Published Aug 8 2016
Using high capacity MLCCs to provide DC power smoothing
Yoshimasa Goto, Product Engineer at Murata, explains how
technical innovation has produced high-capacitance MLCCs,
suitable for smoothing applications.
The vast majority of applications today, use an AC power
adapter or AC/DC power supply to provide a DC voltage to
power all aspects of the design. For some designs, a single DC
supply rail is used to create additional voltage rails as required
using specific power ICs. Capacitors, typically having a high
value above 100 µF and termed smoothing capacitors, are
used to even out fluctuations in the DC voltage, hence
‘smooth’ that result from changing load conditions, load
regulation transients and rectifier dips. Another aspect of
modern day designs, using leading-edge semiconductors is
that these devices are using lower voltage supplies, down to
0.6 VDC in some cases. As a consequence lower impedance
smoothing capacitors are needed to ensure the stability of the
circuitry.
Figure 1. Classification of capacitors according to
structure and composition
Figure 1 illustrates a table of different capacitor types available,
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classified according to their basic structure and the materials
used in the composition. The different advantages and
disadvantages of each one is highlighted. There can be seen to
be a number of advantages to using a multilayer ceramic
capacitor, MLCC. These include their compact and small
dimensions, their high reliability compared to other types of
capacitors, and their low impedance or equivalent series
resistance (ESR). In addition, they are very price competitive.
However, some of the downsides to the MLCC is that they
have a thermal dependence of capacitance and an effective
capacitance value that decreases with voltage application. This
last affect is more commonly termed its DC bias characteristic.
As a consequence most small, high-capacitance capacitors are
multilayer ceramic capacitors, while most of the smoothing
capacitors in use today, which require over a 100µF
capacitance value and low impedance characteristics, are
conductive-polymer electrolytic capacitors.
However, the disadvantages of the MLCC for use as a
smoothing capacitor are being reduced thanks due to ongoing
technical innovation aimed at increasing the capacitance
values even higher. For example, Murata Manufacturing has
already established technologies that allow stable, mass
production of 1,000 or more, 1?m or less high-accuracy
dielectric layers and reduction of their overall thickness. The
result is the ability to reliably mass-produce MLCCs with
capacitance values of 100 µF. An example of a 330 µF
measuring just 3.2 x 2.5 mm is illustrated in Figure 2.
Figure 2. A cross sectional diagram of a 3.2 × 2.5 mm,
330?F MLCC
With the industry trend towards using lower voltage
microcontroller and digital logic devices, the impact that the DC
bias characteristic of an MLCC has on the reduction of the
effective capacitance is diminished. As a result, higher value
MLCCs are now increasingly being considered as a viable
capacitor technology to use for smoothing DC supply voltages.
An interesting perspective towards using MLCCs to replace the
conductive-polymer electrolytic capacitors that have been
traditionally used in smoothing applications is that the MLCC
replacements can have a lower capacitance value. The reason
for this is that multi layer ceramic capacitors have lower
impedance and equivalent series resistance characteristics
compared to the conductive-polymer devices. This is
highlighted in Figure 2.
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Figure 3. Impedance and ESR-frequency characteristics of
conductive-polymer tantalum electrolytic capacitors and
multilayer ceramic capacitors.
The ?gure indicates that in the frequency range above 100
kHz, which is a switching frequency for power ICs used in
digital devices, multilayer ceramic capacitors have a lower
impedance and ESR values than conductive-polymer tantalum
electrolytic capacitors, even if the former have a lower
capacitance than the latter. Also, multilayer ceramic capacitors
are more effective in suppressing high-frequency noise
because at frequencies higher than the resonance frequency,
they have much lower impedance than conductive-polymer
tantalum electrolytic capacitors.
Figure 4. Test results and diagram of the evaluation circuit
In order to validate the capacitor differences an investigation
was carried out on capacitor replacement using an evaluation
board for double data rate (DDR) power ICs for PCs. Figure 4
shows the evaluation circuit and the examination results. In this
evaluation board, DC 1.4 V voltage was used, and two
conductive-polymer tantalum electrolytic capacitors (7.3 × 4.3
mm, 2.0V, 330 µF, M tolerance) were initially used as
smoothing capacitors. Then, these capacitors were replaced
with 150 µF and 220 µF multi-layer ceramic capacitors (3.2 ×
1.6 mm, 6.3 V, M tolerance) to examine voltage ?uctuation
associated with changes in ripple voltage, spike volt-age, and
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load. Before this examination, phase adjustments were made
to ensure the stability of the evaluation board.
The results showed that multi-layer ceramic capacitors tend to
have a lower ripple voltage even though they have a lower
nominal capacitance than conductive-polymer tantalum
electrolytic capacitors. This is probably because at the
switching frequency, multi-layer ceramic capacitors have low
impedance and ESR, and so cause the voltage ?uctuation to
decrease. The results also showed that multi-layer ceramic
capacitors tend to have a lower spike voltage in a similar
manner. This is probably because they have low ESL, and as a
result, suppress high-frequency noise.
However, in a load change test where current was changed
signi?cantly, there was a large voltage ?uctuation when 150 µF
multi-layer ceramic capacitors were used. This is probably
because the load change test has a correlation with the
effective capacitance of capacitors obtained when voltage is
applied. The multi-layer ceramic capacitors used in this test
have a lower nominal capacitance than the conductive-polymer
tantalum electrolytic capacitors, and their effective capacitance
is decreased by DC bias characteristics; this is the reason for
the large voltage ?uctuation in this test. However, the voltage
?uctuation was reduced by using 220 µF high-capacitance
capacitors.
As the use of low-voltage semiconductor devices has been
rapidly increasing, conductive-polymer electrolytic capacitors
featuring high capacitance and low ESR have been widely
used as smoothing capacitors for power ICs that supply DC
power to the semiconductor devices. Size reduction and
long-term reliability, however, are considered more important
for other devices that use these semiconductor devices, such
as server computers, and they are also important for smoothing
capacitors. There is, therefore, a demand for expansion of >
100 µF multi-layer ceramic capacitors that can be more easily
miniaturized, are more reliable, and feature low impedance,
ESR, and ESL.
Murata
www.murata.com/en-eu
By Yoshimasa Goto
Yoshimasa Goto is a Product Manager at Murata Electronics
Singapore. He has been working at Murata for 17 years,
starting as a product engineer for noise suppression products.
He later moved on to MLCCs. Currently, as product manager,
Goto is responsible for the Capacitor, Inductor and Noise
suppression products in the India and ASEAN markets.
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