SEE 3263: ELECTRONIC SYSTEMS

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1 SEE 3263: ELECTRONIC SYSTEMS Chapter 5: Thyristors 1 THYRISTORS Thyristors are devices costructed of four semicoductor layers (pp). Four-layer devices act as either ope or closed switches; for this reaso, they are most frequetly used i cotrol applicatios. Thyristors iclude: Shockley diode, silico-cotrolled rectifier (SCR), diac ad triac. They stay o oce they are triggered, ad will go off oly if curret is too low or whe triggered off. Usage: lamp dimmers, motor speed cotrols, igitio systems, chargig circuits, etc. 2 1

2 The Shockley Diode The 4-layer diode (or Shockley diode) is a type of thyristor that acts somethig like a ordiary diode but coducts cts i the forward directio oly after a certai aode to cathode voltage called the forward-breakover voltage is reached. The symbol remids you that it acts like a diode. It does ot coduct whe it is reverse-biased. p p ode () Cathode (K) Basic Costructio K Schematic Symbol Q 1 Q 2 Equivalet Circuit 3 K Shockley Diode Characteristic Curve The characteristic curve betwee 0 V ad V BR(F) shows the forward blockig regio i.e. off state. Whe the aode-to-cathode voltage, I exceeds V BR(F), coductio occurs. Oce coductio begis, I icreases rapidly ad will cotiue util I is reduced to less tha the holdig curret (I H ). This is the oly way to stop coductio. I I H I S 0 V R S V BR(F) O Off V K V BR(F) = forward-breakover voltage I S = switchig curret I H = holdig curret 4 I 2

3 Shockley Diode pplicatio V BR(F) R S V S D C V C V S > 0 V Relaxatio Oscillator Voltage Waveform Capacitor charges through R S ad discharges through D. 5 Silico-Cotrolled Rectifier (SCR) SCR is aother four-layer pp device. Has 3 termials: aode, cathode, ad gate. I off state, it has a very high resistace. I o state, there is a small o (forward) resistace. pplicatios: motor cotrols, time-delay circuits, heater cotrols, phase cotrols, etc. 6 3

4 SCR cotiue. ode () Gate (G) Cathode (K) p p Basic Costructio G K Schematic Symbol G Q 1 Q 2 K Equivalet Circuit 7 Turig The SCR O +V The positive pulse of curret at the gate turs o Q 2 providig a path for I B1. Q 1 the turs o providig more base curret for Q 2 eve after the trigger is removed. Thus, the device stays o (latches). R Q 1 I I B2 I B1 I G Q 2 I K 8 4

5 Turig The SCR O The SCR ca be tured o by exceedig the forward breakover voltage or by gate curret. Notice that the gate curret cotrols the amout of forward breakover voltage required for turig it o. V BR(F) decreases as I G is icreased. I H0 I H1 I H2 I I G2 >I G1 I G1 >I G0 I G0 =0 V F V BR(F2) V BR(F1) V BR(F0) F SCR characteristic curves for differet I G Values 9 Turig The SCR Off The SCR will coduct as log as forward curret exceeds I H. There are two ways to drop the SCR out of coductio: 1) ode Curret Iterruptio ad 2) Forced Commutatio. G +V +V R R G ode curret ca be iterrupted by breakig the aode curret path (show here), providig a path aroud the SCR, or droppig the aode voltage to the poit that I < I H. (1) ode Curret Iterruptio 10 5

6 Turig The SCR Off +V Force commutatio uses a exteral circuit to mometarily force curret i the opposite directio to forward coductio. G R (2) Forced Commutatio SCRs are commoly used i ac circuits, which forces the SCR out of coductio whe the ac reverses. 11 SCR Characteristics & Ratigs Forward-breakover voltage, V BR(F) : voltage at which SCR eters forward-coductio (o) regio. Holdig curret, I H : value of aode curret for SCR to remai i o regio. Gate trigger curret, I GT : value of gate curret to switch SCR o. verage forward curret, I F(avg) : maximum cotiuous aode curret (dc) that the SCR ca withstad. V R Reverseavalache regio V BR(R) I H I F 0 Forwardcoductio regio (o) for I G = 0 Reverse- blockig regio Forwardblockig regio (off) Reverse-breakdow voltage, I R V BR(R) : maximum reverse voltage before SCR breaks ito avalache. 12 V V F BR(F) 6

