ENGINEERING FEATURE

Alternative Memory Backup via Super Capacitor Technologies

Supercapacitors, often abbreviated to supercaps, have made substantial inroads into power backup designs in battery-powered electronic applications. Supercaps (known also as ultra, electrochemical or double layer capacitors) represent an intermediate stage between conventional capacitors and battery technologies with their low energy but very high power densities. They deliver high peak-pulse power and offer the promise of more efficient battery circuits. This Arrow Innovation article looks at some of the available capacitor technologies using examples from Arrow suppliers, AVX, Cooper Bussmann and Panasonic, in particular.

Enhancing the Power Capability of Primary Batteries

Military uses drove the supercap development, as for so many other advanced technologies, for applications such as battery replacement and battery-power enhancement in the starting engines of armoured vehicles and tanks, in missile guidance systems and mobile communications sets.

Battery power capacity is a limiting factor when an application demands increased current from it at a constant voltage drop across the battery terminals. Designers must connect sufficient cells in parallel to meet anticipated needs, but this increases the size and weight of both battery and finished product and also increases costs.

If the requirement for high power is of short duration only, other approaches are possible. One alternative is using a high-power rechargeable battery charged by a low-power primary cell; the other is a supercap, in many cases delivering a superior solution.

Applications

Applications relying on supercaps are in the home, office and mobile office environments, including computers and peripherals, telephones, DSL modems and routers, digital radios, set top boxes, white goods and kitchen appliances, copiers, utility meters and HVAC control units. Network cards, for connecting PCs, many printers and laptops wirelessly to office and domestic LAN or WAN networks or WiFi points usually incorporate PCMIA modem cards. High speed remote data access presents increased current demands to the card; GSM/GPRS or 3G transmission requires a current of approximately 2A for the pulse duration. The PCMCIA bus alone cannot supply this level of pulsed current, a relatively large capacitance supplies the pulse current to the transmitter and recharges at low current during the interval between pulses.

Technological advances assisted the supercaps’ increasing popularity, such as the availability of smaller form factors, varying packages and lead types and higher capacitance – all available at competitive cost.

Most supercapacitor technology uses a phenomenon of an electric double layer, exploiting the electricity stored when a solid and liquid come into contact. Specifically, the supercap structure comprises two electrodes separated by a semi-permeable membrane in an electrolyte solution. Differences between manufacturers’ products are partly visible in the different media used for this interface.

In the BestCap® from AVX, a proton polymer membrane is used. There are two possible limitations with supercaps, high ESR and high capacitance loss when supplying very short duration current pulses, that BestCap successfully addresses.


Figure 1: AVX BestCap Construction

AVX’s BestCap technology, based on a unique, patented aqueous chemistry and an innovative design, provides excellent high power-pulse characteristics and extremely low leakage current. This series offers high capacitance, even with short pulse applications such as in GSM, GPRS, Edge and PCS based systems. BestCap technology also offers more efficient energy savings for battery circuits, its low ESR results in a high current-handling capability, making this an ideal solution for any portable or wireless device requiring high power availability.

Figure 2 illustrates the severe capacitance loss experienced by several varieties of supercaps, under short pulse width conditions. It also demonstrates how well BestCap retains its capacitance with reducing pulse widths. The characteristic of an equivalent value aluminium electrolytic capacitor, many times the volume of the BestCap, is shown for comparison purposes.


Figure 2: Capacitance Loss for Common Capacitor Types under Short Pulse Width Conditions versus BestCap

Low profile versions are ideally suited to PCMCIA, PDA, DSC and similar applications. Case configurations include three solder-in lead styles, through hole, L-style, planar mount, case only and three case sizes, low profile, low ESR, non-polar, non-organic with a PCB standoff version, insulated body, etc.

The BestCap® is a low profile device from 2.1mm to 6.8mm, available in three case sizes. Capacitance range is from 15mF to 560mF and includes six voltage ratings from 3.6V to 12V.

Cooper Bussmann supplies PowerStor ultra-high capacitance devices it calls ‘aerogel’ capacitors, using an unusual type of carbon foam known as carbon aerogel. The Cooper PowerStor KR Series combines a voltage rating of 5.5V with high capacitance by volume resulting in high-energy density levels in excess of 10 Joules/cm³. This high-energy storage capability, combined with over ten years of operating life, equips the KR Series to maintain supply integrity long enough to complete or end functions in a controlled way when the mains fails or when removing batteries for replacement.


Figure 3: Cooper Bussmann Supercaps

Figure 3 compares the voltage drop in a simulated camera zoom motor system. The system was discharged at 4W for 3s every 3m. The top trace shows the result for two AA alkaline batteries and the bottom trace for the same batteries in parallel with a single 6F supercapacitor. Although the drop is only slightly less, the combination circuit ran three times longer.

Exceptionally low leakage current makes the series ideal for prolonging the operating time of battery-powered equipment, which needs minimised loading. Consumer, office and telecom equipment backup applications are the application targets.

The KR series is available in three industry-standard mechanical packages, cylindrical style, horizontal and vertical mount, each with six capacitance values from 0.1F to 1.5F. Their construction facilitates small low profile packages, 11.5mm to 21.5mm diameter and 5mm or 7.5mm thickness. All are ROHS compliant.

Panasonic has supplied electric double layer, Gold Capacitors, since 1978. Developed by the Panasonic Central Research Laboratory in 1972 they were introduced to the market commercially several years later. Performing like a battery, these capacitors were ideally suited to applications requiring a secondary power source such as backup energy for microprocessors (e.g. real time clock) and energy storage for chargers or solar batteries.


Figure 4: Panasonic’s EP Series Small Electric Double Layer Capacitors

Panasonic sees its Gold Capacitors as ideal replacements for secondary batteries. Their main advantages against a battery are that a quick charge and discharge is available; there is no charge or discharge cycle limitation; it is a green product, RoHS compliant, using no hazardous substances like lead or cadmium; there is therefore no recycling restriction. No charging protection circuit needed, circuit design is easy and they are maintenance free and very long life.

Panasonic Gold Capacitors are available up to 5.5V and up to 70F. Three different packages (coin, stacked coin, radial leaded) are available; design depends on backup current and soldering conditions (flow or reflow). For higher ambient temperature applications, Panasonic offers a +85°C product.

The SMD Gold Capacitor EP series was introduced first in 2002 for reflow soldering. This EP series is possibly the world’s smallest electric double layer capacitor. The size code is 311, 3.8mm diameter with a 1.1mm height excluding terminals. This series reduced in volume by 70% from the previous EN (size code 614) series. Owing to its small size, the EP series is suitable for all small mobile devices, e.g. mobile phones, keyless entry systems, thermo-control units or PDA whose advanced functions require memory backup and clock function backup power.

The rated voltages are 2.6V and 3.3V, 0.033F capacitance, and internal resistance of under 350Ω. The temperature tolerance is -10°C to +60°C and the EP series is suitable for high-temperature lead-free reflow soldering at a peak of +260°C.

Despite all their claims and clear benefits, it is always a careful balancing act to select the right technology, whether battery or supercap, for an application. Designers still need to identify and select suitable battery technologies for their applications. This is where Arrow can help. Not only does Arrow carry an extensive range of products, in a variety of case types, underlying technologies and characteristics, it also offers the support of field application engineers who can advise and guide designers to the most suitable suppliers and supplies depending on the needs of the application.

For further information regarding any of the featured products, please click here