By Kazuyuki Kubota, Product Engineering Manager, and
Shigeki Nishiyama, Product Development Manager, Murata
Manufacturing Co. Ltd., Izumo, Japan, and Karun Malhotra,
Cap Marketing Manager, Murata Electronics, N.A., Smyrna, Ga.
High-voltage multilayer ceramic capacitors are
replacing aluminum electrolytic and film capacitors in many applications where they offer
improved performance.
edium- and high-voltage capacitors
find widespread use as snubbers or
filters in applications, such as switching power supplies for audiovisual
and business equipment (computers,
modems and fax machines) and in lighting ballasts. Their
excellent performance at high frequencies (several ten to
several hundred kilohertz) has made them a choice of
design engineers worldwide. In addition to their use in
power supplies, these capacitors are widely used in industries related to telecommunications, medical, defense and
aerospace, semiconductor and test/diagnostic equipment.
The definition of medium and high voltage varies for
M
Available capacitance
for 250-V rating
Rated voltage available
Polarized
Equivalent series inductance
Equivalent series resistance
Size (volume)
each manufacturer. However, the authors consider capacitors rated at or above 250 V as medium voltage and those
above 2 kV as high voltage (although some would define
capacitors at or above 100 V as medium voltage).
Notwithstanding some design changes, high-voltage
capacitors essentially offer the same advantages as any
other multilayer monolithic ceramic capacitors (MLCCs).
These advantages include high-volumetric capacitance,
extremely low impedance and ESR, and high thermal
stability and reliability. Other benefits include the choice
of several temperature characteristics (X7R, C0G and U2J),
nonpolarity, a choice of chip-style or leaded packaging
(chips are most common) and high performance-to-price
ratio.
Ceramic
Film
Al electrolytic
Because ceramic capacitors cover a
10 pF to 1µF 10 pF to 47µF 10 µF to 1000 µF
broad spectrum, the scope of this article is limited to medium- and high0~50 kV
0~30 kV
0~630 V
voltage capacitors. Furthermore, we
will omit the discussion on tantalum
No
No
Yes
capacitor replacement (MLCCs typiLow
High
Very high
cally replace tantalums at capacitances
Very low
Average
High
up to 220 µF and voltages below 25 V.).
Small
Large
Medium
Capacitance vs. dc voltage
Changes
Stable
Stable
Capacitance vs. frequency
Stable
Stable
Not stable
High
Moderate
Low
Very good
Fair
Fair
Cost (for high voltage, high cap)
Thermal stability
Table 1. Ceramic versus other capacitors.
Power Electronics Technology
May 2004
14
High-Voltage Capacitor
Options
Table 1 shows a comparison of
ceramic capacitors with other types
available in the marketplace.
www.powerelectronics.com
CERAMIC CAPACITORS
80
Temperature rise [°C]
X7R material
Developed material
40
20
Fig. 1. A bank of board-mounted ceramic capacitors can offer high
capacitance at high voltages, such as this combination of six 45-µF,
250-V ceramic capacitors. Each capacitor measures 32 mm (L) × 40
mm (W) × 4.5 mm (T).
0
10
Today, ceramic capacitors account for more than 85%
of the total volume of capacitors sold worldwide. As
mentioned above, they offer the best cost-to-performance
features. And for chip types, the mounting operation can
be highly automated and efficient. Medium- and highvoltage MLCCs have replaced the film capacitors in most
volume applications.
Low-voltage MLCCs are available in case sizes of 01005
and larger; however, the smallest size for a 250-V rated
MLCC is 0603. Advanced manufacturing techniques and
carefully simulated internal designs allow high capacitances in small packages with no external arcing. In fact,
High Voltage
Ceramic Disk
Capacitors
60
15
20
25
Current [arms]
30
Fig. 2. A new dielectric material reduces the ripple current versus
temperature rise versus that of a standard X7R capacitor. Each
capacitor type was tested in an inverter circuit.
most of the MLCC medium- to high-voltage capacitors
do not need to be coated externally. So much so that
Murata now offers UL-certified safety-rated X1/Y2,
X2 and Y3 MLCC chips. Yet, in some niche and specialized applications, ceramics are not the cheapest option
available compared to aluminum electrolytic or film
capacitors.
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CKE is a quality manufacturer of competitively priced high voltage
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■ Voltages up to 60kV
■ Wide range of
temperature
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■ Capacitance
up to
15,000 pF
CKE
P.O. Box 211 ■ Lucernemines, PA 15754
(724) 479-3533 ■ (724) 479-3537 FAX
e-mail: info@cke.com
www.highvoltagecapacitors.com
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35
CERAMIC CAPACITORS
High Voltage, High Power
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Impedance [mW]
1000
■ Non-inductive,
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■ Leading European manufacturer since 1977
■ Custom designs available as well as networks & dividers
■ Competitive pricing, excellent delivery
100
10
X7R material
Newly developed dielectric
1
10
100
Frequency [kHz]
www.ebgusa.com
1000
Fig. 3. Modifying the dielectric composition of a ceramic capacitor can
negate any piezoelectric effects within the range of operating
frequencies.
