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Transcript of BEEE Notes
EEE101-Basic Electrical and Electronics Engineering
UNIT – 4
Power Semiconductor Devices
1. PN Junction diode
2. Zener diode
3. Bipolar Junction Transistors - BJTs
4. Metal Oxide Semiconductor Field Effect Transistors - MOSFETs
5. Insulated Gate Bi-polar Transistors - IGBTs
6. Silicon Controlled Rectifiers - SCRs,
7. Diode AC Switch – DIAC
8. TRIode AC Switch - TRIAC
9. Gate Turn Off Thyristors - GTOs;
10. Switch Mode Power Supply - SMPS
11. Pulse Modulation Techniques
Power Semiconductor Devices
Semiconductor theory
Introduction:
Depending on their conductivity, materials can be classified into three types as conductors,
semiconductors and insulators. Conductor is a good conductor of electricity. Insulator is a poor
conductor of electricity. Semiconductor has its conductivity lying between these two extremes. A
comparatively smaller electric field is required to push the electrons to make it conduct. At low
temperature virtually semiconductor behaves as an insulator. However at room temperature some
electrons move giving conductivity to the semiconductor. AS temperature increases its conductivity
increases hence it has negative temperature co-efficient.
Classification:
Intrinsic semiconductor: A pure semiconductor is called intrinsic semiconductor where even at room
temperature electron-hole pairs are created. Under the influence of electric field, total current through
the semiconductor is the sum of currents due to free electrons and holes.
Extrinsic semiconductor: Current conduction is increased by adding a small amount of impurity to
intrinsic semiconductors, so it becomes extrinsic semiconductors
PN Junction Diode
In a piece of semiconductor material, if one half is doped by P-type and the other half is doped by N-
type impurity, a PN junction is formed. The plane dividing the two halves or zones is called PN junction.
The N-type has high concentration of free electrons while P-type has high concentration of holes.
Therefore at the junction there is a tendency for the free electrons to diffuse over to the P-side and
holes to the N-side (process called diffusion). The net opposite charge in each layer prevents further
diffusion into that layer. Thus a barrier is set up near the junction which prevents further movement of
charge carriers. This is called as potential barrier (0.3V or germanium and 0.7 for silicon).
Under forward bias condition:
When positive terminal of battery is connected to the P-type and negative terminal to the N-type of the
PN junction diode, the bias applied is known as forward bias.
The applied positive potential repels the holes in the P-type region so that the holes move towards the
junction and the applied negative potential repels the electrons in the N-type region and the electrons
move towards the junction(When applied voltage VF is less than V0) and hence the forward current IF is
almost zero. Eventually when the applied potential is more than the internal barrier potential the
barrier will disappear and hence the holes cross the junction from P-type to N-type and the electrons
crss the junction in the opposite direction resulting in relatively large current flow in the external circuit.
Forward bias Reverse bias
A
C V
RL V RL
Reverse bias
region
Forward bias
region
Knee voltage or
cut-in voltage Reverse
Breakdown
voltage IR(μA)
IF(mA)
VF VR
P
N
A
C
Under reverse bias condition:
When the negative terminal of the battery is connected to the P-type and positive terminal is connected
to N-type of the PN junction, the bias applied is known as reverse bias.
Under this condition, holes form the majority carriers of P-side move towards the negative terminal of
the battery and electrons which form the majority carriers of the N-side are attracted towards the
positive terminal of the battery. Hence the width of the depletion region which is depleted of mobile
carriers increases. Thus the electric filed produced by applied reverse bias is in the same direction of
electric field and hence the barrier is increased. Therefore, theoretically no current should flow in the
external circuit. But in practice very small reverse current in the order of microamperes flows under
bias. This current is called as reverse saturation current. The magnitude of reverse saturation current
mainly depends upon junction temperature because the major source of minority carriers is thermally
broken covalent bonds.
For large reverse bias is applied, the free electrons from the N-type moving towards the positive
terminal of the battery acquire sufficient energy to move with high velocity to dislodge valence
electrons from semiconductor atom in the crystal. Thus large number of free electrons are formed
which is commonly called as avalanche of free electrons. This leads to the breakdown of junction
leading to very large reverse current. The reverse voltage at which the junction breakdown is known as
breakdown voltage.
