Cryogenics 45 (2005) 463–467
www.elsevier.com/locate/cryogenics
Performance of capacitors under DC bias at liquid
nitrogen temperature
Ming-Jen Pan
*
US Naval Research Laboratory, Multifunctional Materials Branch, Code 6350, 4555 Overlook Avenue SW, Washington, DC 20375, USA
Received 13 January 2005; accepted 10 March 2005
Abstract
Several commercially available capacitors were evaluated at the liquid nitrogen temperature (77 K). Our primary interest was
their performance under a bias voltage when used as DC link capacitors in cryogenic power electronics. In general, the performance
of polymer capacitors and certain ceramic capacitors is almost independent of the temperature, DC bias, and frequency. On the
other hand, ceramic capacitors based on high dielectric constant materials showed a strong dependence on the boundary conditions.
Aluminum electrolytic capacitors showed a dramatic decrease of capacitance at 77 K, possibly due to the electrolyte being frozen
and therefore losing its conductivity.
2005 Elsevier Ltd. All rights reserved.
Keywords: Dielectric properties; Power applications; Capacitor
1. Introduction
Superconducting systems for electrical power generation, distribution, storage, and use are undergoing field
demonstrations with support from US Department of
Energy, industry, and utility companies [1,2]. The high
energy efficiency and space/weight savings provided by
such superconducting systems are of great interest to
shipboard applications. An integrated electrical power
[3] system operated at cryogenic temperatures would
provide a high power density system for the emerging
all-electric ship, including ship propulsion, auxiliary
power, and weapons.
One of the advantages of a superconductor is its ability to deliver DC power without any loss. In such a DC
distribution configuration, the DC link capacitors are always under a DC bias while filtering transient current
caused by the switching actions of solid state switches.
Although there is literature reporting the dielectric
*
Tel.: +1 202 404 1534; fax: +1 202 404 7176.
E-mail address: pan@anvil.nrl.navy.mil
0011-2275/$ - see front matter 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.cryogenics.2005.03.006
behavior of capacitors at cryogenic temperatures [4–8],
all of it examined the low field dielectric properties
and none addressed the effects of DC bias. In this study,
we focused on the performance of commercial capacitors at liquid nitrogen temperature (77 K) for DC link
capacitor applications.
2. Experimental procedure
Several commercially available polymer film capacitors and ceramic capacitors were chosen for this study
(Table 1). The polymer capacitors we examined include
polypropylene (PP), polyester, polycarbonate, and polyphenylene sulfide (PPS) capacitors. Among them, PP
capacitors are the most common polymer capacitors
due to the wide availability of polypropylene film. Polyester capacitors are usually used for high current and
high dv/dt applications. Both polycarbonate and PPS
capacitors are rated for operating temperature up to
125 C. The ceramic capacitors we tested include X7Rrated ( 55 to 125 C, ±15%), Z5U-rated (10–85 C,
464
M.-J. Pan / Cryogenics 45 (2005) 463–467
Table 1
The list of evaluated capacitors and their dielectric properties without DC bias
Capacitor type
Polypropylene (metallized film)
Polypropylene (foil/film)
Nominal cap. (nF)
10
4.7
Rated voltage
400
400
Polyester
10
400
Polycarbonate
10
50
Polyphenylene sulfide (PPS)
10
100
X7R (company A)
10
100
X7R (company B)
10
100