7 SCR pplicatios SCRs are used i a variety of power cotrol applicatios. Oe of the most commo applicatios is to use it i ac circuits to cotrol a dc motor or appliace because the SCR ca both rectify ad cotrol. The SCR is triggered o the positive cycle ad turs off o the egative cycle. circuit like this is useful for speed cotrol for fas or power tools ad other related applicatios. B R 1 R 2 R 3 R 4 M I 13 SCR pplicatios other applicatio for SCRs is i crowbar circuits (which get their ame from the idea of puttig a crowbar across a voltage source ad shortig it out!) The purpose of a crowbar circuit is to shut dow a power supply i case of over-voltage. SW Fuse DC power supply V OUT Oce triggered, the SCR latches o. The SCR ca hadle a large curret, which causes the fuse (or circuit breaker) to ope. D 2 V TRIG D 1 R 1 R 3 R 2 "Crowbar circuit" 14 7

8 Half-Wave Power Cotrol 0 o -90 o I L V i R 1 R L θ f I P B R 2 D1 I I P = (1 + cosθ ) 2π L( VG) f where θ f = firig agle = 90 0 max. 15 Half-Wave Power Cotrol 0 o -180 o V VS VC R L VCT + V R - I R 1 + V L - θ θ 1 2 θ f t + V S G K VL - + V C C + V D - - t θ f 16 8

9 Silico-Cotrolled Switch (SCS) SCS ca be tured o either by a positive pulse at the cathode or a egative pulse at the aode. SCS ca be tured off by usig pulses of the reversed polarity or by aode curret iterruptio methods. SCS ad SCR are used i similar applicatios. SCS has faster tur-off with pulses o either gate termial; but it has lower maximum curret ad voltage ratigs tha SCR. G K Q 1 G K K G Schematic Symbol Q 2 Equivalet Circuit G K 17 The Diac ad Triac Both the diac ad the triac are types of thyristors that ca coduct curret i both directios (bilateral). They are four-layer devices. The diac has two termials, while the triac has a third termial (gate). The diac is similar to havig two parallel Shockley diodes tured i opposite directios. The triac is similar to havig two parallel SCRs tured i opposite directios with a commo gate. 18 9

10 The Diac The diac is a thyristor that acts like two back-to-back 4-layer diodes. It ca coduct curret i either directio. Because it is bidirectioal, the termials are equivalet ad labeled 1 ad I F p p V R -V BR(R) I H -I H V F V BR(F) 2 2 Basic Costructio Symbol I R Characteristic Curve 19 The Diac The diac coducts curret after the breakdow voltage is reached. t that poit, the diac goes ito avalache coductio, creatig a curret pulse sufficiet to trigger aother thyristor (a SCR or triac). The diac remais i coductio as log as the curret is above the holdig curret, I H. V R -V BR(R) I F I H V F VBR(F) -I H I R Characteristic Curve 20 10

11 Diac Equivalet Circuit 1 R Q 1 Q 3 1 V i 2 Q 2 Q 4 2 Curret ca flow i both directios 21 The Triac The triac is essetially a bidirectioal SCR but the aodes are ot iterchageable. Triggerig is doe by applyig a curret pulse to the gate; breakover triggerig is ot ormally used Get p p 2 Biaa sas G 2 Simbol G Q 1 Q 3 Q 2 Q 4 2 Litar Setara 22 11

12 The Triac Whe the voltage o the 1 termial is positive with respect to 2, a gate curret pulse will cause the left SCR to coduct. Whe the aode voltages are reversed, the gate curret pulse will cause the right SCR to coduct. V BR(R0) V V BR(R1) V BR(R2) I H0 I H1 I H2 II G0 II G1 I G2 II H0 I I I H2 I H1 I G2 I G1 I G0 V BR(F2) V BR(F1) V BR(F0) V 23 Triac pplicatios (Phase-Cotrol Circuit) Triacs are used for cotrol of ac i applicatios like electric rage heatig cotrols, light dimmers, ad small motors. Like the SCR, the triac latches after triggerig ad turs off whe the curret is below the I H, which happes at the ed of each alteatio. D 1 R L 1 V i G Trigger Poit R 1 D 2 Trigger Poit (adjusted by R 1 ) 2 Voltage Waveform across R L 24 12

