Aalborg Universitet
Investigation on the Short-Circuit Behavior of an Aged IGBT Module Through a 6
kA/1.1 kV Non-Destructive Testing Equipment
Wu, Rui; Smirnova, Liudmila; Iannuzzo, Francesco; Wang, Huai; Blaabjerg, Frede
Published in:
Proceedings of the 40th Annual Conference of the IEEE Industrial Electronics Society, IECON 2014
DOI (link to publication from Publisher):
10.1109/IECON.2014.7048996
Publication date:
2014
Document Version
Early version, also known as pre-print
Link to publication from Aalborg University
Citation for published version (APA):
Wu, R., Smirnova, L., Iannuzzo, F., Wang, H., & Blaabjerg, F. (2014). Investigation on the Short-Circuit Behavior
of an Aged IGBT Module Through a 6 kA/1.1 kV Non-Destructive Testing Equipment. In Proceedings of the 40th
Annual Conference of the IEEE Industrial Electronics Society, IECON 2014 (pp. 3367-3373). IEEE Press.
Proceedings of the Annual Conference of the IEEE Industrial Electronics Society
https://doi.org/10.1109/IECON.2014.7048996
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Investigation on the Short-circuit Behavior of an
Aged IGBT Module Through a 6 kA/1.1 kV Non-
Destructive Testing Equipment
Rui Wu
1
, Liudmila Smirnova
2
, Francesco Iannuzzo
1
, Huai Wang
1
, Frede Blaabjerg
1
1
Department of Energy Technology, Aalborg University, Aalborg, Denmark
2
Department of Electrical Engineering, Lappeenranta University of Technology, Finland
rwu@et.aau.dk, liudmila.smirnova@lut.fi,fia@et.aau.dk, hwa@et.aau.dk,fbl@et.aau.dk
Abstract —This paper describes the design and development of
a 6 kA/1.1 kV non-destructive testing system, which aims for
short circuit testing of high-power IGBT modules. An ultra-
low stray inductance of 37 nH is achieved in the
implementation of the tester. An 100 MHz FPGA supervising
unit enables 10 ns level control accuracy of the short-circuit
duration, protection triggering, and acquisition of the electrical
waveforms. Moreover, a protection circuit avoids explosions in
case of failure, making the post-failure analysis possible. A case
study has been carried out on an aged 1.7 kV IGBT power
module. The case study shows the current and voltage
waveforms during short-circuit, as well as the current
mismatch among six inner sections, which demonstrate the
capability and the effectiveness of the proposed setup in the
short-circuit aspect reliability studies of MW-scale power
modules.
Keywords— Insulated Gate Bipolar Transistor (IGBT),
Non-destructive testing (NDT), Short-circuit, Power module,
Degradation.
I. INTRODUCTION
In modern power electronics systems, there are
increasing demands to improve whole system endurance and
safety level while reducing manufacturing and maintenance
costs [1]. According to a questionnaire for manufacturers,
semiconductor devices are considered as the most critical
and fragile component in industrial power electronic
systems [2]. Based on another survey, semiconductor failure
and soldering joint failure in power devices take up 34% of
power electronic system failures [3]. Because Insulated Gate
Bipolar Transistors (IGBTs) are one of the most critical
components as well as the most widely used power devices
in industrial power electronic systems in the range above 1
kV and 1 kW [3], the reliability of IGBTs has drawn more
and more attention. In particular, for critical applications
like MW-level wind turbine systems, the ability of IGBT
withstanding abnormal conditions (e.g. short-circuits or
overloads), is strictly required to achieve sufficient
robustness for the power electronic systems [4].
For the above reasons, reliability assessment of IGBT
module, especially the high-power module for MW-level
application is highly demanded to prevent potential failure.
IGBT failure behaviors can be classified as open-circuit
failure and short-circuit failure. Generally, IGBT open-
circuit failure is considered as not fatal to converters,
because the converter can operate with lower quality of
output with fault-tolerant control. However, short-circuit
failure is critical to converters, since the uncontrolled high-
level short-circuit current may destroy and even explode the
failed IGBT and/or other components in the system [5]. At
present, 10 μs is a typical duration stated for short-circuit
withstanding capability by IGBT manufacturers. However
this value does vary from one IGBT to another due to
manufacturing variations and due to degradations. So far it
is not definitely clear how –and how much the tolerance of
the IGBT to overloads and short circuits could vary during
its entire service life, which is critical for reliability design
and maintenance costs estimation.
