www.we-online.com SUPERCAPACITORS 27 www.power-mag.com Issue 1 2024 Power Electronics Europe Characteristics that boost a supercapacitor’s power There are many uses for supercapacitors beyond emergency power supplies, explains Dr René Kalbitz, Product Manager Capacitors & Resistors Division at Würth Elektronik eiSos GmbH Supercapacitors feature very high charge storage capacity, long service life, short charging times, and fast power delivery. One application is emergency power supply units, but they can also be used for hot-swap and hybrid applications, for example, to support batteries during power peaks. Figure 1 illustrates the typical circuit for such applications. Supercapacitors are energy storage devices, comparable to batteries in many respects. They can be charged from any current limiting energy source and drive electrical applications. Supercapacitors, like any other energy storage system, require a certain technical structure to store and supply energy, including a circuit for charging the supercapacitor under real conditions and an electronic application to be operated. Characteristics When it comes to charging and discharging, two characteristics of supercapacitors need to be considered. Firstly, unlike batteries, the voltage depends on the charged state. The voltage at the component rises or falls as soon as the supercapacitor is charged or discharged. This property is unfavourable for the discharging process, because electronic applications need a constant working voltage. Secondly, supercapacitors can be charged with relatively high currents, which might lead to a semi short-circuit condition for the power supply at the moment of switching on. Although the design-in process for supercapacitors may differ from case to case, the basic procedure is always as follows: The required energy capacity is first calculated on the basis of the expected energy requirement. Determination of the required capacitance (C) depends on the specification of the load, as well as the efficiency of the DC/DC converter, its lowest operating voltage and the charging voltage. The maximum power output is limited by the equivalent series resistance (RESR) and any other resistance (Rp) caused by contacts or intentionally introduced for protection. The charging regime is then determined, and the corresponding charging times are calculated. In the case of constant voltage charging, a protective resistor is selected depending on the specification of the charger. Charging with constant current is more common, however, and has the advantage of shorter charging times. The following example shows how supercapacitors - in this case electric double layer capacitors (EDLC) - can be used as a backup power source. In this scenario, both the actual current source and the application operate at higher voltages than the supercapacitor rated voltage. A step-down converter is used to charge the supercapacitors and a step-up converter to supply the test application. A wireless power transfer (WPT) is used for the application with a simple LED panel as the load. For the following measurements of the voltage and current characteristics during the charging and discharging process, the step-down converter (buck) and the stepup converter (boost) were separated from the supercapacitor. The aim is to operate the application with a power consumption of about P = 0.8W (including conversion losses) for about t = 5 min. A total energy of around E = P • t = 0.8W • 300s = 240J = 0.067Wh is therefore needed. As the converter used has a set charge-cutoff voltage of 2.7V, the capacitance must be at least: Where V1 = fully charged voltage, V2 = lowest useful voltage In the typical circuit, two capacitors are charged in parallel, each with a capacitance of 50F. The total capacitance of the Figure 1: EDLC supercapacitors can be used as a backup power source. <#`#*#! " #g! "#W#g" "# #`#*#! *,+#d \*Kf#g]"W\X#g]"
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