Classification of Thyristors:Thyristors are classified as follows:Thyristors are classified as follows:

1. Phase-controlled thyristors [or Silicon-controlled

rectifiers (SCRs)]

2. Fast switching thyristors (or SCRs)

3. Gate-turn off thyristors (GTOs)

4. Bidirectional triode thyristors (TRIACs)4. Bidirectional triode thyristors (TRIACs)

5. Reverse-conduction thyristors (RCTs)

6. Static induction thyristors (SITHs)

7 Light activated silicon controlled rectifiers (LASCRs)7. Light-activated silicon-controlled rectifiers (LASCRs)

8. FET-controlled thyristors (FET-CTHs)

9. MOS-controlled Thyristors (MCTs)

10. MOS turn-off thyristors (MTOs)

11. Bidirectional phase-controlled thyristors (BCTs)

12. Emiter turn off (control) thyristors (ETOs). e u o (co o ) y s o s ( Os)

13. Integrated gate-commutated thyristors (IGCTs)

Different Types of Thyristors

Phase-Control Thyristor (or SCRs)Control Characteristic: Turn-on with a pulse signal (Current for turn on); Turn-

off with natural commutation (No turn off control).

Switching frequency: Low 60 Hz i.e it is suited for low speed switching

applications.

Turn-off time: 50 to 100 µs.

On-state voltage drop: Varies typically from about 1.15 V for 600 V to 2.5 V

f 4000 V d i d f 5500 A 1200 V th i t it i t i ll 1 25 Vfor 4000 V devices; and for a 5500 A 1200 V thyristor it is typically 1.25 V.

Advantages: Simple turn-on; Latching device; Turn-on gain is very high; Low

cost; high voltage; and high current device.

Disadvantages: Low-switching speed; Most suited for line commutatedDisadvantages: Low-switching speed; Most suited for line commutated

applications between 50 and 60 Hz; cannot be turned-off with gate control.

SCR as an amplifierSCR as an amplifier

The modern thyristors use an amplifying gate, where an

auxiliary thyristor TA is gated on by a gate signal and

then the amplified output of TA is applied as agate signalthen the amplified output of TA is applied as agate signal

to the main thyristor TM.

Fast Switching Thyristors (or SCRs)

Control Characteristic: Turn-on with a pulse signal (Current forControl Characteristic: Turn on with a pulse signal (Current for

turn on); Turn-off with natural commutation (No turn off control).

Switching frequency: Medium 5 kHz, these are used in high speed

switching application with forced commutation such as in inveterswitching application with forced commutation such as in inveter

and chopper circuit.

Turn-off time: 5 to 50 µs (fast turn off time).

O t t lt d L F 2200A 1800 V h i iOn-state voltage drop: Low. For 2200A 1800 V thyristor is

typically 1.7 V. The on-state forward voltage drop varies

approximately as an inverse function of the turn-of time. This type of

thyristor is also known as an inverter thyristor.

dv/dt capability: high dv/dt o typically 1000 V/µs.

di/dt capability: high di/dt o typically 1000 A/µs.p y g yp y µAdvantages: Same as the phase-controlled SCRs, except the turn-off

is faster. The fast turn-off and high di/dt are very important to reduce

the size and weight of commutating and/or reactive circuitthe size and weight of commutating and/or reactive circuit

components.

Disadvantages: Similar to those of phase-controlled SCRs.

Gate-Turn off Thyristors (GTOs)Control Characteristic: Turn-on with a positive pulse signal; Turn-

ff ith ti l i l ( t f b th t d t ffoff with a negative pulse signal (current for both turn-on and turn-off

control).

Switching frequency: Medium 5 kHz.

O b i h hi h l hOn-state voltage drop: Low but it has higher on-state voltage than

that of SCRs. 3.4 V for 550 A 1200 V GTO.

Advantages: Similar to the fast switching thyristors, except it will be

turned-off with a negative gate signal.

