LECTURE 18 (Ch. LECTURE 18 (Ch. 7)7)

THYRISTORS THYRISTORS (SCRs)(SCRs)

ECE 452ECE 452

Power ElectronicsPower Electronics

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Thyristor TypesThyristor Types

Thyristors are manufactured by diffusion.Thyristors are manufactured by diffusion.

The anode current requires a finite time to The anode current requires a finite time to propagate to the whole area of the propagate to the whole area of the junction.junction.

This is from the point near the gate when This is from the point near the gate when the gate signal is applied.the gate signal is applied.

Manufacturers use various gate structures Manufacturers use various gate structures to control the di/dt, turn-on time, and turn-to control the di/dt, turn-on time, and turn-off time.off time.

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Based on physical construction, turn-on and Based on physical construction, turn-on and turn-off behavior, thyristors are classified into turn-off behavior, thyristors are classified into nine categories:nine categories:

Phase-controlled thyristors (SCRs)Phase-controlled thyristors (SCRs) Bidirectional phase-controlled thyristors (BCTs)Bidirectional phase-controlled thyristors (BCTs) Fast-switching thyristors (SCRs)Fast-switching thyristors (SCRs) Light-activated silicon-controlled rectifiers (LASCRs) Light-activated silicon-controlled rectifiers (LASCRs) Bidirectional triode thyristors (TRIACs)Bidirectional triode thyristors (TRIACs) Reverse-conducting thyristors (RCT)Reverse-conducting thyristors (RCT)

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Gate-turn-off thyristors (GTOs)Gate-turn-off thyristors (GTOs) FET-controlled thyristors (FET-CTHs)FET-controlled thyristors (FET-CTHs) MOS-controlled thyristors (MCTs)MOS-controlled thyristors (MCTs) Emitter turn-off (control) thyristors Emitter turn-off (control) thyristors

(ETOs)(ETOs) Integrated gate-commutated thyristors Integrated gate-commutated thyristors

(IGCTs)(IGCTs) MOS-controlled thyristors (MCTs)MOS-controlled thyristors (MCTs) Static induction thyristors (SITHs)Static induction thyristors (SITHs)

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Phase-Controlled Phase-Controlled ThyristorsThyristors

This type of thyristors operates at the line This type of thyristors operates at the line frequency and is turned off by natural frequency and is turned off by natural commutation.commutation.

The turn-off time is of the order of 50 to The turn-off time is of the order of 50 to 100 100 μμs.s.

They are used for low-speed switching They are used for low-speed switching applications, also known as converter applications, also known as converter thyristor.thyristor.

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The on-state voltage varies from The on-state voltage varies from 1.15 V for 600V to 2.5 V for 4000 V 1.15 V for 600V to 2.5 V for 4000 V SCRs, and 1.25 V for 1200 V and SCRs, and 1.25 V for 1200 V and 5500 A thyristor.5500 A thyristor.

The dv/dt is about 1000 V/The dv/dt is about 1000 V/μμs and the s and the di/dt is 500 A/di/dt is 500 A/μμs.s.

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BCTsBCTs It is a new concept for high power It is a new concept for high power

phase control.phase control.

It combines advantages of having It combines advantages of having two antiparallel thyristor in one two antiparallel thyristor in one package.package.

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The maximum voltage rating is as high as 6.5 kV The maximum voltage rating is as high as 6.5 kV at 1.8 kA.at 1.8 kA.

The maximum current rating is 3 kA at 1.8 kV.The maximum current rating is 3 kA at 1.8 kV.

A BCT has two gates; one for turning on the A BCT has two gates; one for turning on the forward current, and one for turning on the forward current, and one for turning on the reverse current.reverse current.

This thyristor is turned on with a pulse current to This thyristor is turned on with a pulse current to one of its gates.one of its gates.

It is turned off if the anode current falls below the It is turned off if the anode current falls below the holding current.holding current.

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Fast-Switching Fast-Switching ThyristorsThyristors

The thyristors are used in high-speed The thyristors are used in high-speed switching applications with forced switching applications with forced commutation. commutation.

They have fast turn-off time, in the range They have fast turn-off time, in the range of 5 to 50 of 5 to 50 μμs. s.

The on-state forward voltage drop is The on-state forward voltage drop is inversely proportional to the turn-off time.inversely proportional to the turn-off time.

