Benefits of Aluminum Polymer Capacitors: Longer Lifetime [Part 2 of 2]

Have you been wondering whether to select an aluminum electrolytic capacitor or an aluminum polymer capacitor for your design? While both types of capacitors seem similar, there are some key differences — especially when it comes to performance and price.

In the previous post in our two-part series on the benefits of aluminum polymer capacitors, we explained the advantage of improved EMC performance. In this second installment, we’ll discuss another important benefit of polymer capacitors: longer lifetime. (To learn more, access the full ANP071 application note!)

How the Output Signal Results in Lower ESR for Aluminum Polymer Capacitors

For the output capacitor of a buck converter, a certain capacitance is required in order to keep the control loop stable (and hence the output voltage stable). If the output voltage reduces the capacitance value, the worst-case scenario is that the converter can no longer meet its specification (for example, during load changes). This must be taken into account, especially when operating with class 2 ceramic capacitor (e.g. X7R and X5R).

The result of the output ripple of the switching regulator fluctuates when only one aluminum electrolytic capacitor is used. The capacitor used is a WCAP-ASLL 865 060 343 004, the same as used previously ( learn more with REDEXPERT). The electrical properties of the capacitor are as follows: Capacitance 47 μF, rated voltage 16 V with an ESR 411 mΩ and ESL 19 nH. The high ESR value results in a peak-to-peak value of 400 mV. At least, this means a voltage ripple of 8 % at an output voltage of 5 V. Even with two aluminum electrolytic capacitors of the same type in parallel, the resulting ESR is still 205.5 mΩ and thus clearly too high.

Another aspect that should not be neglected is the ripple current through the capacitor. This leads to heating of the component —v and leads to the failure of the capacitor. Therefore, the ripple current capability of aluminum electrolytic capacitors must always be checked.

In the case of polymer electrolytic capacitors, due to the low ESR, the heating of the component at the same ripple current is significantly lower in comparison. That means that significantly larger ripple currents are capable without thermally overloading the component. A comparison of the ESR of the aluminum electrolytic capacitor and the ESR of the polymer electrolytic capacitor reveals this.

The residual ripple of the output signal with the polymer capacitor as an output capacitor is measured. The aluminum polymer capacitor used was a WCAP-PSLP 875 105 344 006 ( learn more with REDEXPERT) with a capacitance of 47 μF, rated voltage of 16 V and with an ESR 20.7 mΩ and ESL 3.9 nH.

The peak-to-peak value of the measurement is now only 35 mV and therefore within an acceptable range. The voltage peaks are caused by parasitic inductance during the switching. Since no one would use single aluminum polymer electrolytes alone in a real application, it is advisable to place an MLCC in parallel to the aluminum polymer capacitor. Thus, the parasitic effects can be minimized, and a very clean output signal is achieved, as shown in Figure 14.

The MLCC used was a X7R ceramic with a capacitance of 4.7 μF and rated voltage of 16 V ( learn more with REDEXPERT). If the layout of the PCB will be optimized too, a peak-to-peak value of 20 mV is expected (see Figure 14).

Why Aluminum Polymer Capacitors Have a Longer Lifetime

The lifetime of electrolytic capacitors is very important in many applications, particularly industrial ones. Here, the capacitor is not used as a kind of predetermined breaking point (also called planned obsolescence), as it is in consumer electronics, but as a durable and reliable component.

The life of a capacitor depends on many factors of the application. One important factor is the temperature (or rather thermal load), as it is responsible for the fact that internal structures age over time and the electrical properties deteriorate. This results in increased leakage current, which then increases the ESR, which in turn leads to a further increase of the temperature.

The reason for the temperature increase is the power loss generated by the ESR. If these limits are not exceeded, high lifetime expectancies are possible when the inner temperature load of the component is in a lower range. A comparison of the lifetime of aluminum electrolytic and aluminum polymer capacitors by temperature load is listed here. The bases of this consideration are two formulas. With liquid electrolytic capacitors, the expected lifetime doubles when the temperature at the component is reduced by 10 °C (2). For polymer electrolytic capacitors, the life increases tenfold when the temperature at the component is reduced by 20 °C (1).

The typical maximum expected lifetime varies for different vendor and is between 13 and 15 years. If the specified component temperature for aluminum electrolytic and aluminum polymer capacitors is the same (for example, 2000 h at 105 °C), it can be seen at 85 °C the polymer electrolytic capacitor has a longer lifetime. Only in cases of aluminum electrolytic capacitors with a long specified lifetime at the maximum specified component temperature has a higher intersection point but the point of intersection will always occur (see Figure 15).

The specified hours in this diagram are always the nominal lifetime value of the component at this temperature. Apart from this advantage, of course, the other parameters of the capacitors must be compared. It may be that in a special application the expected lifetime is the same, but the better ESR and ESL are critical to the application and speaks for the aluminium polymer capacitor.

Summary of the Benefits of Aluminum Polymer Capacitors

Because of their construction, aluminum polymer capacitors have significant advantages for electronic applications. Low ESR and low ESL values, in addition to very high expected lifetime, make this technology extremely interesting for many diverse applications.

This is why we urge you to consider the use of aluminum polymer capacitors. They can not only improve the behavior of your design, but also increase the overall performance of your application.

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