POWER ELECTRONICS AND INSTRUMENTATION Class …

transcript

Madhusudhan M

Assistant Professor

Dept. Of ECE

Dr.AIT , Bengaluru-56

POWER ELECTRONICS

AND INSTRUMENTATION

Class-01(Introduction)

DEPARTMENT OF ECE, DR.AIT, BENGALURU

Subject Tit le : Power Electronics and Instrumentat ion

Subject Code : 18EC35

No. of Credits : 03=03:0:0 ( L - T –P)

No. of Lecture Hours/Week : 03

CIE+Assignment+SEE : 45+5+50=100

Total No.of Contact Hours : 39

COURSE OBJECTIVES:The Thyristor circuits.

The applications of power devices in controlled rectifiers, converters and

Choppers.

The types of errors in measurement and the Operation of different Transducers.

Multirange Ammeters, Voltmeters and Multimeters.

Principle of operation of digital measuring instruments and Bridges.

DEPARTMENT OF ECE, DR.AIT, BENGALURU 3

UNIT-01Int roduct ion : History, Power Electronic Systems, Power ElectronicConverters and Applications.

Thyr i s tors : Static Anode-Cathode characteristics Two transistor model ofSCR and Gate characteristics of SCR, Dynamic Turn On switchingcharacteristics, Turn-ON methods, Turn-OFF mechanisms.

Turn -OFF Methods : Natural and Forced Commutation – Class A Class Band Class C.

Gate trigger circuits: Resistance firing circuit, Resistance capacitance firingcircuit.

Uni -Junct ion Trans i s tor : Basic operation and UJT Firing Circuit.

UNIT-02Phase Control led Converter : Control techniques, Singlephase half wave and full wave controlled rectifier with resistiveand inductive loads, effect of freewheeling diode, NumericalExamples.

Choppers: Chopper Classification, Basic Chopper operation:step-down, step-up and step-up/down choppers, NumericalExamples.

Inverters : Classification of Inverters

UNIT-03Pr inc ip les o f Measurement : Static Characteristics, Error inMeasurement, Types of Static Error.

Voltmeters : Introduction, Multi range voltmeter.

Ammeters : DC Ammeter, Multi-range Ammeter.

Dig i ta l Vo l tmeter : Ramp Technique, Dual slope integrating Type DVM,Direct Compensation type and Successive Approximations type DVM.

UNIT-04Digital Instruments: Introduction, Digital Multimeter,Digital frequency meters, Digital measurement of time.

Signal Generators: Function generator, Block diagram ofOscilloscope.

Bridges: Wheatstone’s Bridge, Capacitance and InductanceComparison bridge, Maxwell’s bridge, Wien’s bridge.

UNIT-05Transducers : Introduction, Selecting a transducer, ElectricalTransducer, Resistive Transducer, Resistive position Transducer,Resistance Wire Strain Gauges, Resistance Thermometer,Thermistor, LVDT, Instrumentation Amplifier using TransducerBridge, Temperature indicators using Thermometer, AnalogWeight Scale

Text Book:1. M.D Singh and K B Khanchandani ,“Power Electronics”, 2nd Edition, Tata

McGraw Hill Education Private Limited, 2009

2. H. S. Kalsi, “Electronic Instrumentation”, 3rd Edition, McGraw Hill, 2012

Reference Books:1. Mohammad H Rashid, “Power Electronics, Circuits”, 3rd/4th Edition, Pearson

Education Inc, 2014

2. David A. Bel “Electronic Instrumentation & Measurements”, 2nd Edition,

Oxford UniversityPress PHI, 2006

Course Outcomes:CO1: Define and classify the Power electronic devices and its characteristicsand instrument errors

CO2: Explain the power electronic circuits and principles of measurementtechniques

CO3:Analyse and design different types of rectifiers and converters

CO4: Apply measurement techniques to measure different quantities.

POWER ELECTRONICS

Electronics

Control

In Electrical Engineering can be divided into 3

types:

1.Electronics: study of Semiconductor circuits

for the processing of information at lower power

levels.

2.Power: deals with both rotation and static

equipment for the generation, transmission,

distribution and utilization of vast quantities of

electric power

3.Control: stability and response characteristics

of closed loop systems using feedback on either a

continuous or sampled-data basis.

P o w e r E l e c t r o n i c s “The application of solid state electronics for the

control and conversion of electric power”

Power Electronics is a combination of power,

Electronics and Control

I N T R O D U C T I O N

Electron i cs vs Power E lect roni csElectronics deals with transmission of information/signals in the form of electrical

energy. Whereas power electronics deals with transmission of electrical energy itself.

As the name suggests, easiest answer would be, power electronic devices are used in

high power/ high current scenarios.

Role of power Electron i csThe basic functions of importance for power electronics are:

(1) power conversion, ac to dc, dc to ac, ac to ac,

(2) power conditioning to remove distortion, harmonics, voltage dips and over voltages.

