Azmi Bin Ayup & Zul Mt Lajin

E3065/7/1

THYRISTOR

THYRISTOR

OBJECTIVES

General objective : To understand the concept of thyristor.

Specific objectives : At the end of the unit you should be able to:

· Identify the element of di/dt, dv/dt

· Identify the reverse recovery time for the ON and OFF thyristor method

· Identify the characteristics of thyristor gate

INPUT

7.1 INTRODUCTION OF THYRISTOR

Although transistors can be used as switches, their current carrying capacity is generally small. There are many applications in which it would be advantageous to have a high-speed switch which could handle up to 1000 A. Such a device is known as the thyristor. It also has the advantage of not having any moving parts nor arcing. A thyristor is an electronic device similar to a transistor switch. It has four layers and can only be switched on; it cannot be switched off. Circuits can be used to switch off a thyristor but the most simple arrangement is to let the current fall to zero which arises when used with an a.c. supply.

7.2 PRINCIPLE OF THYRISTOR

The basic parts of the thyristor are its four layers of alternate p-type and n-type silicon semiconductors forming three p-n junctions, A,B and C, as shown in Figure. 7.2(a). The terminals connected to the n1 and p2 layers are the cathode and anode respectively. A contact welded to the p1 layer is termed the gate. The CENELEC Standard graphical symbol for the thyristor is given in, Figure. 7.2 (b). The direction of the arrowhead on the gate lead indicates that the gate contact is welded to a p-region and shows the direction of the gate current required to operate the device. If the gate contact is welded to an n-region, the arrowhead should point outwards from the rectifier.

When the anode is positive with respect to the cathode, junctions A and C are forward-biased and therefore have a very low resistance, whereas junctions B is reverse-biased and consequently presents a very high resistance, of the order of megohms, to the passage of current. On the other hand, if the anode terminal is made negative with respect to the cathode terminal, junction B is forward-biased while A and C act as two reverse-biased junctions in series.

Figure. 7.2 : Thyristor arrangement and symbols

7.3 di/dt PROTECTION

A thyristor requires 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.

The practical devices must be protected against high di/dt. As an example, let us consider the circuit in Figure. 7.3. Under steady-state operation, Dm conducts when thyristor T1 is off. If T1 is fired when Dm is still conducting, di/dt can be very high and limited only by the stray inductance of the circuit.

Figure 7.3: Chopper circuit with di/dt limiting inductors

In practice, the di/dt is limited by adding a series inductor Ls, as shown in Figure. 7.3. The forward di/dt is

Ls

Vs

dt

di

=

where Ls is the series inductance, including any stray inductance.

7.4DV /Dt PROTECTION

If switch S1 in Figure. 7.4 (a) is closed at t = 0, a step voltage will be applied across thyristor T1 and dv/dt may be high enough to turn on the device. The dv/dt can be limited by connecting capacitor Cs, as shown in Figure. 7.4(a). When thyristor T1 is turned on, the discharge current of capacitor is limited by resistor Rs as shown in Figure. 7.4(b).

With an RC circuit known as a snubber circuit, the voltage across the thyristor will rise exponentially as shown in Figure. 7.4(c) and the circuit dv/dt can be found approximately from

RsCs

Vs

T

Vs

dt

dv

632

.

0

632

.

0

=

=

The value of snubber time constant T = Rs Cs can be determined from Eq. (7.4) for a known value of dv/dt. The value of Rs is found from the discharge current ITD.

Rs =

Vs

ITD

It is possible to use more than one resistor for dv/dt and discharging, as shown in Figure. 7.4(d). The load can form a series circuit with the snubber network as shown in Figure. 7.4(e).

Figure 7.4: dv/dt protection circuits

INPUT

Activity 7A

TEST YOUR UNDERSTANDING BEFORE YOU CONTINUE WITH THE NEXT INPUT…!

7.1. Describe briefly what thyristor is.

7.2. Draw a thyristor arrangement and symbols.

7.3What is the purpose of di/dt protection?

Feedback To Activity 7A

7.1. A thyristor is an electronic device similar to a transistor switch. It has four layers and can only be switch on; it cannot be switch off. The basic parts of the thyristor are its four layers of alternate p-type and n-type silicon semiconductors forming three p-n junctions, A,B and C.