Zener diode
When reverse voltage reaches breakdown voltage in a PN diode, the current through the junction and
power dissipated at the junction will be high. Such an operation is destructive and the diode gets
damaged. However, diodes can be designed with adequate power dissipation capability to operate in
the breakdown region. One such diode is Zener diode which is heavily doped than the ordinary diode.
The forward bias condition is same as the ordinary PN diode, but under reverse bias condition,
breakdown of the junction occurs and the breakdown voltage depends upon the amount of doping. If
the diode is heavily doped, depletion layer will be thin and consequently breakdown occurs at lower
reverse voltage, besides the breakdown voltage being sharp. Thus the breakdown voltage can be
selected with the amount of doping. When the reverse bias field across the junction is sufficiently high,
it may exert a strong force on bound electrons to tear them out from a covalent bond. Thus a large
number of electron – hole pairs will be generated through a direct rupture of the covalent bond thereby
resulting in large reverse current at the breakdown voltage. Though Zener breakdown occurs for lower
breakdown voltage and avalanche breakdown occurs for higher breakdown voltage, such diodes are
normally called Zener diode
Application
From the zener diode characteristics, under the reverse bias condition, the voltage across the diode
remains almost constant although the current through the diode increases. Thus the voltage across the
zener diode serves as a reference voltage. Hence the diode can be used as a voltage regulator.
The arrangement shown is useful when it is required to provide a constant voltage across a load
resistance RL where as the input voltage may be varying over a range. As shown, the zener diode is
reverse biased and as long as the input voltage does not fall below Vz, the voltage across the diode will
be constant and hence the load voltage will also be constant.
V RL
A
C
VZ Vo
Reverse bias
region
Reverse
Breakdown
voltage
VZ
VF VR
IF
(mA)
IR(μA)
Power Transistors
The transistors which are used as switching elements are operated in the saturation region resulting in a
low on – state voltage drop. The switching speed of modern transistors is much high. They are
extensively employed in dc – dc and dc – ac converters with inverse parallel-connected diodes to
provide bidirectional current flow. Transistors are normally used in low to medium power applications.
The power transistors can be classified broadly into five categories
1. Bipolar junction transistor (BJT)
2. Metal oxide semiconductor field – effect transistor (MOSFET)
3. Insulated gate bipolar transistors (IGBT)
4. Static induction transistor (SIT)
5. COOLMOS
We will see the first three in brief
Bipolar Junction Transistor (BJT)
A bipolar transistor is formed by adding a second p or n region to a pn junction diode. With two n
regions and one p region, two junctions are formed and it is known as an NPN-transistor. With two p
regions and one n region, it is called as PNP-transistor. The three terminals are named as collector,
emitter and base. A bipolar transistor has two junctions, collector-base junction(CBJ) and base-emitter
junction(BEJ).
For an NPN –type, the emitter side n – layer is made wide, the p – base is narrow and the collector side
n – layer is narrow and heavily doped. For a PNP – type, the emitter side p – layer is made wide, the n –
base is narrow, and the collector side p – layer is narrow and heavily doped.
����
(a) NPN Transistor (b) PNP Transistor
n
p
iC
iE
iB
C
B
E
n
Collector
Emitter
Base
p
n
iC
iE
iB
C
B
E
p
Collector
Emitter
Base
The transfer characteristics of a transistor is as shown There are three operating regions of a transistor:
cutoff, active and saturation.
In the cut-off region, the transistor is off or the base current is not enough to turn it on and both
junctions are reverse biased
In the active region, the transistor acts as an amplifier, where the base current is amplified by a gain and
the collector – emitter voltage decreases with base current. The CBJ is reverse biased and the BEJ is
forward biased.
In the saturation region, the based current is sufficiently high so that the collector – emitter voltage is
low, and the transistor acts as a switch. Both the junctions are forward biased.
Applying Kirchhoff’s law we get
BCEiii +=
(This equation is true regardless of the bias conditions of the junctions)
We define the parameter α as the ratio of the collector current to the emitter current
E
C
i
i=α or
CEii =α
Value of α ranges from 0.9 to 0.999.
