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Application Note
© 2021 ROHM Co., Ltd.
No. 64AN089E Rev.001
2021.10
Switching Regulator Series
Types of Capacitors Used for Output Smoothing of Switching Regulators and
their Precautions
In recent years, it has become a common practice to recommend multilayer ceramic capacitors for the output smoothing of
switching regulators due to the stability of their temperature characteristics and the reduction on the mounting area. On the other
hand, they are increasingly being replaced by low-cost, high-capacity aluminum electrolytic capacitors and conductive polymer
hybrid aluminum electrolytic capacitors. This application note illustrates the use of various capacitors for output smoothing with
simulation results of the output voltage ripple and the gain-phase characteristic of the Open loop transfer function frequency
response.
Impedance Characteristics of Capacitors
It should be noted that an ideal capacitor has only a capacitance component, but a real capacitor has both a resistance and an
inductance component, and the impedance characteristics determined by these components varies greatly depending on the type
and temperature of the capacitor.
C: Capacitance
ESR: Equivalent Series Resistance
ESL: Equivalent Series Inductance
The impedance|Z| of the equivalent circuit in Figure 2
can be expressed as follows, with its frequency response as
a graph shown in Figure 3.
In the lower frequency range, the impedance due to capacitance is dominant,
and in the higher frequency range, the impedance due to inductance is
dominant. The resonant frequency, Fr, is determined by the following
equation: at frequencies below Fr, the impedance decreases due to
capacitive nature, but at higher frequency bands, it becomes inductive and
the impedance increases with increasing frequency.
: Self-resonant frequency
For more information on the impedance characteristics of capacitors, please refer to the application note "Impedance
Characteristics of Bypass Capacitors".
Figure 3. Impedance characteristics of capacitors
Figure 1. Ideal capacitor
Figure 2. Equivalent circuit of a capacitor
considering parasitic components
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Application Note
Types of capacitors used for output smoothing of switching regulators and their precautions
© 2021 ROHM Co., Ltd.
No. 64AN089E Rev.001
2021.10
0.001
0.01
0.1
1
10
100
100 1K 10K 100K 1M 10M 100M
Impedance : [Ω]
Frequency [Hz]
+105 °C
+25 °C
−40 °C
Multi Layer Ceramic Capacitor
22��F
Figure 4. Examples of impedance characteristics of
ceramic capacitors
Impedance Characteristics of Ceramic Capacitors
Figure 4 shows an example of the impedance characteristics of a ceramic
capacitor. The temperature characteristic of the product used in this
example is X7R, so the change in capacitance is ±15% from -55°C to 125°C.
This is relatively small compared to aluminum electrolytic capacitors and
conductive polymer hybrid capacitors, which are discussed below.
The ESR also changes very little at the said temperature variation, which is
also, very small compared to other products. The ESL is small,
around 200 pH, and there is almost no temperature dependence.
As such, it is very stable in terms of impedance characteristics,
but it should be noted that the capacitance change due to DC voltage bias is
significant, and that it is prone to squeal and also, to cracking due to
temperature cycling and shock.
Impedance Characteristics of Aluminum Electrolytic Capacitors
Aluminum electrolytic capacitors offer a higher capacitance at a lower cost
than ceramic capacitors. However, it is important to note the following
characteristics. Figure 5 shows an example of the impedance characteristics
of an electrolytic capacitor. At temperatures as low as -40°C, the graph shows
a significant increase in impedance. This characteristic is seen as
an impedance because aluminum electrolytic capacitors are characterized
by a decrease in capacitance and an increase in ESR at low temperatures.
If the impedance at 100kHz, near the self-resonant frequency, is taken
as the ESR, a change of 1.2Ω @ -40°C is produced, compared
to 30mΩ @ 105°C. As will be discussed later, this change will have a
significant effect on the output voltage ripple and phase characteristics.
The ESL has very little change with temperature, but at 5nH, the value
is larger than the ceramic capacitor, which also influences the
output voltage ripple.
Impedance Characteristics of Conductive Polymer Hybrid Capacitors
Conductive polymer hybrid capacitors are made by fusing conductive polymer
and liquid electrolyte in the electrolyte, resulting in high capacitance and low
ESR. It is also generally cheaper than connecting ceramic capacitors in
parallel to obtain a capacitance value of several hundred µF. Figure 6 shows
an example of the impedance characteristics of a conductive polymer hybrid
capacitor. The capacitance changes by about 30% at the same temperature
change, and the ESR changes by about 12mΩ to 18mΩ. The ESL varies little
with temperature, around 600 pH. The ESR values are close to those of
ceramic capacitors, providing a more stable performance than electrolytic
capacitors when large capacitance values are required
0.001
0.01
0.1
1
10
100 1K 10K 100K 1M 10M 100M
Impedance : [Ω]
Frequency [Hz]
330μF
Aluminum electrolytic capacitor
+105 °C
+25 °C
−40 °C
0.001
0.01
0.1
1
10
100 1K 10K 100K 1M 10M 100M
Impedance : [Ω]
Frequency [Hz]
270μF
Conductive polymer hybrid
aluminum electrolytic capacitor
+105 °C
+25 °C
−40 °C
Figure 5. Examples of impedance characteristics
of electrolytic capacitors
Figure 6. Conductive Polymer Hybrid Capacitors
Impedance characteristics