Datasheet

LTC1922-1
19
where:
C
OSS
= MOSFET D-S capacitance
l
MAG
= magnetizing inductance
f
SW
= switching frequency
D = duty cycle
L
L
= leakage inductance
For a 48V to 3.3V/5V, 200W converter, the following
values were derived:
f
SW
: 300kHz
L
MAG
: 100µH
L
COM
: 0.9µH
L
OUT
: 2.2µH
Turns Ratio (N) = 2.5
Output Capacitors
Output capacitor selection has a dramatic impact on ripple
voltage, dynamic response to transients and stability.
Capacitor ESR along with output inductor ripple current
will determine the peak-to-peak voltage ripple on the
output. The current doubler configuration is advanta-
geous because it has inherent ripple current reduction.
The dual output inductors deliver current to the output
capacitor 180 degrees out of phase, in effect, partially
canceling each other’s ripple current. This reduction is
maximized at high duty cycle and decreases as the duty
cycle reduces. This means that a current doubler con-
verter requires less output capacitance for the same
performance as a conventional converter. By determining
the minimum duty cycle for the converter, worse-case
V
OUT
ripple can be derived by the formula given below.
V I ESR
V ESR
Lf
DD
ORIPPLE RIPPLE
O
OSW
==•
•
••
(– )(– )
2
112
where:
D = minimum duty cycle
f
SW
= oscillator frequency
L
O
= output inductance
ESR = output capacitor series resistance
The amount of bulk capacitance required is usually system
dependent, but has some relationship to output induc-
tance value, switching frequency, load power and dynamic
load characteristics. Polymer electrolytic capacitors are
the preferred choice for their combination of low ESR,
small size and high reliability. For less demanding applica-
tions, or those not constrained by size, aluminum electro-
lytic capacitors are commonly applied. Most
DC/DC converters in the 100kHz to 300kHz range use 20µF
to 25µF of bulk capacitance per watt of output power.
Converters switching at higher frequencies can usually
use less bulk capacitance. In systems where dynamic
response is critical, additional high frequency capacitors,
such as ceramics, can substantially reduce voltage tran-
sients,
Power converter stability is, to a large extent, determined
by the choice of output capacitor. A zero in the converter’s
transfer function is given by 1/(2π • ESR • C
O
). Aluminum
electrolytic ESR is highly variable with temperature, in-
creasing by about 4× at cold temperatures, making the
ESR zero frequency highly variable. Polymer electrolytic
ESR is essentially flat with temperature. This characteris-
tic simplifies loop compensation and allows for a much
faster responding power supply compared to one with
aluminum electrolytic capacitors. Specific details on loop
compensation are given in the Compensation section of
the data sheet.
Power MOSFETs
The full-bridge power MOSFETs should be selected for
their R
DS(ON)
and BV
DSS
ratings. Select the lowest BV
DSS
rated MOSFET available for a given input voltage range
leaving at least a 20% voltage margin. Conduction losses
are directly proportional to R
DS(ON)
. Since the full-bridge
has two MOSFETs in the power path most of the time,
conduction losses are approximately equal to:
2 • R
DS(ON)
• I
2
, where I = I
O
/2N
Switching losses in the MOSFETs are dominated by the
power required to charge their gates, and turn-on and
turn-off losses. At higher power levels, gate charge power
is seldom a significant contributor to efficiency loss. ZVS
operation virtually eliminates turn-on losses. Turn-off
losses are reduced by the use of an external drain to source
OPERATIO
U