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Proceedings of the IEEE Energy Conversion Congress and Exposition (ECCE 2013), Denver, CO, USA, 14-
19 September, 2013.
Reliability of Capacitors for DC-Link Applications – An Overview
Huai Wang
Frede Blaabjerg
Suggested Citation
H. Wang and F. Blaabjerg, "Reliability of capacitors for DC-link applications – An overview," in Proc. IEEE
Energy Convers. Congr. and Expo., 2013, pp. 1866-1873.
Reliability of Capacitors for DC-Link Applications –
An Overview
Huai Wang, IEEE Member, Frede Blaabjerg, IEEE Fellow
Center of Reliable Power Electronics (CORPE), Department of Energy Technology
Aalborg University
Pontoppidanstraede 101, DK-9220 Aalborg, Denmark
hwa@et.aau.dk; fbl@et.aau.dk
Abstract— DC-link capacitors are an important part in the
majority of power electronic converters which contribute to
cost, size and failure rate on a considerable scale. From
capacitor users’ viewpoint, this paper presents a review on the
improvement of reliability of DC-link in power electronic
converters from two aspects: 1) reliability-oriented DC-link
design solutions; 2) conditioning monitoring of DC-link
capacitors during operation. Failure mechanisms, failure
modes and lifetime models of capacitors suitable for the
applications are also discussed as a basis to understand the
physics-of-failure. This review serves to provide a clear picture
of the state-of-the-art research in this area and to identify the
corresponding challenges and future research directions for
capacitors and their DC-link applications.
I. INTRODUCTION
DC-link capacitors are widely used in power converters
to balance the instantaneous power difference between the
input source and output load, and minimize voltage variation
in the DC link. In some applications, they are also used to
provide sufficient energy during the hold-up time. Fig. 1
shows the typical configurations of power electronic
conversion systems with DC-link capacitors. Such
configurations cover a wide range of power electronics
applications, such as in wind turbines, photovoltaic systems,
motor drives, electric vehicles and lighting systems. With
more stringent reliability constrains brought by automotive,
aerospace and energy industries, the design of DC links
encounters the following challenges: a) capacitors are one
kind of the stand-out components in terms of failure rate in
field operation of power electronic systems [1]-[2]; b) cost
reduction pressure from global competition dictates
minimum design margin of capacitors without undue risk; c)
capacitors are to be exposed to more harsh environments
(e.g. high ambient temperature, high humidity, etc.) in
emerging applications and d) constrains on volume and
thermal dissipation of capacitors with the trends for high
power density power electronic systems [3].
The efforts to overcome the above challenges can be
divided into three categories: a) advance the capacitor
technology with improved and pre-determined reliability built
in, b) optimal DC-link design solutions based on the present
DC link
C
AC
DC
DC
DC
(a) AC-DC-DC or DC-DC-AC power converters with a DC-link.
DC link
C
AC
DC
DC
AC
(b) AC-DC-AC power converters with a DC-link.
DC link
C
AC
DC
Resistive
loads
or
Energy
sources
(c) AC-DC or DC-AC power converters with a DC-link.
Fig. 1. Typical configurations of power electronic conversion systems
with DC-link capacitors.
capacitors to achieve proper robustness margin and cost-
effectiveness, and c) implementations of condition monitoring
to ensure reliable field operation and preventive maintenance.
By taking the advantage of the progress in new dielectric
materials and innovative manufacturing process, leading
capacitor manufacturers have been continuously releasing new
generations of products with improved reliability and cost
performance. The proper application of these capacitors for
specific DC-link design is equally important as the operating
conditions (e.g. temperature, humidity, ripple current, voltage)
could significantly influence the reliability of the capacitors.
Compared to the first category, the latter two are more relevant
from the power electronic designers’ perspective, which
therefore will be reviewed in this paper. Moreover, the
comparison of capacitors suitable for DC-link applications are
given. The failure modes, failure mechanisms, corresponding
critical stressors and lifetime models of them are also mapped.
The challenges and opportunities for future research directions
are finally addressed.
The work described in this paper was supported by the grant 12-
131914 from the Danish Council for Independent Research, Denmark.
II. CAPACITORS FOR DC-LINK APPLICATIONS
Three types of capacitors are generally available for DC-
link applications, which are the Aluminum Electrolytic
Capacitors (Al-Caps), Metallized Polypropylene Film
Capacitors (MPPF-Caps) and high capacitance Multi-Layer
Ceramic Capacitors (MLC-Caps). The DC-link design
requires the matching of available capacitor characteristics
and parameters to the specific application needs under
various environmental, electrical and mechanical stresses.
