Unit i

Rama Kishore Bonthu Associate Professor

E-mail: [email protected]

POWER ELECTRONICS

Power Semi Conductor Devices :

Silicon controlled rectifier (SCR) was first introduced in 1957 as a power semi

conductor device. Since then, several other power semi conductor devices have been developed.

SCR --- Silicon Controlled Rectifier

LASCR --- Light Activated SCR

ASCR --- Asymmetrical SCR

RCT --- Reverse Conducting Thyristor

GTO --- Gate-Turn off Thyristor

SITH ---- Static Induction Thyristor

MCT ----- MOS controlled Thysristor

BJT ---- Bipolar Junction Thyristor

MOSFET ---- Metal-Oxide Semiconductor Field Effect Transistor

SIT --- Static Induction Transistor

IGBT --- Insulated gate bilpolar transistor



Based on (i)Turn-on and Turn-off characterstics (ii) gate signal requirements and (iii) degree of controllability, The

power semiconductor devices can be classified as under :

(a) Diodes : These are uncontrolled rectifieng devices. Their on and off states are controlled by power supply.

(b) Thyristors : These have controlled turned-on by a gate signal. After Thyristors are on, They remain in latched-in on

state due to internal regenerative action and gate loss control. These can be turned off by power circuit.

(c) Controllable Switches: These devices are turned-on and turned-off by the application of control signals. Ex: BJT,

MOSFET, GTO, SITH, IGBT, SIT and MCT.

Triac and RCT possess bi-directional current capability where as all other remaining devices (diode, SCR, GTO, BJT,

MOSFET, IGBT, SIT, SITH, and MCT) are unidirectional current devices.

TYPES OF POWER ELECTRONIC CONVERTERS:

A power Electronic Converter is made up of some power semiconductor devices controlled by integrated circuits. There

are six types of power electronic converters as under:



1. Diode Rectifiers: It coverts ac input voltage(1-φ or 3-φ) into a fixed dc voltage. We find diode rectifiers in electric

traction, battery charging, electro plating, electrochemical processing, power supplies, welding and uninterruptible power

supply (UPS) systems.

2. Ac to dc converters (Phase-controlled rectifiers): These convert constant ac voltage to variable dc output voltage.

These rectifiers use line voltage for their commutation, as such these are also called line-commutated or naturally

commutated ac to dc converters. These are used in drives, metallurgical and chemical industries, excitation systems for

synchronous machines etc.

3. DC to dc Converters (DC choppers) : These convert fixed dc input voltage to controllable dc output voltage. The

chopper circuits require forced or load commutation to turn-off the thyristors. Choppers find wide applications in dc

drives, subway cars, trolley trucks, battery driven vehicles etc.

4. DC to ac converters (Inverters): An inverter converts fixed dc voltage to a variable ac voltage. The output may be a

variable voltage and variable frequency. These converters use line, load or forced commutation for turning-off the

thyristors. Inverters find wide use in induction motor and synchronous-motor drives, induction heating, UPS, HVDC

transmission etc.

5. AC to ac converters: These convert fixed ac input voltage into variable ac output voltage. These are of two types :

(a) AC voltage controllers (AC voltage regulators): These convert fixed ac voltage directly to a variable ac voltage at the

same frequency. These are widely used for lighting control, speed control of fans, pumps etc.

(b) Cycloconverters: These convert input power at one frequency to output power at different frequency through one stage

conversion. These are used for low-speed large ac drives like rotary kiln etc.

6. Static Switches : The power semiconductor devices can operate as static switches or contactors. More beneficial than

circuit breakers.

POWER TRANSISTORS:

Power diodes are uncontrolled devices. Their turn-on and trun-off characteristics are not under control.

Power transistors, however, possess controlled characteristics. These are turned on when a current signal is given to base,

or control, terminal. When this control signal is removed, a power transistor is turned off.

Power Transistors are of four types as under:

(i) Bipolar Junction Transistors (BJTs)

(ii) Metal-Oxide-semiconductor field effect transistor (MOSFETs)

(iii) Insulated gate bipolar transistors (IGBTs)

(iv) Static Induction transistors (SITs)

POWER MOSFETS:

Power MOSFETs are of two types; n-channel enhancement MOSFET and p-channel enhancement

MOSFET. Out of these two types, n-channel enhancement MOSFET is more common in use because of higher mobility of

electrons. A power MOSFET has three terminals called drain (D), Source (S) and gate (G). The circuit symbol of n-

channel power MOSFET is as shown in below. Here arrow indicates the direction of electron flow. Power MOSFET, is a

voltage controlled device, is a unipolar device.



On p-substrate, two heavily doped n+ regions are

diffused. An insulating layer of silicon dioxide (SiO2)

is grown on the surface. Source and drain terminals are

embedded in the silicon layer, and in contact with n+

regions as shown above figure. A layer of metal is also

deposited on SiO2 layer so as to form the gate of

MOSFET in between source and drain terminals.

When gate circuit is open, junction between n+ region

below drain and p-substrate is reverse biased by input

voltage VDD . Therefore no current flows from drain to

source and load.

