Here is an app note (PDF!) from Microchip describing the basics of different SMPS
The industry drive toward smaller, lighter and more efficient electronics has led to the development of the Switch Mode Power Supply (SMPS). There are several topologies commonly used to implement SMPS.
This application note, which is the first of a two-part series, explains the basics of different SMPS topologies. Applications of different topologies and their pros and cons are also discussed in detail. This application note will guide the user to select an appropriate topology for a given application, while providing useful information regarding selection of electrical and electronic components for a given SMPS design.