11

FN6280.1

August 7, 2006

General Application Design Guide

This design guide is intended to provide a high-level

explanation of the steps necessary to create a single-phase

power converter. It is assumed that the reader is familiar with

many of the basic skills and techniques referenced below. In

addition to this guide, Intersil provides complete reference

designs that include schematics, bills of materials, and

example board layouts.

Selecting the LC Output Filter

The duty cycle of an ideal buck converter is a function of the

input and the output voltage. This relationship is written as:

The output inductor peak-to-peak ripple current is written as:

20% to 40% of the maximum DC output load current. The

MOSFET switching loss, inductor core loss, and the resistive

loss of the inductor winding. The DC copper loss of the

inductor can be estimated by:

The copper loss can be significant so attention has to be

given to the DCR selection. Another factor to consider when

choosing the inductor is its saturation characteristics at

elevated temperature. A saturated inductor could cause

destruction of circuit components, as well as nuisance OCP

faults.

A DC/DC buck regulator must have output capacitance

which is the sum of the voltage drop across the capacitor

ESR and of the voltage change stemming from charge

moved in and out of the capacitor. These two voltages are

written as:

and

If the output of the converter has to support a load with high

pulsating current, several capacitors will need to be paralleled

The inductance of the capacitor can cause a brief voltage dip

if the load transient has an extremely high slew rate. Low

inductance capacitors constructed with reverse package

geometry are available. A capacitor dissipates heat as a

shared by a sufficient quantity of paralleled capacitors so that

Take into account that the rated value of a capacitor can fade

as much as 50% as the DC voltage across it increases.

Selection of the Input Capacitor

The important parameters for the bulk input capacitance are

the voltage rating and the RMS current rating. For reliable

operation, select bulk capacitors with voltage and current

ratings above the maximum input voltage and capable of

supplying the RMS current required by the switching circuit.

Their voltage rating should be at least 1.25 times greater

than the maximum input voltage, while a voltage rating of 1.5

times is a preferred rating. Figure 6 is a graph of the input

RMS ripple current, normalized relative to output load current,

as a function of duty cycle that is adjusted for converter

efficiency. The ripple current calculation is written as:

Where:

- x is a multiplier (0 to 1) corresponding to the inductor

peak-to-peak ripple amplitude expressed as a

- D is the duty cycle that is adjusted to take into account

the efficiency of the converter which is written as:

In addition to the bulk capacitance, some low ESL ceramic

capacitance is recommended to decouple between the drain

of the high-side MOSFET and the source of the low-side

MOSFET.

D

V

OUT

V

IN

----------------

=

(EQ. 9)

(EQ. 10)

I

PP

V

OUT

1D

–

()

•

F

•

--------------------------------------

=

(EQ. 11)

P

COPPER

I

LOAD

2

DCR

•

=

∆V

ESR

I

•

SR

=

(EQ. 12)

∆V

C

I

PP

8C

•

SW

•

---------------------------------------

=

(EQ. 13)

(EQ. 14)

I

IN_RMS

I

MAX

2

DD

2

–

()

⋅

()

xI

MAX

2

D

12

------

⋅⋅

⎝⎠

⎛⎞

+

I

MAX

-----------------------------------------------------------------------------------------------------

=

D

V

OUT

V

⋅

--------------------------

=

0

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

1

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

0.55

0.6

FIGURE 6. NORMALIZED RMS INPUT CURRENT FOR x = 0.8

DUTY CYCLE

x = 1

x = 0.75

x = 0.50

x = 0.25

x = 0

ISL6269B