When gate is made positive with respect to source, an electric field is established as shown in above figure. Eventually,

induced negative charges in the p-substrate below SiO2 layer are formed thus causing the p layer below gate to become an

induced n layer. These negative charges, called electrons, form n-channel between two n+ regions and current can flow

from drain to source as shown by the arrow. If VGS is made more positive, induced n-channel becomes deeper and

therefore more current flows from D to S. This shows that drain current ID is enhanced by the gradual increase of gate

voltage, hence the name enhancement MOSFET.

In the above figure, the main disadvantage is that conducting n-channel in between drain and sources gives large on-state

resistance. This leads to high power dissipation in n-channel. So that above planar MOSFET construction is feasible only

for low-power MOSFETs.

The below diagram represents the construction of high power MOSFET. It is also known as planar diffused metal-oxide-

semiconductor FET(DMOSFET). On n+ substrate, high resistivity n- layer is grown. The thickness of n- layer determines

the voltage blocking capability of the device. On the other side of n+ substrate, a metal layer is deposited to form the drain

terminal. Now p-regions are diffused in the grown n- layer. Further n+ regions are diffused in p-regions as shown. As

before, SiO2 layer is added, in that metallic source and drain terminals are embedded.

When gate circuit voltage is zero, and VDD is present, n- - p junctions are reverse biased and no current flows from drain to

source. When gate terminal is positive with respect to source, an electric field is established and electrons from n-channel

in the p- regions as shown. With gate voltage is increased, current ID also increases. Length of n-channel can be controlled

and therefore on-resistance can be made low if short length is used for the channel.

In the above figure source is negative and drain is positive. Therefore, electrons flow from source to n+ layer, then through

the n-channel of p-layer and further through n- and n+ layers to drain. The current must flow opposite to the flow of

electrons as indicated in above figure.



PMOSFET Characteristics: The basic circuit diagram for obtaining static characteristics of n-channel PMOSFET, is

shown below. In that the source terminal is taken as common terminal, between in the input and output of a MOSFET.

Transfer Characteristics

Output Characteristics

a) Transfer Characteristics: This characteristic shows the variation of the drain current ID as a function of the gate-

source voltage VGS. VGST is the minimum positive voltage between gate and source to induce n-channel. Thus, for

threshold voltage below VGST , device is in the off state. VGST value is order of 2 to 3 volts.

b) Output Characteristics: It indicates the variation of drain current ID as a function of drain source voltage VDS, with

gate-source voltage VGS as a parameter. For low values of VDS, the graph between ID – VDS is almost linear. This indicates

that constant value of on-resistance RDS = VDS/ID . For given VGS, if VDS is increased, output characteristic is relatively flat.

A load line intersects the output characteristics at ‘A’ and ‘B’. Here ‘A’ indicates fully on condition and ‘B’ fully off-state.

PMOSFET operates as a switch either at ‘A’ or at ‘B’.

When PMOSFET is driven with large gate-source voltage VGS, PMOSFET is turned on, VDS.ON is small. Here PMOSFET

is acted as closed switch(turn-on), driven from cut-off, to active region and then to ohmic region. When PMOSFET is

turned-off, it takes backward journey from ohmic region to cut-off state.

Switching Characteristics:

The switching characteristics of a power MOSFET are

influenced to a large extent by the internal capacitance

of the device and the internal impedance of the gate drive

circuit. At turn-on, there is an initial delay tdr, during which

input capacitance charges to gate threshold voltage VGST.

Here tdn. is called turn-on delay time.

There is further delay tr, called rise time, during which gate

voltage rises to VGSP, a voltage sufficient to drive the

MOSFET into on state. During t r., drain current rises from

zero to full on current ID. Thus, the total turn-on time is ton

= tdn+ tr. The turn-on time can be reduced by using low-

impedance gate drive source.



As MOSFET is a majority carrier device, turn-off process is initiated soon after removal of gate voltage at

time t 1 . The turn-off delay time, td f , is the time during which input capacitance dishrages from overdrive

gate voltage V1 to VGSP. The fall time, t f is the time during which input capacitance discharges from V GSP to

threshold voltage. During t f, drain current falls from ID to zero. So when VGS≤VGST, MOSFET turn-off is

complete

Power MOSFETs are very popular in switched mode power supplies and inverters. They are, at present, available

with 500 V, 140 A rating's..

INSULATED GATE BIPOLAR TRANSISTOR (IGBT):

IGBT has been developed by combining into it the best qualities of both BJT and PMOSFET. Thus an IGBT possesses

high input impedance like a PMOSFET and has low on-state power loss as in a BJT. Further, IGBT is free from second

breakdown problem present in BJT. All these merits have made IGBT very popular amongst power-electronics engineers.

Basic structure:

Below figure illustrates the basic structure of IGBT. It is

constructed virtually in the same manner as a power

MOSFET. There is, however a major difference in the

substrate.

The n+ layer substrate at the drain in a PMOSFET is not

substituted in the IGBT by a p+ layer substrate called

collector C. Like a power MOSFET, an IGBT has also

thousands of basic structure cells connected

appropriately on a single chip of silicon. In IGBT, p+

substrate is called injection layer because it injects holes

into n- layer. The n- layer is called drift region. As in

other semiconductor devices, thickness of n- layer

determines the voltage blocking capability of IGBT. The

p layer is called body of IGBT. The n- layer in between

p+ and p regions serves to accommodate the depletion

layer of pn- junction, i.e. junction J2.