13 The Uijuctio Trasistor (UJT) The UJT cosists of a block of lightly-doped (high resistace) -material with a p-material grow ito its side. It has oly oe p juctio. It has a emitter ad two bases, B 1 ad B 2. It is ofte used as a trigger device for SCRs ad triacs. Base 2 B 2 Emitter p Costructio E Symbol Base 1 B 1 25 THE UJT UJT has oly oe p juctio. It has a emitter ad two bases, B 1 ad B 2. r B1 ad r B2 are iteral dyamic resistaces. The iter-base resistace, r BB = r B1 + r B2. r B1 varies iversely with emitter curret, I E r B1 ca rage from several thousad ohms to tes of ohms E B 2 B 1 r B2 r B1 Equivalet Circuit depedig o I E

14 B 2 V p E V EB1 r B1 _ B 1 Basic UJT Biasig d Equivalet Circuit rb2 η V BB V rb1 = ηv BB η = r B1 /r BB is the itrisic stadoff ratio. + _ V BB η B1 BB If V EB1 < V rb1 + V p, I E = 0 sice p juctio is ot forward biased(v p = barrier potetial of p juctio) t V P = ηv BB + V p, the UJT turs o ad operates i a egative resistace regio up to a certai value of I E. It the becomes saturated ad I E icreases rapidly with V E. 27 UJT Characteristic Curve The UJT is a switchig device; it is ot a amplifier. Whe the emitter voltage reaches V P (the peak poit), the UJT fires, goig through the ustable egative resistace regio to produce a fast curret pulse. V E Cutoff V P Negative resistace Peak poit Saturatio Valley poit V V I P I V I E 28 14

15 pplicatios of UJT UJT ca be used as trigger device for SCRs ad triacs. +V BB Other applicatios iclude o-siusoidal oscillators, sawtooth geerators, phase cotrol, ad timig circuits. R 1 V E V P C V E V R2 R 2 V V V R2 t Relaxatio oscillator t Waveforms for UJT relaxatio oscillator 29 pplicatios of UJT circuit usig a UJT to fire a SCR is show. Whe the UJT fires, a pulse of curret is delivered to the gate of the SCR. The settig of R 1 determies whe the UJT fires. The diode isolates the UJT from the egative part of the ac. B D R 1 V E C UJT R 2 R G R L SCR The UJT produces a fast, reliable curret pulse to the SCR, so that it teds to fire i the same place every cycle

16 Coditios For UJT Oscillator Operatio I the relaxatio oscillator, R 1 must ot limit I E at the peak poit to less tha I P at tur-o, i.e., V BB - V P > I P R 1. To esure tur-off of the UJT at the valley poit, R 1 must be large eough that I E ca decrease below I V, i.e., V BB -V V < I V R 1. So, for proper operatio: VBB VP VBB VV I P > R 1 > I V R 2 is usually << R 1, ad the frequecy of oscillatios is f o V = R1C l V BB BB V V V P 1 31 The Programmable UJT The PUT is a 4-layer thyristors with a gate. It is primarily used as a sesitive switchig device. It is more similar to a SCR (four-layer device) except that its aode-to-gate voltage ca be used to both tur o ad tur off the device. The gate pulse ca trigger a sharp icrease i curret at the output. It ca replace the UJT i some oscillator applicatios

17 PUT Costructio & Symbol ode () +V p Gate (G) p Cathode (K) V i R 1 G K R 2 R 3 Basic Costructio PUT Symbol ad Biasig 33 THE PUT Notice that the gate is coected to the regio +V adjacet to the aode. R 1 The gate is always biased R 2 positive with respect to the cathode. G Whe V -V G > 0.7 V, the V i R 3 PUT turs o. K The characteristic plot of V K versus I is similar to the V E versus I E plot of the UJT

18 THE PUT Characteristic Curve V K (aode-to-cathode voltage) The characteristic of a PUT is similar to a UJT, but the PUT itrisic stadoff ratio ca be programmed with exteral resistors ad the UJT has a fixed ratio. V P V V 0 I P I V I (aode curret) 35 pplicatio of PUT The priciple applicatio for a PUT is for drivig SCRs ad triacs, but, like the UJT, ca be used i relaxatio oscillators. For the circuit to oscillate, R 1 must be large eough to limit curret to less tha the valley curret (I V ). The period of the oscillatios is give by: 1 T = RCl 1 1 η where R3 η = R + R 2 3 +V CC R 1 R 2 C R 4 G K R