Combining the above equations we get
EBii )1( α−=
We define another parameter β as the ratio of the collector current to the base current.
α
αβ
−==
1B
C
i
i
Value of β ranges from 10 to 1000. We can also rewrite the above equation as
IC
IE
IB
RC
VCE
VB
RB +
VCE
– +
VBE
–
Cutoff
VCC
IB
VCE
Active
Saturation
BCii β=
Note that since β is usually very large compared to unity, the collector current is an amplified version of
the base current.
The input and output characteristics of transistor is as shown
(a) Input characteristics (b) Output Characteristics
VBE
IB VCE1 VcE2
VCE2> VCE1
VCE
IBn>IB1> IB0
IC
IB0=0
IB1
IB2
IBn Active
region
Cutoff region
Satura-
tion
region
MOSFET (Metal Oxide Semiconductor Field Effect Transistor or Insulated Gate
Field Effect Transistor)
The MOSFET is a voltage controlled device that works on the depletion capacitor concept. In this a layer
of silicon dioxide is grown on the surface, which act as a dielectric media between gate and the channel.
Based on the channel created between the, the MOSFET is broadly divided as shown.
It has got three terminals, Gate, Drain and source
N-channel MOSFET consists of highly doped ‘P’ type substrate into which two highly doped N regions are
diffused. These ‘N’ regions act as source and drain. A thin layer of insulating silicon dioxide (SiO2) is
grown over the surface of structure and free electrons are cut into the oxide layer, allowing to move
between source and drain
The metal area is overlaid on the entire oxide layer and metal contacts are made to source and drain.
The SiO2 layer insulates the gate from the channel due to which a negligible gate current flows even if
the biasing is applied to gate. So no PN junction is existing in MOSFET and hence known as Insulated
Gate Field Effect Transistor.
Depletion Type:
The depletion type MOSFET can be operated in two different modes: a. depletion mode b.
enhancement mode
Circuit symbol and Circuit
The device operates in this depletion mode, when the gate voltage is negative.
MOSFET
N - Channel
Enhancement
type
P - Channel
Depletion
type
P - Channel N - Channel
++++++
n+ n+ - - - - - - - - - - - - -
Aluminium layer
Silicon layer
Induced n-channel
Source Gate
Drain
P - Substrate
N
VGS
VDS
G
S
D SiO2
Layer
P Drain
Substrate
Source
Gate
N - Channel
Drain
Substrate
Source
Gate
P - Channel
When VGS = 0, a significant current flows for a given VDS
When negative voltage is applied to gate, electrons accumulate on it. If one plate of capacitor (gate) is
negatively charged, induces a positive charge on the other plate. Because of this, free electrons in
vicinity of positive charge area repelled away in the channel
As a result of this, the channel is depleted of free electrons passing through the channel thus the
conduction between source to drain is reduced. Thus as the value of VGs is increased, the value of ID
decreases
The device operated in enhancement mode when the gate voltage is positive
When VGS > 0, the positive gate voltage increases the number of free electrons passing through the
channel. The greater the gate voltage, the greater is the number of free electrons passing through the
channel. This increases ie. Enhances the conduction of channel, this positive gate voltage operation of
MOSFET is called enhancement mode of MOSFET
Drain Characteristics of Depletion type MOSFET
When VDS = 0, no conduction takes place between source to drain. If VGS < 0, and VDS > 0, then drain
current increases upto a point of time when the drain current reaches saturation called pinch off point.
If VDS is increases above this, ID remains constant. For further increase in VDS, avalanche breakdown
occurs in pinch off region and the Drain current increases rapidly
When VGS > 0, the gate induces more electrons in channel side, it is added with the free electron
generated by source. Again the potential applied to gate determines the channel width and maintains
constant current flow in pinch off region as shown
Transfer Characteristics of Depletion type MOSFET
If VGS = 0, the device has a drain current equal to IDSS. Due to this fact only it is called normally – ON
MOSFET
In depletion mode, when VGS = 0, maximum current will flow between source to drain thus ID = IDSS.