Working:

When collector is made positive with respect to emitter, IGBT gets forward biased. With

no voltage between gate and emitter, two junctions between n - region and p region (i.e. junction J2) are reverse

biased; so no current flows from collector to emitter.

When gate is made positive with respect to emitter by voltage VG, with gate-emitter voltage more than the

threshold voltage VGET of IGBT, an n-channel or inversion layer, is formed in the upper part of p region just

beneath the gate. This n-channel short-circuits the n region with n+ emitter regions.Electrons from the n+ emitter

begin to flow to n- drift region through n-channel. As IGBT is forward biased with collector positive and

emitter negative, p+ collector region injects holes into n- drift region. In short, n- drift region is flooded with

electrons from p-body region and holes from p+ collector region. With this, the injection carrier density in n - drift

region increases considerably and as a result, conductivity

of n- region enhances significantly. Therefore, IGBT gets turned on and begins to conduct forward current IC.

Current IC, or IE, consists of two current components : (i) hole current Ih due to injected holes flowing from

collector, p+n-p transistor Q1, p-body region resistance Rby and emitter and (ii) electronic current Ie due to injected

electrons flowing from collector, injection layer p+, drift region n, n-channel resistance Rth, n+ and emitter.



This means that collector, or load, current IC = emitter current IE = Ih + Ie

Major component of collector current is electronic current Ie, i.e. main current path for collector, or load,

current is through p+, n, drift resistance Rd and n-channel resistance Rch. Therefore, the voltage drop in IGBT in

its on-state is

VCE.on = IC.Rch + IC.Rd + Vj1

VCE.on = Voltage drop[in n-channel+across drift in n- region + across forward biased p+n- junction J1]

IGBT Characteristics:

The above circuit shows the various parameters pertaining to IGBT characteristics. Static I-V or output

characteristic shows the variation of collector current IC as a function of collector-emitter voltage VCE, with gate-

emitter voltage VGE as parameter. When the device is off, Junction J2 blocks forward voltage, and in case reverse

voltage appears across collector and emitter, junction J 1 blocks it. VRM is the maximum reverse breakdown

voltage.

The transfer characteristic of an IGBT is a plot of collector current IC versus gate-emitter voltage VGE. When VGE is

less than the threshold voltage VGET , IGBT is in the off-state.

When the device is off, junction J2 blocks forward voltage and in case reverse voltage appears across collector

and emitter, junction J1 blocks it.

Switching Characteristics:

Switching characteristics of an IGBT during

turn-on and turn-off are sketched in below.

The turn-on time is defined as the time

between the instants of forward blocking to

forward on-state. Turn-on time is

composed of delay time td , and rise time

t r , i .e. ton= tdn+ tr. The delay time is defined as

the time for the collector-emitter voltage to fall

from VCE to 0.9 V. Here VCE is the initial

collector-emitter voltage. Time tdn may also

be defined as the time for the collector

current to rise from its initial leakage current

ICE to 0.1IC. Here IC is the final value of

collector current.



The rise time tr is the time during which collector-emitter voltage falls from 0.9 VCE to 0.1 VCE It is also

defined as the time for the collector current to rise from 0.1 I c to its final value I. After time ton, the collector

current is Ic and the collector-emitter voltage falls to small value called conduction drop = VCES, where

subscript S denotes saturated value.

The turn-off time is somewhat complex. It consists of three intervals : (i) delay time, tdf (ii) initial fall time, t f1

and (iii) final fall time, tf2 ; i.e. toff = tdf + tf1.+ tf2.The delay time is the time during which gate voltage falls from

VGE to threshold voltage VGET. As VGE falls to VGET during tdf, the collector current falls from Ic to 0.9 IC. At the

end of tdf, collector-emitter voltage begins to rise. The first fall time t fi is defined as the time during which

collector current falls from 90 to 20% of its initial value i c, or the time during which collector-emitter voltage rises

from VCES to 0.1 VCE.

The final fall time t t2 is the time during which collector current falls from 20 to 10% of or the time during

which collector-emitter voltage rises from 0.1 VCE to final value VCE.

Applications of IGBT

IGBTs are widely used in medium power applications such as dc and ac motor drives, UPS systems, power

supplies and drives for solenoids, relays and contactors. Though. IGBTs are somewhat more expensive than

BJTs, yet they are becoming popular because of lower gate-drive requirements, lower switching losses and

smaller snubber circuit requirements. IGBT converters are more efficient with less size as well as cost, as

compared to converters based on BJTs. Recently, IGBT inverter induction-motor drives using 15-20 kHz

switching frequency are finding favour where audio-noise is objectionable. In most applications, IGBTs will

eventually push out BJTs. At present, the state of the art IGBTs are available upto 1200 V, 500 A.

