Power electronics devices

1. Power Semiconductor Devices
Part-1

2. Switchgear and control panels are found in power generating stations,
transformer stations, distribution substations, commercial and
institutional buildings, industrial plants and factories, refineries.

3. In an electric power system, switchgear is the combination of electrical
disconnect switches, fuses or circuit breakers used to control, protect and
isolate electrical equipment.
The switchgear can
contain as
switching/interrupting
device(s): a circuit breaker
(c.b.), a switch/fuse
combination, a
contactor/fuse combination
(for motor switching), for
low v oltage (l.v.)
assemblies: a switch or
breaker in series with a
contactor (for motor
switching)
Devices in Switchgear
Medium Voltage Transformers insulation up to
40,5kV

4. Circuit
Breaker

5. Cut-away
model of an
oil-filled
high-voltage
circuit
breaker

6. Modern power system networks are very large with hundreds of Buses
Mechanical controlling is not sufficient for them
In current existing systems computers and high-speed electronic
devices are used for control and protection.
In past mechanical devices were used for control purposes
Mechanical devices tend to wear out very quickly compared to static
devices.
Solid State Devices

7. Solid-state device, mean electronic device in which electricity flows
through solid semiconductor crystals (silicon, gallium arsenide,
germanium) rather than through vacuum tubes.
The first solid-state device was the “cat's whisker” (1906), in which a
fine wire was moved across a solid crystal to detect a radio signal.
Solid State Devices
Cat's whisker crystal detector
The cat's whisker detector was the first type of semiconductor diode
Crystals was discovered in 1874 by Karl Ferdinand Braun, and the first crystal
detectors were used to receive radio waves by Braun and Jagadish Chandra Bose in
1894, and improved around 1904

8. Switch without any mechanical movement.
Solid State Devices
Semiconductor Power Switches
silicon controlled rectifier (SCR), also called a
thyristor, by the General Electric Company in 1958.
1000A, 4200V
SCR Rectifier

9. List of Basic Building Blocks

10. General Purpose Semiconductor
Switches
• Diodes
• Power Diodes
• Controlled Rectifiers OR Thyristor
• Diacs
• Triacs
• Transistors as switch
• GTOs
• IGCTs
• MOSFET
• IGBTs

11. Power semiconductor devices can be classified as follows:
1.Uncontrolled turn on and off (Diode)
2.Controlled turn on and uncontrolled turn off (SCR)
3.Controlled turn on and off characteristics (BJT, MOSFET, GTO,
IGBT)
4.Continuous gate signal requirement (BJT, MOSFET, IGBT)
5.Pulse gate requirements (SCR, GTO)
6.Bipolar voltage-withstanding capability (SCR, GTO)
7.Unipolar voltage-withstanding capability (BJT, MOSFET, GTO,
IGBT, MCT)
8.Bidirectional current capability (TRIAC, RCT)
9.Unidirectional current capability (SCR, GTO, BJT, MOSFET,
MCT, IGBT, SITH, SIT, Diode)

12. pn-junction Diode
Logic/Schematic Symbol
12
Epita-xial layer.

13. No Biased pn junction Diode

14. Forward-Biasing Circuit
 The positive terminal of the source is connected to the anode through a
current-limiting resistor.
 The negative terminal of the source is connected to the cathode.
 The forward current (IF) is from anode to cathode as indicated.
 The forward voltage drop (VF) due to the barrier potential is from positive at
the anode to negative at the cathode.
cathodeanode
14

15. Reduction in the Depletion Layer due to
Forward Bias
Switch Closed

16.  The negative terminal of the source is connected to the anode through a
current-limiting resistor.
 The positive terminal of the source is connected to the cathode.
 No forward current (IF) is from anode to cathode as indicated.
 The forward voltage drop (VF) due to the barrier potential is from positive
at the anode to negative at the cathode.
Reverse-Biasing-Circuit
anode
cathode
16

17. Increase in the Depletion Layer due to
Reverse Bias
Open switch

18. Forward Biased Diode
 The diode behaves like a ‘ON’ switch in this mode
 Resistance R and diode’s body resistance limits the current
through the diode
 VBIAS has to overcome VBARRIER (0.7v)in order for the diode to
conduct
18
The diode behaves like a ‘OFF’ switch in this mode
 If we continue to increase reverse voltage VB
breakdown voltage of the diode is reached
 Once breakdown voltage is reached diode conducts
heavily causing its destruction
Reverse Biased Diode

19. Junction Diode Ideal and Real
Characteristics

20. pn-jnction Diode Symbol and Static I-V
Characteristics.

