There are mainly two types of filter capacitors in battery charging circuits: input filter capacitor and output filter capacitor. The AC voltage across the step-down transformer is rectified and often filtered using capacitors to obtain a regulated DC voltage through a voltage regulator chip to charge the battery.
The design of a battery charging circuit with or without filter capacitor depends on many factors including the type of battery and the charging method used. One must follow the safety precautions and have basic knowledge about the batteries being charged. The most common battery charger circuit is the constant-voltage battery charger.
The input capacitor ensures that the voltage regulator chip connected across the transformer is stable and does not oscillate. In a simple battery charging circuit, the battery is directly connected across the output capacitor of the voltage regulator chip. In most cases additional filter capacitors are not required in such circuits.
There are two popular types of electrolytic capacitors: so-called aluminum electrolytics and tantalums. Aluminum electrolytics (AE) employ an ultra-thin dielectric composed of aluminum oxide deposited on a thin, etched aluminum foil. The etched surface increases the dielectric’s surface area, increasing CV density significantly.
Tantalum and Tantalum Polymer capacitors are suitable for energy storage applications because they are very efficient in achieving high CV. For example, for case sizes ranging from EIA 1206 (3.2mm x 1.6mm) to an EIA 2924 (7.3mm x 6.1mm), it is quite easy to achieve capacitance ratings from 100μF to 2.2mF, respectively.
Historically, electrolytic capacitors have been the most popular choice for SMPS filters, especially input filters. They offer very high CV density per package size typically at comparably low cost. The problem is that the high CV density comes at a rather high price in terms of technical disadvantages.