Calculate Lifespan of Electrolytic Capacitors
In this article applicable to all capacitors, which are electrolytic capacitors,
aluminum electrolytic capacitors, component reliability, component
lifespan.
Aluminum
electrolytic capacitors have a reputation as having a short
life expectancy and being unreliable. Part of that reputation is due to a
fifteen-year-old stolen electrolyte formula scandal—but part of it is
because they really don't last as long as other types of capacitors. But
are they really that much worse.
A Scandalous Reputation
In the early 2000s, aluminum electrolytic capacitors (Al e-caps) were
failing at a far higher rate than expected, a phenomenon commonly and
dramatically known as the capacitor plague.
The cause was tracked down to an interesting story of industrial
espionage where a formula for the electrolytic solution of the
capacitors was stolen, but then incorrectly used. The result was
millions of bad capacitors ending up in all sorts of electronic devices.
This story certainly gave Al e-caps a bad reputation as unreliable
parts, and this reputation is further enhanced by the fact that even
properly manufactured Al e-caps have a relatively short lifespan when
compared to film capacitors (which can also have high capacitance and
high voltage ratings like Al e-caps).
For example, the KXG family of Al e-caps from United Chemi-Con (which
includes capacitors with capacitances from 6.8μF to 330μF and voltage
ratings from 160V-450V
DC) has a rated life expectancy of
8,000 to 10,000 hours while the EPCOS B32798 family of film capacitors
(which includes capacitors with capacitances from 18μF to 75μF and
voltage ratings from 250V-400VDC) has a rated life expectancy of 60,000
hours.
Figure 1. Bulging electrolytic capacitors
It would appear that the reputation of Al e-caps as unreliable devices is actually justified. But is that actually the case?
Assessing the Lifespan of Components
To properly assess the lifespan of a component requires more than
just looking at the rated lifespan. The effect of stresses (heat,
current, voltage) that the device will undergo during operation also
needs to be included in the analysis. It turns out that the rated
lifespan is typically given for a case of fairly high stress.
If the device is going to be used at a lower temperature, voltage, or
current, the rated lifespan can be de-rated. This de-rating involves
re-calculating the lifespan based on the actual operating temperature,
voltage, and current, and often times the de-rated lifespan is many
times longer than the rated lifespan.
Manufacturers of capacitors will typically provide de-rating
calculations or graphs based on their research and field testing. Often
times, these calculations are not included in the component
datasheet but are instead found in a separate capacitor characteristics
document (the reason for this is that there is probably just too much
extra information to include in a datasheet).
Defining Capacitor "Failure"
The first thing to determine from the manufacturer is how they define
a failed capacitor. A failure is not usually defined as “doesn’t work
anymore”—it is typically defined as a certain percentage change in the
characteristics of the capacitor. This definition is important to know
so that a proper comparison between capacitors can be made.
For example, Vishay uses standard CECC 30301 for their Al e-caps,
which defines the end of life as being when the capacitance changes by
30%, the dissipation factor changes to more than three times the upper
limit, or if the impedance is more than three times the stated limit.
Meanwhile, TDK defines the end of life for their B32798 series film
capacitors when the capacitance changes by 10%, the dissipation factor
changes to more than four times the upper limit, or if the insulation
resistance drops below 1500MΩ.
Figure 2. A failed capacitor
Defining Capacitor "De-Rating"
The second thing to determine from the manufacturer is how they
de-rate the expected life of a component. In general, capacitors (and
many other devices) can be de-rated as follows:
lifeactual=lifebase×temperatureFactor×voltageFactor×currentFactor
is the life expectancy at the operating temperature, voltage, and current.
lifebase
is the life expectancy at the rated temperature, voltage, and current
temperatureFactor
: For both Al e-caps and film capacitors, a 10
oC decrease in temperature results in a doubling of life expectancy. So the temperature factor is
20.1(Tm−Tc)
. Where Tm is the rated temp and Tc is the operating temp.
voltageFactor
is the derating due to the operating voltage being lower than the
maximum rated voltage. According to United Chemi-Con, voltage has far
less of an effect than temperature on the lifespan of an Al e-cap
(unless the voltage is exceeding the rated maximum)
currentFactor
is the derating due to the operating current being lower than the
maximum rated current. Current has a more significant effect on Al
e-caps than film capacitors because of their higher equivalent series
resistance (ESR). The power loss due to the ESR will be equal to
I2capRESR
- and all of this power loss leads to self heating which will affect lifespan.
Applying Lifespan Calculations: An Example of Capacitor Selection
Armed with this de-rating equation and the de-rating factors, it is
possible to do a proper lifespan comparison between capacitors. To
illustrate the comparison, let’s look at a specific example.
Assume that we are designing an AC-DC converter for a battery charging system. This battery system operates at 400V
DC
and we have determined that we need a 300μF capacitor for the filter of
the converter. After some research to find components that meet these
voltage and capacitance requirements and also have the best combination
of size and cost, we find two possible solutions.
We could use three 100μF Al e-caps from United Chemi-Con (part number
EKXG451ELL101MM40S) or four 75μF film capacitors from EPCOS (part
number B32798G2756K). The tradeoff between these two selections is that
the Al e-caps will cost about $10 total but have a rated lifespan of
10,000 hours, while the film capacitors will cost about $120 total but
have a rated lifespan of 60,000 hours.
If we need this battery charger to last at least four years, it
appears that we will have to select the far more expensive option.
De-Rating to the Rescue
But
wait! We don’t have to use this rated life expectancy because this
system is going to operate below the rated temperature and voltage of
the devices. This means we can de-rate the life expectancy and end up
with a longer life at the operating temperature and voltage than the
rated life.
Once again, the lifespan de-rating calculation is
lifeactual=lifebase×temperatureFactor×voltageFactor×currentFactor
For simplicity, let’s assume the voltage factor and the current
factor are both 1 (they are both probably more than 1) and only de-rate
based on temperature. If the actual life calculated is still not long
enough, we can determine what the actual voltage and current factors are
to provide an even more accurate expected lifespan.
Both of the capacitors have a temperature rating of 105
oC, so if the operating temperature is estimated at 75
oC (which is high for this system), there is a 30
oC difference between the rated and the operating temperature.
This 30
oC difference results in three doublings of the
expected lifespan over the rated lifespan. In other words, the actual
expected lifespan will be
23=8
times more than the rated lifespan. For the Al e-cap, this means that
the expected life will actually be 80,000 hours which is just over 9
years of continuous operation.
Conclusion
Al
e-caps have a reputation as unreliable parts with a short lifespan. This
reputation is justified if you are only looking at the rated lifespan,
but the operating conditions are often much more favorable than the
rated conditions.