21. • When diode is forward biased, it conducts current with a small
forward voltage (Vf) across it (0.2-3V)
• When reversed (or blocking state), a negligibly small leakage
current (uA to mA) flows until the reverse breakdown occurs.
• Diode should not be operated at reverse voltage greater than Vr

22. 22
Typical LED Characteristics
Semiconductor
Material
Wavelength Color VF @ 20mA
GaAs 850-940nm Infra-Red 1.2v
GaAsP 630-660nm Red 1.8v
GaAsP 605-620nm Amber 2.0v
GaAsP:N 585-595nm Yellow 2.2v
AlGaP 550-570nm Green 3.5v
SiC 430-505nm Blue 3.6v
GaInN 450nm White 4.0v
COLORED LED

23. • When a diode is switched quickly from forward to reverse
bias, it continues to conduct due to the minority carriers
which remains in the p-n junction.
• The minority carriers require finite time, i.e, trr (reverse
recovery time) to recombine with opposite charge and
neutralise.
• Effects of reverse recovery are increase in switching losses,
increase in voltage rating, over-voltage (spikes) in inductive
loads
Reverse Recovery

24. Power Electronics and Drives (Version 3-
2003). Dr. Zainal Salam,
UTM-JB
24
Softness factor, Sr
IF
VR
t0
t2
Sr= ( t2 - t1 )/(t1 - t0)
= 0.8
t1
IF
VR
t0
Sr= ( t2 - t1 )/(t1 - t0)
= 0.3
t1 t2
Snap-off
Soft-recovery
SW OFF

25. LARGE t rr SMALL t rr

26. Types of Power Diodes
• Line frequency (general purpose):
– On state voltage: very low (below 1V)
– Large trr (about 25us) (very slow response)
– Very high current ratings (up to 5kA)
– Very high voltage ratings(5kV)
– Used in line-frequency (50/60Hz) applications such as rectifiers
• Fast recovery
– Very low trr (<1us).
– Power levels at several hundred volts and several hundred amps
– Normally used in high frequency circuits

27. POWER DIODES

28. Construction and Characteristics of Power Diodes
Power Diodes of largest power rating are required to conduct
several kilo amps of current in the forward direction with very
little power loss while blocking several kilo volts in the reverse
direction.
POWER DIODES

29. Power diode
• Main requirements:
– reverse voltage vr as high as possible
– voltage drop vF as low as possible
– turn-off speed as high as possible
–trr (reverse recovery time) as low as possible

30. In Power Switching devices : excessive power rating are required to conduct several
kilo amps of current in the forward direction with very little power loss while
blocking several kilo volts in the reverse direction.
Power Devices Switching Requirements
Large blocking voltage requires wide depletion layer in order to restrict the
maximum electric field strength.
Requirement-1
Requirement-2
Solution-1
These two requirements will be satisfied in a lightly doped p-n junction diode of
sufficient width to accommodate the required depletion layer.
1. This will result in a device with high resistively in the forward direction
with heavy power loss and that is unacceptably high.
Cont
Problem in the solution:
2. If forward resistance (and hence power loss) is reduced by increasing the
doping level, reverse break down voltage will reduce (need High).

31. Power Devices Switching Requirements
Solution-2
This drift layer
technique is used
in all normally
constructed
devices to make
them to handle
more power.
This apparent contra-diction in the requirements of a power diode is resolved by:
introducing a lightly doped “drift layer” of required thickness between two
heavily doped p and n layers as shown in Fig below
Power Devices
Lightly doped n-, it will add significant ohmic resistance to the diode when it is
forward biased.