Research efforts have been devoted to the study of the
short-circuit behavior of IGBTs. In [6], the short-circuit
current capability is studied by repetitive low-energy-level
short-circuit tests. It reveals that the short-circuit currents
are reduced with testing time due to the Al metallization
layer degradation and the on-state resistance increase. In [7],
a “critical energy (E
C
)” is proposed to explain the
mechanisms of IGBT failure under repetitive short-circuit
operations. When short-circuit dissipated energy is below E
C
,
IGBT may survive for more than 10
4
times repetitive short-
circuit operations. However, when short-circuit energy is far
beyond E
C
, IGBT may fail after first short-circuit because of
thermal runaway. A further experimental investigation show
that IGBT can turn-off short-circuit successfully but fails
after several microseconds when the short-circuit energy is
lightly higher than E
C
. It is still challenging to determine the
exact value of E
C
, even though many experiments and
numerical simulations have been done in the prior-art
research [8]. In [9], short-circuit current differences among
the IGBT modules from a same production batch are
investigated. However, in the prior-art studies, it is still a
lack of method or setup to experimentally observe the
current capability and current distribution among parallel
chips of high IGBT power modules during short-circuit.
Traditional short-circuit testing circuit includes device
under test (DUT), a set of capacitors and a circuit breaker in
series. Over-current detection is integrated to avoid the DUT
explosion, which cannot totally prevent potential damage
[6]-[7]. Recently, a non-destructive testing concept has been
proposed by precisely control the testing time, allowing
repetitive short-circuit testing of IGBTs without
catastrophically damages [10]-[11]. It is a very cost-
effective way especially for testing high power IGBT
modules. The implementation of non-destructive testing
systems for discrete IGBTs (up to 100 A) and for high
power IGBT modules (up to 2.4 kA) are discussed in [10]
and [11], respectively. The main focus of these testing
systems is to study the discrete IGBTs short-circuit current
behavior, which are lack of the capability to obtain short-
circuit current capability of the high power IGBT modules.
In this paper, a state-of-the-art non-destructive tester
(NDT) is proposed with the capability of 6 kA/1.1 kV
electrical acquisitions during IGBTs short-circuit which is
appropriate for high-power IGBT modules. A Field-
Programmable Gate Array (FPGA) based controller supplies
10 ns precise short-circuit time control and corresponding
accurate electrical measurement. With the advanced busbar
design by Q3D simulations [13], an ultra-low total circuit
stray inductance of 37 nH is achieved in the NDT
implementation. To demonstrate the capability and
performance of the developed testing system, a case study
on the short-circuit behaviors of an aged high power IGBT
module is presented. The paper is organized as follows:
Section II describes the principle and implementation of the
NDT, including low inductance busbar design, FPGA based
timings setting. Section III presents the case study of an
aged 1.7 kV/1 kA IGBT module. It evidently demonstrates
the capability of the testing system in investigating the
electrical behaviors during short-circuits by accurate timing
control and detecting uneven current distribution among
several sections of the power modules. Section V concludes
the paper with discussions.
II. D
ESCRIPTION OF THE NON-DESTRUCTIVE TESTING
SYSTEM
2.1 Structure and Operating Principle
The basic idea of non-destructive testing is performing
repetitive short-circuit tests without destroying IGBTs. It is
challenging because both high voltage and high current are
applied at IGBTs during short-circuits. The power shock can
damage IGBT within several µs. Therefore, the Non-
Destructive Testing (NDT) is constructed in the lab, the
picture and circuit of which are as shown in Fig.1 and Fig. 2.
The NDT structure includes the following parts. A high-
voltage power supply charges up a high-voltage capacitor
bank C
DC
. The stored energy is used to supply for short-
circuits. The on-state series protection switch will be
switched off immediately after the test and save the DUT. A
Computer-Aided-Design (CAD) busbar has been developed
to minimize the overall circuit inductance, including the
intrinsic inductances of the series protection and the
capacitors. It is achieved by optimization of the mutual
coupling of the busbar components. A 100 MHz FPGA
provides the driving signals for the DUT and the switches
for the protection, and also provides the precise time control
for electrical measurement. The remote control and data
acquisition is achieved by a Personal Computer (PC) which
supervises the operation by connecting a LeCroy HDO6054-
MS oscilloscope via an Ethernet link and a FPGA board
through an RS-232 bus. Two commercial IGBT drivers
drive the protection switches and the DUT respectively. In
order to perform short-circuits, the corresponding protection
circuit on the DUT drivers has been deactivated. During
tests, collector current, collector voltage and gate voltage
waveforms are acquired together with the current flowing
through a specific section of the DUT.