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This type of thyristor is also known This type of thyristor is also known as an inverter thyristor.as an inverter thyristor.

They have high dv/dt of 1000 V/They have high dv/dt of 1000 V/μμs s and high di/dt of 1000 A/and high di/dt of 1000 A/μμs.s.

The on-state forward voltage is about The on-state forward voltage is about 1.7 V for a 2200 A, 1800 V SCR.1.7 V for a 2200 A, 1800 V SCR.

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Light-Activated SCRLight-Activated SCR

This device is turned on by direct radiation This device is turned on by direct radiation of light on the silicon wafer.of light on the silicon wafer.

The gate structure is designed to provide The gate structure is designed to provide sufficient gate sensitivity for triggering sufficient gate sensitivity for triggering from practical light sources.from practical light sources.

The LASCRs are used in high-voltage and The LASCRs are used in high-voltage and high-current applications such as HVDC high-current applications such as HVDC and SVC.and SVC.

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LASCR offers complete electrical isolation LASCR offers complete electrical isolation between the light triggering source and the between the light triggering source and the switching device of a power converter, which switching device of a power converter, which floats at a potential as high as a few hundred kV.floats at a potential as high as a few hundred kV.

The voltage rating can be as high as 4 kV at 1500 The voltage rating can be as high as 4 kV at 1500 A with light triggering of less than 100 mW.A with light triggering of less than 100 mW.

The typical di/dt is 250 A/The typical di/dt is 250 A/μμs and the dv/dt could s and the dv/dt could be as high as 2000 V/be as high as 2000 V/μμs.s.

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Bidirectional-Triode Bidirectional-Triode ThyristorsThyristors

A TRIAC can conduct in both directions.A TRIAC can conduct in both directions.

   It is normally used in ac phase control It is normally used in ac phase control

applications such as ac voltage applications such as ac voltage controllers.controllers.

It can be considered as two SCRs It can be considered as two SCRs connected in antiparallel with a connected in antiparallel with a common gate connection.common gate connection.

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Since a TRIAC is a bidirectional device, its Since a TRIAC is a bidirectional device, its terminals cannot be designated as anode or terminals cannot be designated as anode or cathode.cathode.

If MTIf MT22 is positive with respect to MT is positive with respect to MT11, the , the TRIAC can be turned on by applying a TRIAC can be turned on by applying a positive gate signal between G and MTpositive gate signal between G and MT11..

If MTIf MT22 is negative with respect to MT is negative with respect to MT11, the , the TRIAC can be turned on by applying a TRIAC can be turned on by applying a negative gate signal between G and MTnegative gate signal between G and MT11..

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Reverse-Conducting Reverse-Conducting ThyristorsThyristors

In many choppers and inverter circuits, an In many choppers and inverter circuits, an antiparallel diode is connected across an antiparallel diode is connected across an SCR.SCR.

This is to allow the reverse current flow This is to allow the reverse current flow due to an inductive load.due to an inductive load.

The diode clamps the reverse blocking The diode clamps the reverse blocking voltage of the SCR to 1 or 2 V under the voltage of the SCR to 1 or 2 V under the steady-state conditions.steady-state conditions.

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An RCT is a compromise between the An RCT is a compromise between the device characteristics and circuit device characteristics and circuit requirements.requirements.

It is also called an asymmetrical It is also called an asymmetrical thyristor (ASCR).thyristor (ASCR).

The forward blocking is as high as The forward blocking is as high as 2000 V and the current rating goes 2000 V and the current rating goes up to 500 A.up to 500 A.

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Gate-Turn-Off ThyristorsGate-Turn-Off Thyristors

A gate-turn-off thyristor (GTO) like an SCR can A gate-turn-off thyristor (GTO) like an SCR can be turned on by applying a positive gate signal.be turned on by applying a positive gate signal.

GTO can be turned off by a negative gate GTO can be turned off by a negative gate signal.signal.

GTOs have certain advantages over SCRs:GTOs have certain advantages over SCRs: Elimination of the commutating componentsElimination of the commutating components Reduction of electromagnetic noise due to Reduction of electromagnetic noise due to

elimination of the commutation chokeselimination of the commutation chokes

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Faster turn-off timesFaster turn-off times Improved efficiency of the converters Improved efficiency of the converters 

A large initial gate trigger pulse is A large initial gate trigger pulse is required to turn on a GTO.required to turn on a GTO.

