Cell-phone battery charger miniaturization

A fast off-line travel battery charger for cellular telephones is typically as large, or larger than some cellular telephones. This paper presents a miniaturized off-line travel battery charger for cellular phones. The aim of the work was to explore the theoretical and practical limitations of reducing the size of such a battery charger. The difficulty in miniaturizing such a charger arises from the wide input line (85-265 Vac) and load range (0-7.5 V, 0-880 mA) that is required for operation. This makes it difficult to design a highly efficient power conversion circuit for this power (10 W) level. A thermal study of the limitations on the maximum power dissipation allowable provided a lower boundary limit on efficiency for a feasible solution. A topology such as an LLC resonant converter can operate efficiently at high frequencies, and its use of parasitics reduces the need for extra components. New components with the appropriate form factor for denser packaging were used. A special thermal management scheme inside the box was required to keep component temperatures within specifications. A prototype with a volume of 26 cm/sup 3/ (existing solution 58 cm/sup 3/, for 7 V and 700 mA) was built and tested to assess the validity of the preliminary studies, to provide insight into potential practical problems, and to determine the feasibility of a 14 cm/sup 3/ charger.