Application Note

AC Ripple Current Calculations
Vishay Sprague
Application Notes AC Ripple Current
Calculations Solid Tantalum Capacitors
INTRODUCTION
Solid tantalum capacitors are preferred for filtering
applications in small power supplies and DC/DC converters
in a broad range of military, industrial and commercial
systems
including
computers,
telecommunications,
instruments and controls and automotive equipment. Solid
tantalum capacitors are preferred for their high reliability,
long life, extended shelf life, exceptional stability with
temperature and their small size. Their voltage range is 4 to
50 volts for the most common types. Tantalum chip
capacitors for surface mount applications are manufactured
in very small sizes and are compatible with standard
pick-and-place equipment.
The electronics industry has moved to smaller and smaller
power supplies and higher switching frequencies, with an
increased requirement for capacitors with smaller size and
operating characteristics better suited to high frequencies.
This application note briefly describes the construction of
solid tantalum capacitors, the concept of Equivalent Series
Resistance (ESR) and presents calculations for power
dissipation and voltage limitations for both low and high
frequency applications.
Looking closely at the internal structure of the pellet, we see
that it is made of grains of tantalum powder sintered to each
other. A solid tantalum capacitor is equivalent to many small
capacitors in parallel, one for each grain of powder. This
configuration produces a very large surface area, therefore a
large capacitance in a relatively small volume.
TANTALUM PELLET
Simplified View
Tantalum Anode Lead
Tantalum
Ta2O5
MnO2
Carbon
Metallized Outer
Electrode
CONSTRUCTION
The solid tantalum capacitor consists of a sintered tantalum
pellet, the anode, on which a tantalum oxide dielectric is
formed by electrolysis. The pellet is then coated with
manganese dioxide for the cathode. Positive and negative
terminations are attached to this pellet and the assembly
may be conformally-coated or molded.
CONFORMAL COATED SERIES
T
, Solid Electrolyte
Tantalum Chip Capacitors.
ANTAMOUNT®
MOLDED SERIES
Anode Polarity Band
Top
Bottom
Marking
Surface
Side
Cathode Termination
(Silver + Ni/Sn/Plating)
End
Encapsulation
Anode Termination
(Silver + Ni/Sn/Plating)
Silver Adhesive
MnO2/Carbon/Silver
Coating
Sintered Tantalum
Pellet
Epoxy Encapsulation
Anode Polarity Bar
Red Epoxy Tower
Solderable Cathode
Termination
MnO2/Carbon/Silver
Coating
Lead Frame
Solderable
Sintered Tantalum
Anode Termination
Pellet
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For technical questions, contact: tantalum@vishay.com
Document Number: 40031
Revision: 03-Apr-06
AC Ripple Current Calculations
Application Notes AC Ripple Current
Calculations Solid Tantalum Capacitors
Vishay Sprague
EQUIVALENT SERIES RESISTANCE (ESR)
CURRENT LIMITATION (LOW FREQUENCY)
A capacitor offers intemal resistance to AC current, called
the Equivalent Series Resistance (ESR). At lower
frequencies, this is mainly the resistance of the dielectric. At
higher frequencies, the resistance of the manganese dioxide
in the voids between the grains is predominant. Because the
resistivity of manganese dioxide is inversely proportional to
temperature, the ESR of solid tantalum capacitors at high
frequencies decreases as temperature increases.
To find the limiting current Irms, we divide Vrms by the
impedance at the desired frequency.
(3) Irms = Vrms ⁄ Z
using the formula:
(4) Z =
2
X + ESR
2
where X is 1/Cw + Lw (w = 2pf)
POWER DISSIPATION LIMITATION
When AC current is applied to a solid tantalum capacitor, the
resistance (ESR) that opposes the flow of current results in
heat generation, according to the formula:
2
(1) P = I × ESR
Since inductance of a solid tantalum capacitor is usually in
the nanohenry range, the Lw factor becomes important only
when the frequency is higher than a few megahertz. For
filtering applications at 100 kHz and lower, the inductance
factor will generally be ignored in the calculation. At 120 Hz,
the impedance can be determined by calculation.
(5) Z =
The power (P) dissipated in the capacitor results in an
elevation of temperature. The allowable temperature rise of
a capacitor due to power dissipation is determined by
experience. For example, this value is + 20 °C maximum for
molded chip capacitors. This in turn limits the power that the
capacitor can dissipate.
2
( 1 ⁄ 2πfC ) + ( DF ⁄ 2πfC )
2
2
= ( 1 ⁄ 2πfC ) ( 1 + DF )
At 120 Hz, DF2 is relatively small compared with 1 and the
formula can be simplified to:
(6) Z = 1 ⁄ 2πfC
VOLTAGE LIMITATION
The power a capacitor can dissipate is also limited by the
applied DC voltage. The operating voltage should not be
allowed to rise above the rated voltage (nor should it drop
below zero, since the solid tantalum capacitor is a polarized
component). Assuming the capacitor is biased at half the
rated voltage, which is the optimum use condition, the
limiting value of the voltage is, for a sinusoidal waveform:
(2) Vrms = Vpp ⁄ 2 2 = Rv ⁄ 2 2
Vrms for each value of Rv (Rated voltage) are:
RATED VOLTAGE
Vrms MAXIMUM
4
10
20
25
35
40
50
1.42
5.30
7.07
8.84
12.37
14.14
17.68
Document Number: 40031
Revision: 03-Apr-06
More generally, DF values of less than 10 % will not affect
the final result by more than 1 %. It is important to use the
lowest value for C, including the capacitance tolerance. At
120 Hz, the formula can be simplified to:
(7) Irms = 0.266 × CV
where Irms is the maximum permissible rms current in
milliamperes, C the capacitance minus the capacitance
tolerance in microfarads and V the rated voltage in volts. All
above calculations assume the capacitor is properly biased
at half the rated voltage. If this is not the case, Vrms
becomes
(8) Vp ⁄ 2
where Vp = V rated - V bias or V bias, whichever is lower.