Fig. 2 shows a lumped model of capacitors. C, R
s
and L
s
are the capacitance, Equivalent Series Resistance (ESR),
Equivalent Series Inductance (ESL), respectively. The
Dissipation Factor (DF) is tanδ = ωR
s
C. R
p
is the insulation
resistance. R
d
is the dielectric loss due to dielectric
absorption and molecular polarization and C
d
is the inherent
dielectric absorption [4]. The widely used simplified
capacitor model is composed of C, R
s
and L
s
. It should be
noted that the values of them vary with temperature, voltage
stress, frequency and time (i.e. operating conditions). The
absence of the consideration into these variations may lead to
improper analysis of the electrical stresses and thermal
stresses, therefore, also many times unrealistic lifetime
prediction.
The property of dielectric materials is a major factor that
limits the performance of capacitors. Fig. 3 presents the
relative permittivity (i.e. dielectric constant), continuous
operational field strength and energy density limits of Al
2
O
3
,
polypropylene and ceramics, which are the materials used in
Al-Caps, MPPF-Caps and MLC-Caps, respectively [5]. It
can be noted that Al
2
O
3
has the highest energy density due to
high field strength and high relative permittivity. The
theoretical limit is in the range of 10 J/cm
3
and the
commercial available one is about 2 J/cm
3
. Ceramics could
have much higher dielectric constant than Al
2
O
3
and film,
however, it suffers from low field strength, resulting in
similar energy density as that of film.
The three type of capacitors therefore exhibit specific
advantages and shortcomings. Fig. 4 compares their
performance from different aspects in a qualitative way. Al-
Caps could achieve the highest energy density and lowest
cost per Joule, however, with relatively high ESRs, low
ripple current ratings, and wear out issue due to evaporation
of electrolyte. MLC-Caps have smaller size, wider frequency
range, and higher operating temperatures up to 200°C.
However, they suffer from higher cost and mechanical
sensitivity. The recent release of CeraLink series ceramic
capacitors [6] is of interest to extend the scope of MLC-Caps
for DC-link applications. It is based on a new ceramic
materials of antiferroelectric behavior and strong positive
bias effect (i.e. capacitance versus voltage stress). MPPF-
Caps provide a well-balanced performance for high voltage
applications (e.g. above 500 V) in terms of cost and ESR,
capacitance, ripple current and reliability. Nevertheless, they
have the shortcomings of large volume and moderate upper
operating temperature.
The DC-link applications can be classified into high
ripple current ones and low ripple current ones. The ripple
current capability of the three types of capacitors is
approximately proportional to their capacitance values as
shown in Fig. 5. C
1
is defined as the minimum required
capacitance value to fulfill the voltage ripple specification.
For low ripple current applications, capacitors with a total
capacitance no less than C
1
are to be selected by both Al-
Caps solution and MPPF-Caps solution. For high ripple
current applications, the Al-Caps with capacitance of C
1
could not sustain the high ripple current stress due to low
A/μF. Therefore, the required capacitance is increased to C
2
by Al-Caps solution while the one by MPPF-Caps solution is
C
1
. In terms of ripple current (i.e. $/A), the cost of MPPF-
Caps is about 1/3 of that of Al-Caps [7]. It implies the
possibility to achieve a lower cost, higher power density DC-
link design with MPPF-Caps in high ripple current
applications, like the case in electric vehicles [8].
C
ESR
ESL
R
S
L
S
R
D
C
D
R
P
ESR-Equivalent Series Resistance
ESL-Equivalent Series Inductance
Fig. 2. A simplified lumped model of capacitors.
Fig. 3. Energy storage density for various dielectrics (BOPP: Biaxial
Oriented PolyproPylene, which is the preferred film material for capacitors
rated above about 250 V) [5].
Fig. 4. Performance comparisons of the three main types of capacitors for
DC-link applications.
Fig. 5. Capacitance requirement of low ripple current applications and
high ripple current applications.