Recent ceramic capacitor developments have improved
technical characteristics and performance. As these
improvements are implemented in production models,
ceramic capacitor costs are expected to decrease as
volumes increase. Nevertheless, ceramic capacitors offer
effective hybrid solutions. In other words, it’s possible to
improve performance by just replacing a few aluminum
or film capacitors in an application and still keep costs
manageable.
For some high-end applications (typically where space
is a constraint), ceramics are replacing aluminum and film
capacitors to achieve much higher performance. As is the
case for any new product, these capacitors may not offer
the cheapest option when first introduced, but history has
shown their costs to drop almost exponentially once they
are incorporated into the design.
New application requirements in automotive and other
areas are driving development of high-voltage ceramic
capacitors. Since 1998, for example, California has mandated sales of low-emission vehicles and plans call for these
sales to reach 10% of total within a few years. The trend
of low-emission vehicles has caught on in Europe and Japan as well, starting with electrically driven power steering and air conditioners. These and other applications such
as “systems to stop engine idling” have led to the evaluation of dual-voltage batteries as power sources. The control circuits for such motors currently use organic film or
aluminum electrolytic capacitors (1000 µF to 20,000 µF)
as input capacitors for smoothing purposes.
However, as the density of electronics increases in an
automobile, and the performance requirements become
more severe, ceramic capacitors are being considered (and
used in some niche applications) as replacements in such
circuits. A bank of board-mounted capacitors can offer very
high capacitance at high voltages. Fig. 1 shows one typical
example consisting of six 45-µF, 250-V ceramic capacitors.
Each capacitor measures 32 mm (L) × 40 mm (W) × 4.5
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Power Electronics Technology
May 2004
CERAMIC CAPACITORS
Type
Size
(mm)
Rated
voltage
(V)
Nominal
capacitance
(µF)
Temperature
(ºC)
250
45
125
Allowable
current
(Arms)
Recent Technological
Advances
Self Heating. One requirement of any high-frequency capacitor is its ability to withstand
Aluminum
50⌽ × 100
350
1800
105
7
high ripple currents. In almost
electrolytic
all cases, the current rating is
Table 2. Specifications of ceramic and aluminum-electrolytic capacitors.
constrained by the allowable
temperature rise of the capaci1
100
tor. (A rise of 20°C or less is the
Al electrolytic
Al electrolytic
industry norm). The heat generated in
Ceramic
10
Ceramic
0.1
a capacitor is dependent solely on its
1
ohmic resistance (i.e., its ESR). In case
0.01
of an MLCC, the ESR is a function of
0.1
the electrode resistivity and the dielec0.001
0.01
tric loss.
0.0001
0.001
Fig. 2 shows the ripple current ver1000
10,000
100,000 1,000,000
1000
10,000
100,000 1,000,000
sus
temperature rise for a capacitor
Frequency [Hz]
Frequency [Hz]
(a)
(b)
tested in an inverter circuit. Ripple
Fig. 4. Shown here are measurements of impedance (a) and ESR (b) versus frequency for a
current is about 17 Arms for a standard
47-µF, 250-V ceramic capacitor and a 1800-µF, 350-V aluminum-electrolytic capacitor.
X7R capacitor for a 20°C rise (at
20 kHz). However, as the figure shows,
m (T), has a metallic termination and is surface mountthis temperature rise can be reduced significantly by tweakable. The volume of each capacitor in this bank is 100 times
ing the dielectric material. In this case, the newly develthat of the mass-produced 2220 case size. In addition, the
oped paraelectric phase dielectric material has a much lower
ripple-current rating for this bank is 25 Arms.
dielectric loss, suppressing the temperature rise to just
30 × 40 × 5
25
ESR
Impedance
MLCC
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CERAMIC CAPACITORS
below 20°C even for ripple currents
as high as 25 Arms.
Piezoelectric Effect. One minor
drawback for X7R dielectrics has been
their susceptibility to piezoelectricinduced stresses. Although this effect
is marginal and may be neglected
for case sizes smaller than the 2220,
for larger capacitors it can lead to
catastrophic failures caused by cracking. Modifying the dielectric compo-
sition to avoid any piezoelectric
effects within the range of operating
frequencies may skirt this problem
(Fig. 3).
Whereas the standard X7R material shows piezoelectric noise, this is
almost absent from the new ceramic.
These measurements were made
under a 300-V bias at 90°C. Note this
effect is not present in film or aluminum-electrolytic capacitors. With this
Battery
Smoothing capacitor
Fig. 5. This circuit was used to test the noise
absorption characteristics of a high-voltage
aluminum-electrolytic (a) versus a ceramic
capacitor (b).
new development, ceramics now
offer a viable alternative to film and
electrolytic capacitors in the large case
sizes.