Comparison of IGBT with MOSFET:

The relative merits and demerits of IGBT over PMOSFET are enumerated below :

(i) In PMOSFET, the three terminals are called gate, source, drain whereas the corresponding terminals for

IGBT are gate, emitter and collector.

ii) Both IGBT and PMOSFET possess high input impedance.

iii) Both are voltage-controlled devices.

iv) With rise in temperature, the increase in on-state resistance in PMOSFET is much pronounced than it is in IGBT. So,

on-state voltage drop and losses rise rapidly in PMOSFET than in IGBT, with rise in temperature.

v) With rise in voltage rating, the increment in on-state voltage drop is more dominant in PMOSFET than it is in IGBT.

This means IGBTs can be designed for higher-voltage ratings than PMOSFETs.

In view of the above comparison, (a) PMOSFETs are available upto about 500 V, 140 A ratings whereas state of the art

IGBTs have 1200 V, 500 A ratings and (b) operating frequency in PMOSFETs is upto about 1 MHz whereas its value is

upto about 50 kHz in IGBTs.

THYRISTORS

As stated before, Bell Laboratories were the first to fabricate a silicon-based semiconductor device called thyristor. whole

family of semiconductor devices is given the name thyristor. Thus the term thyristor denotes a family of semiconductor

devices used for power control in dc and ac systems. One oldest member of this thyristor family, called silicon-controlled

rectifier (SCR), is the most widely used device. At present, the use of SCR is so vast that over the years, the word thyristor has

become synonymous with SCR.



A thyristor has characteristics similar to a thyratron tube. But from the construction view point, a thyristor (a pnpn device)

belongs to transistor (pnp or npn device) family. The name `thyristor', is derived by a combination of the capital letters from

THYRatron and transISTOR.

TERMINAL CHARACTERISTICS OF THYRISTORS

Thyristor is a four layer, three-junction, p-n-p-n semiconductor switching device. It has three terminals ; anode, cathode and

gate. Basically, a thyristor consists of four layers of alternate p-type and n-type silicon semiconductors forming three junctions

J1, J2 and J3 as shown in fig.(a). The threaded portion is for the purpose of tightening the thyristor to the frame or heat sink

with the help of a nut. For large current applications, thyristors need better cooling ; this is achieved to a great extent by

mounting them onto heat sinks. Gate terminal is usually kept near the cathode terminal. Schematic diagram and circuit

symbol for a thyristor are shown respectively in Figs. (b) and (c). The terminal connected to outer p region is called anode

(A), the terminal connected to outer n region is called cathode(K) and that connected to inner p region is called the gate (G).

An SCR is so called because silicon is used for its construction and its operation as a rectifier (very low resistance in

the forward conduction and very high resistance in the reverse direction) can be controlled. Like the diode, an SCR is

an unidirectional device that blocks the current flow from cathode to anode. Unlike the diode, a thyristor also

blocks the current flow from anode to cathode until it is triggered into conduction by a proper gate signal between gate

and cathode terminals.

SCRs of voltage rating 10 kV and an RMS current rating of 3000 A with corresponding power-handling capacity

of 30 MW are available.

Static I-V Characteristics of a Thyristor:

The circuit diagram for obtaining static I-V characteristics of thyristor as shown above.The anode and cathode are

connected to main source through the load. The gate and cathode are fed from a source E., which provides positive

gate current from gate to cathode.

Here Va is the anode voltage across thyristor terminals A, K and Ia is the anode current. According to SCR I-V

characteristics, a thyristor has three basic modes of operation ; namely, reverse blocking mode, forward blocking

(off-state) mode and forward conduction (on-state) mode.



Reverse Blocking Mode:

When cathode is made positive with respect to anode with switch S open, Fig. 4.2 (a),

thyristor is reverse biased. Junctions J1, J3 are seen to be reverse biased whereas junction J2

is forward biased. A small leakage current of the order of a few milliamperes flows. This

is reverse blocking mode, called the off-state, of the thyristor. If the reverse vol tage

is increased, then at a cr i t ical breakdown level, called reverse breakdown voltage V B R ,

an ava lanche occurs a t J 1 and J 3 and the reverse current increases rapidly. A large

current associated with VBR

gives rise to more losses in the SCR. This may lead to thyristor damage as temperature may exceed its

permissible temperature rise. It should, therefore, be ensured that maximum working reverse voltage

across a thyristor does not exceed VBR.. Reverse avalanche region is shown by PQ.

Forward Blocking Mode:

When anode is positive with respect to the cathode, with gate circuit open, thyristor

is said to be forward biased and junctions J1, J3 are forward biased but junction J 2 is

reverse biased. In this mode, a small current, called forward leakage current, flows. In

case the forward voltage is increased, then the reverse biased junction J2 will have an

avalanche breakdown at a voltage called forward breakover voltage VBO. When forward

voltage is less than VBO, SCR offers a high impedance. Therefore, a thyristor can be treated as

an open switch even in the forward blocking mode.

Forward Conduction Mode :

In this mode, thyristor conducts currents from anode to cathode with a very small voltage drop across it. A thyristor is

brought from forward blocking mode to forward conduction mode -by turning it on by exceeding the forward breakover

voltage or by applying a gate pulse between gate and cathode. In this mode, thyristor is in on -state and behaves

like a closed switch. Voltage drop across thyristor in the on state is of the order of 1 to 2 V depending on the rating

of SCR. This voltage drop increases slightly with an increase in anode current. In conduction mode, anode current

is limited by load impedance alone as voltage drop across SCR is quite small. This small voltage drop VT across

the device is due to ohmic drop in the four layers.

