Tantalum Polymer Capacitors
Achieve Higher Voltage Ratings
T
antalum surface-mount capacitors have gained
widespread favor for bulk decoupling use in both
conventional and switch-mode power supplies
(SMPSs) since their introduction more than
20 years ago. Today, tantalum surface-mount
capacitors are primarily used in SMPSs across multiple
industry segments, most often in applications that have
space restrictions, long stable life expectancy and high-reliability requirements. The characteristics of high-volumetric
efficiency, stable performance and the absence of a wear-out
mechanism continue to drive their popularity in SMPSs,
despite aggressive competition from other alternative dielectric materials such as aluminum and ceramic.
In addition to their benefits, tantalum capacitors have
traditionally had two weaknesses — a susceptibility to ignition when they fail and higher equivalent series resistance
(ESR) than capacitors based on other dielectrics. These
drawbacks were overcome with the introduction of a new
cathode material — an intrinsically conductive polymer —
as a replacement for the conventional manganese dioxide
(MnO2) cathode. But while surface-mount tantalum capacitors built using the MnO2 cathode could be used at voltages
up to 28 V, the tantalums built using polymer cathode material were previously only usable up to 19 V.
However, recent advancements in polymer technology
have permitted development of tantalum polymer capacitors for continuous duty at 20 V to 28 V, which enables them
to address a wider range of power-supply input requirements. To assess the usability of the new tantalum polymer
capacitors in power-supply applications, their electrical
performance and reliability are compared against existing
MnO2 tantalum capacitors.
Tantalum Pros and Cons
All capacitor technologies have their advantages and
disadvantages. Issues such as voltage coefficient and the
30
Power Electronics Technology August 2008
potential cracking of high-capacitance ceramics or the dryout concerns and incompatibility of aluminum electrolytics
to reflow temperatures are weighted and compared against
their advantages to arrive at a technology solution that
best meets the needs of the power-supply design. Primary
among the disadvantages of tantalum capacitors in SMPS
use are the potential for an ignition failure mode and higher
ESR when compared to some alternative dielectrics.
Both these disadvantages are linked to a single construction material within the tantalum capacitor, namely
the use of MnO2 as the cathode. High amounts of oxygen
present in the MnO2 cathode can provide a localized fuel
source that, under the right failure conditions, can result
in an ignition failure.
Also, since MnO2 is a semi-conductive material, it is the
largest contributor to the component’s total ESR value. It is
99
Percent (%)
Components built using the newer, safer, higherperformance conductive-polymer cathodes are
now available for use in 20-V to 28-V power-supply
applications.
By Jayson Young, Technical Product Manager, KO-CAP/
AO-CAP; Jake Qiu, Senior Scientist, Conductive Polymer
Development; and Randy Hahn, Development Director,
Tantalum Materials, KEMET Electronics, Simpsonville, S.C.
95
90
80
70
60
50
40
30
20
10
5
1
DCL 0 hr
DCL 250 hr
DCL 500 hr
DCL 1000 hr
DCL 2000 hr
0.01
0.10
1.00
10.00
DC leakage (µA)
Fig. 1. After 2000 hours of testing for the 85°C Life Test for
New Polymer Process (dc leakage distribution), the 35-V,
15-µF tantalum exhibited a reduction in dc leakage, which
suggests that the dielectric has not degraded.
0808kemet_F1
www.powerelectronics.com
these two characteristics that most often lead power-supply
designers to consider other dielectric solutions.
In the late 1990s, however, the undesirable features
of tantalum surface-mount capacitors were overcome by
the introduction of intrinsically conductive polymer as
a replacement for the MnO2 cathode. The use of conductive polymer offered a new material set 1000 times more
conductive than MnO2 with an absence of readily available
oxygen that could potentially lead to an ignition failure.
With the risk of ignition failure addressed and ESR
values significantly lower than any offerings coming from
a traditional MnO2-style tantalum capacitor, tantalum
polymer capacitors rapidly gained popularity throughout
the design community as engineers quickly took advantage
of this new technology to replace MnO2 tantalum capacitors and multilayer ceramic chip (MLCC) capacitors on the
power supply’s output rails.
