LTC3722-2_15 Datasheet, PDF (22/28 Pages) Linear Technology – Synchronous Dual Mode Phase Modulated Full Bridge Controllers

LTC3722-1/LTC3722-2
OPERATION
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:
N = ^{VIN(MIN) • DMAX}
2 • V_OUT
where:
V_IN(MIN) = Minimum V_IN for operation
D_MAX = Maximum duty cycle of controller (DC_MAX)
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 out-
put. The current doubler configuration is advantageous
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 converter 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
following formula:
V_{O R I P P L E}
=
I_{R I P P L E}
• ESR
=
V_O
L_O •
• ESR
2 • f_SW
(1– D)(1–
2D)
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 inductance
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 applications,
or those not constrained by size, aluminum electrolytic
capacitors are commonly applied. Most DC/DC convert-
ers 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 transients.
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) • I², 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
snubber capacitor and/or a very low resistance turn-off
driver. If synchronous rectifier MOSFETs are used on the
secondary, the same general guidelines apply. Keep in
mind, however, that the BV_DSS rating needed for these can
be greater than V_IN(MAX)/N, depending on how well the
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