Lecture Notes

Thyristors (SCRs) William P. Robbins Professor, Dept. of Electrical and Computer Engineering University of Minnesota 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. SCRs -1 W. P. Robbins

Thyristor (SCR) Geometry Cathode

Gate

N+

J3

19 -3 10 cm

N+ 17 -3 10 cm

P

J2

14 13 10 - 5x10 cm- 3

N J1

P

+

• Cross-sectional

view showing

vertical 30100 µ

orientation of

SCR. 1 0µ

501000

µ

17 -3 10 cm

P

19 -3 10 cm

3050 µ

19 -3 10 cm

Anode

Gate and cathode metallization for slow (phase control) thyristor. cathode

gate

• SCRs with

kiloamp ratings

have diameters

of 10 cm or

greater.

Gate and cathode metallization for fast (inverter grade) SCR wafer distributed gate

wafer

cathode area (metallization not shown)

SCRs -2 W. P. Robbins

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

V A K

cathode

anode

iG gate

• 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 W. P. Robbins

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 = -α1IE1 + ICO1 ; IC2 = -α2IE2 + ICO

3

• IA = IE1 ;

I K = -IE2 = IA + IG

K

• IC1 + IB1 + IE1 = 0

Two transistor equivalent circuit A J 1 J

• IA =

• Blocking state α1 + α2