Yet another advantage gained with the removal of MnO2
was the improvement in the voltage derating requirements
of tantalum surface-mount capacitors. To fully optimize
the reliability of the tantalum dielectric (Ta2O5), a voltage
derating is recommended. Over many years of reliability
analysis, the tantalum capacitor industry, in conjunction
with reliability experts from the military sector, concluded
that a 50% voltage derating of MnO2 tantalum capacitors
yielded acceptable reliability even in the most demanding
applications such as aerospace.
Extensive studies concluded that the primary contributor to failures was damage induced on the dielectric during
the board mounting process.[1-2] This damage was the result
of coefficient of thermal expansion mismatches in the material sets, which placed mechanical stresses on and produced
fault sites in the dielectric. The physical properties of the
MnO2 play a role in this since the material is rigid and in
direct contact with the dielectric, offering no protection
from the expansion and contraction of the other materials
around the anode.
When the MnO2 was replaced with the softer and more
elastic conductive polymer, researchers found that the
dielectric condition after board mounting was much improved and the applied voltages could be greatly increased
with no negative impact on predicted reliability.[2] Over
time, the recommended voltage derating for tantalum
polymers was established at 20% voltage derating or less
(depending on manufacturer and voltage rating). The
predicted reliability of the tantalum polymer capacitor
with a 20% derating was equal to that of a MnO2 tantalum
capacitor with a 50% derating.
While multiple improvements were realized with the
replacement of MnO2 with conductive polymer, tantalum
polymer capacitors did have at least one weak point with
regard to SMPS applications, which was the inability of
manufacturers to produce a reliable design for working
application voltages much above 19 V. This limited the use
of tantalum polymer capacitors to the lower-voltage output
applications of SMPS.
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tantalum capacitors
also be limited to reliability and electrical characteristics
alone with a brief discussion on cost.
Percent (%)
99
95
90
80
70
60
50
40
30
20
10
5
Reliability
In recent years, multiple studies have been conducted
to quantify the reliability of tantalum polymer devices and
compare them to their MnO2 predecessors. These studies
have reached similar conclusions.[3-4] The polymer cathode
systems are at least as reliable as the well-established MnO2
cathode design when used within their recommended
operating ranges.[3-4]
To qualify the new high-voltage tantalum polymer
capacitor, new reliability studies were conducted. One
hundred components were placed on a conventional 85°C
life test and charged at rated voltage (35 V) for 2000 hours.
Initial measurements were taken immediately following
board mounting of the devices. The devices were measured
again after 250, 500, 1000 and 2000 hours of testing.
The reliability of the tantalum polymer capacitor can
best be determined by assessing the dc leakage and ESR
performance of the capacitor over the duration of the life
test. Following 2000 hours of testing, the dielectric showed
no signs of degradation, as evidenced by the reduction in dc
leakage (Fig. 1). In addition, the ESR performance was found
to be stable throughout the 2000 hours of testing (Fig. 2).
This study demonstrates that the high-voltage tantalum
polymer capacitor has reliability characteristics at least as
good as those of currently existing MnO2 and tantalum
polymer capacitors, thus delivering stable and reliable
performance well beyond the life expectancy of any electronic hardware when used at the recommended derated
condition.
Variable
ESR 0 hr
ESR 250
ESR 500
ESR 1000
ESR 2000
1
0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10 0.11 0.12
ESR (7)
Fig. 2. Over the course of 2000 hours of testing for the 85°C
Life Test for New Polymer Process (ESR distribution), the ESR
of the 35-V, 15-µF tantalum remains stable.
At the time tantalum polymer capacitors were introduced, MnO2 tantalum capacitors were being safely used
0808kemet_F2
in voltage input applications up to 28 V. With an industry
demand for higher-voltage tantalum polymer capacitors,
manufacturers quickly began development activities to
provide solutions for the 20-V to 28-V input voltages to
replace MnO2 and high-capacitance MLCCs with these
higher-performance solutions. Despite industry pressures
for a higher-voltage rating in tantalum polymer capacitors,
technical challenges prevented capacitor manufacturers
from delivering the higher-voltage ratings.