32. 1. Heavily doped n+ substrate with doping level of 1019/cm3. This substrate
forms a cathode of the power diode.
2. On n+ substrate, lightly doped n- epitaxial layer is grown. This layer is also
known as drift region. The doping level of n- layer is about 1014/cm3.
3. The PN junction is formed by diffusing a heavily doped p+ region. This p+
region forms anode of the diode. The doping level of p+ region is about
1019/cm3. The thickness of p+ region is 10µm. The thickness of n+ substrate
is 250µm.
+ = Heavily Doped - = Lightly doped
Power Device Construction

33. 4. The thickness of n- drift region depends upon the breakdown voltage
of the diode.
5. The drift region determines the reverse breakdown voltage of the
diode.
6. Its function is to absorb the depletion layer of the reverse biased p+-n-
junction.
7. As it is lightly doped, it will add significant ohmic resistance to the
diode when it is forward biased.
8. For higher breakdown voltages, the drift region is wide.
9. The n- drift region is absent in low power signal diodes.
Power Device Construction

34. 1. When the power diode is forward biased (anode is made positive with respect
to cathode), the holes will be injected from the p+ region into the drift region.
2. Some of the holes combine with the electrons in the drift region. Since
injected holes are large, they attract electrons from the n+ layer.
3. Thus holes and electrons are injected in the drift region simultaneously.
4. Hence resistance of the drift region reduces significantly.
5. Thus diode current goes on increasing, but drift region resistance remains
constant.
1. So on-state losses in the diode are reduced. This phenomenon is called
as Conductivity modulation of drift region.
Working

35. • A power diode has a P-I-N structure
as compared to the signal diode
having a P-N structure.
• (in P-I-N) stands for intrinsic
semiconductor layer to bear the
high-level reverse voltage as
compared to the signal diode
• This intrinsic layer adds more
resistance during forward-biased
condition.
• Thus, power diode requires a proper
cooling arrangement for handling
large power dissipation.
Power diode symbol is the
same as pn diode
POWER DIODES

36. Under reverse bias condition only a small leakage current (less than 100mA
for a rated forward current in excess of 1000A) flows in the reverse
direction.
This reverse current is independent of the applied reverse voltage but highly
sensitive to junction temperature variation.
When the applied reverse voltage reaches the break down voltage, reverse
current increases very rapidly causes avalanche multiplication process.
Power Diode under Reverse Bias Conditions

37. Power diodes now exist with forward current rating of 1A to
several thousand amperes with reverse-recovery voltage
ratings of 50V to 5000V or more.
Power diodes are used in numerous applications including
rectifier, voltage clamper, voltage multiplier and etc. Power diode
symbol is the same as of the signal diode as shown in Fig
Applications

38. SCR Silicon Controlled Rectifier
OR
Thyristor

39. SCR Silicon Controlled Rectifier
Thyristor

40. SCR Silicon Controlled Rectifier or Thyristor
• It has three terminals: Anode, cathode and gate
• Gate is the control terminal while the main current
flows between the anode and cathode.
• Device is a "one way device"
• For AC input, it will only conduct for a maximum
of half the cycle.
• In operation, the thyristor or SCR will not conduct initially. It requires a
certain level of current to flow in the gate to "fire" it.
• Once fired, the thyristor will remain in conduction until the voltage across the
anode and cathode is removed .
• For negative half cycle will be blocked by SCR. Due to rectifier action.
• It will require current in the gate circuit to fire the SCR again.

41. Thyristor
Thyristor (SCR – Semiconductor Controlled Rectifier) is a
controlled semiconductor device of 4-layer PNPN structure with
3 PN junctions.
Thyristor schematic symbol and structure

42. P1
A
G
K
N1
P2P2
N1
N2
a)
NPN
PNP
A
G
K
IG
IK
Ic2
Ic1
IA
V1
V2
b)
Structure Equivalent circuit
Equivalent
circuit
Structure and equivalent circuit of thyristor

43. How a thyristor latches on

44. 1. With no current flowing into the gate, the thyristor is switched off and no
current flows between the anode and the cathode.
2. When a current flows into the gate, it effectively flows into the base
(input) of the lower (n-p-n) transistor, turning it on.
3. Once the lower transistor is switched on, current can flow through it,
activating the base (input) of the upper (p-n-p) transistor, turning that on as
well.
4. Once both transistors are turned on completely ("saturated"), current can
flow all the way through both of them—through the entire thyristor from
the anode to the cathode.
5. Since the two transistors keep one another switched on, the thyristor stays
on—"latches"—even if the gate current is removed
Each transistor acts as the input to the other
See the animation

