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Feb2-07 Rev O

SP7663 Wide Input Voltage Range 6A, 600kHz Buck Regulator

© 2007 Sipex Corporation

APPLICATIONS INFORMATION

and provide low core loss at the high

switching frequency. Low cost powdered-

iron cores have a gradual saturation char-

acteristic but can introduce considerable

AC core loss, especially when the induc-

tor value is relatively low and the ripple

current is high. Ferrite materials, although

more expensive, have an abrupt satura-

tion characteristic with the inductance

dropping sharply when the peak design

current is exceeded. Nevertheless, they

are preferred at high switching frequen-

cies because they present very low core

loss while the designer is only required

to prevent saturation. In general, ferrite

or molypermalloy materials are a better

choice for all but the most cost sensitive

applications.

Optimizing Efficiency

The power dissipated in the inductor is equal

to the sum of the core and copper losses.

To minimize copper losses, the winding

resistance needs to be minimized, but this

usually comes at the expense of a larger

inductor.Corelosseshaveamoresignificant

contribution at low output current where the

copper losses are at a minimum, and can

typically be neglected at higher output cur-

rents where the copper losses dominate.

Core loss information is usually available

from the magnetics vendor. Proper induc-

tor selection can affect the resulting power

supplyefficiencybymorethan15%!

The copper loss in the inductor can be cal-

culatedusingthefollowingequation:

2

L(rMS)

thatcanbecalculatedasfollows:

3

Inductor Selection

There are many factors to consider in se-

lecting the inductor including core material,

inductance vs. frequency, current handling

capability,efficiency,sizeandEMI.Inatypi-

cal SP7663 circuit, the inductor is chosen

primarilyforvalue,saturationcurrentandDC

resistance. Increasing the inductor value will

decrease output voltage ripple, but degrade

transientresponse.Lowinductorvaluespro-

vide the smallest size, but cause large ripple

currents,poorefficiencyandrequiremore

output capacitance to smooth out the larger

ripple current. The inductor must be able

to handle the peak current at the switching

frequencywithoutsaturating,andthecopper

resistance in the winding should be kept as

low as possible to minimize resistive power

loss.Agood compromise between size, loss

and cost is to set the inductor ripple current

to be within 20% to 40% of the maximum

output current.

The switching frequency and the inductor

operating point determine the inductor value

asfollows:

L

=

.

where:

to the maximum output current

Thepeak-to-peakinductorripplecurrentis:

=

.

Once the required inductor value is selected,

theproperselectionofcorematerialisbased

onpeakinductorcurrentandefficiencyre-

quirements. The core must be large enough

not to saturate at the peak inductor current

2