LTC3722-1 LINER [Linear Technology], LTC3722-1 Datasheet - Page 21

• Manufacturer Part Number: LTC3722-1
• Description: Synchronous Dual Mode Phase Modulated Full Bridge Controllers
• Manufacturer: LINER [Linear Technology]

OPERATIO
Selecting the Power Stage Components
Perhaps the most critical part of the overall design of the
converter is selecting the power MOSFETs, transformer,
inductors and filter capacitors. Tremendous gains in effi-
ciency, transient performance and overall operation can
be obtained as long as a few simple guidelines are followed
with the phase shifted full-bridge topology.
Power Transformer
Switching frequency, core material characteristics, series
resistance and input/output voltages all play an important
role in transformer selection. Close attention also needs to
be paid to leakage and magnetizing inductances as they
play an important role in how well the converter will
achieve ZVS. Planar magnetics are very well suited to
these applications because of their excellent control of
these parameters.
Turns Ratio
The required turns ratio for a current doubler secondary is
given below. Depending on the magnetics selected, this
value may need to be reduced slightly.
Turns ratio formula:
where:
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
V
D
N
IN(MIN)
MAX
V
IN MIN
= Maximum duty cycle of controller (DC
(
• 2
= Minimum V
V
)
OUT
•
D
U
MAX
IN
for operation
MAX
)
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
where:
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 20 F
to 25 F 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 MOSFETs
The full-bridge power MOSFETs should be selected for
their R
rated MOSFET available for a given input voltage range
leaving at least a 20% voltage margin. Conduction losses
are directly proportional to R
has two MOSFETs in the power path most of the time,
conduction losses are approximately equal to:
OUT
V
D
f
L
ESR = output capacitor series resistance
2 • R
SW
ORIPPLE
O
ripple can be derived by the formula given below.
DS(ON)
DS(ON)
= minimum duty cycle
= oscillator frequency
= output inductance
LTC3722-1/LTC3722-2
and BV
I
RIPPLE
• I
2
, where I = I
DSS
•
ESR
ratings. Select the lowest BV
DS(ON)
O
L
/2N
O
V
O
• •
2
•
. Since the full-bridge
ESR
f
SW
( – )( –
1
D
21
1 2
372212i
DSS
D
)