brought into conduction very rapidly
because it can tolerate higher di/dt values due to gate
cathode interdigitation. Or in simple words at the
instant of turn on more area gate cathode is available
so therefore you can have high di/dt.
• GTO suffers more power losses when we turn it off
because a large negative pulse it applied at gate to turn
it off. During turn off, the forward voltage of the device
must be limited until the current tails off. If the voltage
rises too fast (reapplied-dv/dt is too fast) not all of the
device will turn off and GTO will fail and destroy. So
during turn off snubber circuit should be used in GTO.

179. Advantages over SCR
• Fully controlled.
• Elimination of commutating components in forced
commutation, resulting in reduction in cost, weight,
and volume.
• Reduction in acoustic and electro-magnetic noise due
to the elimination of commutation chokes.
• Faster turn-off permitting high switching frequencies
and
• Improved efficiency of converters.
• High over current capabilities( High di/dt)

180. Disadvantages as compared to SCR
• Magnitude of latching, holding currents is more. The
latching current of the GTO is several times more as
compared to conventional thyristors of the same rating.
• On state voltage drop and the associated loss is more.
• Due to multicathode structure of GTO, triggering gate
current is higher than that required for normal SCR.
• Gate drive circuit losses are more. Its reverse voltage
blocking capability is less than the forward voltage blocking
capability.
• Turn off losses are significant.
• It requires a minimum gate current to sustain on-state
current.