Disadvantages: Turn-off gain is low between 5 and 8 and it requires a

large gate current to turn-off a large on-state current; there is a long tail

current during turn-off; although a latching device, it requires a

minimum gate current to sustain on-state current.

Advantages of GTOs over SCRs:

1. Elimination commutating components in forcedg p

commutation, resulting in reduction in cost, weight, and

volume,

2 Reduction in acoustic and electromagnetic noise due to the2. Reduction in acoustic and electromagnetic noise due to the

elimination of commutation chokes,

3. Faster turn-off permitting high switching frequencies, and

4 I d ffi i f4. Improved efficiency of converters.

Advantages (in low power application) of GTOs over power

BJTBJTs:

1. A higher blocking voltage capability;

2. A high ratio of peak controllable current to average current,

3. A high ratio of peak surge current to average current,

typically 10:1;

4. A high on-state gain (anode current/gate current), typically4. A high on state gain (anode current/gate current), typically

600; and

5. A pulsed gate signal of short duration.

Bidirectional Triode Thyristors or Triode

AC Semiconductor Switches (TRIACs)C Se co duc o Sw c es ( Cs)A triac is equivalent to a pair of antiparallel connected

SCRs. It has one gate for turning-on in both directions.

It can conduct in both directions and is normally used in

ac phase control (e.g. ac voltage controller)

Control Characteristic: Turn-on applying gate a pulse

signal for current flow in both directions; Turn-off with

natural commutation (Current for turn-on; No turn-off

control).

Switching frequency: Low 60 Hz.

On state oltage drop: LOn-state voltage drop: Low.

Advantages: Same as the phase-controlled SCRs, except

the current can flow in both directions.

Disadvantages: Similar to those of phase-controlledDisadvantages: Similar to those of phase-controlled

SCRs; except for low-power applications.

Advantages of Triacs over Antiparallel SCRsA triac is equivalent to a pair of antiparallel connected SCRsA triac is equivalent to a pair of antiparallel connected SCRs.

1. Triacs can be triggered with positive or negative polarity voltages.

2. A triac needs a single heat sink of slightly larger size, whereas antiparallel

thyristor pair needs two heat sinks smaller sizes, but due to the clearancey p ,

total space required is more for thyristors.

3. A Triac needs a single fuse for protection, which also simplifies

construction.

4. In some dc applications, SCR is required to be connected with a parallel

diode to protect against reverse voltage, whereas a Triac used many work

without a diode, as safe breakdown in either direction is possible.

Disadvantages of Triacs over Antiparallel SCRs1 Triacs have low dv/dt rating compared to SCRs1. Triacs have low dv/dt rating compared to SCRs.

2. SCRs are available in larger rating compared to Triacs.

3. Since a Triac can be triggered in either direction, a trigger circuit with Triac

needs careful consideration.needs careful consideration.

4. Reliability of Triacs is less than that of SCRs.

Reverse Conducting Thyristors (RCTs)RCT is also known as asymmetrical thyristors (ASCRs).y y ( )

The forward blocking voltage varies from 400 to 2000 V and the current

rating goes up to 500 A.

The reverse blocking voltage is typically 30 to 40 V.e eve se b oc g vo tage s typ ca y 30 to 0 V.

The difference between the SCR and RCT is that in RCT has an

integrated reverse diode.

Because of the reverse diode, RCT is not capable of reverse blockingBecause of the reverse diode, RCT is not capable of reverse blocking

thus it is called ASCR.

These devices are advantageous where a reverse or freewheel diode

must be usedmust be used.

Because the SCR and diode never

conduct at the same time they do not

produce heat simultaneously and can

easily be integrated and cooled together.

Reverse conducting thyristors are ofteng y

used in frequency changers and

inverters.

Light-Activated Silicon-Controlled Rectifiers (LASCRs)Control Characteristic: Turn-on by direct radiation on the silicon wafer withy

light. Turn-off with natural commutation.

Switching frequency: Low 60 Hz.