X7R (company C)
10
100
Z5U (company A)
10
50
Z5U (company C)
10
100
NPO (company A)
10
100
NPO (company C)
10
100
1
100
Tantalum (radial)
100
50
Tantalum (axial)
100
50
Al electrolytic
100
50
Mica
+22% to 56%), NPO-rated (temperature stable), and
mica capacitors. X7R capacitors are typically composed
of barium titanate modified with dopants to give high
dielectric constant (K 1000–4000) and temperature
stability. Z5U capacitors usually are made of extra-high
K material (>10,000) with large temperature dependence. In contrast, NPO capacitors are made of non-polar, low K material (100) with extremely stable
temperature response (±30 ppm/C between 55 and
Capacitance (nF)
tan d
Frequency (kHz)
293 K
77 K
293 K
77 K
1
10
100
1
10
100
1
10
100
1
10
100
1
10
100
1
10
100
1
10
100
1
10
100
1
10
100
1
10
100
1
10
100
1
10
100
1
10
100
1
10
100
1
10
100
1
10
100
9.83
9.83
9.83
4.82
4.82
4.81
9.96
9.87
9.69
10.09
10.08
10.05
9.71
9.70
9.69
10.05
9.91
9.67
9.94
9.82
9.64
9.85
9.66
9.36
10.82
10.77
10.56
10.55
10.34
10.03
9.81
9.81
9.81
9.93
9.92
9.92
1.021
1.021
1.020
102.5
100.7
56.1
103.1
102.0
74.3
99.1
91.8
71.6
10.086
10.073
10.104
4.89
4.88
4.89
8.86
8.84
8.86
9.50
9.48
9.48
9.70
9.68
9.70
2.00
1.79
1.59
3.89
3.63
3.35
2.04
1.86
1.68
2.03
1.81
1.60
2.17
1.98
1.79
9.81
9.79
9.82
9.88
9.86
9.86
1.017
1.017
1.017
88.88
44.05
1.74
81.87
25.13
0.88
0.081
0.081
0.081
0.0001
0.0003
0.0019
0.0004
0.0004
0.0004
0.0039
0.0090
0.0150
0.0008
0.0018
0.0061
0.0005
0.0007
0.0002
0.0143
0.0167
0.0232
0.0132
0.0139
0.0159
0.0163
0.0205
0.0308
0.0133
0.0157
0.0227
0.0159
0.0201
0.0304
0.0001
0.0001
0.0001
0.0001
0.0002
0.0005
0.0001
0.0001
0.0001
0.0147
0.0959
0.8654
0.0104
0.0664
0.5881
0.0440
0.0921
0.409
0.0002
0.0004
0.0015
0.0002
0.0002
0.0001
0.0008
0.0008
0.0006
0.0007
0.0008
0.0008
0.0010
0.0007
0.0005
0.0739
0.0796
0.0829
0.0624
0.0685
0.0782
0.0641
0.0689
0.0742
0.0743
0.0801
0.0836
0.0643
0.0691
0.0744
0.0004
0.0005
0.0006
0.0010
0.0009
0.0009
<0.0001
<0.0001
<0.0001
0.1108
0.9987
5.1842
0.1698
1.5038
6.3437
0.0035
0.0028
0.0025
125 C). The last group is electrolytic capacitors (tantalum and aluminum electrolytics), which are characterized by their high capacitance values and strong
frequency dependence. The specifications of each capacitor are listed in Table 1. Note that most of the capacitors, whenever possible, have a capacitance near 10 nF
to reduce measurement error. For more detailed description of each capacitor technology, the readers are referred to the review article by Sarjeant et al. [9].
M.-J. Pan / Cryogenics 45 (2005) 463–467
465
Cap 1 kHz
Cap 10 kHz
Cap 100 kHz
Loss 1 kHz
Loss 10 kHz
Loss 100 kHz
1
0.003
0.96
0.002
0.94
0
0.2
0.4
0.6
0.8
1
0
Normalized DC Bias
Fig. 2. The dielectric properties of the polypropylene (metallized film)
capacitor under DC bias at 77 K. The behavior is representative of all
the polymer capacitors.
Cap 1 kHz
Cap 10 kHz
Cap 100 kHz
Loss 1 kHz
Loss 10 kHz
Loss 100 kHz
Normalized Capacitance
1.2
0.2
1
0.15
0.8
0.6
0.1
0.4
0.05
0.2
0
0
0.2
(a)
0.4
0.6
0.8
1
Normalized DC Bias
Cap 1 kHz
Cap 10 kHz
Cap 100 kHz
Loss 1 kHz
Loss 10 kHz
Loss 100 kHz
Normalized Capacitance
1.2
0.2
1
0.15
0.8
0.6
0.1
0.4
0.05
0.2
0
(b)
0
0
0.2
0.4
0.6
0.8
1
Dielectric Loss (Actual Value)
Thermal shock does not seem to be an issue in this
study––all samples were dipped directly into a liquid
nitrogen bath and none appeared to have any external
physical damage. Moreover, the samplesÕ dielectric
properties at room temperature remained unchanged
after being tested in liquid nitrogen.
