Module 10 SCR - seramporegirlscollege.org Science... · Resonant Pulse Commutation. Complementary...

1

Electronic Science Power Electronics

10. SCR

Module 10

SCR

1. Introduction

2. SCR characteristics

3. Two transistor static and transient models

4. SCR turn-on methods

5. SCR turn-off methods

6. SCR protection and Triggering circuits

7. Gate protection circuits

8. Summary

Learning Objectives

1. To study the SCR Characteristics

2. To understand two Transistor Static and Transient Models

3. To learn the SCR Turn-on and Turn-off methods

4. To study SCR Protections and Gate Triggering Circuits

5. To understand the Gate Protection Circuits

2


10. SCR

1. Introduction

Thyristor is power semiconductor device and is widely used in power electronic circuits. Thyristors are

operated as bi-stable switches, operating from non-conducting to conducting state. There are different

types of thyristors and will be discussed in subsequent section. Silicon controlled rectifier (SCR) is the

most widely used thyristor. SCR can be assumed as ideal unidirectional switch for many applications, but

practical thyristor has certain limitations. A SCR basically is a four layer semiconductor device. It has

pnpn structure with three junctions, J1, J2 and J3. It has three terminals anode, cathode and gate.

Figure 1 Thyristor symbol and three pn-junctions


When the anode voltage is made positive with respect to cathode, junctions J1 and J3 are forward biased

and J2 is reverse biased. Hence only small leakage current flows from anode to cathode. The thyristor then

said to be in off state condition. If anode to cathode voltage VAK is increased to a sufficiently large value,

the reverse biased junction J2 will break. This known as avalanche breakdown and corresponding voltage

is called as forward break over voltage. Since J1 and J3 are already in forward biased, resulting in large

forward current. The device is then in conduction state or ON state. The voltage drop will be due to ohmic

drop in the layers, and it is small about 1V. In ON state the anode current is limited by external resistance

RL. The anode must be more than a value known as latching current IL, in order to maintain the required

amount of carrier flow across the junction. IL is the minimum anode current required to maintain the

thyristor in the ON state immediately thyristor is turned ON. The typical v-i characteristics of the thyristor

is as shown in the Figure 2. Once the thyristor conducts, it behaves like a conducting diode and there is no

control over the device. The device will continue to conduct because there is no depletion layer on J 2 due

to free movement of carriers. If the forward current is reduced below the level known as holding current

IH, a depletion region will develop around J2 due to reduced number of carriers, and thyristor goes into

blocking state. Holding current is always less than latching current.

A

K

G

A K

G

J1 J2 J3

p n p n

3


10. SCR

Figure 2 Thyristor (a) Circuit, (b) v-i characteristics

When cathode voltage is larger than anode voltage J1 and J3 are reverse biased and J2 will be forward

biased. So thyristor will be in OFF state and only current flow will be due to leakage current.

3. Two Transistor model of Thyristor:

The regenerative (latching) action of thyristor due to positive feedback can be explained using the two

transistor model of the thyristor. Thyristor can be considered as two complementary transistors, one NPN

(Q2) and other PNP (Q1) as shown in figure. The collector current IC, current gain α, emitter current IE and

leakage current are related as: CBOEC III . The common base collector current is defined by:

EC II . For transistor Q1, 222 CBOKC III . Using those equations the total anode current can be

written as:

21

212

1

CBOCBOG

A

IIII

The above equation indicates that as if the IG suddenly increased, say from 0 to 1mA, this will

immediately increase α1 and α2. α2 depend on IA and IG. the increase in the values of α1 and α2 would

further increase IA. Therefore there will be regenerative or positive feedback effect. If α1+ α2 tends to

unity, the denominator of above equation approaches to zero, resulting in large IA and thyristor will turn

ON with small gate current.

(a)

RL

VS

+

-

iT

VAK

A

K G

(b)

VAK VBO

iT

IL

IH

Reverse

Breakdown

voltage

Reverse

leakage

current

Forward leakage

current

VBR

4


10. SCR

Figure 3 – Two transistor Model of Thyristor – (a) Basic Structure, (b) Equivalent Circuit

Under transient conditions SCR may turn on without gate trigerring. Such turn on can not be explained

using static two transistor model. In transient model, one has to consider the junction capacitances of

SCR. Figure 4 shows the two transient model of thyristor.