High Voltage Arrives
Today, that limitation has been overcome through advancements in polymer technology resulting in the release
of a higher-voltage tantalum polymer device suitable for
continuous duty at 20 V to 28 V. With this advancement
achieved, many designers are now considering the advantages a tantalum polymer capacitor may offer over alternative solutions to address power-supply input needs.
To assess the relative advantages of the tantalum polymer capacitor, the designer must compare its performance
characteristics to the current solution. Since all capacitor
technologies and performance characteristics cannot be
accounted for in this discussion, only the use of a tantalum
polymer capacitor as a replacement for the commonly used
MnO2 tantalum capacitor will be explored. The topics will
Electrical Performance
The primary goal for developing an intrinsically conductive polymer formulation for higher-voltage tantalum
polymer capacitors was to offer power-supply designers a
device with improved performance characteristics over currently used solutions. While the benefits of a “non-ignition”
failure mode resulted from the use of conductive polymers
as a replacement for MnO2 , the main objective was to reduce
ESR and improve capacitance over frequency. To quantify
the benefits of this technology, a comparison study was conducted using several MnO2 tantalum capacitor technologies
commonly used for higher-voltage applications such as
Component type
(common trade names)
Capacitance
(μF)
Category
voltage (V)
Voltage derating
recommendation
ESR specification
limit (mΩ)
Case size
(L × W × H) (mm)
Polymer Ta
(T521 series)
15
35
20%
100
7.3 × 4.3 × 1.9
(low profile)
MnO2 Ta (commercial)
15
50
50%
700
7.3 × 4.3 × 4.3
MnO2 Ta (low ESR)
15
50
50%
200
7.3 × 4.3 × 4.3
MnO2 Ta
Multi-anode (MAT)
15
50
50%
75
7.3 × 4.3 × 4.3
Table. Component selection for performance comparison.
32
Power Electronics Technology August 2008
www.powerelectronics.com
tantalum capacitors
decreasing temperature). While this may be contradictory
to the way in which metals conduct themselves, the reason
for this negative coefficient is due to the semiconductive
nature of these materials, which is a more dominant factor
than the tantalum metal itself. While the use of polymer in
place of the MnO2 has slightly reduced this impact at cold
temperatures (Fig. 4), the polymer’s negative coefficient to
temperature still outweighs the metallic structure and thus
delivers a similar ESR response to temperature.
0.6
ESR (7)
Commercial MnO2
0.4
Low-ESR MnO2
0.2
0
10 k
Polymer
MAT MnO2
100 k
Frequency (Hz)
1M
Capacitance vs. Frequency
The capacitance behavior was analyzed to determine
the effect of frequency on capacitance. As shown in Fig. 5,
the commercial and low-ESR MnO2 technologies lose 67%
and 40% of capacitance, respectively, at around 300 kHz,
while the MAT device loses only 14% of initial capacitance.
The polymer device demonstrates a capacitance response
similar to that of the MAT device with only a 13% drop in
capacitance at 300 kHz.
Fig. 3. Tantalums based on conductive polymer cathodes
exhibit lower ESR than MnO2 tantalum capacitors at the
common SMPS frequencies of 200 kHz to 800 kHz.
power decoupling on 20-V to 24-V power-input rails.
Component selection for this comparison was based
on the highest capacitance value and lowest ESR com0808kemet_F3
monly available in a 50-V MnO2 tantalum surface-mount
design. Today, the highest CV ratings commonly available
(for the targeted voltage range) in a standard commercial
series design offers up to 15 µF of capacitance in a 7343-43
(7.3-mm 3 4.3-mm 3 4.3-mm) package size with advertised
maximum ESR limits of around 700 mΩ. Low-ESR MnO2
designs are also commonly available with ESR offerings as
low as 200 mΩ. In addition, a significantly more expensive
multi-anode tantalum (MAT) design of the same capacitance value and case size (but whose internal construction
consists of three thinner anodes to reduce ESR) was offered
by several manufacturers with an ESR limit of 75 mΩ.
