TECHNICAL
VIEW
Extending the life of aluminium capacitors: how
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By Marcin Chelminski
Central Applications Engineer, Future Electronics (Poland)
The aluminium capacitor is a common feature of power
supplies, since it is the most suitable type of capacitor for
functions such as energy storage and low- and highfrequency filtering. These functions call for the high
capacitance values and high power ratings that a reasonably
priced aluminium capacitor offers.
Unfortunately, aluminium capacitors are also often the components most
prone to failure: the operating lifetime of an aluminium capacitor will tend
to determine the operating lifetime of the entire power supply. This
means that the designer has to take great care when calculating the
parameters of aluminium capacitors, in order to choose the most
appropriate part for their device. The choice of capacitor also has to
balance cost against performance: after magnetic components,
aluminium capacitors are often the most expensive passive parts in a
power supply. The power engineer’s aim, then, is to select a part that
gives an adequate projected lifetime, without over-specifying the device
and incurring unnecessary expense.
The reliability of an aluminium electrolytic capacitor is generally
measured by its expected life in use. Minor factors which affect the life of
aluminium electrolytic capacitors include humidity, vibration and heat
transmitted through the printed circuit board patterns. But three other
factors have a greater effect on useful life: ambient temperature, ripple
current and applied voltage.
The basic application guidelines for aluminium electrolytic capacitors
say that operating temperature, applied ripple current and applied
voltage should always stay below the specified maximum allowable values.
But these basic guidelines do not provide enough information to
enable the power-supply designer to optimise for long operating life. For
this, the designer needs to estimate the effect on lifetime of variations in
operating conditions within the maximum allowable limits.
By showing how to estimate the effect of changes in operating
conditions, this article provides designers with a guide to extending the
lifetime of aluminium capacitors in any given application. And as it will
show, the latest generation of dedicated power-supply capacitors, which
benefit from the most recent advances in design and materials, can offer
long lifetimes of up to 15 years in a surprisingly wide range of operating
conditions, and without incurring the high cost of capacitors marketed
as ultra-high reliability devices.
Aluminium capacitor failure modes
Aluminium capacitors implement a variety of functions, depending on
their position in the circuit. As an input buffer in an AC-DC converter, an
aluminium capacitor provides energy when the mains input voltage is too
low, or stores energy when it is too high. As an output buffer, an
aluminium capacitor performs filtering, and acts as a current sink for an
inductor.
In operation, these capacitors can fail in a number of ways:
• catastrophic failure occurs when the capacitor completely breaks
down, due to a short or open circuit
• degradation occurs when the capacitor continues to function, but its
performance has deteriorated to some extent. For instance, the
device’s capacitance might fall over time. Whether the change in value
is acceptable or not depends on the requirements of the application. If
the change in value is unacceptable, the device has effectively failed.
A short-circuit between the electrodes can be caused mechanically,
by shock, vibration or stress on the leads. It can also be caused
electrically, by the application of a pulse current or voltage which
exceeds the rated maximum value.
• The common failure modes of aluminium capacitors used in
power supplies
• The most important causes of a capacitor’s failure
• How to calculate a capacitor’s expected lifetime, and to optimise
a system design for long capacitor lifetime
There can be various causes of an open circuit. For instance, if the
capacitor is subjected to too high a force at the time of mounting, the
connection between the lead wire and the tab could be twisted or
distorted. High temperature is also dangerous, either by operating at a
temperature above the rated maximum, or through exposure to
excessive heat transmitted through the circuit board’s tracks, which
vaporises the capacitor’s electrolyte. Similarly, exposing the capacitor to
excessive ripple current causes its internal temperature to rise, drying
the electrolyte.
A drop in capacitance and increased power losses due to high ESR
occur when:
• a reverse voltage is continuously applied
• the capacitor is subjected to a very high number of charge/discharge
cycles
• applied current exceeds the maximum rated ripple current
How to optimise for the application’s conditions
Standard load life test limits applied to aluminium capacitors, at their
rated voltage and maximum rated temperature, typically measure: the
elapsed time until the capacitor suffers a 20% or 30% decrease in
capacitance from its initial value; a 200% or 300% increase in loss
tangent which is a measure of the power losses attributable to the
dielectric; or a 200% increase in leakage current, whichever occurs first.
These standard limits provide a quick but rough means of comparing
the performance of competing devices. But they do not necessarily
reflect the requirements of any given application. So to optimise the
lifetime/cost trade-off and to find the best possible capacitor for a
specific power supply, the designer must calculate the expected life of
capacitors under evaluation in the expected operating conditions of the
application.
Before doing so, it is worthwhile considering how the operating
conditions of the power supply might be modified so as to minimise the
hazard to any aluminium capacitors on the board. The electrical
characteristics of aluminium electrolytic capacitors are more sensitive to
temperature than those of other types of capacitors. This is because of
the liquid electrolyte in aluminium capacitors, the properties of which,
such as conductivity and viscosity, are strongly affected by temperature.
In order to reduce the device’s exposure to high temperatures, the
designer needs to understand the flow of thermal energy through it. This
is shown in Figure 1. Inside the dotted line, all the materials are at the
device’s junction temperature, Tj; outside the dotted line is the ambient
temperature, Ta. The heat generated inside the dotted line is carried
outside it by convection, radiation and conduction.
If the designer can implement a means to improve the heat flow out of
the capacitor, its expected
operating lifetime will be extended.
Indeed, according to the Arrhenius
theory, the life of an aluminium
capacitor doubles with every 10°C
drop in ambient temperature.
