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Design Notes

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  Isolated Flyback Converter Regulates Without an Optocoupler &mdash DN260
  PDF, 75 Kb, File published: Jun 2, 2001
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  Isolated Flyback Converter Regulates without
  an Optocoupler – Design Note 260
  Robert Sheehan
  Introduction
  Designing an isolated power supply can be a daunting
  task. Optocouplers, secondary-side error amplifiers
  and frequency compensation can send you scurrying
  to your distributor catalog to п¬Ѓnd a module. Fortunately,
  there are new IC solutions that make isolated power
  much simpler and less expensive. Here is a gem of a
  circuit that takes the headaches out of your isolated
  power supply design task and at a small fraction of a
  module’s cost. Circuit Description
  A flyback topology was chosen that allows for wide
  variations of input voltage and output current. At the
  heart of the circuit is the LT®1725 isolated flyback controller. This is a fixed frequency, current mode PWM
  controller. It drives the gate of an external MOSFET
  and is powered from the third (bias) winding of the
  flyback transformer.
  The bias winding also provides output voltage feedback
  information. This is done directly through a resistive
  divider, without any rectification or filtering. The unique
  features of the LT1725 allow regulation to be maintained …

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  Power over Ethernet Isolated Power Supply Delivers 11.5W at 90% Efficiency &mdash DN338
  PDF, 80 Kb, File published: May 2, 2004
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  Power over Ethernet Isolated Power Supply
  Delivers 11.5W at 90% Efficiency – Design Note 338
  Jesus Rosales
  fewer switches to turn on and off the better. A push-pull
  converter could be used, but the additional complexity
  is not justified at this power level. A single transistor
  forward converter is another option, but requires
  an additional output inductor and rectifier. A flyback
  converter is the simplest choice. Flyback converters
  are thought to be less efficient than forward and pushpull converters, but that changes when the output is
  synchronously rectified (a MOSFET is used instead of
  a diode to rectify the output). When powering IP telephones, wireless access points,
  PDA charging stations and other PDs (Powered Devices)
  from an Ethernet cable, designers have at most 12.95W
  of available power per the IEEE 802.3af standard. Increased demands for power mandate a very efficient
  power converter especially for classВ 3 devices (consuming between 6.49W and 12.95W). The more power lost
  in the converter, the less power available for the PD.
  The voltage available from the PSE (Power Sourcing
  Equipment) ranges from 44V to 57V, but PDs need to
  operate with as much as 20О© of series wire resistance.
  A PD can never draw more than 350mA or 12.95W
  continuously. With the maximum input current of
  350mARMS, the input voltage can droop as much as …

Articles

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  Simple Isolated Telecom Flyback Circuit Provides Regulation Without Optocoupler &mdash LT Journal
  PDF, 63 Kb, File published: May 1, 2002
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  DESIGN IDEAS Simple Isolated Telecom Flyback
  Circuit Provides Regulation Without
  Optocoupler
  by John Shannon
  Introduction Circuit Operation
  over time. They are also relatively
  slow. Optocoupler shortcomings add
  considerably to the total converter
  design time and ultimately limit performance.
  Consider instead the schematic of
  Figure 1. This is a flyback converter
  based on the LT1725. There are extremely few components and yet a
  high level of functionality. This design is short circuit proof and includes
  an input undervoltage lockout for increased reliability. The performance
  of this converter is shown in Figure 2.
  Output voltage is regulated to within
  1% over a 2:1 input voltage range
  with 10% or greater load. No load
  regulation is within 2% over a 2:1
  input voltage range. This is well within
  the typical requirement of 5% regulation. The LT1725 uses a proprietary technique to regulate an isolated output
  voltage without an optocoupler, thus
  greatly simplifying flyback converter …

Model Line

Manufacturer's Classification

• Power Management > Switching Regulator > Flyback, Forward and Isolated Controllers

Other Names:

LT1725ISTRPBF, LT1725IS TRPBF