Application Bulletin AB-12
Insight into Inductor Current

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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 cur-
rent 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 back-
ground necessary to making an informed selection of an
inductor.
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 pur-
poses of understanding the design of an inductor it is neces-
sary 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):
Figure 1. Basic Switching Action of a Converter
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
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 con-
verter 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 neces-
sarily larger than the output voltage, and so there is a net pos-
itive voltage across the inductor. Conversely, in State 2,
ground is applied to the side of the inductor that was previ-
ously 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
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:
Figure 2. Inductor Current
V
IN
State 1
+
State 2
DC Output Voltage
VL
dI
dt
-----=
I
DC
State 1 State 2
I
PP
Rev 1.0.0.