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The need for portability in devices is ever increasing. And portable devices require a means of storing energy for powering applications. Energy storage is typically limited to alkaline or rechargeable batteries. In either instance, the batteries require replacing or recharging. Applications using rechargeable batteries require a secondary set of charged batteries for when the primary set is being recharged. Therefore, in most typical applications, an investment in two rechargeable battery packs is the norm. But even this strategy is not foolproof when the user is waiting for a forgotten battery pack to recharge.
Portable devices have a wide range of energy-storage needs. Portable tools have high-power, high-energy needs often necessitating hours of run time between charges, whereas some children's toys only require a few seconds of operation between charges. In many applications, alternative energy-/power-sharing strategies may be employed using a technology called ultracapacitors.
Ultracapacitors have found their way into many design engineers' product development strategies for portable devices. These strategies include using batteries and ultracapacitors in parallel or complete battery replacement with ultracapacitors. Parallel strategies have been used to meet high-power/high-energy needs. Replacement strategies are often limited to applications that require only a few seconds of operation between recharging.
So what compels the design engineer to consider the use of ultracapacitors? Several features make ultracapacitors an attractive choice for product design. Ultracapacitors are typically designed for the life of a product. This means the ultracapacitor is a permanent feature of a product, which affords design engineers flexibility in locating the ultracapacitor within the product. Ultracapacitors contain no heavy metals and do not have the environmental concerns as some battery chemistries have. Most portable device applications are concerned with weight. Ultracapacitors are lightweight; a single D-cell ultracapacitor at 2.5 V weighs approximately 60 g, whereas a D-size battery is significantly heavier and depends on chemistry. In addition, for alkaline cells, the voltage is only 1.5 V, requiring additional weight for equivalent voltage. Ultracapacitors can be charged and discharged at extremely high rates compared to batteries. Conventional rechargeable batteries can take several hours to recharge. Newer “rapid-charging” batteries still require several minutes to an hour to be fully charged. An ultracapacitor can be recharged within a matter of seconds depending on the amount of current available for recharge.
Let's focus on how design engineers can take advantage of these attractive features and integrate ultracapacitors in their product designs. Examples will be provided for parallel operation with batteries and ultracapacitors, as well as battery replacement with ultracapacitors. The design engineer must first determine the utilization strategy. Will the best use of ultracapacitors be in a stand-alone system or in parallel with batteries? This decision is based on the run time required for the application between charges. An application with up to 30 sec of run time at peak power or with a few minutes at low power between charges may be a good candidate for ultracapacitors alone. Applications requiring several minutes of operation with peak loads or that only need to be charged periodically benefit most from parallel ultracapacitor/battery implementation. Short run-time applications with ultracapacitors alone are considered first.
Sole Power-Source Applications
A variety of applications are using, or may be suited for, ultracapacitors as a stand-alone power source, including toys, consumer power tools, rechargeable medical devices, flashlights, restaurant paging devices and remote solar-charged devices, to name a few. The first step in incorporating ultracapacitors into a device is determining the appropriate input power available to the ultracapacitor and the required output power from the ultracapacitor. An ultracapacitor is a dc device. More specifically, it is an alternating direct-current device with constantly changing voltage that is dependant on the input/output current. Available sources for charging ultracapacitors in the applications mentioned include batteries (a dc source), power supplies or electrical outlets (an ac source).
Applications such as toys typically use alkaline batteries as the charging source for the remotely powered toy. The ultracapacitor is permanently located within the toy for power during use. The advantage of locating an ultracapacitor within the toy for this application includes weight savings compared to batteries and an intentional desire to have a short run time for the devices, including rechargeable race cars and gliders. The charging stand will have batteries to enable remote recharging of the toy.
In this application, the batteries and ultracapacitors are series linked during the ultracapacitor recharge. Enough batteries are series stacked within the charging stand to provide the necessary voltage to power the end device. In this case, the ultracapacitor charging voltage is limited by the battery.
Alkaline batteries are the primary choice in this application due to the relative cost compared to other chemistries. The alkaline battery voltage is 1.5 V, whereas the ultracapacitor-rated voltage is typically 2.5 V. A common choice is to place two batteries in series to provide 3 Vdc as the charging power supply. This voltage exceeds the rated voltage of the ultracapacitor, thus it would shorten the life of the application if used directly coupled to the ultracapacitor. However, cost and the short design life for toys, this is typically the preferred method. The output of the ultracapacitor is coupled to a motor to power the toy. As the voltage of the ultracapacitor decays, the performance of the motor declines, until at some point the motor becomes inoperable. This example represents the most simplistic implementation of ultracapacitors into a portable device and is atypical. The schematic is shown in Fig. 1.
Other applications such as power tools, rechargeable medical devices and flashlights may require being directly plugged into an ac source or a separate dc source for recharging. Power tools may operate at voltage ranges between 9.6 V and 18 V, depending on the motor size and type of operation. For these and most other applications, additional power conditioning may be necessary on both the input and output of the ultracapacitors. Since the ultracapacitors are a dc device, it is necessary to rectify an ac line voltage. If the ultracapacitor is to be recharged from a dc source, this source may need either boost or buck regulation to provide the proper voltage. On the output of the ultracapacitor, additional dc-dc boost or buck conditioning may be required if the device to be powered needs a constant output voltage.