On-state voltage drop: Low.

Maximum voltage rating: 4 kV

Maximum current rating: 1500 A

Application: used in high-voltage and high-current applications such as HVDC

t i i li d St ti ti lt ti VARtransmission line and Static reactive power or volt ampere reactive VAR

compensation.

dv/dt capability: these thyristors have high dv/dt o typically 2000 V/µs.

di/dt capability: these thyristors have di/dt o typically 250 A/µsdi/dt capability: these thyristors have di/dt o typically 250 A/µs.

Advantages: Same as the phase-

controlled SCRs, except the gate iscontrolled SCRs, except the gate is

isolated and can be remotely

operated.

Disadvantages: Similar to those of

phase-controlled SCRs.

FET-Controlled Thyristors (FET-CTHs)A FET-CTH device combines an MOSFET and a thyristor in parallel.y p

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

This device can turn-on like conventional thyristors, but it cannot be

turned-off by gate control.

MOS-Controlled thyristors (MCTs)An MCT is an improvement over a thyristor with a pair of MOSFETs to turn-on

and turn-off currentand turn off current.

The MCT overcomes several of the limitations of the existing power devices and

promises to be a better switch for the future.

Control Characteristic: Turn-on p-channel MCT with a negative voltage withp g g

respective to anode and turn-off with positive voltage.

Switching frequency: Medium 5 kHz

On-state voltage drop: Medium

Advantages: Integrated the advantages of GTOs and MOSFT gate into a single

device; the power/energy required for the turn-on and turn-off is very small, it

has a low on-state voltage drop.

MOS turn-off thyristors (MTOs)MTO is a combination of a GTO and an MOSFET which together overcome the

limitation of the GTO turn-off capabilitylimitation of the GTO turn off capability.

Control Characteristic: Turn-on with a positive pulse current to the turn on

gate. Turn off with a positive voltage to the turn off gate.

Switching frequency: Medium 5 kHz.g q y

On-state voltage drop: Low.

Maximum voltage rating: 10 kV @ 20 MVA, 4.5 kV @ 500 A.

Maximum current rating: 4 kA @ 20 MVA.

Advantages: Similar to those GTOs, except it can be turn on through the normal

gate and turn off through the MOSFET gate; Due to the MOS gate it requires a

very low turn-off current and the turn-off time is small.

Di d t Si il GTO i h l il d i ffDisadvantages: Similar to GTOs, it has a along tail current during turn-off.

Emiter turn off (control) thyristors (ETOs)ETO is a combination of a GTO and two MOSFET. A MOSFET is connected in

series with GTOseries with GTO.

Control Characteristic: Turn-on with a positive pulse current to the turn on

gate, turn off with a negative pulse voltage to the turn off MOS gate.

Switching frequency: Medium 5 kHz.g q y

On-state voltage drop: Medium

Maximum voltage/ current rating: 6 kV

Advantages: Due to series MOS the transfer of current to the cathode region is

rapid and fast turn off. The series MOSFET has to carry the main anode current

Disadvantages: Similar to GTOs, it has a along tail current during turn-off. The

series MOSFET has to carry the main anode current it increases the on-state

l d b b 0 3 0 5 V d h d i lvoltage drop by about 0.3 to 0.5 V and the conduction losses.

Integrated gate-commutatedthyristors (IGCTs)IGCT d l d ith hit tIGCT developed with an architecture

combining the best features of an

Insulated

Gate Bipolar Transistor (IGBT) and aGate Bipolar Transistor (IGBT) and a

Gate Turn-Off thyristor (GTO).

IGCT can switch with the speed of an

Insulated Gate Bipolar TransistorInsulated Gate Bipolar Transistor

(IGBT) and conduct like a Gate Turn-

Off thyristor (GTO).

IGCT can be turned on and off by they

gate signal, have low conduction loss

and require no dv/dt snubber.