Figure 4 Two Transistor Transient Model of Thyristor

dt

dVC

dt

dCV

dt

VCd

dt

qdi

j

j

j

j

jjj

j

2

2

2

2

222

2

)()(

A

IA

Q1

Q2

G

K

IK

IG

Vj2

Cj1

+

α1

α2

Cj2

Cj3

-

(b) A

IT = IA

Q1

Q2

G

K

IK

IG

IB2

IC1

IB1= IC2

α1

α2

(a)

p

n

P

p

P

n

IT

IK

IG

J1

J2

J3

Q1

Q2

K

A

G

5


10. SCR

The above equation shows the relation between junction current (ij2), junction capacitance (Cj2) and

voltage (Vj2) across the junction J2.

Turn on characteristics of SCR is shown in Figure 5. It consists of delay time, td and rise time tr. When

SCR is triggered the gate current rises, but thyristor does not turn on immediately. It starts turning on with

delay, td. Thus the turn on time is ton = td + tr. The width of triggering pulse must be greater than ton of the

given SCR i.e Gating pulse width, TW > ton.

Figure 5 Turn on characteristics of Thyristor

Turn off characteristics of SCR is complicated than turn on time and is shown in Figure 6. SCR can be

brought back to the blocking state from on state only by reducing the forward current to a level below that

of the holding current. The forward current is reduced by applying a reverse voltage across anode and

cathode and thus forcing the current through the SCR to zero. The SCR has a reverse recovery time trr

which is due to charge storage in the junctions of the SCR. These excess carriers take some time for

recombination resulting in the gate recovery time or reverse recombination time tgr. Thus, the turn-off

time tq is the sum of the durations for which reverse recovery current flows after the application of reverse

voltage and the time required for the recombination of all excess carriers present. At the end of the turn

off time, a depletion layer develops across and the junction can now withstand the forward voltage. The

turn off time is dependent on the anode current, the magnitude of reverse VG applied and the magnitude

and rate of application of the forward voltage. The turn off time for converter grade SCRs is 50 to 100 μs

t

it

IT 0.9 IT

0 0.1 IT

IG

td tr

iG

0 0.1 IG

t

6


10. SCR

and that for inverter grade SCR’s is less than 20 μs. To ensure that SCR has successfully turned off, the

circuit off time tc be greater than SCR turn off time tq.

Figure 6 Turn off characteristics of Thyristor

4. SCR Turn ON Methods

Turn ON method also known as triggering. With anode positive with respect to cathode, a thyristor can be

turned ON by following techniques: Forward voltage triggering, Gate triggering, dv/dt triggering,

temperature triggering and Light triggering.

1. Thermal: During forward blocking, most of the applied voltage is applied across reverse biased

junction J2. This voltage across junction J2 associated with leakage current may raise the temperature

of this junction. With increase in temperature, leakage current through J2 further increases. This

cumulative process may turn ON the thyristor at some high temperature. High temperature triggering

may cause thermal runaway and this is generally avoided.

2. High Voltage Triggering: When breakover voltage VBO across thyristor exceeds than the rated

maximum voltage of the device, thyristor turns ON. At the breakover voltage thyristor anode current

is called as Latching current (IL). Breakover voltage is not normally used as a triggering method,

because it may damage the device. It is destructive turn ON method and should be avoided.

t

t

t1 t2 t3 t4 t5

trr tgr

tq

tc

ia

va

Commutation di/dt Reverse voltage

due to power circuit

On state voltage

drop across SCR

7


10. SCR

3. dv/dt triggering: With forward voltage across anode and cathode of a thyristor, J1, J3 are forward

biased and J2 is reverse biased. This junction J2 acts as capacitor because of the presence of space

charge. As pn junction has capacitance, so larger the junction area larger is the capacitance. If the

charging current is becomes large enough, density of moving current carriers in the device induces

switch ON. This method of triggering is not desirable because high charging current may damage the

thyristor.

4. Gate current triggering: It is simple and efficient method to turn ON thyristor. Refer Figure 7. In

gate triggering method VAK is applied less than VBO. So thyristor is forward biased but not

conducting. When small gate current is applied, thyristor turns on and remain in ON state though gate

voltage is removed. Higher the gate current lower is the forward breakover voltage.

Figure 7 SCR characteristics for different gate trigger currents.

5. Light triggering: In this method, the light particles (photon) are made to strike the reverse biased

junction, which causes an increase in the number of electron-hole pairs and triggering of the thyristor.

For light triggered thyristors, a slot is made in the inner P layer. The light of particular wavelength is

irradiated to this P region through the optical fiber. If the intensity of the light is greater than certain

critical value, the thyristor will turn on. Such thyristor is called as LASCR.

Figure 8 shows the LASCR connection with the load, its structure and equivalent e lectrical circuit.