(3) high speed and/or frequent control of electrical parameters such as currents,

voltage impedance, and phase angle

HISTORY

Metal tank Rectifier, Grid controlled vacuum-tube rectifier, ignitron, phanotron

and thyratron were used for power control Until 1950

Mercury Arc Rectifier in 1900.

In 1948(first electronics revolution began), invention of the silicon transitors at

Bell Telephone Laboratories by Bardeen, Brattain and Shockley.

In 1956, invention of PNPN triggering transistor which was defined as a thyristor

or silicon-controlled rectifier(SCR)

Power source

Power Modulator

Motor Load

Sensing Unit Control Unit

Command

Block Diagram of Power Electronics

Madhusudhan M

Assistant Professor

Dept. Of ECE

Dr.AIT , Bengaluru-56

POWER ELECTRONICS

AND INSTRUMENTATION Class-02 (Introduction)

Power Electronic Converters•Power electronics circuits are also called power convertors

•Convertor- uses a matrix of power semiconductor switches to convert electrical at high efficiency

•Components - Switches, reactive components L,C and transformers

•Switches of two types

• Two terminal devices- Diodes.

• Three terminal devices- Thyristors or Transistors

Classification of ConvertorsThe power converters can be classified into 5 broad categories. They are:

1. Phase controlled rectifiers (AC to DC convertors).

2. choppers (DC to DC convertors).

3. Inverters (DC to AC convertors).

4. Cyclo converters (AC to AC convertors).

5. AC voltage Controllers (AC Regulators)

Phase controlled Rectifiers•These controllers converts fixed AC voltage to a variable DC output voltage.

•Input power from one or more AC voltage/current sources of single or multiple phase and delivers to a load.

•The output variable is a low-ripple dc voltage or dc current and these convertors uses line voltage for their commutation. Hence they are also called as line commutated or naturally commutated ac to dc convertors.

•Application:1. Battery charger circuits.• 2. regulated DC power supply

• 3. DC motor drives

Phase controlled Rectifiers

Choppers (DC to DC converter)•Converts fixed dc input voltage to a variable dc output voltage. The dc output voltage may be different in amplitude than the input source voltage.

• choppers are designed using semiconductor devices such as power transistors, IGBTs, GTOs, power MOSFETs and Thyristors.

•Application:

• Battery driven Vehicles.

• Electric traction

Inverters(DC to AC converter)•Converts a fixed dc voltage to an ac voltage of variable frequency and of fixed or variable magnitude.

•A practical inverter has either a battery, a solar powered dc voltage source.

•Used in very low-power portable electronic systems such as the flashlight discharge system in a photography camera to very high power industrial systems.

•Inverters are designed using semiconductor devices such as power transistors, IGBTs, GTOs, power MOSFETs and Thyristors.

•Applications:

• Uninterruptible power supply(UPS).

• Aircraft and space power supplies.

Cyclo Converters (AC To AC Convertors).•Convert input power at one frequency to output power at a different frequency through one stage conversion.

•Designed using Thyristors and are controlled by triggering signals derived from a control unit.

•Output frequency is a simple fraction such as 1/3, 1/5 and so on of the source frequency.

•They are mainly used for low speed and very high power industrial drives.

AC voltage controllers(AC Regulators)•Converts fixed ac voltage directly to a variable ac voltage at the same frequency using line communication.

•These converters employs a thyristorised voltage controller.

•Application:

• Lighting control

• Speed control of large fans and pumps

POWER ELECTRONICS AND INSTRUMENTATION

Class-03

Madhusudhan M

Assistant Professor

Dept. of ECE, Dr.AIT,

Bengaluru-56

Thyristor

• Two transistor model of SCR.

• Static Anode-Cathode characteristics.

• Gate characteristics of SCR.

• Dynamic Turn On switching characteristics.

• Turn-ON methods.

• Turn-OFF mechanisms.

CONTENTS,

2Dept. of ECE, Dr.AIT, Bengaluru

INTRODUCTION

• Thyristor is a general name given to the family of power semiconductor

switching devices.

• Derived from a combination of capital letters THYRatron and transISTOR

• Characterised by a bistable switching action depending upon the PNPN

regenerative feedback.

• Thyristor is almost universally referred to as the SCR(Silicon Controlled

Rectifiers)

Construction of Thyristor• The structure and symbolic representation of

the Thyristor is as shown in the figure

• Four layer PNPN switching device,

• Three PN junctions J1, J2 and J3.

• Three external terminals namely the anode(A),

cathode(K) and gate(G).

• The anode and cathode are connected to the

main power circuit.

• The gate terminal is provided at the P layer

near the cathode, hence it is known as

cathode gate

• The gate terminal carries a low level gate

current in the direction of gate to cathode

Forward Blocking

state OR off-state of

the device

• When the end P layer is made positive

w.r.t end N layer, the two junctions J1

and J3 are forward biased but, J2

becomes reversed biased(due the

presence of depletion layer current does

not flow through the device).

• Only leakage current(negligibly small in

magnitude)flows through the device due

to the drift if the mobile charges

• The current is insufficient to make the

device conduct.

