A 170 Volt Tantalum HybridTM Capacitor - Engineering Considerations

The electrical performance of Hybrid capacitors (US Patent 5,369,547) using high capacitance density electrochemical capacitor cathodes, electrolytic capacitor anodes and dielectric, and compatible electrolytes is strongly influenced by the nature of the electrolyte employed and the physical characteristics of the electrodes. Management of overall capacitor performance usually involves optimizing one set of material properties against another. This work describes our efforts in producing a tantalum Hybrid capacitor with a 170 V working voltage. INTRODUCTION The Evans Hybrid capacitor combines the best features of both electrochemical and electrolytic capacitors by using an electrochemical capacitor cathode and an electrolytic capacitor anode. Order-of-magnitude increases in volumetric energy density over aluminum electrolytic capacitors have been reported. Employing a dielectric coated anode electrode, the single-cell Hybrid capacitor is able to withstand high breakdown voltages. In contrast to electrochemical capacitors, where cell voltage is limited to the breakdown voltage of the electrolyte, the Hybrid capacitor cell voltage depends on the breakdown voltage of the anode dielectric. Units with >100 working volts/cell are now in use. Hybrid capacitors with porous sintered tantalum anode electrode(s), RuO2 on tantalum foil cathode electrode(s) and acid electrolyte were used in this work. These single-cell devices were sealed in conventional hermetic tantalum cases. A schematic drawing of the internal construction of these capacitors is shown in Figure 1. Figure 2. is a photograph of a one-anode element tantalum-cased tantalum Hybrid capacitor. The physical dimensions of this unit are 1.40 inch diameter by 0.2 inch high. The parts of a disassembled unit are shown in the photograph in Figure 3. The difficulty building reliable high voltage electrochemical capacitors lies in the required use of series-connected cells, because the voltage of an electrochemical capacitor cell is limited to the breakdown potential of the electrolyte, usually <1 volt for aqueous e lec t ro ly tes . I f the Presented to the Seventh International Seminar on Double Layer Capacitors and Similar Energy Storage Devices, December 8 10, 1997, Deerfield Beach, Florida electrical properties of all cells are identical, then the voltage in a series stack is evenly divided. Since normal process variables preclude this condition in a production scenario, cell voltage usually does not divide evenly. The situation can be overcome by reducing the average cell voltage, increasing the number of cells in series. As the number of cells in series rises, further reductions in the average cell voltage are mandated. The performance penalties paid include reduced capacitance, increased ESR and an increased cost. An advantage of the Hybrid capacitor is its ability to handle very highrate charge and discharge. These devices are now in use in pulse-discharge power supplies. In one example of this application, a Hybrid capacitor provides a 200 μs, 150 A discharge at a repetition rate of 50 Hz. With an RC product of just 0.0001 Ω·F, this capacitor is an efficient choice. In contrast, the RC product of most electrochemical capacitors is > 0.1. The RC product, found by multiplying a capacitor’s ESR by its capacitance is a quick way to estimate the likely performance of a device in an application. For example, if the RC product of a capacitor is 1, it is capable of efficient charge-discharge times on the order of 1 second or longer. The RC product is under the direct control of the design engineer. By changing the conductivity of the electrolyte or the thickness of the electrodes, one can influence capacitor Figure 3. Photograph of a dismantled tantalum Hybrid capacitor showing anode, cathode, and case elements. Figure 1. Schematic view showing internal construction of a multiple-anode element tantalum Hybrid capacitor. glass-metal seal