Capacitors may seem simple enough, but specifying them has grown more complex in recent years. The reason why comes down to freedom of choice. The universe of capacitors has expanded greatly over the past few years, in large part due to capacitor designs that take advantage of advances in conductive polymers. These advanced components sometimes use conductive polymers to form the entire electrolyte. Or the conductive polymers can be used in conjunction with a liquid electrolyte in a design known as a hybrid capacitor. Either way, these polymer-based capacitors offer a performance edge over conventional electrolytic and ceramic capacitors when it comes to electrical characteristics, reliability, safety and cost.
Various polymer and hybrid capacitors have distinct sweet spots in terms of their ideal voltages, frequency characteristics, environmental conditions and other application requirements. This article explores how to identify the best uses for each type of advanced capacitor.
Understanding Polymer Capacitor Choices
Polymer capacitors come in four main varieties:
- LAYERED POLYMER ALUMINUM CAPACITORS use conductive polymer as the electrolyte and have an aluminum cathode (see Figure 1). Depending on the model, these capacitors cover a voltage range from 2V to 25V and offer capacitances between 2.2μF and 560μF. The distinguishing electrical characteristic of these polymer capacitors is their extremely low equivalent series resistance (ESR). For example, some of Panasonic’s SP-Cap polymer capacitors have ESR values as low as 3mΩ, among the lowest in the industry. Packaged in a molded resin as compact surface mount devices, these layered polymer capacitors feature a sleek low-profile design.
- WOUND POLYMER ALUMINUM capacitors are also based on conductive polymers and aluminum, but they have a wound foil structure (see Figure 2). These capacitors cover a wider range of voltages and capacitance values than other types of polymer capacitors. Voltages extend from 2.5V to 100V, while capacitances run from 3.3μF to 2700μF. Like the layered polymer capacitors, the wound style has extremely low ESR values. Some of Panasonic’s OS-CON capacitors, for instance, have ESR values below 5mΩ.
- POLYMER TANTALUM CAPACITORS employ a conductive polymer as the electrolyte and have a tantalum cathode (see Figure 3). They span voltages from 1.8V to 35V and capacitances from 2.7μF to 680μF. They too have low ESR values, with some of Panasonic’s POSCAP capacitors exhibiting ESR values as low as 5mΩ. Packaged in a molded resin case, the tantalum polymer capacitors are among the most compact on the market.
- POLYMER HYBRID ALUMINUM CAPACITORS use a combination of a liquid and conductive polymer to serve as the electrolyte (see Figure 4) and aluminum as the cathode. Think of this technical approach as the best of both worlds; the polymer offers high conductivity—and a correspondingly low ESR. The liquid portion of the electrolyte, meanwhile, can withstand high voltages and provide higher capacitance ratings due to its large effective surface area. These hybrid capacitors offer a voltage range from 25V to 80V and capacitances between 10μF and 330μF.
Polymer Capacitor Advantages
Despite differences in their materials and construction, all polymer capacitors feature desirable electrical properties:
- GREAT FREQUENCY CHARACTERISTICS. Due to their ultra low ESR values, polymer capacitors have a low impedance near their resonance point, thereby reducing AC ripple in power circuits. In-house testing reveals as much as a fivefold reduction in peak-to-peak voltage changes when comparing polymer capacitors to conventional low-ESR tantalum varieties.
- STABLE CAPACITANCE. With ceramic models, capacitance drifts in response to temperature changes and DC bias. Polymer capacitors have no such problem and remain stable over time. This stability is particularly important in industrial and automotive applications, which tend to experience fluctuations in operating temperatures.
- ENHANCED SAFETY. Conventional electrolytic capacitors can suffer from safety issues that could cause them to short circuit and fail. The problem arises when electrical or mechanical stresses create defects in the oxide film that forms the capacitor’s dielectric. Polymer capacitors have a self-healing capability that eliminates this failure mode. The repair takes place in response to the joule heating that occurs when a dielectric defect triggers a short circuit. In the case of hybrid capacitors, an additional self-healing mechanism comes into play—because the liquid electrolyte causes current flow near the defect to reoxidize the aluminum. Conventional tantalum capacitors are typically derated in use by 30% to 50% of their labeled voltage to ensure safety, although this practice results in upsizing and increased cost. In contrast, Panasonic guarantees operation at 90% of the full-rated voltage for its polymer capacitors.
Capacitor Evolution Continues
Polymer capacitors have been in production since 1990. But they continue to evolve, both in terms of their electrical characteristics and sizing. Consider Panasonic’s aluminum polymer capacitor line, for example. Upcoming models will drive ESR even lower and capacitance even higher— to 2mΩ and 680μF, respectively. Regarding the tantalum polymer line, new models will offer reductions in ESR in smaller surface mount packages. For example, the 3.5 x 2.8mm B-Size capacitors will likely see a drop in ESR from 9mΩ to 6mΩ. The hybrid capacitor line is evolving too, featuring expanded voltage coverage with new 16V and 100V models. Life cycle and ripple current specifications are also slated for improvement in upcoming product releases.
Hybrid Capacitor Performance Highlights
Driven by miniaturization of electrical components and higher switching frequencies of many electrical devices, hybrid capacitors have started to get more traction due to their stable electrical characteristics at high frequencies. These robust capacitors also have other compelling advantages that make a difference in applications such as computer servers, backup devices and networking gear as well as industrial motors, automotive engine control units, security cameras and LED lighting.
Hybrids are compact. Given the ongoing push to miniaturize electrical equipment, the size of capacitors has taken on a growing importance. Surface mount hybrid capacitors measuring just 6.3 x 5.8mm can handle 35V and offer a capacitance of 47μF. The small size can save a significant amount of board space.
Hybrids maximize reliability. Although capacitors must be small, they also need to hold up under challenging electrical and environmental conditions. By nearly every measure, hybrid capacitors outperform equivalent aluminum electrolytic and polymer capacitors. Hybrids have significantly better endurance and humidity resistance than either their electrolytic or polymer counterparts. They also have significantly higher tolerance for large ripple currents, inrush currents and elevated temperature.
Taken together, the size and reliability produce a strong cost benefit for using hybrid capacitors, in spite of their upfront prices. The higher ripple current specification alone can result in a 20% cost reduction by increasing the capacitor’s life cycle.