THYRISTOR TURN-ON METHODS

With anode positive with respect to cathode, a thyristor can be turned on by any one of the following techniques



(a) Forward voltage triggering

(b) gate triggering

(c) dv/ dt triggering

(d) Temperature triggering

(e) Light triggering.

(a) Forward Voltage Triggering : When anode to cathode forward voltage is increased with gate circuit open,

the reverse biased junction J2 will break. This is known as avalanche breakdown and the voltage at which

avalanche occurs is called forward breakover voltage VBO. At this voltage, thyristor changes from off -state to

on-state. As other junctions J1, J3 are already forward biased, breakdown of junction J2 allows free movement of

carriers across three junctions and as a result, large forward anode-current flows. As stated before, this forward

current is limited by the load impedance. In practice, the transition from off-state to on-state obtained by exceeding

VB0 is never employed as it may destroy the device.

(b) Gate Triggering : Turning on of thyristors by gate triggering is simple, reliable and efficient. when turn-

on of a thyristor is required, a positive gate voltage between gate and cathode is applied. With gate current thus

established, charges are injected into the inner p layer and voltage at which forward breakover occurs is

reduced. The forward voltage at which the device switches to on-state depends upon the magnitude of gate

current. Higher the gate current, lower is the forward breakover voltage.

When posit ive gate current is applied, gate P layer is flooded with electrons from the cathode. This is because

cathode N layer is heavily doped as compared to gate P layer. As the thyristor is forward biased, some of these

electrons reach junction J2. As a result, width of depl e t i on laye r a round junct ion J2 i s r e duc ed . T h i s

c au ses t h e j u nc t i on J2 t o breakdown at an applied voltage lower than forward breakover voltage VBO. If

magnitude of gate current is increased, more electrons wil l reach junct ion J2, as a consequence thyristor

will get turned on at a much lower forward applied voltage.

Fig. (a) shows that for gate current Ig=0,

forward breakover voltage is VBO. For

Ig1, forward breakover voltage, or turn-on

voltage is less than VB0. For Ig2 > Ig1,

forward breakover voltage is still

further reduced. The effect of gate

current on the forward breakover

voltage of a thyristor can also be

illustrated by means of a curve as

shown in Fig.(b).

Typical gate current magnitudes are of the order of 20 to 200 mA.

Once the SCR is conducting a forward current, reverse biased junction J2 no longer exists. As such, no gate

current is required for the device to remain in on -state.

latching current may be defined as the minimum value of anode current which it must attain during turn-on

process to maintain conduction when gate signal is removed.

Once the thyristor is conducting, gate loses control. The thyristor can be turned -off only if the forward

current falls below a low-level current called the holding current, Thus holding current may be defined as the

minimum value of anode current below which it must fall for turning -off the thyristor.



(c)dv/dt Triggering : With forward voltage across the anode and cathode of a thyristor, the two outer junction of J1, J3

are forward biased, but inner junction J2 is reverse biased. This junction J2 has the characteristics of a capacitor due to

charges existing across the junction. If forward voltage Va is suddenly applied, a charging current ic through junction

capacitance Cj may turn on the SCR. Then charging current is

If the rate of rise of forward voltage dVa/dt is high, the charging current would be more. This charging current plays the

role of gate current and turns on the SCR even though gate signal is zero.

(d) Temperature Triggering : During forward blocking, most of the applied voltage appears across

reverse biased junction J2. This voltage across junction J2 associated with leakage current may raise the

temperature of this junction. With increase in temperature, leakage current through junction J2 further increases.

This cumulative process may turn on the SCR at some high temperature.

(e) Light Triggering:

For light-triggered SCRs, a recess (or niche) is made in the inner p-layer. When this

recess is irradiated, free charge carriers (holes and electrons) are generated just like

when gate signal is applied between gate and cathode. If the intensity of this light

thrown on the recess exceeds a certain value, forward-biased SCR is turned on. Such a

thyristor is known as light-activated SCR (LASCR). Light-triggered thyristors have

now been used in high-voltage direct current (HVDC) transmission systems.

In these several SCRs are connected in series-parallel combination and their light-

triggering has the advantage of electrical isolation between power and control circuits.

SWITCHING CHARACTERISTICS OF THYRISTORS:

During turn-on and turn-off processes, a thyristor is subjected to different voltages across it and different currents

through it. Here, first switching characteristics during turn -on are described and then the switching

characteristics during turn-off.

Switching Characteristics during Turn-on:

A forward-biased thyristor is usually turned on by applying a positive gate voltage between gate and cathode.

There is, however, a transition time from forward off -state to forward on state. This transition time called

thyristor turn-on time, is defined as the time during which it changes from forward blocking state to final on-state.

