Contemporary high-power fed electronic circuits must have high capacitance, low Effective Series Inductance (ESL), and low Effective Series Resistance (ESR) capacitors. Conducting polymers are considerably more conductive, even up to 100 times more, when compared to fluid electrolyte and MnO2. The direct benefits of such higher conductivity include better high-frequency performance and lower ESR. Polymer tantalum solid capacitors offer engineers a dependable high-capacitance solution with reliable performance. The maturation of polymer cathode systems in tantalum capacitor technology has provided lower ESR, reductions in the ESL, and packaging density improvements. Polymer-based capacitors eliminate application complications like ignition and squeals, which other capacitors cannot fix.
Panasonic's POSCAP is a polymer capacitor that uses sintered Tantalum as the anode and a highly conductive proprietary polymer as the cathode. This enables POSCAP capacitors to be thin and small, with minute ESR and superior high frequency. These characteristics make it ideal for digital/high-frequency applications. It has high reliability and heat resistance. In this article, we will discuss conductive polymer tantalum solid capacitors and their structure, and why they offer more significant benefits compared to other electrolyte capacitors and applications.
Structure of Conductive Polymer Tantalum Solid Capacitors
Polymer tantalum electrolytic capacitors are tantalum capacitors in which a conductive polymer makes up the electrolyte. The electrolyte ensures electrical conductivity in an electrolytic capacitor, and makes up the counter electrode (cathode). The polymer electrolyte uses either polypyrrole (PPy) or polythiophene (PEDOT or PEDT). Figure 1 illustrates a polymer tantalum capacitor's internal structure. Compressed tantalum powder is pressed around a tantalum wire to make a "pellet" for the anode connection. The pellet and wire combo is subsequently vacuum sintered at elevated temperatures to induce mechanical strength. During sintering, the powder becomes a sponge-like structure, with all its particles interconnected to create a monolithic spatial lattice. Such an arrangement has predictable density and mechanical strength, but is also highly porous, producing a large anode surface area.
A dielectric layer formed by the anodization electrochemical process encases the anode tantalum particle surfaces. This is done by submerging the "pellet" in an extremely weak acid solution, and subsequently applying DC voltage. The oxidized sintered block is then impregnated with polymer precursors to obtain the polymer electrolyte. This polymerized pellet is successively dipped into the conducting graphite and then into silver to better connect to the conducting polymer. This layer accomplishes the capacitor's cathode connection. A synthetic resin then molds the capacitive cell.
Figure 1: Conductive Polymer Tantalum Solid Electrolyte Capacitor Structure
Comparison with Pure Tantalum, Ceramic, and Other Electrolyte Capacitors
Polymer tantalum capacitors have advantages over MnO2 based tantalum capacitors, multilayer ceramic capacitors (MLCCs), and general electrolytic capacitors. We will now compare the polymer tantalum capacitor with other capacitors on a few measurement parameters, such as noise measure, ignition, and profile.
- Noise Measure: Any unwanted signal which accompanies the desired output is called noise. Noise may be caused by a high-frequency AC power supply, AC to DC conversion ripples, any fluctuation in power supply voltages, or the circuit itself. Noise propagation to the load side is barred by the capacitor connected in series with the ground, thereby passing noise to the grounded side. A high-value capacitor with extremely low Equivalent Series Resistor (ESR) is used to suppress maximum noise. Polymer capacitors feature an extremely low ESR compared to other conventional electrolytic capacitors. Polymer conductivity, typically about 100 s/cm, is about 10,000 times when compared to the liquid electrolyte used in standard aluminum electrolytic capacitors. It is also about 1,000 times that of manganese dioxide utilized in tantalum electrolytic capacitors.
- Ignition Measure: Solid electrolytic capacitors like tantalum electrolytic capacitors are susceptible to short circuits and subsequent potential fires. Polymer capacitors (POSCAP), which employ conductive polymer as dielectrics, achieve high reliability by suppressing failures. External stress may damage the dielectric oxide film of electrolytic capacitors. Common Tantalum electrolytic capacitors may short-circuit and catch fire due to this damage. With conductive polymer, leakage current may concentrate in this damaged section, and local Joule heat will be produced. The conductive polymer materials of this damaged section become insulators with a relatively low temperature of roughly 300ºC, suppressing current. Thus, POSCAP can be run as extremely safe capacitors compared to standard tantalum electrolytic capacitors.
- Space Measure: Space measure concerns low profile and volume conservation. Figure 2 compares a polymer tantalum electrolyte capacitor with a general aluminum tantalum capacitor. Aluminum capacitors, due to their tall profile, occupy excess space and cannot be positioned underneath a heat sink. Polymer capacitors (POSCAP) hold roughly 70% less space when compared to a general tantalum capacitor.
Figure 2: Low profile and Space-saving comparison
Comparison with Multilayer Ceramic Capacitors
We will now compare a polymer capacitor with an MLCC, frequently used for noise removal due to their low ESR and ESL.
