Properties and advantages: why polymer for capacitors
Author: Julio GÁLLEGO LÓPEZ, Field Applications Engineer at Rutronik
The use of polymer capacitors in electronic circuits improves performance, reliability, and durability, making them a popular choice in modern electronics.
Polymer capacitors gained popularity in the 2000s. They have been used primarily in electronic equipment where high performance and reliability are essential. Their main applications include computer motherboards, medical, aerospace, consumer and industrial electronics, and automotive electronics. In automotive electronics, hybrid polymer capacitors are gaining popularity. AEC-Q200 capacitors are used in engine control units, infotainment systems, and other critical components requiring a stable power supply and high reliability.
Polymer technology
Polymer capacitors are a subset of electrolytic capacitors. The term "electrolytic capacitors" is derived from the use of an electrochemically formed oxide film on the electrode surface, which acts as a dielectric. Several metals, such as aluminum (Al), tantalum (Ta), niobium (Nb), titanium (Ti), zirconium (Zr), hafnium (Hf), and others, can form a thin, highly insulating oxide. However, only three metals—aluminum, tantalum, and niobium—are currently used in practice.
The oxide film formed on the electrode surface becomes an electrical insulator and functions as a dielectric only when the electrode on which it forms serves as the anode. Therefore, electrolytic capacitors are, in principle, polarity capacitors.
There are two main types of polymer capacitors, aluminum electrolyte and tantalum, which will be discussed in the following sections. Conducting polymers are also used in aluminum capacitors to replace the wet electrolyte. These capacitors have a much lower equivalent series resistance (ESR) and do not dry out over time. The main applications for polymer capacitors are DC-DC conversion/decoupling and automotive power distribution applications.
Aluminum Electrolytic Capacitors
Aluminum electrolytic capacitors are polarized capacitors in which the anode and cathode are made of aluminum. They can have a wet electrolyte, a solid conducting polymer, or a hybrid electrolyte (wet and solid conducting polymer). If these capacitors are polarized, they should not be used in the presence of reverse bias.
In aluminum electrolytic capacitors, both electrodes are made of aluminum. The aluminum anode is separated from the wet electrolyte by an oxide layer, which is a paper sheet saturated with the wet electrolyte. Different types of electrolytes enhance oxidation, operate at higher temperature ranges, and absorb any gases that may form internally. The other aluminum plate, which serves as the cathode, is also present. The robustness of the anode's aluminum sheet depends on whether the capacitor must withstand higher voltages.
The construction of the polymer capacitor is surprisingly similar. It consists of an anode and a cathode, both made of aluminum foil. The dielectric is a layer of aluminum oxide (Al2O3) that acts as an insulator between the anode and the conducting polymer. A non-conducting layer (paper, film, or other insulating material) is placed between the conducting polymer, forming two layers of conducting polymer. Finally, a drying and aging process of up to 8 hours is carried out.
Aluminum-polymer hybrid capacitors combine the characteristics of traditional aluminum electrolytic capacitors and polymer capacitors, leveraging the advantages of each type. The dielectric is a mixture of liquid electrolyte and conductive polymers. The liquid electrolyte helps improve performance at lower frequencies and increases the total capacitance.
Figure 1 shows the similarities and differences in the construction of the various types of polymer capacitors.
Figure 1: Similarities and differences in the construction of polymer capacitors (Source: Kemet)
Self-healing process maintains performance and extends lifespan
Small defects such as pinholes, microcracks, or dielectric breakdown zones can form in the Al oxide layer due to electrical stress, thermal cycling, or mechanical strain. These defects create pathways for leakage current that can degrade the capacitor's performance. When a defect causes an increase in leakage current, the localized area around the defect heats up. The conductive polymer layer reacts to this heat. The heat can cause the polymer to temporarily lose its conductivity in the localized area, effectively isolating the defect. The heat also promotes the regeneration of the alumina dielectric layer at the defect site. This can occur through oxidation of the exposed aluminum at the defect site, where the aluminum reacts with oxygen (often from the polymer or the environment) to form new alumina. The combined effect of the polymer reaction and the oxide regeneration seals the defect and restores dielectric integrity. When the defect is sealed, the leakage current decreases and the capacitor resumes normal operation. In hybrid capacitors, the presence of the liquid electrolyte improves the self-healing process by allowing the aluminum oxide layer to reform more efficiently. Both types of capacitors rely on these self-healing mechanisms to maintain performance and extend their lifespan.
