NCP5211A

http://onsemi.com

10

APPLICATIONS INFORMATION

APPLICATIONS AND COMPONENT SELECTION

Inductor Component Selection

The output inductor may be the most critical component

in the converter because it will directly effect the choice of

other components and dictate both the steady−state and

transient performance of the converter. When selecting an

inductor, the designer must consider factors such as DC

current, peak current, output voltage ripple, core material,

magnetic saturation, temperature, physical size, and cost

(usually the primary concern).

In general, the output inductance value should be as low

and physically small as possible to provide the best transient

response and minimum cost. If a large inductance value is

used, the converter will not respond quickly to rapid changes

in the load current. On the other hand, too low an inductance

value will result in very large ripple currents in the power

components (MOSFETs, capacitors, etc.) resulting in

increased dissipation and lower converter efficiency.

Increased ripple currents will force the designer to use

higher rated MOSFETs, oversize the thermal solution, and

use more, higher rated input and output capacitors. The

converter cost will be adversely affected.

One method of calculating an output inductor value is to

size the inductor to produce a specified maximum ripple

current in the inductor. Lower ripple currents will result in

less core and MOSFET losses and higher converter

efficiency. The following equation may be used to calculate

the minimum inductor value to produce a given maximum

ripple current (

this equation is a minimum because values less than this will

produce more ripple current than desired. Conversely,

higher inductor values will result in less than the maximum

ripple current.

LoMIN + (Vin * Vout) @ Vout (a @ IO,MAX @ Vin @ fSW)

α is the ripple current as a percentage of the maximum

output current (

α = 0.15 for ±15%, α = 0.25 for ±25%, etc)

value is used, the inductor current will swing

±α/2% about

Iout. Therefore, the inductor must be designed or selected

such that it will not saturate with a peak current of (1 +

α/2)

Power dissipation in the inductor can now be calculated

from the RMS current level. The RMS of the ac component

of the inductor is given by the following relationship:

IAC +

IPP

12

where IPP =

IRMS +

IOUT2 ) IAC2

The power dissipation for the inductor can be determined

from:

P

+ IRMS2

ESR

Input Capacitor Selection and Considerations

The input capacitor is used to reduce voltage ripple caused

by the current surges in the top pass transistor.

The input current is pulsing at the switching frequency

going from 0 to peak current in the inductor. The duty cycle

will be a function of the ratio of the input to output voltage

and of the efficiency.

D

+

VO

VI

1

Eff

The RMS value of the ripple into the input capacitors can

now be calculated:

IIN(RMS) + IOUT D * D2

The input RMS is maximum at 50% D, so selection of the

possible duty cycle closest to 50% will give the worst case

dissipation in the capacitors. The power dissipation of the

input capacitors can be calculated by multiplying the square

of the RMS current by the ESR of the capacitor.

Output Capacitor

The output capacitor filters output inductor ripple current

and provides low impedance for load current changes. The

effect of the capacitance for handling the power supply

induced ripple will be discussed here. Effects of load

transient behavior can be considered separately.

The principle consideration for the output capacitor is the

ripple current induced by the switches through the inductor.

discussion of the inductor. This ripple component will

induce heating in the capacitor by a factor of the RMS

current squared multiplied by the ESR of the output

capacitor section. It will also create output ripple voltage.

The ripple voltage will be a vector summation of the ripple

current times the ESR of the capacitor, plus the ripple current

integrating in the capacitor, and the rate of change in current

times the total series inductance of the capacitor and

connections.

The inductor ripple current acting against the ESR of the

output capacitor is the major contributor to the output ripple

voltage. This fact can be used as a criterion to select the

output capacitor.

VPP + IPP

CESR

The power dissipation in the output capacitor can be

calculated from:

P

+ IAC2

CESR

where:

network.

MOSFET & Heatsink Selection

Power dissipation, package size, and thermal solution

drive MOSFET selection. To adequately size the heat sink,

the design must first predict the MOSFET power dissipation.