1.4. Thyristor (SCR) A thyristor is a four layered semicoductor that is ofte used or hadlig large amouts of ower. It ca be tured ON or OFF, it ca regulate ower usig somethig called hase agle cotrol. It has the same characteristics as thyratro tube, it is a solid state device. It is a three juctio semicoductor switchig device ad has three termials aode, cathode ad gate. Threaded stud Aode J 1 Gate J 2 J 3 Cathode Fig.1.11: Schematic of Thyristor Aode A A Gate J 1 J 2 J 3 G G Cathode K K Fig.1.12: Circuit Symbols of Thyristor
The threaded ortio is meat for tighteig the thyristor to the frame with the hel of a ut. (a) Oeratio If the forward break over voltage (V BO ) is exceeded the SCR Self triggers ito coductig state. The resece of gate curret will reduce V BO Normal coditios for thyristor to tur o: The device is i forward blockig state (V AK is ositive) Oce coductig the aode curret is latched. V ak collases to ormal forward voltage dro, tyically 1.5 3 V Fig.1.13: V I Characteristics I reverse biased mode the SCR behaves like a diode
(1) Forward Coductio Mode: A thyristor is brought to the forward coductio mode by exceedig forward break over voltage or by alyig a gate ulse betwee gate ad cathode I this mode thyristor behaves as a closed switch ad the voltage across the thyristor is 1 to 2 V deedig o the ratig of SCR This small voltage dro deeds uo the ohmic dro i the four layers (2) Forward Blockig Mode: I this mode aode is made ositive with resect to cathode as show with gate circuit oe. J 1, J 3 are forward biased ad J 2 is reverse biased. I this mode a small leakage curret results ad icreasig the voltage i forward directio, J 2 will have a avalache breakdow at a forward eak over voltage V BO. For a voltage less tha V BO, thyristor acts as oe switch as it offers a high imedace. + A Forward leakage curret J 1 G S J 2 J 3 _ k (3) Reverse Blockig Mode: Whe cathode is made ositive with resect to aode with the switch S oe as show i the figure, thyristor is reverse biased A G S J 1 J 2 J 3 + k Reverse leakage curret
Juctios J 1, J 3 are see to be reverse biased ad J 2 forward biased. A small leakage curret of a few milli ameres flows. This is the reverse blockig mode called OFF-state of the thyristor. I this mode, icreasig the voltage i reverse directio, avalache breakdow occurs at J 1 ad J 3 at a certai breakdow voltage called reverse breakdow voltage At this voltage, a large reverse curret results which ca lead to thyristor damage. Hece maximum workig voltage should ot exceed V BR. I this mode thyristor offers a high imedace, thus acts as a oe switch. 1.08. A asymmetrical thyristor has (A) Ati arallel coectio of a diode ad thyristor (B) No reverse blockig caability ad small forward blockig caability (C) Small forward blockig caability ad large reverse blockig caability (D) Small reverse blockig caability ad large forward blockig caability. Sol. ASCR is suitable for high frequecy alicatios because its reverse blockig caability is less. Choice (D) 1.09. Reverse coductig thyristors would ossess which amog the followig features (A) High survival voltage (B) Large curret hadlig caacity (C) Faster tur off (D) All of the above Sol. Choice (D) (b) Thyristor Tur-ON Methods (1) Forward Voltage Triggerig: Whe aode to cathode voltage exceeds forward break over voltage thyristor turs ON.