• In the depletion layer, most of the

immobile charge do not constitute any

flow of current5

Dept. of ECE, Dr.AIT, Bengaluru

Reverse blocking state

OR off-state of the

devices

• N-layer is made positive w.r.t P-layer.

• PN junction J2 becomes forwardbiased whereas J1 and J3 becomesreversed biased.

• The junction J1 and J3 does notallow any current to flow through thedevice.

• Only a small amount of leakagecurrent may flow because of the driftof the charges.

• The leakage current is againinsufficient to make the deviceconduct.

Conduction state

OR on state

• The width of the depletion layer at thejunction J2 decreases with the increasein anode to cathode voltage (width isinversely proportional to the voltage)

• When the Voltage between anode andcathode is kept on increasing, a stagecomes, the depletion layer at J2vanishes.(voltage gradient across itsdepletion layer)

• This phenomenon is known as theAvalanche breakdown.

• since J1 and J3 are already forwardbiased, there will be a free carriermovement across all the three junctionsresulting in an large amount of currentflowing through the device from anode tocathode.

Static Anode-Cathode

characteristics of SCR• The figure shows a elementary

circuit diagram for obtaining V-I

characteristics of the SCR.

• Anode and cathode are

connected to the main source

through a load.

• The gate and cathode are fed

from the another source Eg.

• Va is the anode-cathode voltage

and Ia is the anode current.

Dept. of ECE, Dr.AIT, Bengaluru 8

•Reverse Blocking region

•Forward blocking region

•Forward conduction region

Regions of

operation

Reverse Blocking

Region

• Cathode is made positive w.r.t anode,

were Thyristor becomes reversed biased.

• In reverse blocking region, Junction J1

and J3 are reversed biased and J2 is

forward Biased.

• A small leakage current flows.

• If reverse voltage is increased, a critical

breakdown called Reverse Breakdown

Voltage(VBR), an avalanche will occur at

J1 and J3 increasing the current

sharply.

• Power dissipation will lead to the

dangerous level that may destroy the

device.

Forward

Blocking Region• Anode is made positive w.r.t

cathode.

• Junction J1 and J3 are forward

biased while junction J2 is

reversed biased.

• Anode current is small forward

leakage current.

• The region OM represent the

forward blocking region , where

the device does not conduct.

Forward Conduction

Region.• Anode to cathode forward voltage is increased

with the gate circuit kept open, avalanche

breakdown occurs at the junction J2 at a critical

forward break-over voltage(VBO) at point M,

• Device is at conduction State.

• The region MN shows, the voltage across the

device drops from several hundred volts to 1-

2V with a very large amount of the current

flowing through the device.

Dept. of ECE, Dr.AIT, Bengaluru-56

Forward Conduction Region.

• The region of NK of the characteristic

is called as the forward conduction

state.

• Where the anode current is

determined by the external load

impedance.

• When the Thyristor conducts forward

current, it can be regarded as a

closed switch.

Latching Current(IL): the minimum anode current

required to maintain Thyristor in the ON-state

immediately after the Thyristor has been turned ON and

the gate signal has been removed

Holding Current(IH): the minimum anode current to

maintain the Thyristor in the ON-state. The holding

current is less than the latching current i.e, IH<IL.. It is

in the order of Milliampere.

SCR using Two Transistor Model

CLASS-04

Madhusudhan M

Assistant Professor

Dept. of ECE,

Dr.AIT, Bengaluru-56

Thyristor

• Two transistor model of SCR

• Turn-ON methods.

• Turn-OFF mechanisms.

CONTENTS

Two Transistor Analogy • The operation of an SCR can also be

explained by using the two transistor. Thisis know as two transistor analogy of theSCR.

• SCR can be considered as an npn and pnptransistor.

• Collector of one transistor is attached tothe base of the other and vice versa, asshown in the figure(b).

• The model is obtained by splitting the twomiddle layers of the SCR into twoseparate parts, can be seen in figure(a).