The polypropylene and PPS capacitors showed little
change in capacitance when they were cooled from room
temperature to 77 K (Table 1), while the polyester and
polycarbonate capacitors showed a slight decrease
(5–10%) in capacitance in the same temperature range.
All of the polymer film capacitors exhibited virtually no
change in capacitance under DC bias up to the rated
voltage. A representative plot is shown in Fig. 2. The
method of construction (metallized film versus film-foil
combination) did not make any difference in the dielectric behavior of polypropylene capacitors, suggesting
that the effect of thermal expansion mismatch between
polymer film and electrode is insignificant. The dielectric
loss of polymer capacitors is usually slightly lower at
77 K, but independent of the bias voltage.
0.001
0.92
Dielectric Loss (Actual Value)
3. Results and discussion
0.004
0.98
0.9
The dielectric measurements were performed while
the capacitors were immersed in liquid nitrogen. DC
bias from 0 to the rated voltage was applied at 10% steps
using a power amplifier (Kepco Model BOP 1000M).
During the measurements, a blocking circuit, as shown
in Fig. 1, was used to protect the LCR meter (Hewlett-Packard 4284A) from the DC bias voltage. The
capacitance of the high voltage blocking capacitors
was much larger (>20·) than the samples for accurate
measurements. The current limiting resistors and Zener
diodes provided additional protection in the event of a
catastrophic failure of the capacitor under test. Careful
calibration of the LCR meter is critical as the low loss
of some capacitors and parasitic capacitance/inductance
of cables can lead to negative dielectric loss values. The
room temperature properties were measured using a
standard bridge (without the blocking circuit).
0.005
Dielectric Loss (Actual Value)
Fig. 1. A schematic of the measurement circuit used for the DC bias
measurement. Both the high voltage amplifier and the LCR meter were
controlled by a computer through a GPIB interface.
Normalized Capacitance
1.02
0
Normalized DC Bias
Fig. 3. The dielectric properties of the X7R capacitors ((a) company A
and (b) company B) under DC bias at 77 K.
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M.-J. Pan / Cryogenics 45 (2005) 463–467
The X7R and Z5U ceramic capacitors showed a
strong temperature dependence due to their ferroelectric
nature. When cooled from room temperature to 77 K,
both types of capacitors showed a 60–80% decrease in
capacitance. The application of a DC bias caused further decrease in capacitance (Fig. 3), as the bias voltage
tended to hold the domains in place and hence lowered
the contribution of domain wall movement to dielectric
properties. The X7R capacitors showed various degree
of decrease as a function of applied bias voltage. It is
interesting to note that the X7R capacitor that showed
the least amount of decrease during cooling (company
B) exhibited the highest percentage of capacitance drop
under the rated voltage. On the other hand, the Z5U
capacitors showed much smaller decrease under applied
voltage (Fig. 4), because the test temperature was farther
away from the Curie point and therefore contain a more
stable ferroelectric phase. Both X7R and Z5U capacitors tend to have higher dielectric loss at 77 K comparing to room temperature. This is likely due to the
Cap 1 kHz
Cap 10 kHz
Cap 100 kHz
Loss 1 kHz
Loss 10 kHz
Loss 100 kHz
Cap 1 kHz
Cap 10 kHz
Cap 100 kHz
0.2
0.8
0.6
0.1
0.4
0.05
0.2
0
0.2
(a)
0.4
0.6
0.8
1
0.001
0.92
0
0.1
0.4
0.05
0.2
0.8
1
Normalized Capacitance
0.6
0.6
0.2
0.4
0.6
0.8
1
0
Normalized DC Bias
Normalized DC Bias
Loss 1 kHz
Loss 10 kHz
Loss 100 kHz
Fig. 4. The dielectric properties of the Z5U capacitors ((a) company A
and (b) company C) under DC bias at 77 K.