When VGS is increased in negative side, after a certain extend the positive charges induced by gate
completely depletes the channel thus no drain current flows(point A)
ID(mA)
VDS(V)
VGS= –2V
VGS= –1V
VGS= 0V
VGS= 1V Enhance-
ment mode
Depletion
mode
Drain Characteristics
VGS(V) VGS(OFF)
A
ID(mA)
IDSS
B
C
Depletion
mode
Enhance-
ment mode
Transfer Characteristics
In enhancement mode when VGS is increased in positive side, more free electrons are induced in
channel, thus it enhances the electron resulting in increase of ID
Enhancement Type:
Circuit symbol and Circuit
The device operates in this mode, when the gate voltage is positive. The enhancement type MOSFET
has no depletion mode and it operates only in enhancement mode. If differs in construction from the
depletion mode MOSFET in the sense that it has no physical channel. It may be noted that the P type
substrate extends the silicon dioxide layer completely as shown.
The MOSFET is always operated with the positive gate to source voltage. When the VGS = 0, the VDS
supply tries to force free electrons from source to drain. But the presence of P region does not permit
the electrons to pass through it. Thus there is no drain current for VGS = 0. Due to this fact the
Enhancement type MOSFET is called Normally –OFF MOSFET
If some positive voltage is applied to the gate, it induces a negative charge in the P type substrate just
adjacent to the silicon dioxide layer. The induced negative charge produced which would be attracting
the free electrons from the source.
When the gate is positive enough it can attract more number of free electrons. This forms a thin layer
of electrons, which stretches form source to drain. This effect if equivalent to producing a thin layer of
N type channel in the P type substrate. This layer of free electrons is called N type inversion layer. The
minimum gate to source voltage which produces invertion layer is called Threshold voltage. When VGS
is less than threshold voltage no current flows form drain to source. However if VGS is greater than
threshold voltage, inversion layer connects the drain and source and we get significant values of current
Drain characteristics of Enhancement type MOSFET
When VDs = 0, ID = 0. The value of drain current increases with increase in gate to Drain to source
voltage upto saturation value (provided VGS > threshold voltage) after which drain current remains
almost constant value
N
P
N
VGS
VDS
G
S
D SiO2
Layer
ID(mA)
VDS(V)
VGS=Vm
VGS> Vm
Drain Characteristics
VGS(V) VGS(th)
ID(mA)
ID(ON)
Transfer Characteristics
Drain
Substrate
Source
Gate
N – Channel
Drain
Substrate
Source
Gate
P – Channel
Transfer characteristics of Enhancement Type MOSFET
When VGS < threshold voltage, there is no drain current. However in actual practice, an extremely small
value of drain current flows through MOSFET. This current flow is due to the presence of thermally
generated electrons in the P type substrate. When the value of VGs is kept above VGS(th) a significant
drain current flows as shown in figure.
Power MOSFET find increasing applications in low-power high-frequency converters.
IGBT (Insulated-gate bipolar transistors)
An IGBT combines the advantages of BJT and MOSFETs. An IGBT is a voltage controlled device that has
high input impedance like MOSFETs and low on – state conduction losses like BJTs. However the
performance of an IGBT is closer to that of a BJT than an MOSFET. This is due to the p+ substrate, which
is responsible for the minority carrier injection into the n – region.
The symbol and circuit of an IGBT switch is as shown. The three terminals are gate, collector and
emitter instead of gate, drain and source for an MOSFET. Like MOSFET, when the gate is positive with
respect to the emitter for turn – on, n carriers are drawn into the p-channel near the gate region. This
results in a forward bias of the base of the npn transistor, which there by turns on. An IGBT this is
turned on by just applying a positive gate voltage to open the channel for n carriers and is turned off by
removing the gate voltage to close the channel. Typical output characteristic and transfer characteristic
are as shown
(a) Output Characteristics (b) Transfer Characteristics
IGBT is finding increasing application in medium power applications such as DC and AC motor drives,
power supplies, solid state relays and contractors
IC
E
G
C
VCC
VG
RS
RD
RBE
E
C
G
VGE
IC
VCE
IC
VGE1 VGE2
VGE3
VGE5
VGE6
VGE7
VGE7> VGE6> VGE5
SCR (Silicon controlled Rectifier)
The SCR is a prominent member of thyristor family. It is so called because silicon is used for its
construction and its operation as a Rectifier can be controlled. It is widely used as switching device in
power control applications. It can switch ON for variable length of time and delivers selected amount of
power to load. It can control loads, by switching the current OFF and ON up to many thousand times a
second hence, it posses advantage of RHEOSTAT and a SWITCH with none of their disadvantages.
