9020 Datasheet, PDF (20/25 Pages) Fairchild Semiconductor – IGBT Basic II

6. Over-voltage Protection
A. Over-voltage protection
Fig. 10. Turn-off waveform of IGBT with snubber
When a power device turns off rapidly, energy that has been stored in stray inductance is dis-
sipated in the switching device. Because of this, there is voltage overshoot in either side of the
device. The size of this transient voltage is directly related to the size of the stray inductance
and the falling rate of the turn-off current. In particular, large capacity IGBT module switches a
large amount of energy in a short time, and it is possible for a large current to be injected to
lead to the destruction of the device. Such module for a large current is generally made of sev-
eral IGBT chips. Individual chip switches a portion of the current in the amount equivalent to di/
dt as determined by the gate drive circuit. The total amount of the current and di/dt for the
module is equivalent to the sum of the current and di/dt for each chip, respectively. The
moment the device turns off to protect the IGBT during short circuit is the most dangerous
moment. At this time, di/dt could exceed several thousand A/us. To shut down the device to
protect it from a fault condition, care must be taken to minimize the accompanying over-volt-
age, or the device could be destroyed. Over-voltage protection circuit optimized for normal
switching may not be adequate in fault current shut-off process, so there must be additional
response to over-voltage. Snubber circuit is often used as a protection circuit to protect normal
switching. Over-voltage at turn-off can be suppressed by controlling the value of R_G or by
inserting Cg, but these methods hinders other characteristics. As such, snubber circuit is often
used. In the next section, types, characteristics as well as advantages and disadvantages of
the snubber circuit are discussed.
7. Snubber circuit
A. Snubber circuit - Types of snubber circuits and their features
Snubber circuit is a supplementary circuit used in the converter circuit to reduce stress put on
the power semiconductor. The ultimate goal of the snubber circuit is to improve the transient
waveform. The snubber circuit suppresses over-current or over-voltage or improves dv/dt and
di/dt to ease the transient waveform to reduce stress on the device. There are many uses for
snubber circuits, but this discussion will center on its ability to suppress over-current at turn-
off.
Snubber circuit can be divided into those connected in between the DC power supply bus and
ground, and those connected to each IGBT. The first types of circuits include RC snubber cir-
cuits, charge and discharge RCD snubber circuits and discharge-suppressing RCD snubber
circuits, and the second type includes C snubber circuits and RCD snubber circuits. The fol-
lowing are detailed descriptions of each snubber circuit.
20
Rev. A, April 2002