SMPS Capacitors
Application Information on SupraCap
®
SUPRACAP
®
- LARGE CAPACITANCE VALUE MLCs
High speed switch mode power supplies require extremely
low equivalent series resistance (ESR) and equivalent series
inductance (ESL) capacitors for input and output filtering.
These requirements are beyond the practical limits of
electrolytic capacitors, both aluminum and tantalums, but
are readily met by multilayer ceramic (MLCs) capacitors
(Figure 1).
Theoretical SMPS’s output filter capacitor values are in the
range of 6-10 µF/amp at 40KHz and drop to less than
1 µF/amp at 1MHz. Most electrolytic applications use 10 to
100 times the theoretical value in order to obtain lower ESR
from paralleling many capacitors. This is not necessary with
SupraCap
®
MLC capacitors which inherently have ESRs
in the range of milliohms. These extremely low values of
ESR mean low ripple voltage and less self-heating of
the capacitor.
ESR -vs- Frequency
24 uFd Filter Capacitors
Aluminum
Electrolytic
100,000
Low "ESR"
Tantalum
Ceramic
MLC
Output noise spikes are reduced by lowering the filter capac-
itance self-inductance. The ripple current is a triangle wave
form with constant di/dt except when it changes polarity,
then the di/dt is very high. The noise voltage generated by
the filter capacitor is
V
Noise
= L
Capacitor
di/dt
AVX SupraCap
®
devices have inductance value less than 3nH.
Figure 2 compares a 5.6 µF MLC to a 5.6 µF tantalum which
was specially designed for low ESR and ESL. When subjected
to a di/dt of 200 mA/ns the tantalum shows an ESR of 165
mΩ and an ESL of 18nH versus the MLC’s 4 mΩ and 0.3 nH.
These performance differences allow considerable reduction
in size and weight of the filter capacitor.
Additionally, MLCs are compatible with surface mount
technology reflow and assembly techniques which is the
desirable assembly for conversion frequencies exceeding
1 MHz. Electrolytic capacitors (both aluminum and tantalum)
are not compatible with normal vapor phase (VPS) or infrared
(IR) reflow temperatures (205-215°C) due to electrolyte and
structural problems. AVX SupraCap
®
devices are supplied
with lead frames for either thru-hole or surface mount
assembly. The lead frames act as stress relief for differences
in coefficients of expansion between the large ceramic chip
( 10 ppm/°C) and the PC boards.
DSW 16
50mV
50nS
TPOS-7
10,000
ESR (Ohms)
1,000
0.100
0.010
Ta
0.001
0.1
1.0
10.0
100.0
1000.0
10000.0
Frequency (KHz)
Figure 1
CSW 1
MLC
50mV
V=2.0mV
50nS
VZR-0.2
T=25.5nS
Figure 2
7
AVX [ AVX CORPORATION ]
