6.0 THYRISTOR

di/dt, dv/dt and reverse recovery time. Gating requirements;
thyristor gate characteristics, Vg/Ig, gate characteristics upper limit
value and maximum permitted gate voltage.

THYRISTOR

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

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.

 

7.2        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.

7.4       D_V /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

 

7.5       CHARACTERISTICS  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.

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

 

             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∏.

 

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.