Power ElectronicsPower ElectronicsLecture(8)Lecture(8)

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ThyristorsThyristors Most important type of power Most important type of power

semiconductor device.semiconductor device. Have the highest power handling Have the highest power handling

capability.they have a rating of capability.they have a rating of 5000V / 6000A with switching 5000V / 6000A with switching frequencies ranging from 1KHz to frequencies ranging from 1KHz to 20KHz.20KHz.

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Is inherently a slow switching device Is inherently a slow switching device

compared to BJT or MOSFET.compared to BJT or MOSFET. Used as a latching switch that can be Used as a latching switch that can be

turned on by the control terminal but turned on by the control terminal but cannot be turned off by the gate.cannot be turned off by the gate.

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SCRSCR

Symbol of Symbol of Silicon Controlled RectifierSilicon Controlled Rectifier

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StructureStructure

G ate Cathode

J 3

J 2

J 1

Anode

10 cm17 - 3

10 - 5 x 10 cm13 14 - 3

10 cm17 -3

10 cm19 - 3

10 cm19 - 3 10 cm19 -3n+ n+

p -

n–

pp+

10 m

30- 100 m

50- 1000 m

30- 50 m

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Device OperationDevice Operation

Simplified model of a Simplified model of a thyristorthyristor

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Two Transistor Model of SCRTwo Transistor Model of SCR

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The general transistor equations are,1

1

C B CBO

C E CBO

E C B

B E CBO

I I I

I I II I I

I I I

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1 1

1 1

1 1

1

1

Considering PNP transistor of the equivalent circuit,

, , ,

,

1 1

E A C C

CBO CBO B B

B A CBO

I I I I

I I I I

I I I

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2 2 2

2 2

2 2

2

2

Considering NPN transistor of the equivalent circuit,

, ,

2

C C B B E K A G

C k CBO

C A G CBO

I I I I I I I I

I I I

I I I I

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2 1

2 1 2

1 2

From the equivalent circuit, we see that

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C B

g CBO CBOA

I I

I I II

13

1 2

1 2

Case 1: When 0

1

g

CBO CBOA

I

I II

2 1 2

1 2

Case 2: When 0

1

G

g CBO CBOA

II I I

I

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V-I V-I CharacteristicCharacteristic

ss

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Effects of gate currentEffects of gate current

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Turn-on Turn-on CharacteristiCharacteristi

cscs

on d rt t t

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Turn-off Turn-off CharacterisCharacteris

ticstics

Anode currentbegins todecrease

tCtq

t

t

Commutation didt

Recovery Recombination

t1 t2 t3 t4 t5

tr r tgr

tqtc

V A K

I A

tq=device off tim etc=circuit off tim e

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dv/dt dv/dt TriggeringTriggering

2 2 2

2 2 2

2

2

j

j j j

j dV jj

dq di C Vdt dtC dC

Vdt dt

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2 2 2

2 2 2

2

2j j j

j j jj

dq di C Vdt dt

C dV dCV

dt dt

Switching Characteristics (linearized)

Switching Power Loss is proportional to:• switching frequency• turn-on and turn-off times

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Methods of Thyristor Turn-onMethods of Thyristor Turn-on Thermal Turn-on.Thermal Turn-on. Light.Light. High Voltage.High Voltage. Gate Current.Gate Current. dv/dt.dv/dt.

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Thyristor RatingsThyristor RatingsFirst First

SubscriptSubscript Second Second

SubscriptSubscript Third Third

SubscriptSubscript D D off state off state W W working working M M Peak Peak

Value Value T T ON state ON state R R

Repetitive Repetitive F F Forward Forward S S Surge or Surge or

non-repetitivenon-repetitiveR R Reverse Reverse

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Voltage RatingsVoltage Ratings

DWM DRM DSM

RWM RRM RSM

T

V V VV V V

dvVdt

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Current RatingsCurrent Ratings

Taverage TRMS L

H

I I I

diIdt

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Gate SpecificationGate Specification

gt gt

gD RR

thjc

I V

V Q

R

DiodesDiodes Diode Product RangeDiode Product Range

Phase Control ThyristorsPhase Control Thyristors

Fast switching ThyristorsFast switching Thyristors

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Thyristor TypesThyristor Types Phase-control Thyristors (SCR’s).Phase-control Thyristors (SCR’s). Fast-switching Thyristors (SCR’s).Fast-switching Thyristors (SCR’s). Gate-turn-off Thyristors (GTOs).Gate-turn-off Thyristors (GTOs). Bidirectional triode Thyristors Bidirectional triode Thyristors

(TRIACs).(TRIACs). Reverse-conducting Thyristors Reverse-conducting Thyristors

(RCTs).(RCTs).