7. 2.

7.3The practical devices must be protected against high di/dt.

7.5CHARACTERISTICS OF THYRISTOR

Let us now consider the effect of increasing the voltage applied across the thyristor, with the anode positive relative to the cathode. At first, the forward leakage current reaches saturation value due to the action of junction B. Ultimately, a breakover is reached and the resistance of the thyristor instantly falls to a very low value, as shown in Figure. 7.5. The forward voltage drop is of the order of 1 -2 V and remains nearly constant over a wide variation of current. A resistor is necessary in series with the thyristor to limit the current to a safe value.

Figure. 7.5 : Thyristor characteristic

7.6THYRISTOR PRINCIPLE

We shall now consider the effect upon the breakover voltage of applying a positive potential to the gate as in Figure. 7.6(a). When switch S is closed, a bias current, IB, flows via the gate contact and layers p1 and n1 and the value of the breakover voltage of the thyristor depends upon the magnitude of the bias current in the way shown in Figure. 7.6(b). Thus, with IB = 0, the breakover voltage is represented by OA and remains practically constant at this value until the bias current is increased to OB. For values of bias current between OB and OD, the breakover voltage falls rapidly to nearly zero. An alternative method of representing this effect is shown in Figure. 7.6(c).

Figure.7.6(a) : Gate control of thyristor breakover voltage

Figure. 7.6(b) : Variation of breakover voltage with bias current

Figure. 7.6 (c ) : Variation of breakover voltage with bias current, I 8

If the thyristor is connected in series with a non-reactive load, of resistance R, across a supply voltage having a sinusoidal waveform and if it is triggered at an instant corresponding to an angle Ø after the voltage has passed through zero from a negative to positive value, as in Figure. 7.6 d(a) , the value of the applied voltage at that instant is given by

υ = Vm sin Ø

Up to that instant, the voltage across the thyristor has been growing from zero to υ. When triggering occurs, the voltage across the thyristor instantly falls to about 1 – 2V and remains approximately constant while current flows, as shown Figure. 7.6d(a). Also, at the instant of triggering, the current increases immediately from zero to i , where

i = υ – p.d. across thyristor

R

= υ when the p.d. across thyristor « υ

R

If Ø is less than ∏/2, the current increases to a maximum Im and then decreases to the holding value, when it falls instantly to zero, as shown in Figure. 7.6 d(b). The average value of the current over one cycle is the shaded area enclosed by the current wave divided by 2∏.

Figure. 7.6 d : Phase-controlled half-wave rectification

7.7 LIMITATION TO THYRISTOR OPERATION

Because of the nature of the construction of the thyristor, there is some capacitance between the anode and the gate. If a sharply rising voltage is applied to the thyristor, then there is an inrush of charge corresponding to the relaion i = C (dv/dt). This inrush current can switch on the thyristor, and it can arise in practice due to surges in the supply system, for example due to switching or to lighting. Thus thyristors may be inadvertently switched on, and such occurrences can be avoided by providing C – R circuits in order to divert the surges from the thyristors.

Activity 7B

TEST YOUR UNDERSTANDING BEFORE YOU CONTINUE WITH THE NEXT INPUT…!

7.4.What are the characteristics of a thyristor?

7.5Draw a diagram of control thyristor break over voltage.

7.6Draw a diagram of a phase-controlled half-wave rectification.

Feedback To Activity 7B

7.4.

Thyristor characteristic

7.5.

Gate control of thyristor breakover voltage

7.6.

Phase-controlled half-wave rectification

SELF-ASSESSMENT

You are approaching success. Try all the questions in this self-assessment section and check your answers with those given in the Feedback on Self-Assessment 7 given on the next page. If you face any problems, discuss it with your lecturer. Good luck.

Question 7-1

a.Draw and describe briefly the principle of a thyristor.

b.Draw and describe briefly the characteristics of a thyristor .

c. Draw and describe briefly the control of thyristor breakover voltage and voltage with bias current.

d.From Figure. 7.1 below, what will happen if S1 is closed at t =0 ?

Figure 7.1: dv/dt protection circuit

Question 7-2

a.Draw and describe briefly the phase-controlled half-wave rectification.

b.Explain in detail the limitation of a thyristor operation.

