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Date: 08/12/04

SP7652 Wide Input Voltage Range 6A, 600kHz, Buck Regulator

© Copyright 2004 Sipex Corporation

APPLICATIONS INFORMATION

Inductor Selection

There are many factors to consider in selecting

the inductor including core material, inductance

vs. frequency, current handling capability, effi-

ciency, size and EMI. In a typical SP7652 cir-

cuit, the inductor is chosen primarily by operat-

ing frequency, saturation current and DC resis-

tance. Increasing the inductor value will de-

crease output voltage ripple, but degrade tran-

sient response. Low inductor values provide the

smallest size, but cause large ripple currents,

poor efficiency and more output capacitance to

smooth out the larger ripple current. The induc-

tor must be able to handle the peak current at the

switching frequency without saturating, and the

copper resistance in the winding should be kept

as low as possible to minimize resistive power

loss. A good 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 oper-

ating point determine the inductor value as fol-

lows:

(max)

(max )

(max)

)

(

OUT

r

S

IN

OUT

IN

OUT

I

K

F

V

V

V

V

L

−

=

where:

Fs = switching frequency

Kr = ratio of the ac inductor ripple current to the

maximum output current

The peak to peak inductor ripple current is:

L

F

V

V

V

V

I

S

IN

OUT

IN

OUT

PP

(max)

(max)

)

(

−

=

Once the required inductor value is selected, the

proper selection of core material is based on

peak inductor current and efficiency require-

ments. The core must be large enough not to

saturate at the peak inductor current

2

(max)

PP

OUT

PEAK

I

I

I

+

=

and provide low core loss at the high switching

frequency. Low cost powdered iron cores are

inappropriate for 900kHz operation. Gapped

ferrite inductors are widely available for consid-

eration. Select devices that have operating data

shown up to 1MHz. Ferrite materials, on the

other hand, are more expensive and have an

abrupt saturation characteristic with the induc-

tance dropping sharply when the peak design

current is exceeded. Nevertheless, they are pre-

ferred at high switching frequencies because

they present very low core loss and the design

only needs to prevent saturation. In general,

ferrite or molypermalloy materials are better

choice for all but the most cost sensitive appli-

cations.

Optimizing Efficiency

The power dissipated in the inductor is equal to

the sum of the core and copper losses. To mini-

mize copper losses, the winding resistance needs

to be minimized, but this usually comes at the

expense of a larger inductor. Core losses have a

more significant contribution at low output cur-

rent where the copper losses are at a minimum,

and can typically be neglected at higher output

currents where the copper losses dominate. Core

loss information is usually available from the

magnetic vendor. Proper inductor selection can

affect the resulting power supply efficiency by

more than 15-20%!

The copper loss in the inductor can be calculated

using the following equation:

WINDING

RMS

L

Cu

L

R

I

P

2

)

(

)

(

=

can be calculated as follows:

1

2

3