Benefits of Aluminum Polymer Capacitors: Improved EMC Performance [Part 1 of 2]

If you’re looking for an aluminum capacitor for your application, you may be wondering which route to go: aluminum electrolytic or aluminum polymer.

At first glance, both types of capacitors seem similar, but there’s one key distinction: aluminum polymer capacitors use a conductive polymer instead of a liquid electrolyte. This small change makes a big difference. The construction of aluminum polymer capacitors results in superior electrical properties, giving you a much wider range of possibilities (and much better performance) for your application.

In this first post in our two-part series on the benefits of aluminum polymer capacitors, we’ll discuss one of the biggest advantages: improved EMC performance. (To learn more, access the full ANP071 application note!)

What Are Aluminum Polymer Capacitors?

An aluminum polymer capacitor (also called a polymer electrolytic capacitor or polymer e-cap) is a sub-form of the electrolytic capacitor. The special feature of these capacitor types is that a conductive polymer is used instead of a liquid electrolyte. This requires a special processing step, which is carried out during production.

In this chemical reaction, the so-called polymerization, by heating, the still-liquid monomer that has been impregnated in place of electrolyte in the separator paper is crosslinked to a solid polymer. This process is typically done at a temperature of about 100 °C. Once completed, the polymer is solidified indefinitely.

To learn more about the construction and the difference between aluminum electrolytic and aluminum polymer capacitors, read our blog post, “Introduction to Aluminum Capacitors: Traditional Electrolytic vs. Polymer.”

Why a Buck Converter Is Used for Polymer Electrolytic Capacitors

To demonstrate the positive effects of the polymer electrolytic capacitor, a buck converter is used. The input voltage is 12 V, and the output voltage has been set to 5 V. The load is a pure ohm load of 5 Ω. This results to a current of 1 A flowing through the resistor. This setup serves as a basis to make the performance of polymer electrolytic capacitors clear. The design is used for both EMC measurement and voltage ripple output measurements with always the same load.

From the EMC point of view, a buck converter is much more critical at the input side. This is due to the discontinuous current consumption based on the fast switching processes of the semiconductors. As a result of this topology, there is already an "LC filter" at the output, which integrates the discontinuous current on the high side (refer to Figure 2).

The construction and design of the buck converter was based on the specifications of the data sheet and designed with the default values for the coil and capacitors. The inductance values of the coil and the capacitance of the input and output capacitors were verified by the manufacturer's data sheet and with their simulation software. This was especially important when using only one aluminum electrolytic capacitor.

Due to the very high ESR value, the stability of the regulator was impaired. To counteract this effect, a capacitor was additionally attached to the feedback loop. This additional capacity ensures stability even at high ESR values.

How Aluminum Polymer Capacitors Achieve Better EMC Measurement

The measurements were made according to the CISPR 32 standard (which replaces CISPR 22 and 15) in an RF-shielded chamber with the corresponding connection to the ground surface of the cabin. The test receiver was an R&S ESRP 3, and as network simulation, an ENV216 two-wire V-net simulation was available. During the measurement, in the first step, further input filters on the layout were dispensed; only in the last measurement was a T-filter with a separated coil placed. This filter was constructed according to the specifications in the data sheet.

For the first measurement, an aluminum electrolytic capacitor WCAP-ASLL 865 060 343 004 was used for the input capacitor C1 ( 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.

With this, the limit values of CISPR 32 class B are clearly exceeded. There are noise levels of up to 100 dBμV detectable. But where do these interfering signals come from? The capacitor as a real component has parasitic effects, particularly the ESR together with the parasitic effects of the layout (the lead inductance) generate a high frequency voltage drop that can be detected by measurement.

As a first approach to achieve acceptable levels of emissions and stay below the limits, an aluminum polymer capacitor can be used. The electrical properties in terms of capacity and rated voltage of the aluminium polymer capacitor are the same as those of the aluminum electrolytic capacitor.

The design is also equivalent at the capacitance of 47 μF, and the capacitor fits to the original landing pattern. 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 of 20.7 mΩ and ESL of 3.9 nH. Due to the very low ESR and ESL, a corresponding measurement of the interference spectrum is achieved.

It can clearly be seen that by changing only one component, the EMC performance was significantly improved. The voltage drop which is generated by the fundamental frequency, and the first harmonic of this frequency is reduced — thus, less interference is generated. However, the limit could not be met, so further filters have to be placed. The structure of the input filter was based on the information in the data sheet. Therefore, the filter has a corresponding insertion loss (in a 50 Ω system).

The input filter was then included on the PCB, and another measurement performed. The result is shown in Figure 10, where the interaction between the aluminum polymer capacitor and the input filter is visible. The combination of input filter and low ESR and low ESL of the polymer electrolytic capacitance makes it possible to push the level broadband below the limit of class B. Values of less than 40 dBμV (Average & Quasi Peak) are easily possible (compared to the first measurement with around 100 dBμV) — and so, the measurement is passed.

All of this means that aluminum polymer capacitors have the capability of greatly improving the EMC performance of your application — just by changing that one little element.

In our next post, we’ll discuss more benefits of aluminum polymer capacitors, including the longer lifetime compared to aluminum electrolytic capacitors. (To learn even more, access the full ANP071 application note here, or watch a video replay of our webinar, "Aluminum Electrolytic vs. Polymer - Performance in Various Applications, Advantages, and Challenges of both Technologies.")

In the meantime, you can browse our selection of aluminum polymer capacitors online, and request a free sample to see the benefits for yourself!

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