AN-3008 RC Snubber Networks for Thyristor Power Control and ...

APPLICATION NOTE
AN-3008
dV
The noise induced into a circuit is proportional to ------ when
dt
dI
coupling is by stray capacitance, and ---- when the coupling
dt
is by mutual inductance. Best suppression requires the use of
a voltage limiting device along with a rate limiting CRL
snubber. The thyristor is best protected by preventing turn-on
dV
from ------ or breakover. The circuit should be designed for
dt
what can happen instead of what normally occurs.
In Figure 30, a MOV connected across the line protects
many parallel circuit branches and their loads. The MOV
dI
defines the maximum input voltage and ---- through the load.
dt
dV
With the snubber it sets the maximum ------ and peak voltage
dt
across the thyristor. The MOV must be large because there is
little surge limiting impedance to prevent its burn-out.
V MAX
In Figure 32, there is a separate suppressor across each
thyristor. The load impedance limits the surge energy delivered
from the line. This allows the use of a smaller device
but omits load protection. This arrangement protects each
thyristor when its load is a possible transient source.
Figure 34. Limiting Thyristor Voltage
It is desirable to place the suppression device directly
across the source of transient energy to prevent the induction
of energy into other circuits. However, there is no protection
for energy injected between the load and its controlling
thyristor. Placing the suppressor directly across each
thyristor positively limits maximum voltage and snubber
dI
discharge ---- .
dt
Figure 33. Limiting Line Voltage
REV. 4.01 6/24/02 15