45. Thyristor
Steady-State V-I Characteristic of a Thyristor

46. The Four-Layer Diode
The characteristic curve for a 4-layer diode shows the
forward blocking region. When the anode-to-cathode
voltage exceeds Vbarrier, conduction occurs. The switching
current at this point is IS.
On
Off
VBR(F)
VAK
IH
0
IA
IS
Forward-
conduction
region
Forward-
blocking
region
Once conduction begins, it will
continue until anode current is
reduced to less than the holding
current (IH). This is the only way
to stop conduction.
V I Curve
Voltage decreasing
current increasing

47. Relation between Gate Trigger Pulse
and output voltage
t
~
iA
vS
vAK
iG
R
iA
vS
vAK
0 T/2 T

48. Applications of Thyristors:
• AC power control (including lights, motors,etc).
• Over voltage protection crowbar for power supplies.
• AC power switching.
• Control elements in phase angle triggered controllers.
• With in photographic flash lights
Thyristor applications

49. Gate Turn-Off Thyristor (GTO)

50. Gate Turn-Off Thyristor (GTO)
Gate turn-off thyristor (GTO) is a
special type of thyristor. GTOs, are
fully controllable switches which
can be turned on and off by
switching the polarity of the gate
signal.
Slow switching speeds. Used at very high
power levels. Requires elaborate gate
control circuitry (needs a turn-off snubber
circuit).
Turn off: A "negative current" pulse is
applied between the gate and cathode
terminals.
Turn on: A "positive current" pulse is
applied between the gate and cathode
terminals.

51. DIAC

52. (DIode Alternating Current)
Circuit symbol for the DIAC
DIAC is two terminal four layer
semiconductor device that can
conduct current in either direction,
when polarity is active
DIAC conducts current
only after a certain
breakdown voltage has
been exceeded.
Double Diode OR DIAC
The DIAC conducts after a 'break-over'
voltage, designated VBO, is exceeded

53. (DIode Alternating Current)
Circuit symbol for the DIAC
Double Diode OR DIAC
• When the DIAC breakdown voltage
occurs, the resistance of the component
decreases abruptly and this leads to a
sharp decrease in the voltage drop
across the DIAC, and a corresponding
increase in current.
• DIAC will remain in its conducing state
until the current flow through it drops
below a particular value known as the
holding current.
• When the current falls below the
holding current, the DIAC switches
back to its high resistance, or non-
conducting state.
Working

54. DIAC V-I Characteristics curve
The DIAC conducts after a 'break-over'
voltage, designated VBO, is exceeded

55. DIAC
Voltage Current curve (VI curve)

56. Diac negative resistance
In electronics, negative resistance (NR) is a
property of some electrical circuits and devices
in which an increase in voltage across the
device's terminals results in a decrease
in electric current through it

57. • Diac is mainly used in the triac triggering circuits.
• The main applications are:
• It can be used in the lamp dimmer circuit.
• It is used in the heat control circuit.
• It is used in the speed control of a universal motor.
• It is used with triac in series combination for triggering between the two
halves of the device.
Application of Diac
The diac is connected in the gate terminal of the triac.
When the voltage across the gate decreases below a
predetermined value, the gate voltage will be zero and
hence the triac will be turned off.

58. Diac Application
Dimmer Circuit
RC time constant

59. Effects of Temperature on IS

60. TRIAC

61. TRIAC

62. TRIAC
The TRIAC is a three terminal
semiconductor device for controlling
current. It gains its name from the
term TRIode for Alternating Current
TRIAC is a bidirectional device

63. • It has 5 layers of semiconductor
• It can control both positive and negative half cycles of AC signal input
• It is a bidirectional switch.
• The forward and reverse characteristics of TRIAC is similar to forward
characteristics of SCR device.
• Construction of TRIAC is equivalent to 2 separate SCR devices connected in
inverse parallel as shown in the figure
• Similar to the SCR, once the triac is fired into conduction, the gate will lose
all the control. At this stage, the TRIAC can be turned OFF by reducing
current in the circuit below the holding value of current
• The main demerit of TRIAC over SCR is that TRIAC has lower current
capabilities. Typically most of the TRIACs are available in ratings less than
40 Amp and at voltages up to 600 Volt.
TRIAC

