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 (α ⋅ I
O,MAX
). The inductor value calculated by
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.
Lo
MIN
(Vin Vout) Vout( I
O,MAX
Vin f
SW
)
α is the ripple current as a percentage of the maximum
output current (α = 0.15 for ±15%, α = 0.25 for ±25%, etc)
and f
sw
is the switching frequency. If the minimum inductor
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)
⋅ I
O,MAX
.
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:
I
AC
I
PP
12
where IPP = α ⋅ I
O,MAX
.
The total I
RMS
of the current will be calculated from:
I
RMS
I
OUT
2
I
AC
2
The power dissipation for the inductor can be determined
from:
P I
RMS
2
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
V
O
V
I
1
Eff
The RMS value of the ripple into the input capacitors can
now be calculated:
I
IN(RMS)
I
OUT
D D
2
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.
This ripple current was calculated as I
AC
in the above
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.
V
PP
I
PP
C
ESR
The power dissipation in the output capacitor can be
calculated from:
P I
AC
2
C
ESR
where:
I
AC
= ac RMS current
C
ESR
= Effective series resistance of the output capacitor
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.
