Capacitor dielectrics each have their advantages and disadvantages. Aluminium electrolytics offer high capacitance in a small space but are notorious for limited operating life, particularly in harsh environments, due to their wet electrolyte and the ongoing chemical reaction needed to maintain the aluminium oxide dielectric layer. Ceramic capacitors are noisy in acoustic applications and prone to short circuit failure when exposed to over-voltage conditions.
Plastic film capacitors (polyester, polycarbonate, polypropylene and polystyrene) have a wonderful ‘self healing’ characteristic that makes them robust, particularly in response to voltage spikes, but their lower dielectric constants mean that they’re bulkier, at a time when most electronic devices are getting smaller. Tantalum electrolytics pack a lot of capacitance into a small space but they’re relatively expensive and when they fail the results can be explosive, creating a potential fire risk. Capacitors with impregnated paper dielectrics are expensive and big, but robust, so are still found occasionally in high voltage applications.
Just to confuse the situation, the plastic dielectrics mentioned earlier are all types of polymer but the ‘polymer capacitor’, first introduced in 1983, is an altogether different kind of component. Polymer electrolytic capacitors employ a conductive polymer as a solid electrolyte that forms the component’s cathode, rather than as an insulating dielectric layer, which is how it’s used in conventional plastic film capacitors.
The primary types of polymer capacitor are polymer tantalum, polymer metal can, polymer hybrid and polymer aluminium multilayer, and the market for these devices is predicted to grow 30% to 40% in the next five years.
The benefits of polymer capacitors:
The reasons for their popularity vary with the application but in general they offer high capacitance in small packages, long life, high reliability, and a host of other desirable electrical characteristics including:
* Good high frequency performance, including low impedance and ESR (Figure 1)
* High ripple current capability
* Stable operation with respect to changes in temperature and bias voltage (Figure 2)
* Elimination of the acoustic noise created by ceramic capacitors (piezo effect)
* Wide operating temperature range, typically from -55 to +105 or +125 degrees C or more
Figure 1: Murata’s ECAS multilayer polymer capacitors offer lower ESR than other types of polymer capacitor – only multilayer ceramic capacitors are better in this respect
Figure 2: Polymer capacitors are far more stable than multilayer ceramic components, both with respect to bias voltage and temperature variations
Overview of polymer capacitor types:
Polymer tantalum capacitors offer significant cost savings over traditional tantalum capacitors and don’t suffer from the ignition risk that the latter present. They have lower voltage de-rating yet still provide good volumetric efficiency, coupled with low ESR and ESL, so ripple current ratings are high too. The capacitance range is 0.47 to 1500µF at rated voltages from 2.5 to 125Vdc. The ESR can be as low as 5mOhms and ripple current ratings can be 4A or more for some devices.
Polymer metal can capacitors come in values from 3.3 to 4700µF with voltage ratings from 2.5 to 125Vdc. They have lower capacitance per unit volume and higher leakage current than conventional aluminium electrolytic capacitors but this is offset by an ability to withstand high ripple current, low ESR and much longer operating life.
Polymer hybrid capacitors are designed to combine the advantages of aluminium electrolytics with conductive polymer components, primarily the low leakage current of the first with the long operating life, low ESR (down to 14mOhms) and high ripple current capability (up to 4.34A) of the second. They’re sometimes called Polymer Lytic components. Capacitance values range from 6.8 to 1000µF and voltage ratings from 6.3 to 125Vdc.
Polymer aluminium multilayer capacitors are low voltage devices, rated from 2 to 25Vdc, and are available in values from 6.8 to 1000µF. ESR is very low – down to 2mOhms - and ripple currents can exceed 7A in some types.
Avnet Abacus represents the world’s leading manufacturers of polymer capacitors including AVX (Nichicon), Kemet (NEC Tokin), Murata, NIC, Panasonic (Sanyo), Rubycon and Vishay (Holystone).
For more free applications advice or to request samples, get in touch with one of our product specialists in your local language.
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