Module 10 SCR. 2. To understand two Transistor Static and Transient Models. 3. To learn the SCR Turn-on and Turn-off methods

Transcription

1 1 Module 1 SR 1. Inroducion 2. SR characerisics 3. Two ransisor saic and ransien models 4. SR urnon mehods 5. SR urnoff mehods 6. SR proecion and Triggering circuis 7. ae proecion circuis 8. Summary Learning Objecives 1. To sudy he SR haracerisics 2. To undersand wo Transisor Saic and Transien Models 3. To learn he SR Turnon and Turnoff mehods 4. To sudy SR Proecions and ae Triggering ircuis 5. To undersand he ae Proecion ircuis 1. SR

2 2 1. Inroducion Thyrisor is power semiconducor device and is widely used in power elecronic circuis. Thyrisors are operaed as bisable swiches, operaing from nonconducing o conducing sae. There are differen ypes of hyrisors and will be discussed in subsequen secion. Silicon conrolled recifier (SR) is he mos widely used hyrisor. SR can be assumed as ideal unidirecional swich for many applicaions, bu pracical hyrisor has cerain limiaions. SR basically is a four layer semiconducor device. I has pnpn srucure wih hree juncions, J 1, J 2 and J 3. I has hree erminals anode, cahode and gae. J 1 J 2 J 3 p n p n Figure 1 Thyrisor symbol and hree pnjuncions 2. SR characerisics When he anode volage is made posiive wih respec o cahode, juncions J 1 and J 3 are forward biased and J 2 is reverse biased. Hence only small leakage curren flows from anode o cahode. The hyrisor hen said o be in off sae condiion. If anode o cahode volage V is increased o a sufficienly large value, he reverse biased juncion J2 will break. This known as avalanche breakdown and corresponding volage is called as forward break over volage. Since J 1 and J 3 are already in forward biased, resuling in large forward curren. The device is hen in conducion sae or ON sae. The volage drop will be due o ohmic drop in he layers, and i is small abou 1V. In ON sae he anode curren is limied by exernal resisance R L. The anode mus be more han a value known as laching curren I L, in order o mainain he required amoun of carrier flow across he juncion. I L is he minimum anode curren required o mainain he hyrisor in he ON sae immediaely hyrisor is urned ON. The ypical vi characerisics of he hyrisor is as shown in he Figure 2. Once he hyrisor conducs, i behaves like a conducing diode and here is no conrol over he device. The device will coninue o conduc because here is no depleion layer on J 2 due o free movemen of carriers. If he forward curren is reduced below he level known as holding curren I H, a depleion region will develop around J 2 due o reduced number of carriers, and hyrisor goes ino blocking sae. Holding curren is always less han laching curren. 1. SR

3 3 V S i T V Reverse Breakdown volage V BR I L I H i T V BO V R L Reverse leakage curren Forward leakage curren Figure 2 Thyrisor ircui, vi characerisics When cahode volage is larger han anode volage J 1 and J 3 are reverse biased and J 2 will be forward biased. So hyrisor will be in OFF sae and only curren flow will be due o leakage curren. 3. Two Transisor model of Thyrisor: The regeneraive (laching) acion of hyrisor due o posiive feedback can be explained using he wo ransisor model of he hyrisor. Thyrisor can be considered as wo complemenary ransisors, one NPN (Q 2 ) and oher PNP (Q 1 ) as shown in figure. The collecor curren I, curren gain α, emier curren I E and leakage curren are relaed as: I I E I BO. The common base collecor curren is defined by: I I. For ransisor Q E 1, I 2 2I I BO 2. Using hose equaions he oal anode curren can be wrien as: I 2I IBO 1 I BO 2 The above equaion indicaes ha as if he I suddenly increased, say from o 1m, his will immediaely increase α 1 and α 2. α 2 depend on I and I. he increase in he values of α 1 and α 2 would furher increase I. Therefore here will be regeneraive or posiive feedback effec. If α 1 α 2 ends o uniy, he denominaor of above equaion approaches o zero, resuling in large I and hyrisor will urn ON wih small gae curren. 1. SR