3Dept. of ECE, Dr. AIT, Bengaluru

• Collector current of the transistor

𝑇1 𝑏𝑒𝑐𝑜𝑚𝑒𝑠 𝑡ℎ𝑒 𝑏𝑎𝑠𝑒 𝑐𝑢𝑟𝑟𝑒𝑛𝑡 𝑜𝑓 𝑡𝑟𝑎𝑛𝑠𝑖𝑠𝑡𝑜𝑟 𝑇2 𝑎𝑛𝑑 𝑣𝑖𝑐𝑒 𝑣𝑒𝑟𝑠𝑎

𝐼𝑐1 = 𝐼𝑏2 𝑎𝑛𝑑 𝐼𝑏1 = 𝐼𝑐2

𝐼𝑘 = 𝐼𝑎 + 𝐼𝑔… . . 2.1

• We have 𝐼𝑏1 = 𝐼𝑒1 − 𝐼𝑐1…… 2.2

𝐼𝑐1 =∝1 𝐼𝑒1 + 𝐼𝑐𝑜1… . . 2.3

• Where ICO1is the reverse leakage current of the reverse biased

junction J2. substituting Eqn.2.3 in Eqn. (2.2), we get

𝐼𝑏1 = 𝐼𝑒1−∝1 𝐼𝑒1 − 𝐼𝑐𝑜1, 𝐼𝑏1 = 1− 𝛼1 𝐼𝑒1 − 𝐼𝑐𝑜1

• The anode current of the device Becomes the emitter current of

the transistor T1 that is 𝐼𝑎 = 𝐼𝑒1

𝐼𝑏1 = 1−∝1 𝐼𝑎 − 𝐼𝑐𝑜1…… 2.4

Also 𝐼𝑐2 =∝2 𝐼𝑒2 + 𝐼𝑐𝑜2

The cathode current of the SCR becomes the emitter current of the

transist T2, 𝐼𝑘 = 𝐼𝑒2

𝐼𝑐2 =∝2 𝐼𝑘 + 𝐼𝑐𝑜2… . . 2.5

𝐼𝑏1 = 𝐼𝑐2……(2.6)

Dept. of ECE, Dr. AIT, Bengaluru 4

Substituting Eqn. (2.4) and (2.5) in Eqn.(2.6), we have

1−∝1 𝐼𝑎 − 𝐼𝑐𝑜1 =∝2 𝐼𝑘 + 𝐼𝑐𝑜2… 2.7

Substituting Eqn.(2.1) in Eqn.(2.7) we get

1−∝1 𝐼𝑎 − 𝐼𝑐𝑜1 = ∝2 𝐼𝑎 + 𝐼𝑔 + 𝐼𝑐𝑜2

𝐼𝑎 =∝2 𝐼𝑔 + 𝐼𝑐𝑜1 + 𝐼𝑐𝑜2

1 − ∝1 +∝2… 2.8

Assuming that the leakage current of thetransistor T1 and T2 to be negligible small, wehave

𝐼𝑎 =∝2𝐼𝑔

1−(α1+α2)…....(2.9)

Regenerative Action • From Eqn.(2.9), if (α1+α2)=1, the value of anode current Ia becomes infinite.

• The device suddenly latches into the conduction (ON)state from the non-

conduction(OFF) state. This characteristics of the device is known as

Regenerative Action.

• It can also stated as, the gate current Ig is of such a value that (α1+α2)

approaches unity, the device will trigger.

Turn- ON conditions of the SCR

• Thermal. If the temperature of the device is very high, there is an increase in no. of electron-

hole pairs, which increases the leakage current. Increase in the current causes (α1 and α2) to

increase. Due to regenerative action (α1 and α2) may tends to unity and Thyristor may turned on.

• High Voltage. If the forward anode to cathode voltage is greater than the forward breakdown

voltage(VBO), a sufficient leakage current flows to initiate regenerative turn-on.(this type of turn-

on may be destructive and should be avoided)

• Gate Current. If gate current is injected into the base P in the same direction as current Ia across

the J2, the current gain of the NPN transistor be increased independently of the anode to cathode

voltage Va and current Ia.

since α2 depends on (Ia+Ig)and α1 depends on Ia. The total current will now depend on the

Ig and independent means of break over is obtained. The presence of the gate current modifies the

static V-I characteristics.

The figure shows the gate characteristics of a typical SCR.

• First Region. OA lies near the origin and is defined by

the maximum rated junction temperature(usually 125C).

The gate must be operated in this region whenever

forward bias is applied across the Thyristor and triggering

is not necessary.

• Second Region. The minimum value of gate-voltage

and current required to trigger all devices at the minimum

rated junction temperature. This region contains the

actual minimum firing points of all devices. It is a

forbidden region(a signal in this may not always fire all the

devices or never fire any at all).from the figure, OL andOV are the minimum gate-voltage and gate-current limits

respectively.

• Third Region. Largest and shows the limits on the gate

signal for reliable firing. (OP and OQ)

Gate Characteristics of SCR

Gate Characteristics of SCR From the Gate Characteristics of SCR, we have

• ON and OM – characteristics for SCRs of the same

Ratings.

• S,S1 and S2- operating points, most be close as possible to

the permissible Pg curve and must be within the max. and

min. limits of gate voltage and current.

• Es- gate source voltage, Es=OH is drawn as HD.

• Line HD- load line.

• The gradient of the load line, HD=(OH/OD), which will

give the required gate source resistance Rg.

• Line HE- maximum value of the series resistance.

• E- point of intersection of the lines indicating the

minimum gate voltage and gate current.

• Line HC- minimum value of the gate source series

resistance, obtained by drawing a line tangential to the curve

Gate Power Dissipation • Higher the magnitude of gate current pulse, lesser is the time needed to inject the required charge for turning

on the Thyristor. Therefore SCR turn-on time can be reduced by using the gate current of higher magnitude.

