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Recent advances in conductive polymers have resulted in new capacitors which either use polymers to form the entire electrolyte or in conjunction with a liquid electrolyte in a design known as a hybrid capacitor. Either way, these polymer-based devices can perform better than conventional electrolytic and ceramic capacitors in terms of their electrical characteristics, stability, longevity, reliability, safety and overall life cycle cost.
Layered polymer aluminum capacitors use conductive polymer as the electrolyte and have an aluminum anode. They range in voltage range from 2-35V and offer capacitances between 10-560μF. Their distinguishing electrical characteristic is an extremely low ESR - as low as 3mΩ, amongst the lowest in the industry. Packaged as compact surface mount devices, these devices have a low profile and suit applications in a variety of handheld electronic devices.
Wound polymer aluminum capacitors have a wound foil structure. Voltages extend from 2 to 100V, while capacitances run from 3.3 to 2700μF. Like the layered polymer capacitors, the wound style has extremely low ESR values - to below 5mΩ. The wound style devices can also be surface mounted, though they are not quite as compact as layered devices.
Polymer tantalum capacitors employ a conductive polymer as the electrolyte and have a tantalum anode. They span voltages from 2 to 35V and capacitances from 2.7 to 1500μF and again feature low ESR values down to 5mΩ. Packaged in a molded resin case, these devices are amongst the most compact on the market.
Polymer hybrid aluminum capacitors. As their name suggests, these capacitors use a combination of a liquid and conductive polymer to serve as the electrolyte and aluminum as the anode. 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. The hybrid capacitors offer a voltage range from 25 to 80V and capacitances between 10 and 330μF. At 20 to 120mΩ, ESR values for hybrids are higher than other types of polymer capacitors, but still very low considering the higher power applications they address.
Polymer Capacitor Advantages
The four types of polymer capacitors share a collection of desirable electrical properties:
• Great frequency characteristics. Thanks to their ultra low ESR values, polymer capacitors exhibit low impedance near their resonance point. And lower impedance reduces AC ripple in power circuits. Testing has revealed as much as a fivefold reduction in peak-to-peak voltage changes when compared to conventional low-ESR tantalum capacitors.
• Stable capacitance. With ceramic capacitors, 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. Polymer capacitors have a self-healing capability that eliminates this failure mode. Hybrid capacitors feature an additional self-healing mechanism because the liquid electrolyte causes current flow near the defect to reoxidize the aluminum. This feature has important design and cost implications. Conventional tantalum capacitors are normally derated in use by 30 to 50% their labeled voltage to ensure operational safely which results in an upsizing of capacitors and increased cost. In contrast, Panasonic guarantees operation at 100% of the full-rated voltage for its polymer capacitors.
Hybrid Capacitor Performance Advantages
Hybrid capacitors are known for 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. Among the advantages:
Compactness: Surface mount hybrid capacitors measuring just 6.3 x 5.8 mm can handle 35V and offer a capacitance of 47μF. This small size can result in significant PCB space saving: in a recent 48V PSU application, hybrid capacitors occupied just 13% of the board space required by aluminum electrolytic capacitors.
Reliability. Capacitors cannot just 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 hands down. To take a few examples, hybrid capacitors have significantly better endurance and humidity resistance than either their electrolytic or polymer counterparts. Hybrids also have significantly higher tolerance for large ripple currents, inrush currents and elevated temperature.
These size and reliability benefits produce a strong system cost case for using hybrid capacitors, despite higher upfront prices. The higher ripple current specification alone can result in a 20% reduction in cost by increasing the life cycle of the capacitor. In the 48V power supply application we just mentioned, the hybrid capacitors had a total cost 50% lower than the equivalent aluminium electrolytic capacitors, with the savings coming from reductions in board cost, warranty cost and ability to withstand high ripple current.
For further information please check: https://eu.industrial.panasonic.com/