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EE420 POWER

ELECTRONICSChapter 07 ThyristorsLecture 14-15

Dr. Tauseef Tauqeer

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Thyristors

2

Learning objectives

To learn about different type of thyristors,

Limitation of thyristors as switches,

Understand the gate characteristics and gate control

requirement of different type of thyristors.

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Thyristors are family of power semiconductor devices and are usedextensively in the power electronics circuits.

Conventional thyristors are designed with gate controlled turned on

capability but without gate controlled turn-off capability, in which case

the thyristor can recover from its non-conducting state only when the

current is brought to zero by some other means. Gate turn-off thyristors (GTOs) are designed to have both controlled

turn-on and turn-off capability.

Compared to the transistors, thyristors have lower on-state conduction

losses and higher power handling capability.

On the other hand transistors generally have superior switchingperformances in terms of faster switching speed and lower switching

losses.

Advances are being continuously made to achieve devices with the

best of both (i.e. low on-state and switching losses, while increasing

their power handling capability.

7.1: Introduction

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7.2: Thyristor Characteristics

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Figure 7.1: Thyristor symbol and three pn junctions

A thyristor is a four layer semiconductor device of pnpnstructure with three pn junctions. It has three terminals:anode, cathode, and gate.

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Figure 23-1(Mohan): Structural details of generic thyristor, vertical cross section

When the anode voltage is made positive with respect to cathode, the

junction J1 and J3 are forward biased. The junction J2 is reversebiased, and only a small leakage current flows from anode tocathode.

Once a gate signal is applied it triggers the conduction. With higherconcentration of holes in the p-type layer it becomes a +ve feedbackprocess that can sustain even if the gate signal is removed.

(appliedelectric field)

holes

electronselectrons

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When the anode voltage is

made positive with respect tocathode, the junction J1 and

J3 are forward biased. The

junction J2 is reverse biased,

and only a small leakage

current flows from anode tocathode. The thyristor is then

said to be in the Forward

Blocking, or Off State

Condition and the leakage

current is known as off-state

current ID.

If the anode-to-cathode voltage VAKis increased to a sufficiently large

value, the reverse biased junction J2 breaks. This is known as

Avalanche Breakdown and the corresponding voltage is called forward

breakdown voltage VBO.

Figure 7.3: Thyristor circuit and vicharacteristics.

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Because the other junctionsJ1and J3are already forward

biased, there is free

movement of carriers across

all three junctions, resulting

in a large forward anode

current. The device is then in

conducting state, or on state.

The voltage drop would be

due to the ohmic drop in the

four layers and it is small,typically, 1 V. Thyristor circuit and vicharacteristics.

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In the On state, the anode

current is limited by an external

impedance or a resistance RL

as shown in figure.

Thyristor circuit and vicharacteristics.

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Thus the thyristor is alatching device, it latchesinto full conduction in itsforward direction.

Thus as described above, athyristor can be turned on

by increasing the forwardvoltage VAKbeyond VBO, butsuch a turn on could bedestructive.

In practice, the forward

voltage VAK is maintainedbelow VBOand the thyristoris turned on by applying apositive voltage between itsgate and cathode.

Thyristor circuit and vicharacteristics.

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Latching Current IL is the

minimum anode current inthe on-state immediatelyafter the thyristor hasbeen turned on and thegate signal has been

removed.

Once the thyristor isturned on by a gatingsignal and its anodecurrent is greater than the

holding current, the devicecontinues to conduct dueto positive feedback, evenif gating signal is removed. Thyristor circuit and vi

characteristics.

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Once a thyristor conducts, it behaves like a conducting

diode and there is no control over the device. The devicecontinues to conduct because there is no depletion layer onthe junction J2due to free movement of carriers.

However, if the forward anode current is reduced below alevel known as the holding current IH, a depletion region

develops around junction J2 due to reduced number ofcarriers and the thyristor is in blocking state. HoldingCurrent (IH

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Forward voltage drop from 0.5 to 2V Triggered by a gate pulse of few mA.

Switching time 10 s to 400 s

After turn-on, the gate has no effect in turning off.

Have di/dt (up to 1000A/s) and dv/dt (1000V/s) limitations.

Thyristors available up to 6000 V, 6000 A.

Low cost, high efficiency, and high voltage and current capability.

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There are Various methods toturn on the Thyristor. Gate Current

is the most practical one.

If the thyristor is forward biased,

the injection of gate current by

applying positive gate voltage

between the gate and the cathode

terminals turns on the thyristor. As the gate current is

increased, the forward blocking

voltage is decreased.

Figure 7.7: Effect of Gate currenton the forward blocking voltage

7.4: Thyristor Turn ON

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The time delay known as turn on time, ton

is defined as the time

interval between 10% of steady state gate current (0.1 IG) and

90% of the steady state thyristor on-state current (0.9 IT).

Figure 7.8: Turn ON

characteristics

ton= td+ tr,

=delay time + rise time

td is defined as the timeinterval between 10% of gatecurrent and 10% of thyristor

on-state current. tris the time required for the

anode current to rise from10% on-state current to 90%of on-state current.


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The following should be considered in designing the gate

control circuit1. The gate signal should be removed after the thyristor

is turned on. A continuous gating signal wouldincrease the power loss in the gate junction.

2. There should be no gate signal when the thyristor isreverse biased; otherwise the thyristor may fail dueto an increased leakage current.

3. The width of the gate pulse tGmust be longer thanthe time required for the anode current to rise to the

Latching current value IL. In practice, the pulse widthtG is normally made more than the turn-on time ofthe thyristor.

