AB-12

www.fairchildsemi.com Application Bulletin AB-12 Insight into Inductor Current Introduction The design of the main power inductor in a switching power supply provides many challenges to the engineer. Not only must an inductance value be chosen, but also how much current the inductor can handle, the winding resistance, mechanical factors, etc. This Application Bulletin looks at one of these considerations: understanding the effects of DC current on an inductor. This will provide some of the background necessary to making an informed selection of an inductor. controller being used, this may be done in one of two ways: the connection to ground can be made with a diode, or with another (“low-side”) MOSFET. In the latter case, the converter is called “synchronous”. Now consider what happens to the inductor’s current during these two states. In State 1, the input voltage is being applied to one side of the inductor, and the output voltage to the other side. For a buck converter, the input voltage is necessarily larger than the output voltage, and so there is a net positive voltage across the inductor. Conversely, in State 2, ground is applied to the side of the inductor that was previously attached to the input voltage. For a buck converter, the output voltage is necessarily positive, and so there is a net negative voltage across the inductor. We recall that the current through an inductor changes according to dI V = L ---dt Understanding the Function of the Inductor An inducto...