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Static induction Thyristors (SITHs).Static induction Thyristors (SITHs). Light-activated silicon-controlled Light-activated silicon-controlled

rectifiers (LASCRs).rectifiers (LASCRs). FET controlled Thyristors (FET-CTHs).FET controlled Thyristors (FET-CTHs). MOS controlled Thyristors (MCTs).MOS controlled Thyristors (MCTs).

PHASE-CONTROL THYRISTORS : primarily for rectifying PHASE-CONTROL THYRISTORS : primarily for rectifying line frequency voltage and currents (phase controlled line frequency voltage and currents (phase controlled AC and DC motor drivers and high voltage power AC and DC motor drivers and high voltage power transmission). Average current 4000A, blocking transmission). Average current 4000A, blocking voltage 5-7kV and on-state voltage 1.5-3Vvoltage 5-7kV and on-state voltage 1.5-3V

INVERTER-GRADE THYRISTORS: small turn-off times INVERTER-GRADE THYRISTORS: small turn-off times (from a few (from a few µµs to100s to100µµs depends on their blocking s depends on their blocking voltage and on-state voltage drops), and small on-voltage and on-state voltage drops), and small on-state voltage, state voltage,

2500V-1500A.2500V-1500A.

LIGHT-ACTIVATED THYRISTORS: triggered by a pulse LIGHT-ACTIVATED THYRISTORS: triggered by a pulse of light guided by optical fibers to a sensitive region, of light guided by optical fibers to a sensitive region, used primarily in high voltage application such as high used primarily in high voltage application such as high voltage power transmission 4kV-3kAvoltage power transmission 4kV-3kA

DevicesDevices SITH = Static Induction ThyristorSITH = Static Induction Thyristor GTO = Gate Turn Off ThyristorGTO = Gate Turn Off Thyristor MOS = Metal Oxide SemiconductorMOS = Metal Oxide Semiconductor MCT = MOS Controlled ThyristorMCT = MOS Controlled Thyristor MTO = MOS Turn Off ThyristorMTO = MOS Turn Off Thyristor ETO = Emitter Turn Off ThyristorETO = Emitter Turn Off Thyristor IGCT = Insulated Gate Controlled ThyristorIGCT = Insulated Gate Controlled Thyristor TRIAC = Triode ThyristorTRIAC = Triode Thyristor LASCR = Light Activated SCRLASCR = Light Activated SCR

Devices..Devices.. NPN BJT = NPN Bipolar Junction TransistorNPN BJT = NPN Bipolar Junction Transistor IGBT = Insulated Gate Bipolar Junction IGBT = Insulated Gate Bipolar Junction

TransistorTransistor N-Channel MOSFET = N-Channel Metal N-Channel MOSFET = N-Channel Metal

Oxide Silicon Field Effect TransistorOxide Silicon Field Effect Transistor SIT = Static Induction TransistorSIT = Static Induction Transistor RCT = Reverse Conducting ThyristorRCT = Reverse Conducting Thyristor GATT = Gate Assisted Turn Off ThyristorGATT = Gate Assisted Turn Off Thyristor

Power Semiconductor Power Semiconductor Devices, their Symbols & Devices, their Symbols &

CharacteristicsCharacteristics

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DEVICE SYMBOLS & DEVICE SYMBOLS & CHARACTERISTICSCHARACTERISTICS

35Prof. M. Madhusudhan Rao, E&C Dept., MSRIT

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Phase Control Phase Control ThyristorThyristor

These are converter thyristors.These are converter thyristors. The turn-off time tThe turn-off time tq q is in the order of is in the order of

50 to 10050 to 100sec.sec. Used for low switching frequency.Used for low switching frequency. Commutation is natural commutationCommutation is natural commutation On state voltage drop is 1.15V for a On state voltage drop is 1.15V for a

600V device.600V device.

