6-5-3. Effect of capacitor parasitic elements
(1) How does impedance change?
The previous section introduced the fact that a capacitor's
impedance frequency characteristics form a V-shape, and that low
frequencies (left side) and high frequencies (right side) correspond
to electrostatic capacitance and ESL, respectively. You can easily
control the electrostatic capacitance of a capacitor by specifying a
part number. How much of an effect does ESL have?
Fig. 12 shows an example of the impedance measured from several
types of ceramic capacitors with a nominal electrostatic capacitance
of 1,000pF. The figure shows that...
- (a) MLCC (laminate structure)
(rather than a single board)
- (b) Capacitors with shorter leads
- (c) SMD capacitors (rather than
capacitors with leads)
are all closer to the ideal capacitor and have less impedance up
until the higher frequencies. This also demonstrates that ESL
decreases in this order. This tendency is generally seen in all
capacitors—not only in ceramic capacitors. This is because the
main factors behind ESL are the internal electrode and lead
shapes.
When using a capacitor to eliminate emitted noise, it will be used
at a frequency of 30MHz or higher. As shown in the figure, there can
be a tenfold difference or more at this frequency due to differences
in ESL, even if the same 1,000pF capacitor is used.
(2) What is the value of ESL?
So what will be the value of ESL now?
Figure 13 shows the results of calculating the impedance after
changing the ESL, on a 1,000pF capacitor using an equivalent circuit
model. Comparing Fig. 12 and Fig. 13, we can estimate that ESL will
be
- Around 10nH for an MLCC with a 10mm lead ( (2) in Fig. 12)
- 1nH or less for an SMD MLCC with no lead ( (4) in Fig. 12)
- 0.1nH or less for a three-terminal capacitor ( (5) in Fig. 12)
The nH values mentioned here are extremely small values that occur
even on a lead that is only several mm long. Looking at the
frequencies above 100MHz on the graph shows that even this small
amount of inductance has a significant effect.
Note that the three-terminal capacitor shown in (5) in Fig. 12 is a
high performance capacitor with a special structure meant to reduce
ESL. Three-terminal capacitors will be further described in Chapter
8.
(3) Use with as short a capacitor lead as possible
It is better to use a capacitor with little ESL to suppress noise.
As shown in (2) , (3) , and (4) in Fig. 12, the lead should be as
short as possible when using a capacitor (and should be SMD if
possible) .
Actually, during the experiment shown in Fig. 2 in Section 6.4, the
noise reduction effect was changed using the differences in the ESL
of the capacitors themselves, as well as the ESL differences caused
by whether or not a lead was present. If a capacitor is installed
over an approximately 10mm long lead (Section 6.4, Fig. 2 (d) ) ,
the noise reduction effect will be at least 10dB less compared to a
scenario with no lead (Section 6.4, Fig. 2 (c) ) .
(4) Impedance characteristics of electrolytic capacitors
The explanation of capacitor characteristics up until now has mostly
used MLCCs as an example. For applications requiring a large
electrostatic capacitance (such as power leveling), electrolytic
capacitors with a large electrostatic capacitance per volume are
used. The impedance characteristics of electrolytic capacitors
differ slightly from those of MLCCs. Fig. 14 shows some comparison
examples.
Aluminum electrolytic capacitors are sometimes used for power
leveling. Fig. 14 shows that the impedance curve of an aluminum
electrolytic capacitor forms a bowl shape (or U shape) . It also
shows that self-resonance cannot be clearly seen. This means that
the capacitor loss is comparatively large; there will be a
significant ESR in the equivalent circuit in Fig. 7.
(5) What effect does ESR have?
Fig. 15 shows the results of calculating the impedance when ESR is
changed, with a 1μF capacitor as an example. With ESR at 500
megaohms, it is possible to obtain characteristics similar to the
results of measuring the aluminum electrolytic capacitor in Fig. 14
(a) . The impedance characteristics of an electrolytic capacitor can
thus be reproduced by increasing ESR. The impedance corresponding to
the bottom of the bowl-shaped characteristic curve represents the
ESR value.
The ESR of an aluminum electrolytic capacitor can reach up to 1 ohm
or more. The impedance of the capacitor will never be less than that
the ESR; this means that a capacitor with a large ESR is unsuitable
for use in noise suppression.
On the other hand, there are cases where a capacitor used to
suppress noise will create resonance with the surrounding circuit,
causing a malfunction. In this case, ESR can function as a resonance
damping resistor to prevent this malfunction from happening. In this
case, then, a capacitor with a somewhat large ESR would be of
benefit.
(6) Electrolytic capacitors with low ESR
Some electrolytic capacitors are designed so that ESR is minimized.
Examples include tantalum capacitors and conductive polymer
capacitors. The measurement results in Fig. 14 also include examples
using these capacitors. The figure shows that the impedance around
the resonant frequency is smaller than that of an aluminum
electrolytic capacitor.
However, this does not extend to MLCCs, even with these capacitors.
A large capacity MLCC should be selected, or an MLCC should be added
in parallel with an electrolytic capacitor and used, for cases where
noise reduction is important even in applications that require a
large electrostatic capacitance (such as power leveling) .