Supercaps range from vastly larger high voltage/ high-farad arrays to small, low-profile prismatic pulse devices
BY CHRIS REYNOLDS
AVX, Myrtle Beach, SC
http://www.avx.com
Supercapacitor technology has come of age. In less than 10 years, these high-charge electrochemical devices have evolved in two directions from the large, low-voltage cylindrical devices originally designed for dc applications such as voltage hold-up for clocks in microwave ovens and VCRs. At one end of the scale, vastly larger high voltage/ high-farad arrays for hybrid vehicles are now available, and in the other direction a new series of small, low-profile prismatic pulse supercaps have been developed.
The new generation of pulse supercapacitors are characterized by very low ESR (equivalent series resistance) enabling them to supply peak current on demand while trickle charging from primary Li-ion or standard AA or AAA batteries. Their low-profile design enables deployment on small CCAs (PCBs) where they are ideally suited to increasingly power hungry applications such as wireless cards and high-density data transfer in portable devices.
In these applications, peak energies are required that would need double the current available from the primary battery to allow fast transmission of data, or digital signal processing of the megabytes of data in digital SLR cameras and video applications, where flash/strobe functions vie with signal processing of burst-rate exposures and writing to memory of multiple files per second.
Storing huge amounts of charge
Supercapacitors, as implied by their name, can store huge amounts of charge.
Standard capacitors do this by having a dielectric material between the capacitor plates that can be polarized on the application of an electric field. As the internal dipoles align within the dielectric, an electric field is established as measured by the voltage at the plates. The more charge the plates can hold, the higher the capacitance, and the energy stored (in joules) is equal to 0.5(C x V2 ), where C is the capacitance (in farads) and V is the the plate voltage.
Supercapacitors achieve the same result, but by bulk separation and movement of charges, rather than dielectric dipole alignment. The mechanism for moving opposite charges to different sides of a separator is electrochemical in nature and very similar to battery technology. How long the energy will be stored in either standard or supercapacitors will depend on the internal leakage current (as dipoles relax or charges re-combine). How fast the stored energy can be released will depend on the internal resistance of the device.
Traditional materials
Standard capacitor technologies are investing in materials development to improve dielectric constant, dielectric leakage, internal resistance and voltage capability. Likewise with supercapacitors; the original products available were based on high resistance electrochemical systems, giving “battery-like” storage and discharge characteristics, but new material developments have enabled the development of low-ESR devices ideal for pulse applications.
When we calculate the energy stored from dipole alignment in a standard capacitor, we are looking at a purely dc function. But most applications require the capacitor to pass a signal, which means having the plates linked by an ac voltage. The question then becomes how well can the dipole oscillations keep up with the incoming signal frequency to pass it on without distortion? This is what differentiates standard capacitor types and suits them to different applications.
For example, tantalum will respond well in the 100-kHz to 1-MHz range, with bulk capacitance values to 2,200 µF at 6 V, with ESR
Ceramic type II material is also suited to this range, with lower bulk capacitance, but lower ESR (~100 µF / 5 mΩ). Meanwhile, Class I dielectrics will have far higher operating frequencies for RF applications, with single layer devices approaching 10-GHz response for optical systems.
New technology, new applications
Similarly, supercapacitor technologies are evolving to enable a wider range of applications. All have benefited from nano-particle technology (development of high surface area carbon layers), but one of the most exciting developments in recent years has been the introduction of “proton polymer” technology for the separator system. This technology has the following benefits:
• High dc capacitance in the 50mF – 1F range.
• High-capacitance retention at millisecond pulse intervals.
• A wide range of voltage ratings from 3.6 to 15 V (and beyond).
• Low ESR (20 to 300 mΩ).
• Low leakage current (2 to 5-µA range).
• Long cycle life. Deep charge-discharge tests of up to 10 million cycles (or 8 months of nonstop testing) do not show any significant effect on these capacitors.
The packaging of pulse supercapacitors is also evolving, with the emphasis on small footprint and low profile. For example, AVX’s BestCap Series was originally introduced more than eight years ago with a standard 28 x 17-mm outline and a larger 48 x 30-mm version. These had height profiles ranging from 2.0 to 6.0 mm.
