LTC3722-2_15 Datasheet, PDF (11/28 Pages) Linear Technology – Synchronous Dual Mode Phase Modulated Full Bridge Controllers

LTC3722-1/LTC3722-2
OPERATION
3 Internally generated drive signals with programmable
turn-off for current doubler synchronous rectifiers.
Benefit: eliminates external glue logic, drivers, optimal
timing for highest efficiency.
4. Programmable (single resistor) leading edge blanking.
Benefit: prevents spurious operation, reduces external
filtering required on CS.
5. Programmable (single resistor) slope compensation.
Benefit: eliminates external glue circuitry.
6. Optimized current mode control architecture.
Benefit: eliminates glue circuitry, less overshoot at
start-up, faster recovery from system faults.
7. Programmable system undervoltage lockout and hys-
teresis.
Benefit: provides an accurate turn-on voltage for power
supply and reduces external circuitry.
As a result, the LTC3722-1/LTC3722-2 makes the ZVS topol-
ogy feasible for a wider variety of applications, including
those at lower power levels.
The LTC3722-1/LTC3722-2 control four external power
switches in a full bridge arrangement. The load on the
bridge is the primary winding of a power transformer. The
diagonal switches in the bridge connect the primary wind-
ing between the input voltage and ground every oscillator
cycle. The pair of switches that conduct are alternated by
an internal flip-flop in the LTC3722-1/LTC3722-2. Thus,
the voltage applied to the primary is reversed in polarity
on every switching cycle and each output drive signal is
one-half the frequency of the oscillator. The on-time of
each driver signal is slightly less than 50%. The on-time
overlap of the diagonal switch pairs is controlled by the
LTC3722-1/LTC3722-2 phase modulation circuitry (refer
to the Block and Timing Diagrams). This overlap sets the
approximate duty cycle of the converter. The LTC3722‑1/
LTC3722-2 driver output signals (OUTA to OUTF) are
optimized for interface with an external gate driver IC or
buffer. External power MOSFETs A and C require high side
driver circuitry, while B and D are ground referenced and E
and F are ground referenced but on the secondary-side of
the isolation barrier. Methods for providing drive to these
elements are detailed in this data sheet. The secondary
voltage of the transformer is the primary voltage divided
by the transformer turns ratio. Similar to a buck converter,
the secondary square wave is applied to an output filter
inductor and capacitor to produce a well regulated DC
output voltage.
Switching Transitions
The phase-shifted full bridge can be described by four
primary operating states. The key to understanding how
ZVS occurs is revealed by examining the states in detail.
Each full cycle of the transformer has two distinct periods
in which power is delivered to the output, and two “free-
wheeling” periods. The two sides of the external bridge
have fundamentally different operating characteristics that
become important when designing for ZVS over a wide
load current range. The left bridge leg is referred to as the
passive leg, while the right leg is referred to as the active
leg. The following descriptions provide insight as to why
these differences exist.
State 1 (Power Pulse 1)
As shown in Figure 1, State 1 begins with MA, MD and MF
“ON” and MB, MC and ME “OFF.” During the simultane-
ous conduction of MA and MD, the full input voltage is
applied across the transformer primary winding and fol-
lowing the dot convention, V_IN/N is applied to the left side
of LO1 allowing current to increase in LO1. The primary
current during this period is approximately equal to the
output inductor current (LO1) divided by the transformer
turns ratio plus the transformer magnetizing current
(V_IN • t_ON)/(L_MAG • 2). MD turns off and ME turns on at
the end of State 1.
State 2 (Active Transition and Freewheel Interval)
MD turns off when the phase modulator comparator transi-
tions. At this instant, the voltage on the MD/MC junction
begins to rise towards the applied input voltage (V_IN).
The transformer’s magnetizing current and the reflected
output inductor current propels this action. The slew rate
is limited by MOSFET MC and MD’s outputcapacitance
(C_OSS), snubbing capacitance and the transformer inter-
winding capacitance. The voltage transition on the active
leg from the ground reference point to V_IN will always
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