Table 1 lists the main drivers of the different aluminum polymer capacitors.
| Main courses
factors |
Wet aluminum electrolytic capacitors | Aluminum polymer capacitors | Hybrid capacitors |
| Lifespan | Decreasing the temperature by 10°C doubles the shelf life | The shelf life increases tenfold when the temperature drops by 20 °C. | It is calculated in the same way as for the wet electrolytic capacitor |
| Reduction voltage | Derating is not necessary, but can increase shelf life. Wet electrolytic capacitors can operate between 70 and 80% of the rated voltage, while hybrid and solid electrolytic capacitors can operate between 80 and 90% of the rated voltage. |
||
| Rated voltage | Nominal voltages up to 450 V; high-voltage aluminum electrolytic capacitors are available with nominal voltages reaching or even exceeding 600 V. | Nominal voltages up to 63 V, versions up to 100 V available; the trend is toward higher nominal voltages.
|
Versions available up to 125 V |
| DC BIAS | No DC-BIAS influence | ||
| ESR | Typical ESR up to 20 mΩ; tendency for ESR to decrease. | Ultra-low ESR, down to 5 mΩ | Between wet and solid, ESR of about 11 mΩ. |
| Ripple current | It's not the best capacity; . | Very good ripple capacity thanks to its low ESR | Good capacity |
| Vibration | Good vibration behavior | Solid polymer aluminum electrolytic capacitors are stiffer and less able to absorb vibrations, making them more susceptible to mechanical damage in high-vibration environments. Some manufacturers offer reinforced series designed to withstand high vibrations. | |
| Temperature | Max temperature up to 105°C, versions available with max temperature up to 125°C
|
Maximum temperature up to 125 C ° | Tendency to increase temperature up to 150 °C to extend shelf life |
| Stability of capacitance as a function of temperature and frequency | Poor high frequency characteristics; Capacitance drops significantly at 20 kHz; sensitive to temperature changes |
Improved high-frequency performance; stable capacitance across a range of frequencies; Capacitance drops significantly at 1 MHz; good temperature characteristics |
|
| Leakage current
|
Aluminum electrolytic capacitors have a higher leakage current compared to other technologies such as MLCCs and plastic capacitors. The leakage current of an electrolytic capacitor is usually specified by the manufacturer and can be calculated using the empirical formula:
IL= K * C * K: provided by the manufacturer C: Capacitance (F or µF) V: applied Manufacturers' data sheets provide precise information on leakage current, although the exact formula and constants may vary from manufacturer to manufacturer. |
||
Table 1: Main factors of various aluminum polymer capacitors
Tantalum electrolytic capacitors
Tantalum capacitors are polarized capacitors that use a solid electrolyte such as manganese dioxide (MnO₂) or a conductive polymer. However, caution must be exercised when reverse-biasing these types of capacitors. The most notable properties of tantalum are its high ductility, high corrosion resistance, high melting point (3.020°C), high heat and wear resistance, and high biocompatibility. Tantalum capacitors can replace MLCCs (multilayer ceramic capacitors) in certain applications, provided specific application criteria are met.
Solid tantalum capacitors
Solid tantalum (Ta) capacitors use manganese dioxide as the cathode due to its self-healing properties. When defects occur in the dielectric, it becomes non-conductive. The tantalum is separated from the manganese dioxide by an oxide layer called tantalum pentoxide (Ta₂O₅). When this layer is reduced, the manganese dioxide oxidizes the tantalum, forming a new oxide layer. As a result, these capacitors feature exceptional reliability with a virtually infinite service life.