The light can be exposed to the junction J2, which is light activated. In place of electrical signal at the

gate, light is used as a source and the device is worked as a light activated SCR (Photo-SCR). Figure

8(B) shows its equivalent circuit with two transistor model and photodiode in parallel with junction

capacitance.

VAK VBO

iT

IL

IH VBR

V1 V2 V3

VBO > V1 > V2 > V3

IG3 > IG2 > IG1 > IG0

8


10. SCR

Figure 8 LASCR connected to load (a) showing structure and (b) equivalent electrical circuit.

5. SCR Turn OFF Methods

The process of turning off thyristor is called as Commutation. The basic methods are current commutation

and voltage commutation. A thyristor can be turned on by applying a positive voltage about a volt and or

a current of a few tens of milliamps at the gate cathode terminals. But SCR cannot be turned off via gate

terminal. It will be turned off only after the anode current reduced below holding current naturally or by

forced commutation techniques. Thus thyristor commutation can be classified as Natural or Line

Commutation and Forced Commutation.

P

P

N

N

Light

Source

Anode

Cathode

Gate

Silicon

Pellet Case and heat sink

R

Load

VDC

(a)

Anode

C1

Q2

Photo

Diode

R

Q1

D1

Cathode

Gate

Gate trigger

input

Load

(b)

9


10. SCR

Natural Commutation: or Line Commutation occurs only in AC circuits. Natural commutation of

thyristor takes place in AC voltage regulators, Phase controlled rectifiers, cycloconverters etc.

Forced Commutation: This type of commutation is applied to DC circuits.

Line Commutation

In thyristor circuit if the source input voltage is ac, the SCR current goes through a natural zero and

reverse voltage appears across SCR. The device then automatically turns off due to natural behavior of

source voltage. This is known as natural commutation or line commutation. This type of commutation is

applied in ac voltage controllers, phase control rectifiers, and cycloconverters. The timing waveforms of

current and voltage shows SCR for different delay or phase angle are shown in Figure 9.The waveform

for the delay angle α = 0. The delay angle α is defined as the angle between the zero crossing of the input

voltage and instant the thyristor is fired.

Figure 9 – Line Commutated of thyristor – (a) circuit, (b) timing waveforms

RL VS

+

-

iT

VAK

T1

+ -

(a)

π

(c) v

iT

vAK

0

0

0

ωt

Vm

IRR

2π

Leakage Current

trr

π

(b) v

iT

vAK

0

0

0

ωt

Vm

IRR

2π

Leakage Current

trr

10


10. SCR

Forced Commutation

In some thyristor circuits input source voltage is dc. So these thyristors can’t be turn off their own. Some

external circuitry is required to reduce the forward current to zero and turn off thyristor whenever

required. Such external circuits are said to be forced commutated circuits. These forced commutation

techniques are generally used in dc-dc applications and dc-ac applications. The different ways of forced

commutation techniques are Load Side Commutation and Line Side Commutation.

A. Load Side Commutation is further classified as

Self Commutation.

Impulse Commutation.

Resonant Pulse Commutation.

Complementary Commutation.

External Pulse Commutation.

Self Commutation:

Figure 10 Thyristor Self Commutation (a) Circuit, (b) Waveforms

In this type of commutation, a thyristor is turned off due to the natural characteristics of the circuit. The

assumption is the capacitor is initially uncharged. When thyristor T1 is switched on, the capacitor charges

through T1 and L. As the capacitor charges in time LCtt 0 , the charging current becomes zero

and thyristor T1 is switched off itself. Once thyristor is fired, there is a delay of t0 seconds before T1 is

turned off and t0 is called the commutation time of the circuit. This method of turning off of thyristor is

called as self commutation and the thyristor is said to be self commutated.

L

C

T1 +

+

-

-

vc

vL

+

-

vs

LCVs

i(t)

ωmt 0 π

2Vs

ωmt 0

vc(t)

π

t0

11


10. SCR

Impulse Commutation

The impulse commutated thyristor consists of main thyristor T1 and thyristors T2 and T3 are used in forced

communication circuit. It is assumed that capacitor C is charged up to –VS with polarity shown.

Figure 11- Impulse Commutation (a) Basic circuit, (b) Timing waveforms, and (c) diode in series with

inductor for accelerated recharging of capacitor.