III. FAILURE AND LIFETIME OF DC-LINK CAPACITORS
A. Failure Modes, Failure Mechanisms and Critical
Stressors
DC-link capacitors could fail due to intrinsic and
extrinsic factors, such as design defect, material wear out,
operating temperature, voltage, current, moisture and
mechanical stress, and so on. Generally, the failure can be
divided into catastrophic failure due to single-event
overstress and wear out failure due to the long time
degradation of capacitors. The major failure mechanisms
have been presented in [9]-[12] for Al-Caps, [13]-[17] for
MPPF-Caps and [18]-[20] for MLC-Caps. Based on these
prior-art research results, Table I gives a systematical
summary of the failure modes, failure mechanisms and
corresponding critical stressors of the three types of
capacitors.
Table II shows the comparison of failure and self-
healing capability of Al-Caps, MPPF-Caps and MLC-Caps.
Electrolyte vaporization is the major wear out mechanism of
small size Al-Caps (e.g. snap-in type) due to their relatively
high ESR and limited heat dissipation surface. For large
size Al-Caps, the wear out lifetime is dominantly
determined by the increase of leakage current, which is
relevant with the electrochemical reaction of oxide layer
[21]. The most important reliability feature of MPPF-Caps
is their self-healing capability [15]-[16]. Initial dielectric
breakdowns (e.g. due to overvoltage) at local weak points of
a MPPF-Cap will be cleared and the capacitor regains its
full ability except for a negligible capacitance reduction.
With the increase of these isolated weak points, the
capacitance of the capacitor is gradually reduced to reach
the end-of-life. The film layer in MPPF-Caps are in the
order of 10-50 nm which are therefore susceptible to
corrosion due to the ingress of atmospheric moisture. In
[22], the corrosion mechanism is well studied. Figs. 6(a)
and (b) shows the corrosion of the metallized layers of a
degraded film capacitor located in the outer turns and inner
turns of the capacitor roll, respectively. It reveals that severe
corrosion occurs at the outer layers resulting in the
separation of metal film from heavy edge and therefore the
reduction of capacitance. The corrosion in the inner layers
is less advanced as it is less open to the ingress of moisture.
Unlike the dielectric materials of Al-Caps and MPPF-Caps,
TABLE I. OVERVIEW OF FAILURE MODES, CRITICAL FAILURE MECHANISMS AND CRITICAL STRESSORS OF THE THREE MAIN TYPES DC-LINK
CAPACITORS
(WITH EMPHASIS ON THE ONES RELEVANT TO DESIGN AND OPERATION OF POWER CONVERTERS).
Cap. type Failure modes Critical failure mechanisms Critical stressors
Al-Caps
Open circuit
Self-healing dielectric breakdown V
C
, T
a
, i
C
Disconnection of terminals Vibration
Short circuit Dielectric breakdown of oxide layer V
C
, T
a
, i
C
Wear out: electrical
parameter drift
(C, ESR, tanδ, I
LC
, R
p
)
Electrolyte vaporization T
a
, i
C
Electrochemical reaction (e.g. degradation of oxide
layer, anode foil capacitance drop)
V
C
MPPF-Caps
Open circuit (typical)
Self-healing dielectric breakdown V
C
, T
a
, dV
C
/dt
Connection instability by heat contraction
of a dielectric film
T
a
, i
C
Reduction in electrode area caused by
oxidation of evaporated metal due to moisture
absorption
Humidity
Short circuit (with resistance)
Dielectric film breakdown V
C
, dV
C
/dt
Self-healing due to overcurrent T
a
, i
C
Moisture absorption by film Humidity
Wear out: electrical
parameter drift
(C, ESR, tanδ, I
LC
, R
p
)
Dielectric loss V
C
, T
a
, i
C
, humidity
MLC-Caps
Short circuit (typical)
Dielectric breakdown V
C
, T
a
, i
C
Cracking; damage to capacitor body Vibration
Wear out: electrical
parameter drift
(C, ESR, tanδ, I
LC
, R
p
)
Oxide vacancy migration; dielectric puncture;
insulation degradation; micro-crack within ceramic
V
C
, T
a
, i
C,
vibration
V
C
-capacitor voltage stress, i
C
-capacitor ripple current stress, i
LC
-leakage current, T
a
- ambient temperature.
TABLE II. COMPARISONS OF FAILURE AND SELF-HEALING
CAPABILITY OF THE THREE TYPES OF CAPACITORS
.
Al-Caps MPPF-Caps MLCC-Caps
Dominant
failure modes
wear out
open circuit open circuit short circuit
Dominant
failure
mechanisms
electrolyte
vaporization;
electrochemical
reaction
moisture
corrosion;
dielectric loss
insulation
degradation;
flex cracking
Most critical
stressors
T
a
, V
C
, i
C
T
a
, V
C
,
humidity
T
a
, V
C
,
vibration
Self-healing
capability
moderate good no
(a) Separation of metal film from heavy edge by corrosion.
