9020 Datasheet, PDF (13/25 Pages) Fairchild Semiconductor – IGBT Basic II

Section II. IGBT Protections
1. Introduction - IGBT Failure Mechanism
IGBT applications with power converter received high electrical and thermal stress under
short-circuit or turn-off switching of clamped inductive load (CIL). As such, the ability to endure
stress is one of the important requirements.
If there is a large power loss within the device due to electrical stress, much heat is generated
to the limitations in packaging and due to semiconductor’s thermal parameters. It would lead
to thermal breakdown if this continues. Hot-spot generation from impact ionization and current
crowding are the reasons. The existence of parasitic thyristor also has an effect on the robust-
ness of the device. Latch-up of the parasitic thyristor is also a reason for breakdown. Design
for IGBT comes from an understanding of the mechanisms of various stresses that can lead to
the destruction of the device, and the IGBT is optimized to withstand stress from a large cur-
rent. SOA evaluation methods for the device are different for short-circuit in experiment and for
clamped inductive switching stress. IGBT’s short circuit performance generally determines for-
ward bias SOA, while turn-off at clamped inductive load determines reverse bias SOA. Posi-
tion of thermal failure within the latch-up-free PT IGBT chip is known to be different from the
two above switching stress.
There have been much research into preventing device destruction from short-circuit and turn-
off switching stress of CIL. We would like to discuss them in relation to SOA and protection
mechanism.
2. FBSOA and Short Circuit Destruction
A. FBSOA
Forward bias safe operating area (FBSOA or SOA) is generally referred to as the current and
voltage limits where the device can operate normally during on state. FBSOA of IGBT is illus-
trated in Fig. 1. The SOA of IGBT is nearly a rectangle for a short period of time, but as shown
in Fig. 1, FBSOA decreases as on-time increases. Lower limit is determined by DC operation.
Parasitic thyristor latch-up and thermal breakdown are the two major failure mechanisms of
IGBT under extreme stress. Even the device that prevents static latch-up could be prone to
dynamic latch-up. In the device that prevents latch-up, carrier multiplication, which was accel-
erated by thermal effect, becomes the cause of breakdown. It is limited by parasitic thyristor
latch-up at high collector current and dynamic avalanche break-up at high voltage region.
50
Ic MAX. (Pulsed)
10
Ic MAX. (Continuous)
50us
100us
1㎳
1
DC Operation
0.1 ^{Single Nonrepetitive}
Pulse T = 25℃
C
Curves must be derated
linearly with increase
in tem perature
0.01
0.1
1
10
100
Collector-Em itter Voltage, V_CE [V]
Fig. 1. Typical IGBT FBSOA
13
1000
Rev. A, April 2002