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AB-12

Fairchild Semiconductor
Part Number AB-12
Manufacturer Fairchild Semiconductor
Description Application Bulletin AB-12
Published Mar 23, 2005
Detailed Description www.fairchildsemi.com Application Bulletin AB-12 Insight into Inductor Current Introduction The design of the main pow...
Datasheet PDF File AB-12 PDF File

AB-12
AB-12

Overview
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 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 o...


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