Control Characteristic: Turn-on

with a positive pulse current to the

turn on gate. Turn off by applying a

fast rising negative current from a

m ltila ered gate dri er circ itmultilayered gate-driver circuitboard.

Bidirectional phase-controlled thyristors (BCTs)

Triggering DevicesTriggering Devices

Triggering Devices

Unijunction Transistor (UJT)Unijunction Transistor (UJT)

Programmable Unijunction Transistor (UJT)

DIAC (Diode for Alternating Current)

Shockley Diodey

Silicon Unilateral Switch (SUS)

Silicon Bilateral Switch (SBS)

Asymmetrical AC Triggery gg

Unijunction Transistor (UJT)

A unijunction transistor (UJT) is an electronic semiconductor

device that has only one junction and three terminals.

It is made from a bar of lightly doped N-type silicon with aIt is made from a bar of lightly doped N type silicon with a

heavily doped P-zone alloyed into the bar.

The P-zone forms the emitter section

of the transistor and its only junctionof the transistor and its only junction,

hence the name unijunction.

UJT used in general purpose pulse,g p p p ,

timing, sense, and Trigger application.

The terminals name of UJT are:

Emitter (E), Base 1 (B1) and Base 2

(B2).

RB1 and RB2 are called internal

dynamic resistance.

Interbase resistance: R =RInterbase resistance: RBB =RB1

+RB2 (4kΩ ~ 10 KΩ)RB1 > RB2, RB2 is fixed and RB1 is

i bl i t It ia variable resistance. It varies

from 4 kΩ to down 10 Ω. RB1

varies inversely IE.y E

Both B2 and emitter (E) biased

positive relative to B1.1

VBB is fixed and VEE is variable.

BBVBBVBBRBR

BBVBRBR

BRxV η==+= 1

21

1

η is called intrinsic stand off ratio (varies0.5 ~ 0.8 but for a given UJT this value is

fixed).fixed).

VP is called the peak-

i t lt hi h i

DVxVPV +=point voltage which is

the maximum forward

blocking state voltage.g g

IP is called the peak-

point current

corresponding to VP.

If VE<Vp the emitter diode DE is reverse biased and UJT is at

turn-off state. In this state, a small leakage current (IEO) flows

through the emitter and R is at its high value (approximatelythrough the emitter and RB1 is at its high value (approximately

4kΩ).

If VE>Vp, the emitter diode DE is forward biased and UJT is atIf VE Vp, the emitter diode DE is forward biased and UJT is at

turn-on state.

In the peak-point holes

are injected from the

heavily doped P to lightlyy p g y

doped N bar.

Since N-bar is lightly

doped a little number ofdoped, a little number of

holes recombines with

electron.

Due to the increase of holes carrier the current below

the N-bar increases and RB1 is drastically reduced. InB1

this moment Vx decreases and DE becomes more

forward biased, IE increases and VE decreased.

Th l I i j t h l i t B f th d iThe larger IE injects more holes into B1, further reducing

RB1 and so on.

When this regenerative or snowballing process endsWhen this regenerative or snowballing process ends,

RB1 has dropped to a very small value and IE can

become very large, limited by R.y g , y

Because the voltage

decreases as increases ofdecreases as increases of

current in so UJT can be

used as a negative

i t d iresistance device.

The emitter voltage and current reach to the valley point. VV and

IV are the voltage and current corresponding to the valley point.

Aft th ll i t th lt V i l t t t thiAfter the valley point the voltage VE is almost constant so this

region is called saturation region.

Turning OFF: If VEE decreases

the IE is also decreased. When

IE decreases up to IV, UJT

turns off and its operationp

rapidly switches back to cut off

region.

I is same as holding currentIV is same as holding current

of thyristor.

UJT as Relaxation Oscillator

When V is applied the capacitor C is charged through theWhen Vs is applied, the capacitor C is charged through the

resistor R since the emitter circuit of UJT is in open state. The

charging time constant is τ1 = RC.

When the emitter voltage VE, reaches the peak-point voltage Vp,

the UJT turns on and C will discharge through RB1.