- Stable Capacitance Concerning Frequency: Polymer capacitors show similar performance to MLCC capacitors in terms of frequency, but MLCCs cannot achieve high capacitance for the same footprint and volume. The MLCC has capacitance vulnerability to DC bias, due to the ferroelectric dielectric materials making up the MLCC.
- Stable Capacitance Concerning DC Biasing: MLCC capacitance varies with applied DC voltage, and may lead to a 70% or more capacity drop compared to the given datasheet specs. There is no significant variation for polymer capacitors when the applied voltage fluctuates. Such an advantage permits POSCAPs to use a reduced number of parts compared to MLCCs. This not only conserves PCB space, but also lowers costs by reducing part counts and eliminating production steps.
- Stable Capacitance Concerning Temperature: Capacitance growth in polymer capacitors moves in tandem with thermal rise. The temperature traits of MLCCs vary by dielectric type, but all suffer from aging failure manifested by temperature dependency and lower operating temperature needs. Ceramic capacitors are susceptible and brittle to thermal shock, and precautions are necessary to avoid cracks when mounting, particularly for high-capacitance, bigger MLCCs. The typical ceramic capacitor temperature range bracket is in the -40°C to 85°C or 125°C region, where capacitance varies approximately +5% to -40%, with the sweet spot hovering between 5 to 25°C. Due to their superior dielectric materials and working mechanism, polymer capacitors have excellent development potential to achieve higher density, temperature, and field stress ratings.
- Squeals Due to Piezoelectric Effects: MLCCs operate with materials having piezoelectric reactions on dielectrics. Such capacitors micro-vibrate with periodic variations of applied voltage charges. The mounting board resonates and generates squeals (audible band sound). Polymer capacitors (POSCAP) dielectrics lack piezoelectric effects, and thus do not squeal.
- Safety: Solid electrolyte provides an edge over electrolytic capacitors. Electrolyte evaporation is a possibility if a wet electrolytic capacitor is overheated. Pressure builds up within the capacitor during evaporation, and it may burst. Solid polymer capacitors are free of such risks. The capacitor usually either shorts or acts similar to an open circuit when it reaches the end of its functional life. Generally speaking, polymer capacitors are much more reliable than electrolytic capacitors and, specifically, MLCCs.
- Robustness: Cracks found in ceramic surface mount technology (SMT) parts restrict assembly reliability and yields. These cracks are discovered as electrical defects: intermittent contact, capacitance loss, variable resistance, and excess current leaks. This is the reason MLCCs are subjected to multiple reliability tests such as thermal shock, biased humidity tests, and board flex (bending), depending on the targeted applications.
Table 1: Comparison chart for different capacitors
Polymer Tantalum solid electrolyte capacitors (POSCAPs) find extensive use in applications where space is a significant constraint. A few application examples of POSCAP applications include:
- Smartphone/Tablet: Smartphones and Tablets are suitable application examples where a polymer capacitor can substitute for an MLCC. The product set of the two includes high-performance GPU and CPU, premium-quality sound, and high precision LCD screens with HD resolution. The power circuits that drive these components must have MLCC squeaking measures and increased current capability. They demand large capacitance, low profile, and downsized capacitors. POSCAP series capacitors with dimension of 3.5 mm x 2.8 mm and a low profile of 1.1 mm are a viable solution for such demands. Figure 3 illustrates a recommended circuit for these capacitors in tablets and smartphones.
Figure 3: Application of POSCAP in smartphone or tablet
- Monitor Camera: a monitor camera with high picture quality, enhanced communication, compact size, and motor controlled wider viewing angle is another use case example of a polymer capacitor. The power source is designed explicitly for increased LSI load current to run its CPU/GPU, motor, PoE 48V input, and instantaneous power failure measure. All such power circuits require a low ESR, small size, high ripple, and low profile capacitor. A POSCAP capacitor is a better fit for this application, with a low ESR as little as five mΩ and a tiny size of 3.5x2.8 mm. A recommended circuit is shown in figure 4, where a POSCAP capacitor is applied in the CMOS Sensor.
Figure 4: Application of POSCAP in camera
- Built-in-Board: Built-in-Board product trends include factory automation (FA) equipment, DDR memory of bigger capacity, high-performance CPUs/FPGAs to control high-speed drives, medical devices, and machine tools. This board needs a power circuitry that can deliver high current to drive the CPU/FPGA core, DDR memory, and other board components. The power circuit must eliminate input noise. The circuit capacitors, to fulfill this need, must be low profile and of smaller size. Their ESR must be extremely low and tolerate high voltage. POSCAP capacitors are used in this application, as they possess all the relevant features. Figure 5 displays a recommended POSCAP capacitors value to connect with particular output voltages.
Figure 5: POSCAP application in Built-in-Board
Designers take advantage of Conductive Polymer Tantalum Capacitors to shrink their products and obtain new functionality in multiple and diverse applications. These components allow design engineers to reduce the board component population due to reduced ESR, to provide better layout flexibility, and to enable the creation of compact and sleek devices. Such capacitors represent a significant improvement in reliability and safety for critical applications.