The SCR is a four layer, three junction dev ice the layers being alternatively P-type and N-type silicon,
whereas terminals are Anode (A), Cathode(C) and Gate(G). The gate terminal is connected to inner P
layer which is lightly doped and it controls the firing or switching of SCR. The anode is always at a higher
positive potential than the cathode and doping of anode and cathode layers is high.
The operation of SCR is explained by the help of four modes namely
1. Forward blocking mode
2. Forward conducting mode
3. Reverse blocking mode
4. Reverse conducting mode
A
C
G
P
N
P
N
A
C
G
P
N
P
N
A
C
G
PNP
Q1
NPN
Q2
A
C
G
IG
IC1
IE2
IE1
IB2
IC2
IB2
V
R Q1
Q2
CG
A
RL
VACVG
Rin
VBO VAC
IA
VBR
IG=0IG=1
Forward
leakage
Forward
blocking
Forward
conduction
Reverse
leakage
Reverse
conducting
Reverse blocking
Latching
current Holding
current
1. Forward blocking mode (OFF State)
When a positive Voltage is applied between anode A and cathode C of SCR, junctions J1 and J3
are forward biased and junction J2 is reverse biased. Even if forward voltage is applied between anode
and cathode, there is no flow of current from anode to cathode. This is because of junction J2. However
a small amount of current starts flowing from anode to cathode due to the existence of leakage carriers
in the junction. As the applied voltage starts increasing, at certain stage, J2 will undergo avalanche
breakdown and looses its’s blocking capability, thereby behaving as a conductor. So the voltage at which
junction J2 breakdown is called as forward break over voltage or threshold voltage or the critical point at
the avalanche breakdown designated by the letter VBO. When forward voltage is less than VBO, SCR
offers high impedance. In this mode thyristor can be treated as a open switch.
2. Forward Conducting Mode
As J2 breaks down, SCR acts like closed switch; thereby current flowing from anode to cathode increases
irrespective of voltage. When forward voltage becomes greater than VBO, SCR starts conducting and the
anode to cathode voltage decreases quickly to point B, because under this condition the SCR offers very
low resistance hence it drops very low voltage across it. The voltage drop across the SCR during ON state
is of the order of 1V to 2V depending on the rating of SCR. If the value of the gate current IG is increased
from zero, the SCR turns ON even at lower break over voltage. Once the SCR is switched ON then the
gate losses all the control. In the ON state, the anode current is limited by an external impedance or
resistance and it must be more than Latching current in order to maintain the required amount of
carrier flow across the junction.
Hence Latching current is the minimum amount of anode current that it must attain during turn ON
process to keep the SCR in conduction even when the gate signal is removed.
SCR cannot be turned OFF by varying the gate voltage. It is possible only by
1. Reducing the anode current below its holding current. Hence Holding current is the minimum
amount of anode current that it must fall below the normal value to bring the SCR from
conducting state to blocking state.
2. Application of reversal voltage
3. Reverse Blocking mode
When switch S open, if C is make positive with reference to A, junctions J1and J3 are reverse biased and
J2 is forward biased. Due to J2, no current starts flowing from C to A. However small amount of current
starts flows from C to A due to the existence of leakage carriers in the junction J2. If the reverse voltage
is increased, then at a critical breakdown level called Reverse breakdown voltage VBR an avalanche
occurs at J1 and J3 and reverse current increases rapidly, there by acting as conductor. The voltage at
which the junctions J1, J2 and J3 loose its reverse blocking capability is called a Reverse break over
voltage VBR. As the inner regions are lightly doped as compared to outer layers, the thickness of
depletion layer of J2 during forward bias condition will be greater than the total thickness of two
depletion layers at J2 and J3 when the device is reverse biased. Therefore VBO is greater than VBR
4. Reverse conducting mode
After the break over of junctions J1 and J3, SCR acts as a closed switch in the reverse direction, thereby
current flowing from cathode and anode increases irrespective of increasing in voltage. A large current
associated with VBR gives rise to more losses in the SCR outcoming in the form of heat, there by creating
possibility for damaging it. So, by the manufacturers warning, do not operate the SCR in reverse
conduction mode.
