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ADP3198

Rev. A | Page 21 of 32

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. However, using smaller inductors

allows the converter to meet a specified peak-to-peak transient

deviation with less total output capacitance. Conversely, a higher

inductance means lower ripple current and reduced conduction

losses, but more output capacitance is required to meet 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.

()

VID

−

(4)

Equation 5 can be used to determine the minimum inductance

based on a given output ripple voltage.

()

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 what is 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 dc

resistance (DCR), which is used for measuring the phase currents.

A large DCR can cause excessive power losses, though too small

a value can lead to increased measurement error. A good rule is

to have the DCR (RL) 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. Reasonable

tolerances most manufacturers can meet are 15% inductance

and 7% DCR at room temperature. 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 (from Micrometals, Inc., for example,

or Kool Mu® from Magnetics®) 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

•

Intusoft Magnetic Designer Software

•

Designing Magnetic Components for High Frequency

DC to DC Converters

, 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®

•

Coiltronics®

•

Sumida Corporation®

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