64. SCR TRIAC
SCR stands for silicon controlled rectifier.
TRIAC stands for triode for Alternating
Current.
The SCR is unidirectional device. The TRIAC is bidirectional device.
It available in large ratings. It available in smaller ratings.
The SCR control DC power.
The TRIAC control DC as well as AC
power.
The SCR can be triggered by positive gate
voltage only.
The TRIAC can be triggered either by
positive or negative gate voltage.
In SCR only one mode of operation is
possible.
In TRIAC four different modes of
operation is possible.
It is more reliable. It is less reliable.
The SCR conduct current in one direction
only.
The TRIAC conduct current in both the
directions.
It needs two heat sink. It needs only one heat sink.
Difference between SCR and TRIAC

65. Difference between SCR and TRIAC

66. Triac Conduction Waveform
Triac Phase Control
Watch each rising edge

67. Gate firing Input
Cross Reference timing
Voltage at Load
Waveform diagram showing the method of
controlling the input source voltages to be
appeared at the load

68. ADVANTAGES DISADVANTAGES
•Can switch both halves of an AC
waveform
•Single component can be used for
full AC switching
•A TRIAC does not fire
symmetrically on both sides of the
waveform
•Switching gives rise to high level of
harmonics due to non-symmetrical
switching
•More susceptible to EMI problems
as a result of the non-symmetrical
switching
•Care must be taken to ensure the
TRIAC turns off fully when used
with inductive loads
Advantages and disadvantages of TRIAC

69. TRIACs are used in a number of applications. However they tend not to be
used in high power switching applications
Applications
TRIACs are still used for many electrical switching applications:
• Domestic light dimmers
• Electric fan speed controls
• Small motor controls
• Control of small AC powered domestic appliances

70. Power Transistors
100v. 10 Amp

71. Power Transistor
Specs from Data Sheet

72. • The current gain β of power bipolar transistors depends on the collector current IC.
• To raise the current gain, Darlington transistors are used.
• Darlington transistors consist of two transistors connecte the following configuration
that share a common collector.
• The total current gain of
the circuit equals the
product of the current
gain of both devices.
Darlington Power Transistors

73. • In order to block large voltage during “OFF” state a lightly doped
“collector drift region” is introduced between the moderately doped
base region and the heavily doped collector region.
• In order to maintain a large current gain “β” the emitter doping
density is made several orders of magnitude higher than the base
region.
• A power BJT has a vertically oriented alternating layers of n type and p
type semiconductor materials as shown in Fig
• The function of this drift region is similar to that in a Power Diode.
However, the doping density donation of the base region being
“moderate” the depletion region does penetrate considerably into the
base
• Therefore, the width of the base region in a power transistor can not be made as
small as that in a signal level transistor
• The thickness of the base region is also made as small as possible.
Darlington Power Working

74. Power BJT operates in four regions
• Cutoff region-Both BE and
CE junction must be reversed
biased.
• Active region-BE junction
must be forward biased and
CB reverse biased
• Quasi-saturation region-Both
forward biased.
• Hard-saturation region-Both
forward biased.
• Quasi saturation region is a new region in Power BJT
due to lightly doped (n-) drift region
V-I Characteristics curve

75. • SMPS(Switch mode power supply) commonly used in
computers.
• Final audio amplifier in stereo systems.
• Power amplifiers.
• DC to AC inverters.
• Relay and display drivers.
• AC motor speed controllers.
• Power control circuits.
APPLICATIONS OF POWER BJT

76. Insulated gate Bipolar
transistor (IGBT)
It is a modern power transistor with
high-voltage and high-current
capability with moderate forward
voltage drop.
IGBT is more like a thyristor(GTO)
but stays as transistor. Like a
thyristor, it has two transistor
structure but the turn-on and turn-
off is carried out by MOSFET across
its npn transistor.
Salient features:
High impedance gate like MOSFET
and requires small power to switch
Small on-state voltage like BJT
Able to block negative voltage
76
IGBT-Module with a rated current of 1.2
kA and a maximum voltage of 3.3 kV