Two transistor analogy of SCR
The basic operation of SCR can be described by two transistor analogy. The SCR is split into two – three
layer transistors
• As shown Q1 is PNP and Q2 is NPN device interconnected back to back, ie the collector of one
transistor is connected to the base of the other transistor, thus it forms positive feedback and
the collector current of one transistor become base current of other transistor
• Suppose the supply voltage applied across terminals A and C is such that the reverse biased
junction J2 starts breaking down. Then current through the device increases. It means Ie1 begins
so increase, and hence IC1 (IC = αIE), Now IB2 increases since IC1 = IB2, and hence IC2 because IC = βIB.
As IC2 = IB1, now both IE1 and IB1 has increased which further increases IC1. Therefore there is a
regenerative or positive feed back effect. This particular action is called latching action or
regenerative action. Integral regeneration is not possible when the SCR is reverse biased.
Applications
1. Used as a static switch to replace the electromechanical relay
2. Used to control the amount of power delivered to the load
3. Used in power conversion and regulation circuits
4. Used for surge protection
GTO (Gate Turn OFF Thyristor)
A gate turn-off thyristor (GTO) is a special type of thyristor a high-power semiconductor device. GTOs,
as opposed to normal thyristors, are fully controllable switches which can be turned on and off by their
third lead, the GATE lead
Normal thyristors (SCR) are not fully controllable switches. Thyristors are switched ON by a gate signal,
but even after the gate signal is removed, the thyristor remains in the ON-state until any turn-off
condition occurs (which can be the application of a reverse voltage to the terminals, or when the current
flowing through (forward current) falls below a certain threshold value known as the "holding current").
Thus, a thyristor behaves like a normal semiconductor diode after it is turned on or "fired”.
The GTO can be turned-on by a gate signal, and can also be turned-off by a gate signal of negative
polarity
Turn on is accomplished by a "positive current" pulse between the gate and cathode terminals. As the
gate-cathode behaves like PN junction, there will be some relatively small voltage between the
terminals. The turn on phenomenon in GTO is however, not as reliable as an SCR and small positive gate
current must be maintained even after turn on to improve reliability.
Turn off is accomplished by a "negative voltage" pulse between the gate and cathode terminals. Some of
the forward current (about one-third to one-fifth) is "stolen" and used to induce a cathode-gate voltage
which in turn induces the forward current to fall and the GTO will switch off (transitioning to the
'blocking' state). To have an efficient control over gate – controlled turn – Off, the base drive of
transistor 2 must be minimum, that is IB2 must be minimum. This can be obtained by considering αnpn >>
αpnp. In order to obtain the above said condition, the GTO thyristor structure has a thicker n – base
region.
GTO thyristors suffer from long switch off times, whereby after the forward current falls, there is a long
tail time where residual current continues to flow until all remaining charge from the device is taken
away. This restricts the maximum switching frequency to approximately 1 kHz. It may however be
noted that the turn off time of a comparable SCR is ten times that of a GTO. Thus switching frequency of
GTO is much better than SCR
GTO thyristors are available with or without reverse blocking capability. Reverse blocking capability adds
to the forward voltage drop because of the need to have a long, low doped P1 region.
GTO thyristors capable of blocking reverse voltage are known as Symmetrical GTO thyristors,
abbreviated S-GTO. Usually, the reverse blocking voltage rating and forward blocking voltage rating are
the same. The typical application for symmetrical GTO thyristors is in current source inverters
GTO thyristors incapable of blocking reverse voltage are known as asymmetrical GTO thyristors,
abbreviated A-GTO. They typically have a reverse breakdown rating in the tens of volts. A-GTO thyristors
are used where either a reverse conducting diode is applied in parallel (for example, in voltage source
inverters) or where reverse voltage would never occur (for example, in switching power supplies or DC
traction choppers)