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They use amplifying gate thyristor.They use amplifying gate thyristor.

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Fast Switching Fast Switching ThyristorsThyristors

Also called inverter thyristors.Also called inverter thyristors. Used for high speed switching Used for high speed switching

applications.applications. Turn-off time tTurn-off time tqq in the range of 5 to in the range of 5 to

5050sec.sec. On-state voltage drop of typically 1.7V On-state voltage drop of typically 1.7V

for 2200A, 1800V thyristor.for 2200A, 1800V thyristor. High dv/dt and high di/dt rating.High dv/dt and high di/dt rating.

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Bidirectional Triode Bidirectional Triode Thyristors (TRIAC)Thyristors (TRIAC)

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Triac CharacteristicsTriac Characteristics

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Gate Turn-off ThyristorsGate Turn-off Thyristors Turned on by applying positive gate signal.Turned on by applying positive gate signal. Turned off by applying negative gate Turned off by applying negative gate

signal.signal. On state voltage is 3.4V for 550A, 1200V On state voltage is 3.4V for 550A, 1200V

GTO.GTO. Controllable peak on-state current IControllable peak on-state current ITGQTGQ is is

the peak value of on-state current which the peak value of on-state current which can be turned-off by gate control.can be turned-off by gate control.

Gate-Turn-Off Thyristors (GTO)

• Slow switching speeds• Used at very high power levels• Require elaborate gate control circuitry

GTO Turn-Off

• Need a turn-off snubber

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Advantages over SCRsAdvantages over SCRs Elimination of commutating Elimination of commutating

components.components. Reduction in acoustic & Reduction in acoustic &

electromagnetic noise due to electromagnetic noise due to elimination of chokes.elimination of chokes.

Faster turn-off, therefore can be used Faster turn-off, therefore can be used for higher switching frequencies.for higher switching frequencies.

Improved efficiency of converters.Improved efficiency of converters.

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Advantages over BJTsAdvantages over BJTs Higher voltage blocking capabilities.Higher voltage blocking capabilities. High on-state gain.High on-state gain. High ratio of peak surge current to High ratio of peak surge current to

average current.average current. A pulsed gate signal of short duration A pulsed gate signal of short duration

only is required.only is required.

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Disadvantages of GTOsDisadvantages of GTOs On-state voltage drop is more.On-state voltage drop is more. Due to multi cathode structure Due to multi cathode structure

higher gate current is required.higher gate current is required. Gate drive circuit losses are more.Gate drive circuit losses are more. Reverse blocking capability is less Reverse blocking capability is less

than its forward blocking capability.than its forward blocking capability.

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Reverse Conducting Reverse Conducting ThyristorsThyristors

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Anti-parallel diode connected across Anti-parallel diode connected across

SCR on the same silicon chip.SCR on the same silicon chip. This diode clamps the reverse This diode clamps the reverse

blocking voltage to 1 or 2V.blocking voltage to 1 or 2V. RCT also called Asymmetrical RCT also called Asymmetrical

Thyristor (ASCR).Thyristor (ASCR). Limited applications.Limited applications.

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Static Induction ThyristorsStatic Induction Thyristors Turned-on by applying positive gate Turned-on by applying positive gate

voltage.voltage. Turned-off by applying negative gate Turned-off by applying negative gate

voltage.voltage. Minority carrier device.Minority carrier device. Low on-state resistance & low voltage drop.Low on-state resistance & low voltage drop. Fast switching speeds & high dv/dt & high Fast switching speeds & high dv/dt & high

di/dt capabilities.di/dt capabilities.

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Switching time in order of 1 to 6 Switching time in order of 1 to 6

sec.sec. The rating can go upto 2500V / 500A.The rating can go upto 2500V / 500A. Process sensitive.Process sensitive.