Juctio J 2 has a avalache break dow, allows free movemet of carriers across three juctios ad large forward aode curret results Forward V BO ad reverse V BO are of the same magitude ad are temerature deedet but ractically V BR is slightly more tha V Bo. V BR is take as fial voltage ratig of a device After avalache breakdow, if the aode voltage is reduced below V BO, SCR will cotiue coductio of curret. The SCR is the tured off oly by reducig the aode curret below a certai value called holdig curret(i H ). (2) Gate Triggerig: Whe there is a ositive gate voltage betwee gate ad cathode, charges are iducted ito the ier -layer ad V BO is reduced, sice cathode -layer is more heavily doed tha gate -layer As the thyristor is forward biased some of these electros reach J 2 As a result, width of deletio layer shriks, which causes J 2 to breakdow at a alied voltage lower tha forward V BO Icreasig the magitude of gate curret will tur o the thyristor at a lower forward voltage Oce SCR is i coductio, reverse biased J 2 o loger exists ad the coductio of curret from aode to cathode remais uaffected. However if gate curret is reduced to zero before the aode curret attais a value called latchig curret the thyristor will tur-off agai. (c) Switchig Characteristics (1) Switchig Characteristics durig tur-on: A forward biased thyristor is tured ON whe a ositive gate voltage is alied across gate ad cathode, ad it takes some time from forward OFF state to forward ON-state kow as trasitio time. This trasitio time is divided ito three itervals:
(i) Delay time (t d ): It is defied as the time durig which aode voltage falls from V a to 0.9 V a where, V a is the iitial value of aode voltage. (or) It ca be defied as the time durig which aode curret rises from forward leakage curret to 0.1 I a where, I a is the fial value of aode curret. (ii) Rise time (t r ): It is the time required for the forward blockig OFF-state voltage to fall from 0.9 to 0.1 of its iitial value. This time is iversely roortioal to the magitude of gate curret ad its build u rate. (t r ) ca be miimized if high ad stee curret ulses are alied to the gate. di For series RL circuit dt is slow therefore, t r is more ad for RC circuit di is dt higher, causig t r to be less. Due to high aode voltage V a ad large aode curret, tur-o losses are the highest durig rise time. (iii) Sread Time (t ): It is the time required for the forward blockig voltage to fall from 0.1 of its iitial value to the ON-state voltage dro (1 to 1.5 V). After the sread time aode curret attais steady state value ad voltage dro across SCR is equal to the ON-state voltage dro of the order of 1 to 1.5 V. Total tur-o time is tyically of the order 1 to 4 µs deedig uo the aode circuit arameters ad gate sigal wave shaes. (2) Switchig characteristics durig Tur OFF: The rocess ivolvig chage of state of a SCR from the coductio state to forward blockig state is called commutatio rocess or tur-off rocess
If forward voltage is alied to the SCR at the momet its aode curret falls to zero the device is still i coductio sice the carriers ad electros i the four layers still favour coductio. Therefore a thyristor is reverse biased for a fiite eriod after the aode curret has reached zero. Device tur-off time (t q ): It is defied as the time betwee the istat aode curret becomes zero ad the istat SCR regais the forward blockig caability. 1.10. Which amog the followig statemets regardig a Thyristor is/are true. (1) Thyristor ca be tured off by a egative gate ulse (2) Thyristor would remai i its blockig state if the forward aode curret is reduced below its holdig curret. (3) Due to the regeerative effect i a thyristor durig tur o, it ca tur o with a small gate curret ad latch ito coductio (A) 1 ad 2 oly (B) 2 ad 3 oly (C) 1,2, 3 (D) 1 ad 3 oly Sol. Choice (B) Latchig Curret: Latchig curret is that miimum value of aode curret which must be attaied durig tur o rocess to maitai coductio eve after removal of gate sigal. The gate ulse width should be chose such that the aode curret rises above the latchig curret. Holdig Curret: The miimum value of aode curret below which it must fall for turig off the thyristor Geerally latchig curret is more tha the holdig curret. (2 or 3 times more tha the holdig curret)
Tyes of Thyristors (i) Phase cotrolled: - Rectifyig lie frequecy voltage ad curret for ac ad dc motor drives - Large voltage (u to 7 kv) ad curret (uto 4 ka) caability. - low o state voltage dro (1.5 to 3V) (ii) Iverter Grade: - Used i iverter ad choer - Quite fastad ca be tured o usig forced commutatio method. (iii) Light activated: - Similar to hase cotrolled, but triggered by ulse of light. - Normally very high ower ratigs.