© Semiconductor Components Industries, LLC, 2008
June, 2008 − Rev. 3
1 Publication Order Number:
AN1048/D
AN1048/D
RC Snubber Networks
For Thyristor
Power Control and
Transient Suppression
By George Templeton
Thyristor Applications Engineer
INTRODUCTION
Edited and Updated
RC networks are used to control voltage transients that
could falsely turn-on a thyristor. These networks are called
snubbers.
The simple snubber consists of a series resistor and
capacitor placed around the thyristor. These components
along with the load inductance form a series CRL circuit.
Snubber theory follows from the solution of the circuit’s
differential equation.
Many RC combinations are capable of providing accept-
able performance. However, improperly used snubbers can
cause unreliable circuit operation and damage to the semi-
conductor device.
Both turn-on and turn-off protection may be necessary
for reliability. Sometimes the thyristor must function with a
range of load values. The type of thyristors used, circuit
configuration, and load characteristics are influential.
Snubber design involves compromises. They include
cost, voltage rate, peak voltage, and turn-on stress. Practi-
cal solutions depend on device and circuit physics.
STATIC
dV
dt
WHAT IS STATIC
dV
dt
?
Static
dV
dt
is a measure of the ability of a thyristor to
retain a blocking state under the influence of a voltage
transient.
ǒ
dV
dt
Ǔ
s
DEVICE PHYSICS
Static
dV
dt
turn-on is a consequence of the Miller effect
and regeneration (Figure 1). A change in voltage across the
junction capacitance induces a current through it. This cur-
rent is proportional to the rate of voltage change
ǒ
dV
dt
Ǔ
. It
triggers the device on when it becomes large enough to
raise the sum of the NPN and PNP transistor alphas to unity.
Figure 6.1. Model
ǒ
dV
dt
Ǔ
s
I
A
+
C
J
dV
dt
1 * (a
N
) ap)
C
EFF
+
C
J
1*(a
N
)ap)
I
B
P
I
J
I
J
I
K
I
B
N
I
C
N
I
1
I
2
I
C
P
I
A
TWO TRANSISTOR MODEL
OF
SCR
C
J
N
C
J
P
PNP
A
C
G
C
J
N
E
P
B
N
B
P
E
V
t
G
NPN
INTEGRATED
STRUCTURE
K
A
K
dv
dt
APPLICATION NOTE
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