Due to the polymer device’s ability to perform more
reliably closer to its rated voltage, a 35-V-rated polymer
component built with the high-voltage polymer process
with a maximum ESR limit of 100 mΩ was selected. The
advantage of using a lower-rated-voltage polymer device
yielded the additional benefit of a much smaller package
size (low-profile 7343-19). The table summarizes the list of
components that were selected for this evaluation.
Capacitance vs. Temperature
The behavior of capacitance over temperature remains
unaltered by the use of polymer in place of the MnO2 structure since the relationship of capacitance to temperature
relates to the dielectric, which is unchanged. The behavior
of this dielectric correlated to a negative coefficient (capacitance is reduced as temperature is decreased).
This may keep
you cool...
ESR vs. Frequency
To establish a baseline for ESR comparison of the
four component types, initial ESR measurements were
taken from 10 kHz to 1 MHz. The resulting ESR measures
(Fig. 3) demonstrated the advantages of the polymer cathode
when compared to the commercial and low-ESR MnO2
devices. In addition, the polymer design was found to have
only slightly higher ESR than the much larger and more
costly MAT MnO2 design when operating at frequencies
below 30 kHz and showed no significant difference in ESR
at frequencies above 30 kHz. These results show the superior
performance characteristics of the tantalum polymer device
at common SMPS frequencies of 200 kHz to 800 kHz.
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Power Electronics Technology August 2008
tantalum capacitors
600
ESR (m7)
500
400
300
Low-ESR MnO2
200
MAT MnO2
100
0
Polymer
Commercial MnO2
–55
25
Temperature (°C)
85
125
Fig. 4. Tantalums built with MnO2 and conductive polymer exhibit a similar decrease in
ESR as temperature increases.
Capacitance (µF)
20
MAT MnO2 and polymer
15
10
Low-ESR MnO2
0808kemet_F4
5
0
Commercial MnO2
1k
10 k
100 k
Frequency (Hz)
1M
100 M
Capacitance (µF)
Fig. 5. Like the MAT device, the tantalum polymer capacitor experiences only a 13%
drop in capacitance at 300 kHz.
150
100
MAT MnO2 and polymer (overlap)
50
Commercial MnO2 and low-ESR Mn02 (overlap)
0808kemet_F5
0
–55
25
85
125
Temperature (ºC)
Fig. 6. Capacitance overtemperature is primarily a function of dielectric rather than
cathode material, with the minor differences in performance attributed to variations
in tantalum particle size.
As shown in Fig. 6, the slope of each
component selection remains consistent. Minor variations can0808kemet_F6
be detected
between each of the four products;
however, these variations relate more
to the differences in tantalum particle
size used within each design, which
can vary between manufacturers.
Capacitance vs. Voltage
Unlike some dielectrics such as ceramic, tantalum capacitors do not experience capacitance loss with voltage
and maintain the same capacitance
within their voltage range. So when
bias is applied to a tantalum capacitor,
the user can expect to have the nameplate capacitance at all voltage levels
within its voltage rating. This characteristic remains unchanged regardless
of the cathode material selected.
Response of dv/dt
With the assessment of component
performance completed, one can determine how this technology would
be of benefit on higher-voltage powersupply input rails when compared to
MnO2 technologies. The improvements in ESR and capacitance roll-off
can be viewed in a time domain as
shown in the dv/dt plot in Fig. 7.
Using the same four tantalum capacitor technologies as before, a dv/dt
plot was constructed to demonstrate
this element. The dv is expressed as
34
Power Electronics Technology August 2008
volts per ampere, as the current is
another independent variable with
this response. The dt is expressed in
microseconds. As can be seen, there
is no discernable difference in dv between the polymer and multi-anode
MnO2 device up to and beyond 90 µs.
However, the commercial and lowESR MnO2 technologies demonstrate
a more rapid decline in voltage almost
immediately.