4 4 Q 2 I T J 1 n p n I T = I Q 1 α 1 I B1 = I 2 I P p P J 2 J 3 I I 1 I B2 α 2 Q 2 I Q 1 I Figure 3 Two ransisor Model of Thyrisor Basic Srucure, Equivalen ircui Under ransien condiions SR may urn on wihou gae rigerring. Such urn on can no be explained using saic wo ransisor model. In ransien model, one has o consider he juncion capaciances of SR. Figure 4 shows he wo ransien model of hyrisor. I α 1 j1 Q 1 I V j2 j2 Q 2 j3 α 2 I Figure 4 Two Transisor Transien Model of Thyrisor i j2 d( q j d 2 ) d( j2v d j2 ) V j2 d d j2 j2 dv d j2 1. SR

5 5 The above equaion shows he relaion beween juncion curren (i j2 ), juncion capaciance ( j2 ) and volage (V j2 ) across he juncion J 2. Turn on characerisics of SR is shown in Figure 5. I consiss of delay ime, d and rise ime r. When SR is riggered he gae curren rises, bu hyrisor does no urn on immediaely. I sars urning on wih delay, d. Thus he urn on ime is on = d r. The widh of riggering pulse mus be greaer han on of he given SR i.e aing pulse widh, T W > on. i I T.9 IT.1 IT i I.1 I d r Figure 5 Turn on characerisics of Thyrisor Turn off characerisics of SR is complicaed han urn on ime and is shown in Figure 6. SR can be brough back o he blocking sae from on sae only by reducing he forward curren o a level below ha of he holding curren. The forward curren is reduced by applying a reverse volage across anode and cahode and hus forcing he curren hrough he SR o zero. The SR has a reverse recovery ime rr which is due o charge sorage in he juncions of he SR. These excess carriers ake some ime for recombinaion resuling in he gae recovery ime or reverse recombinaion ime gr. Thus, he urnoff ime q is he sum of he duraions for which reverse recovery curren flows afer he applicaion of reverse volage and he ime required for he recombinaion of all excess carriers presen. he end of he urn off ime, a depleion layer develops across and he juncion can now wihsand he forward volage. The urn off ime is dependen on he anode curren, he magniude of reverse V applied and he magniude and rae of applicaion of he forward volage. The urn off ime for converer grade SRs is 5 o 1 μs 1. SR

6 and ha for inverer grade SR s is less han 2 μs. To ensure ha SR has successfully urned off, he circui off ime c be greaer han SR urn off ime q. 6 v a On sae volage drop across SR i a ommuaion di/d Reverse volage due o power circui rr gr q c Figure 6 Turn off characerisics of Thyrisor 4. SR Turn ON Mehods Turn ON mehod also known as riggering. Wih anode posiive wih respec o cahode, a hyrisor can be urned ON by following echniques: Forward volage riggering, ae riggering, dv/d riggering, emperaure riggering and Ligh riggering. 1. Thermal: During forward blocking, mos of he applied volage is applied across reverse biased juncion J2. This volage across juncion J2 associaed wih leakage curren may raise he emperaure of his juncion. Wih increase in emperaure, leakage curren hrough J2 furher increases. This cumulaive process may urn ON he hyrisor a some high emperaure. High emperaure riggering may cause hermal runaway and his is generally avoided. 2. High Volage Triggering: When breakover volage V BO across hyrisor exceeds han he raed maximum volage of he device, hyrisor urns ON. he breakover volage hyrisor anode curren is called as Laching curren (I L ). Breakover volage is no normally used as a riggering mehod, because i may damage he device. I is desrucive urn ON mehod and should be avoided. 1. SR

