RC Snubber Networks - ON Semiconductor
Description
AN1048/D RC Snubber Networks For Thyristor Power Control and Transient Suppression
By George Templeton Thyristor Applications Engineer
http://onsemi.
com
APPLICATION NOTE
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 acceptable performance.
However, improperly used snubbers can cause unreliable circuit operation and damage to the semiconductor 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.
Practical solutions depend on device and circuit physics.
STATIC dV dt
WHAT IS STATIC dV ? dt dV Static is a measure of the ability of a thyristor to dt
ǒdV Ǔ dt
s
DEVICE PHYSICS
Static dV 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 current is proportional to the rate of voltage change dV .
It triggers the device on when it becomes large enough to raise the sum of the NPN and PNP transistor alphas to unity.
A A
IB P
dt
ǒ dt Ǔ
IA PNP V
P
PE NB dv dt CJ G t NE PB C
I1
IC N
CJ P CJ N
IJ
IC
IJ
IB N IK
I2 G
NPN
K TWO TRANSISTOR MODEL OF SCR
dV CJ dt IA + 1 * (aN ) ap) CJ CEFF + 1*(aN)ap)
K INTEGRATED STRUCTURE
retain a blocking state under the influence of a voltage transient.
Figure 6.
1.
ǒdV Ǔ dt
s
Model
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