Advantages
1. It eliminates the external circuitary for switching off the thyristor
2. High speed operation
3. The switching frequency of GTO is much better than SCR.
Disadvantages
1. Larger gate current is required to turn – on
2. GTO suffers from long switch off time.
Applications
The main applications are in variable speed motor drives, high power, inverters and traction
DIAC (DIode AC switch)
A DIAC is a two terminal, three layer, bidirectional device which can be switched OFF state to ON state
for either polarity of applied voltage. It operates like two diodes connected in series. The basic
structure of DIAC is as shown. The two leads are connected to P – region of silicon chip separated by an
N – region. MT1 and MT2 are two main terminals by which the structure of the DIAC is interchangeable.
It is like a transistor which the following basic differences
1. There is no terminal attached to the base layer
2. The doping concentration are identical (unlike a bipolar transistor) to give the device
symmetrical properties
Operation
When a positive or negative voltage is applied the main terminals of a DIAC, only a small leakage current
IBO will glow through the device. If the applied voltage is increased, the leakage current will continue to
flow until the voltage reaches the break over voltage VBO. At this point, avalanche breakdown occurs at
the reverse – biased junction it may be J1 or J2, depending upon the supply connected between MT1 and
MT2 and the device then drops to break back’ voltage Vw as shown.
V- I Characteristics of DIAC
If the applied voltage (positive) is less than VBO a small leakage current IBO flows through the device.
Under this condition, the DIAC blocks the flow of current and effectively behaves as an open circuit. The
voltage VBO is the breakover voltage and usually has a range of 30 to 50 volts.
When the (positive or negative) voltage applied to DIAC is equal to or greater than the break over
voltage then DIAC begins to conduct, due to avalanche breakdown of the reverse biased junction and
the voltage drop across it becomes a few volt, result in which the DIAC current increases sharply and the
volt across the DIAC decreases. Thus the DIAC offers a negative resistance
Applications of DIAC
1. Light dimmer circuits
2. Heat control circuits
3. Universal motor speed control
P N
N
P N
MT2
MT1 MT1
MT2 VBO
IF
TRIAC (TRIode AC switch)
It is a 5 – layered 3 terminal “bidirectional device”, which can be triggered ON by applying either positive
or negative voltages, irrespective of the polarity of the voltage across the terminals A1, A2 and gate. It
behaves like two SCR’s connected in parallel and in opposite direction to each other with a common
gate. Because of the inverse parallel connection the two terminals cannot be identified as anode or
cathode. The anode and gate voltage applied in either direction will fire (ON) a TRIAC because it would
fire at least one of the two SCR’s which are in opposite directions.
Construction
It has three terminals A1, A2 and G. The G is closer to anode A1. It has six doped regions. The schematic
symbol of TRIAC is as shown
Operation.
• When positive voltage is applied to A2, with respect to A1 path of current flow in P1–N1–P2–N2.
The two junction P1 – N1 and P2 – N2 are forward biased whereas N1 – P2 junction is blocked.
The gate can be given either positive or negative voltage to turn ON the TRIAC
i) Positive gate: The positive gate forward biases the P2 – N2 junction and breakdown
occurs as in normal SCR
ii) Negative gate: A negative gate forward biases the P2 – N3 junction and current carriers
are injected into P2 to turn on the TRIAC
• When positive voltage is applied to anode A1, path of current flow is P2 – N1 – P1 – N4. The two
junctions P2 – N1 and P1 – N4 are forward biased whereas junction N1 – P1 is blocked. Conduction
can be achieved by applying either positive or negative voltage to G.
G
A2
A1
P1 N4
N1
P2 N2 N3
A2
G A1
G
A1
A2
VBO VAC
IA
VBo
IG=0IG=1
Forward
leakage
Forward
blocking
Forward
conduction
Reverse
leakage
Reverse
conductiing
Reverse blocking
Latching
current Holding
current
i) Positive gate: The positive gate injects current carriers by forward biasing the P2 – N2
junction and thus initializes the conduction
ii) Negative gate: A negative gate injects current carriers by forward biasing P2 – N3
junction there by triggering conduction, thus there are four TRIAC triggering modes, two
for each of the anodes.