7 7 3. dv/d riggering: Wih forward volage across anode and cahode of a hyrisor, J1, J3 are forward biased and J2 is reverse biased. This juncion J2 acs as capacior because of he presence of space charge. s pn juncion has capaciance, so larger he juncion area larger is he capaciance. If he charging curren is becomes large enough, densiy of moving curren carriers in he device induces swich ON. This mehod of riggering is no desirable because high charging curren may damage he hyrisor. 4. ae curren riggering: I is simple and efficien mehod o urn ON hyrisor. Refer Figure 7. In gae riggering mehod V is applied less han V BO. So hyrisor is forward biased bu no conducing. When small gae curren is applied, hyrisor urns on and remain in ON sae hough gae volage is removed. Higher he gae curren lower is he forward breakover volage. i T I 3 > I 2 > I 1 > I V BR I L I H V 3 V 2 V 1 V BO V V BO > V 1 > V 2 > V 3 Figure 7 SR characerisics for differen gae rigger currens. 5. Ligh riggering: In his mehod, he ligh paricles (phoon) are made o srike he reverse biased juncion, which causes an increase in he number of elecronhole pairs and riggering of he hyrisor. For ligh riggered hyrisors, a slo is made in he inner P layer. The ligh of paricular wavelengh is irradiaed o his P region hrough he opical fiber. If he inensiy of he ligh is greaer han cerain criical value, he hyrisor will urn on. Such hyrisor is called as LSR. Figure 8 shows he LSR connecion wih he load, is srucure and equivalen elecrical circui. The ligh can be exposed o he juncion J 2, which is ligh acivaed. In place of elecrical signal a he gae, ligh is used as a source and he device is worked as a ligh acivaed SR (PhooSR). Figure 8(B) shows is equivalen circui wih wo ransisor model and phoodiode in parallel wih juncion capaciance. 1. SR

8 8 Silicon Pelle node P ase and hea sink R Load N ae P N V D Ligh Source ahode node ae Q 1 1 Phoo Diode D 1 Q 2 R Load ae rigger inpu ahode Figure 8 LSR conneced o load showing srucure and equivalen elecrical circui. 5. SR Turn OFF Mehods The process of urning off hyrisor is called as ommuaion. The basic mehods are curren commuaion and volage commuaion. hyrisor can be urned on by applying a posiive volage abou a vol and or a curren of a few ens of milliamps a he gae cahode erminals. Bu SR canno be urned off via gae erminal. I will be urned off only afer he anode curren reduced below holding curren naurally or by forced commuaion echniques. Thus hyrisor commuaion can be classified as Naural or Line ommuaion and Forced ommuaion. 1. SR

9 9 Naural ommuaion: or Line ommuaion occurs only in circuis. Naural commuaion of hyrisor akes place in volage regulaors, Phase conrolled recifiers, cycloconverers ec. Forced ommuaion: This ype of commuaion is applied o D circuis. Line ommuaion In hyrisor circui if he source inpu volage is ac, he SR curren goes hrough a naural zero and reverse volage appears across SR. The device hen auomaically urns off due o naural behavior of source volage. This is known as naural commuaion or line commuaion. This ype of commuaion is applied in ac volage conrollers, phase conrol recifiers, and cycloconverers. The iming waveforms of curren and volage shows SR for differen delay or phase angle are shown in Figure 9.The waveform for he delay angle α =. The delay angle α is defined as he angle beween he zero crossing of he inpu volage and insan he hyrisor is fired. V S T 1 V R L i T V m v i T I RR π Leakage urren 2π ω v rr v (c) V m π 2π ω i T I RR Leakage urren v rr Figure 9 Line ommuaed of hyrisor circui, iming waveforms 1. SR

10 1 Forced ommuaion In some hyrisor circuis inpu source volage is dc. So hese hyrisors can be urn off heir own. Some exernal circuiry is required o reduce he forward curren o zero and urn off hyrisor whenever required. Such exernal circuis are said o be forced commuaed circuis. These forced commuaion echniques are generally used in dcdc applicaions and dcac applicaions. The differen ways of forced commuaion echniques are Load Side ommuaion and Line Side ommuaion.. Load Side ommuaion is furher classified as Self ommuaion. Impulse ommuaion. Resonan Pulse ommuaion. omplemenary ommuaion. Exernal Pulse ommuaion. Self ommuaion: i() V s L v s T 1 v L v c L π v c () 2V s π ω m ω m Figure 1 Thyrisor Self ommuaion ircui, Waveforms In his ype of commuaion, a hyrisor is urned off due o he naural characerisics of he circui. The assumpion is he capacior is iniially uncharged. When hyrisor T1 is swiched on, he capacior charges hrough T1 and L. s he capacior charges in ime L, he charging curren becomes zero and hyrisor T1 is swiched off iself. Once hyrisor is fired, here is a delay of seconds before T 1 is urned off and is called he commuaion ime of he circui. This mehod of urning off of hyrisor is called as self commuaion and he hyrisor is said o be self commuaed. 1. SR

