In DC applications, capacitors are essentially used as batteries for the storage and release of charge. For long-term charge storage, batteries are the obvious choice due to their low leakage; however, their high internal resistance limits the rate at which their charge can be given up, so high current demand applications often use a rechargeable battery to charge a capacitor downstream, which will then be able to supply charge on-demand to the load. Capacitors for this type of application will have high bulk capacitance with limited high frequency performance, which is characteristic of tantalum and aluminum electrolytics, as well as high dielectric constant (Class II) ceramic X7R and X5R technologies.
Pulse applications are an interesting class that can benefit from both dielectric and non-dielectric capacitor technologies. Supercapacitors, which employ electrochemistry similar to battery technology, are one example of non-dielectric charge storage devices. In this family, when a voltage is applied, charge is stored by the ionization of a medium, and the separated charges go to high-surface-area collectors. Typically, the medium is an organic acid whose ions carry high levels of charge, which enables high equivalent capacitance in the 1F-10F range.
However, the ions’ large molecular size affects their mobility, so they exhibit much slower response times to charge and discharge cycles than dielectric polarization. This delay manifests itself as a high intrinsic ESR in the range of ohms. However, there is a class of pulse supercapacitors that use proton polymer membrane technology. In this technology, protons, which have far more mobility than larger organic acid ions, are used as the charge carriers so that their charge/discharge characteristics in the circuit are closer to that of a bank of electrolytics rather than a standard supercapacitor.
In a repetitive pulse application (such as supplying a 2A pulse to enable a GPRS transmission), the purpose of a pulse supercapacitor is to maintain the voltage in the circuit when the load switches in. At the time of the initial current draw from the first pulse, there will be an instantaneous voltage drop across the load that’s proportional to the ESR of the capacitor. Then, the voltage will continue to decrease as the capacitor releases its charge.
The system will briefly be at an initial DC voltage, but will continue to transmit until the voltage drops below a critical cut-off value (the difference being the allowable voltage droop). Depending on the level of current needed, the pulse duration required, and allowable voltage droop in the system, a combination of high capacitance and low ESR can readily be identified to achieve this and a suitable capacitor selected for the application.
A system will be at an initial voltage and will operate until a minimum voltage level is reached. If the load has a pulse demand with energy supplied from the capacitor, then the capacitor must hold the operating voltage above the minimum value for the duration of the pulse. There will be an immediate voltage drop [ΔV(IR))] due to the ESR, then the voltage will decay [ΔV(Q)] in a time inversely proportional to the capacitance value. The combination of ESR and capacitance will determine the pulse holdup capability.