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 -3 10 cm

N+ 17 10 cm- 3

P

J 2 J1

P

+

501000

m

17 10 cm- 3

P

1 0m 30100 m

14 13 10 - 5x10 cm- 3

N

19 -3 10 cm

3050 µ

19 -3 10 cm

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 A

• SCR triggerable from forward blocking state to on-state by a gate current pulse.

forward on-state I -V

I RW M

i > 0 G

H

• Thyristor latches on and gate cannot turn it off. External circuit must force SCR off. i

G

= 0

BO VH

V B O vAK forward blocking state

Thyristor circuit symbol. + i A

VA K

cathode

anode

iG gate

Copyright © by John Wiley & Sons 2002

• 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 • 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

J

P 2

G

J (N + ) N2

2

• Use Ebers-Moll equations for BJTs • IC1 = -a1IE1 + ICO1 ; IC2 = -a2IE2 + ICO

3

• IA = IE1 ;

I K = -IE2 = IA + IG

K

• IC1 + IB1 + IE1 = 0

Two transistor equivalent circuit A J 1 J

• IA = Q 1

2

Q 2

a IG!+!ICO1!+!ICO2 1!-!!a1!-!a2

• Blocking state a1 + a2