11 LOD LOD 11 Impulse ommuaion The impulse commuaed hyrisor consiss of main hyrisor T 1 and hyrisors T 2 and T 3 are used in forced communicaion circui. I is assumed ha capacior is charged up o V S wih polariy shown. T 1 I m V s v c v S T3 v v c D m V (c) L r T 2 V s v T1 L 1 D 1 V T 1 I m OFF v S T3 v v c D m L r T 2 Figure 11 Impulse ommuaion Basic circui, Timing waveforms, and (c) diode in series wih inducor for acceleraed recharging of capacior. Resonan Pulse ommuaion 1. SR

12 LOD 12 T 1 I m I m i() D m on v c L T 2 v c () v S i() D m V V S T 3 V 1 V 1 2 off 1 c Figure 12 Thyrisor Resonan Pulse ommuaion ircui, Waveforms 1. SR

13 LOD 13 omplemenary ommuaion i c 2V S /R R 1 R 2 2V S /R V S v c v S i c v S T 1 T 2 V S V S v 1 V S T/2 V S v 2 Figure 13 omplemenary ommuaion ircui, Waveforms V S Exernal Pulse ommuaion T 1 T 2 L T 3 I m V S 2V V D m Figure 14 Exernal Pulse ommuaion 1. SR

14 LOD 14 B. Line Side ommuaion v S L T 3 L r T 2 T 1 D m I m Figure 15 Line Side ommuaion The mos imporan parameers: Break over volage Holding curren Turn ON ime Turn OFF ime Maximal forward curren Maximal reverse volage Maximal frequency 6. SR Proecion and Triggering ircuis hyrisor require a minimum ime o spread he curren conducion uniformly hroughou he juncions. If he rae of rise of anode curren is very fas compared o he spreading velociy of a urn on process, a localized hospo heaing will occur due o high curren densiy and he device may fail, as a resul of excessive emperaure. 1. SR

15 Load 15 di/d Proecion L S i T 1 I m R V S D m Figure 16 Thyrisor Swiching circui wih di/d limiing inducors The pracical device mus be proeced agains high di/d. s an example consider he following circui. Under seady sae operaion, D m conducs when T 1 is off. If T 1 is fired when D m is sill conducing, di/d can be very high and limied by he sray inducance of he circui. dv/d Proecion If swich S 1 in fig is closed a =, a sep volage will be applied o hyrisor T 1 and dv/d may be high enough o urn on he device. The dv/d can be limied by connecing capacior s in as shown in fig a. when T 1 is urned on, he discharge curren of capacior is limied by R S as shown in fig b. Wih R circui known as a snubber circui, he volage across hyrisor will rise exponenially and he hyrisor can be proeced. S 1 S 1 S V S S V V S V R S Figure 17 dv/d Proecion circui 1. SR

16 16 Thyrisor riggering circuis: Thyrisors are used for high volage and high power circuis. To conrol he hyrisors various conrol and rigger circuis are employed which operaes a low power. n isolaor circui is required beween an individual hyrisor and gae pulse generaor circui. The isolaion can be achieved by using isolaion ransformer or opocouplers. Opocoupler could be phooransisor or phoo SR. ypical phoo SR is shown in Figure 18. 5V Logic Inpu LOD 23V LED SE LSR Inpu onrol Signal or D T node Oupu onrol onnecions ahode ae Figure 18 ypical phoo SR Pulse ransformers are ofen used o couple a gae rigger pulse circui o high volage hyrisorized circui o obain elecrical isolaion. enerally, hese ransformers are eiher 1:1 (wo windings) or 1:1:1 (hree windings) ypes. Imporan ransformer design facors are a. Primary magneizing inducance should be high enough so ha magneizing curren is low, in comparison wih pulse curren during he pulse ime. b. For unilaeral pulse generaors core sauraion mus be avoided. c. Insulaion beween windings should be high for he applicaions including ransiens. 1. SR