(b) Incomplete edge separation by corrosion.
Fig. 6. Corrosion of the metallized layers of a film capacitor [22].
Fig. 7. Leakage current of a barium titanate-based MLC-Cap under high
temperature and high voltage stresses (ABD: Avalanche BreakDown, TRA:
Thermal RunAway) [18].
the dielectric materials of MLC-Caps are expected to last for
thousands of years at use level conditions without showing
significant degradation [19]. Therefore, wear out of ceramic
capacitors is typically not an issue. However, a MLC-Cap
could be degraded much more quickly due to the
“amplifying” effect from the large number of dielectric
layers [19]. In [23], it has been shown that a modern MLC-
Cap could wear out within 10 years due to increasing
miniaturization through the increase of the number of layers.
Moreover, the failure of MLC-Caps may induce severe
consequences to power converters due to the short circuit
failure mode. The dominant failure causes of MLC-Caps are
insulation degradation and flex cracking. Insulation
degradation due to the decrease of the dielectric layer
thickness results in increased leakage currents. Under high
voltage and high temperature conditions, Avalanche
BreakDown (ABD) and Thermal RunAway (TRA) could
occur, respectively. Fig. 7 shows a study in [18] on the
leakage current characteristics of a MLC-Cap with ABD and
TRA failure. ABD features with an abrupt burst of current
leading to an immediate breakdown, while TRA exhibits a
more gradual increase of leakage current.
B. Lifetime Models of DC-Link Capacitors
Lifetime models are important for lifetime prediction,
online condition monitoring and benchmark of different
capacitor solutions. The most widely used empirical model
for capacitors is shown in (1) which describes the influence
of temperature and voltage stress.
0
00
11
exp
n
a
B
E
V
LL
VKTT
−
⎡⎤
⎛⎞ ⎛ ⎞
⎛⎞
=× × −
⎢⎥
⎜⎟ ⎜ ⎟⎜⎟
⎢⎥
⎝⎠⎝⎠ ⎝ ⎠
⎣⎦
(1)
where
L
and
L
0
are the lifetime under the use condition and
testing condition, respectively.
V
and
V
0
are the voltage at
use condition and test condition, respectively.
T
and
T
0
are
the temperature in Kelvin at use condition and test condition,
respectively. E
a
is the activation energy, K
B
is Boltzmann’s
constant (8.62×10
−5
eV/K), and n is the voltage stress
exponent. Therefore, the values of E
a
and n are the key
parameters to be determined in the above model.
In [24], the E
a
and n are found to be 1.19 and 2.46,
respectively, for high dielectric constant ceramic capacitors.
In [23], the ranges of E
a
and n for MLC-Caps are 1.3 – 1.5
and 1.5 – 7, respectively. The large discrepancies could be
attributed to the ceramic materials, dielectric layer thickness,
testing conditions, etc. With the trend for smaller size and
thinner dielectric layer, the MLC-Caps will be more sensitive
to the voltage stress, implying a higher value of n.
Moreover, under different testing voltages, the value of n
might be different as discussed in [25].
For Al-Caps and film capacitors, a simplified model from
(1) is popularly applied as follows:
0
10
0
0
2
n
TT
V
LL
V
−
−
⎛⎞
=× ×
⎜⎟
⎝⎠
(2)
The derivation of (2) from (1) is discussed in [26]. The
model presented by (2) is corresponding to a specific case of
(1) when E
a
= 0.94 eV and T
0
and T are substituted by 398 K.
For MPPF-Caps, the exponent n is from around 7 to 9.4 used
by leading capacitor manufacturers [27]. For Al-Caps, the
value of n typically varies from 3 to 5 [28]. However, the
![Fig. 9. Ripple current stresses of the 40 mF Al-Caps bank with or without an additional 2 mF film capacitor for a 250 kW inverter application discussed in [32].](/figures/fig-9-ripple-current-stresses-of-the-40-mf-al-caps-bank-with-18zagpp7.webp)
![Fig. 3. Energy storage density for various dielectrics (BOPP: Biaxial Oriented PolyproPylene, which is the preferred film material for capacitors rated above about 250 V) [5].](/figures/fig-3-energy-storage-density-for-various-dielectrics-bopp-2efbzuy1.webp)