9020 Datasheet, PDF (22/25 Pages) Fairchild Semiconductor – IGBT Basic II

d. C snubber circuit
This is the simplest snubber circuit, so it has the advantage of suppressing over-current at
minimum cost. It is effective in mid- to-low current, low power applications, and as the power
level increases, it becomes more likely for the circuit to oscillate as the snubber capacitor and
the main circuit inductance form LC resonance circuit. It is often used in inverters.
e. RCD snubber circuit
It operates in the same manner as the C snubber circuit, but it is different in that it operates
during turn-off switching. It is a circuit that solved oscillation of the C snubber circuit by using
the fast recovery diode. Energy that was stored in DC loop inductance moves to the capacitor
while the IGBT turns off. The snubber diode prevents oscillation from taking place. Charge
from the capacitor is discharged through the snubber resistor. (RC time constant should be
about 1/3 of the switching cycle. (τ = T/3 = 1/3f))
This circuit reduces turn-off voltage transient directly. Switching waveform is significantly
smoother and snubber loss is small. Effect on the turn-on voltage transient is fine, and it has
the advantage of stable wave as the snubber diode blocks oscillation. It is practical in medium
current range, but operation in large capacity IGBT, parasitic inductance increases to present
problems in controlling over-voltage. In such large current applications, discharge-suppressing
RCD snubber circuits are generally used. Functions of the discharge-suppressing RCD snub-
ber circuit are similar to the functions of the RCD snubber circuits, but the discharge-suppress-
ing RCD snubber circuit has the advantage of smaller loop inductance as it is attached to the
collector and the emitter of each device.
This circuit cannot use low inductance snubber capacitors designed to be attached directly to
the IGBT, and the blocking diode added to the protection circuit can increase the total snubber
inductance. Furthermore, if the recovery characteristics of the diode are not good, V_CE over-
shoot and dv/dt at either sides of the IGBT/diode, or the output voltage can oscillate. Turn-off
mechanism is nearly the same as that of the discharge-suppressing RCD snubber circuit.
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Rev. A, April 2002