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