Once the GTO is turned on, forward gate Once the GTO is turned on, forward gate current must be continued for the whole current must be continued for the whole conduction period (1% of the turn on conduction period (1% of the turn on pulse).pulse).

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A GTO requires a relatively high A GTO requires a relatively high negative current pulse to turn off.negative current pulse to turn off.

It also has a higher on-state voltage It also has a higher on-state voltage of 3.4 V for a 550 A and 1200 V of 3.4 V for a 550 A and 1200 V device.device.

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FET-Controlled ThyristorFET-Controlled Thyristor

A FET-CTH device combines A FET-CTH device combines MOSFET and a thyristor.MOSFET and a thyristor.

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If a sufficient voltage is applied to the gate If a sufficient voltage is applied to the gate of the MOSFET (3 V), a triggering current of the MOSFET (3 V), a triggering current for the thyristor is generated.for the thyristor is generated.

It has a high switching speed, high di/dt, It has a high switching speed, high di/dt, and high dv/dt.and high dv/dt.

This device can be turned on like This device can be turned on like conventional thyristors, but it cannot be conventional thyristors, but it cannot be turned off by gate control.turned off by gate control.

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MOS Turn-Off ThyristorMOS Turn-Off Thyristor

It is a combination of a GTO and a It is a combination of a GTO and a MOSFET.MOSFET.

It overcomes limitations of the GTO It overcomes limitations of the GTO turn-off ability.turn-off ability.

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The main drawback of a GTO is that they The main drawback of a GTO is that they require a high pulse current drive circuit for require a high pulse current drive circuit for the low impedance gate.the low impedance gate.

The typical amplitude of the current for the The typical amplitude of the current for the gate circuit is about 35% of the main current.gate circuit is about 35% of the main current.

The MTO provides the same functionality as The MTO provides the same functionality as the GTO but uses a gate drive that needs to the GTO but uses a gate drive that needs to turn on a MOS transistor on and off.turn on a MOS transistor on and off.

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It can handle 10kV and up to 4 kA.It can handle 10kV and up to 4 kA.

It can be used in power applications from 1 to 20 It can be used in power applications from 1 to 20 MVA.MVA.

The MTO is turned on similar to conventional The MTO is turned on similar to conventional SCRs by applying a current pulse to its gate. SCRs by applying a current pulse to its gate.

To turn off the MTO, a gate pulse voltage is To turn off the MTO, a gate pulse voltage is applied to the MOSFET gate.applied to the MOSFET gate.

Turning on the MOSFET shorts the emitter-base Turning on the MOSFET shorts the emitter-base of the npn transistor and turns the SCR off by of the npn transistor and turns the SCR off by stopping the latching process.stopping the latching process.

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Emitter Turn-Off Emitter Turn-Off ThyristorsThyristors

ETO is a MOS-GTO hybrid device that ETO is a MOS-GTO hybrid device that combines the advantages of GTO and combines the advantages of GTO and MOSFET.MOSFET.

ETO has two gates: one normal gate for ETO has two gates: one normal gate for turn-on and one with a series MOSFET for turn-on and one with a series MOSFET for turn-off.turn-off.

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High power ETOs with current and High power ETOs with current and voltage ratings of 4 kA and 6 kV have voltage ratings of 4 kA and 6 kV have been demonstrated.been demonstrated.

ETO is turned on by applying a ETO is turned on by applying a positive voltage to gates 1 and 2.positive voltage to gates 1 and 2.

A negative voltage to the gate will A negative voltage to the gate will turn off the device.turn off the device.

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IGCTIGCT

The internal structure and equivalent The internal structure and equivalent circuit of an IGCT are similar to that circuit of an IGCT are similar to that of GTO.of GTO.

It is turned on by applying a current It is turned on by applying a current to the gate.to the gate.

It is turned off by applying a fast It is turned off by applying a fast rising and high gate current.rising and high gate current.

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MOS-Controlled MOS-Controlled ThyristorThyristor

A MOS-Controlled Thyristor (MCT) A MOS-Controlled Thyristor (MCT) combines the features of a combines the features of a regenerative four-layer thyristor and regenerative four-layer thyristor and a MOS-gate structure.a MOS-gate structure.