Feedback To Self-Assessment

Have you tried the questions? If “YES”, check your answers now.

Answer 7-1:

(a).

Thyristor arrangement and symbols

The basic parts of the thyristor are its four layers of alternate p-type and n-type silicon semiconductors forming three p-n junctions, A,B and C, as shown in Figure above. The terminals connected to the n1 and p2 layers are the cathode and anode respectively. A contact welded to the p1 layer is termed the gate. The CENELEC Standard graphical symbol for the thyristor is given above (b). The direction of the arrowhead on the gate lead indicates that the gate contact is welded to a p-region and shows the direction of the gate current required to operate the device. If the gate contact is welded to an n-region, the arrowhead should point outwards from the rectifier.

When the anode is positive with respect to the cathode, junctions A and C are forward-biased and therefore have a very low resistance, whereas junction B is reverse-biased and consequently presents a very high resistance, of the order of megohms, to the passage of current. On the other hand, if the anode terminal is made negative with respect to the cathode terminal, junction B is forward-biased while A and C act as two reverse-biased junctions in series.

(b).

Thyristor characteristic

Consider the effect of increasing the voltage applied across the thyristor, with the anode positive relative to the cathode. At first, the forward leakage current reaches saturation value due to the action of junction B. Ultimately, a breakover is reached and the resistance of the thyristor instantly falls to a very low value, as shown in the Figure. above. The forward voltage drop is of the order of 1 -2 V and remains nearly constant over a wide variation of current. A resistor is necessary in series with the thyristor to limit the current to a safe value.

(c).

Figure.7.1(a) : Gate control of thyristor breakover voltage

Figure. 7.1(b) : Variation of breakover voltage with bias current

Figure..7.1 (c ) : Variation of breakover voltage with bias current, I 8

Consider the effect upon the breakover voltage of applying a positive potential to the gate as in Figure. 7.1(a). When switch S is closed, a bias current, IB, flows via the gate contact and layers p1 and n1 and the value of the breakover voltage of the thyristor depends upon the magnitude of the bias current in the way shown in Figure. 7.1(b). thus, with IB = 0, the breakover voltage is represented by OA and remains practically constant at this value until the bias current is increased to OB. For values of bias current between OB and OD, the breakover voltage falls rapidly to nearly zero. An alternative method of representing this effect is shown in Figure. 7.1(c).

(d)If switch S1 is closed at t =0, a step voltage will be applied across thyristor T1 and dv/dt may be high enough to turn on the device. The dv/dt can be limited by connecting capacitor Cs.

Answer 7-2

(a)

Figure.7.2 : Phase-controlled half-wave rectification

If the thyristor is connected in series with a non-reactive load, of resistance R, across a supply voltage having a sinusoidal waveform and if it is triggered at an instant corresponding to an angle Ø after the voltage has passed through zero from a negative to positive value, as in Figure. 7.2(a) , the value of the applied voltage at that instant is given by

υ = Vm sin Ø

Up to that instant, the voltage across the thyristor has been growing from zero to υ. When triggering occurs, the voltage across the thyristor instantly falls to about 1 – 2V and remains approximately constant while current flows, as shown Figure. 7.2(a). Also, at the instant of triggering, the current increases immediately from zero to i , where

i = υ – p.d. across thyristor

R

= υ when the p.d. across thyristor « υ

R

If Ø is less than ∏/2, the current increases to a maximum Im and then decreases to the holding value, when it falls instantly to zero, as shown in Figure. 7.2(b). The average value of the current over one cycle is the shaded area enclosed by the current wave divided by 2∏.

(c).Because of the nature of the construction of the thyristor, there is some capacitance between the anode and the gate. If a sharply rising voltage is applied to the thyristor, then there is an inrush of charge corresponding to the relaion i = C (dv/dt). This inrush current can switch on the thyristor, and it can arise in practice due to surges in the supply system, for example due to switching or to lighting. Thus thyristors may be inadvertently switched on, and such occurrences can be avoided by providing C – R circuits in order to divert the surges from the thyristors.

UNIT 7

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Hii !!!!!…..Good Luck and Try your best ….

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CONGRATULATIONS!!!!…..May success be with you always….

CONGRATULATIONS!!!!…..May success be with you always….

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