9020 Datasheet, PDF (21/25 Pages) Fairchild Semiconductor – IGBT Basic II

a. RC snubber circuit
This snubber circuit is effective in turn-off surge voltage and is suitable for chopper circuits. It
is also effective for oscillation by parasitic reactance and dv/dt noise. However, when it is
applied in large capacity IGBT, resistance for the snubber must be set low due to dissipation of
heat, so it has the disadvantage of worsening loading conditions at turn-on. Loss at snubber
itself is quite large, so it is not suitable for high frequency. In very large capacity IGBT circuit, it
is better to use small snubber “RC snubber circuit” along with the main snubber “discharge-
suppressing RCD snubber circuit.” When used together, it helps parasitic oscillation control of
the main snubber loop. Main applications include arc welder and switching power supply.
b. Charge and discharge RCD snubber circuit
This snubber suppresses over-voltage at turn-off to reduce switching losses at turn-off, and its
effectiveness in surge voltage suppression is about average. The snubber capacitor is com-
pletely discharged at turn-on, and it is fully recharged at turn-off. Unlike the discharge-sup-
pressing RCD snubber circuit below which acts as a clamp, this circuit reduces IGBT dv/dt
during turn-off. As such, soft switching is possible, and IGBT loss is reduced. Since the struc-
ture of this circuit is snubber diode added to an RC snubber, snubber resistance can be
increased, which alleviates the load problem at turn-on. It is effective chopper applications,
which uses large current and low DC link voltage. Its advantage also includes no oscillation at
its DC link voltage.
Power loss due to the resistance is as follows:
P
=
(L × I_O2 × f) ⁄ 2 + (C_S × Ed × f) ⁄ 2
=
1
⁄
2
×
C_sn
×
V_p
2
k
×
f_sw
[W]
L: wiring inductance of the main circuit
I_O : collector current at IGBT turn-off
C_S: capacitance of snubber capacitor
Ed: DC supply voltage
f: switching frequency
However, losses from this circuit (mostly from snubber resistance) are significantly larger than
the same in discharging suppressing RCD snubber circuit. As such, it is not suitable for high
frequency switching applications. There is a lot of turn-on losses with bridge configuration.
There are the disadvantages of relatively many parts and difficulties in selecting parts.
c. Discharge-suppressing RCD snubber circuit
Functions of this circuit are similar to those of voltage clamp snubber circuit. Snubber capaci-
tor is charged to the DC link voltage while the IGBT is in conduction, and V_CE rises rapidly
when IGBT is in turn-off. Due to the stray inductance of the DC loop, VCE rapidly rises above
DC link voltage. When V_CE rises above DC link voltage, snubber diode is in forward biased
conduction, and snubber begins operation. Energy stored in stray inductance moves to snub-
ber capacitor. This capacitor absorbs the energy without a rise in voltage.
It has the advantage of small oscillation in DC link voltage, and it is most practical in mid-to-
large current applications. Its effect on the turn-on voltage transient is neither large nor small.
It is ideal for high frequency switching as its losses from the snubber circuits small. Losses
from the circuit is as follows:
P = (L × I_O2 × f) ⁄ 2
[W]
Its disadvantages are that it has many necessary parts and is less than effective on turn-off
surge voltage. It is often used in inverters.
21
Rev. A, April 2002