19 What is itrisic stadoff ratio, ad the period of the circuit? +V CC +20 V η 3 = = = R2 R3 20 k + 10 k = T RCl 1 1 R 10 kω 0.33 G + Ω Ω R 3 1 η 1 = ( 220 kω )( 0.01 µf) l = 0.89 ms R 1 R kω 20 kω C 0.01 µ F R 4 27 Ω K 10 kω What is the frequecy? 1.12 khz 37 The Light-ctivated SCR The light-activated SCR (LSCR) operates essetially as does the covetioal SCR except it ca also be light-triggered. Most LSCRs have a available gate termial for covetioal triggerig. The LSCR is most sesitive to light whe the gate termial is ope. Symbol 38 19

20 Selected Key Terms 4-layer diode Thyristor SCR The type of 2-termial thyristor that coducts curret whe the aode-to-cathode voltage reaches a specified breakover value. class of four-layer (pp) semicoductor devices. Silico-cotrolled rectifier; a type of three termial thyristor that coducts curret whe triggered by a voltage at the sigle gate termial ad remais o util aode curret falls below a specified value. Selected Key Terms LSCR Light-activated silico-cotrolled rectifier; a four layer semicoductor device (thyristor) that coducts curret i oe directio whe activated by a sufficiet amout of light ad cotiues to coduct util the curret falls below a specified value. Diac Triac two-termial four-layer semicoductor device (thyristor) that ca coduct curret i either directio whe properly activated. three-termial thyristor that ca coduct curret i either directio whe properly activated. 20

21 Selected Key Terms SCS Silico-cotrolled switch; a type of four- termial thyristor that has two gate termials that are used to trigger the device o ad off. UJT PUT Uijuctio trasistor; a three termial sigle p juctio device that exhibits a egative resistace characteristic. Programmable uijuctio trasistor; a type of three termial thyristor (physically more like a SCR tha a uijuctio) that is triggered ito coductio whe the voltage at the aode exceeds the voltage at the gate. Quiz 1. The 4-layer (Shockley) diode ca coduct curret if a. the aode-to-cathode voltage exceeds V BR b. a curret pulse is applied to the gate c. both a ad b are correct d. oe of the above 21

22 Quiz 2. The SCR ca coduct curret if a. the aode-to-cathode voltage exceeds V BR b. a curret pulse is applied to the gate c. both a ad b are correct d. oe of the above Quiz 3. bidirectioal thyristor is the a. 4-layer diode b. SCR c. triac d. silico-cotrolled switch 22

23 Quiz 4. thyristor that looks like two back-to-back 4-layer diodes is the a. SCR b. triac c. SCS d. diac Quiz 5. SCR turs off whe the a. gate trigger curret drops below a specified level b. aode curret drops below the holdig curret c. both a ad b are true d. oe of the above 23

24 Quiz 6. The purpose p of a crowbar circuit is to protect a load from a. excessive ripple b. low-voltage c. over-voltage d. all of the above Quiz 7. diac ad triac are similar i that both devices a. ca use breakover triggerig b. ca be used i ac circuits c. are bidirectioal d. all of the above 24

25 Quiz 8. device that has a ustable egative resistace regio is the a. UJT b. diac c. triac d. SCS Quiz 9. The symbol for a silico-cotrolled switch (SCS) is (a) (b) (c) (d) 25

26 Quiz 10. programmable uijuctio trasistor (PUT) is programmed by choosig the a. RC time costat b. gate resistors c. power supply voltage d. cathode resistor Quiz swers: 1. a 6. c 2. c 7. d 3. c 8. a 4. d 9. d 5. b 10. b 26

27 T H E EN D 53 27

R. W. Erickson. Department of Electrical, Computer, and Energy Engineering University of Colorado, Boulder

R. W. Erickson. Department of Electrical, Computer, and Energy Engineering University of Colorado, Boulder R. W. Erickso Departmet of Electrical, Computer, ad Eergy Egieerig Uiversity of Colorado, Boulder Specific o-resistace R o as a fuctio of breakdow voltage V B Majority-carrier device: AARR #$ = kk μμ $