Total turn-on time can be divided into three intervals ; (i) delay time td, (ii) rise time tr and (iii) spread time tp,

(i) Delay time td : The delay time td is measured from the instant at which gate current reaches 0.9 I to the

instant at which anode current reaches 0.14. Here I g and Ia are respectively the final values of gate and

anode currents. The delay time may also be defined as the time during which anode voltage falls from Va to 0.9Va where

Va = initial value of anode voltage. Another way of defining delay time is the time during which anode current rises from

forward leakage current to 0.1 Ia whereIa = final value of anode current. With the thyristor initially in the forward

blocking state, the anode voltage is OA and anode current is small leakage current as shown in below.



As gate current begins to flow from gate to cathode with the application of gate

signal. during delay time td, anode current flows in a narrow region near the gate

where gate current density is the highest.The delay time can be decreased by

applying high gate current and more forward voltage between anode and cathode.

The delay time is fraction of a microsecond. The Above figure represents

Distribution of gate and anode currents during delay time. In that circles

represents conducting area of cathode (i) during td (ii) after tr. (iii) after tp.

(ii) Rise time tr: The rise time tr. is the time taken by the anode current to rise from 0.1 Ia to 0.9Ia. The rise time is also

defined as the time required for the forward blocking off-state voltage to fall from 0.9 to 0.1 of its initial value OA.

However, the main factor determining t,. is the nature of anode circuit. For example, for series RL circuit, the rate of rise

of anode current is slow, therefore, tr is more. For RC series circuit, di/dt is high, tr is therefore, less.

(iii) Spread time tp : The spread time is the time taken by the anode current to rise from 0.9 Ia to Ia. It is also

defined as the time for the forward blocking voltage to fall from 0.1 of its value to the on-state voltage drop (1 to

1.5 V). During this time, conduction spreads over the entire cross-section of the cathode of SCR. After the

spread time, anode current attains steady state value and the voltage drop across SCR is equal to the on -state

voltage drop of the order of 1 to 1.5 V, shown above. Total turn-on time of an SCR is equal to the sum of delay time,

rise time and spread time.

Switching Characteristics during Turn-off:



This dynamic process of the SCR from conduction state to forward blocking state is called commutation process

or turn-off process.

Once the thyristor is on, gate loses control. The SCR can be turned off by reducing the anode current below

holding current. If forward voltage is applied to the SCR at the moment its anode current falls to zero, the device

will not be able to block this forward voltage as the carriers (holes and electrons) in the four layers are still

favourable for conduction. The device will therefore go into conduction immediately even though gate signal is not

applied. In order to obviate such an occurrence, it is essential that the thyristor is reverse biased for a finite

period after the anode current has reached zero.

The turn-off time tq of a thyristor is defined as the time between the instant anode current becomes zero and the instant

SCR regains forward blocking capability. The turn-off time is divided into two intervals ; reverse recovery time t„

and the gate recovery time tgr. i.e, tq = trr + tgr.

At instant t1, anode current becomes zero. After t1, anode current builds up in the reverse direction with the same di /d t

slope as before t1. The reason for the reversal of anode current after t1 is due to the presence of carriers stored in the

four layers. The reverse recovery current removes excess carriers from the end junctions J 1 and J3 between the

instants t1 and t3.

At instant t2, when about 60% of the stored charges are removed from the outer two layers, carrier density

across J1 and J3 begins to decrease and with this reverse recovery current also starts decaying. At instant t3,

when reverse recovery current has fallen to nearly zero value, end junctions J1 and J3 recover and SCR is able to

block the reverse voltage.

At the end of reverse recovery period (t 3- t1), the middle junction J 2 still has trapped charges, therefore,

the thyristor is not able to block the forward voltage at t 3. these trapped charges must decay only by

recombination. This recombination is possible if a reverse voltage is maintained across SCR. The time for the

recombination of charges between t3 and t4 is called gate recovery time tgr. At instant t4, junction J2 recovers and the

forward voltage can be reapplied between anode and cathode.

The thyristor turn-off time tq is applicable to an individual SCR. In actual practice, thyristor (or thyristors)

form a part of the power circuit, The turn-off time provided to the thyristor by the practical circuit is called

circuit turn-off time tc. It is defined as the time between the instant anode current becomes zero and the

instant reverse voltage due to practical circuit reaches zero.

THYRISTOR PROTECTION:

A thyristor must be protected against all abnormal conditions. SCRs are very delicate devices, their protection against

abnormal operating conditions is, therefore, essential.

(i) di/dt protection: When a thyristor is forward biased and is turned on by a gate pulse, conduction of

anode current begins in the immediate neighbourhood of the gate -cathode junction. Thereafter, the current

spreads across the whole area of junction. However, if the rate of rise of anode current, i.e. di/dt, is large as

compared to the spread velocity of carriers, local hot spots will be formed near the gate connection on account

of high current density. This localised heating may destroy the thyristor. Therefore, the rate of rise of anode

current at the time of turn-on must be kept below the specified limiting value. The value of di/dt can be

maintained below acceptable limit by using a small inductor, called di/dt inductor, in series with the anode

circuit.

(ii)dv/dt protection: With forward voltage across the anode and cathode of a thyristor, the two outer

junctions are forward biased but the inner junction is reverse biased. This reverse biased junction J2, has the

characteristics of a capacitor due to charges existing across the junction. In other words, space-charges exist in the

depletion region around junction J2 and therefore junction J2 behaves like a capacitance.