The discharging time constant is τ2=RB1C. τ2 must smaller thanτ1.

The period of oscillation, T, is fairly independent of the dc supply

voltage Vs, and is given by ≈= 1ln1 RCT η−1lnRC

fT

The width tg of triggering pulse is: tg = τ2=RB1C

Resistor RB2 has a value of 100 Ω or greater and can bedetermined approximately from:

sVBR η 4102=

Limiting Value of R

To turn on a UJT should be operate the right of peak-point butTo turn on a UJT should be operate the right of peak point but

to the left of valley point. Thus the maximum value of R is given

by:pVsV

R−=pI

pR =max

The minimum value of R ensure the turn of where IE=IV and

vIvVsVR −=min

E V

VE=VV. Thus the maximum value of R is given by:

So R should be vchosen between

these two limit

that meansthat means

Rmin<R<Rmax.

E l 4 5Example 4.5

Example 17.2

UJT as SCR Trigger

Synchronized UJT Trigger (Ramp Triggering)

The firing angle

can be

controlled up to

150o.

Programmable Unijunction Transistor (PUT)

The PUT is an improved version ofUJT deviceThe PUT is an improved version ofUJT device.

PUT is more similar to an SCR (four-layer device) except

that its anode-to-gate voltage can be used to both turn ong g

and turn off the device.

Applications: Motor control, Oscillators, Relay replacement,

Timers, Pulse shapers, Triggering Pulse etc.

Programmable: R V or I V orProgrammable: RBB, VP or IP, VV or

IV and η.

The gate voltage VG is maintained from the supply by

the resistor divider R1 and R2, and determines the

peak voltage VP.

I th f UJT V i fi d f d i b th dIn the case of UJT, VP is fixed for a device by the dc

supply voltage. But VP of a PUT can be varied by

varying the resistor divider R1 and R2varying the resistor divider R1 and R2.

If VA<VG, the device will remain in its off-state.

If VA> VG + VD, the peak point is reached and theA G D p p

device turns-on.

The peak current IP and the valley current IV both

depend on the equivalent impedance on the gate Rdepend on the equivalent impedance on the gate RG

and the dc supply voltage Vs.

VP is given by: RVP is given by:

sVRR

RPV

21

2+=

The intrinsic stand off ratio is given by:

RV

21

2RR

R

sVPV +==η

PUT as Relaxation Oscillator

R and C control the frequency along with R1 and R2 the periodR and C control the frequency along with R1 and R2. the period

of oscillation T is given approximately by :

⎞⎜⎛ 21ll1 RRCVRCT

⎟⎟⎟⎠⎜⎜

⎜⎜

⎝+=−≈=

1

21lnln1R

RRC

PVsVsVRC

fT

The intrinsic stand off ratio is given by:g y

GRsV

GI )1( η−=R1 and R2 can be found from:

ηη −==12;1

GRRGR

R

Example 4 6Example 4.6

Example 17.3

Advantages of PUT over UJT

1 The switching voltage is easily1. The switching voltage is easily

varied Vp through the voltage

divider resistorsdivider resistors.

2. The PUT can operate low

voltages making it compatible

with integrated circuit.g

3. The peak point current of OUT

is low.is low.

DIAC (Diode for Alternating Current)

The DIAC is a diode that conducts current only after itsThe DIAC is a diode that conducts current only after its

breakdown voltage has been reached momentarily.

A diac is a two terminal five layer semi-A diac is a two terminal five layer semi

conductor bi-directional switching device.

The device consists of two p-n-p-n sections

i ti ll lin anti-parallel.

Diac will conduct when the

voltage applied across the devicevoltage applied across the device

terminals T1 & T2 exceeds the

break over voltage.

A diac phase (ac) control circuit is shown in Fig. 2-59. On either

alternation, the capacitor voltage reaches the breakover

t ti l th di fi d t th t i ONpotentials, the diac fires and gates the triac ON.