For technical questions, contact: tantalum@vishay.com
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23
AC Ripple Current Calculations
Application Notes AC Ripple Current
Calculations Solid Tantalum Capacitors
Vishay Sprague
CURRENT LIMITATION (HIGH FREQUENCY)
At frequencies in the 10 kHz to several hundred kilohertz
range, the power dissipation becomes the limiting factor. The
following formula gives the maximum permissible ripple
current for a sinusoidal wave form:
MULTIPLYING FACTOR
+ 85 °C
+ 125 °C
0.9
0.4
RIPPLE CURRENT/VOLTAGE
CALCULATIONS EXAMPLE
Pmax ⁄ ESR
(9) Irms =
TEMPERATURE
Pmax is the maximum power dissipation the capacitor can
tolerate. The ESR value in the formula is the maximum ESR
of the capacitor at the required frequency. This can be
determined by measuring capacitors and determining a
maximum value by using the mean value and adding 3 or
more standard deviations. Some manufacturers specify the
maximum impedance at 100 kHz or 1 MHz. Either value may
be used in ripple current calculations.
As an example, we will determine the ripple voltage and
power dissipation capability for a 1 µF, ± 20 % tolerance,
35 volt, 293D capacitor.
At 120 Hz:
Vrms = Rv ⁄ 2 2 = 12.37 volts
Irms = Vrms/Z
Power dissipation limits calculated for the most popular
surface mount types of solid tantalum capacitors are:
= 12.37 x 2 x 3.14 x 120 x 0.8 x 10-6
= 0.007 Amp.
Molded Case Chip (293D):
If we used
CASE SIZE
MAXIMUM POWER AT + 25 °C
(WATTS)
A
B
C
D
E
0.075
0.085
0.110
0.150
0.165
Irms =
Pmax ⁄ ESR
With ESR = DF/2pfC
= (04/2 x 3.14 x 120 x 0.8 x 10-6)
= 66 ohms
ESR SCREENING
For parallel operation, the ESR spread can be minimized by
screening. This reduces the risk of excess ripple current
exposure to any one of the capacitors.
Some equipment will only measure impedance. An
impedance limit can be caluclated to insure that the ESR
stays in the required range. Use the formula:
(10) Zmax =
2
Xc + ESR
Irms = Pmax ⁄ ESR =
=
0.080/66
0.035 Amp
At 120 Hz, the voltage is the limiting factor.
At 100 kHz:
2
Irms =
Pmax ⁄ ESR
Xc = 1 ⁄ Cω
Impedance can be measured using an impedance meter and
a fixture that is appropriate for the task. With the most
sophisticated fixtures, several capacitors may be tested at
the same time, reducing the test cycle time.
ESR =
1.5 ohms ( Z = 3 ohms )
Irms = 0.085/1.5 = 0.238 Amp.
CORRECTIVE FACTORS
The calculations for high frequency ripple current are shown
in formula (9) for a sinusoidal waveform and an ambient
temperature of + 25 °C. If the waveform is not sinusoidal, the
ripple current limitations may differ.
Generally speaking, the ripple current limit calculated by
formula (9) can be divided by the duty cycle of the signal. If
the temperature is higher than + 25 °C, the ripple current limit
should also be multiplied by the factors shown:
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At 100 kHz, the typical ESR for a 1 µF/35 volts tantalum is:
If we now look at the maximum ripple voltage, the above
limitation translates into:
Vrms = Z x Irms = 3 x 0.231 = 0.71 volts.
At 100 kHz, the power dissipation is the limiting factor.
For technical questions, contact: tantalum@vishay.com
Document Number: 40031
Revision: 03-Apr-06
AC Ripple Current Calculations
Application Notes AC Ripple Current
Calculations Solid Tantalum Capacitors
Vishay Sprague
TYPICAL CURVES OF IMPEDANCE AND ESR VS FREQUENCY
100
IMPEDANCE
ESR
10
1 µF, 35 V, A CASE
1
4.7 µF, 35 V, C CASE
0.1 100
1K
10K
100K
FREQUENCY
1M
10M
CONCLUSIONS
The industry is moving towards smaller and smaller power
supplies and DC/DC converters operating at higher
frequencies. The three factors shown become more and
more important in capacitor selection.
1. Higher Switching Frequencies: The switching frequency
of power supplies has increased from the 10 kHz range a
decade ago to the 100 kHz range and up today. The ESR of
solid tantalum capacitors is either the same or lower at higher
frequencies and impedance is at a minimum in the 100 kHz
to megahertz range. Higher switching frequencies and the
need for smaller sizes will increase the use of solid tantalum
capacitors.
Document Number: 40031
Revision: 03-Apr-06
2. Surface Mount Technology: The application of surface
mount technology not only reduces the size of power
supplies and converters but also uses the substrate on which
the components are mounted to dissipate some of the heat
generated by the switching elements. Solid tantalum chip
capacitors are well suited for this application. They have
superior operating characteristics, do not leak electrolyte and
are compatible with common automated surface assembly
equipment.
3. Tighter High Frequency Parameters: The reduction of
the maximum ESR of a solid tantalum capacitor may
produce tradeoffs in size or DC characteristics. Rather than
looking at lower ESR in terms of process average, it may be
advisable to try to reduce ESR variation, producing a lower
maximum ESR with a tighter distribution. This improvement
may be achieved by using statistical process control, an
approach already being implemented at Vishay Sprague
Solid Tantalum manufacturing facilities.
For technical questions, contact: tantalum@vishay.com
www.vishay.com
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