If the rate of rise of forward voltage dVa/dt is high, the charging current i will be more. This charging current

plays the role of gate current and turns on the SCR even when gate signal is zero. Such phenomena of turning-

on a thyristor, called dv/dt turn-on must be avoided as i t leads to false opera t ion of the thyr is tor ci rcui t .

For control lable operat ion of the thyr is tor , the rate of rise of forward anode to cathode voltage dVa/dt

must be kept below the specified rated limit. Typical values of dv/dt are 20 — 500 V/µsec.

Design of Snubber Circuits:

A snubber circuit consists of a series combination of resistance Rs and capacitance

in parallel with the thyristor as shown. W h e n switch S is closed, a sudden

voltage appears across the ci rcui t . Capaci tor C s behaves l ike a short

c i rcui t , therefore voltage across SCR is zero. With the passage of time, voltage

across Cs builds up at a slow rate such that dv/dt across Cs and therefore across

SCR is less than the specified maximum dv/dt rating of the device.Before SCR is

fired by gate pulse, Cs charges to full voltage Vs. When the SCR is turned o n ,

c a p a c i t o r d i s c h a r g e s t h r o u g h t h e S C R a n d s e n d s a c u r r e n t

e q u a l t o Vs/ (resistance of local path formed by C 3 and SCR). As this resistance is quite low, the turn-on di/dt

will tend to be excessive and as a result, SCR may be destroyed. In order to limit the magnitude of discharge

current, a resistance Rs is inserted in series with C s as shown inabove Figure.

Thyristor Turn-off Methods:

T h e t u r n -o f f o f a t h y r i s t o r me a n s b r i n g i n g t h e d e v i c e f r o m forward-conduction state to forward-

blocking state. The thyristor turn-off requires that (1) its anode current falls below the holding current and (ii) a

reverse voltage is applied to thyristor for a sufficient time to enable it to recover to blocking state,

Commutation is defined as the process of turning-off a thyristor.

Once thyristor starts conducting, gate loses control over the device, therefore, external means may have to be adopted to

commutate the thyristor.

CLASS A COMMUTATION : LOAD COMMUTATION

For achieving load commutation of a thyristor, the commutating components L and C are connected as shown below.

Here R is the load resistance. For low value of R, L and C are connected in series with R. For high value of R, load R

is connected across C. The essential requirement for both the circuits is that the overall circuit must be

underdamped.

When these circuits are energized from dc, current waveforms as shown are obtained, It is seen that current i first

rise to maximum value and then begins to fall. When current decays to zero and tends to reverse, thyristor T is turned-off on

its own at instant A.

Load commutation is possible in dc circuits and not in ac circuits. Class A, or load, commutation is also called resonant

commutation or self-commutation.

CLASS B COMMUTATION : RESONANT-PULSE COMMUTATION



In class-B, or resonant-pulse, commutation, source voltage Vs charges

capacitor C to voltage Vs. Main thyristor T1 as well as auxiliary

thyristor TA are off. Positive direction of capacitor

voltage vc and capacitor current is are marked. When T1 is turned on

at t = 0, a constant current I0 is established in the load circuit. Here,

for simplicity, load current is assumed constant. Uptill time t1, vc=

Vs, ic= 0, i0 =I0 and iT1=I0. For initiating the commutation of

main thyristor T1, auxiliary thyristor TA is gated at t = t1. With TA on, a

resonant current i0 begins to flow from C through TA, L and back to C.

This resonant current, with time measured from instant t1, is given by

Minus sign before Ipsin w0 t is due to the fact that this current flows

opposite to the reference positive direction chosen in circuit.

After half a cycle of ic from instant t1, ic = 0, vc= –Vs, and iT1=I0.

After π radians from instant t1, i.e. just after instant t2, as ic tends

to reverse, TA is turned off at t2. Resonant current ic now builds up

through C, L, D and T1. As this current ic grows opposite to forward

thyristor current of T1, net forward current iT1 = I0 – ic begins to

decrease. Finally, when ic in the reversed direction attains the value I0,

forward current in T1 (iT1 = I0 –I0 = 0) is reduced to zero and the device

T1 is turned off at t3. As thyristor is commutated by the gradual build up

of resonant current in the reversed direction, this method of commutation

is called current commutation, class-B commutation or resonant-pulse

commutation.

Af ter T 1 i s t urned of f a t t 3 , cons tant cur ren t I 0 f lows f rom V s to load throughC, L and D. Capacitor

begins charging linearly from - Vab to zero at t4 and then to Vs at t5. As a result, at instant t5, when vc= Vs, load

current io = ic = I0 reduces to zero as shown.

It is seen from the waveform of i c that main thyristor T1 is turned off when

Main thyristor T1 is commutated at t3. As constant load current Io charges C linearly from - Vab, at t3 to zero at t4,

SCR T1 is reverse biased by voltage vc for a period (t4 - t3) = tc.

