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PDF ( 数据手册 , 数据表 ) LTC1922-1

零件编号 LTC1922-1
描述 Synchronous Phase ModEGZ12DCFulated Full-Bridge Controller
制造商 Linear
LOGO Linear LOGO 


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LTC1922-1 数据手册, 描述, 功能
LTC1922-1
Synchronous Phase
Modulated Full-Bridge Controller
FEATURES
DESCRIPTIO
s Adaptive DirectSenseTM Zero Voltage Switching
s Integrated Synchronous Rectification Control for
Highest Efficiency
s Output Power Levels from 50W to Kilowatts
s Very Low Start-Up and Quiescent Currents
s Compatible with Voltage Mode and Current Mode
Topologies
s Programmable Slope Compensation
s Undervoltage Lockout Circuitry with 4.2V Hysteresis
and Integrated 10.3V Shunt Regulator
s Fixed Frequency Operation to 1MHz
s 50mA Outputs for Bridge Drive and Secondary Side
Synchronous Rectifiers
s Soft-Start, Cycle-by-Cycle Current Limiting and
Hiccup Mode Short-Circuit Protection
s 5V, 15mA Low Dropout Regulator
s 20-Pin PDIP and SSOP Packages
U
APPLICATIO S
s Telecommunications, Infrastructure Power Systems
s Distributed Power Architectures
s Server Power Supplies
s High Density Power Modules
The LTC®1922-1 phase shift PWM controller provides all
of the control and protection functions necessary to imple-
ment a high performance, zero voltage switched, phase
shift, full-bridge power converter with synchronous recti-
fication. The part is ideal for developing isolated, low
voltage, high current outputs from a high voltage input
source. The LTC1922-1 combines the benefits of the full-
bridge topology with fixed frequency, zero voltage switch-
ing operation (ZVS). Adaptive ZVS circuity controls the
turn-on signals for each MOSFET independent of internal
and external component tolerances for optimal perfor-
mance.
The LTC1922-1 also provides secondary side synchro-
nous rectifier control. The device uses peak current mode
control with programmable slope comp and leading edge
blanking.
The LTC1922-1 features extremely low operating and
start-up currents to simplify off-line start-up and bias
circuitry. The LTC1922-1 also includes a full range of
protection features and is available in 20-pin through hole
(N) and surface mount (G) packages.
, LTC and LT are registered trademarks of Linear Technology Corporation.
DirectSense is a trademark of Linear Technology Corporation.
TYPICAL APPLICATIO
BIAS
SUPPLY
VIN
48V
LTC1922-1
VOUT
3.3V
ISOLATED
FEEDBACK
Efficiency
100
90
VIN = 48V
80
VIN = 36V
70
60
0
10 20 30
LOAD CURRENT (A)
40
1922 • TA01b
1922 TA01a
1







LTC1922-1 pdf, 数据表
LTC1922-1
U
OPERATIO
Phase Shift Full-Bridge PWM
Conventional full-bridge switching power supply topolo-
gies are often employed for high power, isolated DC/DC
and off-line converters. Although they require two addi-
tional switching elements, substantially greater power and
higher efficiency can be attained for a given transformer
size compared to the more common single-ended forward
and flyback converters. These improvements are realized
since the full-bridge converter delivers power during both
parts of the switching cycle, reducing transformer core
loss and lowering voltage and current stresses. The full-
bridge converter also provides inherent automatic trans-
former flux reset and balancing due to its bidirectional
drive configuration. As a result, the maximum duty cycle
range is extended, further improving efficiency. Soft switch-
ing variations on the full-bridge topology have been pro-
posed to improve and extend its performance and
application. These zero voltage switching (ZVS) tech-
niques exploit the generally undesirable parasitic ele-
ments present within the power stage. The parasitic
elements are utilized to drive near lossless switching
transitions for all of the external power MOSFETs.
LTC1922-1 phase shift PWM controller provides enhanced
performance and simplifies the design task required for a
ZVS phase shifted full-bridge converter. The primary
attributes of the LTC1922-1 as compared to currently
available solutions include:
1) Truly adaptive and accurate (DirectSense technology)
ZVS switching delays.
Benefit: higher efficiency, higher duty cycle capability,
eliminates external trim.
2) Internally generated drive signals for current doubler
synchronous rectifiers.
Benefit: eliminates external glue logic, drivers, optimal
timing for highest efficiency.
3) Programmable (single resistor) leading edge blanking.
Benefit: prevents spurious operation, reduces external
filtering required on CS.
4) Programmable (single resistor) slope compensation.
Benefit: eliminates external glue circuitry.
5) Optimized current mode control architecture.
Benefit: eliminates glue circuitry, less overshoot at start-
up, faster recovery from system faults.
6) Proven reference circuits and design tools.
Benefit: substantially reduced learning curve, more time
for optimization.
As a result, the LTC1922-1 makes the ZVS topology
feasible for a wider variety of applications, including those
at lower power levels.
The LTC1922-1 controls 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 winding be-
tween the input voltage and ground every oscillator cycle.
The pair of switches that conduct are alternated by an
internal flip-flop in the LTC1922-1. Thus, the voltage
applied to the primary is reversed in polarity on every
switching cycle and each output drive signal is 1/2 the
frequency of the oscillator. The on-time of each driver
signal is slightly less that 50%. The actual percentage is
adaptively modulated by the LTC1922-1. The on-time
overlap of the diagonal switch pairs is controlled by the
LTC1922-1 phase modulation circuitry. (Refer to Block
and Timing Diagrams) This overlap sets the approximate
duty cycle of the converter. The LTC1922-1 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 the 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 pro-
duce 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.
8







