Power MOSFET Basics: Understanding Gate Charge and Using It To Assess Switching Performance

This is the second in a series of application notes that define the fundamental behavior of MOSFETs, both as standalone devices and as switching devices implemented in a switchmode power supply (SMPS). The first application note provided a basic description of the MOSFET and the terminology behind the device, including definitions and physical structure. This application note goes into more detail on the switching behavior of the MOSFET when used in a practical application circuit and attempts to enable the reader/designer to choose the right device for the application using the minimum available information from the datasheet. The note goes through several methods of assessing the switching performance of the MOSFET and compares these methods against practical results. Several definitions used within the text are drawn from application note AN605 (http://www.element-14.com/community/docs/DOC-18273).