Resonant Pulse Commutation

vc

t 0

Vs

-V0

vT1

t 0

Vs

-V0

tOFF

vS

LOA

D

-

v0

Lr

C

T1

+

vc -

+

Im

T2

T3

Dm

-

+

(a) (b)

-

v0

Lr

C

T1

+

vc -

+

Im

T2

T3

Dm vS

-

+

L1 D1

LOA

D

(c)

12


10. SCR

Figure 12 Thyristor Resonant Pulse Commutation (a) Circuit, (b) Waveforms

C

vc - +

-

L

T1

+

Im

T2

T3

Dm vS i(t) L

OA

D

(a) i(t)

vc (t)

t1

t1 t2

toff tc t0

Dm on

t

t

-V0 -V1

Im

0

0

VC VS

(b)

13


10. SCR

Complementary Commutation

Figure 13 Complementary Commutation (a) Circuit, (b) Waveforms

External Pulse Commutation

Figure 14 External Pulse Commutation

-

L

C

T1

+

LO

AD

+

-

Im

T2 T3

Dm

VS

2V

-

+

V

-

C T1

+

+ - ic

T2

vS

R2 R1

vS

(a)

2VS/R

-VS

-VS

ic

t 0

0 T/2

vc

vt1

vt2

0

0

-2VS/R

VS

VS

VS

-VS

(b)

14


10. SCR

B. Line Side Commutation

Figure 15 Line Side Commutation

The most important parameters:

Break over voltage

• Holding current

• Turn ON time

• Turn OFF time

• Maximal forward current

• Maximal reverse voltage

• Maximal frequency

6. SCR Protection and Triggering Circuits

A thyristor require a minimum time to spread the current conduction uniformly throughout the junctions.

If the rate of rise of anode current is very fast compared to the spreading velocity of a turn on process, a

localized hot-spot heating will occur due to high current density and the device may fail, as a result of

excessive temperature.

-

C

T1

+

+

-

Im

T2

vS Dm

T3

L

Lr

LO

AD

15


10. SCR

di/dt Protection

Figure 16 Thyristor Switching circuit with di/dt limiting inductors

The practical device must be protected against high di/dt. As an example consider the following circuit.

Under steady state operation, D m conducts when T1 is off. If T1 is fired when Dm is still conducting, di/dt

can be very high and limited by the stray inductance of the circuit.

dv/dt Protection

If switch S1 in fig (a) is closed at t=0, a step voltage will be applied to thyristor T1 and dv/dt may be high

enough to turn on the device. The dv/dt can be limited by connecting capacitor Cs in as shown in fig a.

when T1 is turned on, the discharge current of capacitor is limited by RS as shown in fig b. With RC

circuit known as a snubber circuit, the voltage across thyristor will rise exponentially and the thyristor can

be protected.

Figure 17 dv/dt Protection circuit

VAK

(b)

CS

VS

+

-

A

K G

S1

RS

(a)

CS VS

+

-

VAK

A

K G

S1

R

Dm VS

C

T1 i Im LS

+

-

Lo

ad

16


10. SCR

Thyristor triggering circuits :

Thyristors are used for high voltage and high power circuits. To control the thyristors various control and

trigger circuits are employed which operates at low power. An isolator circuit is required between an

individual thyristor and gate pulse generator circuit. The isolation can be achieved by using isolation

transformer or optocouplers. Optocoupler could be phototransistor or photo SCR. A typical photo SCR is

shown in Figure 18.

Figure 18 A typical photo SCR

Pulse transformers are often used to couple a gate trigger pulse circuit to high voltage thyristorized circuit

to obtain electrical isolation. Generally, these transformers are either 1:1 (two windings) or 1:1:1 (three

windings) types.

Important transformer design factors are -

a. Primary magnetizing inductance should be high enough so that magnetizing current is low, in

comparison with pulse current during the pulse time.

b. For unilateral pulse generators core saturation must be avoided.

c. Insulation between windings should be high for the applications including transients.

TA Anode

Cathode

Gate

Output Control

Connections

Input Control

Signal AC or DC

LED LASCR

CASE

A

K 230VAC

Logic

Input

+5V

G

LOAD

17


10. SCR

d. Inter-winding capacitance should be low because it may provide path for undesirable stray signals at

high frequencies.

Short Pulse Triggering

If triggering pulse is wide, the size and weight of the pulse transformer required is high. The circuit

shown in Figure 19 is used to convert wide pulse to short pulse. The differentiator converts the input

pulse to short pulse which is further amplified using transistorized circuit as shown in timing diagram.

Diode D1 allows only positive triggering pulse to the base of transistor and diode Dm acts as freewheeling

diode.

Figure 19 Pulse Transformer isolation: Short Pulse

Long Pulse Triggering

If triggering pulse is too short, it may not trigger the thyristor. The circuit shown in Figure 20is used to

convert short pulse to wide pulse. The integrator converts the input pulse to wide pulse which is further

amplified using transistorized circuit as shown in timing diagram. Diode Dm acts as freewheeling diode.

