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Power Supplies

Power when it’s needed T

Super capacitors are increasingly being used to meet peak power requirements in portable devices, as Markus Huschens explains

he trend towards small, well designed and feature rich consumer electronics devices is driving

innovation across the whole electronics industry. But needless to say, the design challenges required of the power source appear to be coming back to basic physics. The battery to power a portable device is chosen based on the functionality it has to support. Whether it’s a 300 mAh, 600 mAh or even a 1,000 mAh there is a limit to the maximum current that a single battery can provide. This has the impact of potentially limiting the feature specification of the device being designed. Of course there are solutions to resolve that, but they are neither easy or elegant and tend to require limited space. But surely, that isn’t something you want in a portable device. You only need higher power or sometimes a burst of power for staring an engine for example. Or is it? One recent innovative example is the use of LED flash on smartphones or in compact digital cameras in place of a Xenon flash. Then the required current cannot be supplied by the battery alone, simply because you need a higher current than the battery can provide. If, for example, you chose two LEDs to provide a flash capability on a mobile phone you would need approximately 2 to 4 Amps for a time of about 33-40 milli-seconds. A conventional battery cannot do that, so a designer then has to decide, how to power the flash. An option might be to use two batteries. But nobody does it that way because the batteries are quite expensive and that would considerably increase the form factor and may be a bit over-design for a peak-current assist to a set function. So what is the solution? Recent advances in capacitor technology might provide a means of obtaining that peak power source. Super capacitors, or electrical double-layer capacitors (EDLC) are essentially energy storage devices. As compared to conventional or advanced batteries, an EDLC has a high power density and a far longer cycle life. When first introduced, such devices were used

mostly for low power applications such as memory backup but recent electrochemical and packaging innovations have significantly improved the power capability at the same time as reducing the package size.

An example device is Murata’s EDLED (electrical double-layer energy device). Providing 2.75 volts output, it has a flexible discharge ability of 500uAh to 2As and measures only 18.5 x 20.5 mm. Most importantly such a device has a very low equivalent series resistance (ESR) making super capacitors very attractive for specific power assist functions in small form factors. Typically an EDLC uses the phenomenon of storing an electrical charge in a double layer that occurs when a solid and a liquid come into contact. Figure 1 shows how an EDLC is made up of a separator, electrolyte, activated carbon and current collector. Almost always activated carbon is used as the electrode, and because of its extremely high specific surface area and relatively low cost, ultra high capacitances are possible within a small package size. Also, because there are no chemical reactions taking place, an EDLC can achieve a very high cycle life.

Figure 3:Definition of the target

Discharge profile Figure 3 shows the discharge profile using a constant current model. Using the Murata EDLED device SingleCell type as an example, it allows a peak operating voltage of 2.75V. 5V for popular requirements can be addressed by the DualCell EDLED device. For an application requiring a higher operating voltage it is necessary to use multiple devices in series. However, this results in an increase in ESR. Alternatively, by putting cells into parallel the capacitance can be increased and this has the added advantage of lowering ESR. EDLCs will find many more applications. One interesting potential application might be with POS terminals, the type we increasingly use to pay our bill in a restaurant. These portable devices require charging up to at least three times a day depending on workload. In the sense of the overall storage of energy, the batteries used in these appliances do not meet the

energy profile expectations. By using EDLCs it might be possible to reach more power efficient designs with less battery recharging To power other portable devices, in the past mainly the Li-Ion battery technology could follow the increasing demand for high currents, but it might not keep track with the present speed of additional functionality and requirements for peak current demands. So microfuelcells and other innovative energy storage technologies will become more popular in the future. For the dynamic peak power assist the SuperCapacitor technology is available now and is helping solve these problems in competitive and attractive small form factors.

Murata Electronics |

Markus Huschens is New Business Research Manager at Murata Electronics Europe

Figure 2:Equivalent circuit of an EDLC

Having determined that an EDLC might be able to supply enough energy for a burst of power, the crucial decision for an engineer is to estimate how long is the burst of power required for. Up to 500ms, you could use one or more EDLCs to achieve the demand. Going into detail for your design, determining the exact capacitance of the EDLC, and the number of cells required depends on a number of factors. These factors include the maximum and minimum operating voltage of your application, the average current, the peak current, environmental operating temperature, the time that the peak is required for and, finally, the expected life time of the application.

Figure1:Construction of an EDLC 34 July/August 2011 Components in Electronics

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