In looking at a time interval of 30 µs,
it can be seen that the MAT MnO2 and
polymer capacitors experience a voltage drop of around 2.5 V/A. However,
the commercial MnO2 and low-ESR
MnO2 capacitors experience a dv of
4.3 V/A and 3.2 V/A, respectively,
within the same time domain.
By targeting a specific application
need, a piece count assessment can be
conducted. For this exercise, a dv/dt of
less than 1 V/A per 30 µs was selected.
To maintain this requirement, the
piece count of each capacitor technology was increased until the dv/dt was
met. As shown in Fig. 8, the minimum
piece count necessary to maintain this
dv/dt was 5x commercial MnO2, 4x
low-ESR MnO2, 3x MAT MnO2 and
3x polymer capacitors.
Cost Factors
Considering that much of the cost
associated with tantalum surfacemount capacitors comes from the tantalum itself, it can quickly be concluded that the use of fewer capacitors and
a smaller case size will translate into
a lower total-cost solution. As shown
in the table, the use of the 15-mF part
with a MnO2 design required the use
of a 7343-43 case size versus the use of
a 7343-19 case size in a polymer design
(55% less package volume).
In Fig. 8, the assessment of a
specific dv/dt application need of
1 V per 30 µs has also shown the piece
count reduction that may occur when
replacing MnO2 with polymer. While
the added cost of polymer processing
versus MnO2 processing does shift the
manufacturing costs and therefore the
selling price of the polymer capacitor,
many designers have concluded that
in their own unique power-supply
www.powerelectronics.com
dv (V/A)
tantalum capacitors
0.00
-1
-2
-3
-4
-5
-6
-7
-8
-9
0
The da Vinci Way
MAT MnO2 and polymer
Low-ESR MnO2
Commercial MnO2
10
20
30
40
50
60
70
80
90
dt (µs)
MAT MnO2
0808kemet_F7
(3 pieces)
and polymer (3 pieces)
dv (V/A)
-500.00
Commercial MnO2 (5 pieces)
-1.00
NEW!
Low-ESR MnO2
(4 pieces)
-1.50
da Vinci designed The Flying Screw,
-2.00
-2.50
but never saw it fly. If a transformer
0
10
20
30
40
50
dt (µs)
60
70
80
90
or inductor has your latest design
stalled in mid air, call the Power
Team at Datatronics. We are the
magnetics innovator with years of
applications, the use of the high-voltage polymer part has translated into
a lower-cost solution over currently
0808kemet_F8
used MnO2 devices. In addition to
cost, the use of the tantalum polymer
capacitor has also translated into more
board space, the potential for a lowerprofile power-supply design and
superior performance over currently
utilized capacitor technologies.
What’s Next
The initial release of a high-voltage
tantalum polymer capacitor was limited to a 15-mF, 35-V design in the
lower-profile package design with
an ESR limit of 100 mΩ to 125 mΩ.
Future products currently in development include higher-capacitance
values (22 mF and 33 mF) in larger
case sizes (7342-28 case sizes) and
lower ESR values (45 mΩ to 60 mΩ).
These efforts will focus on 35-V-rated
devices intended for use in 20-V to
28-V power-supply applications.
www.powerelectronics.com
Looking even further ahead, there
also are development activities to
deliver higher-voltage ratings, which
could potentially lead to the use of tantalum polymer capacitors in powersupply applications with operating
voltages as high as 46 V. PETech
References
1. Prymak, J. “Derating Differences
in Tantalum-MnO2 vs. TantalumPolymer vs. Aluminum-Polymer,”
2003 CARTS Conference.
2. Prymak, J. “Improvements with
Polymer Cathodes in Aluminum and
Tantalum Capacitors,” IEEE 2001APEC Conference 2001.
3. Reed, E. “Characterization of Tantalum Polymer Capacitors,” NASA Electronic Parts and Packaging Program,
NEPP Task 1.21.5, Phase 1, FY05.
4. Reed, E. “Characterization of Tantalum Polymer Capacitors,” NASA Electronic Parts and Packaging Program,
NEPP Task 1.21.5, Phase 2, 2006.
35
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Power Electronics Technology August 2008