A

0 t

V1

+VCC G

K

R

Q1

D1

C1

R1

Dm N1 N2

(a) v1

t 0

V

VA

t 0

t 0

VGK

(b)

18


10. SCR

Figure 20 Pulse Transformer Isolation: Long Pulse

Pulse Train Generator

In thyristorized ac circuits with inductive load, load current lags behind the voltage. In phase angle

control, it may happen that when triggering pulse is applied at low de angle, thyristor is reverse biased.

Therefore thyristor will remain off even though gate pulse is applied. To avoid this generally pulse train

is used to trigger the thyristor. Pulse train is generated using blocking oscillator as shown in Figure 21.

When control input voltage is high, pulse train will be generated and when it is low, pulse train will be

terminated.

A

+VCC G

K

R

Q1

D1

C1

R1

Dm N1 N2

-

+

C

-

+

v1

(a) v1

t 0

V

VA

t 0

Vm

t 0

Vm

VGK

(b)

19


10. SCR

Figure 21Pulse Transformer isolation: Pulse Train Generator

Pulse Train Generator using timer and AND logic

Pulse train can be generated using timer and AND logic as shown in Figure 21. To avoid this generally

pulse train is used to trigger the thyristor. Pulse train is generated using blocking oscillator as shown in

Figure 22. When control input voltage is high, pulse train will be generated and when it is low, pulse train

will be terminated. V1 is the gating input and if it is high it will allow the pulsed to pass further.

Figure 22 Pulse Transformer isolation: Pulse Train Generator using timer and AND logic

+VCC

R

Q1 R1

Dm

v1

G

K

N1 N2

v2

Oscillator

AND

(a)

v1

t 0

V

t 0

VGK

(b)

+VCC

R

Q1

D1

C1

R1

Dm

-

+

v1

N3

G

K

N1 N2

-

+

(a) v1

t 0

V

t 0

VGK

(b)

20


10. SCR

Figure 23

Figure 24

7. Gate Protection Circuits

The gate signals are generally provided through pulse transformers for isolation purpose. The voltage

pulse may be of high voltage/ may be bipolar. Hence it may happen the power dissipation at the gate high.

Pulse may contain high frequency noise. In such case, there may be undesirable turn on of SCR. In order

to avoid such effects, it is necessary to protect the gate. Various gate protection schemes are used which

are discussed as follows.

LO

AD

230V

/ 5

0H

z

HOT

NEUTRAL

TRIGGER

RG

R1

+

T1

- XENON

FLASH

LAMP

OPTICAL

FIBER L

OA

D

23

0V

/ 5

0H

z

HOT

NEUTRAL

+5V

RS R1 R2

C T1

21


10. SCR

Table 1 Gate protection circuits

a) Gate cathode Resistance Increase dv/dt capability.

Keep gate damped to assure VDRM capability.

Lowers turn off time (tq).

Increases the latching and holding current.

b) Gate cathode Capacitance Increase dv/dt capability.

Remove high frequency noise.

Increases turn on and turn off time.

Lowers gate signal rise time.

Lowers di/dt capability.

c) Gate cathode Inductance

Decrease DC gate sensitivity.

Decreases turn off time (tq).

d) Series Resistor

Limits the gate current

Lowers dissipation in gate junction

e) Series Zener:

Decreases threshold Sensitivity.

Affects the signal rise time and di/dt rating. Isolates the gate.

f) Gate cathode LC Resonant circuit

Frequency Selection.

Unless the circuit is damped the positive and negative gate current may inhibit conduction or bring about sporadic anode current.

g) Reverse Diode

Supply reverse bias in off period.

Protects gate and gate supply for reverse transients.

Lowest turn off time.

h) Capacitive coupling

Isolates gate circuit DC component.

In narrow gate pulses and low impedance sources, the Igt is followed by reverse gate signals which may inhibit conduction.

22


10. SCR

8. Summary

SCR. is silicon controlled rectifier . It is widely used in ac to dc conversion. It has the capability

of controlling the output voltage. The SCR


2. Two transistor static and transient models

3. SCR turn-on methods

4. SCR turn-off methods

5. SCR protection and Triggering circuits

6. Gate protection circuits

7. Summary

Module 10 SCR - seramporegirlscollege.org Science... · Resonant Pulse Commutation. Complementary...

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Transcript of Module 10 SCR - seramporegirlscollege.org Science... · Resonant Pulse Commutation. Complementary...