17 17 d. Inerwinding capaciance should be low because i may provide pah for undesirable sray signals a high frequencies. Shor Pulse Triggering If riggering pulse is wide, he size and weigh of he pulse ransformer required is high. The circui shown in Figure 19 is used o conver wide pulse o shor pulse. The differeniaor convers he inpu pulse o shor pulse which is furher amplified using ransisorized circui as shown in iming diagram. Diode D 1 allows only posiive riggering pulse o he base of ransisor and diode D m acs as freewheeling diode. V v 1 D m N1 N2 V R V 1 V1 R 1 D 1 Q 1 V Figure 19 Pulse Transformer isolaion: Shor Pulse Long Pulse Triggering If riggering pulse is oo shor, i may no rigger he hyrisor. The circui shown in Figure 2is used o conver shor pulse o wide pulse. The inegraor convers he inpu pulse o wide pulse which is furher amplified using ransisorized circui as shown in iming diagram. Diode D m acs as freewheeling diode. 1. SR

18 18 V v 1 V D m N1 N2 R V m V R 1 D 1 Q 1 V m V v 1 1 Figure 2 Pulse Transformer Isolaion: Long Pulse Pulse Train eneraor In hyrisorized ac circuis wih inducive load, load curren lags behind he volage. In phase angle conrol, i may happen ha when riggering pulse is applied a low de angle, hyrisor is reverse biased. Therefore hyrisor will remain off even hough gae pulse is applied. To avoid his generally pulse rain is used o rigger he hyrisor. Pulse rain is generaed using blocking oscillaor as shown in Figure 21. When conrol inpu volage is high, pulse rain will be generaed and when i is low, pulse rain will be erminaed. 1. SR

19 19 V R V v 1 N3 Dm N1 N2 V R 1 D 1 Q 1 v 1 1 Figure 21Pulse Transformer isolaion: Pulse Train eneraor Pulse Train eneraor using imer and ND logic Pulse rain can be generaed using imer and ND logic as shown in Figure 21. To avoid his generally pulse rain is used o rigger he hyrisor. Pulse rain is generaed using blocking oscillaor as shown in Figure 22. When conrol inpu volage is high, pulse rain will be generaed and when i is low, pulse rain will be erminaed. V 1 is he gaing inpu and if i is high i will allow he pulsed o pass furher. V R V v 1 Dm N1 N2 v 1 ND R 1 Q 1 V v 2 Oscillaor Figure 22 Pulse Transformer isolaion: Pulse Train eneraor using imer and ND logic 1. SR

20 23V / 5Hz LOD 23V / 5Hz LOD 2 5V HOT R S R 1 R 2 T 1 NEUTRL Figure 23 TRIER OPTIL FIBER HOT R 1 XENON FLSH LMP R T 1 NEUTRL Figure ae Proecion ircuis The gae signals are generally provided hrough pulse ransformers for isolaion purpose. The volage pulse may be of high volage/ may be bipolar. Hence i may happen he power dissipaion a he gae high. Pulse may conain high frequency noise. In such case, here may be undesirable urn on of SR. In order o avoid such effecs, i is necessary o proec he gae. Various gae proecion schemes are used which are discussed as follows. 1. SR

21 21 Table 1 ae proecion circuis a) ae cahode Resisance Increase dv/d capabiliy. eep gae damped o assure V DRM capabiliy. Lowers urn off ime ( q ). Increases he laching and holding curren. b) ae cahode apaciance Increase dv/d capabiliy. Remove high frequency noise. Increases urn on and urn off ime. Lowers gae signal rise ime. Lowers di/d capabiliy. c) ae cahode Inducance Decrease D gae sensiiviy. Decreases urn off ime ( q ). d) Series Resisor Limis he gae curren Lowers dissipaion in gae juncion e) Series Zener: Decreases hreshold Sensiiviy. ffecs he signal rise ime and di/d raing. Isolaes he gae. f) ae cahode L Resonan circui Frequency Selecion. Unless he circui is damped he posiive and negaive gae curren may inhibi conducion or bring abou sporadic anode curren. g) Reverse Diode Supply reverse bias in off period. Proecs gae and gae supply for reverse ransiens. Lowes urn off ime. h) apaciive coupling Isolaes gae circui D componen. In narrow gae pulses and low impedance sources, he I g is followed by reverse gae signals which may inhibi conducion. 1. SR

22 22 8. Summary SR. is silicon conrolled recifier. I is widely used in ac o dc conversion. I has he capabiliy of conrolling he oupu volage. The SR 1. SR characerisics 2. Two ransisor saic and ransien models 3. SR urnon mehods 4. SR urnoff mehods 5. SR proecion and Triggering circuis 6. ae proecion circuis 7. Summary 1. SR