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ADP3189

Rev. 0 | Page 23 of 36

INDUCTOR SELECTION

The choice of inductance for the inductor determines the ripple

current in the inductor. Less inductance leads to more ripple

current, which increases the output ripple voltage and conduction

losses in the MOSFETs, but allows using smaller inductors and,

for a specified peak-to-peak transient deviation, less total output

capacitance. Conversely, a higher inductance means lower

ripple current and reduced conduction losses, but requires

larger inductors and more output capacitance for the same

peak-to-peak transient deviation.

In any multiphase converter, a practical value for the

peak-to-peak inductor ripple current is less than 50% of

the maximum dc current in the same inductor. Equation 4

shows the relationship between the inductance, oscillator

frequency, and peak-to-peak ripple current in the inductor.

Equation 5 can be used to determine the minimum inductance

based on a given output ripple voltage.

()

VID

−

(4)

()

RIPPLE

VID

−

≥

(5)

Solving Equation 5 for an 8 mV p-p output ripple voltage yields

()

kHz

330

0.432

mΩ

1.0

1.3

−

≥

If the resulting ripple voltage is less than that designed for,

the inductor can be made smaller until the ripple value is met.

This allows optimal transient response and minimum output

decoupling.

The smallest possible inductor should be used to minimize

the number of output capacitors. For this example, choosing a

320 nH inductor is a good starting point and gives a calculated

ripple current of 11 A. The inductor should not saturate at the

peak current of 35.5 A and should be able to handle the sum of

the power dissipation caused by the average current of 30 A in

the winding and core loss.

Another important factor in the inductor design is the DCR

(RL), which is used for measuring the phase currents. A large

DCR can cause excessive power losses, while too small a value

can lead to increased measurement error. A good rule is to have

the DCR be about 1 to 1½ times the droop resistance (RO). This

example uses an inductor with a DCR of 1.4 mΩ.

DESIGNING AN INDUCTOR

Once the inductance and DCR are known, the next step is to

either design an inductor or to find a standard inductor that

comes as close as possible to meeting the overall design goals.

It is also important to have the inductance and DCR tolerance

specified to control the accuracy of the system. 15% inductance

and 7% DCR, at room temperature, are reasonable tolerances

most manufacturers can meet.

The first decision in designing the inductor is choosing the

core material. Several possibilities for providing low core loss

at high frequencies include the powder cores (for example,

Kool-Mμ® from Magnetics, Inc. or from Micrometals) and the

gapped soft ferrite cores (for example, 3F3 or 3F4 from Philips).

Low frequency powdered iron cores should be avoided due to

their high core loss, especially when the inductor value is

relatively low and the ripple current is high.

The best choice for a core geometry is a closed-loop type such

as a potentiometer core; PQ, U, or E core; or toroid. A good

compromise between price and performance is a core with

a toroidal shape.

Many useful magnetics design references are available for

quickly designing a power inductor, such as

•

Magnetic Designer Software

Intusoft (www.intusoft.com)

•

Designing Magnetic Components for High-Frequency DC-

DCConverters, by William T. McLyman, Kg Magnetics,

Inc., ISBN 1883107008

Selecting a Standard Inductor

The following power inductor manufacturers can provide design

consultation and deliver power inductors optimized for high

power applications upon request.

•

Coilcraft

www.coilcraft.com

•

Coiltronics

www.coiltronics.com

•

Sumida Electric Company

www.sumida.com

•

Vishay Intertechnology

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