MULTILAYER POLYMER (MLP) CAPACITORS
APPLICATIONS
•
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42 Volt Automotive
SMPS Off-Line
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RFI/EMI Suppression
Max RMS Current vs. Capacitance
MLP Capacitors @ 500KHz
Max I rms
• Ultra Low ESR and ESL
• High Ripple Current
Handling
• Stable Under AC & DC
Voltage
• Telecom Grade - No Aging
• Low Profile For Surface
Mount
• Robust Mechanical and
Electrical Design
• 50 to 500 Volts
20.0
18.0
16.0
14.0
12.0
10.0
8.0
6.0
4.0
2.0
0.0
0
5
10
15
20
Cap Value (microfarads)
Max RMS Current vs. Capacitance Value
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Power Electronics Technology
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20
High-Frequency
Performance
This section compares some high
frequency characteristics of a ceramic
capacitor versus an electrolytic type.
Table 2 lists the sample parameters.
Although the rated voltages of
MLCCs are lower than that of aluminum electrolytic, they are guaranteed
to the same specifications because of
the much higher breakdown voltage
margins for MLCCs.
Fig. 4 illustrates the impedance and
ESR characteristics of the two capacitors specified in Table 2. It is easily
noticeable that the MLCC offers a
much better solution, especially at
frequencies exceeding 100 kHz. The
ceramics achieve this at capacitances
almost two orders of magnitude lower
than the electrolytics.
Under identical measurement conditions, the rise in surface temperature for MLCC versus aluminum
electrolytic was measured at 20 kHz
and 15 A rms ripple current. The
temperature rise was 0.8°C for the
MLCC versus 3.1°C for the Al electrolytic. When considering the fact
that the temperature rise inside the
Al electrolytic is higher than at the
surface, the advantages of the MLCC
become obvious.
Noise absorption characteristics
were evaluated based on the circuit
shown in Fig. 5. Other conditions
include a primary side voltage of
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CERAMIC CAPACITORS
Al electrolytic
10
8
8
6
6
4
4
Volts
Volts
10
2
2
0
0
-2
-2
-4
-0.1
-0.05
0
0.05
Time [msec]
0.1
MLCC in parallel
-4
-0.1
-0.05
0
0.05
Time [msec]
0.1
Fig. 6. Shown here are voltage measurements across the capacitor in Fig. 5 for the high-voltage
aluminum electrolytic (a) and a ceramic capacitor (b). Despite having much lower capacitance,
the ceramic device outperforms the aluminum electrolytic.
300
Al electrolytic
Ceramic
250
Volts
200
150
100
50
0
0.0499
0.0501
0.0503
Time [msec]
0.0505
Fig. 7. These voltage measurements were taken
across the emitter and collector in Fig. 5.
150 Vdc and a switching frequency of
20 kHz.
Figs. 6 and 7 show the voltage
waveforms across each capacitor and
emitter/collector. In spite of having
1/20th the capacitance under bias of the
electrolytic, the superior surge absorption characteristics of the MLCC are
self-evident. However, because of the
lower effective capacitance of the
MLCC, the ripple voltage is higher
and its effect on the battery must be
evaluated. Due to their extremely low
ESR at higher frequencies, MLCCs
have very high ripple-current capabilities. This, along with their higher
rated temperatures, makes them an
attractive choice. Their volumetric
capacitance is higher compared to
film capacitors, but smaller than
electrolytics. However, the electrolytic
capacitance is rated at room temperature and 1 kHz. Because at higher temperatures and/or frequencies the capacitance drops significantly due to
high ESR for aluminum electrolytics,
their effective capacitance is the same
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as MLCCs in actual operation.
Overall, MLCCs offer a better solution in terms of size and configuration flexibility compared to aluminum
electrolytics and film capacitors (especially for primary snubbers in high
ripple-current situations).
Although it was not possible to list
all the types of medium- to high-voltage ceramic capacitors along with
their applications in the scope of this
article, the article illustrates the basic
advantages of MLCC as well as recent
advances in technology. As with all
ceramic capacitors, high-voltage
MLCCs boast very low ESR and ESL,
have flexibility of size and configuration, and high thermal stability and
volumetric capacitance. Their performance in the high-frequency region
is excellent.
Of course, these capacitors cannot
compete with aluminum capacitors
(in terms of cost) in low-frequency
regions where the most important
characteristic is the bulk capacitance.
In addition, the drop in capacitance
with dc bias for X7R MLCC dielectrics is also a drawback compared to
film or aluminum capacitors, but is an
insignificant parameter at higher frequencies (although C0G and U2J dielectrics do not display any drop in
capacitance). Medium- to high-voltage MLCCs are already widely used in
the electronic industry, and their usage in field of power electronics is expected to grow significantly in the
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