Silicon controlled rectifier (SCR)
forward-breakover voltage
Thyristors
12
Silicon controlled rectifier (SCR)
Turning SCR off
When the gate returns to 0 V after the trigger pulse is
removed, the SCR cannot turn off; it stays in the forward-
conduction region.
The series switch in part
(a) simply reduces the
anode current to zero
and causes the SCR to
turn off.
The parallel switch in
part (b) routes part of
the total current away
from the SCR, thereby
reducing the
anode current to a value
less than
I
H
.
Thyristors
13
•
Holding current, I
H
.
Value of anode current below which the SCR switches
from the forward conduction region to the forward
blocking region.
I
H
increases with decreasing values of I
G
, and is
maximum for I
G
= 0.
Silicon controlled rectifier (SCR)
SCR Specifications
Thyristors
14
•
Circuit fusing (I
2
t) rating
Indicates the maximum forward surge current
capability of the
device.
If this rating is exceeded, the device will be destroyed due to
excessive power.
I
S
surge current
,
and
t
max
max
imum duration of
surge current
.
I
2
t
(
rating
)
t
max
I
S
2
where
One SCR application is as a surge protection device,
and the circuit fusing rating is critical.
Silicon controlled rectifier (SCR)
SCR Specifications