• The gate pulse width is take as equal to or greater than SCR turn-on time,

• if T is the pulse width as shown in the figure, then we have

𝑇 ≥ 𝑡𝑜𝑛With pulse firing, if the frequency of firing f is known, the peak instantaneous gate power dissipation Pgmax can be

obtained as 𝑃𝑔𝑚𝑎𝑥 = 𝑉𝑔𝐼𝑔 =𝑃𝑔𝑎𝑣

𝑓𝑇… . . 2.10

𝑓 =1

𝑇1= 𝑓𝑟𝑒𝑞𝑢𝑒𝑛𝑐𝑦 𝑜𝑓 𝑓𝑖𝑟𝑖𝑛𝑔 𝑂𝑅 𝑝𝑢𝑙𝑠𝑒 𝑟𝑒𝑝𝑖𝑡𝑖𝑜𝑛 𝑟𝑎𝑡𝑒 𝑖𝑛 𝐻𝑧

T= pulse width in second

Duty cycle : ratio of pulse-on period to the period time of pulse.

Duty cycle is given by,

𝛿 =𝑇

𝑇1= 𝑓𝑇… . 2.11

From Eqn.(2.10), 𝑃𝑔𝑎𝑣

𝛿≤ 𝑃𝑔𝑚𝑎𝑥

CLASS-05

Madhusudhan M

Assistant Professor

Dept. of ECE,

Thyristor

• Turn-on methods.

• Turn-off mechanisms.

CONTENTS

2Dept. of ECE, Dr.AIT, Bengaluru-56

Turn-on methods1. Thermal Triggering(Temperature Triggering).If the temperature of the device is very high, there is an increase in no. of electron-hole pairs, which

increases the leakage current. Increase in the current causes (α1 and α2) to increase. Due to regenerative

action (α1 and α2) may tends to unity and Thyristor may turned on.

2. Forward voltage Triggering (High Voltage).If the forward anode to cathode voltage is greater than the forward breakdown voltage(VBO), a sufficient

leakage current flows to initiate regenerative turn-on.(this type of turn-on may be destructive and should be

avoided)

3. Radiation Triggering(Light Triggering ).If light is allowed to strike the junctions of the Thyristor, the electron-hole pair increases, resulting which

Thyristor may be turned on. The light activated Thyristor are turned on by allowing the light to strike the

silicon wafers.

4. dv/dt triggering.With the forward voltage across the anode and cathode of the device, the junction J1 and J3 are forward

biased and junction J2 is reversed biased. the reversed biased junction has characteristics of a capacitor due to

charges existing across the junction.

If the voltage impressed across the device is denoted byV, the charge by Q and the capacitance by Cj, then we

𝒊𝒄 =𝒅𝑸

𝒅𝒕=𝒅 𝑪𝒋𝑽

𝒅𝒕= 𝑪𝒋 ∗

𝒅𝑽

𝒅𝒕+ 𝑽 ∗

𝒅𝑪𝒋

𝒅𝒕the rate of change of junction capacitance may be negligible as the junction capacitance is almost constant.

The contribution of the charge current by the later term is negligible. Hence

𝑖𝑐 = 𝐶𝑗 ∗𝑑𝑉

𝑑𝑡Therefore, if the ia the rate of change of voltage across the device is larger, the device may turn on even

though the voltage appearing across the device is small.

5. Gate Triggering. By applying a positive signal at the gate terminal of the device, it can be triggered much

before the specified break over voltage.

For gate triggering, three types of signal can be used, they are D.C signals, pulse signals or A.C signals

DC gate triggering. Dc voltage between gate and cathode is applied. Where gate becomes positive w.r.t the

cathode. When the applied voltage is sufficient to produce the required gate current, the device starts conducting.

Disadvantage:1. no isolation between the power and the control circuit..

2. continuous dc signal causes more gate power losses.

AC gate triggering. A reverse voltage is applied between the gate and the cathode terminal between the negative

half of the cycle.

Advantage: provides proper isolation between the power and control circuit.

Disadvantage: A separate transformer is required to step down the A.C supply.

Pulse gate triggering. Most popular methods of triggering. The gate drive consists of a singal pulse appearing

periodically or a sequence of high frequency pulses. This is known as carrier frequency gatting.

Advantage:1. gate losses are very much reduced.

2. isolation is been provided between main gate supply and gatting signals

Dynamic turn-on switching Characteristics.

The turn-on time ton of the SCR is subdivided into three

distinct periods, called delay time, rise time and spread time.

These time periods are defined in terms of the waveforms of

the anode voltage and current obtained in the circuit.

Delay Time(td): time between the instant at which the gate

current reaches 90% of its final value and the instant at which

the anode current reaches 10% of its final value.

The time during which anode voltage falls from Va to 0.9Va,

where Va is the initial value of the anode voltage.

Rise time(tr): the time required for the anode current to rise

from 10 to 90% of its final value.

time required for the forward blocking off state voltage to fall

from 0.9 to 0.1 of its initial value.