Such a drop has a direct effect on
the designer’s lifetime calculation
when the heat generated by
resistive losses, such as in timing
circuits, is negligible.
Fig. 1: The three ways in which heat escapes from a power supply’s capacitor
[Source: Vishay, Engineering Solutions document ‘Aluminum capacitors in power supplies’, page 9.]
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to calculate the effect of operating conditions
Here, the relationship between lifetime and ambient temperature is:
L = L0 2
n
Table 1 shows an example calculation for a part from the NRZJ series of
aluminium capacitors from NIC Components, the NRZJ182M35V12.5X35.
The information available from the datasheet is:
Tmax = 105°C
L0 = 1000 hours
I0 = 4.12A
Tmax – Ta
10
where:
L = estimated life [hours]
L0 = life at rated temperature [hours]
Lmax = rated temperature [°C]
La = ambient temperature [°C]
The calculation was applied to estimate the device’s operating lifetime at
an ambient temperature of 95°C. NIC Components specifies the value of
∆Tj0 at 7°C.
An example of an expected life calculation according to this equation is
shown in Figure 2.
I100kHz [A]
Life Expectancy at 95°C
∆Tj [°C]
Hours
years
2.5
2.6
29,752
3.4
3.0
3.7
27,120
3.1
3.5
5.1
24,121
2.8
4.0
4.12
6.6
7.0
20,825
20,000
2.4
2.3
4.5
8.4
17,327
2.0
Table 1: Lifetime calculation for the NRZJ182M35V12.5X35 capacitor from NIC Components
Fig. 2: Calculation of various capacitors’ expected life over a broad range of ambient temperatures
This equation needs to be modified in the case of aluminium capacitors
in power supplies, however, since resistive losses are an important
factor. The effect of ripple current on lifetime estimates can be calculated
as follows:
L = L0 2
n
Tmax – Ta
10
∆T
∆T
–
(
10
–
(0.25
∆T
)
10
–
(0.25
∆T ) )
2
j0
n
n
j0
j
where:
∆Tj0 = temperature rise caused by rated ripple current [°C]. This is the
maximum differential temperature from the core to the external
capacitor case. This value will vary depending on the materials used in
the device’s construction. There is no easy way for the user to
calculate this value, so manufacturers provide this information on
request. Every series of capacitors, and sometimes even different
case sizes within a series, will have a different value for ∆Tj0 .
∆Tj = temperature rise caused by the actual ripple current [°C],
expressed as:
∆Tj = ∆Tj0
(II )
100kHz
where:
I100kHz = actual ripple current at 100kHz
I0 = rated ripple current at 100kHz
PW
NRE-JL
(Nichicon)
(NIC Components)
(Vishay)
Temperature Range [°C]
-55 to 105
-40 to 105
-55 to 105
Ripple rated at 105°C 100kHz [mArms]
3680
3450
3000
Useful Life at 105°C [Hours]
8000
10000
10000
0.019
j
n
n
Superior performance of specialist power-supply capacitors
A comparison of the lifetime of an aluminium electrolytic capacitor such
as the NRZJ series, specially designed for use in power supplies, with
that of a general purpose capacitor will show a dramatic difference in
lifetime when used in an AC-DC or DC-DC converter. Specialised powersupply capacitors can be obtained from leading manufacturers such as
Nichicon and Vishay as well as NIC Components, shown in Table 2.
These manufacturers offer miniature capacitors with a wide temperature
range, very long useful life, high ripple-current capability and low
impedance. Some of them are AEC-Q200 qualified for use in automotive
applications.
Nichicon offers the PS, CS, PH, PX, PW and PA series, NIC
Components the NRZJ and NRE-JL series, and Vishay the 150 RMI and
136 RVI series.
2
0
Note that, if ∆Tj >20, the designer should contact the manufacturer’s
representative for further guidance.
A lifetime estimation calculated using the equation above includes a
margin of error; it is not a value guaranteed by the manufacturer.
Therefore manufacturers of aluminium capacitors recommend that
designers should allow a wide safety margin between the calculated
value and the intended service life. Also, even if the calculation indicates
an expected lifetime of more than 15 years, manufacturers recommend
limiting the service life expectation to 15 years.
150 RMI
Impedance at 20°C 100kHz [Ω]
0.015
0.019
Impedance at -10°C 100kHz [Ω]
0.03
0.056
0.044
Case Size [mm]
18 x 35.5
16 x 31.5
18 x 31
Table 2: Comparison of 3,300µF aluminium capacitors rated for 35V
Clearly then, a lifetime performance comparison will help the designer to
find the best device for the application. If, however, a device cannot be
found which offers the required combination of performance and price,
the designer can consider the option of cooling, in order to extend the
life of a cheaper capacitor with inferior performance.
The basic method of cooling a capacitor is to mount it in free space.
The natural circulation of air around the capacitor will provide sufficient
cooling for most applications.
If this is not sufficient, a heatsink will increase the flow of heat from the
device. The most common type of heatsink is an aluminium extrusion
attached to the closed end of the capacitor.
As shown above, whichever capacitor is used in a power supply, the
designer can ensure that the device survives for its average rated lifetime
by regulating the temperature, ripple current and applied voltage to
within the manufacturer’s specified limits.
The above information is specific to NRE‐JL series from NIC Components For information on ‘ΔTj0’ (temperature rise caused by rated ripple current) for other For more information e-mail
NIC product series, please contact NIC Technical Support at tpmg@niccomp.com
info@my-ftm.com
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