3 - Power conditioning for notebook and palmtop systems

Abstract

Palmtop computer power supply designs present an entirely separate set of problems from notebook computers. Notebook machines typically use a 9–15 V NiCad stack for the power source. Palmtop machines, due to their extremely small size, have room for only two or four AA cells. The palmtop machines require much longer operating time in sleep mode, since they presently do not have disk drives. They also have lithium batteries for backup power when the AA cells are dead or being replaced. The use of these disposable batteries generates a separate set of problems from notebook computers. Unlike power supply systems powered by rechargeable NiCad or NiMH batteries, high efficiency power converter circuits are not necessarily optimum for use with disposable batteries. Since rechargeable batteries have very low output impedance, the most efficient converter circuits result in maximum operating time. Disposable cells, on the other hand, have relatively high internal impedance, which results in maximum battery life when the battery load is low and relatively constant. Power supply converters that minimize both the loss in the converter circuit and the effect of battery internal resistance give longest system operating life. Some of the four-cell designs presented in this chapter are optimized for low peak battery current to lengthen the disposable battery life. Also, the systems shown here provide power conditioning with high efficiency and low part count.