The time is inversely proportional to the magnitude of the gate

current. For series RL circuit, rate of rise of anode current is

slow, tr is more. For series RC circuit, di/dt is high thus tr is

Tr is minimized if high and steep current pulses are applied to

the gate

Spread Time (Ts): time required for the forward blocking

voltage to fall from 0.1 to its value to the on-state voltage

drop(1-1.5V). After spread time anode current attains the

steady state value and voltage drop across the SCR is equal to

the on-state voltage drop.

Turn-on Time(ton):the sum of the delay time, rise-time and

spread time. It is typically 1 to 4usec depending upon the anode

circuit parameters and gate signal waveforms. width of the

firing pulse should be in the range of 20 to 100usec.

SCR carries a large forward current which results in a high

instantaneous power dissipation creating hot-spots which could

destroy the device. Therefore it necessary to limit the rat of rise

of current

Normally, a small inductor called di/dt inductor is inserted in

the anode circuit to limit the di/dt of the anode current..

Turn-off Mechanism (Turn-off Characteristics)

CLASS-06Madhusudhan M

Assistant Professor

Dept. of ECE,

Thyristor

• Turn-on methods.

• Turn-off mechanisms.

CONTENTS

Turn-on methods1. Thermal Triggering(Temperature Triggering).If the temperature of the device is very high, there is an increase in no. of electron-hole pairs, which

increases the leakage current. Increase in the current causes (α1 and α2) to increase. Due to regenerative

action (α1 and α2) may tends to unity and Thyristor may turned on.

2. Forward voltage Triggering (High Voltage).If the forward anode to cathode voltage is greater than the forward breakdown voltage(VBO), a sufficient

leakage current flows to initiate regenerative turn-on.(this type of turn-on may be destructive and should be

avoided)

3. Radiation Triggering(Light Triggering ).If light is allowed to strike the junctions of the Thyristor, the electron-hole pair increases, resulting which

Thyristor may be turned on. The light activated Thyristor are turned on by allowing the light to strike the

silicon wafers.

4. dv/dt triggering.With the forward voltage across the anode and cathode of the device, the junction J1 and J3 are forward

biased and junction J2 is reversed biased. the reversed biased junction has characteristics of a capacitor due to

charges existing across the junction.

If the voltage impressed across the device is denoted byV, the charge by Q and the capacitance by Cj, then we

𝒊𝒄 =𝒅𝑸

𝒅𝒕=𝒅 𝑪𝒋𝑽

𝒅𝒕= 𝑪𝒋 ∗

𝒅𝑽

𝒅𝒕+ 𝑽 ∗

𝒅𝑪𝒋

𝒅𝒕the rate of change of junction capacitance may be negligible as the junction capacitance is almost constant.

The contribution of the charge current by the later term is negligible. Hence

𝑖𝑐 = 𝐶𝑗 ∗𝑑𝑉

𝑑𝑡Therefore, if the ia the rate of change of voltage across the device is larger, the device may turn on even

though the voltage appearing across the device is small.

5. Gate Triggering. By applying a positive signal at the gate terminal of the device, it can be triggered much

before the specified break over voltage.

For gate triggering, three types of signal can be used, they are D.C signals, pulse signals or A.C signals

DC gate triggering. Dc voltage between gate and cathode is applied. Where gate becomes positive w.r.t the

cathode. When the applied voltage is sufficient to produce the required gate current, the device starts conducting.

Disadvantage:1. no isolation between the power and the control circuit..

2. continuous dc signal causes more gate power losses.

AC gate triggering. A reverse voltage is applied between the gate and the cathode terminal between the negative

half of the cycle.

Advantage: provides proper isolation between the power and control circuit.

Disadvantage: A separate transformer is required to step down the A.C supply.

Pulse gate triggering. Most popular methods of triggering. The gate drive consists of a singal pulse appearing

periodically or a sequence of high frequency pulses. This is known as carrier frequency gatting.

Advantage:1. gate losses are very much reduced.

2. isolation is been provided between main gate supply and gatting signals

The turn-on time ton of the SCR is subdivided into three

distinct periods, called delay time, rise time and spread time.

These time periods are defined in terms of the waveforms of

the anode voltage and current obtained in the circuit.

Delay Time(td): time between the instant at which the gate

current reaches 90% of its final value and the instant at which

the anode current reaches 10% of its final value.

The time during which anode voltage falls from Va to 0.9Va,

where Va is the initial value of the anode voltage.

Rise time(tr): the time required for the anode current to rise

from 10 to 90% of its final value.

time required for the forward blocking off state voltage to fall

from 0.9 to 0.1 of its initial value.

The time is inversely proportional to the magnitude of the gate

current. For series RL circuit, rate of rise of anode current is

slow, tr is more. For series RC circuit, di/dt is high thus tr is

Tr is minimized if high and steep current pulses are applied to

the gate

Spread Time (Ts): time required for the forward blocking

voltage to fall from 0.1 to its value to the on-state voltage

drop(1-1.5V). After spread time anode current attains the

steady state value and voltage drop across the SCR is equal to

the on-state voltage drop.