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Light-Activated Silicon Light-Activated Silicon Controlled RectifiersControlled Rectifiers

Turned-on by direct light radiation on Turned-on by direct light radiation on silicon wafer.silicon wafer.

Gate structure is sensitive for Gate structure is sensitive for triggering from practical light sources.triggering from practical light sources.

Used in high voltage and high current Used in high voltage and high current applications. Example: HVDC applications. Example: HVDC transmission, Static reactive power transmission, Static reactive power compensation.compensation.

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Offers complete electrical isolation Offers complete electrical isolation

between light triggering source & between light triggering source & power circuit.power circuit.

Rating could be has high as 4KV / Rating could be has high as 4KV / 1500A.1500A.

di/dt rating is 250A / di/dt rating is 250A / sec.sec. dv/dt rating is 2000V / dv/dt rating is 2000V / sec.sec.

Photo-SCR coupled isolatorPhoto-SCR coupled isolator

Bipolar Junction Transistors (BJT)

• Used commonly in the past• Now used in specific applications• Replaced by MOSFETs and IGBTs

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FET Controlled FET Controlled ThyristorsThyristors

Combines a Combines a MOSFET & a MOSFET & a thyristor in thyristor in parallel as parallel as shown.shown.

High switching High switching speeds & high speeds & high di/dt & dv/dt.di/dt & dv/dt.

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Turned on like conventional Turned on like conventional

thyristors.thyristors. Cannot be turned off by gate control.Cannot be turned off by gate control. Application of these are where Application of these are where

optical firing is to be used.optical firing is to be used.

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MOS-Controlled MOS-Controlled ThyristorThyristor

New device that has become New device that has become commercially available.commercially available.

Basically a thyristor with two Basically a thyristor with two MOSFETs built in the gate structure.MOSFETs built in the gate structure.

One MOSFET for turning ON the MCT One MOSFET for turning ON the MCT and the other to turn OFF the MCT.and the other to turn OFF the MCT.

MCT

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FeaturesFeatures Low on-state losses & large current Low on-state losses & large current

capabilities.capabilities. Low switching losses.Low switching losses. High switching speeds achieved due High switching speeds achieved due

to fast turn-on & turn-off.to fast turn-on & turn-off. Low reverse blocking capability.Low reverse blocking capability.

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Gate controlled possible if current is Gate controlled possible if current is

less than peak controllable current.less than peak controllable current. Gate pulse width not critical for Gate pulse width not critical for

smaller device currents.smaller device currents. Gate pulse width critical for turn-off Gate pulse width critical for turn-off

for larger currents.for larger currents.

MOSFETMOSFET

66Prof. M. Madhusudhan Rao, E&C Dept., MSRIT

MOSFETs

• Easy to control by the gate• Optimal for low-voltage operation at high switching frequencies• On-state resistance a concern at higher voltage ratings

IGBT

IGBTIGBT

77Prof. M. Madhusudhan Rao, E&C Dept., MSRIT

Advantages of IGBTAdvantages of IGBT Combines the advantages of BJT & MOSFETCombines the advantages of BJT & MOSFET High input impedance like MOSFETHigh input impedance like MOSFET Voltage controlled device like MOSFETVoltage controlled device like MOSFET Simple gate drive, Lower switching lossSimple gate drive, Lower switching loss Low on state conduction power loss like BJTLow on state conduction power loss like BJT Higher current capability & higher switching Higher current capability & higher switching

speed than a BJT. ( Switching speed lower speed than a BJT. ( Switching speed lower than MOSFET)than MOSFET)

78Prof. M. Madhusudhan Rao, E&C Dept., MSRIT

Applications of IGBTApplications of IGBT ac and dc motor controls.ac and dc motor controls. General purpose inverters.General purpose inverters. Uninterrupted Power Supply (UPS).Uninterrupted Power Supply (UPS). Welding Equipments.Welding Equipments. Numerical control, Cutting tools.Numerical control, Cutting tools. Robotics & Induction heating.Robotics & Induction heating.

79Prof. M. Madhusudhan Rao, E&C Dept., MSRIT

MCT

Comparison of Controllable Switches

Summary of Device Capabilities