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The gate structure can be represented by a p-The gate structure can be represented by a p-channel MOSFET and an n-channel MOSFET.channel MOSFET and an n-channel MOSFET.

An MCT has:An MCT has: Low voltage drop during conductionLow voltage drop during conduction Fast turn-on time (0.4 Fast turn-on time (0.4 μμs) and fast turn-off time s) and fast turn-off time

(1.25 (1.25 μμs)s) Low switching lossesLow switching losses Low reverse voltage blocking capabilityLow reverse voltage blocking capability High gate input impedanceHigh gate input impedance

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Static Induction Static Induction ThyristorThyristor

The characteristics of a SITH are The characteristics of a SITH are similar to those of a MOSFET.similar to those of a MOSFET.

A SITH is normally turned on by A SITH is normally turned on by applying a positive gate voltage like applying a positive gate voltage like normal thyristors, and is turned off normal thyristors, and is turned off by application of a negative voltage by application of a negative voltage to its gate.to its gate.

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SITH has low on-state resistance or voltage SITH has low on-state resistance or voltage drop.drop.

A SITH has fast switching speeds and high A SITH has fast switching speeds and high dv/dt and di/dt capabilities.dv/dt and di/dt capabilities.

The switching time is in the order of 1 to 6 The switching time is in the order of 1 to 6 μμs.s.

The voltage rating is as a high a 2500 V and The voltage rating is as a high a 2500 V and the current rating is limited to 500 A.the current rating is limited to 500 A.

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Series Operation of Series Operation of ThyristorsThyristors

For high voltage applications, two or more For high voltage applications, two or more thyristors can be connected in series.thyristors can be connected in series.

Due to manufacturing, the characteristics Due to manufacturing, the characteristics of thyristors of the same type are not of thyristors of the same type are not identical.identical.

The following figure shows the off-state The following figure shows the off-state characteristics of two thyristors.characteristics of two thyristors.

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For the same off-state current, their For the same off-state current, their off-state voltage differ.off-state voltage differ.

The voltage sharing is accomplished The voltage sharing is accomplished by connecting resistors across each by connecting resistors across each thyristor.thyristor.

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For equal voltage sharing, the off-For equal voltage sharing, the off-state currents differ as shown below.state currents differ as shown below.

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Let there be nLet there be nss thyristors in the string. thyristors in the string.

The off-state current of thyristor TThe off-state current of thyristor T11 is I is ID1D1, , and that of other thyristors are equal (if and that of other thyristors are equal (if they share equal voltages) such that:they share equal voltages) such that:

IID2D2 = I = ID3D3 = I = IDnDn & I & ID1D1 < I < ID2D2

Since TSince T11 has the least off-state current, T has the least off-state current, T11 will share higher voltage (if they don’t will share higher voltage (if they don’t share equal voltages).share equal voltages).

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The value of resistor R is calculated from:The value of resistor R is calculated from:

Where, Where,

will be maximum when is will be maximum when is

maximum or when is zero.maximum or when is zero.

s

DssD n

IRnVV

)1(1

12 DDD III

1DV DI1DI

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During the turn-off, the differences During the turn-off, the differences in stored charge cause differences in in stored charge cause differences in the reverse voltage sharing.the reverse voltage sharing.

s

DssDS n

RInVV 2

(max)

)1(

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The thyristor with the least recovery charge, The thyristor with the least recovery charge, or reverse recovery time, will face the highest or reverse recovery time, will face the highest transient voltage.transient voltage.

It will become necessary to use an RC It will become necessary to use an RC network across each thyristor, and R limits network across each thyristor, and R limits the discharge current.the discharge current.

The same RC network is used for both The same RC network is used for both transient voltage sharing and dv/dt protection.transient voltage sharing and dv/dt protection.

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The transient voltage across the The transient voltage across the thyristor is obtained from:thyristor is obtained from:

])1(

[1

])1(

[1

1

2(max)

11

11

12

C

QnV

nV

C

QnV

nV

C

Q

C

QQIRV

ss

sDT

ss

sD

D

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A derating factor which is normally A derating factor which is normally used to increase the reliability of the used to increase the reliability of the string is defined as:string is defined as:

(max)

1DSs

s

Vn

VDRF

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