V- I characteristics
• As seen in SCR, TRIAC exhibits same forward blocking and forward conducting characteristics
like SCR but for either polarity of voltage applied to terminal (A1 or A2). TRIAC has latch current
in either direction hence the switching ON is effected by raising the applied voltage to breakover
voltage. The TRIAC can be made to conduct in either direction. No matter what bias polarity,
characteristic of TRIAC are those of forward biased SCR.
• If the applied voltage of one of the main terminal is increased above zero, a very small current
flows through the device, under this condition the TRIAC is OFF, it will be continued until the
applied voltage reaches the forward breakover voltage
• If the anode to cathode voltage exceeds the breakover voltage, the SCR turns ON and anode to
cathode voltage decreases quickly to point ‘B’, because under this condition the SCR offers very
low resistance hence it drops very low voltage across it. At this stage the SCR allows more
current to flow through it, the amplitude of the current is depending upon the supply voltage
and load resistance connected in the circuit
• The same procedure is repeated for forward blocking state with the polarity of main terminals
interchanged.
Applications
TRIAC is a bidirectional device hence it is used in many industrial applications such as i) phase
control ii) heater control iii) light dimmer control iv) speed control of motors. It is also used to
control ac power to a load by switching ON and OFF during positive and negative half cycle of input
ac power.
Power Conditioning equipments
All electronic circuits need DC power supply either from battery or power back units. It may not
be economical and convenient to depend upon battery power supply. Hence, many electronic
equipment contain circuits which convert the AC supply voltage into DC voltage at the required level.
The unit containing these circuits is called the Linear Mode Power Supply (LPS). In the absence of AC
main supply, the DC supply from battery can be converted into required AC voltage which may be used
by computer and other electronic systems for their voltage which may be used by computer and other
electronic systems for their operation. Also, in certain applications, DC to DC conversion is required.
Such a power supply unit that converts DC into AC or DC is called Switched Mode Power Supply (SMPS)
Switch Mode Power Supply (SMPS)
The SMPS operating from mains, without using an input transformer at line frequency 50
Hz is called “off – line switching supply” in which the AC mains is directly rectified and filtered and the
DC voltage so obtained is then used as an input to a switching type DC to DC converter.
In a switching power supply, the active device that provides regulation is always operated in a
switched mode, i.e it is operated either in cut – off or in saturation. The input DC is chopped at a high
frequency using an active device like BJT, power MOSFET or SCR and the converter transformer. The
transformed chopped waveform is rectified and filtered. A sample of the output voltage is used a s the
feedback signal for the drive circuit for the switching transistor to achieve regulation.
The main feature of SMPS is the elimination of physically massive power transformers and other power
line magnetic. The net result is a smaller, lighter package and reduced manufacturing cost, resulting
primarily from the elimination of the 50Hz components
Control
element
Output
Unregulated input Control
element
Sampling
network
Voltage
reference
Oscillator
Err
AMP
+
–
Pulse Modulation Techniques
In order to transmit a large number of signals simultaneously through a single channel in an
efficient manner, pulse modulation techniques are employed. Pulse modulation techniques yield better
signal to nose rations at the receiving end and hence they are highly immune to noise.
Here, a train of rectangular pulses is considered to be a carrier signal. In Pulse modulation
technique, the continuous waveform of the message signal is sampled at regular intervals. Information
regarding the message signal is transmitted only at the sampling times. Hence for proper recovery of
the message signal at the receiving end, the sampling rate should be greater than a specified value
which is given by the sampling theorem
There are totally four types of pulse modulation. They are
1. Pulse Amplitude modulation (PAM)
2. Pulse Time Modulation (PTM)
3. Pulse Width Modulation (PWM)
4. Pulse Position Modulation (PPM)
Pulse Width Modulation (PWM)
PWM is also called as Pulse Duration Modulation (PDM) or Pulse Length Modulation (PLM). In
PWM as shown, the amplitude and starting time of each pulse is fixed, but the width of each pulse is
made proportional to the amplitude of signal at that instant. The pulses of PWM are of varying width
and therefore of varying power content. Even if synchronization between transmitter and receiver fails,
PWM still works whereas PPM does not
Modulating wave
Pulse Carrier
PWM Wave