0.005
1
0.004
0.98
0.003
0.96
0.002
0.94
0.001
0.92
0.9
(b)
0
0
0.2
0.4
0.6
0.8
1
Dielectric Loss (Actual Value)
Normalized Capacitance
0.002
0.94
1.02
Dielectric Loss (Actual Value)
0.8
0.4
0.003
0.96
Cap 1 kHz
Cap 10 kHz
Cap 100 kHz
0.15
0.2
0.004
0.98
0.9
0.2
0
1
Loss 1 kHz
Loss 10 kHz
Loss 100 kHz
1
0
0.005
(a)
1.2
(b)
0
Normalized DC Bias
Cap 1 kHz
Cap 10 kHz
Cap 100 kHz
1.02
Normalized Capacitance
0.15
Loss 1 kHz
Loss 10 kHz
Loss 100 kHz
Dielectric Loss (Actual Value)
1
Dielectric Loss (Actual Value)
Normalized Capacitance
1.2
0
increase in ferroelectric coercive field and hence more
pronounced hysteresis at low temperatures.
As mentioned earlier, NPO capacitors are made of
non-polar, low K material (100) with extremely stable
dielectric response in regard to temperature, therefore it
is not surprising to find that there was little change in
capacitance during cooling. Their dielectric properties
also remained constant under applied voltages at different frequencies (Fig. 5). Another oxide-based capacitor
with similar behavior is mica capacitor, which is fabricated from the mineral muscovite KAl2(SiAl)O10(OH)2.
In NPO and mica capacitors, the permittivity is almost
all due to electronic and ionic contributions, unlike ferroelectrics in which the permittivity is mostly due to dipoles and domain wall movement.
The last group we examined is electrolytic capacitors,
including tantalum and aluminum electrolytic capacitors. The tantalum capacitors are composed of Ta2O5
dielectric and the solid electrolyte MnO2. Their dielectric
properties were almost constant under DC bias voltage
0
Normalized DC Bias
Fig. 5. The dielectric properties of (a) NPO capacitor and (b) mica
capacitor under DC bias at 77 K.
M.-J. Pan / Cryogenics 45 (2005) 463–467
Cap 1 kHz
Cap 10 kHz
Cap 100 kHz
Loss 1 kHz
Loss 10 kHz
Loss 100 kHz
10
1
8
0.8
6
0.6
4
0.4
2
0.2
0
0
0.2
0.4
0.6
0.8
1
Dielectric Loss (Actual Value)
Normalized Capacitance
1.2
0
Normalized DC Bias
Fig. 6. The dielectric properties of the tantalum capacitors under DC
bias at 77 K.
(Fig. 6), but mildly dependent on temperature and
strongly dependent on frequency. As the Ta2O5 dielectric is non-polar, the measured dielectric behavior is
attributed to the limitation of the MnO2 electrolyte in
conducting electrical current. Similarly the aluminum
electrolytic capacitor showed a dramatic decrease in
capacitance from room temperature to 77 K (Table 1).
We attributed this to its liquid electrolyte being frozen
and not being able to conduct electrical current. Note
that due to the large capacitance of electrolytic capacitors relative to the blocking capacitors in the measurement circuit, the measurement error is expected to be
large. However, the general trend of degradation at
77 K and under DC bias is unmistakable.
All of the commercially available capacitors characterized above showed cryogenic capacitance equal to
or lower than room temperature capacitance. Therefore,
the energy density of capacitors at cryogenic temperatures remains an issue as at room temperature. To improve the performance of dielectrics at a reasonable
cost, a ceramic–polymer composite is a feasible approach. One can engineer the composition of ferroelectric materials to place their Curie points in the vicinity
of 77 K such that the dielectric constant is higher than
10,000, e.g., barium titanate–strontium titanate solid
solutions [10,11]. Subsequently, by mixing the high K
ferroelectric powder with a polymer matrix, one can in-
467
crease the dielectric constant of the polymer while maintaining the ease of fabrication.
Compared to room temperature operations, capacitors in cryogenic power electronics are subject to additional electrical stress due to improved switching
characteristics of semiconductor devices at cryogenic
temperatures (high dv/dt and di/dt). As a result, the failure mode at cryogenic temperature may change and
predictions of capacitor lifetime become questionable.
We plan to address this issue in the near future.
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