AB-12| Application Note

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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 inductor is often described as being part of an LC filter at the output of a switching power supply (with the "C" being the output capacitors). Although this is correct, for the purposes of understanding the design of an inductor it is necessary to have deeper insight into the inductor's operation. In a buck converter (the type used by all Fairchild switching controllers), one end of the inductor is attached to the output voltage, which is DC. The other end is alternately attached to the input voltage or ground, the alternation occurring at the switching frequency (see Figure 1):
VIN State 1 DC Output Voltage + State 2

Thus when the voltage across the inductor is positive (State 1), the inductor current increases; when the voltage across the inductor is negative (State 2), the inductor current decreases. The net current through the inductor is shown in Figure 2:

IDC

IPP

Figure 1. Basic Switching Action of a Converter
State 1 State 2

In State 1, the connection is made to the input voltage: this is done by turning on a ("high-side") MOSFET. In State 2, the connection is made to ground. Depending on the type of

Figure 2. Inductor Current

Rev 1.0.0.