The self-healing process can release oxygen, which in extreme cases can lead to combustion. However, tantalum capacitors are well suited for applications requiring operation at higher temperatures.
In these capacitors, the conductive surface area significantly affects the capacitance (directly proportional), while the thickness of the dielectric inversely affects the capacitance. Despite their thinness, tantalum capacitors are robust (dielectric breakdown: 470 V/mm), allowing for relatively high-voltage applications.
| VR | Dielectric thickness (nm) | |
| Ta | MLCC | |
| 2 | 20,7 | 600 |
| 4 | 27,6 | 600 |
| 6 | 36,8 | 600 |
Table 2: Comparison of dielectric thickness between tantalum (Ta) capacitors and MLCCs
Table 2 shows a comparison of dielectric thickness between Ta and MLCC capacitors. In particular, MLCC capacitors require a larger surface area and size to achieve high capacitance due to their thicker dielectric.
Solid Conductive Tantalum Polymer
Conducting polymers began replacing MnO₂ in tantalum capacitors in the mid-1990s due to the polymers' higher conductivity, which translates into significantly lower equivalent series resistance (ESR). The transition from MnO₂ to conducting polymers offers several notable advantages, one of which is the self-healing mechanism.
If a dielectric breakdown occurs during operation (resulting in a short circuit or leakage path), the high current density at the defect site causes localized heating. This heat causes the conductive polymer to oxidize, rendering it non-conductive and effectively sealing the defect. This oxidation restores the insulating properties, preventing further failures and allowing the capacitor to continue operating. Notably, these capacitors are considered safer because their self-healing process does not generate oxygen, minimizing the risk of inflammation, as shown in Figure 2. The main applications are DC-DC rail voltage converters. Table 3 lists the main drivers for the various tantalum capacitors.
Figure 2: Advantages of Ta polymer capacitors: The conductivity of Ta polymer is higher than that of MnO2, and the polymer is also non-flammable. (Source: Kemet)
| Main courses
factors |
Tantalum MnO2 | Tantalum polymer |
| Reduction voltage | Requires a derating voltage of approximately 50%. | 10% applied Voltage less than 10 V
20% applied Voltage greater than 10 V |
| Frequency behavior | They do not perform well at high frequencies; capacitance drops significantly at 10 kHz. | Good characteristics at high frequencies, especially around 100 kHz; if performance is required at 1 MHz, MLCCs are the best choice. |
| Wear mechanism/life | Unlimited shelf life without aging | The cathode material wears down due to moisture and oxidation. As a result, the components will gradually degrade over time. The only way to prevent this degradation is to use airtight packaging. |
| Capacitance volume and energy density efficiency | They maximize capacitance per volume and energy density; they achieve higher capacitance in smaller volumes and at higher voltages than other technologies. | |
| ERS, ripple current and leakage current
|
The polymer has a significantly lower ESR than MnO₂, allowing it to handle high ripple currents. However, both tantalum polymer and MnO₂ have higher leakage currents than other technologies. Tantalum capacitors are not suitable when low leakage current is essential for maximum battery performance. | |
| Robustness and piezoelectric noise | No cracking when bent, similar to; no piezoelectric noise | |
Table 3: Keys to the different tantalum capacitors
Weak points of tantalum polymer
Tantalum polymer capacitors offer several advantages over traditional electrolytic capacitors, making them desirable for a variety of applications. However, they have some drawbacks and cannot be used in all scenarios.
The use of polymer capacitors is not recommended for frequencies near or above 1 MHz, temperatures above 150°C, or where maximum battery life depends on low leakage current.
Polymer capacitors are not suitable if the voltage is higher than 48 V DC, the application requires ultra-low ESR (<< 4 mΩ), low capacitance (< 0,68 µF) or reverse bias.
Conclusion
Polymer capacitors offer low ESR, high stability and reliability, high ripple current handling, increased safety, improved low-voltage performance, and improved frequency characteristics. The use of polymer capacitors in electronic circuits improves performance, reliability, and durability, making them a popular choice in modern electronics.
