Specifying capacitors might seem a simple task, but it has actually become more complicated in recent years. The reason is growing freedom of choice. The universe of capacitors has expanded greatly over the past few years, in large part because of the new capacitor technologies which have provided many advances in conductive polymer solutions.
These advanced capacitors 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 in the parameters of:
- Electrical characteristics
- Lifetime cost
The various polymer and hybrid capacitors have different sweet spots in terms of voltage, frequency characteristics, environmental conditions and other application requirements. This Design Note describes the best uses for each type of advanced capacitor. It also highlights specific applications in which a polymer or hybrid capacitor will outperform traditional electrolytic or ceramic capacitors.
The Varieties of Polymer Capacitor
Polymer capacitors come in four main varieties, including hybrid aluminum capacitors. Each type has different electrolytic and electrode materials, packaging and application targets.
Layered polymer aluminum capacitors use conductive polymer as the electrolyte and have an aluminum cathode. These capacitors cover a voltage range from 2V to 35V, 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 Panasonic SP-Cap™ polymer capacitors have ESR values as low as 3mΩ. They are suitable for use in handheld electronic devices or other applications requiring a low-profile capacitor that will not interfere with a nearby heat-sink.
Wound polymer aluminum capacitors are also based on conductive polymers and aluminum, but they have a wound foil structure, as shown in Figure 1. The wound polymer 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 2,700μF.
Fig. 1: The construction of a wound polymer aluminum capacitor
Like the layered polymer capacitors, the wound style has very low ESR values. Panasonic OS-CON™ capacitors, for instance, have ESR values below 5mΩ. The wound style can be surface mounted, but they are not as small as layered capacitors.
Polymer tantalum capacitors employ a conductive polymer as the electrolyte and have a tantalum cathode, as shown in Figure 2. They span voltages from 2V to 35V and capacitances from 3.9μF to 1,500μF. They, too, have low ESR values.
Fig. 2: The construction of a polymer tantalum capacitor
POSCAP™ capacitors from Panasonic feature ESR values as low as 5mΩ. Packaged in a molded resin case, the tantalum polymer capacitors are among the most compact on the market. The Panasonic POSCAP M size, for example, measures just 2.0mm x 1.25mm.
Polymer hybrid aluminum capacitors use a combination of a liquid and conductive polymer as the electrolyte, and aluminum as the cathode. The polymer offers high conductivity, and a correspondingly low ESR. The liquid portion of the electrolyte, meanwhile, can withstand high voltages and provides higher capacitance ratings due to its large surface area.
The hybrid capacitors offer a voltage range from 25V to 80V and capacitances between 10μF and 330μF. At 20mΩ to 120mΩ, ESR values for hybrids are higher than for other types of polymer capacitors, but still very low considering that they are suitable for use in relatively high-power applications.
The Advantages of Polymer Capacitors
Despite differences in their materials and construction, the four types of polymer capacitors share desirable electrical properties.
Good frequency characteristics. Thanks to their low ESR values, polymer capacitors have a low impedance near their resonance point, and lower impedance reduces AC ripple in power circuits by as much as five times by comparison with conventional low-ESR tantalum capacitors, as shown in Figure 3.
Fig. 3: Lower ESR reduces AC ripple in high-frequency switching power converters
Stable capacitance. Unlike ceramic capacitors which suffer from capacitance drift in response to temperature changes and DC bias, polymer capacitors remain stable over time. This stability is particularly important in industrial and automotive applications, which tend to experience fluctuations in operating temperatures.
Hybrid capacitors add another dimension to capacitance stability. They keep a stable capacitance in the face of common operating conditions, such as high frequencies and low temperatures, which reduce the capacitance of conventional liquid electrolytic capacitors, as shown in Figure 4.
Capacitance vs. Frequency
ESR vs. Temperature
Fig. 4: Hybrid capacitors offer stable capacitance at high frequencies and stable ESR, even at temperatures as low as -55°C
Greater safety. Conventional electrolytic capacitors can suffer from electrical or mechanical stresses which produce defects or discontinuities in the oxide dielectric film. Polymer capacitors have a self-healing capability which eliminates this failure mode.
Typical tantalum capacitors are normally de-rated in use by 30-50% of their labeled voltage to ensure that they operate safely. This de-rating, while a common and accepted engineering practice, increases cost because a higher capacitance value has to be specified. By contrast, Panasonic guarantees the operation of its polymer capacitors at 90% of the full rated voltage.
Robust Capacitors for Industrial Use
The increased use of electronics in industrial applications has created a need for more robust capacitor solutions. These demanding applications often have unforgiving operating environments which are not friendly to conventional capacitor types such as aluminum electrolytic.
Capacitors using polymer technology, such as Panasonic’s OS-CON and hybrid models, are ideally suited for these applications because they offer a combination of:
- Long life
- Low ESR
- High ripple current
- High-temperature operation
- High voltage
- High capacitance
Industrial applications that can benefit from advanced polymer and hybrid capacitors include motor drives, power inverters and specialty lighting. Controller applications can take advantage of polymer-based capacitors as well. POSCAP polymer tantalum and SP-Cap models from Panasonic offer similar electrical characteristics to the OS-CON and hybrid models, but also have compact form factors which make it easy to integrate them into industrial control systems.
Hybrid Capacitor Performance Advantages
Driven by the miniaturization of electrical components and the rise in switching frequencies in many electrical devices, hybrid capacitors are becoming increasingly popular. Hybrids are known for their stable electrical characteristics at high frequencies. These robust capacitors also have other strong advantages in applications such as computer servers, back-up devices and networking gear as well as industrial motors, automotive engine control units, security cameras and LED lighting.
Among the advantages are:
Small size. Surface-mount hybrid capacitors measuring just 6.3mm x 5.8mm can handle 35V and offer a capacitance of 47μF. The small size can save a significant amount of board space. In a 48V power-supply design, hybrid capacitors from Panasonic occupied just 13% of the board space required by aluminum electrolytic capacitors.
Reliability. By nearly every measure, hybrid capacitors outperform equivalent aluminum electrolytic and polymer capacitors in reliability. For instance, hybrid capacitors have markedly better endurance and humidity resistance than either their electrolytic or polymer counterparts. Hybrids also have significantly higher tolerance for large ripple currents, in-rush currents and high temperature.
In combination, the size and reliability advantages of hybrid capacitors result in a strong cost benefit, in spite of the higher purchase price of hybrid capacitors. The higher ripple current specification alone can result in a 20% reduction in cost by increasing the lifetime of the capacitor.
In a 48V power-supply design, the use of hybrid capacitors can cut cost in half compared to the use of equivalent aluminum electrolytic capacitors. The savings come from reductions in board cost and warranty cost, combined with the ability to withstand high ripple currents.