77. The Unijunction Transistor (UJT)
The UJT consists of a a block of lightly-doped
(high resistance) n-material with a p-material
grown into its side. It is often used as a trigger
device for SCRs and triacs.
The UJT is a switching device; it is
not an amplifier. When the emitter
voltage reaches VP (the peak
point), the UJT “fires”, going
through the unstable negative
resistance region to produce a fast
current pulse.
B2
E
B1
Negative
resistance
IE
IVIP
VV
VP Peak
point
VE
Cutoff Saturation
Valley point

78. The Unijunction Transistor (UJT)
The equivalent circuit for a UJT shows that looks like a
diode connected to a voltage divider. The resistance of the
lower divider (r’B1) is inversely proportional to the emitter
current. When the pn junction is first
forward-biased, the junction
resistance of r’B1 suddenly appears
to drop, and a rush of current occurs.
–
r′B2
r′B1
E
B2
B1
+
VEB1
+ –
Vpn
–
+
VBB
ηVBB
IE
An important parameter is h, which is the
intrinsic standoff ratio. It represents the ratio
of r’B1 to the interbase resistance r’BB with no
current.

79. The Unijunction Transistor (UJT) Application
A circuit using a UJT to fire an SCR is shown. When the
UJT fires, a pulse of current is delivered to the gate of the
SCR. The setting of R1 determines when the UJT fires. The
diode isolates the UJT
from the negative part
of the ac. R1
VE
R2C
B
A
R
R
L
G
D
UJT
SCR
The UJT produces a fast,
reliable current pulse to the
SCR, so that it tends to fire
in the same place every
cycle.

80. The Programmable Unijunction Transistor (PUT)
The PUT is a 4-layer thyristor with a gate. It is
primarily used as a sensitive switching device.
The gate pulse can trigger a sharp increase in
current at the output.
A
G
K
The characteristic of a PUT is
similar to a UJT, but the PUT
intrinsic standoff ratio can be
“programmed” with external
resistors and the UJT has a fixed
ratio.
VAK (anode-to-cathode voltage)
VP
VV
IP IV0 IA
(anode current)

81. The Programmable Unijunction Transistor (PUT)
The principle application for a PUT is for driving SCRs and
triacs, but, like the UJT, can be used in relaxation oscillators.
For the circuit to oscillate, R1 must be large
enough to limit current to less than the valley
current (IV). The period of the oscillations is given
by:
1
1
ln
1
T R C
h


where
3
2 3
R
R R
h 

R3
G
K
A
R1 R2
R4
C
+VCC
F=1/T

82. The Programmable Unijunction Transistor (PUT)
1
1
ln
1
T R C
h


3
2 3
10 k
20 k + 10 k
R
R R
h

  
  
R3
G
K
A
R1 R2
R4
C
+VCC
0.01 Fm
220 k
10 k
20 k
27 
+20 V
What is intrinsic standoff ratio, and the period of the circuit?
0.33
  
1
220 k 0.01 μF ln
1 0.33
  

0.89 ms
What is the frequency? 1.12 kHz

83. Construction • An UJT is made up of n-type
silicon base to which p-type
emitter is embedded.
• The n-type base is lightly doped
,while the p-type is heavily
doped.
• The base is divided into two
parts:
1. base-one B1
2. base-two B2
UJT

84. Construction When the Voltage VBB is applied
across the two base .The
Voltage across the point A will
be:
VAB1 = VBB .RB1
RB1+RB2
ɳ= RB1 (intrinsic stand-off
ratio)
RB1+RB2
So,
VAB1 = ɳ. RB1
Equivalent Diagram of
UJT

85. Construction
The value of ɳ lies between 0.51
to 0.82.
The resistance RBB can be easily
measured with the help of can
multimeter by keeping the
emitter open circuited.
The resisitance of RB2 is less than
RB1 , because emitter is near to
B2 .
Circuit Symbol

86. V-I Characteristics of UJT
V-I characteristics of UJT

87. V-I Characteristics of UJT
Circuit of V-I characteristics

88. Crowbar protection is a fail-safe protection mechanism
which shorts circuits the output of a power supply under
failure conditions such as overvoltage. Crowbar
protection can also refer to a circuit which has its sole
purpose to cause a fuse to blow by subjecting it to high
current.
A fail-safe in engineering is a design feature or practice that in the event of a
specific type of failure, inherently responds in a way that will cause no or
minimal harm to other equipment, the environment or to people.