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7.5: Thyristor Turn OFF

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A Thyristor that is in the on state, can be

turned off by reducing the forward current toa level below the holding current IH. There

are various techniques for turning off

(commutation techniques) a thyristor.

In all the commutation techniques, the anode current is maintainedbelow the holding current for a sufficient long time so that all the

excess carriers in the four layers are swept out or recombined.

Due to two outer pn-junctions, J1and J3, the turn-off characteristics

would be similar to that of a diode, exhibiting reverse recovery time

trr, and peak reverse recovery current IRR.

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timeionrecombinat

currentrecoveryreverse

rc

rr

t

t

17

rcrrq ttt

Turn-off time,

In line-commutated converter circuit where the input voltage isalternating, a reverse voltage appears across the thyristor

immediately after the forward current goes through the zero value.

This reverse voltage accelerates the turn-off process by sweeping

out the excess carriers from the pn-junctions J1and J3.

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D T f T

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rcrrq ttt

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Turn-off time,

The inner pn-junction J2requires a time known as recombination time trcto

recombine excess carriers. A negative reverse voltage would reduce thisrecombination time. trc is dependent on the magnitude of the reverse

voltage.

The turn off time tqis the sum of reverse recovery time trrand recombination

time trc.At the end of turn-off, a depletion layer develops across junction J2

and the thyristor recovers its ability to withstands forward voltage. In all thecommutation techniques, a reverse voltage is applied across the thyristor

during the turn-off process.

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The turn off time tq is the minimum value of time interval between

the instant when the on-state current has decreased to zero and the

instant when the thyristor is capable of withstanding forward voltage

without turning on. tqdepends on the peak value of on-state current

and instantaneous on state voltage.

Reverse recovery charge QRRis the amount of charge that has to be

recovered during turn-off process. Its value is determined from the

area enclosed by the path of reverse recovery current. The value of

QRR depends on the rate of fall of on-state current and the peak

value of on-state current before turn-off. QRR causes corresponding

energy loss within the device.

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The manufacturer use various gate structures to

control di/dt, turn on time, and turn off time.

Thyristor can be easily turned on with a short pulse.For turning off they require special drive circuitry orspecial internal structure to aid in the turn-off process.

There are several versions of thyristors with turn-offcapability and the goal of any new device is to improvethe turn-off capability.

With the emergence of new devices with both turn-onand turn-off capability, the device with just the turn-oncapability is referred to as Conventional Thyristor orjus the Thyristor.

Other members of the thyristor or silicon-controlledrectifier (SCR) family have other names based onacronyms.

7.6: Thyristor Types

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7.6: Thyristor Types

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1) Phase-controlled thyristor (or SCR)

2) Bidirectional phase-controlled thyristor (BCT)

3) Fast switching thyristor (or SCR)

4) Light-activated silicon-controlled rectifier (LASCR)

5) Bidirectional triode thyristor (TRIAC)

6) Reverse-conducting thyristor (RCT)

7) Gate turn-off thyristor (GTO)

8) FET-controlled thyristor (FET-CTH)

9) MOS turn-off thyristor (MTO)

10)Emitter turn-off (control) thyristor (ETO)11)Integrated gate-commutated thyristor (IGCT)

12)MOS-controlled thyristor (MCT)

13)Static Induction Thyristor (SITH)

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Controlled Rectifier

In diode rectifier, the DC voltage at the output is fixed and cannot be controlled and thus called non-controlled rectifier

In Controlled Rectifier, the output DC voltage is adjustable

Phase controlled thyristors are used

The output voltage of the thyristor rectifier is varied bycontrolling the delay or firing angle of thyristors

A phased control rectifier can be turned on by applying a shortpulse to the gate and turned off by natural line commutation

In case of highly inductive load, it is turned off by firinganother thyristor of the rectifier during the negative cycle ofthe input voltage

Examples: DC Welders, DC Motor Drives, Battery Charging,DC Power Supply etc.

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( )

( )

( )

2 2

( )

1sin

2

1 cos2

1sin

2

1 sin 2

2 2

o dc m

m

o dc

o dc

o rms m

m

V V d

V

VI

R

V V d

V

Single Phase Half Wave Rectifier (1/3)

Load is purely resistive, R



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Single Phase Half Wave Rectifier (2/3)

Load is purely / highly inductive, L

Average voltage across inductor is zero

From to , the energy will be stored in inductor

Beyond , Ldi/dt becomes negative and forcing thethyristor to remain on

-ev Ldi/dt will be equal to +ve Ldi/dt

The duration during the positive and negative cycle will e

the same i.e. -for +ve cycle and 2--for ev cycle So the thyristor conducts from to 2-



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Freq=50 Hz Cycle

11.78msec

Angel Calculations10msec=180deg

11.78msec=212deg

212-180=>32.14deg

Maths

= tan1

= 32.14deg

= 2 + ()2

Z=5.904 Ohm

=

with lag

311/5.904=52.7A

VL

Vs VR

IR,L

V1

VSINE R1

5

D1

DIODE

VRload

Current

V1(+)

L1

10mHL1(1)

Single Phase Half Wave Rectifier

RL load (3/3)


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Single Phase Full Wave Rectifier


RL load

( )

( )

( )

2 2

( )

( )( )

2sin2

2cos

2sin

2

2

o dc m

m

o dc

o dc

o rms m

m

s

o rms

o rms

V V d

V

VI

R

V V d

VV

VI

If =0 thenVdc=2Vm/

If =/2 thenVdc=0

If = thenVdc=-2Vm/

So, Vdc=+ve for 0< < /2

And, Vdc=-ve for /2 <

14-15 thyristors (1)

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