Tantalum Capacitors in Power Supply Applications

The capacitance range for tantalum capacitors spans from singular microfarads (uFd) through a thousand microfarads. It is an electrolytic type of capacitor that can utilize either a liquid type of electrolyte or a solid state material as the cathode connection. The anode construction utilizes the high porosity of the pressed tantalum material to create an enormous amount of surface area to produce the highest volumetric efficiency of any of the commercial capacitors available. In the solid state devices, these can be readily packaged in a molded plastic package that yields a mechanically consistent surface mount package for high speed automated board processes. Circuitry that utilizes some of the newer switch mode power supply designs (SMPS), requires capacitance values for their filter circuits that lie well within the capacitance range of tantalum capacitors. With the volumetric efficiency, capacitance range and surface mount packaging, the tantalum capacitor has become a standard device for this application. certed efforts as well as through a natural refinement of materials, processes, and designs. For the standard commercial product, the three ohms per volt recommendation was soon dropped to one ohm per volt, and the standard line today is 0.1 ohm per volt. New types of capacitors utilizing design, process and materials specifically to achieve a lower inherent resistance for the power supply application is fairly common among all the manufacturers. With resistance as the target, earlier design changes have dropped the resistive element by 50%. Some of the most recent designs offer another 50%, or even higher degrees of improvement. For these capacitors, there is no suggested series circuit resistance — they don’t need any. How these designs differ from the standard design is detailed in this report. Concentrated Effort on ESR For the power supply applications, the capacitor must act as an energy source for part of the time. With a pulse modulated switching power supply, the output is fed directly from the input source for only a fraction of the total time. During the period the switch is open and disconnects the input current from the load, the capacitor must act as the energy source. The voltage cannot be exactly the same level, during this period as the capacitor is losing charge and the Tantalum History in Power Applications The history of these capacitors in this application is not entirely without failures. Early versions of the surface mount devices were not intended for power supply applications. The “normal” recommendation for these devices included that the circuit offer at least 3 ohms per volt as a series resistance, based on the voltage rating. With the filter circuit requirements in the power supply, this recommendation was either challenged with development efforts of the users and manufacturers, or was totally ignored. This device in the surface mount package was a large enough temptation to result in many early attempts to use the tantalum capacitor in power filter applications, regardless of the recommendations. Most of the results may have been successful, but the failures generated a lot of bad press for this capacitor. This series resistance requirement was not only a manifestation of the inherent parasitic resistance of the capacitor; but also the inconsistency with internal interconnects. All capacitors are not perfect and this parasitic resistive element is mostly the result of using imperfect conductor materials used in the plate elements of the capacitor. The manufacturers of the tantalum capacitors have responded to this application need through con-