Turn-on Time(ton):the sum of the delay time, rise-time and

spread time. It is typically 1 to 4usec depending upon the anode

circuit parameters and gate signal waveforms. width of the

firing pulse should be in the range of 20 to 100usec.

SCR carries a large forward current which results in a high

instantaneous power dissipation creating hot-spots which could

destroy the device. Therefore it necessary to limit the rat of rise

of current

Normally, a small inductor called di/dt inductor is inserted in

the anode circuit to limit the di/dt of the anode current..

Turn-off Mechanism (Turn-off Characteristics)

Power Electronics and Instrumentation

MADHUSUDHAN M

ASSISTANT PROFESSOR

DEPT. OF ECE,

DR. AIT, BENGALURU-56

CLASS-07

•Natural Commutation.

• Forced Commutation.

•Class A.

•Class B.

•Class C.

Turn-OFFMethods:

Turn-Off Methods

• A Thyristor can only operate in two modes: it is either in the OFF state or in the ON state. By itself it cannot

control the level of current and voltage in the circuit.

• Control can only be achieved, when the Thyristor is switched ON and OFF (Commutation is center to this

switching process)

• Commutation. The transfer of the current from one path to another path.

• In Thyristor circuit, the term is used to describe the process of transferring the current from one Thyristor to

another. Were the Thyristor current is reduced to zero and enables turn-off condition.

• All Thyristor circuit involves cyclic or sequential switching of Thyristors.

Turn-Off Methods

COMMUTATION

Natural Commutation

Forced commutation

Class A-self commutated by resonating the

Class B- self commutation by

an LC circuit.

Class C-complementary Commutation.

Class D-Auxiliary

commutation.

Class E-External Pulse

commutation.

Class F- A.C line Commutation

Turn-Off Methods

Natural Commutation

• In AC circuits the current always passes through zero every half cycle.

• As current passes through zero naturally, a reverse voltage will appear across the device. This immediately

turns-off the device. This process is called as natural commutation (Since no external circuit is required)

Forced commutation

• In DC circuits, for switching off the Thyristors, the forward current should be forced to be zero by means of

some external circuits. This process is called as forced Commutation.

• The external circuits required for it is known as commutation circuit.

• The components which constitute the commutating circuit is called commutating components (Inductor

and Capacitor).

• A reverse voltage is developed across the device by means of a commutating circuits that immediately brings

the forward current in the device to zero, thus turning off the device.

• The classification of the methods of forced commutation is based on the arrangements of the following:

• commutating components

• Manner in which zero current is obtained in SCR

Forced Commutation

Class A- self commutation by resonating the load(Resonant

Commutation)

• In the circuit, The forward current passing through the device is reduced to the less than

the level of holding current of the device. Hence this method is also known as the current

commutation.

• A underdamped resonant circuit which is exited by DC source is as shown.

• From the current waveform, when the current reached zero at the point K, a reverse

current flows in the circuit. Where the device ia automatically turned-off.

• When the Thyristor is in turn-on condition, the capacitor will charge. When the capacitor

reaches the value equal to the supply voltage, the device will soon decay to the value less

than the holding current.

Class-A

Forced Commutation

Class B- self commutation by LC circuit.

Forced Commutation

Class C- Complementary commutation.

MADHUSUDHAN M

ASSISTANT PROFESSOR

DEPT. OF ECE,

CLASS-08

•Resistance firing circuit

•Resistance capacitance firing circuit.

Gate Trigger Circuits

Gate Trigger circuit

RESISTANCE FIRING CIRCUIT

Fig(a):R-firing Circuit Fig(b):Voltage waveforms

𝑅𝑀𝐼𝑁 ≥𝐸𝑀𝐴𝑋𝐼𝑔𝑚

𝑅𝑏 ≤𝑅𝑣 + 𝑅𝑀𝐴𝑋 𝑉𝑔 𝑀𝐴𝑋

𝐸𝑀𝐴𝑋 − 𝑉𝑔 𝑀𝐴𝑋

Resistance Firing Circuit

• Gate current is applied by the AC supply voltage via es, Rmin, Rv and D1.

• During the positive half of the input cycle, Anode and Cathode will be forward biased but the

Thyristor wont be under the conduction stage unless Ig(min) is obtained.

• During the positive half of the input cycle, the diode (D1) will be under forward biased. As the

supply voltage is increased, the gate current will be increased to Ig(min). Where the Thyristor

come under conduction stage. Voltage drop across the load will be equal to the supply voltage

i.e, EL=es.

• As the supply voltage reaches to the value of zero, the voltage drop across the load and the

Thyristor will be zero. as the supply voltage becomes further negative the voltage drop across

the load will be zero.

• Rmin. To maintain the minimum current for triggering the Thyristor.

• Rb. Is also called stabilizing resistance. Provides a maximum possible gate voltage.

• Rv varies the resistance in the gate circuit.