Circuit turn-off time for main thyristor,

the magnitude of reverse voltage Vab across main thyristor T1, when it gets commutated, is given by

Vab = Vs cos ωo(t3-t2)

CLASS C COMMUTATION COMPLEMENTARY COMMUTATION: In this type of commutation, a thyristor

carrying load current is commutated by transferring its load current to another incoming thyristor.Below Figure

illustrates an arrangement employing complementary commutation. In this figure, firing of SCR T1 commutates T2



and subsequently, firing of SCR T2 would turn off T1.

When T1 is turned on at t=0, current through R1 is i1=Vs/R1 and through R2 is ic=Vs/R2, so that thyristor T1 current iT1 =

i1+ic = Vs(1/R1 + 1/R2) begins to flow. Capacitor C begins charging through R 2 from vc=0.

The charging current through the circuit V s, C and R2 is given by



CLASS D COMMUTATION : IMPULSE COMMUTATION:

In this class D, or impulse commutation, T1 and TA are called main and auxiliary thyristors respectively.

Initially, main thyristor T1 and auxiliary thyristor TA are

off and capacitor is assumed charged to voltage Vs with

upper plate positive. When T1 is turned on at t = 0,

source voltage Vs is applied across load and load

current I0 begins to flow which is assumed to

remain constant. With T1 on at t = 0, another

oscillatory circuit consisting of C, T1., L and D is

formed where the capacitor current is given by

When ωo t = π, ic = 0. Between 0 < t < (π/ωo), iT1 = I0 +Ip sin ωo t. Capacitor voltage changes from + V s to - V s co-

sinusoidally and the lower plate becomes positive. At ω0t=π , ic = 0, iT1 = I0 and vc = -Vs.

At t1, auxiliary thyristor TA is turned on. Immediately after TA is on, capacitor voltage Vs applies a reverse voltage across

main thyristor T1 so that vT1 = -Vs at t1 and SCR T1 is turned off and iT1= 0. The load current is now carried by C and TA.

Capacitor gets charged from - Vs to Vs with constant load current I0. When vc = Vs, ic = 0 at t2, thyristor TA is turned off.

During the time TA is on from t1 to t2, vc = vT1, ic = -I0 and i0 = I0 . For main thyristor T1, circuit turn-off time is tc, as shown

in Fig.



With the firing of thyristor TA, a reverse voltage V is suddenly applied across T1 ; this method of commutation is

therefore, also called voltage commutation. With sudden appearance of reverse voltage across T1, its current is

quenched. As an auxiliary thyristor TA is used for turning-off the main thyristor T1, this type of commutation is also

known as auxiliary commutation.

When thyristor TA is turned on, capacitor gets connected across T1 to turn it off, this type of commutation is therefore,

also called parallel-capacitor commutation.

CLASS E COMMUTATION : EXTERNAL PULSE COMMUTATION

In this type of commutation, a pulse of current is obtained from a separate

voltage source to turn off the conducting SCR. The peak value of this current

pulse must be more than the load current. In the circuit, Vs is the voltage of the

main source and VI is the voltage of the auxiliary supply. When thyristor T1 is

conducting and load is connected to source Vs. When thyristor T3 is turned on

at t = 0; V1, T3, L and C form an

oscillatory circuit. Therefore, C is charged to a vol tage + 2V 1 wi th upper plate posi t ive at 𝑡 = √𝐿𝐶 and as

oscillatory current falls to zero. Thyristor T3 gets commutated. For turning off the main thyristor T 1, thyristor T2 is

turned on. With T2 on, T1 is subjected to a reverse voltage equal to Vs-2V1 and T1 is turned off. After T1 is off,

capacitor discharges through the load

CLASS F COMMUTATION : LINE COMMUTATION

This type of commutation is also known as natural commutation. Here, the thyristor carrying the load current is

reverse biased by the ac source voltage and the device is turned-off when anode current falls below the holding current

(assumed nearly zero).

In this figure, thyristor T is fired at firing angle equal

to zero, i.e. when ωt = 0, vs = 0. During the positive

half-cycle, v0 = vs and waveshape of load current i0 is

identical with the waveshape of vo for a resistive load.

At ωt = π, vs = 0, v0 = 0 and i0 = 0; therefore T gets

turned off at this instant. From ωt = π to ωt = 2π,

T is reverse biased for a period t c = π/ω sec, longer

than the thyristor turn-off time tq. Here tc is called the

circuit turn-off time.

Another method of classification of thyristor commutation technique is as tinder

(1) Line commutation class F

(2) Load commutation : class A

(3) Forced commutation class B, C and D

(4) External-pulse commutation : class E.

In line, or natural, commutation, natural reversal of ac supply voltage commutates the conducting thyristor. As stated

before, line commutation is widely used in ac voltage controllers, phase-controlled rectifiers and step-down

cycloconverters.

In load commutation, L and C are connected in series with the load or C in parallel with the load such that overall load

circuit is under damped. Load commutation is commonly employed in series inverters.

In forced commutation, the commutating components L and C do not carry load current continuously. So class B, C and D

commutation constitute forced commutation techniques. As stated before, in forced commutation, forward current of the

thyristor is forced to zero by external circuitry called commutation circuit. Forced commutation is usually employed in dc

choppers and inverters.

Unit i

Engineering

Transcript of Unit i