LTC1922-1 equivalent, schematic
LTC1922-1
U
OPERATIO
2.0
RS = 0.025
1.8 VIN = 48V
1.6
VO = 3.3V
LO = 2.2µH
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
0 5 10 15 20 25 30 35 40
OUTPUT CURRENT (A)
1922 • F09
Figure 9. RSENSE Power Loss vs IOUT
If RAMP and CS are connected together:
RCS
=
0.4V
(125µA •RSLOPE)
IP (PEAK)
IP(PEAK) =
IO(MAX)
2 • N • EFF
+
VIN(MAX) • 2 •DMIN
LMAG • f CLK
+
VO(1– DMIN)
LOUT • f CLK• N
where: N = Transformer turns ratio
If RAMP and CS are separated
RCS
=
0.4V
IP (PEAK)
Current Transformer Sensing
A current sense transformer can be used in lieu of resistive
sensing with the LTC1922-1. Current sense transformers
are available in many styles from several manufacturers.
A typical sense transformer for this application will use a
1:50 turns ratio (N), so that the sense resistor value is N
times larger, and the secondary current N times smaller
than in the resistive sense case. Therefore, the sense
resistor power loss is about N times less with the trans-
former method, neglecting the transformers core and
copper losses. The disadvantages of this approach
include, higher cost and complexity, lower accuracy, core
reset/max duty cycle limitations and lower speed. Never-
theless, for very high power applications, this method is
preferred. The sense transformer primary is placed in the
same location as the ground referenced sense resistor, or
between the upper MOSFET drains in the (MA, MC) and
VIN. The advantage of the high side location is a greater
immunity to leading edge noise spikes, since gate charge
current and reflected rectifier recovery current are largely
eliminated. Figure 10 illustrates a typical current sense
transformer based sensing scheme. RS in this case is
calculated the same as in the resistive case, only its value
is increased by the sense transformer turns ratio. At high
duty cycles, it may become difficult or impossible to reset
the current transformer. This is because the required
transformer reset voltage increases as the available time
for reset decreases to equalize the (volt • seconds) applied.
The interwinding capacitance and secondary inductance
of the current sense transformer form a resonant circuit
that limits the dV/dT on the secondary of the CS trans-
former. This in turn limits the maximum achievable duty
cycle for the CS transformer. Attempts to operate beyond
this limit will cause the transformer core to “walk” and
eventually saturate, opening up the current feedback loop.
Common methods to address this limitation include:
1. Reducing the maximum duty cycle by lowering the
power transformer turns ratio.
2. Reducing the switching frequency of the converter.
3. Employ external active reset circuitry.
4. Using two CS transformers summed together.
5. Choose a CS transformer optimized for high frequency
applications.
MB
SOURCE
MD
SOURCE
RAMP
RSLOPE
N:1
RS
CURRENT
TRANSFORMER
CS
OPTIONAL
FILTERING
1922 F10
Figure 10. Current Transformer Sense Circuitry
16










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