Demystifying Buck Inductors

Jul 1, 2001 12:00 PM
By Jim Holdahl and Terry VanConant, CoEv, Watertown, S.D.

A buck inductor is nothing more than a wound inductor normally used in switchmode power applications to step a dc voltage source down to a lower level. However, it's just a little more complex than that. First, the term “buck” refers specifically to how it's used within the designed application.

For instance, the buck inductor “bucks” the output voltage to a lower level while its cousin, the boost inductor, “boosts” the output voltage to a higher level. Of course, this means the buck/boost component would either “buck” or “boost” the output voltage higher or lower than that of the input voltage.

In a typical buck circuit, when the switch is closed, current starts to flow through to the output, steadily rising at a rate governed by the inductance (see figure). According to Lenz's law, Δi=EΔt/L, or the change in current through an inductor is equal to voltage times the change in time divided by the inductance. As the current steadily rises through R_L, the output voltage develops proportionally. When you reach a predetermined voltage limit, the control IC turns off the switch and the magnetic field around the inductor collapses continuing to feed the output circuit until a minimum voltage condition is reached and the control IC once again turns on the switch repeating the cycle.

When specifying a buck converter inductor, the first item you must determine is the minimum inductance. You do this by taking into account the inductor ripple current (typically double the minimum output current), the switch ON/OFF times, frequency of operation, voltage drop across the rectification device, and the maximum input voltage. As an example, the Table lists the buck converter specifications for calculating the minimum required inductance:

Using the maximum input voltage, the following equation gives the minimum “on-time” of the switch. This is a critical parameter, since it will be the driving factor behind the minimum inductance required, as you will see in the later equations:

Where:
T_ON(min) = Minimum switch on time
V_O = Output voltage
V_IN(max) = Maximum input voltage
V_SW = Voltage drop across the switching device
f_s = Switching frequency

By selecting a value of 1A for the inductor ripple current (generally twice the minimum value of dc output current), we can then solve for the minimum inductance:

Where:
L_MIN = Minimum inductance
I_R = Inductor ripple current Therefore:

Therefore, the inductor requiring specification for this particular application should have an inductance value of at least 68 μH as well as a current rating with a comfortable safety margin above the maximum operating current.

Although this is a somewhat simplified version of the process, this guide should provide you with the means to get a reasonably fast and accurate calculation for specifying a buck inductor.

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