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Lecture Notes Thyristors (SCRs) OUTLINE • SCR construction and I-V characteristics. • Physical operation of SCRs. • Switching behavior of SCRs • dv/dt and di/dt limitations and methods of improving them. • SCR drive circuit considerations. Copyright © by John Wiley & Sons 2002 SCRs -1 Thyristor (SCR) Geometry Cathode Gate N+ J3 19 10 cm -3 N+ 17 10 cm P 19 10 cm 10 -3 -3 30100 J 2 14 13 10 - 5x10 cm N 501000 -3 J1 P P + 17 10 cm -3 19 10 cm -3 3050 µ Anode Gate and cathode metallization for slow (phase control) thyristor. cathode gate • Cross-sectional view showing vertical orientation of SCR. • SCRs with kiloamp ratings have diameters of 10 cm or greater. Gate and cathode metallization for fast (inverter grade) SCR wafer distributed gate wafer Copyright © by John Wiley & Sons 2002 cathode area (metallization not shown) SCRs -2 Thyristor I-V Characteristics i • SCR triggerable from forward blocking state to on-state by a gate current pulse. A forward on-state I I -V i >0 G H • Thyristor latches on and gate cannot turn it off. External circuit must force SCR off. i =0 G BO RW M V BO vAK VH forward blocking state Thyristor circuit symbol. + i A VA K • Current to several kiloamps for V(on) of 24 volts. • Blocking voltages to 5-8 kilovolts. • VBO = breakover voltage ; I BO = breakover current • VH = holding voltage I H = holding current cathode anode iG gate Copyright © by John Wiley & Sons 2002 • Maximum junction temperature = 125 C limited by temperature dependence of VBO. SCRs -3 SCR Model and Equivalent Circuit One dimensional SCR model. A • BJTs in equivalent circuit in active region. P1 J 1 (N - ) N1 • Use Ebers-Moll equations for J 2 P 2 G J 3 (N +) N2 BJTs • IC1 = - 1 IE1 + I CO1 ; I C2 = - 2 IE2 + I CO • IA = I E1 ; I K = -I E2 = I A + I G K • IC1 + I B1 + I E1 = 0 Two transistor equivalent circuit A • IA = J 1 Q 1 J 2 IG + I CO1 + I CO2 11 - 2 • Blocking state Q 2 J 1 + 2 << 1 G 3 K Copyright © by John Wiley & Sons 2002 • At breakover 1 + 2 - 1 SCRs -4 Thyristor Turn-on Process • In forward blocking state, both BJTs active. • If 1 + 2 < 1, connect ion is stable. • If VAK = VBO or if positive gate current pulse is applied 1 + 2 becomes equal to unity and ci rcuit connection beco mes unstable and SCR switches on. p 1 J1 n + J + - 2 + J 3 Holes attracted by ne gative charge o f inje cted ele ctro ns + 1 + + + p 2 - J 2 d epletion width - no gate current J 2 d epletion width - with gate current n2 Electrons injected in resp onse to gate current f low Copyright © by John Wiley & Sons 2002 • Negative charge of electrons swept into n 1 layer partially compensate positive charge of ionized donors exposed by growth of depletion of junction J2. • Growth of depletion reduces width of bases of Qnpn and Q pnp and thus increases 1 and 2. • Holes attracted by first wave of injected elctrons attract additional electrons and so on - regenerative action. SCRs -5 Thyristor On-state Latchup SCR with negative gate current • Negative gate current causes lateral voltage drops as indicated which lead to current crowding in center of cathode. K Negative gate current N G + N + - P N - - + + • Conventional SCRs (phase control) have large area cathodes - negative gate current cannot remove stored charge from center of large cathode area. • SCR stays latched on in spite of negative gate current. P A Copyright © by John Wiley & Sons 2002 • External circuit must force anode current to negative values in order that enough stored charge be removed from SCR so that it can turn off. SCRs -6 Thyristor On-state Operation G N 2 K P 2 N 1 P 1 A x total carrier density N N A 1 1 D 2 N A 2 N D1 x • On-state: all three junctions forward biased and BJTs in equivalent circuit saturated. • On-state stable because saturated BJTs have 1 + 2 << 1. • On-state voltage V AK(on) = Vj1 - V j2 + Vj3 + Vn Copyright © by John Wiley & Sons 2002 SCRs -7 Thyristor Turn-on Behavior i (t) G T A t di v A TB i (t) A vB T C v t C d(on) Io F dt Io t tr t ps v (t) AK control t v A vB v • td(on) = turn-on delay time; time required for charge injection by gate current to make 1 + 2 = 1. C t • Time intervals that T i • tr = time required for anode current to reach on-state value. Anode current rate-of-rise diF/dt limited by external inductance. can be on A • tps = time required for plasma to spread over whole cathode area from cathode periphery near gate. G Delay or trigger angle Copyright © by John Wiley & Sons 2002 t • VAK does not attain on-state value until complete area of cathode is conducting. SCRs -8 Thyristor Turn-off Behavior diR t dt iA ( t ) t2 I t1 3 t IR R 4 Turn-off waveforms dv F dt V REV v (t) AK recovery time t > t q t 3 • SCR turn-off quite similar to power diode turn-off. • Anode current rate-of-fall controlled by external inductance. • Reverse voltage overshoot caused by external inductance. • Junction J1 is blocking junction in reverse bias. J 3 has low breakdown voltage (20-40 volts) because of the heavy doping on both sides of the junction. Copyright © by John Wiley & Sons 2002 SCRs -9 Thyristor di/dt Limit at Turn-on i G K G N2 N2 P2 i (t) G N 1 P 1 i A t A • SCR first turns on at cathode pe riphe ry n earest gate. • Current constricted to small areas during initial pha ses of turnon, td(on) and tr. • Use shaped gate current pulse for rapid turn-on. • If anode cu rrent rate-of-rise, di F/dt, not kept le ss than some specified maximum, current density in constricted area will be too large. • Localized pow er dissipation too high and thermal runaway likely. Copyright © by John Wiley & Sons 2002 SCRs -10 Thyristor Re-applied dv/dt Limits dv F dt v (t) AK • Removal of all stored charge in SCR requires a minimum time tq. VF • Application of positive dVF/dt larger than a specified value before tq results in a pulse of positive anode current which may produce unintentioned turn-on of the SCR. t V REV forward recover y current iA(t) t • Avoidance of unintentioned turn-on requires dVF/dt < dVF,max/dt and remaining in reverse bias for a minimum time tq. Rate effect Copyright © by John Wiley & Sons 2002 SCRs -11 Methods of Improving Thyristor di/dt Rating • Interdigitated gate-cathode structure used to greatly increase gate-cathode periphery. • Distance from periphery to center of any cathode region significantly shortened. • Ability of negative gate current to break latching condition in on-state increased. • Combination of pilot thyristor, diode, and iterdigitated gate-cathode geometry tgermed a gate-assisted turn-off thyristor or GATT • Use of pilot thyristor to increase turn-on gate current to main thyristor. • Larger gate current increases amount of initial conducting area of cathode and thus improves diF/dt capabiities. • Diode allowes negative gate current to flow from main SCR. Copyright © by John Wiley & Sons 2002 SCRs -12 Improvement in dv/dt Rating Via Cathode Shorts • Current thru Cj2 indistinguishable from positive gate current with respect to turn-on of SCR. • If current thru Cj2 bypasses junction J 3, then SCR will not be turned on by the large displacement currents. • Cathode shorts provide this desirable bypa ss. Most effective with interdigitaated gate-cathode geometry. dVF • dt significantly increased. max Copyright © by John Wiley & Sons 2002 SCRs -13 Thyristor Gate Trigger Requirements V G K V G G trigger circuit load line maximum gate power diss ipation Equivalent circuit of minimum temperature SCR drive circuit RG maximum temperature I minimum trigger current I G 2 G 1 V G R GG V + G G IG i (t) G Gate current must be on for a specified minimum time interval (few tens of microseconds) to guarantee SCR turn-on Copyright © by John Wiley & Sons 2002 SCRs -14