RESISTANCE-CAPACITANCE FIRING CIRCUIT:

Fig(b):Voltage waveformsFig(a):RC-firing Circuit

𝑉𝑔𝑡 = 𝑉𝑔 𝑚𝑖𝑛 + 𝑉𝐷1

RESISTANCE-CAPACITANCE FULL-WAVE TRIGGERING CIRCUIT:

Fig(a):RC-firing Circuit Fig(b):Voltage waveforms

UNIJUNCTION TRANSISTOR

Fig(A):Basic UJT Structure

Fig(B): Symbol and Equivalent Circuit of UJT Fig(C): UJT biasing Circuit.

𝑉𝑥 = (𝑅𝐵1∗ 𝑉𝐵𝐵)/(𝑅𝐵1 + 𝑅𝐵2)

𝑉𝑥 = 𝜂𝑉𝐵𝐵

Fig(B): V-I characteristicsFig(A): Equivalent circuit of UJT

Fig(B): Waveforms

Fig(A): Circuit Diagram

UJT Relaxation Oscillator

UJT as an SCR Trigger

𝑉𝐵1 𝑜𝑓𝑓 < (𝐼𝑔 ∗ 1𝐾Ω + 𝑉𝑔)

Synchronized UJT Triggering

MADHUSUDHAN M

ASSISTANT PROFESSOR

DEPT. OF ECE,

CLASS-08

•Resistance firing circuit

•Resistance capacitance firing circuit.

•UJT

RESISTANCE FIRING CIRCUIT

Fig(a):R-firing Circuit Fig(b):Voltage waveforms

𝑅𝑀𝐼𝑁 ≥𝐸𝑀𝐴𝑋𝐼𝑔𝑚

𝑅𝑏 ≤𝑅𝑣 + 𝑅𝑀𝐴𝑋 𝑉𝑔 𝑀𝐴𝑋

𝐸𝑀𝐴𝑋 − 𝑉𝑔 𝑀𝐴𝑋

Resistance Firing Circuit

• Gate current is applied by the AC supply voltage via es, Rmin, Rv and D1.

• During the positive half of the input cycle, Anode and Cathode will be forward biased but the

Thyristor wont be under the conduction stage unless Ig(min) is obtained.

• During the positive half of the input cycle, the diode (D1) will be under forward biased. As the

supply voltage is increased, the gate current will be increased to Ig(min). Where the Thyristor

come under conduction stage. Voltage drop across the load will be equal to the supply voltage

i.e, EL=es.

• As the supply voltage reaches to the value of zero, the voltage drop across the load and the

Thyristor will be zero. as the supply voltage becomes further negative the voltage drop across

the load will be zero.

• Rmin. To maintain the minimum current for triggering the Thyristor.

• Rb. Is also called stabilizing resistance. Provides a maximum possible gate voltage.

• Rv varies the resistance in the gate circuit.

RESISTANCE-CAPACITANCE FIRING CIRCUIT:

Fig(b):Voltage waveformsFig(a):RC-firing Circuit

𝑉𝑔𝑡 = 𝑉𝑔 𝑚𝑖𝑛 + 𝑉𝐷1

RESISTANCE-CAPACITANCE FULL-WAVE TRIGGERING CIRCUIT:

Fig(a):RC-firing Circuit Fig(b):Voltage waveforms

Fig(B): Waveforms

MADHUSUDHAN M

ASSISTANT PROFESSOR

DEPT. OF ECE,

CLASS-11

•Uni-Junction Transistor

•Problems

Fig(B): Waveforms

Problems

Example-01:

The latching current of a Thyristor circuit is 50mA. The duration of the firing pulse is 50us. Will

the Thyristor get fired?

Solution:

𝑖 𝑡 =𝑉

𝑅(1 − 𝑒−

𝑡𝜏)

𝜏 =𝐿

𝑅=0.5

20= 0.025𝑠

At t=50us, the value of the current is given by

𝑖 50𝑢𝑠 =100

20 1 − 𝑒−50𝑢0.025

𝑖 50𝑢𝑠 = 9.99𝑚𝐴

Since the current value is less than the latching current, the circuit does not get fired or the

circuit wont enter its conduction stage .

MADHUSUDHAN M

ASSISTANT PROFESSOR

DEPT. OF ECE,

CLASS-13

Problems - 01

Example-01:

The latching current of a Thyristor circuit is 50mA. The duration of the firing pulse is 50µs. Will

the Thyristor get fired?

Solution:

𝑖 𝑡 =𝑉

𝑅(1 − 𝑒−

𝑡𝜏)

𝜏 =𝐿

𝑅=0.5

20= 0.025𝑠

At t=50us, the value of the current is given by

𝑖 50µ𝑠 =100

20 1 − 𝑒−50µ0.025

𝑖 50µ𝑠 = 9.99𝑚𝐴

Since the current value is less than the latching current, the circuit does not get fired or the

circuit wont enter its conduction stage .

Problems - 02

Problems - 01

Problems - 03

Problems - 04

Problems - 05

Problems - 06

Problems - 07

Problems - 08

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