Now, an even smaller outline is available (20 x 15 mm) with a still-smaller version in development (15 x 12 mm). The construction is extremely robust, having a precision steel outer body and the internal element epoxy sealed in place.
The internal element is built with environmentally friendly, solvent-free, aqueous materials with multiple cells making a homogenous matrix. Because of the solid casing and homogenous internal element, parts can withstand in excess of 1,000-g shock/acceleration, while the internal element has an operating temperature range from –40° to 75°C.
Fig. 1. Actual capacitance vs. pulse width.
A supercapacitor’s electrical characteristics (see Fig. 1 ), combined with a low-profile format ideally suit them for digital wireless applications. One such consumer application is the wireless card, either in PCMCIA or USB configurations where supercapacitors provide the necessary pulse energy to support the current-on-demand required for GPRS and EDGE transmissions while being trickle-charged from a notebook or PDA Li-ion battery. Because they can support these pulses over a wider operating range than primary batteries (and extend battery life by 200% to 300%), they can improve efficiency in many devices and there are a number of heavy-duty wireless card applications that benefit as a result, one being remote optical scanners.
An important factor in wireless transmission is the effective capacitance at the transmission pulse rate. Many supercapacitors with high dc capacitance suffer from limited effective capacitance retention as transmission pulse rates and/or duty cycles increase.
The proton polymer system used by some supercapacitors provide a high effective capacitance, meaning that high duty cycles (for example, GPRS-8 to GPRS-10) can be supported with a lower dc capacitance rating, which in turn requires a lower trickle-charging energy budget for the device — a very important factor in turn-on time for some handheld devices. The range of rated voltages (from 3.6 to 5.5 V) for these applications means that a pulse supercapacitor can be used across the GSM chip (3.5 V), across the Li-ion battery (4.5 V) or at the dc/dc output (5.5 V). The 5.5-V deployment allows for holdup of additional circuitry, or elimination of other sub-circuitry (LDOs, etc.).
Other energy-demanding applications include high-end digital cameras, which have a high-energy budget due to all the ancillary systems (zoom, focus and processor support during caching of large data files during multiple burst operation), but also require instantaneous power to fire the flash when required, without draining the other applications.
As portable devices, cameras can be used in quite hostile environments wherever datalogging, inventory control, or package monitoring is required, which often includes a wide temperature range and shock or vibration. In cases where the device can be dropped while in operation, the prime concern is not just the survivability of the instrument, but whether any data dropout will occur. In battery-only operated devices, the resultant “battery-chatter” can mean loss of critical data; having a supercapacitor hard-soldered in place ensures that no discontinuity in operation as a result of severe shock.
This level of robustness and electrical characteristics mean that they can support many industrial-grade battery-assisted applications where pulse power current on demand is required, from wireless transmission to electromechanical applications. These include remote installation and wireless control of automated valves, automatic metering systems, remote RFID readers and remote security systems. In many of these systems the primary cell may not be a battery, as it could also be a solar cell.
Another emerging application is batteryless operation. While supercapacitors operate as a secondary device in most systems, if only a trickle charge is required then sometimes a primary (or solar) cell is not needed; there are a number of ways that movement or vibration can be used to generate energy, from piezo to inductive devices. Energy harvesting systems are now available that can be deployed on any mechanically vibrating system and can provide voltage output to a storage device. In these applications, proton polymer supercapacitors have another advantage: higher voltage ratings (from 7 to 15 V) are available as discrete devices and unlike other types of supercapacitors in series configuration there is no need for the balancing resistors.
As discussed earlier, the energy stored in a capacitor is equal to 0.5(C x V2 ). As this is a function of the square of the voltage, in any system designed to store energy doing so at higher voltage results in much greater efficiency. The capacitor in these systems has a dual function: (1) to store the harvested energy and also use it to provide current for any remote device that is being supported; and (2) In the case of wireless devices, high-capacitance retention at high duty cycles is another way the proton polymer technology adds to the efficiency of the system.
Pulse supercapactors have come a long way since their introduction less than 10 years ago and are now used in a wide variety of applications — from consumer products to remote industrial systems. In next 10 years there is very reason to expect that pulse supercapacitors will become as ubiquitous as dielectric types. ■
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