Fig. 1 shows a photograph of the laboratory setup, where
the dimension is around 1 m x 1 m x 1 m. The electrical
schematic of the NDT with the commutation Loop 1 and
Loop 2 is shown in Fig. 2. It includes the DUT, the series
protection, parallel protection, load inductance L
load
, DC link
capacitance C
DC
, a high voltage power supply V
DC
, Schottky
diodes, negative-voltage capacitance C
NEG
with
corresponding negative voltage supply V
NEG
.
The operating principle is as follows: as shown in Fig. 2,
the power circuit is divided into two loops –Loop 1, (the
main loop in the Table I) including the series protection, and
Loop 2, including a parallel protection; the DUT is located
in the common branch. Table I gives the specifications of
major components in Fig.2. The tester is operated in a
standard single-shot way, so that the energy stored in the
capacitors C
DC
is used for the tests. C
DC
and C
NEG
are
composed of five and three capacitors in parallel,
respectively, in order to reduce the intrinsic stray
inductances. The same principle has been adopted for the
two switches of the series protection, the two switches of the
Fig. 1. Picture of the Non-Destructive Testing setup using the power circuit
in Fig. 2.
TABLE I. THE SPECIFICATIONS AND MAIN COMPONENTS APPLIED IN THE
POWER CIRCUIT PARAMETERS.
haracteristic Value
Maximum voltage 1.1 kV
Maximum current 6 kA
C
DC
capacitors 5 x 1100 µF
Stray inductance of the main loop 37 nH
Devices in series protection 2 x Dynex DIM1500ESM33-
TS000, 3 kA, 3.3 kV
Devices in parallel protection 2 x Mitsubishi CM1200HC-66H
2.4 kA, 3.3 kV
C
N
EG
capacitors 3 x 1100 µF
Schottky diodes 5 x 170 V, 1.2 kA
parallel protection and the five Schottky diodes.
Loop 2 is designed to improve the performance of the
NDT. The “non-destructive testing” means that the series
protection is activated right after the commutation to prevent
the DUT from explosions in case of failure. This capability
is strictly dependent on the series protection’s capability to
cut the current flowing through the DUT to zero
immediately after the test. However, the turn-off transition
of the series protection is non-ideal because IGBT switches
have current tails, which would continue flowing through
the DUT. To avoid this effect and divert the current tail, the
parallel protection is fired up together with the activation of
the series one. As demonstrated in [12], to improve the
parallel intervention as well, a negative voltage biases a
capacitor bank C
NEG
in order to enhance the voltage fall
promptness during IGBT turn-on. Furthermore, to avoid a
negative current flowing through the DUT, the Schottky
diode bank is placed in series.
There are two different short-circuit types: Type 1 short-
circuit happens during the IGBT turn-on, while Type 2
short-circuit happens when the IGBT is at on-state, as
illustrated in Fig. 3. The NDT can provide both short-circuit
types by different configuration and control timing schemes.
Control timing schemes for the two types short-circuit will
be further illustrated in Section II, Part 3.
2.2 Low Inductance Busbar Layout
The high current slope during turn-on and turn-off short-
circuits (at kA/us level) can cause voltage spikes, thus the
stray inductance should be controlled at a very low level (at
nH). The busbar design of the NDT power circuit is
illustrated in Fig. 4. Fig. 4 (a) provides a 3D view of the
NDT: the DUT is the black module in the lower right corner,
and the two capacitor arrays C
DC
(5x) and C
NEG
(3x) are
located under busbar. Parallel and series protection are also
behind the busbar. Schottky diodes are mounted in five
square windows close to the DUT. Colors adopted for the
busbar layers are coherent among Figs. 2, 4 (a) and 4 (b).
An original upside-down-T-shaped busbar is adopted, as
illustrated in Fig. 4 (b). The cross section of the whole
busbar system has been reported, together with some blocks
indicating the main components of the circuit. The adopted
geometry allows observing the DUT behaviors by
measuring equipment during the commutations.
