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ADP1071-1 Datasheet(Fiches technique) 14 Page - Analog Devices
Numéro de pièce
AD [Analog Devices]
Rev. B | Page 14 of 27
THEORY OF OPERATION
The ADP1071-1/ADP1071-2 are PWM, current mode, fixed
frequency, synchronous flyback controllers designed for isolated
dc-to-dc power supplies. Analog Devices iCouplers are integrated
in the ADP1071-1/ADP1071-2 to eliminate the bulky signal
transformers and optocouplers that transmit signals over the
isolation boundary. Integrating the iCouplers reduces system
design complexity, cost, and component count and improves
overall system reliability. With the integrated isolators and
MOSFET drivers on both the primary and the secondary side, the
ADP1071-1/ADP1071-2 offer a compact system level design
and yield a higher efficiency than a diode rectified flyback
converter at heavy loads.
Traditionally in a forward or flyback converter, a discrete
optocoupler is used in the feedback path to transmit the signal
from the secondary to the primary side, and an external trans-
former is used for transmitting the PWM signal from the
primary to the secondary side for synchronous rectification.
However, the current transfer ratio (CTR) of the optocouplers
degrades over time and over temperature and so the optocoupler
must be replaced every 5 to 10 years, depending on the manu-
facturing quality and optocoupler grade that determines the
initial CTR. The ADP1071-1/ADP1071-2 eliminate the use of
an optocoupler and signal transformer, thus reducing system
cost, PCB area, and complexity, while improving system reliability
without the issue of CTR degradation of the optocouplers.
The ADP1071-1/ADP1071-2 controllers offer a complete
solution for an isolated dc-to-dc power supply by integrating
the 5 kV isolators and the primary and secondary control
circuitries in one package.
The PWM controls are performed on the primary side by sensing
the input peak current cycle by cycle with a sense resistor at the
source of the main switching MOSFET. The output of the converter
is sensed by the secondary circuitry, which sends the feedback
and PWM signals to the primary side via the 5 kV integrated
isolators for a complete control loop solution.
The primary circuitry in the ADP1071-1/ADP1071-2 includes
an 8 V LDO, input current sensing, bias circuit, and MOSFET
drivers including an active clamp reset driver, slope compensation,
external frequency synchronization, PWM generator, and a
programmable maximum duty cycle setting. The primary side
also has pins for differential sensing of the current sense signal.
The secondary circuitry includes the feedback compensation, a
5 V LDO regulator, an internal reference, two MOSFET drivers for
synchronous rectification, and a dedicated pin for overvoltage
protection. Additionally, the secondary side features differential
output voltage sensing and a programmable LLM setting.
The integrated iCouplers carry out the communications between
the primary and secondary sides by transmitting the feedback
signal and the PWMs over the isolation barrier.
The feedback signal and timing of synchronous rectifier PWMs
are transmitted between the primary and the secondary sides,
or between the secondary and primary sides, through the
Couplers using a proprietary transmission scheme.
The ADP1071-1/ADP1071-2 also offer features such as input
current protection, OVP, UVLO, precision enable with adjustable
hysteresis, OTP, LLM, and tracking.
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