A variety of methods are available for charging the ultracapacitor from either an ac or dc source. The selection would be based on efficiency, cost and charge time, and would be determined by the design engineer for the specific application. Examples for charging ultracapacitors are provided depending on the source available. For a portable device, the charger system electronics are most likely located externally to the portable device.
Unlike batteries, ultracapacitors may be charged at the same rate that they are discharged. An ultracapacitor with zero charge may seem like a short circuit to the charging source because of the low impedance of the ultracapacitors. Most low-cost power supplies perceive the ultracapacitor as a short circuit and fold back the output current. The low-series inductance also makes stabilizing switch-mode chargers simple. Many off-the-shelf components such as linear regulators are available for charging ultracapacitors. These components have been designed to accommodate needs for battery systems and are often superior to passive charging networks, which are usually slow and inefficient. Therefore, active charging topology is preferred for faster, efficient charging if necessary for the application.
A dc-dc constant-current regulator is the simplest form of active charging. It can be operated as either a buck or boost regulator depending on the application. The buck regulator is the preferred topology for larger fast-charge, high-duty-cycle applications due to the continuous charge current. Ultracapacitor heating is proportional to current squared times the duty cycle. The application requirements would dictate the preferred methodology. A circuit example is provided in Fig. 2, representing a constant-current charging scenario. A simple constant-current charger may be built with standard power supply ICs. The current limit would be set to the required charge current, and the voltage limit would be set to the maximum required voltage.
When charge time is critical, constant power charging provides the fastest charge method. It can transfer all the available power from the charge source into the energy-storage capacitors. Drawing a constant current from the source at a constant voltage is a simple implementation of constant power charging. This usually requires a maximum switching current of 2.5 times the nominal to prevent overloading the switching circuitry when the ultracapacitor voltage is below 40% of maximum. A schematic for a circuit (patent pending) that performs this function is shown in Fig. 3.
If a device is to be directly charged from an ac source, it is often difficult to cover the wide dynamic range requirements for charging ultracapacitors from a varying ac power line. An example circuit is provided in Fig. 4 (U.S Patent 6,912,136) for ac charging scenarios. The circuit uses the L/V characteristics of the switching transformer to set the switching frequency. Full output current at 0 V on capacitor C1 is capable with this design with no risk of saturating the magnetics.
The switch Q1 turns on, charging the primary of T1 to a preset current limit. Q1 then turns off, permitting the energy stored in T1 to discharge through D1 into the ultracapacitor C1. When the secondary current has discharged to a preset lower limit, then Q1 will turn on again, repeating the cycle. The time required to charge T1 is inversely proportional to the instantaneous line voltage and the ultracapacitor voltage at C1.
The combination of low line voltage and low ultracapacitor voltage produces the lowest switching frequency. The highest switching frequency occurs when the ac power line is at maximum voltage and the full charge voltage is on the ultracapacitor. Depending on the application, the switching frequency can cover a range of 20-to-1. When C1 reaches its maximum voltage, the voltage sensor will drive the control circuit into discontinuous operation.
The output voltage of the ultra-capacitor to supply the required electronic application will vary as current is drawn from the capacitor module. As previously mentioned, if the application is voltage sensitive, a dc-dc boost converter stage may be added after the ultracapacitor, enabling a constant input voltage to the device. If the ultracapacitor module is allowed to operate between its full-rated voltage and half-rated voltage, the module can supply 75% of its available energy. The selection of an appropriate dc-dc converter is dependent on the operating voltage range of the system and the wattage requirement for the application.
Ultracapacitors Reduce Battery Constraints
Applications in which frequent recharging is not possible and peak loads are common may be suitable for a parallel battery/ultracapacitor configuration. Paralleling an ultracapacitor with a battery can significantly improve the run time of the device by prolonging the battery life or enabling lower cost batteries to be used rather than more advanced chemistries. Examples of applications benefiting from this arrangement include digital cameras and power tools.
When an ultracapacitor is placed in parallel with a battery, it is preferred to place a current limiting resistor or blocking diode between the battery and ultracapacitor. This prevents excessive currents between the two, allowing the ultracapacitor to serve the purpose of providing the peak power to the loads and preventing the ultracapacitor from drawing excessive current during recharge from the battery. In this arrangement, the ultracapacitor merely serves as a buffer for the battery.
In this example, all of the ultracapacitor charging is provided by the battery. If the battery is rechargeable, appropriate charger design for the battery is required for the specified chemistry. Once the peak load is removed (defined by the current-limiting resistor selected), the battery will take over as the primary energy source for the application. Therefore, dc-dc converters are only required on the input of the application electronics if the working voltage of the electronics is less than the battery voltage.
Designing ultracapacitors into portable devices need not add additional complexity to the overall product design. Determining the best methodology is defined by the application needs. How much run time is required? How frequent is charging allowed? What is the operating voltage range of the electronic components? What peak loads are required? Identifying these requirements will lead to the proper selection of the described arrangements for batteries and ultracapacitors, charging scenarios and output conditioning.