Short-circuit tests by the manufacturer is normally
performed without series protection, in order to keep the
circuit total inductance at an acceptably low value. The
presence of the series protection in the NDT main loop
increases the circuit stray inductance. Therefore, the design
of a low inductance busbar is becoming more critical for the
a) b)
c) d)
e) f)
Fig. 4. Busbar design of the power circuit: a) 3D view of the core of the
N
DT setup including the busbar; b) principle cross-section of the busbar
layers: Loop 1 is the horizontal one, Loop 2 is the vertical one (see Fig. 2);
c) Ansys Q3D simulated distribution of the Loop 1 inductance and d) of the
Loop 2 inductance; e) virtual prototype of the NDT setup; f) physical
p
rototype of the NDT setup.
Fig. 2. Detailed schematic of the power circuit of the Non-Destructive
Tester.
Fig. 3. Two types of short-circuits: Type 1 occurs during turn-on, Type 2
occurs during conduction state.
NDT.
The design of the NDT busbar has been performed with
3D FEM modelling tool. Ansys Q3D [13] simulation
outputs have been reported in Figs. 4 (c) and 4 (d),
respectively for Loop 1 and Loop 2. Finally, Figs. 4 (e) and
4 (f) illustrate the whole power circuit design and the picture
of the constructed setup, respectively. Based on the Q3D
simulations, the calculated equivalent stray inductances of
the Loop 1 busbar is 8.6 nH, and the inductance of the Loop
2 busbar is estimated to be 9.4 nH. According to the
datasheets, parasitic inductance of the DUT, the Schottky
diode, the DC capacitor, the series protection, and the
parallel protection, are 10 nH, 10 nH, 100 nH (maximum
value), 10 nH, and 10 nH, respectively [14]-[16]. Therefore,
the total inductances of the commutation Loop 1 and Loop 2
are 45.7 nH and 59.4 nH, respectively, as shown in Figs. 4
(c) and 4 (d). These low inductance values are achieved by
the placement of the components in a way with negative
mutual coupling among them and by the application of a
200 μm Mylar isolation foil for the busbar.
The low stray-inductance design is verified by
experimental switching waveforms as shown in Fig. 5. It
can be noted that the peak value of V
CE
is 1253 V and the
DC voltage is 900 V. The current is decreased linearly by
351.9 A in 36.8 ns. Therefore, the obtained stray inductance
of Loop 1 is:
nH
A
nsVV
L
Loop
37
9.351
8.36)9001253(
1
(1)
It implies that the simulation result is relatively
conservative compared to the experimental one (i.e., 81 %
of the simulated value). It is worth noting that the value of
37 nH includes intrinsic inductance of the IGBT modules
for series protection and the capacitor bank C
DC
. The 19 %
difference compared to the simulation result is mainly due
to the parasitic inductance in the capacitor. In the simulation,
the maximum value of 100 nH of each capacitor in the
datasheet is used, while the measured inductance of each
capacitor is 40 nH.
2.3 FPGA Operation Timings Setting
As discussed in section 2.1, short-circuits are classified
as Type 1 and Type 2, as illustrated in Fig. 3. The circuit
configuration and control timing schemes for the two types
short-circuit are illustrated as follows:
For Type 1 short-circuit which happens at the device
turn-on transient, the load inductance L
load
is removed.
Before tests, the series protection is in on-state and the
parallel protection is off-state. Loop 1 has the stray
inductance only and the Schottky diodes behave almost
ideally, so the DUT is connected directly to the C
DC
capacitors. During the tests, the DUT falls to short-circuit
when it is triggered. After the precise controlled time by 100
MHz FPGA, the DUT short-circuit is switched off by series
protection IGBTs. At the same time, the parallel protection
is turned on to avoid the undesirable tail current through
DUT. The corresponding control time sequences of the
series and parallel protections and DUT are as shown in Fig.
6 (a). The negative voltage V
NEG
can speed up the parallel
protection, and the Schottky diodes can avoid a current flow
from the DUT to the negative voltage.
Type 2 short-circuit happens during on-state of the
device, and the load inductance L
load
is required to obtain
high current. At first, the series protection is off-state while
DUT is turned on. The series protection diodes operate as
freewheeling for the load inductance. In this case, the
current flowing through the DUT is determined by the load
inductance. Then, the series protection is turned on and
triggered by an on-state short-circuit to DUT. Finally, short-
circuit is ended with series protection turned off and parallel
protection turned-on. The corresponding control time
sequences of series, parallel protections and DUT are as
a)
b)
Fig. 6. Timing settings for two types short-circuits: a) Timing diagram for
Type 1 short-circuit; b) Timing diagram for Type 2 short-circuit.
Fig.5. Experimental waveforms of short-circuit turn-off used for measuring
the busbar total inductance.
